The AP2 clathrin adaptor protein complex regulates the abundance of GLR-1 glutamate receptors in the ventral nerve cord of Caenorhabditis elegans

The endocytic adaptor AP-2 maintains Purkinje cell function by balancing cerebellar parallel and climbing fiber synapses

The loss of cerebellar Purkinje cells is a hallmark of neurodegenerative movement disorders, but the mechanisms remain enigmatic. We show that endocytic adaptor protein complex 2 (AP-2) is crucial for Purkinje cell survival. Using mouse genetics, viral tracing, calcium imaging, and kinematic analysis, we demonstrate that loss of the AP-2 μ subunit in Purkinje cells leads to early-onset ataxia and progressive degeneration. Synaptic dysfunction, marked by an overrepresentation of parallel fibers (PFs) over climbing fibers (CFs), precedes Purkinje cell loss. Mechanistically, AP-2 interacts with the PF-enriched protein GRID2IP, and its loss triggers GRID2IP degradation and glutamate δ2 receptor (GLURδ2) accumulation, leading to an excess of PFs while CFs are reduced. The overrepresentation of PFs increases Purkinje cell network activity, which is mitigated by enhancing glutamate clearance with ceftriaxone. These findings highlight the role of AP-2 in regulating GRID2IP levels in Purkinje cells to maintain PF-CF synaptic balance and prevent motor dysfunction.

The adaptor protein 2 (AP2) complex modulates habituation and behavioral selection across multiple pathways and time windows

Animals constantly integrate sensory information with prior experience to select behavioral responses appropriate to the current situation. Genetic factors supporting this behavioral flexibility are often disrupted in neuropsychiatric conditions, such as the autism-linked ap2s1 gene which supports acoustically evoked habituation learning. ap2s1 encodes an AP2 endocytosis adaptor complex subunit, although its behavioral mechanisms and importance have been unclear. Here, we show that multiple AP2 subunits regulate acoustically evoked behavior selection and habituation learning in zebrafish. Furthermore, ap2s1 biases escape behavior choice in sensory modality-specific manners, and broadly regulates action selection across sensory contexts. We demonstrate that the AP2 complex functions acutely in the nervous system to modulate acoustically evoked habituation, suggesting several spatially and/or temporally distinct mechanisms through which AP2 regulates escape behavior selection and performance. Altogether, we show the AP2 complex coordinates action selection across diverse contexts, providing a vertebrate model for ap2s1's role in human conditions including autism spectrum disorder.

Synaptogenesis: unmasking molecular mechanisms using Caenorhabditis elegans

The nematode Caenorhabditis elegans is a research model organism particularly suited to the mechanistic understanding of synapse genesis in the nervous system. Armed with powerful genetics, knowledge of complete connectomics, and modern genomics, studies using C. elegans have unveiled multiple key regulators in the formation of a functional synapse. Importantly, many signaling networks display remarkable conservation throughout animals, underscoring the contributions of C. elegans research to advance the understanding of our brain. In this chapter, we will review up-to-date information of the contribution of C. elegans to the understanding of chemical synapses, from structure to molecules and to synaptic remodeling.

VER/VEGF receptors regulate AMPA receptor surface levels and glutamatergic behavior

Several intracellular trafficking pathways contribute to the regulation of AMPA receptor (AMPAR) levels at synapses and the control of synaptic strength. While much has been learned about these intracellular trafficking pathways, a major challenge is to understand how extracellular factors, such as growth factors, neuropeptides and hormones, impinge on specific AMPAR trafficking pathways to alter synaptic function and behavior. Here, we identify the secreted ligand PVF-1 and its cognate VEGF receptor homologs, VER-1 and VER-4, as regulators of glutamate signaling in C. elegans. Loss of function mutations in ver-1, ver-4, or pvf-1, result in decreased cell surface levels of the AMPAR GLR-1 and defects in glutamatergic behavior. Rescue experiments indicate that PVF-1 is expressed and released from muscle, whereas the VERs function in GLR-1-expressing neurons to regulate surface levels of GLR-1 and glutamatergic behavior. Additionally, ver-4 is unable to rescue glutamatergic behavior in the absence of pvf-1, suggesting that VER function requires endogenous PVF-1. Inducible expression of a pvf-1 rescuing transgene suggests that PVF-1 can function in the mature nervous system to regulate GLR-1 signaling. Genetic double mutant analysis suggests that the VERs act together with the VPS-35/retromer recycling complex to promote cell surface levels of GLR-1. Our data support a genetic model whereby PVF-1/VER signaling acts with retromer to promote recycling and cell surface levels of GLR-1 to control behavior.

