Cell-type-specific regulation of neuronal intrinsic excitability by macroautophagy

Neuroactive steroid effects on autophagy in a human embryonic kidney 293 (HEK) cell model

Neuropsychiatric and neurodegenerative disorders are correlated with cellular stress. Macroautophagy (autophagy) may represent an important protective pathway to maintain cellular homeostasis and functionality, as it targets cytoplasmic components to lysosomes for degradation and recycling. Given recent evidence that some novel psychiatric treatments, such as the neuroactive steroid (NAS) allopregnanolone (AlloP, brexanolone), may induce autophagy, we stably transfected human embryonic kidney 293 (HEK) cells with a ratiometric fluorescent probe to assay NAS effects on autophagy. We hypothesized that NAS may modulate autophagy in part by the ability of uncharged NAS to readily permeate membranes. Microscopy revealed a weak effect of AlloP on autophagic flux compared with the positive control treatment of Torin1. In high-throughput microplate experiments, we found that autophagy induction was more robust in early passages of HEK cells. Despite limiting studies to early passages for maximum sensitivity, a range of NAS structures failed to reliably induce autophagy or interact with Torin1 or starvation effects. To probe NAS in a system where AlloP effects have been shown previously, we surveyed astrocytes and again saw minimal autophagy induction by AlloP. Combined with other published results, our results suggest that NAS may modulate autophagy in a cell-specific or context-specific manner. Although there is merit to cell lines as a screening tool, future studies may require assaying NAS in cells from brain regions involved in neuropsychiatric disorders.

GABA-mediated inhibition in visual feedback neurons fine-tunes Drosophila male courtship

Vision is critical for the regulation of mating behaviors in many species. Here, we discovered that the Drosophila ortholog of human GABA A -receptor-associated protein (GABARAP) is required to fine-tune male courtship by modulating the activity of visual feedback neurons, lamina tangential cells (Lat). GABARAP is a ubiquitin-like protein that regulates cell-surface levels of GABA A receptors. Knocking down GABARAP or GABA A receptors in Lat neurons or hyperactivating them induces male courtship toward other males. Inhibiting Lat neurons, on the other hand, delays copulation by impairing the ability of males to follow females. Remarkably, the human ortholog of Drosophila GABARAP restores function in Lat neurons. Using in vivo two-photon imaging and optogenetics, we show that Lat neurons are functionally connected to neural circuits that mediate visually-guided courtship pursuits in males. Our work reveals a novel physiological role for GABARAP in fine-tuning the activity of a visual circuit that tracks a mating partner during courtship.

Effect of chronic intermittent ethanol vapor exposure on RNA content of brain-derived extracellular vesicles

Extracellular vesicles (EVs) are important players in normal biological function and disease pathogenesis. Of the many biomolecules packaged into EVs, coding and noncoding RNA transcripts are of particular interest for their ability to significantly alter cellular and molecular processes. Here we investigate how chronic ethanol exposure impacts EV RNA cargo and the functional outcomes of these changes. Following chronic intermittent ethanol (CIE) vapor exposure, EVs were isolated from male and female C57BL/6J mouse brain. Total RNA from EVs was analyzed by lncRNA/mRNA microarray to survey changes in RNA cargo following vapor exposure. Differential expression analysis of microarray data revealed a number of lncRNA and mRNA types differentially expressed in CIE compared to control EVs. Weighted gene co-expression network analysis identified multiple male and female specific modules related to neuroinflammation, cell death, demyelination, and synapse organization. To functionally test these changes, whole-cell voltage-clamp recordings were used to assess synaptic transmission. Incubation of nucleus accumbens brain slices with EVs led to a reduction in spontaneous excitatory postsynaptic current amplitude, although no changes in synaptic transmission were observed between control and CIE EV administration. These results indicate that CIE vapor exposure significantly changes the RNA cargo of brain-derived EVs, which have the ability to impact neuronal function.

