Synapse Formation & Function
1. Synaptic cell-adhesion molecules (CAMs) are a family of neuronal proteins that establish physical interactions between the pre- and postsynaptic compartments, and thereby promote synapse formation and specify synapse parameters. Mutations in synaptic CAMs lead to major encephalopathies including autism and schizophrenia, which affect millions of people worldwide. Therefore, understanding the cellular function and molecular mechanism of synaptic CAMs is an important question. We are interested in understanding how individual synaptic CAMs can interact and coordinate with other synaptic proteins to exert general vs. specific synaptic effects. Using single, double and triple conditional KO (cKO) of Neuroligin genes, a class of postsynaptic CAMs, we have demonstrated that they directly influence the synaptic clustering of neurotransmitter receptors, and modulate AMPAR, NMDAR, and GABAAR –mediated basal synaptic transmissions (Chanda et al., 2017). We are currently investigating the cellular mechanisms of how different Neuroligin genes define various synaptic properties, using extensive molecular, biochemical, and electrophysiological analyses.
2. In addition, we are also broadly interested in understanding the fundamental principles of how synapses form, how they function and maintain their properties. We have previously demonstrated that synapses undergo activity-driven and neuromodulator-dependent functional changes, which last for a very short period of time (few milliseconds to seconds), also known as short-term synaptic plasticities (STPs). Using calcium imaging and electrophysiology techniques, we have demonstrated that postsynaptic AMPAR saturation and desensitization, pre- and postsynaptic neuromodulation via metabotropic GABABR and mGluR, can determine the fidelity of synaptic transmission at a model auditory synapse (Chanda et al., 2010a; Chanda et al., 2010b; Chanda et al., 2011).
Selected References:
Chanda S#, Hale WD, Zhang B, Wernig M, and Südhof TC. Unique versus redundant functions of Neuroligin genes in shaping excitatory and inhibitory synapse properties. J Neurosci (2017); PMID: 28607166. #Corresponding author.
Chanda S and Xu-Friedman MA. Excitatory modulation in the Cochlear Nucleus through group I metabotropic glutamate receptor activation. J Neurosci (2011); PMID: 21593328.
Chanda S and Xu-Friedman MA. Neuromodulation by GABA converts a relay into a coincidence detector. J Neurophysiol (2010); PMID: 20702743.
Chanda S and Xu-Friedman MA. A low-affinity antagonist reveals saturation and desensitization in mature synapses in the auditory brainstem. J Neurophysiol (2010); PMID: 20107122.
Programmed Neurogenesis & Neurodevelopment
1. In order to gain mechanistic insights into human neurodevelopment, we have recently devised novel techniques that can efficiently generate human neuronal subtypes from somatic cells (e.g. fibroblast, blood T-cell etc.) and embryonic/induced pluripotent (ES/iPS) stem cells. In this method, also known as direct neuronal reprogramming, we use proneural and lineage-specific transcription factors to impose neuronal fate into non-neuronal cells. These transdifferentiated neurons suppress donor cell programs and acquire neuronal identity quickly after transgene induction, continue to mature in vitro as evidenced by a gradual enhancement in dendritic arborization, express bona fide neuronal markers, and also exhibit functional parameters of mature neurons (e.g. action-potential firing, robust synaptogenesis, and highly reliable synaptic transmission).
2. Using cellular reprogramming approaches, we have recently identified key molecules that act as pioneering factors and promote neurogenesis from different donor cell types (e.g. Ascl1; see Chanda et al., 2014). We have also screened for accessory transcription factors that can define neural subtype specificity (e.g. Dlx2/5; see Yang et al., 2017). In addition, we have discovered molecular roadblocks that can suppress neuronal pathways and inhibit functional maturation (e.g. FoxO3; see Ahlenius et al., 2016). Moreover, we have identified molecular determinants that can protect neuronal identity by actively suppressing undesired cellular programs (e.g. Myt1l, see Mall et al., 2017). The reprogrammed neurons thus provide a unique and well-defined system to assess the functional properties of various neuronal genes in human cellular environment.
Selected References:
Yang N*, Chanda S*, Marro S, Ng YH, Janas JA, Haag D, Ang CE, Tang Y, Flores Q, Mall M, Wapinski O, Li M, Ahlenius H, Rubenstein JL, Chang HY, Buylla AA, Südhof TC, and Wernig M. Generation of pure GABAergic neurons by transcription factor programming. Nature Methods (2017); PMID: 28504679. *Co-first author.
