Dissecting the in vivo role of Huntingtin in Rab vesicle movement on microtubules

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Huntington's disease (HD) is a devastating, dominantly inherited neurodegenerative disease clinically characterized by chorea and cognitive impairment due to loss of striatal neurons. Currently there are no effective treatments/cures for HD. Most therapeutic treatments currently used are aimed at dissolving/dissociating aggregates and preventing cell death, common neuropathology seen at the end stage of disease. Although the HD protein, huntingtin (HTT) is critical for viability, the complexity of HTT-mediated associations indicates multiple functions. Thus the challenge is to unravel the primary function of HTT, which when disrupted initiates disease. Previous work put forth a tantalizing proposal that disruption of axonal transport within long, narrow-caliber axons is an early event that causes protein accumulations that elicit cell death, ultimately resulting in neuronal dysfunction observed in HD. Our long-term goal is to understand how HTT-mediated axonal transport defects initiates disease pathways. The objective here, which is our next step in the pursuit of this goal, is to determine how HTT influences the transport of a specific sub class of vesicles (Rab proteins). Our central hypothesis is that disruption of Rab vesicle transport within axons mediated by HTT can contribute to early neuropathology observed in HD. There are two clear predictions of this hypothesis; 1: HTT and Rab proteins are on the same vesicles and 2: Rab vesicles use kinesin-1 and dynein motors for movement on microtubules (MT). In this context our specific aim is to identify how HTT influences Rab proteins for MT-dependent transport within axons. We have 5 specific objectives, 1: determine how HTT influences Rab proteins, 2: test the prediction that HTT and Rab11, Rab32 and RabX4 are on the same vesicle, 3: test the prediction that Rab32 and RabX4 are both on the Rab11 vesicle, 4: test the prediction that Rab32 and RabX4 use kinesin-1 and dynein motors for movement on MT, and 5: test the prediction that mutant HTT disrupts Rab-mediated functions. A comprehensive in vivo approach will be used to dissect the physiological role of HTT in Rab vesicle transport in an organism. The rationale for the proposed research is that once the mechanisms of how HD disease is initiated by perturbations in Rab transport by mutant HTT are known, new and innovative approaches against HD can be developed. Therefore identifying how HTT normally functions in neurons will have a significant impact on providing novel target pathways for developing effective preventive and therapeutic interventions, which are currently unavailable for HD. Thus our work is innovative, in our opinion because it represents a new and substantive departure from the status quo, namely the approach of detailing the role of HTT using in vivo dynamics of vesicle movement in a living organism. The proposed research is significant, because it is expected to vertically advance and expand our understanding of how disease pathways initiate, which will significantly alter current knowledge. The knowledge acquired will dramatically propel the development of numerous pharmacological or genetic modifiers against axonal defects or to restore Rab function (impact).