The establishment, maintenance, and repair of neuronal connections depend on coordinated neuronal responses to a variety of guidance cues that direct their projections to appropriate targets. My research is focused on understanding how connectivity in the nervous system is initially generated during neural development, how these connections are modified in the adult to provide plasticity at the structural level, and how these or compensatory connections can be reestablished following neuronal injury or degeneration. Our goal is to define the cellular and molecular logic underlying the function of neuronal guidance cues during neural development and in the adult nervous system. We also wish to know whether similar mechanisms are used in the development of non-neuronal tissues. To address these issues we use genetic, molecular, cell biological, and biochemical approaches in both invertebrates and vertebrates to understand the underlying molecular basis of neuronal growth cone guidance.
What are the extrinsic and intrinsic factors that mediate repulsive guidance events? Several families of guidance cues and their receptors sculpt developing neural circuits. One of the largest is the semaphorin family of proteins, several members of which are robust neuronal repellents. Our previous work on neural development in the fruit fly Drosophila has allowed us to define the role of semaphorin-mediated repulsion in axonal pathfinding, to understand how the output of semaphorin repulsive receptors is modulated, and to elucidate the intracellular signaling cascades that steer growth cones in response to these guidance cues. Our work on rodent models of neural development and regeneration has helped us to define semaphorin receptors, to understand the logic by which a large and diverse family of guidance cues establishes highly specific neuronal connectivity patterns in a variety of neural systems, and to address the roles played by repellents in the regulation of dendritic morphology and synapse function.
For example, to understand how guidance cues coordinately regulate the assembly of neural connections, we have focused on the relatively simple developing Drosophila nervous system. We found that plexin B (PlexB), one of two Drosophila plexin receptors, plays a key role in both central and peripheral axon pathfinding, functions that are in part complementary to those of the other Drosophila plexin, PlexA. The secreted semaphorins Sema-2a and Sema-2b, but not the related protein transmembrane Sema-1a, utilize PlexB to regulate embryonic axon guidance events. Sema-2b organizes both pre- and postsynaptic components of a specific sensory circuit. These same sensory and CNS interneuron axons are repelled by Sema-2a, providing a foundation for future work on the molecular basis of plexin attraction and repulsion. Several of our projects are devoted to understanding the intracellular signaling events that transduce these attractive and repulsive semaphorin signaling events, and ongoing work is focused on cytosolic signaling molecules downstream of plexin receptor signaling in both flies and mice.
The organization of complex connections into intricate laminar structures occurs in many neural systems. We find in the mouse that repulsive guidance plays a critical role in organizing select retinal connectivity and in directing overall retinal lamination. A central question is how laminar-specific targeting of retinal neurons within the inner plexiform layer (IPL) arises during development. We have found that a transmembrane repulsive guidance cue, the protein Sema6A, and its PlexA4 receptor organize retinal projections that normally target the outermost sublaminae of the IPL. Sema6A expression during retinal development defines a zone avoided by PlexA4-expressing amacrine cell neurites (Figure 2). This suggests that complex neuronal lamination develops through initial establishment of broad zones demarcated by guidance cues, here repellents, that are subsequently refined as the retina develops. We are interested in determining whether similar mechanisms direct laminar organization elsewhere in the CNS, and we are investigating this issue in the mammalian neocortex.
Our work on transmembrane semaphorins and their receptors in visual system retinal development has allowed us to zero in on circuits devoted to direction selectivity and image stabilization. Based on our recent observations showing that Sema6A and its plexin A receptors also regulate starburst amacrine cell morphology and their functional connections with direction selective retinal ganglion cells, we now have unique access to components of the image stabilization circuit within the brainstem and beyond, perhaps to the control of eye movement. Our current work characterizes these circuits, both anatomically and functionally, and provides insight into the logic underlying unique visual system computations initiated by distinct populations of retinal ganglion cells. These connections involve not only synapses in the retina, but also connections to retinal ganglion cell axon targets in the brain. Recent results reveal ligand-receptor interactions that are critical for the formation of these connections to brain stem targets, and their absence results in visual system connectivity and behavioral defects.
It is clear that classical neuronal guidance cues also can regulate distinct features of dendritic morphology and synapse structure. We have found that this is the case for secreted semaphorins in the mouse, which regulate dendrite growth and the development of excitatory synapses in vivo. In collaboration with David Ginty (HHMI, Harvard Medical School), we have investigated the roles played by secreted emaphoring guidance cues in CNS synapse formation and neuronal morphology in vivo. We have analyzed mice harboring targeted mutations in secreted semaphorins and their respective receptors, and we observe dramatic and selective defects in spine morphology and number in both the hippocampus and neocortex (Figure 3). This is the result of selective distribution of cell surface receptors within distinct dendritic domains, and likely underlies distinct in vivo functions of these structurally related extrinsic cues. Genome-wide association studies link certain emaphoring guidance cues and their receptors to autism disorders, motivating our recent investigation into guidance cue signaling cascades that define novel functions of these ligands in the regulation of dendritic morphology and synapse. Finally, our recent observations indicate that these guidance cues and their receptors influence not only synaptogenesis, but also synaptic plasticity. Current work addresses the direct association of these guidance cue receptors with glutamate receptors and demonstrates that these ligand-receptor interactions play critical roles in regulating homeostatic scaling responses to increases in neuronal activity.
This research is also supported by grants from the National Institutes of Health.
As of March 7, 2016