Developmental regulation of thalamus-driven pauses in striatal cholinergic interneurons

In response to salient sensory cues, the tonically active striatal cholinergic interneuron (ChI) exhibits a characteristic synchronized "pause" thought to facilitate learning and the execution of motivated behavior. We report that thalamostriatal-driven ChI pauses are enhanced in ex vivo brain slices from infantile (P10) mice, with decreasing expression in preadolescent (P28) and adult (P100) mice concurrent with waning excitatory input to ChIs. Our data are consistent with previous reports that the adult ChI pause is dependent on dopamine signaling, but we find that the robust pausing at P10 is dopamine independent. Instead, elevated expression of the noninactivating delayed rectifier Kv7.2/3 current promotes pausing in infantile ChIs. Because this current decreases over development, a parallel increase in Ih further attenuates pause expression. These findings demonstrate that cell intrinsic and circuit mechanisms of ChI pause expression are developmentally determined and may underlie changes in learning properties as the nervous system matures.

Regulation of α-synuclein homeostasis and inflammasome activation by microglial autophagy

Autophagy clears protein aggregates, damaged cellular organelles, and pathogens through the lysosome. Although autophagy is highly conserved across all cell types, its activity in each cell is specifically adapted to carry out distinct physiological functions. The role of autophagy in neurons has been well characterized; however, in glial cells, its function remains largely unknown. Microglia are brain-resident macrophages that survey the brain to remove injured neurons, excessive synapses, protein aggregates, and infectious agents. Current studies have demonstrated that dysfunctional microglia contribute to neurodegenerative diseases. In Alzheimer's disease animal models, microglia play a critical role in regulating amyloid plaque formation and neurotoxicity. However, how microglia are involved in Parkinson's disease (PD) remains poorly understood. Propagation of aggregated α-synuclein via cell-to-cell transmission and neuroinflammation have emerged as important mechanisms underlying neuropathologies in PD. Here, we review converging evidence that microglial autophagy maintains α-synuclein homeostasis, regulates neuroinflammation, and confers neuroprotection in PD experimental models.

Macroautophagy in CNS health and disease

Macroautophagy is an evolutionarily conserved process that delivers diverse cellular contents to lysosomes for degradation. As our understanding of this pathway grows, so does our appreciation for its importance in disorders of the CNS. Once implicated primarily in neurodegenerative events owing to acute injury and ageing, macroautophagy is now also linked to disorders of neurodevelopment, indicating that it is essential for both the formation and maintenance of a healthy CNS. In parallel to understanding the significance of macroautophagy across contexts, we have gained a greater mechanistic insight into its physiological regulation and the breadth of cargoes it can degrade. Macroautophagy is a broadly used homeostatic process, giving rise to questions surrounding how defects in this single pathway could cause diseases with distinct clinical and pathological signatures. To address this complexity, we herein review macroautophagy in the mammalian CNS by examining three key features of the process and its relationship to disease: how it functions at a basal level in the discrete cell types of the brain and spinal cord; which cargoes are being degraded in physiological and pathological settings; and how the different stages of the macroautophagy pathway intersect with diseases of neurodevelopment and adult-onset neurodegeneration.

Dendritic autophagy degrades postsynaptic proteins and is required for long-term synaptic depression in mice

The pruning of dendritic spines during development requires autophagy. This process is facilitated by long-term depression (LTD)-like mechanisms, which has led to speculation that LTD, a fundamental form of synaptic plasticity, also requires autophagy. Here, we show that the induction of LTD via activation of NMDA receptors or metabotropic glutamate receptors initiates autophagy in the postsynaptic dendrites in mice. Dendritic autophagic vesicles (AVs) act in parallel with the endocytic machinery to remove AMPA receptor subunits from the membrane for degradation. During NMDAR-LTD, key postsynaptic proteins are sequestered for autophagic degradation, as revealed by quantitative proteomic profiling of purified AVs. Pharmacological inhibition of AV biogenesis, or conditional ablation of atg5 in pyramidal neurons abolishes LTD and triggers sustained potentiation in the hippocampus. These deficits in synaptic plasticity are recapitulated by knockdown of atg5 specifically in postsynaptic pyramidal neurons in the CA1 area. Conducive to the role of synaptic plasticity in behavioral flexibility, mice with autophagy deficiency in excitatory neurons exhibit altered response in reversal learning. Therefore, local assembly of the autophagic machinery in dendrites ensures the degradation of postsynaptic components and facilitates LTD expression.