Mall M, Kareta MS, Chanda S, Ahlenius H, Perotti N, Zhou B, Grieder SD, Ge X, Drake S, Ang CE, Walker BM, Vierbuchen T, Fuentes DR, Brennecke P, Nitta KR, Jolma A, Steinmetz LM, Taipale J, Südhof TC, and Wernig M. Myt1l safeguards neuronal identity by actively repressing many non-neuronal fates. Nature (2017); PMID: 28379941.
Ahlenius H*, Chanda S*, Webb AE, Yousif I, Karmazin J, Prusiner SB, Brunet A, Südhof TC, Wernig M. FoxO3 regulates neuronal reprogramming of cells from postnatal and aging mice. Proc Natl Acad Sci USA (2016); PMID: 27402759. *Co-first author.
Chanda S*, Ang CE*, Davila J*, Pak C, Mall M, Lee QY, Ahlenius H, Jung SW, Südhof TC and Wernig M. Generation of induced neuronal cells by the single reprogramming factor Ascl1. Stem Cell Reports (2014); PMID: 25254342. *Co-first author.
Disease Models & Pathogenic Mechanisms
1. Genetic mutations in synaptic proteins can alter their molecular functions, which may ultimately lead to severe neurological disorders including neurodegenerative diseases, cognitive dysfunctions, and intellectual disabilities. Understanding the cellular mechanisms of these pathogenic mutations can offer new avenues for therapeutic strategies. We are currently investigating how autism-associated mutations in different Neuroligin genes can differentially affect their cellular properties and contribute to impairments in synapse formation and/or function. We use electrophysiological analysis and high-resolution synaptic imaging to characterize mouse and human (reprogrammed) neurons carrying Neuroligin mutations. We demonstrated that pathogenic mutations in Neuroligin genes can directly affect the synaptic localization of neurotransmitter receptors, and cause aberrant synaptic transmission (Chanda et al., 2013; Chanda et al., 2015; Marro et al., 2019).
2. In addition to genetic mutations, exposure to teratogenic drugs during pregnancy may also interfere with fetal brain development and maturation. One such exogenous chemical is the short-chain fatty acid valproic acid (VPA). VPA is an effective and commonly prescribed drug for epilepsy and bipolar disorder. However, children born to mothers who were treated with VPA during pregnancy exhibit an increased incidence of neurological diseases, especially neural tube defects and fetal valproate syndrome, which includes severe cognitive defects and autism spectrum disorder (ASD). Using ES cell-derived human neurons, we have recently developed an in vitro platform to assess VPA-toxicity during early neurodevelopment. We have demonstrated that VPA-treatment of human neurons at early developmental stages impairs their morphological and functional maturation. These pathogenic phenotypes were mediated by VPA-induced transcriptional changes in developing neurons (Chanda et al., 2019).
Selected References:
Chanda S#, Ang CE, Lee QY, Ghebrial M, Haag D, Shibuya Y, Wernig M, and Südhof TC#. Direct reprogramming of human neurons identifies MARCKSL1 as a pathogenic mediator of valproic acid-induced teratogenicity. Cell Stem Cell (2019); PMID: 31155484. #Co-corresponding author.
Samuele G. Marro*, Soham Chanda*, Nan Yang, Justyna A. Janas, Giulio Valperga, Justin Trotter, Bo Zhou, Sean Merrill, Issa Yousif, Hannah Shelby, Hannes Vogel, M. Yashar S. Kalani, Thomas C. Südhof, and Marius Wernig. Neuroligin-4 regulates excitatory synaptic transmission in human neurons. Neuron (2019); PMID: 31257103. *Co-First Author.
Chanda S, Aoto J, Lee S, Wernig M, and Südhof TC. Pathogenic mechanism of an autism-associated Neuroligin mutation involves altered AMPA-receptor trafficking. Mol Psychiatry (2015); PMID: 25778475.
Chanda S, Marro S, Wernig M, and Südhof TC. Neurons generated by direct conversion of fibroblasts reproduce synaptic phenotype caused by autism-associated neuroligin-3 mutation. Proc Natl Acad Sci USA (2013); PMID: 24046374.