Presynaptic Endosomal Cathepsin D Regulates the Biogenesis of GABAergic Synaptic Vesicles

Disrupted synaptic gene expression in Fabry disease: Findings from RNA sequencing

Fabry disease (FD) is a rare X-linked lysosomal storage disorder caused by a deficiency in the enzyme α-galactosidase A. This defect leads to the progressive accumulation of glycosphingolipids, resulting in kidney, heart, and nervous system damage, which contributes to significant morbidity and mortality. Early diagnosis is essential to prevent irreversible damage and optimize treatment strategies. Recent research aims to provide a better understanding of FD pathophysiology to improve management approaches. This study is an international, multicenter, cross-sectional analysis that used RNA sequencing (RNA-seq) to compare blood samples from 50 FD patients and 50 age- and sex-matched healthy controls. The objective was to identify gene expression patterns and investigate secondary cellular pathways affected by lysosomal dysfunction. Among the more than 400 differentially expressed genes detected, 207 were protein-coding genes, most of which were overexpressed in the FD cohort. Functional enrichment analysis highlighted processes related to synaptic function, specifically concerning chemical synaptic transmission and membrane potential regulation. Identified genes included those related to voltage-gated ion channels, neurotransmitter receptors, cell adhesion molecules, scaffold proteins, and proteins associated with synaptic vesicles and neurotrophic signaling, all linked to lipid rafts. Notable identified genes included those encoding voltage-gated potassium channel genes (KCNQ2, KCNQ3, KCNMA1) and ionotropic receptor genes involved in glutamatergic (GRIN2A, GRIN2B) and GABAergic systems (GABRA4, GABRB1, GABRG2, GABRQ). These findings suggest that lysosomal dysfunction contributes to synaptic defects in FD, paving the way for further research into the role of synaptic pathology and lipid rafts in the underlying pathogenesis and clinical outcomes in FD.

Mendelian randomisation analysis to explore the association between cathepsins and bipolar disorder

INTRODUCTION

Bipolar disorder is a psychiatric condition characterized by the coexistence of depression and mania. Diagnosis of this disorder can be challenging due to limited pathologic and experimental tools. Treatment compliance is often poor due to medication side effects. Although cathepsin is known to play a significant role in diseases such as tumors and osteoporosis, its role in psychiatric disorders is not yet fully understood.

OBJECTIVE

The aim of this study was to investigate the relationship between cathepsin in the blood circulation and bipolar disorder.

METHODS

The causal relationship between cathepsin and different subtypes of bipolar affective disorder was explored using bidirectional Mendelian randomization analysis and multivariate analysis.

RESULTS

It was found that cathepsin H level was a protective factor for type II bipolar disorder. No potential causal relationship was found between cathepsin H and type I bipolar disorder, but cathepsin B changes with the development of type I bipolar disorder. A causal relationship was found between cathepsin H and cerebral dopamine neurotrophic factor.

CONCLUSIONS

In conclusion, cathepsin H may be a diagnostic target for bipolar II disorder and may play a guiding role in clinical diagnosis. Cathepsin H may have an effect on BD through cerebral dopamine neurotrophic factor.

GPR158 in pyramidal neurons mediates social novelty behavior via modulating synaptic transmission in male mice

Impairment in social communication skills is a hallmark feature of autism spectrum disorder (ASD). The role of G-protein-coupled receptor 158 (GPR158) in ASD remains largely unexplored. In this study, we observed that both constitutive and cell-/tissue-specific knockouts of Gpr158 in pyramidal neurons or the medial prefrontal cortex (mPFC) result in impaired novelty preference, while sociability remains unaffected in male mice. Notably, the loss of GPR158 leads to a significant decline in excitatory synaptic transmission, characterized by a reduction in glutamate vesicles, as well as the expression and phosphorylation of GluN2B in the mPFC. We successfully rescue the phenotype of social novelty deficits either by reintroducing GPR158 in the mPFC of Gpr158 deficient mice or by chemogenetic activation of pyramidal neurons where Gpr158 is specifically ablated. Our findings indicate that GPR158 in pyramidal neurons plays a specific role in modulating social novelty and may represent a potential target for treating social disorders.

