Environmental Adversity, Endoplasmic Reticulum Stress, and Neurogenesis

Abstract

Environmental adversity experienced during the prenatal period can include maternal nutritional deficiency, infectious agents, heavy metals, industrial chemicals, air pollution, medication, alcohol exposure, and substance use, as well as maternal factors such as diabetes. If these adversities occur during certain developmental time windows, they can significantly impact fetal development and have long-lasting neurobehavioral deficits. However, molecular mechanisms underlying the impact of environmental adversity remains unclear. The process by which new neurons form in the brain is neurogenesis. In certain brain regions neurogenesis continues throughout the lifespan and is essential for continued neurodevelopment and good mental health. Appropriate cellular responses to both extrinsic and intrinsic stressors require maintenance of the proteome, which relies on homeostasis of the endoplasmic reticulum (ER). Perturbations of ER homeostasis, such as the depletion of nutrients and disturbances in calcium or redox status, lead to abnormal accumulation of misfolded proteins and induce ER stress, which is monitored by the unfolded protein response (UPR). UPR is an adaptive reaction that restores protein homeostasis or triggers apoptotic cell death. Recent research indicates that ER stress during development can impair neurogenesis. We hypothesize that ER stress-mediated disruption of neurogenesis underlies the neurobehavioral deficits caused by environmental adversity. In this review, we discuss evidence of the impact that environmental adversities have on neurogenesis and the involvement of ER stress. We also discuss crosstalk across ER stress, oxidative stress, autophagy, and neuroinflammation, as well as potential therapeutic strategies that target ER stress/UPR for the treatment of neurobehavioral deficits associated with environmental adversities.

Introduction

Early life adversity (ELA) is a major risk factor for mental illness, potentially including neurodevelopmental disorders (NDDs), but the underlying neurobiological mechanisms are poorly understood [1–3]. NDDs are psychiatric or behavioral conditions whose onset has been traced to prenatal stages of brain development, with clinical symptoms presenting in childhood and adulthood [4, 5]. NDDs are heterogeneous and can result from the interaction of genetic abnormalities and ELA [1, 3, 6, 7]. They include autism spectrum disorders; attention-deficit/hyperactivity disorder (ADHD); intellectual disability; epilepsy, fetal alcohol spectrum disorder (FASD); and microcephaly [5, 8–11]. The shared clinical symptom of all NDDs is cognitive dysfunction, which can manifest as behavioral deficits, social deficits, and sleep difficulties. NDDs can also have disorder-specific symptoms, such as seizures in epilepsy. One important cause of ELA is environmental adversity during development. When environmental adversities occur during embryonic or fetal stages, development of the brain may be affected, predisposing the individual to neurological and psychiatric diseases later in life. Such developmental effects are called environmental programming [12]. Environmental adversities that can lead to environmental programming include maternal stress, malnutrition, medication, perinatal infections, substance abuse/alcohol misuse, and exposure to air pollutants or industrial chemicals [5, 13]. These adversities are associated with an increase in risk of NDD that persists into adolescence and adulthood [12, 14, 15].

Brain development is a highly orchestrated process that occurs during prenatal and early postnatal life, when complex neural circuits are being formed and refined through an interplay of excitatory and inhibitory neural input, synaptogenesis, synaptic pruning, myelination, and neurogenesis. Adversities that influence these processes during sensitive periods of development can thus disrupt brain development and have long-lasting and pervasive effects on brain structures, thereby leading to neurobehavioral deficits. Therefore, the developing central nervous system (CNS) is particularly sensitive to environmental adversities [6]. However, the cellular and molecular mechanisms that underlie the interaction of genetic abnormalities and environmental adversities that disrupts brain development remain unknown. Neurogenesis is critical to brain development, with new functional neurons being produced by neural stem cells (NSCs). Neurogenesis involves cell division, cell differentiation, cell migration, and integration of new neurons into existing brain structures. Recent research suggests that environmental adversities may cause endoplasmic reticulum (ER) stress in the brain, and that this impairs neurogenesis and brain development. In this review, we discuss recent evidence of the impact of environmental adversities on neurogenesis and its interaction with ER stress. We further discuss the interplay among ER stress, oxidative stress, autophagy and neuroinflammation, as well as potential therapeutic strategies based on targeting ER stress/UPR for the treatment of NDDs that are associated with environmental adversities.

1. Neurogenesis

1.1. Developmental neurogenesis

Neurodevelopment is an intricate and highly coordinated process characterized by the proliferation, differentiation, migration, and spatial organization of the progenitor cells that give rise to the nervous system. The vertebrate CNS originates from the neural tube, which is derived from the ectoderm. Typical features of NSCs, which are neuroepithelial in nature, are tight junctions and polarization along the apical-basal axis [16]. NSCs have the potential to undergo self-renewing symmetric divisions, a process that increases the size of the precursor cell pool during early development while the neural plate forms. Following neural tube closure, neuroepithelial cells give rise to radial glial (RG) cells through asymmetric division, and RG cells produce multiple cell types during cortical neurogenesis [17].

Embryonic RG cells line the neural-tube lumen as a single layer, producing the primary neural germinative zone, which is known as the ventricular zone (VZ). Initially, RG cells undergo symmetric divisions to produce additional RG cells, expanding the proliferative population. During cortical neurogenesis, RG cells undergo two fundamental forms of cell division: direct neurogenesis (dNG) and indirect neurogenesis (iNG). In dNG, RG cells undergo asymmetric divisions, producing a self-renewed RG cell and a neuronal precursor cell that migrates outward from the lumen along a neighboring RG cell. These early neuronal precursors develop primarily into large, projecting-type neurons. In iNG, RG asymmetric division produces an intermediate progenitor (IP), which then undergoes symmetric division to generate two neurons. Although dNG occurs along the full length of the neural tube, iNG is restricted to the telencephalon, producing cells of specifically the forebrain, particularly the cerebral cortex [18].

As the neural wall thickens, the number of primary RG cells gradually decreases, and their derivatives generate a new germinative zone, the subventricular zone (SVZ) [19]. The SVZs along the entire neuroaxis generate various progenitors and precursors, including smaller projecting-type neurons, interneurons, astrocytes, and oligodendrocytes. The SVZ also produces enough stem cells to maintain neurogenic capacity. Novel precursors migrate from the SVZs to their final destinations along defined routes in the developing neural parenchyma [20].

A fundamental feature of cortical neurogenesis is temporal patterning. Neural-tube formation and closure proceeds with a delay from the zone of the branchial arches (the region of the future hindbrain) in both the rostral and caudal directions. Within each neural domain, the production and maturation of neural cells on the ventral face always precede those of the cells on the dorsal part. Thus, the RG cells change their competence to produce different types of neurons over the course of corticogenesis, with the first neurons that are generated from RG cells populating the deepest layers of the cortex and having specific gene expression profiles and distinct connectivity properties. The neurons that are born at later stages of corticogenesis acquire other identities, within progressively more superficial layers, and have different gene expression and connectivity patterns [21].

Cellular heterogeneity is further increased by ongoing regional specification, resulting in well-distinguishable domains with characteristic gene expression patterns along both the anteroposterior and dorsoventral axes of the growing CNS [22]. Differential expression of positional master genes leads to diverging patterns of expression of downstream genes, including those that encode adhesion receptors and extracellular matrix proteins. This differential expression results in the formation of morphological boundaries between expression domains, such that the developing CNS is composed of segments called neuromeres [23]. Along the anteroposterior body axis, transversal segments delineate the primary brain vesicles (the prosencephalon, mesencephalon, and rhombencephalon) first, and smaller neuromeres later [20].