Systemic LPS-induced microglial activation results in increased GABAergic tone: A mechanism of protection against neuroinflammation in the medial prefrontal cortex in mice

Neuroinflammation with excess microglial activation and synaptic dysfunction are early symptoms of most neurological diseases. However, how microglia-associated neuroinflammation regulates synaptic activity remains obscure. We report here that acute neuroinflammation induced by intraperitoneal injection of lipopolysaccharide (LPS) results in cell-type-specific increases in inhibitory postsynaptic currents in the glutamatergic, but not the GABAergic, neurons of medial prefrontal cortex (mPFC), coinciding with excessive microglial activation. LPS causes upregulation in levels of GABA_AR subunits, glutamine synthetase and vesicular GABA transporter, and downregulation in brain-derived neurotrophic factor (BDNF) and its receptor, pTrkB. Blockage of microglial activation by minocycline ameliorates LPS-induced abnormal expression of GABA signaling-related proteins and activity of synaptic and network. Moreover, minocycline prevents the mice from LPS-induced aberrant behavior, such as a reduction in total distance and time spent in the centre in the open field test; decreases in entries into the open arm of elevated-plus maze and in consumption of sucrose; increased immobility in the tail suspension test. Furthermore, upregulation of GABA signaling by tiagabine also prevents LPS-induced microglial activation and aberrant behavior. This study illustrates a mode of bidirectional constitutive signaling between the neural and immune compartments of the brain, and suggests that the mPFC is an important area for brain-immune system communication. Moreover, the present study highlights GABAergic signaling as a key therapeutic target for mitigating neuroinflammation-induced abnormal synaptic activity in the mPFC, together with the associated behavioral abnormalities.

mTOR-related synaptic pathology causes autism spectrum disorder-associated functional hyperconnectivity

Postmortem studies have revealed increased density of excitatory synapses in the brains of individuals with autism spectrum disorder (ASD), with a putative link to aberrant mTOR-dependent synaptic pruning. ASD is also characterized by atypical macroscale functional connectivity as measured with resting-state fMRI (rsfMRI). These observations raise the question of whether excess of synapses causes aberrant functional connectivity in ASD. Using rsfMRI, electrophysiology and in silico modelling in Tsc2 haploinsufficient mice, we show that mTOR-dependent increased spine density is associated with ASD -like stereotypies and cortico-striatal hyperconnectivity. These deficits are completely rescued by pharmacological inhibition of mTOR. Notably, we further demonstrate that children with idiopathic ASD exhibit analogous cortical-striatal hyperconnectivity, and document that this connectivity fingerprint is enriched for ASD-dysregulated genes interacting with mTOR or Tsc2. Finally, we show that the identified transcriptomic signature is predominantly expressed in a subset of children with autism, thereby defining a segregable autism subtype. Our findings causally link mTOR-related synaptic pathology to large-scale network aberrations, revealing a unifying multi-scale framework that mechanistically reconciles developmental synaptopathy and functional hyperconnectivity in autism.

