Neuronal Autophagy in Synaptic Functions and Psychiatric Disorders

Abstract

Homeostatic maintenance of physiological functions is fundamental to organismal well-being. Disruption or imbalance in homeostasis results in functional disturbances at molecular, cellular, and tissue levels, leading to manifestation as physical and mental illnesses. Homeostatic imbalance is caused by a range of pathophysiological mechanisms, including disrupted reduction-oxidation (redox) reactions, inflammatory responses, metabolic disturbances, or failure in quality control of cellular proteins and organelles. However, the roles for the protein/organelle quality control in the regulation of behaviors, in particular of cognitive processes, have not been well documented until recent reports that finally support this concept. The frontline studies in neuroscience have revealed that synaptic components (e.g., synaptic proteins, organelles, neurotransmitters and their receptors) are selectively degraded by autophagy, a cellular recycling machinery implicated in surveillance and quality control of proteins and organelles responsible for the maintenance of cellular homeostasis. Apart from the canonical role of autophagy in supporting cell viability, synaptic autophagy appears to regulate synapse remodeling and plasticity. Consistently, emerging evidence suggests novel roles of autophagy in memory encoding, information processing, or cognitive functions. In this article, we will overview recent progress in understanding the roles of neuronal autophagy in homeostatic maintenance of synaptic functions, with particular focus on how disruptions in these processes may contribute to the pathophysiology of psychiatric disorders.

Introduction

A number of pathophysiological events could converge on disrupted homeostasis of physiological functions, leading to a wide range of diseases or pathological consequences (1,2). In the nervous system, redox deregulation, inflammation, metabolic disturbances, or failure in quality control of cellular proteins and organelles have been deeply implicated in neurological and neurodegenerative disorders (3–6). Neuropsychiatric disorders are no exceptions; emerging evidence suggests that the roles for the protein/organelle quality control in the regulation of behaviors, in particular of cognitive processes, and in psychiatric manifestations may become a frontline topic in biological psychiatry.

Among the mechanisms that ensure homeostasis of the protein or organelle quality, autophagy has drawn much attention in recent years. Autophagy is a Greek synonym for ‘self-eating’, originally named by Christian de Duve in 1963 based on his discovery of digestive organelles within cells, including the lysosome and the autophagic vacuole termed the autophagosome. More recently, mechanisms of autophagy started to be unveiled by Yoshinori Ohsumi two decades ago, sparked by the discovery of a series of AuTophaGy-related (ATG) genes conserved through evolution, culminating in the general concept that autophagy is fundamental to the maintenance of cellular and organismal homeostasis (7). Among three main forms of autophagy [e.g., macroautophagy, microautophagy, chaperone-mediated autophagy (CMA)], macroautophagy is the major catabolic process, in which old cytoplasmic proteins, lipids, or damaged organelles are either non-selectively or selectively engulfed into the autophagosome and subsequently delivered to the lysosome for degradation (7,8) (Fig. 1). The amino acids and lipids generated through this digestive process can then be recycled to fuel biosynthetic machinery, and contribute to continuous rejuvenation of proteins or organelles and generation of cellular energy. In addition, autophagy can be dynamically induced in response to cellular stress modalities, such as nutrient deprivation, oxidative stress, and accumulation of misfolded or aggregated proteins (9,10). Recent studies show that post-mitotic neurons are particularly vulnerable to disrupted homeostasis caused by autophagy deficiency (11,12). Reflecting these critical roles of autophagy in the maintenance of cellular homeostasis, deregulated autophagy has been implicated in a wide range of human diseases (13,14).

Fig. 1. Schematic diagram of macroautophagy.

Upon cellular stress conditions triggered by a range of stress modalities (e.g., nutrient starvation, oxidative stress, accumulation of misfolded or aggregated proteins), mammalian target of rapamycin (mTOR) signaling is suppressed, leading to a cascade of events that contribute to the induction of macroautophagy (e.g., 1. Assembly of autophagy regulatory protein complex on the isolation membrane; 2. Elongation of the isolation membrane to form autophagosomes; 3. Autophagosome trafficking to fuse with the lysosome; 4. Maturation of autolysosome and degradation). Critical autophagy regulatory proteins are depicted in green.

