Severe injury may result in disconnection between the peripheral and central nervous system. Regeneration of the central portion of sensory neurons into the spinal cord is notoriously poor in adult mammals, with low regenerative drive and an unpermissive central environment, most likely resulting in persistent loss of sensory function. A variety of strategies have been addressedto augment regeneration, including application of growth promoting factors, counteraction of inhibitory molecules, and provision of growth permissive substrates. Stem cells have been investigated in these contexts, as well as for the possibility of providing new neurons to act as a relay between the periphery and spinal cord. Here we have investigated different sources of neural stem cells for their ability to form neurons and glia after transplantation to the periphery; to project axons into the spinal cord; and to assist regeneration of surviving sensory neurons. These have been performed at two locations: the "dorsal root ganglion cavity", and the transitional zone following dorsal root avulsion. Neurons and glia were generated form mouse boundary cap neural crest stem cells and embryonic stem cell derived ventral spinal cord progenitors, and in addition to this, regeneration of sensory fibers was observed after transplantation of human fetal spinal cord derived progenitors and human embryonic stem cell derived ventral spinal cord progenitors. Further, delivery of neurotrophic factor mimetics via mesoporous silica nanoparticles proved a valuable tool for stem cell survival and differentiation. While technological advances make in vivo differentiation a realistic goal, our findings indicate that so far assisting regeneration of host sensory fibers to reconnect with the spinal cord by transplantation of stem cells is a more reliable strategy.

Injury and neurodegenerative conditions of the spinal cord can lead to paralysis and loss of sensation. Cell therapeutic approaches can restore sensory innervation of the spinal cord following injury and protect spinal cord cells from degeneration. This thesis primarily focuses on the restoration of deaffarented sensory fibres following injury to the dorsal root and spinal cord. These injuries lead to the formation of a non-permissive glial scar that prevents sensory axons from reinnervating spinal cord targets. It takes advantage of a dorsal root injury model that closely mimics spinal root avulsion injuries occurring in humans. In the first part of the thesis, three different neural progenitor types from human or murine sources are tested for their regenerative properties following their transplantation to the site of dorsal root avulsion injury. In the second part, the ability of murine neural progenitors to protect spinal motor neurons from a neurodegenerative process is tested. In the first original research article, I show that human embryonic stem cell derived neural progenitors are able to restore sensorimotor functions, mediated by the formation of a tissue bridge that allows ingrowth of sensory axons into the spinal cord. In the second research article, I present that murine boundary cap neural crest stem cells, a special type of neural progenitor that governs the entry of sensory axons into the spinal cord during development, are unable to form a permissive tissue bridge. This is possibly caused by the contribution of transplant derived ingrowth non-permissive glial cells. In the third research article, I show that human neural progenitors derived from foetal sources are capable of stimulating sensory ingrowth and that they ameliorate the glial scar. When this approach is combined with the delivery of sensory outgrowth stimulating neurotrophic factors, these cells fail to form a permissive tissue bridge and fail to modify the glial scar. In the final research article, murine boundary cap neural crest stem cells are shown to protect motor neurons, which harbor an amyotrophic lateral sclerosis causing mutation, from oxidative stress. Oxidative stress is a pathological component of amyotrophic lateral sclerosis in human patients. Taken together, this thesis provides first evidence that sensory regeneration following a spinal root avulsion injury can be achieved by transplantation of human neural progenitors. In addition, it introduces murine boundary cap neural crest stem cells as interesting candidates for the cell therapeutic treatment of amyotrophic lateral sclerosis.