1.2. Adult neurogenesis:

As discussed above, neurogenesis, defined here as a process of generating functional neurons from precursors, was traditionally thought to occur only during embryonic and perinatal stages in mammals. The concept that neurons could be generated in the CNS of adult animals first emerged with reports in the 1960s that neurons are generated in the postnatal rodent hippocampus [24]. Adult neurogenesis is now recognized as a dynamic and ongoing process in the mammalian brain, and this has profound implications for our understanding of brain plasticity, learning, and memory, as well as for developing therapeutic interventions for neurological disorders [25]. In the adult brain, neurogenesis occurs primarily in two well-characterized niches: the subgranular zone (SGZ) of the dentate gyrus (DG) within the hippocampus, and the SVZ that lines the lateral ventricles. NSCs in these regions give rise to progenitor cells that differentiate into neurons, astrocytes, and oligodendrocytes [25]. In adult mammals, neural progenitor cells in these regions proliferate and differentiate into neurons. During adult neurogenesis, the SGZ of the DG is where new granule cells become integrated into local neuronal networks, and the SVZ of the lateral ventricles is where neural progenitor cells give rise to cells that migrate toward the olfactory bulb [26, 27].

Continuous neurogenesis in the adult SGZ and SVZ is believed to be essential for the physiological functions of the mammalian hippocampus and the olfactory bulb. Neurogenesis in the adult hippocampus has been linked to crucial affective and cognitive behaviors, including learning, memory retention, pattern recognition, and memory clearance [28]. However, the functional significance of adult neurogenesis extends beyond basic cognitive processes. Research suggests that neurogenesis may be required for mood regulation and emotional resilience, and that deficiencies in neurogenesis contribute to psychiatric disorders such as depression and anxiety [29]. Furthermore, adult neurogenesis may be required for brain repair and regeneration in neurodegenerative diseases and traumatic brain injury [30, 31]. The regulation of adult neurogenesis is influenced by various factors, including exposure to environmental enrichment, physical activity, aging, stress, and exposure to neurotrophic factors [32]. For example, exercise has been shown to promote neurogenesis in the hippocampus, leading to improvements in learning and memory [33]. Conversely, chronic stress can inhibit neurogenesis and impair cognitive function [29].

2. ER stress and neurogenesis

2.1. ER stress and the unfolded protein response (UPR)

The ER is an intracellular compartment that plays a major role in protein folding and processing, membrane biosynthesis, and the storage and release of calcium, as well as serving as the first stop in the secretory pathway. As such, the ER is an important regulator of protein quality control and cellular homeostasis. Also, as a source of lipids for membrane biosynthesis, it is essential during neurogenesis and synaptic plasticity. ER stress is defined as perturbations to ER homeostasis and is characterized by the accumulation of aberrant proteins because the ER protein folding capacity fails to keep up with cellular demand [34], [35, 36]. A broad range of pathological conditions, such as glucose deprivation, toxin exposure, altered lipid homeostasis, viral infection, redox imbalance, and calcium homeostasis disruption, lead to the accumulation of improperly folded proteins within the ER lumen and evoke ER stress [35, 37]. ER stress is usually resolved by the activation of two adaptive programs: the UPR and the ER-associated protein degradation (ERAD) pathway. The UPR is the major cellular response induced by ER stress, and it consists of three distinct but interconnected intracellular signal transduction pathways (Fig. 1A). Signaling by each of these pathways is initiated by ER-resident transmembrane sensor proteins: inositol-requiring kinase 1 (IRE1, α and β isoforms), protein kinase-like ER kinase (PERK), or activating transcription factor 6 (ATF6, α and β isoforms) [34]. These transmembrane proteins each has an ER-luminal sensor domain and a cytosolic effector domain and can transmit the protein folding status within the ER to other cellular compartments via intracellular signaling pathways. In the case of IRE1, its endoribonuclease activity cleaves the X-Box binding protein-1 (XBP1) mRNA, initiating translation of the spliced XBP1 protein (XBP1s), which is a potent transcription factor. PERK phosphorylates the eukaryotic translation initiator factor-2α (eIF2α) at serine 51, enabling it to interact with eIF2β and trigger the inhibition of general protein translation while selectively promoting translation of the activating transcription factor 4 (ATF4). ATF4, in turn, activates the proapoptotic C/EBP homologous protein (CHOP) and other UPR target genes [34]. The ATF6 pathway is triggered by the site-1 and 2 proteases (S1P and S2P), which release its cytosolic region (ATF6f), making it a potent transcription factor [35].

Fig. 1: Effects of ER stress and the UPR on neurogenesis.

A: A broad range of pathological conditions, including glucose deprivation, toxin exposure, altered lipid homeostasis, viral infection, redox imbalance, and calcium homeostasis disruption, lead to the accumulation of unfolded or improperly folded proteins within the ER lumen. This, in turn, induces ER stress and activates UPR pathways. The UPR pathways are initiated by three ER-resident transmembrane sensor proteins: inositol-requiring kinase 1 (IRE1, both α and β isoforms; blue pathway), protein kinase-like ER kinase (PERK, red pathway), and activating transcription factor 6 (ATF6, both α and β isoforms, green pathway). These transmembrane sensor proteins have a sensor domain within the ER lumen sensor domain and an effector domain in the cytosol and are thus able to communicate the protein folding status inside ER to other cellular compartments via intracellular signaling pathways. In non-stressed cells, all the sensor proteins remain inactive because they are bound to an ER chaperone, 78-kDa glucose-regulated protein (GRP78), through their N-terminus. In cells under ER stress, GRP78 dissociates from these sensor proteins, triggering their activation. UPR activity typically leads to increased expression of ER chaperone proteins, improved degradation of misfolded proteins and quality control, and attenuated protein translation. However, severe ER stress and failed attempts to achieve ER homeostasis result in a shift towards apoptotic cell death. B: ER stress and the UPR may impact neurogenesis through diverse outcomes, including increased activation of UPR pathways and inflammation, and disruption of intracellular calcium homeostasis and protein synthesis.

In non-stressed cells, each of the UPR sensor protein is kept inactive by binding of the ER chaperone 78-kDa glucose-regulated protein (GRP78) to its N-terminus. Under conditions of ER stress, GRP78 dissociates from these sensor proteins, initiating their activation. Most UPR outputs lead to increased expression of this ER chaperone, improving the degradation of misfolded proteins as well as quality control mechanisms, and attenuating protein translation [34]. However, severe ER stress and failed attempts to achieve ER homeostasis result in a shift towards apoptotic cell death [34, 37]. UPR dysfunctions and ER stress contribute to many neurodegenerative diseases that are characterized by the accumulation and aggregation of misfolded proteins. Examples include Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and prion disease [34, 37]. Despite the clear impact of ER stress on brain development and neurogenesis, this condition has not been thoroughly investigated and thus the mechanisms responsible for its negative effects are unclear.

2.2. Impact of ER stress on neurogenesis

During neurogenesis, the proliferation, migration, and differentiation of NSCs raise the demand for protein synthesis. The ER is a central control hub for protein homeostasis (proteostasis), and it impacts a wide range of cellular processes required for brain function. Most studies have focused on the role of ER stress in neurodegenerative diseases that are marked by abnormal protein aggregation. Recent evidence indicates that ER proteostasis contributes to brain development and may significantly impact neurogenesis [38, 39](Fig. 1B). Here, we review emerging evidence that links ER stress with impaired neurogenesis and its relevance to human disorders.

