Release activity-dependent control of vesicle endocytosis by the synaptic adhesion molecule N-cadherin

Convergent depression of activity-dependent bulk endocytosis in rodent models of autism spectrum disorder

BACKGROUND

The key pathological mechanisms underlying autism spectrum disorder (ASD) remain relatively undetermined, potentially due to the heterogenous nature of the condition. Targeted studies of a series of monogenic ASDs have revealed postsynaptic dysfunction as a central conserved mechanism. Presynaptic dysfunction is emerging as an additional disease locus in neurodevelopmental disorders; however, it is unclear whether this dysfunction drives ASD or is an adaptation to the altered brain microenvironment.

METHODS

To differentiate between these two competing scenarios, we performed a high content analysis of key stages of the synaptic vesicle lifecycle in primary neuronal cultures derived from a series of preclinical rat models of monogenic ASD. These five independent models (Nrxn1+/-, Nlgn3-/y, Syngap+/-, Syngap+/Δ-GAP, Pten+/-) were specifically selected to have perturbations in a diverse palette of genes that were expressed either at the pre- or post-synapse. Synaptic vesicle exocytosis and cargo trafficking were triggered via two discrete trains of activity and monitored using the genetically-encoded reporter synaptophysin-pHluorin. Activity-dependent bulk endocytosis was assessed during intense neuronal activity using the fluid phase marker tetramethylrhodamine-dextran.

RESULTS

Both synaptic vesicle fusion events and cargo trafficking were unaffected in all models investigated under all stimulation protocols. However, a key convergent phenotype across neurons derived from all five models was revealed, a depression in activity-dependent bulk endocytosis.

LIMITATIONS

The study is exclusively conducted in primary cultures of hippocampal neurons; therefore, the impact on neurons from other brain regions or altered brain microcircuitry was not assessed. No molecular mechanism has been identified for this depression.

CONCLUSION

This suggests that depression of activity-dependent bulk endocytosis is a presynaptic homeostatic mechanism to correct for intrinsic dysfunction in ASD neurons.

The correct connectivity of the DG-CA3 circuits involved in declarative memory processes depends on Vangl2-dependent planar cell polarity signaling

In the hippocampus, dentate gyrus granule cells connect to CA3 pyramidal cells via their axons, the mossy fibers (Mf). The synaptic terminals of Mfs (Mf boutons, MfBs) form large and complex synapses with thorny excrescences (TE) on the proximal dendrites of CA3 pyramidal cells (PCs). MfB/TE synapses have distinctive "detonator" properties due to low initial release probability and large presynaptic facilitation. The molecular mechanisms shaping the morpho-functional properties of MfB/TE synapses are still poorly understood, though alterations in their morphology are associated with Down syndrome, intellectual disabilities, and Alzheimer's disease. Here, we identify the core PCP gene Vangl2 as essential to the morphogenesis and function of MfB/TE synapses. Vangl2 colocalises with the presynaptic heparan sulfate proteoglycan glypican 4 (GPC4) to stabilise the postsynaptic orphan receptor GPR158. Embryonic loss of Vangl2 disrupts the morphology of MfBs and TEs, impairs ultrastructural and molecular organisation, resulting in defective synaptic transmission and plasticity. In adult, the early loss of Vangl2 results in a number of hippocampus-dependent memory deficits including characteristic flexibility of declarative memory, organisation and retention of working / everyday-like memory. These deficits also lead to abnormal generalisation of memories to salient cues and diminished ability to form detailed contextual memories. Together, these results establish Vangl2 as a key regulator of DG-CA3 connectivity and functions.

Synaptic endocytosis in adult adipose stromal cell-derived neurons

Synapses are essential for facilitating the transmission of information between neurons and for executing neurophysiological processes. Following the exocytosis of neurotransmitters, the synaptic vesicle may quickly undergo endocytosis to preserve the structural integrity of the synapse. When converting adipose-derived stromal cells (ADSCs) into neurons, the ADSCs have already demonstrated comparable morphology, structure, and electrophysiological characteristics to neurons. Nevertheless, there is currently no published study on the endocytotic function of neurons that are produced from ADSCs. This study aimed to examine synaptic endocytosis in neurons derived from ADSCs by qualitatively and quantitatively analyzing the presence of Ap-2, Clathrin, Endophilin, Dynamin, and Hsc70, which are the key proteins involved in clathrin-mediated endocytosis (CME), as well as by using FM1-43 and cadmium selenide quantum dots (CdSe QDs). Additionally, single-cell RNA sequencing (scRNA-seq) was used to look at the levels of both neuronal markers and markers related to CME at the same time. The results of this study provide evidence that synapses in neurons produced from ADSCs have a role in endocytosis, mainly through the CME route.

