Axonal transport is essential to development and function of the nervous system. Axonal transport relies on motor proteins (kinesins and dynein) to move protein, membrane and RNA cargos along microtubules. It is especially important to long and often highly branched axons that requires building blocks made in the cell body or signals received at axonal terminals to be transported for long distance. Recently studies have identified many regulatory mechanisms, including the interactions between motor proteins with lattice-bound microtubule associated proteins (MAPs), in different axonal regions. However, how axonal transport is regulated to steer cargos into and out of branches is not well understood. This is an outstanding problem as axonal branches are present throughout the nervous system. They not only define neuronal shape, but also control synaptic connectivity and specificity, influence structural plasticity, and promote functional regeneration after injury. The proposed study will tackle this under-studied problem by building on our long-term interest in branch morphogenesis and cytoskeleton regulation as well as a recent discovery of a MAP in branch development and transport regulation. Our preliminary data showed that transport at branch junctions is highly selective as cargos are preferentially transported into growing branches. In addition, we also found that MAP7, a MAP that is localized to branch junctions and interacts with the plus end motor kinesin-1, influences transport behavior and branch growth. We thus hypothesize that axonal transport at branch junctions is controlled by a selective routing mechanism that is mediated by specific motor-MAP interactions. To test this hypothesis, we will: 1) establish a functional link between selective transport and branch growth; 2) dissect the mechanism mediated by MAP7; and 3) establish selective routing as a common feature in axonal transport. By focusing on an important region of the axon that has not been studied in the past, these studies will not only fill in a gap in our understanding of axonal transport, but also provide new insights into synaptic development and function. Given the importance of axonal transport in many neurological and neurodegenerative disorders, and the association of MAP7 and kinesin-1 with epilepsy and ALS, our proposed studies of a basic neuronal cell biological problem will provide new knowledge to uncover disease mechanisms, and thus are highly relevant to the NIH mission to understand and enhance human health.
The goal of the proposed research is to understand the mechanism regulating microtubule-based transport into and out of axonal branches that are critical to synaptic development and function. Following our recent study of branch development and transport regulation at branch junctions, we will use molecular, imaging, and optogenetic approaches to investigate how transport is selectively routed for functional needs of the branch, especially via novel interactions between microtubule-associated proteins and motor proteins. Results from the proposed study will not only establish a general feature of axonal transport, but also shed lights on the mechanisms critical to synaptic function and dysfunction.