Both in vitro and in vivo studies indicate that ER stress disrupts neurogenesis. ER proteostasis is essential for NSCs because of the high demand for protein synthesis during the their proliferation and differentiate [38]. Maternal diabetes induces neural-tube defects by suppressing neurogenesis in the developing neuroepithelium. An in vitro study mimicking maternal diabetes demonstrated that high glucose suppresses NSC differentiation by significantly reducing expression of the neuron marker Tuj1 and the glial cell marker GFAP, as well as decreasing the numbers of Tuj1⁺ and GFAP⁺ cells. The ER stress inhibitor, 4-phenylbutyric acid (4-PBA), abolished the inhibitory effect of high glucose on NSC differentiation. Similarly, an antioxidant reversed the decrease in Tuj1 and GFAP expression triggered by high glucose, and it also restored the numbers of neurons and glial cells differentiated from NSCs, indicating that oxidative stress and the resulting ER stress mediate the inhibitory effect of high glucose on NSC differentiation [40]. In another in vitro model of neuronal differentiation, neurons were differentiated from mouse embryonic carcinoma P19 cells by treatment with retinoic acid [41]. Exposure of these cells to the ER stress inducer, tunicamycin, led to a marked increase in levels of ER stress, which in turn led to an increase in expression of the neuronal marker βIII-tubulin at 8-days of differentiation. However, the neurites of cells positive for the dendrite marker microtubule-associated protein-2 (MAP-2) retracted in response to ER stress, with both dendrite length and MAP-2 expression markedly reduced but the number of surviving mature neurons remaining the same. Therefore, ER stress caused aberrant neuronal differentiation from NSCs, followed by inhibition of neurite outgrowth. Further evidence suggested that the effects of ER stress on NSC differentiation were mediated by HRD1, an ERAD-related E3 ubiquitin ligase [41]. In contrast, a neuritogenic compound isolated from a plant used in Chinese herbal medicine, Desmodium sambuense, enhanced neurite outgrowth in cultured PC12 cells by inducing ER stress and activating BDNF-TrkB signaling pathway [42].

Results from in vivo studies also support that ER stress disrupts neurogenesis during development. In a mouse model of autism spectrum disorders in which valproic acid (VPA) is injected into pregnant mice, autism-like behaviors and strong ER stress were induced in the cerebral cortex and hippocampus. ER stress caused by VPA injection or tunicamycin treatment during pregnancy inhibited the extension of dendrites and axons in primary cultures derived from the cerebral cortex of E14.5 mice, indicating that ER stress induced abnormal maturation of neurons in the developing brain [43]. Similarly, infection of cortical neuron progenitors of mice with the mosquito-borne Zika virus (ZIKV), which can cause congenital microcephaly resulted in strong ER stress. Subsequent disruption of both neurogenesis and apoptotic neuronal death led to cortical microcephaly [44], and administration of the ER stress inhibitors GSK2656157 and 4μ8C (specific inhibitors of PERK and IRE1α, respectively) prevented ZIKV-induced cortical microcephaly [44]. Another in vivo study demonstrates that ER stress impairs neurogenesis in the hippocampus. The DG in the hippocampal formation of the mammalian brain is crucial for higher brain functions, such as learning, memory, and motivation. As discussed above, during postnatal life, neurogenesis continues in the SGZ of the DG. In the SGZ, type-1 NSCs differentiate into type-2a, type-2b, and type-3 progenitor cells. The type-3 cells undergo a final round of mitosis and then differentiate into immature GCs, and then to mature GCs [45, 46]. Prenatal exposure of pregnant mice to aluminum (Al) impaired postnatal neurogenesis in the hippocampus. Specifically, it caused ER stress and impaired the capacity of NSCs for self-renewal, and this was accompanied by slowing of the cell cycle, and ultimately in exhaustion of the NSC pool in the hippocampal SGZ. Moreover, Al inhibited neurogenesis by inducing GC apoptosis which was also mediated by ER stress [47]. These results suggest that developmental exposure to Al irreversibly affects postnatal neurogenesis in the hippocampus and that ER stress plays a pivotal role in the process. Additional in vivo study further supports the involvement of ER stress in neurogenesis. Fatty acid synthase (Fasn) plays an important role in lipid metabolism and the proliferation of NSCs. In a mouse model of the human Fasn variant, a reduction of neurogenesis and ER stress in the hippocampal DG was observed [48]. Using 2-D and 3-D brain organoids, Bower et al demonstrated that the changes in lipid metabolism induced by the Fasn variant caused ER stress, which then inhibited the proliferation of NSCs [48].

ER stress also affects neurogenesis in the adult brain, particularly in the hippocampus. For example, traumatic brain injury (TBI) in adult mice caused ER stress, loss of newborn hippocampal neurons, and changes in dendritic arbors. Treatment with the ER stress inhibitor, guanabenz, or knockout of CHOP ameliorated TBI-triggered disruption of both adult hippocampal neurogenesis and memory [49]. Also, in adult mice long-term obesity induced ER stress in the hippocampus and inhibited the generation of doublecortin (DCX)-positive immature neurons in the DG. Moreover, in cells differentiating from hippocampal neurospheres in vitro, ER stress induced by thapsigargin (an ER stress inducer) treatment led to a decrease in levels of the DCX mRNA [50]. Similarly, Wu et al demonstrated that spinal cord injury in adult mice led to a long-term reduction in the number of newly-generated immature neurons in the hippocampal DG, and that this was accompanied by neuronal ER stress [51]. Exposure to the insecticide deltamethrin triggered ER stress-mediated suppression of adult hippocampal neurogenesis, which has the potential to contribute to learning and memory deficits in mice [52]. Lipopolysaccharide (LPS) inhibited adult hippocampal neurogenesis and induced depression-like behaviors. It also induced ER stress and activated PERK [53]. The PERK inhibitor ISRIB reduced the proinflammatory responses of microglia in the context of LPS-induced brain inflammation, preserved hippocampal neurogenesis, and improved depression-like behavioral outcomes following LPS challenge [53], suggesting that ER stress plays a pivotal role in LPS-impaired adult hippocampal neurogenesis and associated neurobehaviors.

2.3. Mechanisms underlying ER stress-induced disruption of neurogenesis

2.3.1. Regulatory role of UPR proteins and associated genes:

ER stress induces the expression of UPR proteins and UPR-associated proteins. UPR activation may be required to increase the capacity for protein folding or to maintain proteostasis in the ER during brain development and neurogenesis [38]. However, these UPR proteins and some UPR-related genes may additionally contribute to the regulation of neurogenesis in the absence of ER stress (Fig. 1B). UPR proteins are thought to contribute to the differentiation, self-renewal, and survival of NSCs [38]. Specifically, during brain development, the expression of UPR-related genes tends to be higher during early vs. later stages [38], suggesting that they contribute to the regulation of brain development and neurogenesis. This was supported by observations that the expression of ATF6, ATF4, XBP1, and several ER chaperones (including calreticulin, GRP78, GRP94 and ER protein (ERp) 57) was high in the embryonic brain and declines significantly in the adult brain [35, 38, 54]. Notably, the dynamic change in ATF4 expression contributed to proper cortical neurogenesis by regulating activity of Cyclin D promoter and thereby controlling cell-cycle progression in the earliest neural progenitors [55]. In addition, overexpression of ATF4 impaired the migration of newly generated neurons. In vitro studies using mouse embryonic stem cells revealed that PERK was activated during their differentiation into neurons and inhibition of PERK disrupted this differentiation [38].

