MOLECULAR PERTURBATIONS IN SYNUCLEINOPATHY DISORDERS: INSIGHTS FROM PRE-CLINICAL TO HUMAN NEUROPATHOLOGY
Parkinson’s disease (PD) is a devastating neurodegenerative disorder that affects 10 million people worldwide and is characterized by pronounced motor symptoms. Dementia with Lewy Bodies (DLB) involves both cognitive and motor deficits and affects ~1 million people in the United States. To date there is no cure for PD or DLB, and current treatments address only a subset of the symptoms that define these diseases. PD and DLB are ‘synucleinopathies’, defined as disorders involving the accumulation in patients’ brains of Lewy bodies. Lewy bodies are cellular inclusions that consist largely of aggregated species of alpha-synuclein (aSyn), a presynaptic protein that exists as both cytosolic and membrane-bound forms. Pathophysiological findings suggest that aggregated aSyn is involved in neurodegeneration in PD and DLB. However, mechanisms by which aSyn forms neurotoxic aggregates, and neurotoxic processes that distinguish different synucleinopathies such as PD and DLB, are poorly understood. To address these gaps, we have (i) designed a protocol to establish a primary cell culture model that can recapitulate key neuropathological features of PD, (ii) examined effects of expressing aSyn variants in a rat model of PD, and (iii) examined the expression profiles of neuroprotective genes in PD and DLB brain specimens.
In the first part of my thesis, I describe the development of an optimized protocol to prepare primary midbrain and cortical cultures from rat embryonic brains for the study of PD and other synucleinopathies. The establishment of cellular models that simulate specific aspects of neuropathology can enable the characterization of molecular perturbations that lead to dopaminergic (DA) neuronal death. Our primary midbrain mixed culture model provides an outstanding opportunity to explore therapeutic strategies to rescue DA neurons from toxicity elicited by a range of PD-related insults. In addition, our primary cortical mixed cultures can be used to model cortical neuropathology in various CNS disorders including synucleinopathies.
A number of mutations in the gene that codes for aSyn are associated with familial, early-onset forms of PD. A major goal of my thesis research is to characterize neurotoxic effects of a recently discovered familial substitution, A53E. This mutant was chosen based on the rationale that the introduction of a negatively charged residue at position 53 could potentially interfere with aSyn-membrane interactions and favor A53E aggregation, as we described for other familial aSyn mutants. For the first time, we have reproduced the neurotoxicity of A53E seen in human patients by expressing the mutant protein in rat midbrain. Rats injected unilaterally in the substantia nigra (SN) with rAAV encoding A53E and another familial mutant, A53T, but not rAAV encoding WT aSyn or a vector-control (‘stuffer’) virus, exhibited a significant motor impairment. Immunohistochemical analysis at 14 weeks after the viral injection revealed that brain sections from aSyn-expressing rats exhibit key features reminiscent of neuropathology in human PD, including nigral dopaminergic neuron loss (confirmed by unbiased stereology), striatal terminal depletion, and aSyn inclusion formation. In addition, it was determined that WT aSyn and the A53E and A53T mutants invaded the non-injected substantia nigra, implying that expressed aSyn protein can spread throughout the brain in the rat rAAV-aSyn model. These results yield insights into the molecular basis for the neurotoxicity of A53E and shed light on a potential role for membrane-induced aSyn aggregation in PD pathogenesis in vivo, thus setting the stage for developing therapies to slow neurodegeneration in the brains of familial and idiopathic PD patients.
aSyn neurotoxicity varies with the expression of neuroprotective proteins, and misfolded aSyn affects cellular functions and gene expression. These observations suggest that differential gene expression patterns can inform us about similarities and differences in pathogenic mechanisms of different synucleinopathy disorders. A third phase of my thesis research was aimed at determining the expression levels of a panel of candidate neuroprotective genes in post-mortem brain samples from DLB and PD patients and age-matched controls (5 individuals in each group). mRNAs encoding the following proteins were quantified via qRT-PCR in homogenates prepared from the frontal cortex and the BA24 region encompassing the cingulate gyrus: DJ-1, a protein with antioxidant and chaperone activities; PGC1α, a master regulator of mitochondrial biogenesis and oxidative metabolism; MsrA, an antioxidant enzyme responsible for repairing oxidatively damaged proteins; and ATP13A2, a lysosomal protein involved in autophagy. In addition to yielding new insights into differential gene expression patterns in cortex versus cingulate gyrus, the data revealed differences in mRNA expression levels in DLB versus non-DLB cortical tissue. Although levels of all four neuroprotective mRNAs were increased (or showed a trend towards being increased) in DLB cortex, Western blot analysis revealed that only the DJ-1 and PGC1α proteins showed a trend towards being up-regulated, whereas levels of ATP13A2 and MsrA were unchanged. These findings suggest that there is a failure to induce cellular antioxidant responses and lysosomal autophagy at the protein level in DLB cortex, and in turn this failure could contribute to neuropathology. Interestingly, analysis of the same panel of neuroprotective genes in PD cortical samples did not show significant differences in mRNA or protein levels compared to control samples, suggesting that different neuroprotective mechanisms are induced in DLB versus PD cortex. These studies shed light on brain-region specific changes in gene expression associated with different synucleinopathy disorders, and they set the stage for developing new diagnostic tests and therapeutic strategies.