The recently discovered family of neurotrophic compounds, which includes nerve growth factor (NGF), neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), and neurotrophin 4/5 (NT 4/5), are among the most powerful naturally occurring substances known to regulate neuron survival and function. Their discovery and initial characterization have generated great optimism for their use as potent therapies for neurological disorders. However, much remains unknown about how these compounds function during embryonic development and in the adult nervous system. The broad, long term objective of this proposal is to understand the role NGF, NT-3, and BDNF in the development and function of sensory perception in the mammalian nervous system. To do this we have used the Kl4 epidermal keratin gene promoter linked to the cDNA sequences encoding NGF, NT-3, BDNF, and an antisense NGF cDNA, to produce transgenic mice that overexpress these mRNAs in skin. The K14 regulatory sequences are expressed in the embryonic skin beginning at El4, the time at which endogenous levels of neurotrophins decline and neuronal cell death is initiated. Transgene expression continues in the adult. Skin normally produces these substances but at significantly lower levels than in the transgenic mice. Thus, these unique animals provide a model system in which to study the effects of overexpression (and reduced expression in the case of NGF) in a mammalian system.
The specific aims are to test four hypotheses: 1) Changes in neurotrophin expression cause anatomical alterations in cutaneous and muscle sensory organs, neuron survival, and in central projections of primary afferents, 2) The level of neurotrophin expression influences sensory neuron phenotype, 3) Neurotrophins modulate thresholds of sensory responses, 4) Overexpression of neurotrophins induce formation of novel sympathetic projections to sensory neurons that contribute to chronic pain syndromes. These experiments will employ anatomical (morphometry, cell counting, retrograde tract tracing, immunocytochemistry), molecular (PCR, Northern analysis, in situ hybridization, ELISA) and behavioral (thermal and mechanical sensation testing) methodologies to characterize the sensory nervous system of each transgenic mouse line. This model system provides a novel approach to study the developing peripheral nervous system and its sensory function in the adult. In addition, these studies will provide data that is essential to our understanding of the action of these compounds and their applicability to treatment of neuropathies.
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