Involvement of the UPR in neurogenesis is apparent during corticogenesis. The cerebral cortex contains layers of neurons that are sequentially generated by distinct lineage-related progenitors. At the onset of corticogenesis, apical progenitors are born, and their asymmetric division directly gives rise to neurons. Later, neurogenesis is indirect, with intermediate progenitors (IPs) emerging and giving rise to projection neurons of all cortical layers. Activation of the PERK-eIF2α-ATF4 signaling pathway by experimental deletion of Elp3, which is a subunit of the Elongator complex, led to microcephaly and a decrease in the number of IPs. In addition, ATF4 knockdown resulted in increases in both the number of IPs generated and the amount of indirect neurogenesis [56]. These findings suggested that UPR proteins can regulate neurogenesis during corticogenesis. Progressive attenuation of the expression of UPR-related genes, such as ATF4 in cortical progenitors, is a physiological signal for the proliferation of IPs and the promotion of indirect neurogenesis [56]. Also, Mimura and colleagues showed that mice expressing a mutant form of GRP78 display a disordered outside-in pattern of layer formation (the inverse of the normal and highly ordered inside-out layer formation) in the cerebral cortex. These mice also had developmental defects of the cerebellum [57]. Cultured hippocampal neurons derived from XBP1 knockout mice exhibited significantly impaired BDNF-induced neurite extension and branching [54]. In addition to contributing to corticogenesis, the UPR is involved in development of the cerebellum. For example, in the postnatal rat, the expression of both UPR and ER stress-associated proteins was regulated during development of the cerebellar cortex [58]. The activation of PERK and IRE1 was first observed in the developing cerebellar granule cell precursors. The density of UPR protein-positive cells decreased significantly as development of the cerebellar cortex progressed [58]. In the mutant GRP78-expressing mice, growth of the cerebellum was delayed and migration of cells in the external granule layers and Purkinje cells of the cerebellum was impaired [57].

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an ER-resident protein that has neurotrophic properties. Its expression is induced by ER stress, and it has been implicated in various neuropathological processes. During brain development MANF expression is regulated in a spatiotemporal manner [59, 60], with high expression in cells of neural lineage, including NSCs in the developing brain [61]. In culture, MANF-deficient NSCs are viable but they have deficits in neurite extension upon neuronal differentiation. In vivo, MANF deficiency leads to slower neuronal migration and impaired neurite outgrowth, suggesting that it is a critical regulator of neurite growth and neuronal migration during cortical development [61]. Consistent with this notion, in an in vitro model of neuronal differentiation, MANF deficiency impaired neurite outgrowth [62], inhibited Akt, Erk, mTOR, and P70S6, and impaired protein synthesis. In contrast, MANF overexpression facilitated neurite outgrowth by activating Akt, Erk, mTOR, and P70S6. Also, neurite outgrowth in response to MANF was eliminated by pharmacological blockade of Akt, Erk or mTOR. These findings suggest that MANF positively regulates neurite outgrowth by activating the Akt/mTOR and Erk/mTOR signaling pathways, as well as protein synthesis [62]. MANF is also involved in adult neurogenesis [63]. In adult mice, MANF deficiency led to increased BrdU labeling and the number of Ki-67-positive cells in both the hippocampal SGZ and the lateral ventricle SVZ. It also upregulated proliferative activity and was accompanied by decreases in the levels of cell-cycle inhibitors (p15 and p27), and increases in the levels marker proteins of G2/M phase (phospho-histone H3) and neural progenitor markers (Sox2 and NeuroD1) in the brain [63].

In addition to contributing to NSC proliferation and neuronal differentiation, UPR proteins participate in Schwann-cell differentiation. PERK and eiF2α have been shown to regulate Schwann cell differentiation and myelination via mechanisms independent of ER proteostasis [64]. Phosphorylation of eIF2α promotes Schwann-cell differentiation and myelination by mediating crosstalk between the eIF2α and MEK/ERK pathways in the absence of significant ER stress [64].

2.3.2. Calcium signaling:

Calcium (Ca²⁺) signaling plays essential roles in various steps of neurogenesis, both during development and in the adult; these steps include neural induction, cell proliferation, cell migration, NSC differentiation, and NSC survival [65, 66] (Fig. 1B). Much evidence correlates Ca²⁺ oscillations with the fate determination of NSCs. For example, Ca²⁺ oscillations of higher frequency lead to increased differentiation of hippocampus-derived NSCs [66]. As discussed above, ER is essential for Ca²⁺ homeostasis. There is a reciprocal interaction between ER stress and Ca²⁺ signaling. Ca²⁺ signaling pathways are shaped by interactions among metabotropic signaling cascades, intracellular Ca²⁺ stores, ion channels, and a multitude of downstream effector proteins that activate specific genetic programs. Disturbance of cytosolic and ER Ca²⁺ homeostasis leads to ER stress and activation of the UPR [67]. As a result, some cytotoxic effects of dysregulated Ca²⁺ levels, such as apoptosis, are mediated by ER stress/UPR and disrupted proteostasis [67–69]. For instance, inhibiting ER Ca²⁺-ATPase disrupts Ca²⁺homeostasis, resulting in UPR and cell death [69]. Much research has focused on the effects of disrupted Ca²⁺ homeostasis on ER stress. However, this interaction may be reciprocal (Fig. 1B). Specifically, ER stress and some UPR proteins appear to interact with Ca²⁺ regulators to influence Ca²⁺ levels in the cytosol and ER. For example, small molecules targeting the UPR reverted ER stress and ER Ca²⁺ homeostasis in cardiomyocytes [68]. One of the main Ca²⁺ signaling pathways is store-operated Ca²⁺ entry (SOCE), which is responsible for the flow of Ca²⁺ into cells in response to the depletion of ER Ca²⁺ stores. When SOCE is activated, it both refills ER stores and evokes sustained Ca²⁺ signals [70]. SOCE is regulated by several proteins in the ER and plasma membrane (PM), including stromal interaction molecule proteins (STIM), ORAI Ca²⁺ channels, and transient receptor potential canonical channels. SOCE and its molecular regulators contribute to NSC function and play an important role in their proliferation, differentiation, and maturation [70], [71]. There is evidence that ER stress and the UPR can interact with SOCE and alter Ca² homeostasis. The UPR protein IRE1 interacts with STIM1, promotes contact between the ER and PM, and activates SOCE [72]. This indicates direct crosstalk between SOCE and the UPR via IRE1, which acts as key regulator of ER Ca²⁺ and proteostasis. An in vivo study also supports that ER stress can interact with SOCE. The infection of swine with porcine reproductive and respiratory syndrome virus (PRRSV) caused dysregulated Ca²⁺ homeostasis by inducing ER stress [73]. PRRSV induced ER stress and formed a close ER-PM contacts, activating SOCE and causing the ER to take up extracellular Ca²⁺, which was then released into the cytoplasm [73]. Importantly, pharmacological inhibition of ER stress can block this action of PRRSV, supporting the notion that crosstalk exists between ER stress and SOCE.