Single-Cell Transcriptomics of Parkinson's Disease Human In Vitro Models Reveals Dopamine Neuron-Specific Stress Responses

The advent of induced pluripotent stem cell (iPSC)-derived neurons has revolutionized Parkinson's disease (PD) research, but single-cell transcriptomic analysis suggests unresolved cellular heterogeneity within these models. Here, we perform the largest single-cell transcriptomic study of human iPSC-derived dopaminergic neurons to elucidate gene expression dynamics in response to cytotoxic and genetic stressors. We identify multiple neuronal subtypes with transcriptionally distinct profiles and differential sensitivity to stress, highlighting cellular heterogeneity in dopamine in vitro models. We validate this disease model by showing robust expression of PD GWAS genes and overlap with postmortem adult substantia nigra neurons. Importantly, stress signatures are ameliorated using felodipine, an FDA-approved drug. Using isogenic SNCA-A53T mutants, we find perturbations in glycolysis, cholesterol metabolism, synaptic signaling, and ubiquitin-proteasomal degradation. Overall, our study reveals cell type-specific perturbations in human dopamine neurons, which will further our understanding of PD and have implications for cell replacement therapies.

Autophagy in neuronal physiology and disease

Neural circuit functions critically depend on homeostatic regulation and quality control of neuronal proteins and organelles. Emerging evidence shows that autophagy, cellular clearance machinery, selectively degrades or controls homeostasis of both pre- and post-synaptic components (e.g. synaptic proteins, organelles, neurotransmitters, and their receptors), thereby regulating synaptic remodeling, neurotransmission, and neuroplasticity. Along with its well-known role in supporting neuronal cell viability and neurodevelopment, autophagy is now implicated in a wide range of neuronal physiology throughout neuronal lifetime, including higher-order brain functions such as information processing, memory encoding, or cognitive functions. Here, we review recent literature on the roles of neuronal autophagy in homeostatic maintenance of synaptic functions and discuss how disruptions in these processes may contribute to the pathophysiology of neurodevelopmental and psychiatric disorders.

Synaptojanin1 deficiency upregulates basal autophagosome formation in astrocytes

Macroautophagy dysregulation is implicated in multiple neurological disorders, such as Parkinson's disease. While autophagy pathways are heavily researched in heterologous cells and neurons, regulation of autophagy in the astrocyte, the most abundant cell type in the mammalian brain, is less well understood. Missense mutations in the Synj1 gene encoding Synaptojanin1 (Synj1), a neuron-enriched lipid phosphatase, have been linked to Parkinsonism with seizures. Our previous study showed that the Synj1 haploinsufficient (Synj1^+/−) mouse exhibits age-dependent autophagy impairment in multiple brain regions. Here, we used cultured astrocytes from Synj1-deficient mice to investigate its role in astrocyte autophagy. We report that Synj1 is expressed in low levels in astrocytes and represses basal autophagosome formation. We demonstrate using cellular imaging that Synj1-deficient astrocytes exhibit hyperactive autophagosome formation, represented by an increase in the size and number of GFP-microtubule-associated protein 1A/1B-light chain 3 structures. Interestingly, Synj1 deficiency is also associated with an impairment in stress-induced autophagy clearance. We show, for the first time, that the Parkinsonism-associated R839C mutation impacts autophagy in astrocytes. The impact of this mutation on the phosphatase function of Synj1 resulted in elevated basal autophagosome formation that mimics Synj1 deletion. We found that the membrane expression of the astrocyte-specific glucose transporter GluT-1 was reduced in Synj1-deficient astrocytes. Consistently, AMP-activated protein kinase activity was elevated, suggesting altered glucose sensing in Synj1-deficient astrocytes. Expressing exogenous GluT-1 in Synj1-deficient astrocytes reversed the autophagy impairment, supporting a role for Synj1 in regulating astrocyte autophagy via disrupting glucose-sensing pathways. Thus, our work suggests a novel mechanism for Synj1-related Parkinsonism involving astrocyte dysfunction.

