Trophic factors, neuroprotection and spinal cord repair

Abstract

Background: There is no cure for paralysis. The only drug therapy presently in use, acute high-dose methylprednisolone, has only modest effects on outcome. There is therefore a great need for new therapies. Trophic factors, capable of stimulating neurite outgrowth and supporting cell survival, are excellent candidates to be involved in repair strategies of spinal cord injuries. Members of the NGF and GDNF families play crucial roles during nervous system development, and are involved in cell survival and differentiation. In the adult nervous system they influence e.g. synapse plasticity, pain sensation; after injury these factors support survival and regeneration. The GDNF family members GDNF, NTN, PSP and ART signal through co-receptors GFR[alpha]-1, 2, 3, 4 and the proto oncogene RET.

Another trophic factor studied in the present thesis, VEGF, is best known to act on endothelial cells in blood vessels. VEGF may signal by binding to Flk-1, Flt-1 or neuropilins. Attempts to repair injured spinal cord must address several complex issues such as minimizing secondary degeneration, glial scar formation, and cyst formation; optimizing the function of surviving axon fibers and restoration of functional connections. If trophic factors are to be involved in therapies, it is likely that a combination of different factors win have to be used.

Aims: To develop spinal cord repair strategies with guidance from studies of trophic factors and their receptors in the spinal cord and related brain nuclei.

Results: The first part of the present thesis describes the cloning of two novel GDNF receptors, GFR[alpha]-2 and GFR[alpha]-3, and the cellular expression of these and related ligands in the nervous system and peripheral organs (papers I-IV). GFR[alpha]-2 mRNA was found to be widely distributed in the CNS, including spinal cord gray matter as well as in DRG neurons. In the periphery, high mRNA levels were found in the testis and kidney. NTN mRNA was notably expressed in developing striatum, cerebral cortex and brainstem. GFR[alpha]-3 mRNA was expressed mainly by peripheral ganglia and nerves.

The second part of the thesis deals with neurotrophic factor expression after different kinds of perturbations, such as physical activity (paper V) and spinal cord injury (paper VI). While sparse upregulations of neurotrophic factors were found in neurons after spinal cord injury, Schwann cells of nerve roots and meningeal cells had markedly increased NGF and GDNF mRNA levels.

Finally, different approaches to improve recovery after spinal cord injury were studied. Grafting of neurotrophic factor-producing dental pulp cells to injured spinal cord was shown to increase cell old vival and induce some nerve fiber growth (paper VII). Olfactory ensheathing cells were shown to express certain neurotrophic factors, stimulate neurite outgrowth in vitro, enhance survival and preservation of morphology of spinal cord tissue in oculo, and improve functional recovery after spinal cord injury (paper VIII). Acutely delivered VEGF improved functional recovery and decreased secondary degeneration after spinal cord contusion injury. Studies on blood vessel density, VEGF receptor expression, effects on neurite outgrowth and cell proliferation were undertaken to further characterize the effects of VEGF (paper IX).