In this review article, we will overview: 1) current understanding of the canonical role of neuronal autophagy in the maintenance of cellular homeostasis, hence supporting neuron survival and neurodevelopment; 2) emerging evidence for neuronal autophagy in regulating synaptic functions and neuroplasticity, and then discuss 3) how neuronal autophagy regulates memory or specific dimensions of behaviors, such as cognition, as well as their dysfunctions in psychiatric disorders.

Canonical roles of autophagy in neuronal viability and neurodevelopment

Basal autophagy activity is critical to the maintenance of neuronal homeostasis and viability. Evidence supporting this view originally came from cell biology and animal studies. In post-mitotic neurons, the autophagosomes are continuously formed at the distal end of the axon and undergo unidirectional transport along microtubules toward the soma, which is enriched with lysosomes and biosynthetic machinery responsible for producing building blocks for life (e.g., amino acids, lipids) (15–18). Mice with nervous system-specific or neuronal cell type-specific ablation of core autophagy genes (i.e., Atg5, Atg7) exhibit dystrophic axonal swelling and progressive degeneration of axon termini filled with aberrant membranous structures yet no sign of autophagosome formation (19–21). Affected neurons have prominent accumulation of abnormal cytosolic proteins and ubiquitin-positive aggregates, leading to progressive neuronal death during the early postnatal development in these animals. The results indicate that continuous quality control of diffuse cytosolic proteins via basal constitutive autophagy is critical to neuronal survival. Additional evidence for the role of autophagy-related genes in neuronal viability is summarized in Table 1.

Table 1.

Autophagy genes and neurodegeneration

Gene	Type of gene mutation	Species	Phenotype	Ref
Atg5, Atg7	Nervous system-specific knockout (KO)	Mouse	Axonal swelling and degeneration; progressive postnatal loss of neurons (pyramidal neurons in the cerebral cortex and hippocampus, Purkinje cells in the cerebellum); organismal lethality	19,20
Atg5, Atg7	Purkinje neuron-specific KO	Mouse	Axonal swelling and degeneration; progressive postnatal loss of Purkinje cells in the cerebellum and motor coordination decline; organismal lethality	21,120
Fip200	Nervous system-specific KO	Mouse	Purkinje cell degeneration; progressive cerebellar ataxia; organismal lethality	121
Ulk1/Ulk2	Nervous system-specific double KO	Mouse	Progressive loss of neurons (pyramidal neurons in the hippocampus CA1 region); elevated endoplasmic reticulum stress (activation of unfolded protein response pathway); organismal lethality	122
Endophilin A	Endophilin 1/2 double KO, Endophilin 1/3 double KO	Mouse	Progressive impairment in motor coordination, ataxia, neurodegeneration (motor cortex, hippocampus)	66
	Endophilin A KO	Drosophila	Loss of dopaminergic neurons	43
Synaptojanin 1	Point mutation (R258Q)	Drosophila	Impaired autophagosome maturation, neurodegeneration (dopaminergic neuron loss)	44
ATG5	Homozygous missense mutation	Human	Congenital ataxia; mental retardation; developmental delay	123
WDR45	de novo mutations	Human	Neurodegeneration with brain iron accumulation (NBIA); abnormal iron deposition in substantia nigra and globus pallidus; developmental delay; neurological deterioration (parkinsonism, dystonia, early onset dementia)	124–126

Although loss of autophagy causes neuronal death (19–21), and altered autophagy functions are associated with aging-associated neurodegenerative disorders, such as Alzheimer’s, Huntington’s, and Parkinson’s diseases (AD, HD, PD) (11,22,23), autophagy deficiency may not simply represent the pathophysiological mechanisms in these disorders. Notably, the autophagosomes are reported to accumulate in vulnerable neurons in these human diseases (22,23), whereas autophagy deficiencies modeled in mice show no sign of autophagosome formation (11,19–21). Studies on human neurodegenerative disorders provide support for additional pathobiological mechanisms downstream of autophagy deregulation, including defective axonal transport (24,25) and attenuated lysosomal function (26–29), together contributing to buildup of autophagosomes and ultimate neuronal death observed in these disorders. Further discussion on this topic can be found in additional reviews (30,31).

