Analysis of cutaneous MRGPRD free nerve endings and C-LTMRs transcriptomes by RNA-sequencing

Molecular Structure, Expression and Role of TAFA4 and its Receptor FPR1 in the Spinal Cord

TAFA chemokine like family member 4 (TAFA4, also named FAM19A4) is a member of the TAFA chemokine like ligand or FAM19A family, which includes TAFA1, TAFA2, TAFA3, TAFA4, and TAFA5 (or FAM19A1, FAM19A2, FAM19A3, FAM19A4, and FAM19A5). They are also referred to as neurokines and are involved in the regulation of a diverse range of cellular processes, including chemotaxis of macrophages, phagocytosis, and release of reactive oxygen species (ROS). TAFA4 is a marker of C-low-threshold mechanoreceptors and is expressed predominantly in nociceptors, such as dorsal root ganglia (DRG). TAFA4 has been implicated in the sensory perception of pain in the spinal cord. Mice with deficiency of TAFA4 demonstrate altered excitability in lamina IIi neurons in DRG in addition to increased mechanical and chemical nociception following inflammation or injury. As a secreted protein, TAFA4 binds to cell surface receptor formyl peptide receptor 1 (FPR1), a G protein-coupled receptor to mediate the chemoattraction of macrophages, phagocytosis, and the inflammatory profile of macrophages. It also interacts with cell surface neurexin to mediate signalling across the synapse. Further understanding the mechanisms by which this conserved protein family regulates diverse biological processes such as in neuronal functions, inflammation, and tissue fibrosis will help to design therapeutic targets for the treatment of TAFA related diseases such as spinal cord injury and neuro-inflammatory disorders.

2D <em>vs</em> 3D morphological analysis of dorsal root ganglia in health and painful neuropathy

Dorsal root ganglia (DRGs) are clusters of sensory neurons that transmit the sensory information from the periphery to the central nervous system, and satellite glial cells (SGCs), their supporting trophic cells. Sensory neurons are pseudounipolar neurons with a heterogeneous neurochemistry reflecting their functional features. DRGs, not protected by the blood brain barrier, are vulnerable to stress and damage of different origin (i.e., toxic, mechanical, metabolic, genetic) that can involve sensory neurons, SGCs or, considering their intimate intercommunication, both cell populations. DRG damage, primary or secondary to nerve damage, produces a sensory peripheral neuropathy, characterized by neurophysiological abnormalities, numbness, paraesthesia and dysesthesia, tingling and burning sensations and neuropathic pain. DRG stress can be morphologically detected by light and electron microscope analysis with alterations in cell size (swelling/atrophy) and in different subcellular compartments (i.e., mitochondria, endoplasmic reticulum, and nucleus) of neurons and/or SGCs. In addition, neurochemical changes can be used to portray abnormalities of neurons and SGC. Conventional immunostaining, i.e., immunohistochemical detection of specific molecules in tissue slices, can be employed to detect, localize and quantify particular markers of damage in neurons (i.e., nuclear expression of ATF3) or SGCs (i.e., increased expression of GFAP), markers of apoptosis (i.e., caspases), markers of mitochondrial suffering and oxidative stress (i.e., 8-OHdG), markers of tissue inflammation (i.e., CD68 for macrophage infiltration) etc. However classical (2D) methods of immunostaining disrupt the overall organization of the DRG, thus resulting in the loss of some crucial information. Whole-mount (3D) methods have been recently developed to investigate DRG morphology and neurochemistry without tissue slicing, giving the opportunity to study the intimate relationship between SGCs and sensory neurons in health and disease. Here, we aim to compare classical (2D) vs whole-mount (3D) approaches to highlight “pros” and “cons” of the two methodologies when analysing neuropathy-induced alterations in DRGs.

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 GABA_A 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.

Peripheral and Central Mechanisms of Itch

Itch is a unique sensory experience that is encoded by genetically distinguishable neurons both in the peripheral nervous system (PNS) and central nervous system (CNS) to elicit a characteristic behavioral response (scratching). Itch interacts with the other sensory modalities at multiple locations, from its initiation in a particular dermatome to its transmission to the brain where it is finally perceived. In this review, we summarize the current understanding of the molecular and neural mechanisms of itch by starting in the periphery, where itch is initiated, and discussing the circuits involved in itch processing in the CNS.