Using recently validated GFR1-3 antibodies[13],[21], we performed immunohistochemical staining on lumbar 4 (L4) DRG. by nerve growth factor (NGF), GDNF and ARTN, but not NRTN. Moreover, up-regulation NIK of ATF3 (a marker of neuronal injury)in vitrocould be prevented by NGF and ARTN, but not by GDNF Borussertib or NRTN. The lack of NRTN efficacy was correlated with quick and near-complete loss of GFR2 immunoreactivity. By retrogradely-labeling cutaneous afferentsin vivoprior to nerve cut, we exhibited that GFR2-positive neurons switch phenotype following injury and begin to express GFR3 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 GFR2 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. == 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). The GFL receptor Borussertib complex consists of Ret, a receptor tyrosine kinase, in combination with a member Borussertib of the GFR family of glycophosphatidylinositol-linked receptors (GFR1-4). More recently, neural cell adhesion molecule (NCAM) and integrin have been implicated as potential co-receptors for GFL[1],[2],[3],[4]. Following peripheral nerve injury, successful regeneration requires a program of gene expression that includes changes in growth factor receptor expression[5],[6],[7].In vivostudies of sensory neurons following peripheral axotomy have found increases in the percentage of neurons expressing GFR1 and GFR3 mRNA (the receptors for GDNF and ARTN, respectively) and a decrease in the Borussertib percentage of GFR2 (the receptor for NRTN)[8],[9]. It has been proposed that injury-induced alterations in peripheral growth factor expression lead to changes in GFR expression in sensory neurons[8], the inference being that GFL can regulate the level of their cognate receptors. The majority of GFR1 and 2 neurons are non-peptidergic (i.e., do not express the pro-inflammatory neuropeptides calcitonin gene-related peptide (CGRP) or material P (SP)) C-fibers, whereas virtually all GFR3-expressing neurons are peptidergic and 80% also express TrkA, the tyrosine kinase receptor for nerve growth factor (NGF)[10],[11],[12],[13],[14]. GFL protect against some of the pathological effects of nerve injury, including loss of neuropeptide expression and decreased conduction velocity[15]. In addition, 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 not only an effective marker of injured neurons, but also a driver of peripheral nerve regeneration[18]. Recent evidence indicates that, like GDNF and NGF, ARTN can reverse some effects of nerve injury[19]. However, unlike those factors, ARTN substantially enhances functional recovery after dorsal root injury as well as peripheral nerve injury, and this recovery includes neurons that do not normally express the receptor GFR3[20]. The present study shows that NGF and ARTN regulate ATF3 expression and neuronal survivalin Borussertib vitro, whereas NRTN is usually ineffective.In vivo, we demonstrate that in some cells the loss of detectable GFR2 is replaced by GFR3 expression and that this may be regulated by Runx1, a transcription factor critical for differentiation of nociceptor subtypes. These changes have important functional effects for nociceptive transduction in that neurons down-regulating GFR2 subsequently begin to expressde novofunctional TRPV1 channels. These results indicate that injury-evoked changes in GFR receptor expression alter the efficacy of the GDNF family members, and may explain the unexpected gain of function for ARTN and the loss of function for NRTN during sensory neuron regeneration. == Results == == Distribution of GFR1-3-expressing sensory neurons in vivo and 24 hin vitro == Initial experiments were designed to determine the proportion of neurons expressing GFR1-3 proteinin situ(i.e., tissue sections of DRG) and in dissociated neurons, to elucidate the extent to which the phenotype of cultured neurons was representative of thein vivocondition. Using recently validated GFR1-3 antibodies[13],[21], we performed immunohistochemical.