Multifaceted targets of cannabidiol in epilepsy: Modulating glutamate signaling and beyond

Cannabidiol has been reported to interact with broad-spectrum biological targets with pleiotropic pharmacology including epilepsy although a cohesive mechanism is yet to be determined. Even though some studies propose that cannabidiol may manipulate glutamatergic signals, there is insufficient evidence to support cannabidiol direct effect on glutamate signaling, which is important in intervening epilepsy. Therefore, the present study aimed to analyze the epilepsy-related targets for cannabidiol, assess the differentially expressed genes with its treatment, and identify the possible glutamatergic signaling target. In this study, the epileptic protein targets of cannabidiol were identified using the Tanimoto coefficient and similarity index-based targets fishing which were later overlapped with the altered expression, epileptic biomarkers, and genetically altered proteins in epilepsy. The common proteins were then screened for possible glutamatergic signaling targets with differentially expressed genes. Later, molecular docking and simulation were performed using AutoDock Vina and GROMACS to evaluate binding affinity, ligand-protein stability, hydrophilic interaction, protein compactness, etc. Cannabidiol identified 30 different epilepsy-related targets of multiple protein classes including G-protein coupled receptors, enzymes, ion channels, etc. Glutamate receptor 2 was identified to be genetically varied in epilepsy which was targeted by cannabidiol and its expression was increased with its treatment. More importantly, cannabidiol showed a direct binding affinity with Glutamate receptor 2 forming a stable hydrophilic interaction and comparatively lower root mean squared deviation and residual fluctuations, increasing protein compactness with broad conformational changes. Based on the cheminformatic target fishing, evaluation of differentially expressed genes, molecular docking, and simulations, it can be hypothesized that cannabidiol may possess glutamate receptor 2-mediated anti-epileptic activities.

Role of adaptor protein complexes in generating functionally distinct synaptic vesicle pools

The synaptic vesicle (SV) cycle ensures the release of neurotransmitters and the replenishment of SVs to sustain neuronal activity. Multiple endocytosis and sorting pathways contribute to the recapture of the SV membrane and proteins after fusion. Adaptor protein (AP) complexes are among the critical components of the SV retrieval machinery. The canonical clathrin adaptor AP2 ensures the replenishment of most SVs across many neuronal populations. An alternative AP1/AP3-dependent process mediates the formation of a subset of SVs that differ from AP2 vesicles in molecular composition and respond preferentially during higher frequency firing. Furthermore, recent studies show that vesicular transporters for different neurotransmitters depend to a different extent on the AP3 pathway and this affects the release properties of the respective neurotransmitters. This review focuses on the current understanding of the AP-dependent molecular and functional diversity among SVs. We also discuss the contribution of these pathways to the regulation of neurotransmitter release across neuronal populations.

Transport-dependent maturation of organelles in neurons

Some organelles show a spatial gradient of maturation along the neuronal process where more mature organelles are found closer to the cell body. This gradient is set up by progressive maturation steps that are aided by differential organelle distribution as well as transport. Autophagosomes and endosomes mature as they acquire lysosomal membrane proteins and decrease their luminal pH as they are retrogradely transported towards the cell body. The acquisition of lysosomal proteins along the neuronal processes likely occurs through fusion or membrane exchange events with Golgi-derived donor transport carriers that are transported anterogradely from the cell body. The mechanisms by which endosomes and autophagosomes mature might be applicable to other organelles that are transported along neuronal processes. Defects in axonal transport may also contribute to the accumulation of immature organelles in neurons. Such accumulations have been seen in neurons of neurodegenerative models.