2.3.3. Neuroinflammation:

It is well known that neuroinflammation affects neurogenesis both during development and in the adult [74,75, 76]. For example, neuroinflammation induced by IL-1β administration during the early postnatal period acutely reduces the proliferation of Tbr2⁺ neural progenitors in the DG of the hippocampus [77]. Exposure to anesthetics during pregnancy affects neuronal development by inducing neuroinflammation and disrupting neurogenesis [78]. Several anesthetics (including sevoflurane isoflurane, enflurane, and halothane) induces the proinflammatory IL-6 mRNA in the fetal brain, and this significantly alters the number of neuronal precursor cells in the SVZ [78].

ER stress can stimulate the innate immune defenses and induce neuroinflammation in the CNS [58, 79, 80]. Each branch of the UPR has been shown to increase the production and release of pro-inflammatory mediators. It is believed that all three UPR pathways activate nuclear factor-kB (NF-kB), a transcription factor that drives expression of proinflammatory cytokines such as TNFa, IL-1, IL-6, and IL-8 [80, 81]. These cytokines, in turn, may amplify activity of the NF-kB pathway in a feed-forward loop. The UPR-associated protein MANF modulates inflammation by interacting with NF-kB in vitro and in vivo [59]. The positive feedback loop between ER stress and neuroinflammation has clear implications for some neurodegenerative diseases [81]. ER stress-mediated neuroinflammation has been shown to negatively impact neurogenesis. For example, the pyrethroid insecticide deltamethrin inhibited hippocampal neurogenesis and induced neuroinflammation and learning deficits in adult mice [82, 83]. It appears that the deltamethrin-induced neuroinflammation is mediated by ER stress in the brain. Spinal cord injury led to a long-term reduction in the number of newly-generated immature neurons in the DG of the hippocampus, accompanied by neuronal ER stress and neuroinflammation, which may mediate spinal cord injury-induced inhibition of hippocampal neurogenesis [51].

2.3.4. Protein synthesis:

De novo protein synthesis is essential for neurogenesis and brain development. During neurogenesis, the demand for protein synthesis is increased to accommodate the need for the proliferation, migration, and differentiation of NSCs. The ER is a central hub that controls proteostasis, regulating a wide range of cellular processes required for brain development [39, 84]. Some UPR proteins and ER stress-associated proteins contribute directly or indirectly to the regulation of protein synthesis, secretion, and degradation [34, 85] (Fig. 1B). For example, the ER stress-inducible protein MANF promoted in vitro neurite outgrowth, probably by promoting protein synthesis [86]. MANF deficiency impaired neurite outgrowth, which was preceded by reduced de novo protein synthesis [61, 62].

3. Environmental adversities and ER stress

Environmental adversity during development can disrupt neurogenesis and contribute to the onset of NDDs. Various forms of adversity—such as maternal stress and obesity, substance abuse, alcohol exposure, viral infections, air pollution, and exposure to heavy metals, industrial chemicals, pesticides, or certain medications (Fig. 2)—have been shown to impair ER homeostasis and induce ER stress. The detrimental effects of these environmental factors on CNS development may be mediated through ER stress and the UPR. However, current knowledge in this area remains limited, and the precise mechanisms by which these environmental insults trigger ER stress are not yet fully understood. Therefore, there is an urgent need for more targeted research to elucidate underlying cellular/molecular mechanisms. Here we summarize the existing evidence that links environmental adversities to the induction of ER stress and UPR in the developing CNS.

Fig. 2: Effects of environmental adversity on ER stress and neurogenesis.

A: Environmental adversity during development can affect neurogenesis and cause NDDs. Environmental challenges shown in the other ring can affect ER homeostasis and induce ER stress in the CNS. B: Direct interactions among ER stress, oxidative stress, neuroinflammation, and autophagy impact neurogenesis and the pathogenesis of NDDs.

3.1. Substance abuse

Prenatal substance exposure is a worldwide public health concern that is growing. Exposure of a fetus to substances including methamphetamine (METH), opioids, cannabis, cocaine, and marijuana can affect brain development and thus can have an impact on neurobehavioral and cognitive performance throughout the lifespan [87–89]. One potential underpinning of substance abuse-induced changes in neurobehavioral and cognitive performance is impaired neural plasticity and neurogenesis [90]. As discussed above, activation of ER stress significantly influences brain development and neurogenesis. Increasing evidence indicates that ER stress mediates the effects of substance abuse on fetal brain development and neurobehavioral deficits [87, 91], and the neuropsychiatric effects of alcohol, METH, cocaine, opioid, and kratom may be caused by persistent ER stress and the UPR [91]. For example, cocaine exposure can induce ER stress in both the developing and adult brain [87, 92]. Cocaine has been proposed to affect prenatal brain development by inducing ER stress and the UPR pathways [87]. In vitro, cocaine-induced ER stress resulted in inhibition of the proliferation of neural progenitor cells, premature differentiation of neurons, and apoptosis of microglial cells [87]. METH is an illegal amphetamine-type psychostimulant that is abused worldwide and causes serious public health problems. METH exposure induces ER stress in the developing and adult brain [87, 93], and its neurotoxic effects are thought to be mediated through multiple mechanisms including ER stress [93, 94]. Morphine exposure induces ER stress in the mouse brain, affecting neuroplasticity, and anxiety-like behaviors can be induced by morphine withdrawal [87, 95].

3.2. Alcohol exposure

Alcohol exposure during pregnancy can result in FASD, and the CNS is particularly susceptible to alcohol-induced damage. FASD is diagnosed at an alarmingly high rate, affecting 2%−5% of live births in the United States. It is the most common non-heritable cause of mental disability and results in tremendous personal and societal cost [96]. Children with FASD can experience neurodevelopmental, physical, psychological, and behavioral impairments [97]. Developmental exposure to alcohol affects fetal brain development and inhibits neurogenesis, resulting in long-lasting neurobehavioral deficits later in life [98, 99]. Alcohol exposure also affects adult neurogenesis [100]. Several mechanisms have been proposed to contribute to alcohol-induced impairment of brain development; they include inhibition of neurotrophic/neurotransmitter signaling, enhancement of naturally occurring programmed cell death, induction of oxidative stress, and neuroinflammation. Recent research indicates that alcohol-induced ER stress in the developing and mature CNS and ER stress may underlie alcohol-induced NDD [101–108]. The expression of UPR proteins and ER stress-inducible neurotrophic factors such as MANF is developmentally regulated [60, 109]. The immature brain appears more sensitive to ER stress; as the brain matures, it becomes increasingly resistant to ER stress and alcohol-induced neuronal damage [108–110]. The temporal window of neuronal sensitivity to ER stress correlates with alcohol-induced neurodegeneration in the developing brain [109]. Microcephaly is a neurodevelopmental disorder in which an infant’s head is much smaller and associated with intellectual deficiency, one of the most important clinical signs of FASD. Microcephaly has also been associated with ER stress [8], consistent with ER stress playing a critical role in alcohol-induced structural damage to the CNS, as well as in neurobehavioral deficits observed in FASD.

3.3. Exposure to heavy metals and industrial chemicals:

Some heavy metals, such as cadmium, mercury, lead, and aluminum, as well as industrial chemicals, can cross the placental barrier and accumulate in the fetal bloodstream. Prenatal exposure to these metals and chemicals has been linked to NDD. Here we discuss the evidence that exposure to these toxins impairs brain development and that the toxins interact with ER stress in the CNS.

