Weill Medical College of Cornell University
Ulrich Hengst, Ph.D.
Postdoctoral Associate

Spinal cord injury is a major cause of disability, and there is still no highly effective treatment. The devastating clinical consequences reflect a failure of the injured axons to reconnect to their targets due to inhibitory substances in the side of injury of the spinal cord. Molecular studies have suggested that these substances lead to enhanced activity of RhoA. RhoA functions as a molecular switch: if it is off, the axons can grow, but as soon as it is on, it causes the growth cones to collapse and axons to retract. The prevailing model is that the substances that prevent axons from reestablishing their connections in the injured spinal cord switch RhoA on. A major question is: how is RhoA regulated in axons? One of the most unexpected discoveries of recent years is that some proteins are manufactured within the growth cone itself, rather than in the neuron cell body and subsequently trafficked, as had previously been presumed. The research we propose is based on our surprising finding that the template messenger RNA (mRNA) for RhoA is localized to growth cones, and axonal RhoA protein levels rise in response to Sema3A, a collapsing agent. These findings suggest that local RhoA translation may be a molecular mechanism fundamental to spinal cord injury. Our finding that Sema3A-mediated growth cone collapse involves local translation of RhoA mRNA proposes a novel and previously unknown mechanism of control of axonal elongation. However, several questions remain: Is RhoA mRNA translation triggered by myelin-derived axonal retraction molecules such as MAG or Nogo-66? What intracellular signaling molecules couple receptor activation to RhoA mRNA translation? The experiments in this proposal seek to determine the relevance of the RhoA translation pathway in an in vitro model of spinal cord injury signaling that utilizes explant cultures of embryonic rat sensory neurons and myelin-derived inhibitors of regeneration. We will use neurotropic viruses and pharmacological approaches to study the mechanism of local RhoA translation in growth cones in response to collapsing agents. We will extend our work by identifying the role of microRNA pathways in the regulation of RhoA mRNA translation. microRNAs are a recently discovered class of non-coding RNAs, thought to act as repressors of mRNA translation. We have indications that the regulation of local RhoA translation in axons is mediated by microRNAs. Together, these experiments will help establish the feasibility of our long-term goals of a spinal cord injury therapy that uses microRNAs and/or neurotropic RNA viruses, to modulate RhoA translation or signaling directly at the site of axonal injury. The development of microRNAs-based therapies and of approaches for viral introduction of microRNAs is a major area of academic and industrial research. Approaches that target the mRNA regulatory pathway in growth cones could have significant use to ameliorate the degree of morbidity in spinal cord injury.