Redundant Postsynaptic Functions of SynCAMs 1-3 during Synapse Formation

SynCAM1 deficiency in the hippocampal parvalbumin interneurons contributes to sevoflurane-induced cognitive impairment in neonatal rats

AIMS

Sevoflurane is widely used for general anesthesia in children. Previous studies reported that multiple neonatal exposures to sevoflurane can induce long-term cognitive impairment in adolescent rats, but the underlying mechanisms were not defined.

METHODS

Postnatal day 6 (P6) to P8 rat pups were exposed to 30% oxygen with or without 3% sevoflurane balanced with air. The Y maze test (YMT) and Morris water maze (MWM) tests were performed in some cohorts from age P35 to assess cognitive functions, and their brain samples were harvested at age P14, 21, 28, 35, and 42 for measurements of various molecular entities and in vivo electrophysiology experiments at age P35.

RESULTS

Sevoflurane exposure resulted in cognitive impairment that was associated with decreased synCAM1 expression in parvalbumin (PV) interneurons, a reduction of PV phenotype, disturbed gamma oscillations, and dendritic spine loss in the hippocampal CA3 region. Enriched environment (EE) increased synCAM1 expression in the PV interneurons and attenuated sevoflurane-induced cognitive impairment. The synCAM1 overexpression by the adeno-associated virus vector in the hippocampal CA3 region restored sevoflurane-induced cognitive impairment, PV phenotype loss, gamma oscillations decrease, and dendritic spine loss.

CONCLUSION

Our data suggested that neonatal sevoflurane exposure results in cognitive impairment through decreased synCAM1 expression in PV interneurons in the hippocampus.

Alterations of presynaptic proteins in autism spectrum disorder

The expanded use of hypothesis-free gene analysis methods in autism research has significantly increased the number of genetic risk factors associated with the pathogenesis of autism. A further examination of the implicated genes directly revealed the involvement in processes pertinent to neuronal differentiation, development, and function, with a predominant contribution from the regulators of synaptic function. Despite the importance of presynaptic function in synaptic transmission, the regulation of neuronal network activity, and the final behavioral output, there is a relative lack of understanding of the presynaptic contribution to the pathology of autism. Here, we will review the close association among autism-related mutations, autism spectrum disorders (ASD) phenotypes, and the altered presynaptic protein functions through a systematic examination of the presynaptic risk genes relating to the critical stages of synaptogenesis and neurotransmission.

A synaptomic analysis reveals dopamine hub synapses in the mouse striatum

Dopamine transmission is involved in reward processing and motor control, and its impairment plays a central role in numerous neurological disorders. Despite its strong pathophysiological relevance, the molecular and structural organization of the dopaminergic synapse remains to be established. Here, we used targeted labelling and fluorescence activated sorting to purify striatal dopaminergic synaptosomes. We provide the proteome of dopaminergic synapses with 57 proteins specifically enriched. Beyond canonical markers of dopamine neurotransmission such as dopamine biosynthetic enzymes and cognate receptors, we validated 6 proteins not previously described as enriched. Moreover, our data reveal the adhesion of dopaminergic synapses to glutamatergic, GABAergic or cholinergic synapses in structures we named “dopamine hub synapses”. At glutamatergic synapses, pre- and postsynaptic markers are significantly increased upon association with dopamine synapses. Dopamine hub synapses may thus support local dopaminergic signalling, complementing volume transmission thought to be the major mechanism by which monoamines modulate network activity.

The neurotransmitter dopamine is an important regulator of brain function. Here the authors describe “dopamine hub synapses”, where dopamine transmission may act in synergy with other neurotransmitters.

Differential Contribution of Cadm1-Cadm3 Cell Adhesion Molecules to Peripheral Myelinated Axons