3.3.1. Cadmium (Cd):

Cd exposure during pregnancy can affect fetal development and result in NDD [111–114]. Cd exposure has been reported to cause neural-tube defects, fetal-growth restriction, and microcephaly [111, 112]. Cd also impairs the growth of the axons of cortical neurons in the fetal rat [111]. Maternal Cd exposure induces ER stress and UPR activity in the placenta [115]. It is believed that Cd-induced neurotoxicity is mediated by ER stress [116].

3.3.2. Methylmercury (MeHg):

MeHg is one of greatest hazard to the developing fetus. Prenatal exposure can cause NDD and have long-lasting neurobehavioral consequences [117–119]. In rats, prenatal MeHg exposure induces neuronal apoptosis and cell-cycle arrest, accompanied by ER stress, in the cerebral cortex [119]. In mouse, MeHg-induced neuronal apoptosis in the brain is mediated by activation of the UPR [118]. In the study by Pan et al., pretreatment of rats with the ER stress-specific inhibitor 4-PBA restored MeHg-induced axonal shortening and alleviated apoptosis, cell-cycle arrest, and DNA methylation [119]. Also, this study suggested that MeHg-induced neurotoxicity is mediated by activation of the CHOP/c-Jun/GADD45A signaling pathway [119].

3.3.3. Lead (Pb):

Pb is a ubiquitous toxic element, and exposure to it during pregnancy can cause developmental abnormalities in the brain, resulting in intellectual impairment, neurobehavioral deficits, and neurological disorders [120, 121]. In the mouse spleen, Pb-mediated toxicity has been associated with ER-driven apoptosis and autophagy [122]. In rat brain, Pb exposure induces ER stress, which is believed to be the cause of neurotoxicity [116]. Notably, blocking ER stress by administering naturally occurring flavonoids known as proanthocyanidins, was reported to improve Pb-induced cognitive impairments [123].

3.3.4. Aluminum (Al):

Prenatal exposure to Al affects hippocampal neurogenesis. In mice it causes ER stress and impairs the self-renewal capacity of NSCs. This effect is accompanied by slowing of the cell cycle, which ultimately results in exhaustion of the NSC pool in the SGZ of the hippocampus. Moreover, Al causes ER stress-mediated granule cell apoptosis in mice [47]. In rats, oral exposure to Al suppresses NSC proliferation and increases the number of mature granule cells during adult hippocampal neurogenesis [124]. Al exposure also causes cellular damage and reduces the proliferation and migration of neural progenitor cells, resulting in global inhibition of neural progenitor cell differentiation and neurogenesis [125]. Al induces ER stress in cultured neuronal cell lines as well as rat and rabbit brains, and its neurotoxicity is mediated partly by ER stress [126–128].

3.3.5. Bisphenol A (BPA):

BPA is an environmental endocrine disruptor that can mimic estrogen and disrupt the hormonal balance. It is found in many everyday items, including plastic products, food and beverage containers, and thermal paper receipts. Clinical and epidemiological studies have identified correlations between early BPA exposure and developmental disorders, such as ADHD and autism spectrum disorders [129, 130]. Higher maternal urinary BPA concentrations at mid-gestation correlate with altered brain structures in offspring [131]. Studies using vertebrate animal models demonstrated that developmental exposure to BPA can impair multiple aspects of neuronal development, including NSC proliferation and differentiation, synapse formation, and synaptic plasticity [132–135]. Among the molecular mechanisms by which BPA exposure could potentially disrupt brain development, ER stress is important. BPA induces ER stress and the UPR, as indicated by up-regulation of PERK, IRE1, ATF6, XBP-1, CHOP, and GRP78 in both mice and a cultured mouse neuronal cell line [136, 137]. In sheep, gestational exposure to BPA caused ER stress in the placenta and trophoblasts and resulted in fetal growth restriction [138].

3.3.6. Benzophenones (BPs):

BPs are widely used as supplements in personal care products and have been shown to affect neurodevelopment. In mice, prenatal exposure to the BP metabolite 4-hydroxybenzophenone (4HBP) impaired hippocampus development and caused cognitive dysfunction in offspring [139]. 4HBP induced ER stress-mediated apoptotic signaling and inflammatory response in NSCs of the hippocampus. Mechanistically, 4HBP activated PERK, which induced CHOP and stimulated NFκB signaling, thereby promoting inflammation and apoptosis. In both mice and a human brain organoid model, genetic or pharmacological inhibition of the PERK pathway significantly attenuates 4HBP-induced NFκB signaling and neurodevelopmental abnormalities [139].

3.4. Exposure to pesticides:

Pesticides such as organophosphate (OP), pyrethroids (PYRs), and organochlorine (OC) have been shown to affect brain development and are associated with NDD [140–143]. For example, in both humans and animal models, prenatal exposure to OP correlates with impaired neurodevelopment. In humans, prenatal OP exposure is associated with alterations in white-matter structures [14]. A functional study of prenatal OP exposure and brain activation in humans showed that, in addition to leading to structural changes in the brain, prenatal OP exposure is associated with altered patterns of brain activity during tasks of executive function, suggesting that OP may affect cognitive function at the neural level [140]. In mouse, the OP pesticide chlorpyrifos induces ER stress in cultured primary cortical neurons, and this mediates chlorpyrifos-induced neuronal apoptosis [144].

PYRs are the second most widely used pesticides in agriculture, after OP. PYRs are lipophilic chemicals that can cross the placenta and blood-brain barrier (BBB), potentially disrupting brain development [142, 145]. Exposure to PYRs during the third trimester can significantly reduce a child’s mental development index [143]. Prenatal exposure to PYR has also been associated with ADHD-related traits at ages 2–4 years [146]. Exposure to PYRs leads to ER stress-mediated neuroinflammation in the hippocampus [82]. In mice, PYRs cause ER stress and disrupt hippocampal neurogenesis [83, 147]. In addition, prenatal exposure of mice to fenvalerate (FEN), a synthetic PYR insecticide, resulted in depression-like behavior in adulthood [148]. FEN also induces ER stress in the fetal brain [148]. Dieldrin (DLD) is an OC pesticide that remains a human health concern due to high lipid bioaccumulation, and it has been epidemiologically associated with an increased risk for Parkinson’s disease. DLD-induced neurotoxicity involves impaired mitochondrial bioenergetics and ER stress in dopaminergic neurons [149].

3.5. Exposure to anesthetics:

In 2016, the US Food and Drug Administration issued a warning that repeated or prolonged use of general anesthesia and sedation drugs (including sevoflurane, propofol, ketamine, barbiturates, and benzodiazepines) during pregnancy, especially in the third trimester, can negatively impact fetal neurodevelopment [150–152]. Children prenatally exposed to general anesthesia for maternal surgery had an increased risk of being diagnosed with disruptive or internalizing behavioral disorders including ADHD, behavioral disorders, developmental speech or language disorders, and autism [153, 154]. Animal studies confirmed that fetal exposure to anesthetics disrupts neurogenesis, induces neurodegenerative changes, and impairs neuronal function, causing neurobehavioral deficits [78, 150, 155–163]. These anesthetics can induce ER stress in the fetal and mature brain, as well as in cultured neuronal cells. For example, in cultured primary neurons, the neurotoxicity induced by lidocaine and isoflurane was mediated by ER stress [155, 156]. Sevoflurane induces ER stress and inhibits neurogenesis in the hippocampus of developing rat brain, and it has been associated with neurobehavioral deficits [160]. In the developing mouse brain, sevoflurane-induced neurotoxicity is mediated by ER stress [164] and inhibition of ER stress with 4-PBA attenuates sevoflurane-induced learning and memory deficits [160]. Similarly, in neonatal rats, sevoflurane induces ER stress in the brain, and it then activates neuronal and microglial autophagy that contribute to learning and memory impairment later in life [165].

