Karin Harnesk's research while affiliated with Karolinska Institutet and other places

Activation of the complement system has been implicated in both acute and chronic states of neurodegeneration. However, a detailed understanding of this complex network of interacting components is still lacking. Large-scale global expression profiling in a rat F2(DAxPVG) intercross identified a strong cis-regulatory influence on the local expression of complement receptor 2 (Cr2) in the spinal cord after ventral root avulsion (VRA). Expression of Cr2 in the spinal cord was studied in a separate cohort of DA and PVG rats at different time-points after VRA, and also following sciatic nerve transection (SNT) in the same strains. Consequently, Cr2 −/− mice and Wt controls were used to further explore the role of Cr2 in the spinal cord following SNT. The in vivo experiments were complemented by astrocyte and microglia cell cultures. Expression of Cr2 in naïve spinal cord was low but strongly up regulated at 5–7 days after both VRA and SNT. Levels of Cr2 expression, as well as astrocyte activation, was higher in PVG rats than DA rats following both VRA and SNT. Subsequent in vitro studies proposed astrocytes as the main source of Cr2 expression. A functional role for Cr2 is suggested by the finding that transgenic mice lacking Cr2 displayed increased loss of synaptic nerve terminals following nerve injury. We also detected increased levels of soluble CR2 (sCR2) in the cerebrospinal fluid of rats following VRA. These results demonstrate that local expression of Cr2 in the central nervous system is part of the axotomy reaction and is suggested to modulate subsequent complement mediated effects.

The complement system is activated in a wide spectrum of CNS diseases and is suggested to play a role in degenerative phenomena such as elimination of synaptic terminals. Still, little is known of mechanisms regulating complement activation in the CNS. Loss of synaptic terminals in the spinal cord after an experimental nerve injury is increased in the inbred DA strain compared with the PVG strain and is associated with expression of the upstream complement components C1q and C3, in the absence of membrane attack complex activation and neutrophil infiltration. To further dissect pathways regulating complement expression, we performed genome-wide expression profiling and linkage analysis in a large F2(DA × PVG) intercross, which identified quantitative trait loci regulating expression of C1qa, C1qb, C3, and C9. Unlike C1qa, C1qb, and C9, which all displayed distinct coregulation with different cis-regulated C-type lectins, C3 was regulated in a coexpression network immediately downstream of butyrylcholinesterase. Butyrylcholinesterase hydrolyses acetylcholine, which exerts immunoregulatory effects partly through TNF-α pathways. Accordingly, increased C3, but not C1q, expression was demonstrated in rat and mouse glia following TNF-α stimulation, which was abrogated in a dose-dependent manner by acetylcholine. These findings demonstrate new pathways regulating CNS complement expression using unbiased mapping in an experimental in vivo system. A direct link between cholinergic activity and complement activation is supported by in vitro experiments. The identification of distinct pathways subjected to regulation by naturally occurring genetic variability is of relevance for the understanding of disease mechanisms in neurologic conditions characterized by neuronal injury and complement activation.

Local CNS inflammation takes place in many neurological disorders and is important for autoimmune neuroinflammation. Microglial activation is strain-dependent in rats and differential MHC class II expression is influenced by variations in the Mhc2ta gene. Despite sharing Mhc2ta and MHC class II alleles, BN and LEW.1N rats differ in MHC class II expression after ventral root avulsion (VRA). We studied MHC class II expression and glial activation markers in BN rats after VRA. Our results demonstrate that MHC class II expression originates from a subpopulation of IBA1(+), ED1(-), and ED2(-) microglia. We subsequently performed a genome-wide linkage scan in an F2(BNxLEW.1N) population, to investigate gene regions regulating this inflammatory response. Alongside MHC class II, we studied the expression of MHC class I, co-stimulatory molecules, complement components, microglial markers and Il1b. MHC class II and other transcripts were commonly regulated by gene regions on chromosomes 1 and 7. Furthermore, a common region on chromosome 10 regulated expression of complement and co-stimulatory molecules, while a region on chromosome 11 regulated MHC class I. We also detected epistatic interactions in the regulation of the inflammatory process. These results reveal the complex regulation of CNS inflammation by several gene regions, which may have relevance for disease.

