Perturbed MAPK signaling in ASD: Impact of metal neurotoxicity

Abstract

The mitogen-activated protein kinase (MAPK) pathways are intracellular signaling pathways necessary for regulating various physiological processes, including neurodevelopment. The developing brain is vulnerable to toxic substances, and metals, such as lead, mercury, nickel, manganese, and others, have been proven to induce disturbances in the MAPK signaling pathway. Since a well-regulated MAPK is necessary for normal neurodevelopment, perturbation of the MAPK pathway results in neurodevelopmental disorders, including autism spectrum disorder (ASD). ASD affects brain parts responsible for communication, cognition, social interaction, and other patterned behaviors. Several studies have addressed the role of metals in the etiopathogenesis of ASD. Here, we briefly review the MAPK signaling pathway and its role in neurodevelopment. Furthermore, we highlight the role of metal toxicity in the development of ASD and how perturbed MAPK signaling may result in ASD.

Introduction

The mitogen-activated protein kinase (MAPK) pathways are prominent families of intracellular signaling pathways controlling various intracellular functions and development during cell proliferation. They are serine-threonine kinases that regulate various cellular activities [1,2]. Deviation from the regular control of the MAPK signaling pathway has been associated with the development of several human diseases, including autism spectrum disorder (ASD), Parkinson’s disease, Alzheimer’s disease, and various forms of cancer [1,3] (Figure 1). Neurodevelopmental disorders (NDDs) reflect various conditions resulting from deviation from optimal brain development, commonly due to a fetus’s exposure to toxic substances or genetic disorders [4]. ASD is an NDD with an early-appearing social communication disorder that affects brain parts responsible for communication, social interaction, cognition, and other patterned behaviors [4,5]. Juvenile health has been dramatically influenced by the environment, as several NDDs have been attributed to exposure of developing fetus to various toxic substances in the external environment. Such neurotoxic substances are predominantly, but not limited to, industrial chemicals. Metals as one of the major environmental toxicants have been linked to aberrant neurodevelopment and may represent a significant cause of NDD. Reports have shown a close relationship between levels of environmental toxicants and increased frequency of ASD [4,6,7]. Here, we review the role of the MAPK signaling pathway in the pathogenesis of ASD and the neurotoxic impact of metal exposures.

Figure 1.

MAPK in normal and perturbed neurodevelopment. Optimum developmental conditions will favor a well-regulated MAPK signaling, therefore normal intracellular functions and development, and consequently normal neurodevelopment. Exposure of the developing brain to neurotoxic substances results in perturbed MAPK signaling, resulting in NDDs such as ASD. ASD is characterized by a defect in social communication and restricted, repetitive sensory-motor behavior.

Overview of MAPK signaling

Four distinct cascades share the MAPK signaling pathway. These cascades, named according to their MAPK class components, include the extracellular signal-regulated kinases (ERK1/2), Jun amino-terminal kinases (JNK1/2/3), p38-MAPK, and ERK5 [8]. Different extracellular and intracellular stimuli activate these pathways. Such triggers include cytokines, hormones, peptide growth factors, and several cellular stressors, such as oxidative stress. The MAPK signaling pathway regulates several activities, including cellular proliferation, differentiation, migration, survival, aging, and death [1,8]. In the nervous system, MAPK plays a variety of roles. MAPK/extracellular signal-regulated kinases (ERK) mediate antiapoptotic actions of vasopressin in hippocampal neurons. Likewise, plasmalogenmediated cell survival is dependent on the activation of MAPK/ERK pathways [8]. The phosphor-inositode-3 kinase (PI3K) and MAPK play crucial roles in brain angiogenesis. Activated by the insulin signaling pathway, PI3K and MAPK are involved in stimulating the hypoxia-inducible angiogenesis factor [9]. MAPK is also involved in the regulation of myelination and remyelination in the central nervous system by interacting with other intracellular pathways [10]. MAPK signaling is also essential in regulating renal differentiation, cardiovascular and digestive system development, immune responsiveness, and many other systems [8,11]. Deviation from the typical regulation of the MAPK signaling pathway is associated with numerous human diseases’ etiology. ERK signaling is involved in several steps of cancer development. It is a decisive pathway for the survival of human cancer cells, metastasis, and resistance to treatments. In contrast, the JNK pathway is involved in neuronal apoptosis in Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis [1,12] (Figure 2). In a study conducted by Rosina et al. to identify differential expression of mechanistic target of rapamycin (mTOR) and MAPK pathways in patients affected by moderate and severe idiopathic autism, it was discovered that there was an increase in the activity of mTOR and MAPK pathways in these patients by analyzing the peripheral blood at the protein level, which showed the impact of perturbed MAPK signaling in the etiology of ASD [13].

