The incidence of chronic pain is estimated to be 20-25% worldwide, thus making it a serious health concern. In order to elaborate alternative therapeutic strategies, we study the mechanisms by which a specific class of low threshold mechanoreceptors, the C-LTMRs, modulate pain transmission within spinal networks.
Pain is a physiological signal that contributes to the protection of individuals from potentially harmful stimuli. In pathological conditions, however, maladaptive plasticity of primary afferent neurons conveying sensory information, or of neuronal networks transmitting this information towards the brain, can induce long lasting intense pain sensations without any protective value. Sensory and noxious information is transmitted from the periphery of the organism by morphologically and functionally specialized neurons. Among these neurons, a specific class of mechanoreceptors, the C-Low Threshold Mechanoreceptors (C-LTMRs), has received recent emphasis through studies unraveling their role in the modulation of pain transmission. Hence, this particular population of neurons is likely to constitute an important yet unexplored target for the treatment of chronic pathological pain. <br />The C-LTMR project aims at developing new tools in order to decipher the role of C-LTMRs in controlling nociceptive information transmission and integration within spinal cord. In line with these fundamentals investigations, we aim at developing new therapeutic approaches for chronic pain relief in patients.<br />
his project relies on the development of novel and groundbreaking techniques in the field of sensory systems physiology. The first technological development will allow us to quickly establish the gene expression profile of several subtypes of sensory neurons (among which the C-LTMRs), in different physio-pathological contexts. Using fluorescent cell sorting followed by deep RNA sequencing, we can target few proteins whose expression is specifically altered in a category of sensory neurons in chronic pain conditions. Using genetic engineering, we can elaborate models with altered expression of these proteins, and then study the consequences of this alteration on
Spinal pain networks operation.
The second major technological approach is to develop genetically engineered mice in which C-LTMRs can be specifically and precisely manipulated with light beams. Using these tools which are currently under heavy development, we will be able to understand how these neurons control the integration of nociceptive information with other sensory modalities.
After 18 month, two striking results have emerged from the collaboration of the 3 teams involved in this joint ANR.
First, we identified a protein whose expression is restricted to low threshold mechanoreceptors. We provide evidence that this protein plays a key by turning transient pain, such as the pain induced by inflammatory agents, into long lasting unrecoverable chronic pain. This striking discovery will open the door for the development of new therapeutic strategies in the treatment of chronic pain patients.
Second, we have established a first functional sketch of the neuronal circuits recruited by C-LTMRs to modulate nociceptive information transmission and alleviate pain. Namely, our results identify spinal inhibitory interneurons as a key partner of C-LTMRs and highlight an exquisite interaction between C-LTMRs, spinal inhibitory interneurons, and spinal microglial cells.
Our experimental strategy as well as our promising preliminary results have considerably enriched our molecular and cellular knowledge of pain pathways, and we can now propose several roles for C-LTMRs in modulating pain transmission. A first sketch of the neuronal circuits involved in C-LTMRs mediated pain attenuation is now available, which will undoubtefully result in the development of new therapeutic approaches for chronic pain relief.
A first paper was recently submitted (Neuron), in which we identified a protein expressed by low threshold mechanoreceptors as a pivot for the development of chronic mechanical allodynia. Another publication identifying the spinal target neurons of C-LTMRs and their complex interaction with microglia is now close to being submitted.
Pain is a physiological signal that contributes to the protection of individuals from potentially harmful stimuli. In pathological conditions, however, maladaptive plasticity of primary afferent neurons conveying sensory information, or of neuronal networks transmitting this information towards the brain, can induce long lasting intense pain sensations without any protective value. Sensory and noxious information is transmitted from the periphery of the organism by morphologically and functionally specialized neurons. Among these neurons, a specific class of mechanoreceptors, the C-Low Threshold Mechanoreceptors (C-LTMRs), has received recent emphasis through studies unraveling their role in the modulation of pain transmission. Hence, this particular population of neurons is likely to constitute an important yet unexplored target for the treatment of chronic pathological pain.
Our project will combine molecular, cellular, electrophysiological and behavioral approaches to investigate how C-LTMRs control the integration of nociceptive information in spinal cord, and how their dual function in mediating both pleasant aspects of touch and unpleasant painful information can be achieved. Using optogenetics, we will decipher neuronal and synaptic pathways involved in the modulation of pain transmission by C-LTMRs. In parallel, we will first launch a wide genome screen combining FACS sorting and RNA Seq to identify novel genes specifically expressed in C-LTMRs, alter their expression using knock-down and overexpression experiments, and determine their roles in modulating cell excitability, spinal networks physiology, and pain sensation. The project comprises 2 main tasks addressed by three partner teams.
In Task 1: We will combine genetic, morphological and functional approaches to resolve the organization of spinal networks processing C-LTMRs-triggered information, and how, within these networks, C-LTMRs-triggered information interferes with noxious information to attenuate its propagation to higher brain structures. i) Using genetically engineered mice expressing the trans-synaptic tracer WGA specifically in C-LTMRs, we will perform a first characterization of spinal neurons connected to C-LTMRs. ii) By combining spinal slice patch clamp recordings with cre-lox AAV based strategy to target the expression of channel rhodopsin to C-LTMRs, we will confirm the identification of spinal interneurons receiving inputs from C-LTMRs, characterize the pharmacology, short and long term plasticity of these inputs. iii) We will determine how these inputs propagate to the spinal projection neurons identified by retrograde labelling. iv) Finally, using highly challenging dual optical control, we will decipher the mechanisms by which noxious (controlled with TRPV1 permeant ion channel photoswitch) and non-noxious (controlled with genetically targeted channelrhodopsin expression in C-LTMRs) information gets integrated in dorsal horn networks.
In Task 2: We will determine how the fine tuning of nociceptive transmission by C-LTMRs is altered in chronic pain conditions. i) We will use FACS sorting followed by RNA Seq to identify selective markers of C-LTMRs, characterize their expression in neuropathic animals using qRT-PCR and histological techniques, thus enabling the selection of few functionally relevant genes. ii) Using knock-down and overexpression approaches, we will characterize how these genes shape the activity of C-LTMRs and their response to mechanical or chemical stimuli, in control and neuropathic animals. iii) We will determine how these markers alter sensory-nociceptive transmission in spinal networks and pain behavior in acute and neuropathic pain models.
This project will not only extend our knowledge on the physiology of C-LTMRs but also on the role of this unique population of primary sensory neurons in pain processing very likely leading to the identification of new potential therapeutic targets.
Monsieur Yves Le Feuvre (Institut interdisciplinaire de neurosciences)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
IGF Institut de Genomique Fonctionnelle
CNRS DR 12 IBDM CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION PROVENCE ET CORSE - INSTITUT DE BIOLOGIE DU DEVELOPPEMENT DE MARSEILLE
IINS Institut interdisciplinaire de neurosciences
Help of the ANR 414,499 euros
Beginning and duration of the scientific project: December 2014 - 36 Months