Short and long sleeping mutants reveal links between sleep and macroautophagy

Sleep is a conserved and essential behavior, but its mechanistic and functional underpinnings remain poorly defined. Through unbiased genetic screening in Drosophila, we discovered a novel short-sleep mutant we named argus. Positional cloning and subsequent complementation, CRISPR/Cas9 knock-out, and RNAi studies identified Argus as a transmembrane protein that acts in adult peptidergic neurons to regulate sleep. argus mutants accumulate undigested Atg8a(+) autophagosomes, and genetic manipulations impeding autophagosome formation suppress argus sleep phenotypes, indicating that autophagosome accumulation drives argus short-sleep. Conversely, a blue cheese neurodegenerative mutant that impairs autophagosome formation was identified independently as a gain-of-sleep mutant, and targeted RNAi screens identified additional genes involved in autophagosome formation whose knockdown increases sleep. Finally, autophagosomes normally accumulate during the daytime and nighttime sleep deprivation extends this accumulation into the following morning, while daytime gaboxadol feeding promotes sleep and reduces autophagosome accumulation at nightfall. In sum, our results paradoxically demonstrate that wakefulness increases and sleep decreases autophagosome levels under unperturbed conditions, yet strong and sustained upregulation of autophagosomes decreases sleep, whereas strong and sustained downregulation of autophagosomes increases sleep. The complex relationship between sleep and autophagy suggested by our findings may have implications for pathological states including chronic sleep disorders and neurodegeneration, as well as for integration of sleep need with other homeostats, such as under conditions of starvation.

Nilotinib restores memory function by preventing dopaminergic neuron degeneration in a mouse model of Alzheimer's Disease

What happens precociously to the brain destined to develop Alzheimer's Disease (AD) still remains to be elucidated and this is one reason why effective AD treatments are missing. Recent experimental and clinical studies indicate that the degeneration of the dopaminergic (DA) neurons in the Ventral Tegmental Area (VTA) could be one of the first events occurring in AD. However, the causes of the increased vulnerability of DA neurons in AD are missing. Here, we deeply investigate the physiology of DA neurons in the VTA before, at the onset, and after onset of VTA neurodegeneration. We use the Tg2576 mouse model of AD, overexpressing a mutated form of the human APP, to identify molecular targets that can be manipulated pharmacologically. We show that in Tg2576 mice, DA neurons of the VTA at the onset of degeneration undergo slight but functionally relevant changes in their electrophysiological properties and cell morphology. Importantly, these changes are associated with accumulation of autophagosomes, suggestive of a dysfunctional autophagy, and with enhanced activation of c-Abl, a tyrosine kinase previously implicated in the pathogenesis of neurodegenerative diseases. Chronic treatment of Tg2576 mice with Nilotinib, a validated c-Abl inhibitor, reduces c-Abl phosphorylation, improves autophagy, reduces Aβ levels and - more importantly - prevents degeneration as well as functional and morphological alterations in DA neurons of the VTA. Interestingly, the drug prevents the reduction of DA outflow to the hippocampus and ameliorates hippocampal-related cognitive functions. Our results strive to identify early pathological brain changes in AD, to provide a rational basis for new therapeutic interventions able to slow down the disease progression.

Detailed analysis of Japanese patients with adenosine deaminase 2 deficiency reveals characteristic elevation of type II interferon signature and STAT1 hyperactivation

BACKGROUND

Deficiency of adenosine deaminase 2 (DADA2) is an autosomal recessive inflammatory disease caused by loss-of-function mutations in both alleles of the ADA2 gene. Most patients with DADA2 exhibit systemic vasculopathy consistent with polyarteritis nodosa, but large phenotypic variability has been reported, and the pathogenesis of DADA2 remains unclear.

OBJECTIVES

This study sought to assess the clinical and genetic characteristics of Japanese patients with DADA2 and to gain insight into the pathogenesis of DADA2 by multi-omics analysis.

METHODS

Clinical and genetic data were collected from 8 Japanese patients with DADA2 diagnosed between 2016 and 2019. ADA2 variants in this cohort were functionally analyzed by in vitro overexpression analysis. PBMCs from 4 patients with DADA2 were subjected to transcriptome and proteome analyses. Patient samples were collected before and after introduction of anti- TNF-α therapies. Transcriptome data were compared with those of normal controls and patients with other autoinflammatory diseases.