Figure 2.

MAPK signaling pathway. In the MAPK signaling pathway, external stimuli initiate MAP4K/GTPases’ activation, which in turn phosphorylates and activates MAP3K. Activated MAP3K phosphorylates and activates MAP2K, which in turn phosphorylates and activates MAPK. Activated MAPK phosphorylates various other proteins in the cell, which consequently results in cellular responses. Other members of the MAPK family signaling pathway include ERK, p38, and JNK.

MAPK pathway neurodevelopmental perturbations

The MAPK signaling axis comprises a minimum of three components: MAP3K, MAP2K, and MAPK. The MAP2Ks are phosphorylated and activated by MAP3Ks. Activated MAP2Ks, in turn, phosphorylate and trigger MAPKs, which phosphorylate several other proteins, including transcription factors such as Elk-1, c-jun, ATF2, and p53 [1]. MAPK cascades are primarily activated by extracellular growth factors that activate receptors on the cell surface. These surface receptors are receptor tyrosine kinase, and G protein–coupled receptors. Many subfamilies of receptor tyrosine kinase can initiate the MAPK cascades. MAPK activation mediated by G protein–coupled receptor results in the activation of phospholipase C, which, in turn, increases intracellular calcium that activates protein kinase C leading to Raf activation of Raf and the MAPK pathways [2]. In response to cellular stresses such as oxidative, osmotic, hypoxic, and genotoxic stress or proinflammatory cytokines, such as tumor necrosis factor-α and interleukin-1β, the JNK and p38 pathways are activated. In the ERK signaling pathway, Raf isoforms such as A-Raf, B-Raf, or Raf-1 activates MEK1/2, which, in turn, activates ERK1 or ERK2 (ERK1/2) [1,14,15].

MAPK signaling has been linked to both developmental and maturation processes in the central nervous system. During neurodevelopment, the optimal quantity of progenitor cells needs to be generated and is associated with the formation of necessary neuronal circuits and synapses required for normal brain functions. Perturbations in any of these processes alter normal brain development and consequently result in NDD [2]. Fibroblast growth factor and neurotrophin signaling are the drivers of MAPK signaling during neurodevelopment. During the cerebral cortex development, the ERK plays a crucial role in driving the neural stem cell population [16–18].

Moreover, ERK signaling plays a crucial role in developing the brain’s supporting cells, such as oligodendrocytes and glial cells. Although ERK loss of ERK2 does not affect oligodendrocytes’ proliferation, the decreased expression of ERK/MAPK may result in delayed maturation of oligodendrocytes [2,19]. In ASD, ERK/MAPK pathways are a central hub that interacts with genes and copy number variants. Various genes and copy number variants have been identified and shown to be associated with ASD. Both syndromic and nonsyndromic forms of ASD have been linked to perturbed ERK/MAPK signaling pathway. In ASD, disorders associated with dysregulated ERK/MAPK pathways are collectively identified as ‘RASopathies,’ reflecting a mutation in the genes that encode the signaling pathway elements, which activate the ERKs [2].