Cell adhesion proteins of the Cadm (SynCAM/Necl) family regulate myelination and the organization of myelinated axons. In the peripheral nervous system (PNS), intercellular contact between Schwann cells and their underlying axons is believed to be mediated by binding of glial Cadm4 to axonal Cadm3 or Cadm2. Nevertheless, given that distinct neurons express different combinations of the Cadm proteins, the identity of the functional axonal ligand for Cadm4 remains to be determined. Here, we took a genetic approach to compare the phenotype of Cadm4 null mice, which exhibit abnormal distribution of Caspr and Kv1 potassium channels, with mice lacking different combinations of Cadm1-Cadm3 genes. We show that in contrast to mice lacking the single Cadm1, Cadm2, or Cadm3 genes, genetic ablation of all three phenocopies the abnormalities detected in the absence of Cadm4. Similar defects were observed in double mutant mice lacking Cadm3 and Cadm2 (i.e., Cadm3 ^-/- /Cadm2 ^-/-) or Cadm3 and Cadm1 (i.e., Cadm3 ^-/- /Cadm1 ^-/-), but not in mice lacking Cadm1 and Cadm2 (i.e., Cadm1 ^-/- /Cadm2 ^-/-). Furthermore, axonal organization abnormalities were also detected in Cadm3 null mice that were heterozygous for the two other axonal Cadms. Our results identify Cadm3 as the main axonal ligand for glial Cadm4, and reveal that its absence could be compensated by the combined action of Cadm2 and Cadm1.SIGNIFICANCE STATEMENT Myelination by Schwann cells enables fast conduction of action potentials along motor and sensory axons. In these nerves, Schwann cell-axon contact is mediated by cell adhesion molecules of the Cadm family. Cadm4 in Schwann cells regulates axonal ensheathment and myelin wrapping, as well as the organization of the axonal membrane, but the identity of its axonal ligands is not clear. Here, we reveal that Cadm mediated axon-glia interactions depend on a hierarchical adhesion code that involves multiple family members. Our results provide important insights into the molecular mechanisms of axon-glia communication, and the function of Cadm proteins in PNS myelin.

Oligodendrocytes express synaptic proteins that modulate myelin sheath formation

Vesicular release from neurons promotes myelin sheath growth on axons. Oligodendrocytes express proteins that allow dendrites to respond to vesicular release at synapses, suggesting that axon-myelin contacts use similar communication mechanisms as synapses to form myelin sheaths. To test this, we used fusion proteins to track synaptic vesicle localization and membrane fusion in zebrafish during developmental myelination and investigated expression and localization of PSD95, a dendritic post-synaptic protein, within oligodendrocytes. Synaptic vesicles accumulate and exocytose at ensheathment sites with variable patterning and most sheaths localize PSD95 with patterning similar to exocytosis site location. Disruption of candidate PDZ-binding transsynaptic adhesion proteins in oligodendrocytes cause variable effects on sheath length and number. One candidate, Cadm1b, localizes to myelin sheaths where both PDZ binding and extracellular adhesion to axons mediate sheath growth. Our work raises the possibility that axon-glial communication contributes to myelin plasticity, providing new targets for mechanistic unraveling of developmental myelination.

Downregulating SynCAM and MPP6 expression is associated with ovarian cancer progression

Synaptic cell adhesion molecules (SynCAMs) are single transmembrane proteins that belong to the immunoglobulin superfamily of cell adhesion molecules. In the present study, a decrease in SynCAM levels in ovarian tumor tissues compared with normal tissues is reported; the downregulation was accompanied by the grade malignancy. The observations suggested that SynCAM may be essential for important novel functions in ovarian cancer. Further experiments showed that low SynCAM expression inhibited membrane palmitoylated protein 6 (MPP6) expression, a member of the palmitoylated membrane protein subfamily of peripheral membrane-associated guanylate kinases. In addition, low levels of MPP6 in ovarian tumor tissues correlated with shorter patient survival. A SynCAM-regulated pathway may provide molecular targets for the treatment of ovarian cancer and novel biomarkers to be used in clinical diagnosis.

Axoglial Adhesion by Cadm4 Regulates CNS Myelination

The initiation of axoglial contact is considered a prerequisite for myelination, yet the role cell adhesion molecules (CAMs) play in mediating such interactions remains unclear. To examine the function of axoglial CAMs, we tested whether enhanced CAM-mediated adhesion between OLs and neurons could affect myelination. Here we show that increased expression of a membrane-bound extracellular domain of Cadm4 (Cadm4dCT) in cultured oligodendrocytes results in the production of numerous axoglial contact sites that fail to elongate and generate mature myelin. Transgenic mice expressing Cadm4dCT were hypomyelinated and exhibit multiple myelin abnormalities, including myelination of neuronal somata. These abnormalities depend on specific neuron-glial interaction as they were not observed when these OLs were cultured alone, on nanofibers, or on neurons isolated from mice lacking the axonal receptors of Cadm4. Our results demonstrate that tightly regulated axon-glia adhesion is essential for proper myelin targeting and subsequent membrane wrapping and lateral extension.

