New synaptic organizing proteins and their roles in excitatory and inhibitory synapse development

The brain consists of billions of neurons. During development, these neurons must migrate to their proper position and form connections with neighboring neurons to form networks. The specificity and maturation of these connections, or synapses, are critical for proper brain function, including learning, memory and cognition. Many cell adhesion molecules (CAMs) are involved in the formation and maturation of synapses, including the well-characterized neuroligin-neurexin pair. In this study, two new synapse modifying proteins, calsyntenin and MDGA, are characterized using in vitro assays and primary hippocampal neuron cultures. Calsyntenin-3 was identified in an un-biased screen to search for new synaptogenic proteins. It is a post-synaptic transmembrane protein that induces the formation of excitatory and inhibitory presynaptic specializations in contacting axons via extracellular cadherin and LNS domains. Overexpression of calsyntenin-3 in neurons increases presynaptic protein clustering. Interestingly, calsyntenin-3 binds to α-neurexins with high affinity, suggesting presynaptic induction is mediated through trans-synaptic signaling with neurexins. MDGAs are a family of synaptic GPI-linked proteins that bind neuroligin-2 with high affinity. MDGA1 blocks the presynaptic induction activity of neuroligin-2, through blocking binding to neurexins, via extracellular immunoglobulin domains. Overexpression of MDGA1 in neurons specifically decreases inhibitory synapses, while knockdown increases inhibitory synapses. Interestingly, like other synaptic proteins including neurexin and neuroligin, MDGAs have recently been linked to autism spectrum disorders and schizophrenia. Thus, the characterization of the synapse-promoting calsyntenin-3 and the synapse-reducing MDGA1 shed new light on the mechanisms by which synaptogenesis is regulated. Investigating the complex interplay between molecular players during synaptogenesis is critical not only for understanding normal brain development, but also for providing insight into neurodevelopmental disorders.Medicine, Faculty ofGraduat

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

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.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 β (TRβ) 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, TRβ 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 TRβ gene promoter activity. Examination of endogenous T(3)-responsive promoters (TRβ 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.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