Neuronal autophagy also plays important roles during neuronal development, for example in neural tube closure (32), axon outgrowth or pathfinding (33–35), and synapse formation (36–41), as summarized in Table 2. For example, in mice deficient for Alfy, an adaptor protein responsible for selective autophagy, some axons at the corpus callosum and anterior commissure fail to cross the midline, in part due to failure to respond to axon guidance cues, resulting in abnormal development of interhemispheric axon tracts (33). The results suggest that autophagy facilitates membrane recycling or turnover of signaling components, including the membranes carrying the axon guidance cue receptors, at the leading edge of the axon or in the migrating growth cone, together contributing to axonal development (33–36). In addition, autophagy pathways promote assembly of presynaptic compartments (38–40), in part by regulating synaptic vesicle clustering (39), and also facilitate post-synaptic maturation, by mediating elimination of excess dendritic spines (i.e., spine pruning)(41). These lines of evidence suggest the involvement of defective autophagy in neurodevelopmental disorders.

Table 2.

Autophagy genes and neurodevelopment

Gene	Type of gene mutation	Species	Phenotype	Ref
Ambra1	KO	Mouse	Neural tube closure defect; imbalance in neuronal proliferation and differentiation	32
Alfy	KO	Mouse	Abnormal development of interhemispheric axon tracts (corpus callosum, anterior commissure, hippocampal commissure); failure of axons to respond to the axon guidance cue (Netrin-1); midline crossing defects	33
Atg9	Nervous system-specific KO	Mouse	Abnormal development of corpus callosum and anterior commissure	34
Wdr47	KO	Mouse	Abnormal development of corpus callosum	35
Atg1	P-element insertion and imprecise excision of the locus	Drosophila	Defective axonal transport of synaptic vesicle precursors; presynaptic assembly deficits at the neuromuscular junction (NMJ); reduced number of synaptic boutons and smaller size of NMJ	36,37
Atg1, Atg2, Atg18	P-element insertion and imprecise excision of the locus	Drosophila	Presynaptic assembly deficits at NMJ; reduced number of synaptic boutons and smaller size of NMJ	38
Atg9	EMS-induced point mutations	C. elegans	Defective clustering of synaptic vesicles in presynaptic specializations	39
Atg7	Motor neuron-specific KO	Mouse	Accelerated neuromuscular denervation during early development in ALS model mice (SOD[G93A]); non-cell autonomous suppression of ALS-related phenotypes at later stages of disease progression in SOD[G93A] mice	40
Tsc2	Heterozygous KO	Mouse	Developmental spine pruning defects; excessive dendritic spines in cortical pyramidal neurons	41

Role of autophagy in synapse remodeling and synaptic plasticity

Maintaining the integrity of proteins (e.g., synaptically localized proteins, neurotransmitters, and their receptors) and organelles (e.g., synaptic vesicles, mitochondria) is crucial to sustain neuronal functionality throughout their lifetime, which could span over a century in the case of humans. Besides, highly polarized and extended morphologies of neurons pose a unique spatial challenge for coordinating the local homeostatic need for autophagic clearance. Recent cellular imaging studies revealed that the autophagosomes are not only constitutively generated in axons (17,18), but are also increased in number upon synaptic activity, either through increased local biogenesis or recruitment of pre-existing autophagosomes to the site of synaptic activation (42–45). The data raise the possibility that neuronal autophagy regulates synaptic functions and neuroplasticity by interpreting both intrinsic and extrinsic demands for maintaining homeostasis of nervous system functions.

