Effects of insulin like growth factor-1 (IGF-1) Over Expression on Purkinje Cell Survival in SCA-I Transgenic Mice.

This is a summary of research funded by NAF in 2000.

Insulin like growth factor-1 (IGF-1) belongs to a group of protein molecules present in the nervous system that have an important role in the maintenance and growth of nerve cells. IGF-1 and its receptors are abundant in the cerebellum Purkinje cells make IGF-1, and IGF-1 is also produced by other nerve cells that project on to Purkinje cells. In Purkinje cells, IGF-1 seems to have an important role in keeping cells healthy, which includes regulation of calcium levels and production of calcium binding protein calbindin.

IGF-1 and its related molecules are also involved in neurodegenerative processes in which IGF-1 dependent cell functions are affected. Our previous studies have shown that the process of nerve cell death in human then known as olivopontocerebellar atrophy (OPCA), and in lurcher mutant mouse is associated with changes in IGF-1 receptor functions. Other researchers have shown that patients with late-onset cerebellar ataxias with degeneration in their IGF-1 containing neurons also show marked changes in the peripheral IGF system. Further, Purkinje cells in Purkinje cell degeneration (PCD) mice also contain significantly low levels of IGF-1 suggesting the involvement of IGF-1 related processes in nerve cell degeneration.

Therefore, the present study was initiated to test the hypothesis that the over-production of IGF-1 will rescue SCA-1 Purkinje cell from degeneration. To achieve this, double mutant mice were generated by mating SCA-1 transgenic mice with IGF-1 transgenics. These IGF-1 transgenic mice are genetically manipulated to make more IGF-1 as compared to normal mice, and have an increase in their brain weight and number of Purkinje cells in the cerebellum. The SCA-1 transgenic mice were obtained from Drs. Harry Orr and Huda Zoghbi. The IGF-1 transgenic mice were obtained from Dr. D' Ercole at the University of North Carolina at Chapel Hill. The double mutants survived after birth, and various biochemical, histochemical and behavioral studies were conducted on these double mutants and littermate SCA-1 transgenic mice to test our proposed hypothesis.

We looked at balance in the double mutants, littermate SCA-1 transgenic, IGF-1 transgenic and normal (with no mutation) mice. These studies were done at the age of five weeks. We measured how long an animal can stay on the rotating rod. The double mutants performed remarkably well as compared to the SCA-1 transgenic mice, but had somewhat lower scores than the normal mice. We looked at the shape, size, and production of calcium binding proteins in Purkinje cells of double mutants at various ages after birth. Purkinje cells in double mutants showed delayed SCA-1 pathology. The immunostaining of calcium binding proteins calbindin and parvalbumin in Purkinje cells was also compared between the double mutants and littermate SCA-1 transgenic mice. At seven weeks, double mutants showed stronger parvalbumin staining in Purkinje cell bodies and their processes as compared to the SCA-1 transgenics. The interneurons also stained positive for parvalbumin and had more intense localization of this protein than what was observed in SCA-1 transgenics Calbindin in 13-weekold double mutants was widely distributed throughout the dendritic tree of Purkinje cells. Whereas, in littermate SCA-1 transgenics the staining was limited to the lower branches of the tree in most of Purkinje cells. However, even in double mutants, the intensity of overall staining of calbindin was lower than that seen in normal mice suggesting that the adverse effects of ataxin-1 were still present. The other marker proteins for Purkinje cells also showed marked improvement in the immunostaining suggesting that Purkinje cells in double mutants were healthier than those in SCA-1 transgenic mice. To further confirm these observations the bio-chemical experiments are being performed. Also, we will be further analyzing these double mutants, as they grow older.

These studies indicate that IGF-1 may have a protective role in ataxin-1 mediated Purkinje cell degeneration. IGF-1 and/or the drugs that could mimic the effects of IGF-1 could be potential candidates for devising treatment strategies in patients with SCA-1.

We are grateful for the support of the National Ataxia Foundation as we continue to understand the mechanisms of nerve cell degeneration in cerebellar ataxias.

Parminder J. S. Vig, PhD University of Mississippi Medical Center Jackson, MS