Sensation, behavior, and cognition all depend on the proper function of neuronal connections called synapses. Synapses that use the neurotransmitter glutamate to signal between nerve cells are the most abundant type in our brain. Presynaptic neurons release glutamate, which activates glutamate receptors on postsynaptic neurons. Dysfunction of glutamate synapses leads to several neurological disorders, and changing their strength–in part by altering glutamate receptors numbers on the surface of the postsynaptic cell—provides the cellular basis of learning and memory. Much remains to be learned about how factors released from other cell types affects synaptic communication. We took advantage of light-activated molecular switches engineered into specific sensory neurons of C. elegans worms to trigger a behavioral reflex that depends on glutamate synapses. Using this behavior, we identified proteins called VER-1 and VER-4 as important for glutamate synapse function. We found that worms missing these VER proteins or their activator PVF-1 have reduced levels of glutamate receptors at the postsynaptic surface and defects in glutamate-dependent behaviors. Our results suggest that inter-tissue cross-talk between muscle PVF-1 and neuronal VERs is important for controlling the number of glutamate receptors at the cell surface, robust neuronal communication and behavioral responses.

The Snail transcription factor CES-1 regulates glutamatergic behavior in C. elegans

Regulation of AMPA-type glutamate receptor (AMPAR) expression and function alters synaptic strength and is a major mechanism underlying synaptic plasticity. Although transcription is required for some forms of synaptic plasticity, the transcription factors that regulate AMPA receptor expression and signaling are incompletely understood. Here, we identify the Snail family transcription factor ces-1 in an RNAi screen for conserved transcription factors that regulate glutamatergic behavior in C. elegans. ces-1 was originally discovered as a selective cell death regulator of neuro-secretory motor neuron (NSM) and I2 interneuron sister cells in C. elegans, and has almost exclusively been studied in the NSM cell lineage. We found that ces-1 loss-of-function mutants have defects in two glutamatergic behaviors dependent on the C. elegans AMPA receptor GLR-1, the mechanosensory nose-touch response and spontaneous locomotion reversals. In contrast, ces-1 gain-of-function mutants exhibit increased spontaneous reversals, and these are dependent on glr-1 consistent with these genes acting in the same pathway. ces-1 mutants have wild type cholinergic neuromuscular junction function, suggesting that they do not have a general defect in synaptic transmission or muscle function. The effect of ces-1 mutation on glutamatergic behaviors is not due to ectopic cell death of ASH sensory neurons or GLR-1-expressing neurons that mediate one or both of these behaviors, nor due to an indirect effect on NSM sister cell deaths. Rescue experiments suggest that ces-1 may act, in part, in GLR-1-expressing neurons to regulate glutamatergic behaviors. Interestingly, ces-1 mutants suppress the increased reversal frequencies stimulated by a constitutively-active form of GLR-1. However, expression of glr-1 mRNA or GFP-tagged GLR-1 was not decreased in ces-1 mutants suggesting that ces-1 likely promotes GLR-1 function. This study identifies a novel role for ces-1 in regulating glutamatergic behavior that appears to be independent of its canonical role in regulating cell death in the NSM cell lineage.

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.

AP-2 reduces amyloidogenesis by promoting BACE1 trafficking and degradation in neurons

Cleavage of amyloid precursor protein (APP) by BACE-1 (β-site APP cleaving enzyme 1) is the rate-limiting step in amyloid-β (Aβ) production and a neuropathological hallmark of Alzheimer's disease (AD). Despite decades of research, mechanisms of amyloidogenic APP processing remain highly controversial. Here, we show that in neurons, APP processing and Aβ production are controlled by the protein complex-2 (AP-2), an endocytic adaptor known to be required for APP endocytosis. Now, we find that AP-2 prevents amyloidogenesis by additionally functioning downstream of BACE1 endocytosis, regulating BACE1 endosomal trafficking and its delivery to lysosomes. AP-2 is decreased in iPSC-derived neurons from patients with late-onset AD, while conditional AP-2 knockout (KO) mice exhibit increased Aβ production, resulting from accumulation of BACE1 within late endosomes and autophagosomes. Deletion of BACE1 decreases amyloidogenesis and mitigates synapse loss in neurons lacking AP-2. Taken together, these data suggest a mechanism for BACE1 intracellular trafficking and degradation via an endocytosis-independent function of AP-2 and reveal a novel role for endocytic proteins in AD.

Cell-specific non-canonical amino acid labelling identifies changes in the de novo proteome during memory formation

The formation of spatial long-term memory (LTM) requires the de novo synthesis of distinct sets of proteins; however, a non-biased examination of the de novo proteome in this process is lacking. Here, we generated a novel mouse strain, which enables cell-type-specific labelling of newly synthesised proteins with non-canonical amino acids (NCAAs) by genetically restricting the expression of the mutant tRNA synthetase, NLL-MetRS, to hippocampal neurons. By combining this labelling technique with an accelerated version of the active place avoidance task and bio-orthogonal non-canonical amino acid tagging (BONCAT) followed by SWATH quantitative mass spectrometry, we identified 156 proteins that were altered in synthesis in hippocampal neurons during spatial memory formation. In addition to observing increased synthesis of known proteins important in memory-related processes, such as glutamate receptor recycling, we also identified altered synthesis of proteins associated with mRNA splicing as a potential mechanism involved in spatial LTM formation.