Synaptic plasticity is defined as the activity-dependent modification or adaptation of synaptic strength to achieve lasting changes in synaptic efficacy (46). Several well-known forms of synaptic plasticity include long-term potentiation (LTP), long-term depression (LTD), and homeostatic plasticity (i.e., synaptic homeostasis) (46,47). Mechanistically, synaptic plasticity entails a broad spectrum of morphological and biochemical changes in presynaptic neurotransmitter release machinery, postsynaptic receptor composition, activity of signal transduction pathways, gene expression, as well as local proteome regulation at the synapse, including de novo protein synthesis and degradation (46–52). Protein degradation is particularly important to counterbalance protein synthesis, allowing tight control of local synaptic proteome to fine-tune synaptic activities (51,52). In addition to proteins, lipids and synaptic organelles (e.g., synaptic vesicles, mitochondria) are also important players for regulating synaptic plasticity. These synaptic components are susceptible to wear and damage due to remarkably high frequencies of neuronal firing (i.e., action potentials, being delivered up to 50 pulses per second) (53,54). Consequently, synapses are the site of high demand for cellular catabolic activities, requiring efficient degradation systems to maintain local proteome/organelle integrity and sustain synaptic functions (55,56).

Until recently, little attention has been paid to the potential role of autophagy in synaptic plasticity, due in part to a classical view of autophagy as a non-selective, bulk degradation system. Accumulating evidence now shows that neuronal autophagy could achieve selective targeting of synaptic components. For example, synaptic vesicles and mitochondria are targeted by respective autophagic adaptor proteins and degraded through macroautophagy (57,58). A subset of presynaptic proteins with a specific amino acid motif (e.g., KFERQ) are selectively targeted by the molecular chaperone [i.e., Heat shock cognate 70 (Hsc70)] and degraded through microautophagy or CMA (59–61). Along with the ubiquitin/proteasomal system (UPS) that targets ubiquitinated proteins for degradation (62–64), neuronal autophagy could target a broader spectrum of synaptic components, together contributing to the regulation of neurotransmission and synaptic plasticity. Besides, given the intrinsic nature of autophagy as a mechanism for producing bioenergetic and biosynthetic materials (e.g., amino acids, lipids, and other metabolic building blocks), this machinery appears well suited to regulation of synaptic functions, allowing local autonomic control of synaptic plasticity in axons and dendrites distant from the neuronal soma (55,56).

Recent studies on local proteome in autophagy-deficient model organisms began to reveal a link between synaptic autophagy and synaptic plasticity (see schematic view in Fig. 2). In the presynaptic compartment, neuronal activities are shown to control autophagosome biogenesis and maturation, using several adaptor proteins or positive/negative regulators enriched at the presynaptic terminal [e.g., leucine-rich repeat kinase 2 (LRRK2), Endophilin A1, Synaptojanin 1, Bassoon] (42–44,65,66). For example, Endophilin A1, once phosphorylated by LRRK2, promotes local synaptic macroautophagy by recruiting core autophagy proteins necessary for autophagosome formation (43). Induced autophagosomes can selectively target synaptic vesicles (57,65,67); pharmacological acute activation of macroautophagy reduces the number of synaptic vesicles at axon terminals, and macroautophagy depletion (i.e., Atg7-knockout in dopaminergic neurons) increases evoked dopamine release in mice (67). Thus, presynaptic macroautophagy likely regulates neurotransmission by controlling the size of the synaptic vesicle pool available for neurotransmitter release. In addition, macroautophagy selectively targets mitochondria when mitochondrial outer membrane proteins are ubiquitinated (through Pink1-dependent activation of E3 ubiquitin ligase, Parkin; see additional reviews for details (68,69)). This form of selective autophagy (aka mitophagy) may regulate local energy supply or calcium buffering capacity, thereby contributing to synaptic functions and plasticity (70).

Fig. 2. Regulators of pre- and post-synaptic autophagy.