RESULTS

Five novel ADA2 variants were identified in these 8 patients and were confirmed pathogenic by in vitro analysis. Anti-TNF-α therapy controlled inflammation in all 8 patients. Transcriptome and proteome analyses showed that upregulation of type II interferon signaling was characteristic of DADA2. Network analysis identified STAT1 as a key regulator and a hub molecule in DADA2 pathogenesis, a finding supported by the hyperactivation of STAT1 in patients' monocytes and B cells after IFN-γ stimulation.

CONCLUSIONS

Type II interferon signaling and STAT1 are associated with the pathogenesis of DADA2.

Involvement of Autophagy in Levodopa-Induced Dyskinesia

Background

Autophagy is intensively studied in cancer, metabolic and neurodegenerative diseases, but little is known about its role in pathological conditions linked to altered neurotransmission. We examined the involvement of autophagy in levodopa (l‐dopa)‐induced dyskinesia, a frequent motor complication developed in response to standard dopamine replacement therapy in parkinsonian patients.

Methods

We used mouse and non‐human primate models of Parkinson's disease to examine changes in autophagy associated with chronic l‐dopa administration and to establish a causative link between impaired autophagy and dyskinesia.

Results

We found that l‐dopa‐induced dyskinesia is associated with accumulation of the autophagy‐specific substrate p62, a marker of autophagy deficiency. Increased p62 was observed in a subset of projection neurons located in the striatum and depended on l‐dopa‐mediated activation of dopamine D1 receptors, and mammalian target of rapamycin. Inhibition of mammalian target of rapamycin complex 1 with rapamycin counteracted the impairment of autophagy produced by l‐dopa, and reduced dyskinesia. The anti‐dyskinetic effect of rapamycin was lost when autophagy was constitutively suppressed in D1 receptor‐expressing striatal neurons, through inactivation of the autophagy‐related gene protein 7.

Conclusions

These findings indicate that augmented responsiveness at D1 receptors leads to dysregulated autophagy, and results in the emergence of l‐dopa‐induced dyskinesia. They further suggest the enhancement of autophagy as a therapeutic strategy against dyskinesia. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society

FOXP1 negatively regulates intrinsic excitability in D2 striatal projection neurons by promoting inwardly rectifying and leak potassium currents

Heterozygous loss-of-function mutations in the transcription factor FOXP1 are strongly associated with autism. Dopamine receptor 2 expressing (D2) striatal projection neurons (SPNs) in heterozygous Foxp1 (Foxp1^+/-) mice have higher intrinsic excitability. To understand the mechanisms underlying this alteration, we examined SPNs with cell-type specific homozygous Foxp1 deletion to study cell-autonomous regulation by Foxp1. As in Foxp1^+/- mice, D2 SPNs had increased intrinsic excitability with homozygous Foxp1 deletion. This effect involved postnatal mechanisms. The hyperexcitability was mainly due to down-regulation of two classes of potassium currents: inwardly rectifying (K_IR) and leak (K_Leak). Single-cell RNA sequencing data from D2 SPNs with Foxp1 deletion indicated the down-regulation of transcripts of candidate ion channels that may underlie these currents: Kcnj2 and Kcnj4 for K_IR and Kcnk2 for K_Leak. This Foxp1-dependent regulation was neuron-type specific since these same currents and transcripts were either unchanged, or very little changed, in D1 SPNs with cell-specific Foxp1 deletion. Our data are consistent with a model where FOXP1 negatively regulates the excitability of D2 SPNs through K_IR and K_Leak by transcriptionally activating their corresponding transcripts. This, in turn, provides a novel example of how a transcription factor may regulate multiple genes to impact neuronal electrophysiological function that depends on the integration of multiple current types - and do this in a cell-specific fashion. Our findings provide initial clues to altered neuronal function and possible therapeutic strategies not only for FOXP1-associated autism but also for other autism forms associated with transcription factor dysfunction.

