Toshifumi Tomoda, M.D., Ph.D. Research
Axonal Trafficking and Neurodegeneration
Initial neuronal polarity determines progenitor cell division and axon/dendrite partitioning, which ultimately ensures functional polarity of neurons, synapse and brain network formation. We study axonal trafficking mechanisms that underlie neuronal polarity establishment, using molecular cell biology, biochemistry, imaging and Drosophila / mouse genetics.
Role of Axonal Trafficking in Neuronal Polarity Establishment and Axon/Synapse Formation
Neural circuitry develops through coordinated outgrowth of axons and the formation of synaptic connections with their appropriate targets. This neuronal wiring is fundamental to our brain function, which includes the regulation of sensorimotor activities and behaviors, language acquisition, and the development of personality. The current focus of my lab is to understand the mechanisms by which neurons form axons and synapses. Neurons are one of the cell types that display most extensive morphological and functional differentiation during development. Cell biological studies have provided evidence that a series of cellular steps are involved in axon / synapse formation. These include neuronal polarity determination, growth cone guidance, cytoskeletal organization, neuronal membrane organization, and local protein synthesis and degradation.
Neurons are one of the most highly polarized cell types. They receive signals through dendrites and send output signals toward the axons. During development, neuronal progenitors undergo asymmetric cell divisions to establish initial polarity, and the neurons that commit to terminal differentiation need to partition themselves into axonal as well as dendritic compartments to ensure functional polarity, which appears to be maintained throughout life. Axonal and dendritic trafficking is a key mechanism to explain how the neuronal polarity is generated and maintained during development. We are also interested in how intrinsic and extrinsic cues influence the development of the initial polarity and how they contribute to maintain it.
Recent advances in the field have begun to highlight a potential role of endocytic membrane cycling in axon formation. While the overall membrane organization appears important during neuronal morphogenesis, how the axonal/synaptic membranes are regulated during axon/synapse formation remains elusive. We have preliminary evidence that the directed axonal transport of neuronal membranes is a driving force for proper axon / synapse formation. We are currently interested in clarifying a novel regulatory role of phosphorylation and ubiquitination that controls axonal trafficking, in particular, the mechanism by which the membrane cargo is attached to the kinesin motor complex in a phosphorylation-dependent manner. By using a combination of biochemistry, molecular and cell biology, live imaging, and mouse and Drosophila genetics, we are studying a novel mechanism of axonal trafficking that is important for neuronal polarity establishment as well as for axon / synapse formation (see sample movies). The study will also provide insights into the pathophysiology of neurological disorders in which abnormal axonal trafficking has been implicated.
Genetic studies have started to uncover a link between genes that regulate early neuronal development with a variety of neurological disorders, such as brain tumors, chronic neuropathies, neurodegenerative disorders, autism or psychiatric illness. In particular, abnormal intracellular trafficking of proteins and lipids has been linked to a number of neurological symptoms. An axonal subtype of Charcot-Marie-Tooth disease, that affects peripheral nervous system and manifests progressive neuropathic muscle atrophy, has been reported to be caused by a mutation in microtubule motor KIF1B or by mutations in the small GTPase late endosomal protein Rab7. A subtype of Niemann-Pick diseases, a congenital sphingolipidosis with central nervous system degeneration, is caused by mutations in NPC-1 gene, whose function is postulated to be a cholesterol transporter that facilitates the delivery of lipids from endosomal-lysosomal system to trans-Golgi networks. In addition, recent evidence demonstrates that huntingtin and its associated protein HAP1 participate in intracellular trafficking and that polyglutamine expansion affects vesicular transport, suggesting an involvement of altered trafficking in the pathogenesis of Huntington’s disease. While we investigate basic mechanisms of how axonal trafficking is regulated during development, we also plan to study how defective polarity or defective intracellular trafficking machinery leads to neural dysfunctions.  Through our studies, we hope to obtain an insight into the pathophysiology of ‘neurodevelopmental’ disorders such as schizophrenia and autistic spectrum disorders, and also to contribute to the diagnosis or effective treatment of such neural dysfunctions by intervening in the disease-specific mechanisms.
Role of an Autophagic Machinery in Neuorodegeneration
Another focus of my lab is to study neurodegenerative disorders that are characterized by a novel form of cellular degradation pathway called autophagy. Dysregulated autophagic membrane organization has been reported for a variety of degenerative diseases including Alzheimer’s disease and Huntington’s disease as well as a form of muscle degeneration (i.e. inclusion body myopathy).  Autophagy critically depends on intracellular trafficking machineries that appear to be essential for maintaining cellular homeostasis.  We are currently studying how dysregulated autophagy contributes to or modify cellular degeneration, in a hope to develop a novel therapeutic means to cure degenerative diseases.