Neuronal activity upregulates autophagosome formation in the presynaptic compartment, using several presynaptically enriched adaptors (e.g., Endophilin A, Bassoon) and regulatory proteins (e.g., Synaptojanin 1). Example of the synaptic vesicle is depicted; synaptic vesicles selectively tagged by an adaptor protein (e.g., Rab26) are recognized by the pre-autophagosomal structure (①) and engulfed by the autophagosome (②), which is subsequently delivered to the lysosome for degradation (③). Endophilin A and synaptojanin 1 cooperatively recruit autophagy regulatory proteins on the autophagosomal membrane, and Bassoon negatively regulates this process by sequestering Atg5. In the postsynaptic compartment, neuronal activity also induces macroautophagy, causing endocytic removal of neurotransmitter receptors (e.g., AMPAR, GABA_AR) from the plasma membrane (④), possibly via regulation of receptor adaptor proteins (e.g., PICK1, PSD-95, SHANK3). Subsequently, the endosome fuses with the autophagosome (⑤) to form the amphisome, which is delivered to the lysosome for degradation (⑥). Surface levels of GABA_AR could also be regulated by the steady-state levels of p62; cytosolic p62 proteins are normally sequestered by the autophagosome (⑦) and degraded by the lysosome (⑧); however, elevated p62 expression due to reduced autophagy activity causes sequestration of GABARAP (as shown by a dotted line in the post-synaptic compartment), leading to downmodulation of surface levels of GABA_AR.

Autophagy also regulates synaptic activity in the postsynaptic compartment (Fig. 2). Synaptic stimulation induces autophagosome formation or recruitment within dendrites (45). In addition, low-dose activation of N-methyl-D-aspartate (NMDA) receptors induces LTD via macroautophagy-dependent degradation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (45). Similarly, lysosomes are recruited to synapses in response to synaptic activities, leading to AMPA receptor sequestration and reduction in synaptic activity and spine density (71). Inhibitory neurotransmitter receptors (i.e., γ-aminobutyric acid (GABA)_A receptors (GABA_ARs)) are also regulated by macroautophagy; upon denervation at the neuromuscular junction in C. elegans, GABA_ARs expressed on the muscle cells are selectively sorted via endocytosis to traffic from the postsynaptic membrane surface to the autophagosome, whereas acetylcholine receptors in the same cells do not (72). This suggests a role of macroautophagy in selective sorting of GABA_ARs, hence attenuated inhibitory neurotransmission. Further supporting a role of autophagy in synaptic plasticity, brain-derived neurotrophic factor (BDNF), an inducer of LTP, regulates autophagy in hippocampal neurons; suppression of autophagy is sufficient to rescue LTP defects and memory impairment caused by BDNF deficiency in mice (73,74). Since several post-synaptic scaffolding proteins (i.e., PICK1, PSD-95, and SHANK3) are found to be included in the autophagosomes, synaptic plasticity may be regulated by autophagic degradation of these proteins (73,74). Furthermore, recent evidence shows that TrkB, a receptor for BDNF, is localized on the autophagosomal membrane, arguing for the role of the autophagosome in transducing BDNF signals, hence neuroplasticity (75). Importantly, the above-mentioned synaptic components targeted for autophagic degradation are implicated in the regulation of synaptic plasticity (48,76).

Neuronal autophagy in the regulation of memory, cognition, and psychiatric manifestations

Although we discussed the roles of autophagy in synaptic plasticity, what is the direct evidence supporting the role of autophagy in higher brain functions, such as learning, memory or cognition? Cellular autophagic activity is known to gradually decrease during normal aging (77). Spermidine, an endogenous substance with autophagy-inducing activity, has been reported to extend longevity in many species (78), and could protect from aging-associated memory impairment and metaplasticity (i.e., ultrastructural increase of presynaptic active zones) (79), by blocking the build-up of presynaptic active zone proteins (e.g., Bruchpilot, Rim-binding protein, Unc-13) and suppressing excessive neurotransmitter release (80–83). A recent study further showed that macroautophagy activity within the Drosophila learning and memory center (i.e., mushroom body) is responsible for restricting brain-wide metaplasticity by maintaining the expression of NPY, a neuropeptide expressed in interneurons (84). This suggests the non-cell autonomous role of neuronal autophagy in protecting the brain from synapse aging and highlights the critical role of the high-level brain integration center in dictating overall brain health. In addition, mice with the nervous system-specific deletion of Wdr45, an ortholog of Atg18 essential for autophagosome formation, exhibit impaired learning and memory (85). Pharmacological upregulation of autophagy using rapamycin, an inhibitor of mammalian target of rapamycin (mTOR) that normally suppresses autophagy, is shown to alleviate deficits in synaptic plasticity and improve cognition in drug- or stress-induced rodent models of cognitive impairment (86,87). Recent evidence from an auditory fear reconsolidation model in mice demonstrated that autophagy induction could enhance erasure of a reconsolidation-resistant auditory fear memory, providing a potential therapeutic target for alleviating anxiety (88). These lines of evidence suggest that autophagy has an important role in some forms of synaptic plasticity underlying memory formation.

