The specific α-neurexin interactor calsyntenin-3 promotes excitatory and inhibitory synapse development.

Perturbations of cell surface synapse-organizing proteins, particularly α-neurexins, contribute to neurodevelopmental and psychiatric disorders. From an unbiased screen, we identify calsyntenin-3 (alcadein-ß) as a synapse-organizing protein unique in binding and recruiting α-neurexins, but not ß-neurexins. Calsyntenin-3 is present in many pyramidal neurons throughout cortex and hippocampus but is most highly expressed in interneurons. The transmembrane form of calsyntenin-3 can trigger excitatory and inhibitory presynapse differentiation in contacting axons. However, calsyntenin-3-shed ectodomain, which represents about half the calsyntenin-3 pool in brain, suppresses the ability of multiple α-neurexin partners including neuroligin 2 and LRRTM2 to induce presynapse differentiation. Clstn3⁻/⁻ mice show reductions in excitatory and inhibitory synapse density by confocal and electron microscopy and corresponding deficits in synaptic transmission. These results identify calsyntenin-3 as an α-neurexin-specific binding partner required for normal functional GABAergic and glutamatergic synapse development.

Interaction between autism-linked MDGAs and neuroligins suppresses inhibitory synapse development.

Rare variants in MDGAs (MAM domain-containing glycosylphosphatidylinositol anchors), including multiple protein-truncating deletions, are linked to autism and schizophrenia, but the function of these genes is poorly understood. Here, we show that MDGA1 and MDGA2 bound to neuroligin-2 inhibitory synapse-organizing protein, also implicated in neurodevelopmental disorders. MDGA1 inhibited the synapse-promoting activity of neuroligin-2, without altering neuroligin-2 surface trafficking, by inhibiting interaction of neuroligin-2 with neurexin. MDGA binding and suppression of synaptogenic activity was selective for neuroligin-2 and not neuroligin-1 excitatory synapse organizer. Overexpression of MDGA1 in cultured rat hippocampal neurons reduced inhibitory synapse density without altering excitatory synapse density. Furthermore, RNAi-mediated knockdown of MDGA1 selectively increased inhibitory but not excitatory synapse density. These results identify MDGA1 as one of few identified negative regulators of synapse development with a unique selectivity for inhibitory synapses. These results also place MDGAs in the neurexin-neuroligin synaptic pathway implicated in neurodevelopmental disorders and support the idea that an imbalance between inhibitory and excitatory synapses may contribute to these disorders.

Modulation of thyroid hormone-dependent gene expression in Xenopus laevis by INhibitor of Growth (ING) proteins.

BACKGROUND: INhibitor of Growth (ING) proteins belong to a large family of plant homeodomain finger-containing proteins important in epigenetic regulation and carcinogenesis. We have previously shown that ING1 and ING2 expression is regulated by thyroid hormone (TH) during metamorphosis of the Xenopus laevis tadpole. The present study investigates the possibility that ING proteins modulate TH action. METHODOLOGY/PRINCIPAL FINDINGS: Tadpoles expressing a Xenopus ING2 transgene (Trans(ING2)) were significantly smaller than tadpoles not expressing the transgene (Trans(GFP)). When exposed to 10 nM 3,5,3'-triiodothyronine (T(3)), premetamorphic Trans(ING2) tadpoles exhibited a greater reduction in tail, head, and brain areas, and a protrusion of the lower jaw than T(3)-treated Trans(GFP) tadpoles. Quantitative real time polymerase chain reaction (QPCR) demonstrated elevated TH receptor ß (TRß) and TH/bZIP transcript levels in Trans(ING2) tadpole tails compared to Trans(GFP) tadpoles while TRα mRNAs were unaffected. In contrast, no difference in TRα, TRß or insulin-like growth factor (IGF2) mRNA abundance was observed in the brain between Trans(ING2) and Trans(GFP) tadpoles. All of these transcripts, except for TRα mRNA in the brain, were inducible by the hormone in both tissues. Oocyte transcription assays indicated that ING proteins enhanced TR-dependent, T(3)-induced TRß gene promoter activity. Examination of endogenous T(3)-responsive promoters (TRß and TH/bZIP) in the tail by chromatin immunoprecipitation assays showed that ING proteins were recruited to TRE-containing regions in T(3)-dependent and independent ways, respectively. Moreover, ING and TR proteins coimmunoprecipitated from tail protein homogenates derived from metamorphic climax animals. CONCLUSIONS/SIGNIFICANCE: We show for the first time that ING proteins modulate TH-dependent responses, thus revealing a novel role for ING proteins in hormone signaling. This has important implications for understanding hormone influenced disease states and suggests that the induction of ING proteins may facilitate TR function during metamorphosis in a tissue-specific manner.