Towards cell replacement therapy in Parkinson’s disease. Proteoglycans and Nogo-A as modulators of axonal growth in midbrain dopaminergic neurons

Abstract: Parkinson’s disease (PD) is currently the second most common neurodegenerative disorder (after Alzheimer’s disease). PD is diagnosed on its motor symptoms, which include akinesia, bradykinesia, rigidity, postural imbalance and resting tremor. It is well established that the motor symptoms develop due to lack of dopamine in the striatum. This is caused by progressive degeneration of dopaminergic neurons (DNs) in the midbrain (substantia nigra pars compacta), which innervate the striatum. Although initially efficacious, the therapeutic effect of various anti-parkinsonian pharmacological treatments is limited to five to ten years. Hence, the new therapeutic strategies for PD are highly warranted. The concept of cell replacement therapy in PD is based on the idea that the diminished dopamine levels in the striatum would be restored by the functional DNs, obtained from donated, electively aborted fetuses, and subsequently injected into the patients’ striatum. In several clinical trials, about 400 patients received such therapy worldwide. Although the therapeutic effects varied, some individuals experienced major motor improvement even for up to 16 years. Other PD patients did not respond so favorably and even developed adverse effects, i.e. graft-induced dyskinesia. The clinical trials are currently on hold. In the first project, we have analyzed the survival and integration of the graft in a patient who underwent the transplantation in Lund, in 1987, as one of the first transplanted cases in the world. Clinically, the patient did not show the symptomatic relief in response to the transplantation. Examining the postmortem brain, we observed a very small surviving graft. In addition, we also detected signs of PD-like pathology in the transplanted DNs. Nonetheless, the graft survived in this brain for 22 years and such a long graft-life has never before been reported. It is certain that embryonic tissue will not be routinely used as a cell source for transplantation therapy in PD. Yet, stem cells bear a great potential for future PD therapy. Recent publications show that DNs of the midbrain subtype could be differentiated from stem cells in a time-efficient manner. Moreover, those DNs could form well surviving tumor-free grafts which reverted the PD-like motor symptoms in animal models. Nonetheless, the differentiation of embryonic stem cells to DNs is still not fully understood and therefore difficult to control. Proteoglycans may be engaged in differentiation of DNs, as they govern the formation of the nervous system in developing vertebrate embryo. My second project aimed at defining the genes encoding proteoglycans and the enzymatic machinery fine-tuning their structure during the DN differentiation. From around 2000 proteoglycan-related genes, we identified two (neurocan and HS3ST5) that potentially could enhance the differentiation efficiency of DNs from stem cells. Our results can serve as a starting point for the further functional studies. After transplantation, the grafted DNs have to survive, and also integrate with the host brain, i.e. grow neurites, form synapses and release dopamine in a regulated fashion. In the third project, we studied how Nogo-A protein affects the survival and neurite growth in DNs. Nogo-A is a strong growth-inhibitory protein in the brain and spinal cord. Interestingly, in recent years, the growth- and survival-supportive role of Nogo-A in various types of neurons has been shown. We have demonstrated here for the first time the Nogo-A roles in the substantia nigra pars compacta DNs. A careful analysis of all data coming from initial clinical trials, studies on animal parkinsonian models, and the emergence of recent safe and effective dopaminergic differentiation protocols, collectively imply that the cell replacement approach in PD holds a great therapeutic potential. Understanding of the mechanisms governing the DNs differentiation, survival and neurite growth will help develop safe and efficient cell replacement therapies in PD. The work presented in this thesis will hopefully contribute to such advancement.