Evidence has begun to accumulate that autophagy deficits are linked to neuropsychiatric conditions (Table 3). First evidence for a causal role of attenuated macroautophagy in biology relevant to neuropsychiatric disorders came from a study on mice heterozygous for Tsc2 loss of function (Tsc2^+/−), a model for a rare variant of autism spectrum disorders (ASDs) (41). Because Tsc2 is a negative regulator of mTOR, Tsc2^+/− mice have constitutively elevated mTOR activity, hence attenuated autophagy, exhibiting excessive dendritic spine formation. This parallels with increased dendritic spine density and reduced developmental spine pruning in layer V pyramidal neurons in postmortem ASD temporal lobe. Activation of autophagy by rapamycin rescues spine pruning deficits and ASD-like behaviors (i.e., impaired social interaction) in Tsc2^+/− mice, but not in neuronal autophagy-deficient mice (i.e., Tsc2^+/−;Atg7-conditional KO), suggesting that the deficit in mTOR-regulated macroautophagy is causal for developmental spine pruning defects and underlies the pathophysiology of ASD.

Table 3.

Autophagy genes and psychiatric conditions

Gene	Type of gene mutation	Species	Phenotype	Ref
Wdr45	Central nervous system-specific KO	Mouse	Impaired spatial working memory; impaired recall of fear memory	85
Tsc2	Heterozygous KO	Mouse	ASD-like imapired social interaction behavior; dendritic spine pruning defect; elevated mTOR activity and reduced autophagy flux	41
Fmr1	KO	Mouse	Aberrant dendritic spine structure; deficits in synaptic plasticity and cognition; elevated mTOR activity and reduced autophagy flux	89
Ulk1	KO	Mouse	Ethanol exposure-dependent cognitive deficit in novel object recognition	127
Ulk2	Heterozygous KO	Mouse	Deficits in sensorimotor gating and cognitive flexibility; elevated p62 expression in cerebral pyramidal neurons; reduced surface expression of GABAARs; imbalanced excitatory-inhibitory balance	90
Atg7	Post-adolescent KO	Mouse	Impaired social interaction behavior; deficits in inhibitory neurotransmission; elevated p62 expression and reduced surface expression of GABAARs	96
Ulk1, Ulk2, Ulk4	Copy number variation (CNV)	Human	CNVs of Ulk1, Ulk2, and Ulk4 genes in Icelandic schizophrenia and bipolar patients	128
Beclin1	N/A	Human	Reduced expression of Beclin1 in hippocampus in postmortem brains of patients with schizophrenia	113
p62	N/A	Human	Elevated expression of p62 protein in olfactory neuronal cells biopsied from patients with schizophrenia and bipolar disorder	90

Furthermore, a mouse model of Fragile X syndrome (FXS), the most frequent form of heritable intellectual disability and autism, also exhibits enhanced activity of mTOR, hence reduced autophagy (89). Activation of autophagy by silencing the expression of Raptor, a component of mTOR complex 1, largely restores aberrant spine structure, synaptic plasticity, and cognition in fragile X mice, suggesting that impaired macroautophagy is causally related to the FXS phenotypes.

