Glial Cells, Proteoglycans and the Regulation of Axonal Growth

Ph.D. Washington University

One of the most critical events in the life of a developing neuron is the growth of an axonal processes. If a developing neuron fails to grow an axon or if that axon fails to make a synaptic connection with its appropriate target, that neuron dies. How axons find their target cells remains an essential question in developmental neurobiology. Moreover, an understanding of the mechanisms that allow neurons to grow axons during development will aid in the design of therapies that encourage damaged axons to regenerate.

Proteoglycans comprise one class of extracellular and cell surface molecules that can modulate axonal growth. One of several proteoglycans currently under study in the lab is a high molecular weight chondroitin-sulfate proteoglycan known as NG2. NG2 is expressed on the surfaces of bipotential glial precursor cells in developing and mature animals. When tissue culture surfaces are coated with the NG2 proteoglycan, neurons do not adhere to these surfaces. When surfaces are coated with a mixture of NG2 and cell adhesion molecules known to promote axonal growth, the growth of axons is inhibited relative to substrates containing only the cell adhesion molecules. Thus, the NG2 proteoglycan can inhibit axonal outgrowth. These in vitro observations suggest that the glial cells which carry the NG2 proteoglycan in vivo can provide negative guidance cues to growing axons.

We are currently studying the mechanisms by which NG2 inhibits axonal growth. We have identified a putative neuronal receptor for the NG2 core protein and are studying how activation of this receptor leads to an inhibition of axonal extension. Molecular biological techniques are being used to map functional domains of the NG2 core protein and to study the ability of these different domains to interact with the neuronal cell surface. The peptides generated in these studies will be used as probes to better identify and ultimately clone the NG2 receptor.

In the adult brain, the NG2 proteoglycan is expressed on the surfaces of a class of oligodendroglial precursor cells known formerly as O2A progenitor cells. Why the adult brain should contain a large population of oligodendrocyte precursor cells is unknown. One clue to these functions of these cells comes from our ongoing studies of brain injury and nerve regeneration. When the brain is experimentally damaged, oligodendrocyte precursor cells begin to synthesize DNA and up-regulate their expression of the NG2 proteoglycan. Our current studies seek to define the cell-cell interactions and chemical signals that trigger the injury-induced responses of oligodendrocyte precursor cells and to better define the role of these cells during experimental models of neurodegenerative diseases.

Selected Publications

• Levine, J.M. (1994) Increased expression of the NG2 proteoglycan after brain injury. J. Neurosci., 14:4716-4730.
  
  
• Dou, C.L. and J.M. Levine (1994) Inhibition of neurite growth by the NG2 chondroitin sulfate proteoglycan. J. Neurosci., 14:7616-7628.
  
  
• Dou, C.L. and J.M. Levine (1995) Differential effects of glycosaminoglycans on neurite growth on L1 and laminin substrates. J. Neurosci. 15(12). 1995. 8053-8066.