Neuronal Autophagy Regulates Presynaptic Neurotransmission by Controlling the Axonal Endoplasmic Reticulum

Neurons are known to rely on autophagy for removal of defective proteins or organelles to maintain synaptic neurotransmission and counteract neurodegeneration. In spite of its importance for neuronal health, the physiological substrates of neuronal autophagy in the absence of proteotoxic challenge have remained largely elusive. We use knockout mice conditionally lacking the essential autophagy protein ATG5 and quantitative proteomics to demonstrate that loss of neuronal autophagy causes selective accumulation of tubular endoplasmic reticulum (ER) in axons, resulting in increased excitatory neurotransmission and compromised postnatal viability in vivo. The gain in excitatory neurotransmission is shown to be a consequence of elevated calcium release from ER stores via ryanodine receptors accumulated in axons and at presynaptic sites. We propose a model where neuronal autophagy controls axonal ER calcium stores to regulate neurotransmission in healthy neurons and in the brain.

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Neuronal autophagy controls the endoplasmic reticulum (ER) in axons

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Loss of neuronal autophagy leads to increased excitatory neurotransmission

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Increased neurotransmission is due to elevated calcium release from ER stores

Mechanisms of neuronal survival safeguarded by endocytosis and autophagy

Multiple aspects of neuronal physiology crucially depend on two cellular pathways, autophagy and endocytosis. During endocytosis, extracellular components either unbound or recognized by membrane-localized receptors (termed "cargo") become internalized into plasma membrane-derived vesicles. These can serve to either recycle the material back to the plasma membrane or send it for degradation to lysosomes. Autophagy also uses lysosomes as a terminal degradation point, although instead of degrading the plasma membrane-derived cargo, autophagy eliminates detrimental cytosolic material and intracellular organelles, which are transported to lysosomes by means of double-membrane vesicles, referred to as autophagosomes. Neurons, like all non-neuronal cells, capitalize on autophagy and endocytosis to communicate with the environment and maintain protein and organelle homeostasis. Additionally, the highly polarized, post-mitotic nature of neurons made them adopt these two pathways for cell-specific functions. These include the maintenance of the synaptic vesicle pool in the pre-synaptic terminal and the long-distance transport of signaling molecules. Originally discovered independently from each other, it is now clear that autophagy and endocytosis are closely interconnected and share several common participating molecules. Considering the crucial role of autophagy and endocytosis in cell type-specific functions in neurons, it is not surprising that defects in both pathways have been linked to the pathology of numerous neurodegenerative diseases. In this review, we highlight the recent knowledge of the role of endocytosis and autophagy in neurons with a special focus on synaptic physiology and discuss how impairments in genes coding for autophagy and endocytosis proteins can cause neurodegeneration.

Natural Compounds and Autophagy: Allies Against Neurodegeneration

Prolonging the healthy life span and limiting neurological illness are imperative goals in gerontology. Age-related neurodegeneration is progressive and leads to severe diseases affecting motility, memory, cognitive function, and social life. To date, no effective treatments are available for neurodegeneration and irreversible neuronal loss. Bioactive phytochemicals could represent a natural alternative to ensure active aging and slow onset of neurodegenerative diseases in elderly patients. Autophagy or macroautophagy is an evolutionarily conserved clearing process that is needed to remove aggregate-prone proteins and organelles in neurons and glia. It also is crucial in synaptic plasticity. Aberrant autophagy has a key role in aging and neurodegeneration. Recent evidence indicates that polyphenols like resveratrol and curcumin, flavonoids, like quercetin, polyamine, like spermidine and sugars, like trehalose, limit brain damage in vitro and in vivo. Their common mechanism of action leads to restoration of efficient autophagy by dismantling misfolded proteins and dysfunctional mitochondria. This review focuses on the role of dietary phytochemicals as modulators of autophagy to fight Alzheimer's and Parkinson's diseases, fronto-temporal dementia, amyotrophic lateral sclerosis, and psychiatric disorders. Currently, most studies have involved in vitro or preclinical animal models, and the therapeutic use of phytochemicals in patients remains limited.