Metal neurotoxicity and ASD

Exposure to heavy metals may result in aberrant effects on the nervous system, especially the developing brain, susceptible to environmental factors. Metals may disrupt the strictly regulated brain development process, resulting in neurodevelopment disorders, such as ASD (Figure 3). These metals, including lead, mercury, nickel, and manganese, have been shown to play critical roles in the pathogenesis of ASD during neurodevelopment [4]. Exposure to these metals during the early stages of fetal development may result in brain disorder at concentrations much lower than those affecting adults’ brain function [20]. The blood level of these metals is higher in children with ASD. A study by Li et al. supports the vital role of heavy metal exposure in the etiology of ASD [21].

Figure 3.

Mechanisms of metal-induced perturbed MAPK in ASD. Overexposure to metals (Pb, Hg, Mn, Ni) during neurodevelopment triggers perturbations in MAPK signaling and may contribute to ASD onset. Pb causes increased phosphorylation of ERK1/2 and p38 of the MAPK family. Hg activates MAPK and PKA/CREB pathway, followed by upregulation of c-fos and brain-derived neurotrophic factor (BDNF). Mn induces oxidative stress, increased p38, ERK1/2, JNK1/2/3, and caspase activities, and Ni is also a contributor to imbalanced antioxidants in oxidative stress.

Lead (Pb)

Lead, soft bluish-white metal with no known functional role in the body, is historically used in alloys, construction materials, paints, pigments, and other metals’ coating elements. With technological advancement, its utilization in electricity, communication, and as an antiknock agent in planes increased [4,22]. The consumption of lead-contaminated water during pregnancy and lactation has been linked with NDD. The most common means of lead toxicity among children is the ingestion of paint chips or dust containing lead [22]. Severe lead blood level initiates neuronal damage in ASD pathogenesis [23]. El-Ansary et al. [24] reported a significant rise of mercury and lead with a concomitant decrease in selenium levels in RBCs of children with ASD compared to healthy control potential lead-induced neurotoxicity in ASD. Exposure of the fetus to lead during pregnancy, even at a low dose, may result in brain damage leading to adverse neurobehavioural development in the offsprings mediated via epigenetic changes [22].

Mercury (Hg)

Mercury, a ubiquitous naturally occurring element, has a wide range of uses in industry and manufacturing processes [25]. Mercury intoxication has been linked to brain damage in developing fetuses. ASD children have a higher susceptibility to heavy metal intoxication than non-ASD children. The role of Hg in the pathogenesis of ASD includes, but not limited to, degeneration of microtubule, inflammation of the neurons, lipid peroxidation and oxidative stress, dysfunctions in the cellular mitochondrion, inhibition of glutamic acid decarboxylase activity, decrease in the level of reduced glutathione, increase in the level of oxidized glutathione, perturbed calcium homeostasis and signaling, and increase in the level of proinflammatory cytokines in the brain [26]. Levels of mercury in urine, blood, and hair were positively associated with ASD [27]. Also, mercury toxicity can initiate immunologic changes such as specific brain antibodies’ production [26]. A reduced mercury level and brain antibodies were recommended as a therapeutic approach to treat ASD [28]. However, the pathophysiological etiologies associated with the development of ASD remain controversial. Gil-Hernandez et al. [29] reported no relationship between mercury neurotoxicity and the etiology of ASD based on no difference in urinary mercury levels between ASD and non-ASD children. This observation was also confirmed by Wright et al. [30]. However, a comprehensive report on the relationship between mercury and autism shows that the vast majority (74%) of such studies indicates both direct and indirect association between mercury and ASD onset [31].

Nickel (Ni)

Nickel is a transition element with a significant contribution to plants’ morphological and physiological functions, eukaryotes, algae, and bacteria [32]. However, at high amounts, nickel alters various metabolic processes in plants, inhibiting enzymatic activities, and the process of photosynthesis. Humans’ exposure to nickel-polluted environments may result in multiple pathological effects, including lung fibrosis, cancer, cardiovascular, and renal diseases [33]. The neurotoxic effects of nickel are associated with its propensity to cause oxidative stress [4,34]. Nickel decreases the activity of superoxide dismutase and catalase, thereby increasing reactive oxygen species [35]. Nickel has also been linked to epigenetic effects by causing DNA methylation, histone modification, and microRNA expression, resulting in altered gene expression [33]. Ijomone et al. [34] concluded that nickel induces developmental neurotoxicity by causing degeneration of cholinergic, dopaminergic, and GABAergic neurons. Perturbation in these neurotransmitter systems has been linked to ASD [4].

