A Conserved Tyrosine Residue in Slitrk3 Carboxyl-Terminus Is Critical for GABAergic Synapse Development

Human mutations in SLITRK3 implicated in GABAergic synapse development in mice

This study reports on biallelic homozygous and monoallelic de novo variants in SLITRK3 in three unrelated families presenting with epileptic encephalopathy associated with a broad neurological involvement characterized by microcephaly, intellectual disability, seizures, and global developmental delay. SLITRK3 encodes for a transmembrane protein that is involved in controlling neurite outgrowth and inhibitory synapse development and that has an important role in brain function and neurological diseases. Using primary cultures of hippocampal neurons carrying patients' SLITRK3 variants and in combination with electrophysiology, we demonstrate that recessive variants are loss-of-function alleles. Immunostaining experiments in HEK-293 cells showed that human variants C566R and E606X change SLITRK3 protein expression patterns on the cell surface, resulting in highly accumulating defective proteins in the Golgi apparatus. By analyzing the development and phenotype of SLITRK3 KO (SLITRK3-/-) mice, the study shows evidence of enhanced susceptibility to pentylenetetrazole-induced seizure with the appearance of spontaneous epileptiform EEG as well as developmental deficits such as higher motor activities and reduced parvalbumin interneurons. Taken together, the results exhibit impaired development of the peripheral and central nervous system and support a conserved role of this transmembrane protein in neurological function. The study delineates an emerging spectrum of human core synaptopathies caused by variants in genes that encode SLITRK proteins and essential regulatory components of the synaptic machinery. The hallmark of these disorders is impaired postsynaptic neurotransmission at nerve terminals; an impaired neurotransmission resulting in a wide array of (often overlapping) clinical features, including neurodevelopmental impairment, weakness, seizures, and abnormal movements. The genetic synaptopathy caused by SLITRK3 mutations highlights the key roles of this gene in human brain development and function.

SLITRK2 variants associated with neurodevelopmental disorders impair excitatory synaptic function and cognition in mice

SLITRK2 is a single-pass transmembrane protein expressed at postsynaptic neurons that regulates neurite outgrowth and excitatory synapse maintenance. In the present study, we report on rare variants (one nonsense and six missense variants) in SLITRK2 on the X chromosome identified by exome sequencing in individuals with neurodevelopmental disorders. Functional studies showed that some variants displayed impaired membrane transport and impaired excitatory synapse-promoting effects. Strikingly, these variations abolished the ability of SLITRK2 wild-type to reduce the levels of the receptor tyrosine kinase TrkB in neurons. Moreover, Slitrk2 conditional knockout mice exhibited impaired long-term memory and abnormal gait, recapitulating a subset of clinical features of patients with SLITRK2 variants. Furthermore, impaired excitatory synapse maintenance induced by hippocampal CA1-specific cKO of Slitrk2 caused abnormalities in spatial reference memory. Collectively, these data suggest that SLITRK2 is involved in X-linked neurodevelopmental disorders that are caused by perturbation of diverse facets of SLITRK2 function.

Convergence of adenosine and GABA signaling for synapse stabilization during development

[Figure: see text].

Proper synaptic adhesion signaling in the control of neural circuit architecture and brain function

Trans-synaptic cell-adhesion molecules are critical for governing various stages of synapse development and specifying neural circuit properties via the formation of multifarious signaling pathways. Recent studies have pinpointed the putative roles of trans-synaptic cell-adhesion molecules in mediating various cognitive functions. Here, we review the literature on the roles of a diverse group of central synaptic organizers, including neurexins (Nrxns), leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs), and their associated binding proteins, in regulating properties of specific type of synapses and neural circuits. In addition, we highlight the findings that aberrant synaptic adhesion signaling leads to alterations in the structures, transmission, and plasticity of specific synapses across diverse brain areas. These results seem to suggest that proper trans-synaptic signaling pathways by Nrxns, LAR-RPTPs, and their interacting network is likely to constitute central molecular complexes that form the basis for cognitive functions, and that these complexes are heterogeneously and complexly disrupted in many neuropsychiatric and neurodevelopmental disorders.