mTOR Suppresses Macroautophagy During Striatal Postnatal Development and Is Hyperactive in Mouse Models of Autism Spectrum Disorders

Macroautophagy (hereafter referred to as autophagy) plays a critical role in neuronal function related to development and degeneration. Here, we investigated whether autophagy is developmentally regulated in the striatum, a brain region implicated in neurodevelopmental disease. We demonstrate that autophagic flux is suppressed during striatal postnatal development, reaching adult levels around postnatal day 28 (P28). We also find that mTOR signaling, a key regulator of autophagy, increases during the same developmental period. We further show that mTOR signaling is responsible for suppressing autophagy, via regulation of Beclin-1 and VPS34 activity. Finally, we discover that autophagy is downregulated during late striatal postnatal development (P28) in mice with in utero exposure to valproic acid (VPA), an established mouse model of autism spectrum disorder (ASD). VPA-exposed mice also display deficits in striatal neurotransmission and social behavior. Correction of hyperactive mTOR signaling in VPA-exposed mice restores social behavior. These results demonstrate that neurons coopt metabolic signaling cascades to developmentally regulate autophagy and provide additional evidence that mTOR-dependent signaling pathways represent pathogenic signaling cascades in ASD mouse models that are active during specific postnatal windows.

Autophagic bias in the striatum

Macroautophagy/autophagy is implicated in the maintenance of normal neuronal activity through the regulation of synaptic function and plasticity. However, differences in autophagic degradation within different classes of neurons have not been examined. We have recently demonstrated that autophagy plays very different roles in the two closely related principal neurons of the striatum - the spiny projection neurons of the direct (dSPN) and indirect (iSPN) pathways. Behavioral and electrophysiological experiments revealed that the absence of autophagy in either of these SPN pathways produces unique effects on motor learning, dendritic length, and intrinsic excitability. Specifically, autophagy is required for the normal development of synaptic inputs onto dSPNs, while being required for intrinsic excitability in iSPNs. In iSPNs, this occurs through the regulation of the activity of the KCNJ/Kir2 ion channel, and provides a first demonstration of autophagic control of neuronal intrinsic excitability.

ABBREVIATIONS

ASD: autism spectrum disorders; dSPNs: direct pathway spiny projection neurons; iSPNs: indirect pathway spiny projection neurons; Kir2: inwardly rectifying potassium channel 2.

mTOR Suppresses Macroautophagy During Striatal Postnatal Development and Is Hyperactive in Mouse Models of Autism Spectrum Disorders

Macroautophagy (hereafter referred to as autophagy) plays a critical role in neuronal function related to development and degeneration. Here, we investigated whether autophagy is developmentally regulated in the striatum, a brain region implicated in neurodevelopmental disease. We demonstrate that autophagic flux is suppressed during striatal postnatal development, reaching adult levels around postnatal day 28 (P28). We also find that mTOR signaling, a key regulator of autophagy, increases during the same developmental period. We further show that mTOR signaling is responsible for suppressing autophagy, via regulation of Beclin-1 and VPS34 activity. Finally, we discover that autophagy is downregulated during late striatal postnatal development (P28) in mice with in utero exposure to valproic acid (VPA), an established mouse model of autism spectrum disorder (ASD). VPA-exposed mice also display deficits in striatal neurotransmission and social behavior. Correction of hyperactive mTOR signaling in VPA-exposed mice restores social behavior. These results demonstrate that neurons coopt metabolic signaling cascades to developmentally regulate autophagy and provide additional evidence that mTOR-dependent signaling pathways represent pathogenic signaling cascades in ASD mouse models that are active during specific postnatal windows.