Manganese (Mn)

Manganese is an essential trace element necessary for several physiological processes. However, manganese accumulation or impaired hepatobiliary excretion can result in neurotoxic effects and NDD [36]. Several studies supported the association between ASD and Mn exposure as measured by air distribution, hair, urine, and Mn’s blood levels, although with some conflicting findings [37–39]. Manganese overexposure results in oxidative stress causing an imbalance between reactive oxygen species and antioxidants. In addition, ASD has been linked to a perturbation of the dopaminergic system [40]. Interestingly, Mn overexposure is established to trigger dopaminergic dysfunctions; hence it is considered an environmental risk factor for ASD [4,41].

MAPK signaling and metal neurotoxicity in ASD pathogenesis

MAPK signaling pathways are altered during neurodevelopment because of the fetus’s overexposure to several metals, which have neurotoxic effects on the developing brain (Figure 3). There are several mechanisms by which metal neurotoxicity causes perturbed MAPK signaling in ASD. According to Leal et al., lead modulates the ERK1/2 and p38 of MAPK signaling in the cerebellum of Brazilian catfish Rhamdia quelen, showing this metal’s potential role in perturbed MAPK signaling [42]. The study also reported that both in vivo and in vitro lead exposure resulted in significantly increased phosphorylation of both MAPKs [42]. Fujimural and Usuki suggested that methylmercury (MeMercury) induces neuronal degeneration by causing hyperactivity of site-specific neurons induced by the activation of MAPK and PKA/CREB pathways followed by upregulation of c-fos and brain-derived neurotrophic factor [43]. Cordova et al. showed that overexposure of fetus to Mn during critical neurodevelopment stages results in dysfunction in motor coordination with a parallel rise in oxidative stress markers, increased phosphorylation of p38 (MAPK), and activity of caspase in the striatum [44]. In a study conducted by Peres et al. in immature rats, Mn led to the activation of MAPK signaling (ERK1/2 and JNK1/2/3) in slices of immature hippocampus and striatum, suggesting a potential role of Mn in perturbed MAPK signaling in the juvenile or developing brain [45]. Some studies also suggest metal-induced neuroinflammation as a mechanism for perturbed MAPK in ASD [4,46]. Zhu et al. highlighted the roles of PKA and Ca²⁺-dependent p38 activation in protecting against manganese neuronal apoptosis [47]. The study emphasized that p38 MAPK/CREB (cAMP response element-binding protein) activation through PKA activation and increased cellular Ca2+ aided in mitigating Mn-induced neuronal apoptosis regulating brain-derived neurotrophic factor, thereby elucidating the process of Mn-induced neurotoxicity and the impact of the MAPK signaling pathway [47].

Conclusion

MAPK signaling plays a crucial role in neurodevelopment. As discussed in this article, the developing fetus is vulnerable to overexposure to certain substances such as metals. Overexposure to these metals results in a perturbed MAPK signaling pathway, consequently altering neurodevelopment’s standard process, thereby resulting in neurodevelopment disorders such as ASD. These metals use several mechanisms to cause disturbances in the normal process of neurodevelopment. Some cause an imbalance between antioxidants and free radicals, resulting in oxidative stress, while some have epigenetic effects, leading to alterations in the genes coding for the MAPK signaling pathway. Several studies found a potential relationship between metal overexposure and ASD, explaining the potential role of these metals in the pathogenesis of ASD. However, the relationship between these metals and ASD and metal exposure remains controversial and requires future studies. Further research is needed to provide additional evidence on the relationship between metal exposure and ASD and elucidate the role of metal-induced perturbed MAPK in the etiopathogenesis of ASD.