Strategies for identifying genes that play a role in spinal cord regeneration

A search for genes that promote or block CNS regeneration requires numerous approaches; for example, tests can be made on individual candidate molecules. Here, however, we describe methods for comprehensive identification of genes up- and down-regulated in neurons that can and cannot regenerate after injury. One problem concerns identification of low-abundance genes out of the 30,000 or so genes expressed by neurons. Another difficulty is knowing whether a single gene or multiple genes are necessary. When microchips and subtractive differential display are used to identify genes turned on or off, the numbers are still too great to test which molecules are actually important for regeneration. Candidates are genes coding for trophic, inhibitory, receptor and extracellular matrix molecules, as well as unknown genes. A preparation useful for narrowing the search is the neonatal opossum. The spinal cord and optic nerve can regenerate after injury at 9 days but cannot at 12 days after birth. This narrow window allows genes responsible for the turning off of regeneration to be identified. As a next step, sites at which they are expressed (forebrain, midbrain, spinal cord, neurons or glia, intracellular or extracellular) must be determined. An essential step is to characterize proteins, their levels of expression, and their importance for regeneration. Comprehensive searches for molecular mechanisms represent a lengthy series of experiments that could help in devising strategies for repairing injured spinal cord.