Cytoskeletal proteins and early neurodegenerative mechanisms in Alzheimer's disease

Alzheimer’s Disease (AD) is a devastating neurodegenerative disorder that is histopathologically characterized by several hallmark lesions, including extracellular plaques comprised of amyloid beta (AB) peptides, neuropil threads and intra-neuronal neurofibrillary tangles, both of which are comprised of hyperphosphorylated microtubule-associated (MAP) protein tau. Neuropil threads are one of the earliest lesions observed in AD brain and the extent of their presence correlates with cognitive decline and disease progression. In addition, rod-like aggregates of actin and actin depolymerizing factor (ADF)/cofilin (‘AC’ or ‘cofilin’ rods) have also been described throughout the neuropil of AD brains. Sporadic AD accounts for over 90% of all AD cases and although aging has been identified as the most significant risk factor for developing this form of the disease, the pathogenic mechanisms involved in initiation of sporadic AD remain poorly understood. Decreased mitochondrial function and increased oxidative stress are common features of the aging brain and a growing body of evidence suggests these mitochondrial changes may play a central role in the pathogenesis of sporadic AD. The key question therefore is can mitochondrial dysfunction induce histopathological features of AD and if so, by what mechanisms? Answers to these important questions may be pivotal in the development of more effective AD therapeutics. This thesis investigated the effects of mitochondrial dysfunction on the interrelationships between two AD-related cytoskeletal systems, MAP/tau and AC-actin. Employing primary neuron culture models, organotypic brain slice cultures and a range of immuno—labeling and microscopy techniques, we show that mitochondrial dysfunction rapidly induces rod-Iike aggregations of activated AC throughout neurites that subsequently recruit AD-relevant epitopes of phosphorylated MAP/tau. The resulting cytoskeletal complexes closely resemble neuropil threads observed in human AD brain. Mechanistically, the relationship between these inclusions was explored through use of actin modifying drugs, knockdown of ADF/cofilin in primary neuron culture and knockout of tau in transgenic mouse brain slices. Overall, the results suggest that during neuronal stress, AC rods form rapidly and serve as a nucleation seed for subsequent recruitment of phosphorylated MAP/tau. Moreover, this initial recruitment is specific to MAP/tau phosphorylated in the functional microtubuIe-binding domain which is of significance, since this is one of the first phosphosites identified during the early pathogenesis of AD tau pathology. Furthermore, treatments with synthetic or naturally-secreted preparations of AB peptides induced the same effects in primary neuron and brain slice cultures, thus suggesting that the major histopathologies of AD may all be reconciled in one common pathway. The studies reported here provide evidence suggesting that mitochondrial dysfunction is central to the pathogenesis of two AD-related cytoskeletal pathologies: MAP/tau neuropil threads and AC rods. Moreover, the results presented here show for the first time that these two neuritic inclusions are closely interrelated and together implicate a disrupted cytoskeletal network that may account for the widespread axonal transport deficits and axonal degeneration characteristic of this disease. To that extent, we propose that association of MAP/tau and AC-actin proteins constitutes one of the earliest events in the pathogenesis of sporadic AD