Charles Greer, PhD

We study mechanisms mediating the complex pathfinding and synaptogenesis of axons and dendrites during early development and how those mechanisms may degrade during aging. In the olfactory system, ~1,000 subpopulations of sensory neurons express different odor receptors. The sensory neuron axons segregate in the olfactory bulb based on odor receptor expression, producing a highly specific molecular map. Understanding the molecular basis of this segregation of axons is one of our primary goals.
In parallel, we study the targeting and differentiation of dendrites to understand the mechanisms that regulate their highly specific interactions with small subsets of the sensory neuron axons. Unique to the olfactory system, ongoing adult neurogenesis generates new populations of sensory neurons in the olfactory epithelium and interneurons centrally. The molecular differentiation and integration of these adult generated neurons into synaptic circuits is an ongoing interest in the lab.

Current Research Program

A major goal of my laboratory is understanding the basic mechanisms that contribute to the establishment of orderly topographic maps within the CNS, during both normal development and during regenerative events following injury or disease. We have been focusing our efforts on the olfactory system, in part because of the complexity of the map between the olfactory epithelium and the olfactory bulb. Axons sort into 1,000 functional subsets that are targeted with high specificity to olfactory bulb glomeruli.

Using both in vivo and in vitro models, we are currently isolating several mechanisms that may contribute to this complex reorganization including laminin, tenascin and the expression of putative odor receptors in growth cones. In related studies we continue to characterize a unique population of glial cells, ensheathing cells, found in the olfactory nerve. While elsewhere in the mature CNS glia are an impediment to axon growth, the ensheathing cell glia support axon extension and targeting throughout life.

We recently demonstrated that the growth promoting effects of ensheathing cells are not limited to olfactory receptor neurons but are also seen in other populations of neurons. Particularly exciting, our recent studies demonstrate that the ensheathing cells remain pluri-potential and that when implanted into demyelinated spinal cord can adopt a myelinating phenotype which remyelinates the axons and contributes to a restoration of normal conduction velocities.

In parallel studies we are examining the molecular and synaptic organization of the olfactory bulb glomeruli. Using RT-PCR we are mapping the distribution of subsets of olfactory receptor cell axons in glomeruli to gain insights into the topography of odor-ligand maps in the olfactory bulb. In addition, working with colleagues, we are using a GFP tag to test hypotheses regarding the specificity of synaptic organization within glomeruli. Second, using antibodies synaptic vesicle related proteins and confocal microscopy we have begun to describe a hitherto unrecognized segregation of local and projection synaptic circuits in the glomeruli.

It may be that this segregation underlies the lateral inhibitory systems that are believed to be operative in the olfactory system. Beyond my colleagues in Neurosurgery, I maintain active collaborations with the following Departments at Yale: Neurology, Neurobiology, Anesthesiology and Ophthamology. In addition, I have collaborative studies with members of the faculty at Columbia University, Emory University, The Rockefeller, University of Maryland and University of Colorado.

Relationship of Research to Neurological Disease
Increasingly, the neurological sciences are focusing on intervention strategies that will both limit the debilitating consequences of injury/disease as well as increase the probability of successful regeneration of pathways and connections in the CNS. In order to facilitate these processes it is necessary for us to understand the molecular and cellular events operative during axon extension, target selection and synapse formation. The studies described above directly assess those questions and, particularly in the case of the ensheathing cells, offer the possibility of clinical application in the near future.

Fate mapping of olfactory bulb projection neurons

The role of axon:axon adhesion in establishing sensory maps

Cell surface and diffusible molecules influencing the extension and convergence of axons

The timing and molecular mechanisms mediating the development of 3-layer piriform cortex