TAFA4 relieves injury-induced mechanical hypersensitivity through LDL receptors and modulation of spinal A-type K+ current.

Pain, whether acute or persistent, is a serious medical problem worldwide. However, its management remains unsatisfactory, and new analgesic molecules are required. We show here that TAFA4 reverses inflammatory, postoperative, and spared nerve injury (SNI)-induced mechanical hypersensitivity in male and female mice. TAFA4 requires functional low-density lipoprotein receptor-related proteins (LRPs) because their inhibition by RAP (receptor-associated protein) dose-dependently abolishes its antihypersensitive actions. SNI selectively decreases A-type K+ current (IA) in spinal lamina II outer excitatory interneurons (L-IIo ExINs) and induces a concomitant increase in IA and decrease in hyperpolarization-activated current (Ih) in lamina II inner inhibitory interneurons (L-IIi InhINs). Remarkably, SNI-induced ion current alterations in both IN subtypes were rescued by TAFA4 in an LRP-dependent manner. We provide insights into the mechanism by which TAFA4 reverses injury-induced mechanical hypersensitivity by restoring normal spinal neuron activity and highlight the considerable potential of TAFA4 as a treatment for injury-induced mechanical pain.

Loss of bhlha9 Impairs Thermotaxis and Formalin-Evoked Pain in a Sexually Dimorphic Manner.

C-LTMRs are known to convey affective aspects of touch and to modulate injury-induced pain in humans and mice. However, a role for these neurons in temperature sensation has been suggested, but not fully demonstrated. Here, we report that deletion of C-low-threshold mechanoreceptor (C-LTMR)-expressed bhlha9 causes impaired thermotaxis behavior and exacerbated formalin-evoked pain in male, but not female, mice. Positive modulators of GABAA receptors failed to relieve inflammatory formalin pain and failed to decrease the frequency of spontaneous excitatory post-synaptic currents (sEPSCs) selectively in bhlha9 knockout (KO) males. This could be explained by a drastic change in the GABA content of lamina II inner inhibitory interneurons contacting C-LTMR central terminals. Finally, C-LTMR-specific deep RNA sequencing revealed more genes differentially expressed in male than in female bhlha9 KO C-LTMRs. Our data consolidate the role of C-LTMRs in modulation of formalin pain and provide in vivo evidence of their role in the discriminative aspects of temperature sensation.

Disruption of TRPV3 Impairs Heat-Evoked Vasodilation and Thermoregulation: A Critical Role of CGRP.

Sensing environmental temperature is a key factor allowing individuals to maintain thermal homeostasis via thermoregulatory mechanisms, including changes to skin blood flow. Among transient receptor potential channels, transient receptor potential vanilloid 3 (TRPV3) is a heat-activated cation channel highly expressed in keratinocytes. However, the role of TRPV3 in triggering heat-evoked cutaneous vasodilation is unknown. Using a murine in vivo model of local acute environmental heat exposure in the skin, we show that TRPV3 is involved in the local thermoregulatory control of skin blood flow by initiating the release of calcitonin gene-related peptide and nitric oxide in response to local heating of the skin. In addition to their contribution in local heat-evoked vasodilation, TRPV3, calcitonin gene-related peptide, and nitric oxide also contribute to internal body temperature stability during passive whole-body heating. This study provides in vivo demonstration of the role of TRPV3 as a strong modulator of cutaneous vascular thermoregulatory mechanisms.

Acute heat-evoked temperature sensation is impaired but not abolished in mice lacking TRPV1 and TRPV3 channels.

