The GDNF-receptor GFR-alpha-1 : Neural expression patterns and regulation in stroke

Sammanfattning: Background: Neurotrophic factors in the mammalian nervous system are important for development and survival of neurons and for maintenance and plasticity of synaptic connections. Additionally, they have been shown to prevent neurodegeneration after damage to the central nervous system (CNS). GDNF (glial cell line-derived neurotrophic factor), a member of the TWO super-family is one such factor, derived from the glial cell line B49 and shown to have neurotrophic effects in both the central and peripheral nervous system. In addition to dopaminergic neurons, GDNF promotes survival and differentiation of several classes of non-dopaminergic neurons. A receptor termed GDNF receptor-alpha (GFR-alpha-1) is expressed in the cells that respond to GDNF. Following the cloning of GFR-alpha-1 in situ hybridization studies were performed to determine the pattern of expression of GFR-alpha-1 as well as its anatomical relationship with its signaling component, Ret. Coexistence of GFR-alpha-1 and Ret has been shown in several regions of the central nervous system, although important disparities are also evident. It appears that GDNF first binds to GFR-alpha-1, a glycosyl-phosphatidyl-inositol-linked protein, after which the GDNF/GFR-alpha-1 complex either associates with and activates Ret, or leads to Ret-independent signaling. Aims: 1. To characterize the phenotype of cells which express GFR-alpha-1 in various brain regions in the adult rat brain using anatomical techniques. 2. To study the regulation of GFR-alpha-1 in the adult rat brain in two stroke models, rats and GFR-alpha-1 +/+ and +/- mice using middle cerebral artery ligation and in situ hybridization. Results: In paper 1, we characterized the phenotype of cells which express the GFR-alpha-1 receptor in hippocampus and the dentate gyrus. Detection of GFR-alpha-1 in principal neurons and in a specific subpopulation of GABA neurons, the parvalbumin-containing neurons, suggests a complex involvement of GDNF signaling through GFR-alpha-1 in the adult hippocampus. In paper II, we studied phenotypic characteristics of GFR-alpha-1 mRNA-expressing cells in substantia nigra and the ventral tegmenal area. Neurons in these areas in which the GFR-alpha-1 gene was active were found to be both dopaminergic and GABAergic. In paper Ill, we showed that GFR-alpha-1 is expressed in different types of cells in a number of additional CNS regions. These findings will help us to better understand the nature of the target cells for, and spectrum of activity of, GDNF in several different brain regions. In paper IV, using in situ hybridization histochemistry we examined the spatial and temporal differences in upregulation of GFR-alpha-1 and c-Ret after focal ischemia. The selective and time-dependent upregulation of these receptors in different brain regions suggests that there may be regional differences in the efficacy of GDNF to offer neuroprotection in stroke. In paper V, we produced focal ischemia in GFR-alpha-1 transgenic mice for 45 min and looked at the expression of this receptor at 0, 6, 12 and 24 hours after reperfusion. Again, changes were both time-dependent and region-specific. Conclusions: GDNF and related molecules that act by binding to GFR-alpha-1 subserve multiple functions in the normal adult brain and GFR-alpha-1 availability regulates the neuroprotective actions of GDNF in ischemic stroke.