The CHD Protein Kismet Restricts the Synaptic Localization of Cell Adhesion Molecules at the Drosophila Neuromuscular Junction

The appropriate expression and localization of cell surface cell adhesion molecules must be tightly regulated for optimal synaptic growth and function. How neuronal plasma membrane proteins, including cell adhesion molecules, cycle between early endosomes and the plasma membrane is poorly understood. Here we show that the Drosophila homolog of the chromatin remodeling enzymes CHD7 and CHD8, Kismet, represses the synaptic levels of several cell adhesion molecules. Neuroligins 1 and 3 and the integrins αPS2 and βPS are increased at kismet mutant synapses but Kismet only directly regulates transcription of neuroligin 2. Kismet may therefore regulate synaptic CAMs indirectly by activating transcription of gene products that promote intracellular vesicle trafficking including endophilin B (endoB) and/or rab11. Knock down of EndoB in all tissues or neurons increases synaptic FasII while knock down of EndoB in kis mutants does not produce an additive increase in FasII. In contrast, neuronal expression of Rab11, which is deficient in kis mutants, leads to a further increase in synaptic FasII in kis mutants. These data support the hypothesis that Kis influences the synaptic localization of FasII by promoting intracellular vesicle trafficking through the early endosome.

Flying under the radar: CDH2 (N-cadherin), an important hub molecule in neurodevelopmental and neurodegenerative diseases

CDH2 belongs to the classic cadherin family of Ca2+-dependent cell adhesion molecules with a meticulously described dual role in cell adhesion and β-catenin signaling. During CNS development, CDH2 is involved in a wide range of processes including maintenance of neuroepithelial integrity, neural tube closure (neurulation), confinement of radial glia progenitor cells (RGPCs) to the ventricular zone and maintaining their proliferation-differentiation balance, postmitotic neural precursor migration, axon guidance, synaptic development and maintenance. In the past few years, direct and indirect evidence linked CDH2 to various neurological diseases, and in this review, we summarize recent developments regarding CDH2 function and its involvement in pathological alterations of the CNS.

Hemisynapse Formation Between Target Astrocytes and Cortical Neuron Axons in vitro

One of the most fundamental organizing principles in the mammalian brain is that neurons do not establish synapses with the other major cell type, the astrocytes. However, induced synapse formation between neurons and astrocytes appears conceivable, because astrocytes are well known to express functional ionotropic glutamate receptors. Here, we attempted to trigger synapse formation between co-cultured neurons and astrocytes by overexpressing the strongly synaptogenic adhesion protein LRRTM2 in astrocytes physically contacted by cortical axons. Interestingly, control experiments with immature cortical astrocytes without any overexpression resulted in the induction of synaptic vesicle clustering in contacting axons (hemisynapse formation). This synaptogenic activity correlated with the endogenous expression of the synaptogenic protein Neuroligin1. Hemisynapse formation was further enhanced upon overexpression of LRRTM2 in cortical astrocytes. In contrast, cerebellar astrocytes required overexpression of LRRTM2 for induction of synaptic vesicle clustering in contacting axons. We further addressed, whether hemisynapse formation was accompanied by the appearance of fully functional glutamatergic synapses. We therefore attempted to record AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) in innervated astrocytes using the whole-cell patch-clamp technique. Despite the endogenous expression of the AMPA receptor subunits GluA2 and to a lesser extent GluA1, we did not reliably observe spontaneous AMPA mEPSCs. In conclusion, overexpression of the synaptogenic protein LRRTM2 induced hemisynapse formation between co-cultured neurons and astrocytes. However, the formation of fully functional synapses appeared to require additional factors critical for nano-alignment of presynaptic vesicles and postsynaptic receptors.

Insights Into Central Nervous System Glial Cell Formation and Function From Zebrafish

The term glia describes a heterogenous collection of distinct cell types that make up a large proportion of our nervous system. Although once considered the glue of the nervous system, the study of glial cells has evolved significantly in recent years, with a large body of literature now highlighting their complex and diverse roles in development and throughout life. This progress is due, in part, to advances in animal models in which the molecular and cellular mechanisms of glial cell development and function as well as neuron-glial cell interactions can be directly studied in vivo in real time, in intact neural circuits. In this review we highlight the instrumental role that zebrafish have played as a vertebrate model system for the study of glial cells, and discuss how the experimental advantages of the zebrafish lend themselves to investigate glial cell interactions and diversity. We focus in particular on recent studies that have provided insight into the formation and function of the major glial cell types in the central nervous system in zebrafish.