The discovery of heat-sensitive Transient Receptor Potential Vanilloid ion channels (ThermoTRPVs) greatly advanced our molecular understanding of acute and injury-evoked heat temperature sensation. ThermoTRPV channels are activated by partially overlapping temperatures ranging from warm to supra-threshold noxious heat. TRPV1 is activated by noxious heat temperature whereas TRPV3 can be activated by warm as well as noxious heat temperatures. Loss-of-function studies in single TRPV1 and TRPV3 knock-out mice have shown that heat temperature sensation is not completely abolished suggesting functional redundancies among these two channels and highlighting the need of a detailed analysis of TRPV1::TRPV3 double knock-out mice (V1V3dKO) which is hampered by the close proximity of the loci expressing the two channels. Here we describe the generation of a novel mouse model in which trpv1 and trpv3 genes have been inactivated using bacterial artificial chromosome (BAC)-based homologous recombination in embryonic stem cells. In these mice, using classical thermosensory tests such hot plate, tail flick and the thermotaxis gradient paradigms, we confirm that TRPV1 is the master channel for sensing noxious heat temperatures and identify a cooperative role of TRPV1 and TRPV3 for sensing a well-defined window of acute moderate heat temperature. Using the dynamic hot plate assay, we unravel an intriguing and unexpected pronounced escape behavior in TRPV1 knock-out mice that was attenuated in the V1V3dKO. Together, and in agreement with the temperature activation overlap between TRPV1 and TRPV3 channels, our data provide in vivo evidence of a cooperative role between skin-derived TRPV3 and primary sensory neurons-enriched TRPV1 in modulation of moderate and noxious heat temperature sensation and suggest that other mechanisms are required for heat temperature sensation.

The timing of emergence of muscle progenitors is controlled by an FGF/ERK/SNAIL1 pathway.

In amniotes, the dermomyotome is the source of all skeletal muscles of the trunk and the limbs. Trunk skeletal muscles form in two sequential stages: in the first stage, cells located at the four borders of the epithelial dermomyotome delaminate to generate the primary myotome, composed of post-mitotic, mononucleated myocytes. The epithelio-mesenchymal transition (EMT) of the central dermomyotome initiates the second stage of muscle formation, characterised by a massive entry of mitotic muscle progenitors from the central region of the dermomyotome into the primary myotome. The signals that regulate the timing of the dermomyotome EMT are unknown. Here, we propose that this process is regulated by an FGF signal emanating from the primary myotome, a known source of FGF. The over-expression of FGF results in a precocious EMT of the dermomyotome, while on the contrary, the inhibition of FGF signalling by the electoporation of a dominant-negative form of FGFR4 delays this process. Within the dermomyotome, FGF signalling triggers a MAPK/ERK pathway that leads to the activation of the transcription factor Snail1, a known regulator of EMT in a number of cellular contexts. The activation or the inhibition of the MAPK/ERK pathway and of Snail1 mimics that of FGF signalling and leads to an early or delayed EMT of the dermomyotome, respectively. Altogether, our results indicate that in amniotes, the primary myotome is an organizing center that regulates the timely entry of embryonic muscle progenitors within the muscle masses, thus initiating the growth phase of the trunk skeletal muscles.

FGFR4 signaling is a necessary step in limb muscle differentiation.

In chick embryos, most if not all, replicating myoblasts present within the skeletal muscle masses express high levels of the FGF receptor FREK/FGFR4, suggesting an important role for this molecule during myogenesis. We examined FGFR4 function during myogenesis, and we demonstrate that inhibition of FGFR4, but not FGFR1 signaling, leads to a dramatic loss of limb muscles. All muscle markers analyzed (such as Myf5, MyoD and the embryonic myosin heavy chain) are affected. We show that inhibition of FGFR4 signal results in an arrest of muscle progenitor differentiation, which can be rapidly reverted by the addition of exogenous FGF, rather than a modification in their proliferative capacities. Conversely, over-expression of FGF8 in somites promotes FGFR4 expression and muscle differentiation in this tissue. Together, these results demonstrate that in vivo, myogenic differentiation is positively controlled by FGF signaling, a notion that contrasts with the general view that FGF promotes myoblast proliferation and represses myogenic differentiation. Our data assign a novel role to FGF8 during chick myogenesis and demonstrate that FGFR4 signaling is a crucial step in the cascade of molecular events leading to terminal muscle differentiation.