Postdoctoral positions available in the Tourtellotte Lab to study transcriptional regulatory mechanisms in nervous system development.
Gene regulation in nervous system development and function; nerve-muscle induction mechanisms; sympathetic nervous system development and growth factor signaling
Nervous system processes that involve receptor-ligand interactions exert their control on neurons by activating signal transduction pathways that modulate gene expression. The earliest genetic events typically involve the induction of immediate early genes (IEGs) that often encode transcription factors or cytokines. In the nervous system for example, the rapid regulation of downstream genes by IEG transcription factors is thought to play a critical role in physiological and structural plasticity required for cognitive processes such as learning and memory. Our work focuses on a family of IEG transcription factors (Egr1, Egr2, Egr3 and Egr4) that are highly induced within neurons by synaptic activity. Egr proteins are essential for a wide variety of processes in the nervous system including neuron precursor proliferation and differentiation, long-term synaptic potentiation (LTP), learning and memory, neuronal kindling, circadian rhythm generation and neurotrophin signaling.
We are interested in defining how these transcriptional regulators influence development and function of the mammalian nervous system, how the related Egr proteins interact where they are co-expressed and to what extent they mediate growth factor signaling and learning and memory processing. We are currently using a variety of contemporary molecular-genetic techniques including microarray analysis, recombinant virus mediated gene transfer and transgenic mouse models to understand the repertoire of target genes regulated by Egr regulatory proteins in three processes where they have an essential role:
I. Regulation of muscle stretch receptor development. In work pioneered by our laboratory, we found that Egr3 is upregulated in a small number of muscle cells after they are innervated by sensory axons during muscle development. The sensory axon contacted myotubes are fate specified by Egr3 to become muscle spindle stretch receptors that are necessary for normal stretch reflexes and coordinated limb movements. Current work is focused on defining the role of Egr3 in fate specifying myotubes to differentiate into spindle stretch receptors and to define the regulatory programs that are engaged to mediate their morphogenesis. This work has important implications for understanding the signaling mechanisms that are required to stabilize the interactions between sensory neurons and mechanoreceptors; similar interactions which are often disrupted in human sensory neuropathies.
Egr3 is essential for development of muscle stretch receptors. (A) Muscle spindle stretch receptors are sensory organs embedded in vertebrate skeletal muscles that provide limb position information (proprioception) to the central nervous system. They consist of encapsulated muscle fibers innervated by specialized sensory and motor axons. (B) Egr3 is expressed at high levels in myonuclei of muscle spindle fibers. Three color confocal micrograph from intact muscle demonstrates Egr3 expression in muscle spindle fiber myonuclei (blue), motor and sensory innervation (pan-neurofilament, green) and motor endplates ( a -bungarotoxin, red). (C, D) Egr3-deficient mice lack muscle spindles and have severe abnormalities in walking (sensory ataxia).
II. Sympathetic nervous system development. The sympathetic nervous system is critical for organ homeostasis. Sympathetic neurons depend upon target tissue derived growth factors such as nerve growth factor (NGF) for survival during development. Egr gene expression is regulated in sympathetic neurons by NGF and other growth factor signaling. Thus, we are examining the extent to which Egr genes such as Egr3 are necessary for normal sympathetic nervous system development and we are defining their role gene regulation mediated by neurotrophin (eg: NGF and NT-3) signaling.
Egr3 is required for normal sympathetic nervous system development. (A, B) We generated a novel transgenic reporter mouse to express axon localized b -galactosidase (lacz) in all sympathetic neurons and then mated it to Egr3 knockout mice to generate double transgenic mice. The axon localized lacz makes it possible to examine in detail sympathetic defects in (A') Egr3-deficient mice and compare the innervation pattern to (A) wild type mice (salivary gland sympathetic innervation shown). While sympathetic axons are not completely absent in (B') Egr3-deficient mice, they are markedly decreased in number and there is decreased axon branching compared to (B) wild mice.
III. Learning and memory processing. Egr transcription factors are regulated by synaptic activity and Egr1 is widely recognized to be involved in gene regulation required for learning and memory. We are characterizing the role of other Egr transcription factors in learning and memory using a variety of novel gain-of-function and conditional loss-of-function mutant mice and characterizing their behavior in learning and memory. We are examining the role of a variety of potential target genes regulated by Egr proteins in learning and memory.
Egr3 is required for normal hippocampal LTP and memory. (A) The magnitude of LTP is decreased in Egr3-deficient Schaffer collateral synapses relative to wild type mice. Short term and long term hippocampal-dependent memory are markedly decreased in Egr3-deficient mice relative to wild type in (B) fear conditioning and (C) novel object recognition. (* = p < 0.001; ** = p < 0.01)