Molecular mechanisms of axon morphogenesis during development and regeneration
The overall goal of our research is to understand the molecular mechanisms underlying development of the mammalian nervous system. Specifically, we are interested in understanding how neurons generate their complex morphology and form proper circuitries during development and how neurons regenerate to restore connections after brain or spinal cord injuries. In these studies, We take multi-faceted experimental approaches including:
1. different model systems: in vitro culture of primary neurons from embryonic/adult mice, and in vivo models of brain development and nerve regeneration
2. In vitro and in vivo gene electroporation
3. high-resolution fluorescent microscopy and live cell imaging of mammalian neurons
1. Regulation of neuronal cytoskeleton to control axon growth, guidance, and regeneration: The complicated behaviors of an animal arise from intrinsic neural networks, which include the precise connections between neurons in the form of circuits. During development of the nervous system, axon growth guided by a myriad of extracellular cues is one of the major events that contribute to the formation of such neural circuits. We are interested in how signals from the guidance cues ultimately converge onto the neuronal cytoskeleton, the building block of axons, to direct axon growth. Our previous study has identified a microtubule plus end tracking (MT +TIP) protein APC to be a key regulator of axon growth downstream of the nerve growth factor. Our ongoing studies of APC reveal that specific knocking out APC in the mouse brain cause severe defects in brain development. We also identified a novel function of APC in regulation of axon branching. In addition to APC, we are studying another MT +TIP protein named CLASP. By RNAi mediated loss of function studies, we find that CLASP plays important roles in both axon and dendrite development of cortical neurons. Specifically, knocking down of CLASP drastically promotes axon growth but disrupts dendrite development. By using in utero electroporation, we are now exploring the in vivo roles of APC or CLASP in neuron migration, axon/dendrite morphogenesis during cortical development.
2. New strategies to promote axon regeneration: Another interest of the lab is to find ways to promote axon regeneration after brain and spinal cord injuries. One reason that CNS neuron cannot regenerate is that they lose their intrinsic capacity to support axon growth after they mature. In contrast to CNS neurons, adult PNS neurons have the highest intrinsic capacity of axon growth after peripheral nerve injuries. Thus, one goal of our study is to understand how peripheral axotomy activates the intrinsic ability of PNS neurons to support axon regeneration via controlling gene expression. Another reason that CNS axon regeneration fails is the presence of multiple inhibitory molecules in the CNS to prevent axon regeneration. Because all CNS inhibitors converge onto the nerve growth cone to inhibit axon regeneration, we are now testing whether direct targeting the growth cone cytoskeleton is able to promote axon regeneration. For instance, we have found that specific manipulation of growth cone microtubules is able to antagonize the inhibitory effects of several CNS inhibitors on axon regeneration, such as myelin-based inhibitors and CSPGs. Together we hope that a combination of gene expression regulation with growth cone cytoskeleton manipulation will offer a novel approach to promote CNS regeneration.
We are grateful for the generous support for our research provided by the following agencies: Whitehall Foundation, March of Dimes, Christopher and Dana Reeve Foundation, and NARSAD.