On the influence of glia on neurite outgrowth from dopamine neurons in the nigrostriatal system

Abstract

Interactions between glial cells and neurons are important during fetal development, in the maintenance of the normal functions in tile central nervous system (CNS), arid in response to injuries or diseases. Some of die motor disturbances in Parkinson's disease, a neurodegenerative disorder where one of the hallmarks is degeneration of dopamine (DA) neurons and concomitant depletion of DA in striatum, can be alleviated by intrastriatal grafting of embryonic DA tissue.

Clinical trials of embryonic cell grafting have been hampered by availability of embryonic tissue, limited graft survival and restricted reinnervation of host striatum. Mile low cell survival can be partly overcome by engraftment of higher cell numbers and by neuroprotective and/or neurotrophic agents, the striatal reinnervation remains incomplete. The response of the adult host glial cells as well as tile role of immature glia accompanying grafted embryonic neurons needs to be elucidated. Neural stein cells, with their ability to develop into different neuronal subtypes, constitute an interesting alternative source of cells for therapeutic grafting. Information about cell induction mechanisms arid the role of glia in these processes is needed in order to develop possible future cellbased treatment strategies.

The overall objective of this thesis has been to study the influence of glia oil neurite outgrowth from DA neurons in the nigrostriatal system during development and after transplantation by using in vitro arid in vivo techniques, and in situ hybridization.

Influence of astrocyte migration arid proliferation oil dopamine neurite outgrowth from rat fetal ventral mesencephalon (VM) was studied in organotypic cultures. Two types of DA neurite Outgrowth were found, one initial transient wave of neurite formation that precedes the migration of astrocytes from the tissue slice arid is not target directed, arid a later wave of neuritic outgrowth that is closely associated with astrocytes. Different subsets of astrocytes were found to be present in the fetal VM explants arid had different effects on neurite growth. GFAP positive astrocytes are involved in axon elongation while S 100 positive astrocytes are associated with neurite branching arid also present as DA neurites innervate striatal target tissue.

Secondly, the influence of co-grafting embryonic VM cells with olfactory en-sheathing cells (OEC) or astrocytes on cell survival, striatal reinnervation and motor behaviour was investigated in the rodent 6-hydroxydopamine lesion model of Parkinson's disease. OEC are glial cells of the Olfactory nerve With unique growth promoting properties able to support regeneration of axons when grafted to sites of corticospinal injury. Adding OEC to the VM grafts enhanced striatal reinnervation and functional restoration through a mechanism that is not exclusively cell-cell contact dependent.

Thirdly, tile spatiotemporal expression of glia and DA neuron specific genes was investigated in the embryonic human VM by in situ hybridization. It was found that early glia with characteristics of radial glia arid immature astrocytes were present from an early time point of mesencephalic development.

Taken together, this thesis has shown that glial cells have profound effects oil DA neurons throughout development arid after transplantation. Clarifying mechanisms behind Such effects may help in the development of cell-based treatments arid protective strategies for Parkinson's disease.