Human genes, homologous with genes in smallest congenic fragment on rat chromosome 8. Diseases associated with the human genes are given in the right column. List of human homologues to the rat genes located within the R3 congenic fragment. No association of any human gene to complex neurodegenerative disease. (0.11 MB DOC)

Major histocompatibility complex (MHC) class II is of critical importance for the induction of immune responses. Levels of MHC class II in the nervous system are normally low, but expression is up-regulated in many disease conditions. In rat and human, variation in the MHC class II transactivator gene (C2ta) is associated with differential expression of MHC class II and susceptibility to autoimmune disease. Here we have characterized the response to facial nerve transection in 7 inbred mouse strains (C57BL/6J, DBA/2J, 129X1/SvJ, BALB/cJ, SJL/J, CBA/J, and NOD). The results demonstrate differences in expression of C2ta and markers for MHC class I and II expression, glial activation, and T cell infiltration. Expression levels of C2ta and Cd74 followed similar patterns, in contrast to MHC class I and markers of glial activation. The regulatory region of the C2ta gene was subsequently sequenced in the four strains (C57BL/6/J, DBA/2J, SJL/J and 129X1/SvJ) that represented the phenotypical extremes with regard to C2ta/Cd74 expression. We found 3 single nucleotide polymorphisms in the type I (pI) and type III (pIII) promoters of C2ta, respectively. Higher expression of pI in 129X1/SvJ correlated with the pI haplotype specific for this strain. Furthermore, congenic strains carrying the 129X1/SvJ C2ta allele on B6 background displayed significantly higher C2ta and Cd74 expression compared to parental controls. We conclude that genetic polymorphisms in the type I promoter of C2ta regulates differential expression of MHC class II, but not MHC class I, Cd3 and other markers of glial activation.

Damage to nerve cells and axons leading to neurodegeneration is a characteristic feature of many neurological diseases. The degree of genetic influence on susceptibility to axotomy-induced neuronal death has so far been unknown. We have examined two gene regions, Vra1 and Vra2, previously linked to nerve cell loss after ventral root avulsion in a rat F2 intercross between the DA and PVG inbred rat strains. In this study, we use two generations (G8 and G10 cohorts) of an advanced intercross line between DA and PVG(av1) to reproduce linkage to Vra1 and to fine-map this region. By isolating the effect from Vra1 in congenic strains, we demonstrate that Vra1 significantly regulates the loss of motoneurons after avulsion. The regulatory effect mediated by Vra1 thus resides in a congenic fragment of 9 megabases. Furthermore, we have used the advanced intercross lines to give more support to Vra2, originally detected as a suggestive QTL. The results demonstrated here show that naturally occurring allelic variations affect susceptibility to axotomy-induced nerve cell death. Vra1 and Vra2 represent the first quantitative trait loci regulating this phenotype that are characterized and fine mapped in an advanced intercross line. In addition, congenic strains provide experimental evidence for the Vra1 effect on the extent of injury-induced neurodegeneration. Identification of the underlying genetic variations will increase our understanding of the regulation and mechanisms of neurodegeneration.

Presentation of Ag bound to MHC class II (MHC II) molecules to CD4+ T cells is a key event in adaptive immune responses. Genetic differences in MHC II expression in the rat CNS were recently positioned to allelic variability in the CIITA gene (Mhc2ta), located within the Vra4 locus on rat chromosome 10. In this study, we have examined reciprocal Vra4-congenic strains on the DA and PVGav1 backgrounds, respectively. After experimental nerve injury the strain-specific MHC II expression on microglia was reversed in the congenic strains. Similar findings were obtained after intraparenchymal injection of IFN-gamma in the brain. Expression of MHC class II was also lower on B cells and dendritic cells from the DA.PVGav1-Vra4- congenic strain compared with DA rats after in vitro stimulation with IFN-gamma. We next explored whether Vra4 may affect the outcome of experimental autoimmune disease. In experimental autoimmune encephalomyelitis induced by immunization with myelin oligodendrocyte glycoprotein, DA.PVGav1-Vra4 rats displayed a lower disease incidence and milder disease course compared with DA, whereas both PVGav1 and PVGav1.DA-Vra4 rats were completely protected. These results demonstrate that naturally occurring allelic differences in Mhc2ta have profound effects on the quantity of MHC II expression in the CNS and on immune cells and that this genetic variability also modulates susceptibility to autoimmune neuroinflammation.