Egr3 Dependent Sympathetic Target Tissue Innervation in the Absence of Neuron Death

Abstract

Nerve Growth Factor (NGF) is a target tissue derived neurotrophin required for normal sympathetic neuron survival and target tissue innervation. NGF signaling regulates gene expression in sympathetic neurons, which in turn mediates critical aspects of neuron survival, axon extension and terminal axon branching during sympathetic nervous system (SNS) development. Egr3 is a transcription factor regulated by NGF signaling in sympathetic neurons that is essential for normal SNS development. Germline Egr3-deficient mice have physiologic dysautonomia characterized by apoptotic sympathetic neuron death and abnormal innervation to many target tissues. The extent to which sympathetic innervation abnormalities in the absence of Egr3 is caused by altered innervation or by neuron death during development is unknown. Using Bax-deficient mice to abrogate apoptotic sympathetic neuron death in vivo, we show that Egr3 has an essential role in target tissue innervation in the absence of neuron death. Sympathetic target tissue innervation is abnormal in many target tissues in the absence of neuron death, and like NGF, Egr3 also appears to effect target tissue innervation heterogeneously. In some tissues, such as heart, spleen, bowel, kidney, pineal gland and the eye, Egr3 is essential for normal innervation, whereas in other tissues such as lung, stomach, pancreas and liver, Egr3 appears to have little role in innervation. Moreover, in salivary glands and heart, two tissues where Egr3 has an essential role in sympathetic innervation, NGF and NT-3 are expressed normally in the absence of Egr3 indicating that abnormal target tissue innervation is not due to deregulation of these neurotrophins in target tissues. Taken together, these results clearly demonstrate a role for Egr3 in mediating sympathetic target tissue innervation that is independent of neuron survival or neurotrophin deregulation.

A Sympathetic Neuron Autonomous Role for Egr3-Mediated Gene Regulation in Dendrite Morphogenesis and Target Tissue Innervation

“…4 C ). In addition, Egr3 cKO mice had peripheral sympathetic target tissue innervation abnormalities that closely recapitulated the innervation abnormalities observed in germline Egr3 KO mice (Li et al, 2011). For example, sympathetic innervation (demonstrated by TH immunofluorescence) to heart, spleen, kidney, and pineal gland was significantly reduced in Egr3 cKO mice (Fig.…”

“…In addition, mice lacking both Egr3 and Bax (the latter, which prevents apoptotic neuron death) have innervation abnormalities that resemble those occurring in mice lacking both NGF and Bax (Glebova and Ginty, 2004; Li et al, 2011), further suggesting that Egr3 regulates, at least in part, gene expression induced by NGF signaling in sympathetic neurons to influence target tissue innervation. However, previous studies indicate that Egr3 is also expressed in blood vessels along which sympathetic axons travel (Shneider et al, 2009) and in Schwann cells that myelinate some axons in nerves through which unmyelinated sympathetic axons pass during development (Gao et al, 2007).…”

“…Mice lacking Egr3 in all tissues have sympathetic innervation abnormalities and dysautonomia, but whether its expression in sympathetic neurons is required for normal target tissue innervation and/or what target genes it regulates is not known (Eldredge et al, 2008; Li et al, 2011). Here, we show that loss of Egr3 expression specifically in sympathetic neurons leads to abnormal neurite outgrowth in response to NGF signaling in vitro and sympathetic neuron dendrite and axon abnormalities in vivo .…”

A neuron autonomous role for the familial dysautonomia geneELP1in sympathetic and sensory target tissue innervation

“…Whole mount TH immunohistochemistry was performed as previously described (Li et al, 2011) but with slight modification (see methods in the supplementary material).…”

Intrinsic Axonal Growth and the Drive for Regeneration

“…Interestingly, Egr3 null mice have degenerating muscle spindles, (Tourtellotte and Milbrandt, 1998) which fail to express NT-3, which results in type Ia afferents failing to reach their targets in the muscle spindle (Chen et al, 2002). Also, Egr3 is required for sympathetic axons to properly innervate their targets (Eldredge et al, 2008) and these defects appear to be dependent on growth, but not survival since Egr3 null mice crossed onto a Bax null background fail to innervate several sympathetic nervous system targets including the heart, spleen, kidney and pineal gland (Li et al, 2011). Egr3 is also required for proper innervation of salivary glands and heart where NGF and NT-3 expression persist in the absence of Egr3 suggesting that Egr3 regulates axon growth via neurotrophin-independent factors in these contexts.…”