Axons and nerve terminals are unique subcellular structures of the neuron that play a critical role in the development and maintenance of neural connectivity. One of the central tenets in neuroscience is that the protein constituents of these distal neuronal compartments are synthesized in the nerve cell body and subsequently transported to their ultimate sites of function. Hence, the structure and function of these highly specialized distal domains of the neuron are totally dependent on slow anterograde axoplasmic transport. Although the majority of neuronal mRNAs are indeed transcribed and translated in the neuronal cell soma, recent findings have established the presence of a diverse population of mRNAs in the distal structural/functional domains of the neuron to include the axon and presynaptic nerve terminal. It has also become well established that proteins synthesized from these mRNAs play a key role in the development of the neuron and the function of the axon and nerve terminal, including navigation of the axonal growth cone, synthesis of membrane receptors employed as axon guidance molecules, axon transport, synapse formation, and in activity-dependent synaptic plasticity. In previous studies, we reported that several nuclear-encoded mitochondrial mRNAs were present in the axon and the presynaptic nerve terminal and that approximately 25% of the total protein synthesized locally in the nerve terminal was destined for the mitochondria. Based upon these findings, we hypothesized that the local protein synthetic system plays a critical role in the maintenance of the mitochondrial population and ultimately, the function of the axon and presynaptic nerve terminal. Currently, we are testing this working hypothesis using rat primary sympathetic neurons cultured in multicompartment Campenot chambers. Results of these studies establish that acute inhibition of the local protein synthetic system significantly diminishes the membrane potential of mitochondria, and also reduced the ability of mitochondria to maintain basal levels of axonal ATP and restore levels of axonal ATP after KCl-induced depolarization. Moreover, the inhibition of local protein synthesis for more than six hours significantly reduced the viability of the axons, as judged by the stucture's ability to thrive in the cell culture system. Most recently, we have discovered that cytochrome c oxidase IV mRNA is present in the axon. This protein is a rate-limiting factor in the mitochondrion's ability to generate of ATP in the cell. Surprisingly, the expression of COXIV in the axon is being regulated by a brain-specific microRNA which binds to a signal sequence present in the 3' untranslated region of the mRNA. This microRNA is a member of a family of small noncoding RNA molecules that regulate the expression of genes that are involved in various fundamental biological processes such as cell growth and differentiation. Modulation of axonal COXIV expression by this RNA moiety has marked effects on the axon's metabolic activity and its capacity to generate energy. The silencing of the local expression of COXIV also has profound effects on the growth of the axon. Taken together, these results indicate that the local protein synthetic system plays a key role in mitochondrial function and the growth and maintanence of the axon. We hope that this line of investigation will augment our understanding of the molecular mechanisms that underlie neuronal development, regeneration, and plasticity and generate new avenues of research into the pathophysiology of mental illness.
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