Immunoglobulin-Like Receptors and Their Impact on Wiring of Brain Synapses

Synapse formation is mediated by a surprisingly large number and wide variety of genes encoding many different protein classes. One of the families increasingly implicated in synapse wiring is the immunoglobulin superfamily (IgSF). IgSF molecules are by definition any protein containing at least one Ig-like domain, making this family one of the most common protein classes encoded by the genome. Here, we review the emerging roles for IgSF molecules in synapse formation specifically in the vertebrate brain, focusing on examples from three classes of IgSF members: ( a) cell adhesion molecules, ( b) signaling molecules, and ( c) immune molecules expressed in the brain. The critical roles for IgSF members in regulating synapse formation may explain their extensive involvement in neuropsychiatric and neurodevelopmental disorders. Solving the IgSF code for synapse formation may reveal multiple new targets for rescuing IgSF-mediated deficits in synapse formation and, eventually, new treatments for psychiatric disorders caused by altered IgSF-induced synapse wiring.

4.1Ba is necessary for glutamatergic synapse formation in the sensorimotor circuit of developing zebrafish

During the process of synapse formation, thousands of proteins assemble at prospective sites of cell-cell communication. Although many of these proteins have been identified, the roles they play in generating functional connections during development remain unknown. 4.1 scaffolding proteins have been implicated in synapse formation and maturation in vitro, but in vivo studies for some family members have suggested these proteins are not important for this role. We examined the role of family member 4.1B because it has been implicated in glutamatergic synaptogenesis, but has not been described in vivo. We identified two 4.1B genes in zebrafish, 4.1Ba and 4.1Bb, by sequence comparisons and synteny analysis. In situ hybridization shows these genes are differentially expressed, with 4.1Ba expressed primarily in the nervous system and 4.1Bb expressed in the nervous system and muscle, but not the spinal cord. We focused our studies on 4.1Ba in the spinal cord. 4.1Ba knockdown reduced the number of glutamatergic synapses at caudal primary motor neurons and caused an increase in the duration of touch-evoked coiling. These results suggest 4.1Ba is important for the formation of functional glutamatergic synapses in the developing zebrafish spinal cord.

A Modular Organization of LRR Protein-Mediated Synaptic Adhesion Defines Synapse Identity

Pyramidal neurons express rich repertoires of leucine-rich repeat (LRR)-containing adhesion molecules with similar synaptogenic activity in culture. The in vivo relevance of this molecular diversity is unclear. We show that hippocampal CA1 pyramidal neurons express multiple synaptogenic LRR proteins that differentially distribute to the major excitatory inputs on their apical dendrites. At Schaffer collateral (SC) inputs, FLRT2, LRRTM1, and Slitrk1 are postsynaptically localized and differentially regulate synaptic structure and function. FLRT2 controls spine density, whereas LRRTM1 and Slitrk1 exert opposing effects on synaptic vesicle distribution at the active zone. All LRR proteins differentially affect synaptic transmission, and their combinatorial loss results in a cumulative phenotype. At temporoammonic (TA) inputs, LRRTM1 is absent; FLRT2 similarly controls functional synapse number, whereas Slitrk1 function diverges to regulate postsynaptic AMPA receptor density. Thus, LRR proteins differentially control synaptic architecture and function and act in input-specific combinations and a context-dependent manner to specify synaptic properties.

Cell adhesion and matricellular support by astrocytes of the tripartite synapse

Astrocytes contribute to the formation, function, and plasticity of synapses. Their processes enwrap the neuronal components of the tripartite synapse, and due to this close interaction they are perfectly positioned to modulate neuronal communication. The interaction between astrocytes and synapses is facilitated by cell adhesion molecules and matricellular proteins, which have been implicated in the formation and functioning of tripartite synapses. The importance of such neuron-astrocyte integration at the synapse is underscored by the emerging role of astrocyte dysfunction in synaptic pathologies such as autism and schizophrenia. Here we review astrocyte-expressed cell adhesion molecules and matricellular molecules that play a role in integration of neurons and astrocytes within the tripartite synapse.