Excitatory Synaptic Transmission in Ischemic Stroke: A New Outlet for Classical Neuroprotective Strategies

Stroke is one of the leading causes of death and disability in the world, of which ischemia accounts for the majority. There is growing evidence of changes in synaptic connections and neural network functions in the brain of stroke patients. Currently, the studies on these neurobiological alterations mainly focus on the principle of glutamate excitotoxicity, and the corresponding neuroprotective strategies are limited to blocking the overactivation of ionic glutamate receptors. Nevertheless, it is disappointing that these treatments often fail because of the unspecificity and serious side effects of the tested drugs in clinical trials. Thus, in the prevention and treatment of stroke, finding and developing new targets of neuroprotective intervention is still the focus and goal of research in this field. In this review, we focus on the whole processes of glutamatergic synaptic transmission and highlight the pathological changes underlying each link to help develop potential therapeutic strategies for ischemic brain damage. These strategies include: (1) controlling the synaptic or extra-synaptic release of glutamate, (2) selectively blocking the action of the glutamate receptor NMDAR subunit, (3) increasing glutamate metabolism, and reuptake in the brain and blood, and (4) regulating the glutamate system by GABA receptors and the microbiota–gut–brain axis. Based on these latest findings, it is expected to promote a substantial understanding of the complex glutamate signal transduction mechanism, thereby providing excellent neuroprotection research direction for human ischemic stroke (IS).

PINK1-mediated Drp1S616 phosphorylation modulates synaptic development and plasticity via promoting mitochondrial fission

Dynamic change of mitochondrial morphology and distribution along neuronal branches are essential for neural circuitry formation and synaptic efficacy. However, the underlying mechanism remains elusive. We show here that Pink1 knockout (KO) mice display defective dendritic spine maturation, reduced axonal synaptic vesicles, abnormal synaptic connection, and attenuated long-term synaptic potentiation (LTP). Drp1 activation via S616 phosphorylation rescues deficits of spine maturation in Pink1 KO neurons. Notably, mice harboring a knockin (KI) phosphor-null Drp1^S616A recapitulate spine immaturity and synaptic abnormality identified in Pink1 KO mice. Chemical LTP (cLTP) induces Drp1^S616 phosphorylation in a PINK1-dependent manner. Moreover, phosphor-mimetic Drp1^S616D restores reduced dendritic spine localization of mitochondria in Pink1 KO neurons. Together, this study provides the first in vivo evidence of functional regulation of Drp1 by phosphorylation and suggests that PINK1-Drp1^S616 phosphorylation coupling is essential for convergence between mitochondrial dynamics and neural circuitry formation and refinement.

Synaptic Vesicle Recycling and the Endolysosomal System: A Reappraisal of Form and Function

The endolysosomal system is present in all cell types. Within these cells, it performs a series of essential roles, such as trafficking and sorting of membrane cargo, intracellular signaling, control of metabolism and degradation. A specific compartment within central neurons, called the presynapse, mediates inter-neuronal communication via the fusion of neurotransmitter-containing synaptic vesicles (SVs). The localized recycling of SVs and their organization into functional pools is widely assumed to be a discrete mechanism, that only intersects with the endolysosomal system at specific points. However, evidence is emerging that molecules essential for endolysosomal function also have key roles within the SV life cycle, suggesting that they form a continuum rather than being isolated processes. In this review, we summarize the evidence for key endolysosomal molecules in SV recycling and propose an alternative model for membrane trafficking at the presynapse. This includes the hypotheses that endolysosomal intermediates represent specific functional SV pools, that sorting of cargo to SVs is mediated via the endolysosomal system and that manipulation of this process can result in both plastic changes to neurotransmitter release and pathophysiology via neurodegeneration.

Cell Heterogeneity Uncovered by Single-Cell RNA Sequencing Offers Potential Therapeutic Targets for Ischemic Stroke

Ischemic stroke is a detrimental neurological disease characterized by an irreversible infarct core surrounded by an ischemic penumbra, a salvageable region of brain tissue. Unique roles of distinct brain cell subpopulations within the neurovascular unit and peripheral immune cells during ischemic stroke remain elusive due to the heterogeneity of cells in the brain. Single-cell RNA sequencing (scRNA-seq) allows for an unbiased determination of cellular heterogeneity at high-resolution and identification of cell markers, thereby unveiling the principal brain clusters within the cell-type-specific gene expression patterns as well as cell-specific subclusters and their functions in different pathways underlying ischemic stroke. In this review, we have summarized the changes in differentiation trajectories of distinct cell types and highlighted the specific pathways and genes in brain cells that are impacted by stroke. This review is expected to inspire new research and provide directions for investigating the potential pathological mechanisms and novel treatment strategies for ischemic stroke at the level of a single cell.