3.6. Maternal stress and obesity:

Maternal stress and early life stress affect brain development and increase the risk of mental disorders [166–168]. One potential cause of maternal stress- and early life stress-induced NDD is impaired neurogenesis [166, 168]. For example, exposure to glucocorticoids, the stress hormones that trigger the well-known fight-or-flight response during development, alters the identity of NSCs and their ability to proliferate, and this has long-lasting consequences for the brain architecture and educational attainment [169]. Early life stress can be caused by maternal separation. In pre-adolescent mice, both repeated and single maternal separation have been reported to alter specifically the morphology of microglia in the prefrontal cortex and neurogenesis in the hippocampus [170]. In juvenile rats, repeated maternal separation enhanced activation of the UPR and apoptosis in the medial prefrontal cortex. In both juvenile and preadolescent rats, but not adults, repeated separation also modulated the expression of apoptotic markers [171]. Thus, modulation of ER stress and UPR processes could potentially underlie susceptibility or resilience to early life stress [171].

Both maternal malnutrition and obesity are also considered maternal stressors and affect brain development, causing NDD [172]. The adverse effects of maternal malnutrition and obesity on mental health may be mediated by the disruption of neurogenesis [173]. It has been reported that maternal malnutrition and obesity induce ER stress in the brain [174]. For example, in a rat model, maternal protein malnutrition caused by a low-protein diet affected hippocampal functions and induced ER stress in the hippocampus of juvenile rats [175]. In obese mice, maternal obesity induces ER stress in the hypothalamus of neonates [174, 176]. In a rat model of maternal obesity, prenatal exposure to a high-fat diet induced ER stress in the DG and cornu ammonis 1 (CA1) regions of the hippocampus [177]. In the offspring of obese dams, treatments of neonates with the ER stress-relieving drug tauroursodeoxycholic acid (TUDCA) improved metabolic and neurodevelopmental deficits [176].

3.7. Air pollution:

In addition to the above discussed environmental adversities, exposure to air pollution is a risk factor for fetal brain development and potentially also for NDD [178–181]. Fetuses and infants are more vulnerable than adults to the effects of air pollution. For example, in mice developmental exposure to diesel exhaust inhibited hippocampal neurogenesis and altered the organization of the cortical lamina [182]. In rodent models more generally, prenatal exposure to traffic-related air pollution (TRAP) increased depressive behaviors. Exposure to TRAP during gestation and early postnatal development altered neurogenesis in the hippocampus of young adult mice [181]. Similarly, in rats prenatal and early-life exposure to TRAP induced hippocampal vascular leakage and impaired neurogenesis, and it was also associated with behavioral deficits [183]. ER stress has been proposed to contribute to air pollution-induced neurological dysfunction [183]. In mice, maternal exposure to low doses of carbon nanoparticles, a model particle of air pollution, induced ER stress in the developing mouse brain of offspring [184]. Benzene is a major air pollutant. Sources of benzene in ambient air include cigarette smoke, e-cigarette vaping, and evaporation of benzene-containing petrol. In mice, chronic exposure to low levels of benzene induced a severe metabolic imbalance and was associated with hypothalamic inflammation and ER stress [185]. Pharmacological inhibition of the ER stress response by inhibiting the IRE1α-XBP1 pathway significantly alleviates benzene-induced glial inflammatory responses [185].

4. Crosstalk among ER stress, oxidative stress, autophagy, and neuroinflammation:

The ER is involved in essential cellular processes including the maintenance of intracellular Ca²⁺ homeostasis, the folding of newly synthesized secretory and membrane proteins, and the post-translational modification of proteins. However, disturbances of ER homeostasis and ER stress trigger not only the UPR but also other cellular stress signal cascades. Indeed, the evidence for crosstalk among ER stress, oxidative stress, autophagy, and neuroinflammation is strong [186–190]. For example, morphine has detrimental effects on synaptic regulation when oxidative stress, ER stress, and autophagy in the hippocampus are induced sequentially [87], indicating that these stress signal cascades interact. It is also believed that the interaction between ER stress and oxidative stress is synergistic [186]. Antioxidants such as N-acetylcysteine (NAC) stand out as effective agents for counteracting hippocampal ER stress [177]. Also, a great deal of evidence supports the interaction between ER stress and autophagy. ER stress induces autophagy in the brain [189], and in neonatal rats ER stress-activated autophagy of neurons and microglial cells contributes to postoperative cognitive dysfunction [165]. In the developing rat brain, autophagy was activated as a protective compensatory response for ER stress induced by exposure to silver nanoparticles [191]. The UPR pathways emerged as critical regulators of neuroinflammation [58]. Pharmacological inhibition of ER stress by manipulation of the IRE1α-XBP1 pathway significantly alleviated benzene-induced glial inflammatory responses [185]. In a mouse model of depression, treatment with the PERK inhibitor ISRIB reduced LPS-induced brain neuroinflammation and subsequently preserved hippocampal neurogenesis, reducing depression-like behavior [53]. ISRIB treatment also reduced levels of pro-inflammatory cytokines, including IL-1β, IL-6, and IL-18 in the brain [53], suggesting that the UPR and inflammation interact. Thus, although the current review focuses on ER stress/UPR in neurogenesis, the contribution of other cellular stress signal pathways and cascades should not be overlooked.

5. Potential therapeutic approaches:

Because ER stress can contribute to many human diseases including obesity, liver disorders, cancer, diabetes, and neurodegeneration, many efforts have been made to develop therapies and treatments that target ER proteostasis and the UPR [34, 192, 193]. Indeed, progress in developing small molecules that directly target the three main UPR signaling branches and translation by phosphorylated eIF2α has been rapid and encouraging (Fig. 3). To target IRE1α signaling, several specific inhibitors of the IRE1α RNase and IRE1α kinase, as well as allosteric modulators, have been developed. The novel IRE1 kinase inhibitor Z4P was recently developed, and it can permeate the BBB, is non-toxic, and has high specificity. It effectively inhibited the growth of glioblastoma and prevented relapse in vivo when administered together with the standard chemotherapeutic [194]. PERK is an ER-resident protein kinase that phosphorylates eIF2α at serine 51 (p-eIF2α). The most widely studied PERK inhibitors are GSK2606414 and GSK2656157; both are well characterized and were the first reported molecules with high potency and pharmacokinetic properties suitable for in vivo use. Effective PERK activators (CCT020312, MK-28) have also been developed [195]. In addition to PERK inhibitors/activators, some small molecules were designed to target eIF2α, because when this protein is phosphorylated, it reduces translation, attenuating the load of nascent proteins flowing into the ER. Three additional kinases are known to phosphorylate eIF2α, including protein kinase R (PKR), heme-regulated eIF2α kinase (HRI), and general control nonderepressible 2 kinase (GCN2). Collectively, this pathway is referred to as the ‘integrated stress response’ (ISR). Some small molecules have been developed to target the ISR: eIF2α phosphatase inhibitors (Salubrinal, Guanabenz, Sephin 1); and eIF2B activators (ISRIB, Dibenzoylmethane, 2BAct, and Trazodone). These agents have promising neuroprotective effects in vitro and in vivo. For example, METH-mediated neurotoxicity of hippocampal neurons is mediated by ER stress [196], and the PERK inhibitor GSK2656157 and IRE1α inhibitor STF-0803010 significantly alleviate METH-induced death of hippocampal neurons [196]. ISRIB was reported to alleviate the depression-like behavior induced by chronic restraint stress by preserving hippocampal neurogenesis in mice [197]. ER stress and the UPR are involved in the regulation of cancer stem cells (CSCs), which are the most malignant cells in tumors. PERK- and IRE1-targeted therapy using specific inhibitors and activators was found to inhibit self-renewal of CSCs or induce their differentiation [195]. To target ATF6 pathways, various ATF6 modulating compounds have been developed; these can either specifically block or stimulate ATF6α activity [34]. Some of them can pass the BBB and offer neuroprotection against ER stress-induced CNS damage. For example, after cardiac arrest and resuscitation, the UPR is activated in various organs including the mouse brain [198]. Also, a selective the ATF6 pathway-selective activator 147 improved outcomes for cardiac arrest, including the survival rate [198].

