Biology and Biomedical Sciences: Neurosciences
Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
ABSTRACT OF THE DISSERTATION Isoform-specific roles of Extracellular Signal-Regulated Kinases in pain by Benedict Joseph Alter Doctor of Philosophy in Biology and Biomedical Sciences Neurosciences Washington University in St. Louis, 2012 Professor Robert Gereau, Chairperson The extracellular signal-regulated kinase: ERK) isoforms, ERK1 and ERK2, are believed to be key signaling molecules in nociception and nociceptive sensitization. Studies utilizing inhibitors targeting the shared ERK1/2 upstream activator, mitogen-activated protein kinase kinase: MEK), and transgenic mice expressing a dominant negative form of MEK have established the importance of ERK1/2 signaling. However, these techniques do not discriminate between ERK1 and ERK2. To dissect the function of each isoform in pain, mice with a targeted genetic deletion of ERK1: ERK1-/-) and mice with a conditional deletion of ERK2 in nociceptors: ERK2f/f;NaV1.8-Cre) were used. Although both isoforms are activated: phosphorylated) following inflammation, deletion of ERK1 had no effect in several models of chemical nociception, inflammatory pain, and neuropathic pain. In contrast, conditional deletion of ERK2 in nociceptors attenuates nociceptive spontaneous behavior in the second phase of the formalin test, reduces inflammatory mechanical hypersensitivity, and eliminates heat hypersensitivity due to the inflammatory mediator, nerve growth factor: NGF). Nociceptive sensitization was not reduced in all models tested, since ERK2f/f;NaV1.8-Cre mice developed robust heat hypersensitivity to other inflammatory and chemical insults. Biochemical analysis of both lines revealed that eliminating one ERK isoform led to elevated phosphorylation of the remaining isoform at baseline, which could explain the lack of a phenotype in ERK1-/- mice. However, this is probably not the case since the elevation in spinal cord ERK2 phosphorylation above baseline following noxious stimulation is not affected by the deletion of ERK1. It is also possible that other intracellular signaling cascades may compensate for the loss of ERK1. This seems less probable since systemic MEK inhibition attenuates formalin-induced spontaneous nociceptive behaviors similarly in ERK1-/- and WT littermate controls. These experiments demonstrate an isoform-specific role for ERK2 on the behavioral level. On the cellular level, ERK2 is also partially required for innervation of the epidermis by peptidergic nociceptive afferents that express the NGF receptor, TrkA. The partial reduction in epidermal innervation is not accompanied by cell loss, suggesting a defect in axon growth or maintenance. Fiber loss is unlikely to account for all behavioral phenotypes observed in ERK2f/f;NaV1.8-Cre mice since remaining epidermal fibers have previously been shown to play important roles on the behavioral level. Additionally, ERK1-/- mice have exaggerated NGF-induced heat hypersensitivity but do not have altered epidermal innervation. Overall, these data highlight the complicated interplay between ERK1 and ERK2 in vivo and suggest that ERK2 is the critical isoform for nociceptor sensitization.
Alter, Benedict, "Isoform-Specific Roles of Extracellular Signal-Regulated Kinases in Pain" (2012). All Theses and Dissertations (ETDs). 680.