Project Summary:
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), the molecular pathways
specifying this identity remain a fundamental mystery in developmental neurobiology. This process is
presumably regulated 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.
Semaphorins (Semas) and Plexins are families of widely expressed and functionally versatile transmembrane or
secreted ligands and their transmembrane receptors that first gained attention for their roles in axon guidance
during nervous system development. Class 4 Semas and Plexin-B receptors are expressed in mammalian
hippocampus (in excitatory and inhibitory neurons and in glia) and promote synapse formation both during
development and in adulthood.
Our previous studies revealed Sema4D to be one of few molecules having synaptogenic function restricted to
inhibitory synapses; the extracellular domain of Sema4D induces inhibitory synapse formation on a rapid
timescale (~30 mins) through the Plexin-B1 receptor. Additionally, Sema4A promotes formation of both
inhibitory and excitatory synapses in hippocampus via the Plexin-B1 or Plexin-B2 receptors, respectively. The
ability to rapidly drive synapse formation by application of Sema4A or Sema4D, as well as the observations that
these Semas regulate inhibitory and excitatory synapse formation in unique ways requiring different Plexin-B
receptors, are central to our proposed strategy to identify the distinct signaling conformations that
instruct synapse identity (excitatory or inhibitory) downstream of Plexin-B receptor
engagement. Further, we previously showed that Sema4D protein application increases
resilience to seizure in adult mice in vivo, a finding that supports the potential clinical impact of
this work.
The proposed research uses a combination of transgenic mice, organotypic hippocampal slice culture, gene
knockdown, and expression of mutant forms of Plexin-B receptors to examine the divergent roles of 1) unique
Plexin-B signaling domains, 2) a novel mechanism gating Plexin-B1 signaling in cis, and 3) contributions from
Plexin-B coreceptors, in excitatory and inhibitory synapse formation promoted by class 4 semaphorins.
