Artemin Overexpression in Skin Enhances Expression of TRPV1 and TRPA1 in Cutaneous Sensory Neurons and Leads to Behavioral Sensitivity to Heat and Cold

Abstract

Artemin, a neuronal survival factor in the glial cell line-derived neurotrophic factor family, binds the glycosylphosphatidylinositolanchored protein GFR␣3 and the receptor tyrosine kinase Ret. Expression of the GFR␣3 receptor is primarily restricted to the peripheral nervous system and is found in a subpopulation of nociceptive sensory neurons of the dorsal root ganglia (DRGs) that coexpress the Ret and TrkA receptor tyrosine kinases and the thermosensitive channel TRPV1. To determine how artemin affects sensory neuron properties, transgenic mice that overexpress artemin in skin keratinocytes (ART-OE mice) were analyzed. Expression of artemin caused a 20.5% increase in DRG neuron number and increased the level of mRNA encoding GFR␣3, TrkA, TRPV1, and the putative noxious colddetecting channel TRPA1. Nearly all GFR␣3-positive neurons expressed TRPV1 immunoreactivity, and most of these neurons were also positive for TRPA1. Interestingly, acid-sensing ion channel (ASIC) 1, 2a, 2b, and 3 mRNAs were decreased in the DRG, and this reduction was strongest in females. Analysis of sensory neuron physiological properties using an ex vivo preparation showed that cutaneous C-fiber nociceptors of ART-OE mice had reduced heat thresholds and increased firing rates in response to a heat ramp. No change in mechanical threshold was detected. Behavioral testing of ART-OE mice showed that they had increased sensitivity to both heat and noxious cold. These results indicate that the level of artemin in the skin modulates gene expression and response properties of afferents that project to the skin and that these changes lead to behavioral sensitivity to both hot and cold stimuli.

Topographically specific regeneration of sensory axons in the spinal cord

“…Because unmyelinated axons lack NgR expression, they might be less responsive to a blockade of myelin-associated inhibition than myelinated axons (11). In contrast, CGRP expression recovers following ART treatment, consistent with expression of the specific ART receptor, GFRα3, on unmyelinated neurons (9,12,13), suggesting that these afferents are regenerating (Fig. 3E).…”

“…Based on immunohistochemical staining, GFRα3 is expressed largely on unmyelinated neurons. Variable expression on myelinated neurons, ranging from 0 to 14% of the total number of myelinated neurons in the DRG, has been reported (9,12,13). Behavioral recovery attributed to regeneration of unmyelinated axons is more robust than that by myelinated axons, consistent with a larger fraction of unmyelinated neurons expressing GFRα3 (9).…”

Inflammatory and neuropathic cold allodynia are selectively mediated by the neurotrophic factor receptor GFRα3

“…S5F) (P > 0.05). GFRα3 is found exclusively in TRPV1-positive afferents (27,32), but we observed no difference in TRPV1 expression in Gfrα3 −/− mice (Fig. S5 A and F).…”

“…contr, Contralateral; ipsi, ipsilateral; ns, not significant; oxal, oxaliplatin. ***P < 0.001. hyperalgesia as well as altered expression of molecules involved in sensory transduction (27,28). However, these studies involved either genetically induced artemin overexpression in the periphery (27) or multiple plantar injections of exogenous artemin (28), making it unclear if artemin-GFRα3 signaling influences afferent expression phenotypes under physiological conditions.…”

“…***P < 0.001. hyperalgesia as well as altered expression of molecules involved in sensory transduction (27,28). However, these studies involved either genetically induced artemin overexpression in the periphery (27) or multiple plantar injections of exogenous artemin (28), making it unclear if artemin-GFRα3 signaling influences afferent expression phenotypes under physiological conditions. Thus, to determine if the lack of a cold allodynic phenotype in Gfrα3 −/− mice is a result of alterations in sensory afferent development caused by the absence of GFRα3, we used quantitative PCR to determine expression of array markers involved in cold thermosensation (29).…”