Unique dynamics and exocytosis properties of GABAergic synaptic vesicles revealed by three-dimensional single vesicle tracking

Maintaining the balance between neuronal excitation and inhibition is essential for proper function of the central nervous system. Inhibitory synaptic transmission plays an important role in maintaining this balance. Although inhibitory transmission has higher kinetic demands compared to excitatory transmission, its properties are poorly understood. In particular, the dynamics and exocytosis of single inhibitory vesicles have not been investigated, due largely to both technical and practical limitations. Using a combination of quantum dots (QDs) conjugated to antibodies against the luminal domain of the vesicular GABA transporter to selectively label GABAergic (i.e., predominantly inhibitory) vesicles together with dual-focus imaging optics, we tracked the real-time three-dimensional position of single GABAergic vesicles up to the moment of exocytosis (i.e., fusion). Using three-dimensional trajectories, we found that GABAergic synaptic vesicles traveled a shorter distance prior to fusion and had a shorter time to fusion compared to synaptotagmin-1 (Syt1)-labeled vesicles, which were mostly from excitatory neurons. Moreover, our analysis revealed that GABAergic synaptic vesicles move more straightly to their release sites than Syt1-labeled vesicles. Finally, we found that GABAergic vesicles have a higher prevalence of kiss-and-run fusion than Syt1-labeled vesicles. These results indicate that inhibitory synaptic vesicles have a unique set of dynamics and exocytosis properties to support rapid synaptic inhibition, thereby maintaining a tightly regulated coordination between excitation and inhibition in the central nervous system.

Post-Golgi carriers, not lysosomes, confer lysosomal properties to pre-degradative organelles in normal and dystrophic axons

Lysosomal trafficking and maturation in neurons remain poorly understood and are unstudied in vivo despite high disease relevance. We generated neuron-specific transgenic mice to track vesicular CTSD acquisition, acidification, and traffic within the autophagic-lysosomal pathway in vivo, revealing that mature lysosomes are restricted from axons. Moreover, TGN-derived transport carriers (TCs), not lysosomes, supply lysosomal components to axonal organelles. Ultrastructurally distinctive TCs containing TGN and lysosomal markers enter axons, engaging autophagic vacuoles and late endosomes. This process is markedly upregulated in dystrophic axons of Alzheimer models. In cultured neurons, most axonal LAMP1 vesicles are weakly acidic TCs that shuttle lysosomal components bidirectionally, conferring limited degradative capability to retrograde organelles before they mature fully to lysosomes within perikarya. The minor LAMP1 subpopulation attaining robust acidification are retrograde Rab7⁺ endosomes/amphisomes, not lysosomes. Restricted lysosome entry into axons explains the unique lysosome distribution in neurons and their vulnerability toward neuritic dystrophy in disease.

Symmetrical arrangement of proteins under release-ready vesicles in presynaptic terminals

Controlled release of neurotransmitters stored in synaptic vesicles (SVs) is a fundamental process that is central to all information processing in the brain. This relies on tight coupling of the SV fusion to action potential-evoked presynaptic Ca2+ influx. This Ca2+-evoked release occurs from a readily releasable pool (RRP) of SVs docked to the plasma membrane (PM). The protein components involved in initial SV docking/tethering and the subsequent priming reactions which make the SV release ready are known. Yet, the supramolecular architecture and sequence of molecular events underlying SV release are unclear. Here, we use cryoelectron tomography analysis in cultured hippocampal neurons to delineate the arrangement of the exocytosis machinery under docked SVs. Under native conditions, we find that vesicles are initially "tethered" to the PM by a variable number of protein densities (∼10 to 20 nm long) with no discernible organization. In contrast, we observe exactly six protein masses, each likely consisting of a single SNAREpin with its bound Synaptotagmins and Complexin, arranged symmetrically connecting the "primed" vesicles to the PM. Our data indicate that the fusion machinery is likely organized into a highly cooperative framework during the priming process which enables rapid SV fusion and neurotransmitter release following Ca2+ influx.

