Molecular mechanisms of inhibitory synapse formation

In the mammalian central nervous system, two main types of synapses – glutamatergic and GABAergic – play opposing roles in exciting or inhibiting the postsynaptic cell. While it is critical that upon cell-cell contact, new synapses form the correct postsynaptic specialization (excitatory or inhibitory), it remains a fundamental mystery in developmental neurobiology which molecular pathways specify this identity. Synapse formation is canonically characterized by several steps: initial contact, accumulation of pre- and post-synaptic molecular machinery, stabilization of the synaptic contact followed by maturation, and in some cases elimination by pruning. Synapse formation is regulated at least in part by trans-synaptic ligand/receptor partners that belong to protein families (e.g. Neuroligins/Neurexins, Ephrins/Ephs, Semaphorins/Plexins) which have been demonstrated to regulate both excitatory and inhibitory synapse formation (Sudhof, 2018).

Semaphorins are a large family of secreted and transmembrane proteins (Pasterkamp and Giger, 2009); Semaphorins and their receptors (Plexins and Neuropilins) have been implicated in a variety of developmental processes including retinal lamination, neuronal migration, and vascular and heart morphogenesis (Kruger et al., 2005; Matsuoka et al., 2011; Tran et al., 2007; Yazdani and Terman, 2006). Studies from our lab (Acker et al., 2018; Kuzirian et al., 2013; McDermott et al., 2018; Raissi et al., 2013) and others have demonstrated that Semaphorins and their receptors are critical mediators of synaptogenesis (Ding et al., 2012; Duan et al., 2014; Joo et al., 2013; Mizumoto and Shen, 2013; O'Connor et al., 2009; Tran et al., 2009; Uesaka et al., 2014). In particular, Sema4D and Plexin-B1 are transmembrane proteins that are broadly expressed in hippocampal neurons (McDermott et al., 2018); interaction between Sema4D and Plexin-B1 activates the Plexin-B1 intracellular GTPase activating protein (GAP) domain, triggering downstream signal transduction events (Pasterkamp, 2012; Tran et al., 2009). Our functional studies demonstrate that Sema4D signals through its extracellular Sema domain to promote GABAergic synapse formation via the Plexin-B1 receptor (Kuzirian et al., 2013; McDermott et al., 2018), possibly via an interaction with Plexin-B1 expressed by both the pre- and postsynaptic neurons.

Specifically we demonstrated that addition of the soluble, extracellular domain (ECD) of Sema4D is sufficient to initiate GABAergic synapse formation in rodent hippocampus within 30 minutes and further, that these newly formed synapses appear functional within 3 hours of Sema4D addition both in vivo and in vitro (Acker et al., 2018; Kuzirian et al., 2013). This effect requires the presence of Plexin-B1 (Kuzirian et al., 2013), a high-affinity receptor for Sema4D (Tamagnone et al., 1999). In addition, live-imaging studies from the Wierenga lab demonstrated that Sema4D addition to hippocampal slice stabilizes transient GABAergic pre-synaptic boutons on a similarly rapid time scale (Frias et al., 2019). Importantly, Sema4D signaling has no effect on glutamatergic synapse formation (Kuzirian et al., 2013; McDermott et al., 2018; Paradis et al., 2007). Thus, our discovery represents the first description of a ligand-receptor pair that selectively regulates GABAergic synapse formation on a rapid time-scale.

In addition to our studies of Sema4D/Plexin-B1 signaling, we also showed that the ECD of Sema4A promotes formation of either inhibitory and excitatory synapses via signaling through the Plexin-B1 and Plexin-B2 receptors, respectively (McDermott et al., 2018). The distinct functions of the Sema4 ligands and Plexin-B receptors present us with a unique opportunity to discover the signal transduction mechanisms that differentially instruct Sema4/Plexin-B to promote excitatory versus inhibitory synapse formation in the hippocampus.