Recent studies show another causal link of attenuated autophagy to neurobiological and behavioral changes relevant to psychiatric manifestations. In neuronal cells sampled from living human subjects via nasal biopsy, macroautophagic activity is reported to be downregulated in patients with schizophrenia and bipolar disorder compared with healthy controls (90). In mice with reduced expression of Ulk2 (Ulk2^+/−), an ortholog of Atg1 implicated in autophagy induction, expression levels of sequestosome-1/p62, an adaptor protein responsible for selective autophagy, are upregulated predominantly in pyramidal neurons of the prefrontal cortex (PFC) as a result of attenuated autophagy (90). Ulk2^+/− mice exhibit behavioral deficits relevant to psychosis, including disrupted sensorimotor gating (i.e., attenuated prepulse inhibition of acoustic startle stimuli) and impaired cognitive flexibility. Mechanistically, p62 binds with GABA_A receptor-associated protein (GABARAP) (91), a protein that regulates endocytic trafficking of GABA_ARs (92) (Fig. 2); thus, elevated p62 expression in Ulk2^+/−pyramidal neurons appears to sequester a higher proportion of GABARAP and limit the amount of GABARAP available for surface presentation of GABA_ARs, including α5 subunit-containing GABA_AR implicated in memory and cognitive functions (93,94). Consequently, Ulk2^+/− pyramidal neurons have imbalanced excitatory–inhibitory neurotransmission, which may in part underlie the sensorimotor gating deficit and cognitive impairment observed. Notably, Ulk2^+/− neurons selectively down-modulates surface GABA_AR levels without affecting surface NMDA receptor levels (90). Because mice deficient for α5-GABA_AR also show sensorimotor gating deficit and cognitive inflexibility phenotypes (94,95) similar to those seen in Ulk2^+/− mice, the data suggest that Ulk2, p62 and α5-GABA_AR may function together to regulate cognitive functions. Additionally, a similar mechanism by which elevated p62-dependent sequestration of GABARAP family proteins downregulates the levels of surface GABA_AR is confirmed in another autophagy-deficient mouse model (i.e., post-adolescent Atg7-conditional KO), which exhibits deregulated inhibitory neurotransmission and impaired social interaction behaviors (96).

Although the above-mentioned models of monogenic mutations causally link attenuated autophagy to psychiatric conditions, a question remains as to the extent to which attenuated autophagy is prevalent in psychiatric diseases in general. Enrichment analysis (see Supplementary materials for details) indicated that Genome-wide association study (GWAS) risk genes for brain disorders [schizophrenia, bipolar disorder, psychosis, AD, PD, dementia, attention-deficit/hyperactivity disorder (ADHD), autism, cognitive decline, and depression] are over-represented in autophagy-related pathways from gene ontology (GO) biological processes: (i) negative regulation of autophagy (p-value: 0.017); (ii) positive regulation of autophagy (p-value: 9.0E-5); and (iii) umbrella pathway consisting of core autophagy genes, autophagosome maturation, autophagosome assembly, negative regulation of autophagy, and positive regulation of autophagy (p-value: 3.1E-4) (Table S1). In addition, Gene set enrichment analysis (GSEA) using RNA-Seq data from CommonMind Consortium (97) indicated that the autophagy pathway from GO biological process was significantly downregulated in patients with schizophrenia compared to healthy controls (Table S2).

Future perspective

These studies have broader implications in neurological and neuropsychiatric disorders. Elevated levels of p62 proteins or p62-positive protein aggregates in the nervous system may represent a molecular signature shared across these two disease categories. For example, a rodent model of 22q11.2 chromosomal deletions, which are at high risk of developing schizophrenia or early-onset PD (98,99), exhibits elevated expression of p62 and α-synuclein proteins in PFC or substantia nigra, presumably due to attenuated autophagy caused by elevated mTOR activity (100). Given the finding that p62 protein levels could modulate surface GABA_AR levels (90,96), elevated p62 pathology may serve as a mechanism underlying deregulated GABAergic inhibitory neurotransmission and associated cognitive impairment observed across neurological and neuropsychiatric disorders, including aging (101–106). In addition, α-synuclein and p62 are often co-localized in Lewy bodies (107), a hallmark of PD pathophysiology (70). α-synuclein is a presynaptically enriched protein implicated in neurotransmitter release, and is targeted by CMA (59). Therefore, deregulated expression of α-synuclein due to deficits in the autophagic process may underlie neuropsychiatric symptoms associated with PD in general or with 22q11.2 deletion syndrome in particular.

