Establishment of Neuronal Connectivity in the Visual System
Recently, we have discovered several neuronal guidance cues and their receptors that are critical for the establishment of neural circuits in the mammalian retina as well as between retinal ganglion cell axons and their various targets in the brain. We wish to understand how these cues serve these functions and how they regulate visual system processing.
As part of this project, we are undertaking in-depth analyses of a select class of interneuron called a star burst amacrine cell and the direction-selective retinal ganglion cells that form synapses with them. We have mouse mutants in the genes that encode these cues and receptors as well as mutants in which these neurons are genetically labeled. Work on this project will involve dynamic imaging of these cells in the intact retina, analysis of axonal trajectories in various mouse mutants, and assessment of receptor expression and subcellular distribution.
Regulation of Cortical Neuronal Morphology
Our laboratory has a long-standing interest in understanding how neuronal guidance cues, through their interactions with their receptors, orchestrate the establishment of neuronal connectivity. We have found that a secreted repulsive axonal guidance cue called semaphorin 3F (Sema3F) functions in the postnatal mouse brain to regulate dendritic spine morphology: Sema3F–/– mice show a dramatic increase in dendritic spine number and grossly altered morphology, as do mice harboring a mutation in the gene encoding the Sema3F receptor neuropilin-2 (Npn-2). This phenotype is restricted to the primary apical dendrite of layer V cortical pyramidal neurons. To understand this exquisite subcellular sequestration of the response to Sema3F via selective localization of its Npn-2 receptor, we have developed in vivo and in vitro assays to understand the basis of this key regulatory event in spine morphogenesis and synaptogenesis. We also now have evidence that this guidance cue signaling pathway influences glutamate receptor cell surface distribution and is critical for homeostatic scaling responses to changes in neuronal activity.
The project will involve the use of altered guidance cue receptors in neuronal cell culture paradigms and, with the assistance of researchers in the laboratory, in utero electroporation of mouse embryonic cortex. The goal is to uncover the molecular mechanisms underlying Npn-2 receptor localization and its neuronal morphology regulatory functions in select regions of cortical pyramidal neurons. There may also be some biochemical assessment of associations between guidance cue receptors and glutamate receptors. This work will contribute to our understanding of how neural circuits are established during postnatal cortical development. Similar experimental approaches may be used to investigate the involvement of a novel neuronal cytosolic protein that we have recently found has a role in regulating synaptogenesis and dendritic spine morphology. These research directions should provide experience with approaches directed toward understanding how connectivity is established in the developing brain.