The axonal endolysosomal and autophagic systems

Neurons, because of their elaborate morphology and the long distances between distal axons and the soma as well as their longevity, pose special challenges to autophagy and to the endolysosomal system, two of the main degradative routes for turnover of defective proteins and organelles. Autophagosomes sequester cytoplasmic or organellar cargos by engulfing them into their lumen before fusion with degradative lysosomes enriched in neuronal somata and participate in retrograde signaling to the soma. Endosomes are mainly involved in the sorting, recycling, or lysosomal turnover of internalized or membrane-bound macromolecules to maintain axonal membrane homeostasis. Lysosomes and the multiple shades of lysosome-related organelles also serve non-degradative roles, for example, in nutrient signaling and in synapse formation. Recent years have begun to shed light on the distinctive organization of the autophagy and endolysosomal systems in neurons, in particular their roles in axons. We review here our current understanding of the localization, distribution, and growing list of functions of these organelles in the axon in health and disease and outline perspectives for future research.

Ultrastructural studies of the neurovascular unit reveal enhanced endothelial transcytosis in hyperglycemia‐enhanced hemorrhagic transformation after stroke

Aims

Pre‐existing hyperglycemia (HG) aggravates the breakdown of blood–brain barrier (BBB) and increases the risk of hemorrhagic transformation (HT) after acute ischemic stroke in both animal models and patients. To date, HG‐induced ultrastructural changes of brain microvascular endothelial cells (BMECs) and the mechanisms underlying HG‐enhanced HT after ischemic stroke are poorly understood.

Methods

We used a mouse model of mild brain ischemia/reperfusion to investigate HG‐induced ultrastructural changes of BMECs that contribute to the impairment of BBB integrity after stroke. Adult male mice received systemic glucose administration 15 min before middle cerebral artery occlusion (MCAO) for 20 min. Ultrastructural characteristics of BMECs were evaluated using two‐dimensional and three‐dimensional electron microscopy and quantitatively analyzed.

Results

Mice with acute HG had exacerbated BBB disruption and larger brain infarcts compared to mice with normoglycemia (NG) after MCAO and 4 h of reperfusion, as assessed by brain extravasation of the Evans blue dye and microtubule‐associated protein 2 immunostaining. Electron microscopy further revealed that HG mice had more endothelial vesicles in the striatal neurovascular unit than NG mice, which may account for their deterioration of BBB impairment. In contrast with enhanced endothelial transcytosis, paracellular tight junction ultrastructure was not disrupted after this mild ischemia/reperfusion insult or altered upon HG. Consistent with the observed increase of endothelial vesicles, transcytosis‐related proteins caveolin‐1, clathrin, and hypoxia‐inducible factor (HIF)‐1α were upregulated by HG after MCAO and reperfusion.

Conclusion

Our study provides solid structural evidence to understand the role of endothelial transcytosis in HG‐elicited BBB hyperpermeability. Enhanced transcytosis occurs prior to the physical breakdown of BMECs and is a promising therapeutic target to preserve BBB integrity.

Quantitative Synaptic Biology: A Perspective on Techniques, Numbers and Expectations

Synapses play a central role for the processing of information in the brain and have been analyzed in countless biochemical, electrophysiological, imaging, and computational studies. The functionality and plasticity of synapses are nevertheless still difficult to predict, and conflicting hypotheses have been proposed for many synaptic processes. In this review, we argue that the cause of these problems is a lack of understanding of the spatiotemporal dynamics of key synaptic components. Fortunately, a number of emerging imaging approaches, going beyond super-resolution, should be able to provide required protein positions in space at different points in time. Mathematical models can then integrate the resulting information to allow the prediction of the spatiotemporal dynamics. We argue that these models, to deal with the complexity of synaptic processes, need to be designed in a sufficiently abstract way. Taken together, we suggest that a well-designed combination of imaging and modelling approaches will result in a far more complete understanding of synaptic function than currently possible.