Attenuated autophagy may have therapeutic and diagnostic implications in neurological and neuropsychiatric disorders. Reduced surface α5-GABA_AR levels may provide a potential therapeutic target for remediating cognitive impairment downstream of attenuated autophagy or elevated p62 pathology, possibly via augmenting GABAergic neurotransmission using, for example, positive allosteric modulators directed at α5-GABA_AR (108,109). A recent study reported that several classes of antidepressants (e.g., amitriptyline, fluoxetine) commonly activate autophagy via slow accumulation of sphingomyelin and ceramide in neurons, and that direct inhibition of sphingomyelin synthase exerts rapid accumulation of ceramide, autophagy activation, and behavioral reversal in the stress-induced depression model (110). To evaluate efficacy of such autophagy-targeting therapeutic strategies, it is critically important to monitor autophagy activity in live brains. While the current methods of monitoring neuronal autophagy largely depend on measuring LC3/Atg8 protein levels in the brain in animal models (7), this approach may not be applicable to humans. Instead, monitoring levels of these or relevant counterpart proteins in surrogate tissues may be useful. For example, we have shown clinical utility of using olfactory neuronal cells biopsied from patients (111,112), and reported attenuated autophagy (i.e., p62 overexpression) in these cells from patients with sporadic schizophrenia and bipolar disorder (90). In addition, levels of activity-dependent neuroprotective protein (ADNP), a binding partner of LC3/Atg8, are shown to be significantly elevated in the peripheral lymphocytes of patients with schizophrenia compared to healthy controls (113).

Besides autophagy, the UPS plays a major role in homeostatic control of protein quality (62,63). Although the roles of the UPS in psychiatric disorders are not clearly understood yet, a recent report showed that a subset of the postmortem brains from patients with schizophrenia display significantly elevated levels of insoluble proteins and protein ubiquitination, arguing for pathological involvement of the UPS in some cases of psychiatric disorders (114). Another recent report also showed increased aggregate formation of GABARAP family proteins in a subset of postmortem brains from patients diagnosed as ASD (96). These studies are consistent extension and generalization of a notion that elevated insolubility of specific proteins, such as DISC1, occurs in the brains with psychiatric disorders (115–117). Mechanistically, DISC1 protein has an aggregate-prone property and sequestrates its interacting proteins into insoluble fractions, leading to a loss of function of key interacting proteins such as phosphodiesterase-4 and the associated changes in emotionality (118,119). An outstanding question is how autophagy and the UPS are related with each other under an overall picture of protein quality control and, in turn, contribute to psychiatric disorders.

We have witnessed part of the strategies that neuronal autophagy uses in regulating synaptic functions and network activity. This includes activity-dependent degradation of neurotransmitters and their receptors, selective surface presentation of neurotransmitter receptors, and morphological regulation of synaptic spines. Furthermore, these recent findings provide insight into how neuronal autophagy regulates higher brain functions, such as cognition, mood, and social interaction. Future studies at molecular, cellular, circuit, and behavioral levels are warranted to provide insight into the detailed mechanisms for autophagy-mediated brain functions, and to further address how deregulated autophagy underlies specific dimensions of symptomatic outcomes in neurological and psychiatric disorders. Depending on the cell types (e.g., excitatory or inhibitory neurons), the brain regions (e.g., PFC or substantia nigra), or the circuits (e.g., corticolimbic or nigrostriatal projection) that are principally affected, we predict to see varying phenotypic consequences in specific behavioral or cognitive dimensions. Finally, in-depth understanding of the circuit-wide alterations following deregulated neuronal autophagy is critical to devise novel therapeutic approaches for these pathological conditions.