Transsynaptic N-Cadherin Adhesion Complexes Control Presynaptic Vesicle and Bulk Endocytosis at Physiological Temperature

At mammalian glutamatergic synapses, most basic elements of synaptic transmission have been shown to be modulated by specific transsynaptic adhesion complexes. However, although crucial for synapse homeostasis, a physiological regulation of synaptic vesicle endocytosis by adhesion molecules has not been firmly established. The homophilic adhesion protein N-cadherin is localized at the peri-active zone, where the highly temperature-dependent endocytosis of vesicles occurs. Here, we demonstrate an important modulatory role of N-cadherin in endocytosis at near physiological temperature by synaptophysin-pHluorin imaging. Different modes of endocytosis including bulk endocytosis were dependent on N-cadherin expression and function. N-cadherin modulation might be mediated by actin filaments because actin polymerization ameliorated the knockout-induced endocytosis defect. Using super-resolution imaging, we found strong recruitment of N-cadherin to glutamatergic synapses upon massive vesicle release, which might in turn enhance vesicle endocytosis. This provides a novel, adhesion protein-mediated mechanism for efficient coupling of exo- and endocytosis.

Postsynaptic Neuroligin-1 Mediates Presynaptic Endocytosis During Neuronal Activity

Fast, high-fidelity neurotransmission and synaptic efficacy requires tightly regulated coordination of pre- and postsynaptic compartments and alignment of presynaptic release sites with postsynaptic receptor nanodomains. Neuroligin-1 (Nlgn1) is a postsynaptic cell-adhesion protein exclusively localised to excitatory synapses that is crucial for coordinating the transsynaptic alignment of presynaptic release sites with postsynaptic AMPA receptors as well as postsynaptic transmission and plasticity. However, little is understood about whether the postsynaptic machinery can mediate the molecular architecture and activity of the presynaptic nerve terminal, and thus it remains unclear whether there are presynaptic contributions to Nlgn1-dependent control of signalling and plasticity. Here, we employed a presynaptic reporter of neurotransmitter release and synaptic vesicle dynamics, synaptophysin-pHluorin (sypHy), to directly assess the presynaptic impact of loss of Nlgn1. We show that lack of Nlgn1 had no effect on the size of the readily releasable or entire recycling pool of synaptic vesicles, nor did it impact exocytosis. However, we observed significant changes in the retrieval of synaptic vesicles by compensatory endocytosis, specifically during activity. Our data extends growing evidence that synaptic adhesion molecules critical for forming transsynaptic scaffolds are also important for regulating activity-induced endocytosis at the presynapse.

Functions of p38 MAP Kinases in the Central Nervous System

Mitogen-activated protein (MAP) kinases are a central component in signaling networks in a multitude of mammalian cell types. This review covers recent advances on specific functions of p38 MAP kinases in cells of the central nervous system. Unique and specific functions of the four mammalian p38 kinases are found in all major cell types in the brain. Mechanisms of p38 activation and downstream phosphorylation substrates in these different contexts are outlined and how they contribute to functions of p38 in physiological and under disease conditions. Results in different model organisms demonstrated that p38 kinases are involved in cognitive functions, including functions related to anxiety, addiction behavior, neurotoxicity, neurodegeneration, and decision making. Finally, the role of p38 kinases in psychiatric and neurological conditions and the current progress on therapeutic inhibitors targeting p38 kinases are covered and implicate p38 kinases in a multitude of CNS-related physiological and disease states.

Differential Properties of the Synaptogenic Activities of the Neurexin Ligands Neuroligin1 and LRRTM2