Inactivation of GABAA receptor is related to heat shock stress response in organism model Caenorhabditis elegans

The mechanisms underlying oxidative stress (OS) resistance are not completely clear. Caenorhabditis elegans (C. elegans) is a good organism model to study OS because it displays stress responses similar to those in mammals. Among these mechanisms, the insulin/IGF-1 signaling (IIS) pathway is thought to affect GABAergic neurotransmission. The aim of this study was to determine the influence of heat shock stress (HS) on GABAergic activity in C. elegans. For this purpose, we tested the effect of exposure to picrotoxin (PTX), gamma-aminobutyric acid (GABA), hydrogen peroxide, and HS on the occurrence of a shrinking response (SR) after nose touch stimulus in N2 (WT) worms. Moreover, the effect of HS on the expression of UNC-49 (GABAA receptor ortholog) in the EG1653 strain and the effect of GABA and PTX exposure on HSP-16.2 expression in the TJ375 strain were analyzed. PTX 1 mM- or H2O2 0.7 mM-exposed worms displayed a SR in about 80 % of trials. GABA exposure did not cause a SR. HS prompted the occurrence of a SR as did PTX 1 mM or H2O2 0.7 mM exposure. In addition, HS increased UNC-49 expression, and PTX augmented HSP-16.2 expression. Thus, the results of the present study suggest that oxidative stress, through either H2O2 exposure or application of heat shock, inactivates the GABAergic system, which subsequently would affect the oxidative stress response, perhaps by enhancing the activity of transcription factors DAF-16 and HSF-1, both regulated by the IIS pathway and related to hsp-16.2 expression.

Quantitative proteomics analysis of zebrafish exposed to sub-lethal dosages of β-methyl-amino-L-alanine (BMAA)

The non-protein amino acid β-methylamino-L-alanine (BMAA) is a neurotoxin present in microalgae and shown to accumulate in the food web. BMAA has been linked to the complex neurodegenerative disorder of Guam and to increased incidents sporadic ALS. Two main neurotoxic routes are suggested; an excitotoxic by acting as an agonist towards glutamate receptors and a metabolic by misincorporating into cellular proteins. We have used zebrafish, an increasingly used model for neurodegenerative diseases, to further identify signaling components involved in BMAA-induced toxicity. Zebrafish embryos were exposed to sub-lethal dosages of BMAA and a label-free proteomics analysis was conducted on larvae 4 days post fertilization. The exposed larvae showed no developmental abnormalities, but a reduced heart rate and increased expression of GSK3 isoforms. Search towards a reviewed database containing 2968 entries identified 480 proteins. Only 17 of these were regulated 2-fold or more in the exposed larvae. Seven of these proteins could be associated to glutamate receptor signaling and recycling. The remaining nine have all been linked to disturbance in protein homeostasis, reactive oxygen species (ROS) development or neuronal cell death. We also found that BMAA influenced the endocannabinoid system by up-regulation of fatty acid amide hydrolase (FAAH) and that FAAH inhibitor URB597 reduced the BMAA effect on heart rate and GSK3 expression.

Carboxyl-Terminal SSLKG Motif of the Human Cystinosin-LKG Plays an Important Role in Plasma Membrane Sorting

Cystinosin mediates an ATP-dependent cystine efflux from lysosomes and causes, if mutated, nephropathic cystinosis, a rare inherited lysosomal storage disease. Alternative splicing of the last exon of the cystinosin sequence produces the cystinosin-LKG isoform that is characterized by a different C-terminal region causing changes in the subcellular distribution of the protein. We have constructed RFP-tagged proteins and demonstrated by site-directed mutagenesis that the carboxyl-terminal SSLKG sequence of cystinosin-LKG is an important sorting motif that is required for efficient targeting the protein to the plasma membrane, where it can mediate H⁺ coupled cystine transport. Deletion of the SSLKG sequence reduced cystinosin-LKG expression in the plasma membrane and cystine transport by approximately 30%, and induced significant accumulation of the protein in the Golgi apparatus and in lysosomes. Cystinosin-LKG, unlike the canonical isoform, also moves to the lysosomes by the indirect pathway, after endocytic retrieval from the plasma membrane, mainly by a clathrin-mediated endocytosis. Nevertheless, silencing of AP-2 triggers the clathrin-independent endocytosis, showing the complex adaptability of cystinosin-LKG trafficking.