Cathepsins in the Pathophysiology of Mucopolysaccharidoses: New Perspectives for Therapy

Cathepsins (CTSs) are ubiquitously expressed proteases normally found in the endolysosomal compartment where they mediate protein degradation and turnover. However, CTSs are also found in the cytoplasm, nucleus, and extracellular matrix where they actively participate in cell signaling, protein processing, and trafficking through the plasma and nuclear membranes and between intracellular organelles. Dysregulation in CTS expression and/or activity disrupts cellular homeostasis, thus contributing to many human diseases, including inflammatory and cardiovascular diseases, neurodegenerative disorders, diabetes, obesity, cancer, kidney dysfunction, and others. This review aimed to highlight the involvement of CTSs in inherited lysosomal storage disorders, with a primary focus to the emerging evidence on the role of CTSs in the pathophysiology of Mucopolysaccharidoses (MPSs). These latter diseases are characterized by severe neurological, skeletal and cardiovascular phenotypes, and no effective cure exists to date. The advance in the knowledge of the molecular mechanisms underlying the activity of CTSs in MPSs may open a new challenge for the development of novel therapeutic approaches for the cure of such intractable diseases.

Neuropathophysiology of Lysosomal Storage Diseases: Synaptic Dysfunction as a Starting Point for Disease Progression

About two thirds of the patients affected with lysosomal storage diseases (LSD) experience neurological manifestations, such as developmental delay, seizures, or psychiatric problems. In order to develop efficient therapies, it is crucial to understand the neuropathophysiology underlying these symptoms. How exactly lysosomal storage affects biogenesis and function of neurons is still under investigation however recent research highlights a substantial role played by synaptic defects, such as alterations in synaptic spines, synaptic proteins, postsynaptic densities, and synaptic vesicles that might lead to functional impairments in synaptic transmission and neurodegeneration, finally culminating in massive neuronal death and manifestation of cognitive symptoms. Unveiling how the synaptic components are affected in neurological LSD will thus enable a better understanding of the complexity of disease progression as well as identify crucial targets of therapeutic relevance and optimal time windows for targeted intervention.

The Role of Cathepsins in Memory Functions and the Pathophysiology of Psychiatric Disorders

Cathepsins are proteases with functions in cellular homeostasis, lysosomal degradation and autophagy. Their role in the development of neurodegenerative diseases has been extensively studied. It is well established that impairment of proper cathepsin function plays a crucial role in the pathophysiology of neurodegenerative diseases, and in recent years a role for cathepsins in mental disorders has emerged given the involvement of cathepsins in memory function, hyperactivity, and in depression- and anxiety-like behavior. Here we review putative cathepsin functions with a special focus on their role in the pathophysiology of psychiatric diseases. Specifically, cathepsins are crucial for maintaining cellular homeostasis, particularly as part of the autophagy machinery of neural strategies underlying acute stress response. Disruption of cathepsin functions can lead to psychiatric diseases such as major depressive disease (MDD), bipolar disorder, and schizophrenia. Specifically, cathepsins can be excreted via a process called secretory autophagy. Thereby, they are able to regulate extracellular factors such as brain-derived neurotrophic factor and perlecan c-terminal fragment LG3 providing maintenance of neuronal homeostasis and mediating neuronal plasticity in response to acute stress or trauma. In addition, impairment of proper cathepsin function can result in impaired synaptic transmission by compromised recycling and biogenesis of synaptic vesicles. Taken together, further investigations on cathepsin functions and stress response, neuroplasticity, and synaptic transmission will be of great interest in understanding the pathophysiology of psychiatric disorders.