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 are 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 the axoplasmic transport of protein synthesized in the parental cell bodies. Although the majority of neuronal mRNAs are indeed transcribed and translated in the neuronal cell body, it is now well-established that there exists a diverse population of mRNAs and noncoding microRNAs in the distal structural/functional domains of the neuron to include the axon and presynaptic nerve terminal. It has also become well-accepted that proteins synthesized from these mRNA templates play a key role in the development of the neuron, as well as the long-term viability and function of the axon and nerve terminal. In previous studies, we reported the surprising observation that the axon contained numerous nuclear-encoded mitochondrial mRNAs and that approximately 25% of the total protein synthesized in the presynaptic nerve terminal was shipped into this organelle. The inhibition of the local synthesis of these proteins had a profound negative impact on local mitochondrial function to include the inhibition of ATP production and elevation of the synthesis of damaging reactive oxygen species (ROS). In previous studies, we also identified a 38-nucleotide sequence (cis-acting regulatory element) that directs the trafficking of select nuclear-encoded mRNAs to the axon. We refer to th8is element as a zip-code that is present in the 3 untranslated region of the mRNAs. Results of recent cell culture studies indicate that the over-expression of the zip-code disrupts the normal trafficking of the endogenous mRNAs, inhibits the function of the local mitochondria, as well as manifests deleterious effects on axonal growth and axonal viability. This year, we completed the initial behavioral and physiological characterization of a transgenic mouse line that over expresses the zip-code specifically in glutaminergic neurons located in the cerebral cortex. These transgenic animals manifest anxiety- and depressive-like behaviors that are exacerbated by stress (Kar et al., 2014). At present, we are evaluating the effects of the over-expression of the zip-code on the transgenic animals cognitive behavior. This past year, we discovered that mRNAs encoding two protein synthesis translation initiation factors were also present in the axon. The results generated from this portion of our research program established that the local expression of these factors regulates the activity of the intra-axonal protein synthetic system per se. Moreover, the inhibition of local expression of these key proteins has a profound inhibitory effect on axon growth, as well as the long-term viability of the axon. A peer-reviewed paper describing these new findings was recently published (Kar et al., 2013). Last, we continue to conduct our exploratory, high-risk investigation designed to test the working hypothesis that the synthesis and release/reuptake of the catecholamine neurotransmitters are regulated locally in the axon and presynaptic nerve terminal. In this regard, our preliminary data indicate that the mRNAs encoding all the enzymes of the catecholamine biosynthesis pathway are present in the axons of sympathetic neurons, and are locally translated. Additionally, the levels of these mRNAs in the polysome fraction, as well as, levels of axonal norepinephrine could be induced 2-to 4-fold by elevating the levels of cyclic AMP in the axon, a finding that suggests that the intra-axonal synthesis of these enzymes and neurotransmitter can be modulated by synaptic activity. Papers describing these provocative new findings have been presented at two prestigious international conferences by my postdoctoral fellow, Dr. Noreen Gervasi, and a manuscript is now in preparation. Taken together, these results deived fom this years investigation of the local local protein synthetic system indicate that this system plays a key role in the regulation of local mitochondrial activity and axon growth, as well as neurotransmitter metabolism. We anticipate 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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