Αlpha-Synuclein as a Mediator in the Interplay between Aging and Parkinson's Disease.

Accumulation and misfolding of the alpha-synuclein protein are core mechanisms in the pathogenesis of Parkinson's disease. While the normal function of alpha-synuclein is mainly related to the control of vesicular neurotransmission, its pathogenic effects are linked to various cellular functions, which include mitochondrial activity, as well as proteasome and autophagic degradation of proteins. Remarkably, these functions are also affected when the renewal of macromolecules and organelles becomes impaired during the normal aging process. As aging is considered a major risk factor for Parkinson's disease, it is critical to explore its molecular and cellular implications in the context of the alpha-synuclein pathology. Here, we discuss similarities and differences between normal brain aging and Parkinson's disease, with a particular emphasis on the nigral dopaminergic neurons, which appear to be selectively vulnerable to the combined effects of alpha-synuclein and aging.

Nigrostriatal overabundance of α-synuclein leads to decreased vesicle density and deficits in dopamine release that correlate with reduced motor activity.

α-Synuclein (α-syn) is a presynaptic protein present at most nerve terminals, but its function remains largely unknown. The familial forms of Parkinson's disease associated with multiplications of the α-syn gene locus indicate that overabundance of this protein might have a detrimental effect on dopaminergic transmission. To investigate this hypothesis, we use adeno-associated viral (AAV) vectors to overexpress human α-syn in the rat substantia nigra. Moderate overexpression of either wild-type (WT) or A30P α-syn differs in the motor phenotypes induced, with only the WT form generating hemiparkinsonian impairments. Wild-type α-syn causes a reduction of dopamine release in the striatum that exceeds the loss of dopaminergic neurons, axonal fibers, and the reduction in total dopamine. At the ultrastructural level, the reduced dopamine release corresponds to a decreased density of dopaminergic vesicles and synaptic contacts in striatal terminals. Interestingly, the membrane-binding-deficient A30P mutant does neither notably reduce dopamine release nor it cause ultrastructural changes in dopaminergic axons, showing that α-syn's membrane-binding properties are critically involved in the presynaptic defects. To further determine if the affinity of the protein for membranes determines the extent of motor defects, we compare three forms of α-syn in conditions leading to pronounced degeneration. While membrane-binding α-syns (wild-type and A53T) induce severe motor impairments, an N-terminal deleted form with attenuated affinity for membranes is inefficient in inducing motor defects. Overall, these results demonstrate that α-syn overabundance is detrimental to dopamine neurotransmission at early stages of the degeneration of nigrostriatal dopaminergic axons.

Prevention of the initial host immuno-inflammatory response determines the long-term survival of encapsulated myoblasts genetically engineered for erythropoietin delivery.

The present study investigates the respective roles of both the host immune response and the metabolic requirements in determining the long-term survival of erythropoietin-secreting myoblasts within encapsulating polymer membranes. Hollow-fiber capsules loaded with a high density of erythropoietin-secreting C(2)C(12) myoblasts survived poorly in the subcutaneous tissue of syngeneic mice, inducing variable hematocrit responses. To determine the role and the nature of the host response, recipients were treated with anti-inflammatory (diclofenac) and immunosuppressive (dexamethasone, FK506) agents. Only immunosuppressive drugs led to improved graft survival after 5 weeks of implantation, indicating an immune process as the cause of cell death. Furthermore, transient blocking of this process allowed long-term preservation of the implanted cells (> 100 days). The formation of necrotic cell cores inside densely packed devices elicited a local chronic inflammatory reaction. Hence, implants were designed to limit early cell death by inserting a supporting matrix and decreasing the number of loaded cells. The most efficient erythropoietin delivery was obtained with matrix-containing capsules that had received the lowest myoblast density. These results highlight the critical role of initial engraftment in the long-term acceptance of encapsulated myoblasts and the need to limit early cell death in the device to prevent subsequent host immuno-inflammatory responses.