Mechanisms of specificity in neuronal activity-regulated gene transcription.

Abstract

In the nervous system, activity-regulated gene transcription is one of the fundamental processes responsible for orchestrating proper brain development–a process that in humans takes over 20 years. Moreover, activity-dependent regulation of gene expression continues to be important for normal brain function throughout life; for example, some forms of synaptic plasticity important for learning and memory are known to rely on alterations in gene transcription elicited by sensory input. In the last two decades, increasingly comprehensive studies have described complex patterns of gene transcription induced and/or repressed following different kinds of stimuli that act in concert to effect changes in neuronal and synaptic physiology. A key theme to emerge from these studies is that of specificity, meaning that different kinds of stimuli up- and down regulate distinct sets of genes. The importance of such signaling specificity in synapse-to-nucleus communication becomes readily apparent in studies examining the physiological effects of the loss of one or more forms of transcriptional specificity – often, such genetic manipulations result in aberrant synapse formation, neuronal cell death, and/or cognitive impairment in mutant mice. The two primary loci at which mechanisms of signaling specificity typically act are 1) at the synapse – in the form of calcium channel number, localization, and subunit composition – and 2) in the nucleus – in the form of transcription factor expression, localization, and post-translational modification. My dissertation research has focused on the mechanisms of specificity that govern the activity-regulated transcription of the gene encoding Brain-derived Neurotrophic Factor(Bdnf). BDNF is a secreted protein that has numerous important functions in nervous system development and plasticity, including neuronal survival, neurite outgrowth, synapse formation, and long-term potentiation. Due to Bdnf’s complex transcriptional regulation by various forms of neural stimuli, it is well positioned to function as a transducer through which altered neural activity states can lead to changes in neuronal physiology and synaptic function. In this dissertation, I show that different families of transcription factors, and even different isoforms or splice variants within a single family, can specifically regulate Bdnf transcription in an age- and stimulus-dependent manner. Additionally, I characterize another mechanism of synapse-to-nucleus signaling specificity that is dependent upon NMDA-type glutamate receptor subunit composition, and provide evidence that the effect this signaling pathway has on gene transcription is important for normal GABAergic synapse formation. Taken together, my dissertation research sheds light on several novel signaling mechanisms that could lend specificity to the activity-dependent transcription of Bdnf exon IV. My data indicate that distinct neuronal stimuli can differentially regulate the Calcium-Response Element CaRE1 within Bdnf promoter IV through activation of two distinct transcription factors: Calcium-Response Factor (CaRF) and Myocyte Enhancer Factor 2 (MEF2). Furthermore, individual members of the MEF2 family of transcription factors differentially regulate the expression of Bdnf, and different MEF2C splice variants are unequally responsive to L-type voltage-gated calcium channel activation. Additionally, I show here for the first time that the NMDA-type glutamate receptor subunit NR3A (also known as GluN3A) is capable of exerting an effect on NMDA receptor-dependent Bdnf exon IV transcription, and that changes in the expression levels of NR3A may function to regulate the threshold for activation of synaptic plasticity-inducing transcriptional programs during brain development. Finally, I provide evidence that the transcription factor CaRF might function in the regulation of homeostatic programs of gene transcription in an age- and stimulus-specific manner. Together, these data describe multiple novel mechanisms of specificity in neuronal activity-regulated gene transcription, some of which function at the synapse, others of which function in the nucleus.