Phenotypic Switching of Nonpeptidergic Cutaneous Sensory Neurons following Peripheral Nerve Injury

Ting Wang,Erica S Schwartz,Derek C Molliver,Qiufu Ma,Brian M Davis,Omar Abdel Samad,Fu-Chia Yang,Xiaotang Jing,Rafael Linden

doi:10.1371/journal.pone.0028908

Abstract

In adult mammals, the phenotype of half of all pain-sensing (nociceptive) sensory neurons is tonically modulated by growth factors in the glial cell line-derived neurotrophic factor (GDNF) family that includes GDNF, artemin (ARTN) and neurturin (NRTN). Each family member binds a distinct GFRα family co-receptor, such that GDNF, NRTN and ARTN bind GFRα1, -α2, and -α3, respectively. Previous studies revealed transcriptional regulation of all three receptors in following axotomy, possibly in response to changes in growth factor availability. Here, we examined changes in the expression of GFRα1-3 in response to injury in vivo and in vitro. We found that after dissociation of adult sensory ganglia, up to 27% of neurons die within 4 days (d) in culture and this can be prevented by nerve growth factor (NGF), GDNF and ARTN, but not NRTN. Moreover, up-regulation of ATF3 (a marker of neuronal injury) in vitro could be prevented by NGF and ARTN, but not by GDNF or NRTN. The lack of NRTN efficacy was correlated with rapid and near-complete loss of GFRα2 immunoreactivity. By retrogradely-labeling cutaneous afferents in vivo prior to nerve cut, we demonstrated that GFRα2-positive neurons switch phenotype following injury and begin to express GFRα3 as well as the capsaicin receptor, transient receptor potential vanilloid 1(TRPV1), an important transducer of noxious stimuli. This switch was correlated with down-regulation of Runt-related transcription factor 1 (Runx1), a transcription factor that controls expression of GFRα2 and TRPV1 during development. These studies show that NRTN-responsive neurons are unique with respect to their plasticity and response to injury, and suggest that Runx1 plays an ongoing modulatory role in the adult.

Highlights

glial cell line-derived neurotrophic factor (GDNF) family ligands (GFL) are neurotrophic factors that regulate the development and functional phenotype of peripheral sensory neurons in the dorsal root ganglia (DRG)
Immunoreactivity for TRPV1 was examined because it is expressed in 95% of GFRa3 neurons, but in only a minority of neurons labeled by the plant lectin IB4, that is extensively colocalized with GFRa1 and a2 [15]
A large number of cells immunoreactive for GFRa3 and TRPV1 were observed in lumbar 4 (L4) DRG of naıve mice (Fig. 1G, H, I), whereas IB4 was primarily localized in GFRa2-positive (Fig. 1M, N, O) and small-diameter GFRa1-positive neurons (Fig. 1J, K, L), confirming previous studies [8,15,22]

Summary

Introduction

GDNF family ligands (GFL) are neurotrophic factors that regulate the development and functional phenotype of peripheral sensory neurons in the dorsal root ganglia (DRG). It has been proposed that injury-induced alterations in peripheral growth factor expression lead to changes in GFRa expression in sensory neurons [8], the inference being that GFL can regulate the level of their cognate receptors. GFL protect against some of the pathological effects of nerve injury, including loss of neuropeptide expression and decreased conduction velocity [15]. Both GDNF and NGF have been shown to inhibit expression of the transcription factor ATF3 [15,16], which is normally induced in injured sensory neurons [17]. ATF3 is an effective marker of injured neurons, and a driver of peripheral nerve regeneration [18]

Methods

Results

Conclusion