Fig. 3: Potential therapeutic approaches that target ER stress and the UPR.

Specific therapeutic strategies for preventing or treating NDD could potentially be developed by targeting ER stress and the UPR. Candidate strategies include developing small molecules that directly target the three main branches of the UPR signaling branches. Alternatively, it might be possible to reduce protein misfolding and ER stress using chemical chaperones, such as 4-PBA, TUDCA, and trehalose. Side effects might be overcome, and thus safety increased, by targeting ER stress/UPR specifically in the affected tissue and organ using a gene therapy approach. Targets in addition to components of UPR pathways that might be useful include ER stress-associated neurotrophic factors, such as and MANF and CDNF.

An alternative to targeting UPR pathways for alleviating ER stress is to reduce protein misfolding by administering chemical chaperones (Fig. 3). These chaperones include 4-PBA, TUDCA, and trehalose, which can stabilize protein structure, and they have been shown to reduce the levels of ER stress in various disease models. For example, in the developing mouse brain 4-PBA protects immature neurons against alcohol-Induced damage by alleviating ER stress [108]. As discussed earlier, pretreatment with 4-PBA restored MeHg-induced axonal shortening and alleviated apoptosis, cell-cycle arrest, and DNA methylation [119]. Neonatal treatment with TUDCA improved metabolic and neurodevelopmental deficits in the offspring of obese dams [176].

Pharmacological modulation of the UPR is challenging because of the physiological importance of these pathways in various organs. However, local targeting of ER stress responses in the affected tissue and organ via gene therapy is emerging as a possible solution for overcoming side effects (Fig. 3). The delivery of ER chaperones, active UPR components, or siRNAs using adeno-associated virus (AAV) has had outstanding beneficial effects in several disease models (e.g., of neurodegenerative disorders, eye disorders, and metabolic diseases) [199]. For example, a gene-transfer approach that uses AAV to deliver XBP1 and ATF6f fusion protein to the brain enhanced neuroprotection in preclinical models of neurodegeneration [200].

Some ER stress-associated neurotrophic factors (NTFs), such as and MANF and cerebral dopamine neurotrophic factor (CDNF), are also promising targets for therapy of diseases related to ER stress and the UPR (Fig. 3). MANF and CDNF are members of a new family of NTFs that have a unique structure and functions. MANF and CDNF localize to the ER, where they regulate protein homeostasis and UPR signaling [15]. MANF is broadly expressed in developing and mature tissues of the nervous system, and they are upregulated in response to ER stress. A growing body of evidence from various experimental models of neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, and stroke supports the notion that MANF protects neurons from ER stress-associated complications by restoring ER homeostasis and regulating the UPR [201, 202]. Using a model of brain development that uses early postnatal mouse pups, we demonstrated that increasing MANF expression in the brain by intracerebroventricular injection of an AAV construct carrying MANF ameliorated alcohol-induced neuronal apoptosis [203]. Similarly, in a mouse model of Parkinson’s disease, delivery of CDNF by AAV-mediated gene transfer protects dopaminergic neurons and regulates ER stress and inflammation [204].

Since some NDDs may be caused by ER stress/UPR activation in the brain following environmental adversity and targeting ER stress/UPR pathways is a promising avenue for the development of improved therapies. It will be important to evaluate and develop effective and safe therapeutic approaches in future research.

Highlights.

• Environmental stressors can disrupt brain development and impair neurogenesis

• Endoplasmic reticulum (ER) stress plays a key role in neurogenesis impairment

• The effects of environmental stressors on neurogenesis may be mediated through the interaction among ER stress, autophagy, oxidative stress, and neuroinflammation

• Targeting ER stress could represent a promising therapeutic strategy

Abbreviation:

AAV

Adeno-associated virus

ADHD

Attention-deficit/hyperactivity disorder

Al

Aluminum

ATF4

Activating transcription factor 4

ATF6

Activating transcription factor 6

BBB

Blood brain barrier

BP

Benzophenones

BPA

Bisphenol A

CA1

Cornu ammonis 1

Cd

Cadmium

CDNF

Cerebral dopamine neurotrophic factor

CHOP

C/EBP homologous protein

CNS

Central nervous system

CSCs

Cancer stem cells

DCX

Doublecortin

DG

Dentate gyrus

eIF2α

Eukaryotic Initiation factor 2 alpha

ER

Endoplasmic reticulum

ERAD

ER-associated protein degradation

Erk

Extracellular signal-regulated kinase

ELA

Early life adversity

FASD

Fetal alcohol spectrum disorder

Fasn

Fatty acid synthase

FEN

Fenvalerate

GCN2

General control nonderepressible 2 kinase

GFAP

Glial fibrillary acidic protein

GRP78

78 kDa glucose-regulated protein

HRI

Heme-regulated eIF2α kinase

4HBP

Metabolite 4-hydroxybenzophenone

ISR

Integrated stress response

IP

Intermediate progenitor

dNG

Direct neurogenesis

iNG

Indirect neurogenesis

IRE1α

Inositol-requiring kinase 1 alpha

LPS

Lipopolysaccharide

MANF

Mesencephalic astrocyte-derived neurotrophic factor

MAP-2

Microtubule-associated protein-2

MeHg

Methylmercury

METH

Methamphetamine

NAC

N-acetylcysteine

NDD

Neurodevelopmental disorders

NF-kB

Nuclear factor-kB

NSCs

Neural stem cells

NTFs

Neurotrophic factors

OC

Organochlorine

OP

Organophosphate

P70S6

Ribosomal protein S6 kinase beta-1

Pb

Lead

PERK

Protein kinase-like ER kinase

PKR

Protein kinase R

PRRSV

Respiratory syndrome virus

PYR

Pyrethroids

4-PBA

4-Phenylbutyric acid

RG

Radial glial

SGZ

Subgranular zone

SOCE

store-operated calcium entry

SVZ

Subventricular zone

TBI

Traumatic brain injury

TUDCA

Tauroursodeoxycholic acid

Tuj1

Class III beta-tubulin

TRAP

Prenatal traffic-related air pollution

UPR

Unfolded protein response

VPA

Valproic acid

VZ

Ventricular zone

XBP1

X-box binding protein 1

Data Availability:

No datasets were generated or analyzed during the current study.