Genetic dissection of models for neuroinflammation and neurodegeneration

Abstract

Many central nervous system (CNS) diseases are characterized by inflammation and nerve cell loss, but the exact relationship between these phenomena is not known. The complex etiology of CNS disorders includes a genetic component. MHC class II molecules are key mediators of immune activation and variations in MHC class II genes are the main genetic determinant of several complex autoimmune disorders. Gene regions regulating complex phenotypes can be mapped in experimental animal crosses. In this thesis, the genetic regulation of the response to mechanical nerve injury and experimental autoimmune encephalomyelitis (EAE) was investigated in populations of intercrossed mice and rats.

The effect of a previously identified gene region, Vra4, on MHC class II expression was characterized in DA.PVG1av1-Vra4 and PVG1av1DA-Vra4 congenic rat strains after ventral root avulsion (VRA). Vra4 contains the class II transactivator gene, Mhc2ta, which is a transcription factor for MHC class II. The influence of the same region was tested in EAE. The results show that Vra4 regulates MHC class II on microglia after VRA, as well as risk and severity of EAE. In addition, IFN-g inducible class II expression on antigen presenting cells (APCs) is dependent on the Vra4 region. Similar results were obtained in a study of inbred mouse strains, where differential MHC class II expression was observed in the facial nucleus after axotomy of the facial nerve.

Congenic strain experiments and sequencing of C2ta strongly indicate that polymorphisms in the regulatory region of the pI promoter are regulating this trait. Vra4/C2ta had no effect on expression of microglial markers, co-stimulatory molecules or MHC class I, nor T cell infiltration. Additional genetic influence on MHC class II expression was mapped to chromosomes 1 and 7 in a F2 cross between BN and LEW.1N rats, two strains which share Mhc2ta haplotype but display differential MHC class II expression after VRA. Analysis of other inflammatory markers in this cross revealed common regulation of several immune related molecules by the same gene region, which may suggest upstream effects.

Finally, the genetic impact on nerve cell death following VRA by two gene regions previously detected in a F2(DAxPVGc) cross, Vra1 and Vra2, was fine mapped in 2 generations of an advanced intercross line (AIL) between DA and PVG1av1 rats, as well as in a panel of Vra1 congenic strains. The effect of Vra1 on neurodegeneration was reproduced in both AIL populations. Increased support was given by the congenic strains, where PVG alleles in the Vra1 region on DA background resulted in significantly reduced neuronal loss. These studies also narrowed down the Vra1 region from 54 to 9 Mb. Vra2 displayed suggestive linkage to neurodegeneration only in one AIL cohort, but showed an additive effect on the phenotype together with Vra1.

To conclude, these results show that neuroinflammation and neurodegeneration are influenced by genetic factors. Identification of genes and pathways will increase our understanding of the molecular pathways of human complex disease.