Synaptic cell adhesion molecules are well established to exhibit synaptogenic activity when overexpressed in target cells, indicating that they are involved in formation and functional maturation of synapses. The postsynaptic adhesion proteins Neuroligin1 and LRRTM2 both induce synaptic vesicle clusters in presynaptic axons in vitro by transsynaptically interacting with neurexins. In neurons, this is accompanied by the induction of glutamatergic, but not GABAergic synapses. Although the synaptogenic activity of Neuroligin1 has been well characterized, the properties of the synaptogenic activities of other synaptic adhesion molecules are largely unknown. In this paper, we now compared characteristics of the synaptogenic activities of Neuroligin1 and LRRTM2 upon overexpression in cultured mouse cortical neurons. Individual cortical neurons were transfected with Neuroligin1 and LRRTM2 expression plasmids, respectively, and synaptic vesicle clustering in contacting axons was examined by immunostaining for the vesicle membrane protein VAMP2. In immature neurons at 6–7 days in vitro (DIV) both Neuroligin1 and LRRTM2 exhibited strong synaptogenic activity. However, upon further neuronal differentiation only LRRTM2 retained significant synaptogenic activity at 12–13 DIV. A similar differential developmental maturation of the synaptogenic activities of Neuroligin1 and LRRTM2 was observed for the induction of glutamatergic synapses, which were detected by co-immunostaining for VGLUT1 and Homer1. Most interestingly, the synaptogenic activity of Neuroligin1 was strongly dependent on the expression and function of the synaptic adhesion molecule N-cadherin in immature neurons. In contrast, the synaptogenic activity of LRRTM2 was independent of N-cadherin expression and function in both immature (6–7 DIV) and more mature neurons (14–15 DIV). Taken together, our results with overexpression in cultured cortical neurons revealed striking differences in the properties of the synaptogenic activities of Neuroligin1 and LRRTM2, although both transsynaptically interact with presynaptic neurexins.

Astrocyte lineage cells are essential for functional neuronal differentiation and synapse maturation in human iPSC-derived neural networks

Human astrocytes differ dramatically in cell morphology and gene expression from murine astrocytes. The latter are well known to be of major importance in the formation of neuronal networks by promoting synapse maturation. However, whether human astrocyte lineage cells have a similar role in network formation has not been firmly established. Here, we investigated the impact of human astrocyte lineage cells on the functional maturation of neural networks that were derived from human induced pluripotent stem cells (hiPSCs). Initial in vitro differentiation of hiPSC-derived neural progenitor cells and immature neurons (glia+ cultures) resulted in spontaneously active neural networks as indicated by synchronous neuronal Ca²⁺ transients. Depleting proliferating neural progenitors from these cultures by short-term antimitotic treatment resulted in strongly astrocyte lineage cell-depleted neuronal networks (glia- cultures). Strikingly, in contrast to glia+ cultures, glia- cultures did not exhibit spontaneous network activity. Detailed analysis of the morphological and electrophysiological properties of neurons by patch clamp recordings revealed reduced dendritic arborization in glia- cultures. In addition, a reduced action potential frequency upon current injection in pyramidal-like neurons was observed, whereas the electrical excitability of multipolar neurons was unaltered. Furthermore, we found a reduced dendritic density of PSD95-positive excitatory synapses, and more immature properties of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) miniature excitatory postsynaptic currents (mEPSCs) in glia- cultures, suggesting that the maturation of glutamatergic synapses depends on the presence of hiPSC-derived astrocyte lineage cells. Intriguingly, addition of the astrocyte-derived synapse maturation inducer cholesterol increased the dendritic density of PSD95-positive excitatory synapses in glia- cultures.

Idelalisib for optimized CD19-specific chimeric antigen receptor T cells in chronic lymphocytic leukemia patients

Despite encouraging results with chimeric antigen receptor T (CART) cells, outcome can still be improved by optimization of the CART cell generation process. The proportion of less-differentiated T cells within the transfused product is linked to enhanced in vivo CART cell expansion and long-term persistence. The clinically approved PI3Kδ inhibitor idelalisib is well established in the treatment of B cell malignancies. Besides B cell receptor pathway inhibition, idelalisib can modulate T cell differentiation and function. Here, detailed longitudinal analysis of idelalisib-induced effects on T cell phenotype and function was performed during CART cell production. A third generation CD19.CAR.CD28.CD137zeta CAR vector system was used. CART cells were generated from peripheral blood mononuclear cells of healthy donors (HDs) and chronic lymphocytic leukemia (CLL) patients. Idelalisib-based CART cell generation resulted in an enrichment of less-differentiated naïve-like T cells (CD45RA+CCR7+), decreased expression of the exhaustion markers PD-1 and Tim-3, as well as upregulation of the lymph node homing marker CD62L. Idelalisib increased transduction efficiency, but did not impair viability and cell expansion. Strikingly, CD4:CD8 ratios that were altered in CART cells from CLL patients were approximated to ratios in HDs by idelalisib. Furthermore, in vivo efficacy of idelalisib-treated CART cells was validated in a xenograft mouse model. Intracellular TNF-α and IFN-γ production decreased in presence of idelalisib. This effect was reversible after resting CART cells without idelalisib. In summary, PI3Kδ inhibition with idelalisib can improve CART cell products, particularly when derived from CLL patients. Further studies with idelalisib-based CART cell generation protocols are warranted.