The Structure And Function Of Specialized Cutaneous Schwann Cells In Pain And Pruritus Sensations

More and more studies have shown that glial cells play an important role in the initiation and maintenance of chronic pain.Schwann cells（SCs）are glial cells in the peripheral nervous system,whose main function is to support,nourish,and form myelin sheath of nerve fibers;Schwann cells are also involved in the development of chronic pain.The primary sensory neurons in dorsal root ganglion（DRG）play a key role in the perception and regulation of pain,itch,and other sensory information.A-fibers of DRG neurons are completely wrapped by myelin sheath,while C-fibers have no myelin sheath.Recently,a study published in Science found that there is a kind of specialized cutaneous Schwann cells in the skin,which can participate in the regulation of the perception of nociceptive information related to C-fibers,and may be considered as a new organ for pain perception.However,the three-dimensional structure of the specialized cutaneous Schwann cells is not clear,and its function in chronic pain and pruritus is still unknown.In this study,we used the CUBIC（Clear,Unobstructed Brain/Body Imaging Cocktails and Computational analysis）method to transparentize the skin tissue and perform 3D reconstruction.We found that there were Plp1⁺specialized cutaneous Schwann cell in the skin.The cell bodies of specialized cutaneous Schwann cells are mainly located in the dermis-epidermis junction area of the skin;they emit processes from each other and form a mesh-like network structure;many processes extend into the epidermis layer and have a close connection with A-fiber or C-fiber primary sensory nerve terminals.Optogenetic activation of specialized cutaneous Schwann cells（Plp1-Ch R2）significantly reduced the threshold of mechanical pain in mice,and induced nociceptive reflex behaviors such as foot withdrawal,flinching,holding,and licking in mice,but it did not produce obvious aversive emotion related behaviors involving emotion-motivation such as jumping,vocalization,and place avoidance.Moreover,the behavioral responses induced by optogenetic activation of specialized cutaneous Schwann cells was significantly weaker than that induced by optogenetic activation of C-fibers（SNS-Ch R2）.The behavioral responses induced by optogenetic activation of Plp1⁺Schwann cells could be significantly inhibited by pharmacologically selective blocking of A-fibers or C-fibers.These results suggest that specialized cutaneous Schwann cells may be involved in the regulation of nociceptive reflex behaviors by affecting the excitability of A-fibers and C-fibers of DRG neurons.In addition,we used several chronic pain models to analyze and compare the morphological and functional changes of specialized cutaneous Schwann cells.We found that:in the model of chronic inflammatory pain induced by CFA（Complete Freund’s adjuvant）,the morphology and distribution of specialized cutaneous Plp1⁺Schwann cells did not change significantly,and there was no obvious abnormality in the behavior of foot withdrawal,flinching,and holding caused by optogenetic activation of specialized cutaneous Schwann cells.In the model of neuropathic pain induced by CCI（Chronic constriction injury）,the morphology and distribution of specialized cutaneous Schwann cells did not change significantly,but the behavioral response induced by optogenetic activation of Schwann cells was slightly increased.The results showed that there was no obvious abnormality in the morphology and distribution of specialized cutaneous Schwann cells in PAX（paclitaxel）-induced chemotherapy pain model,and the behavioral responses caused by optogenetic activation of specialized cutaneous Schwann cells were slightly decreased.In STZ（Streptozotocin）-induced diabetic neuropathic pain model,there was no obvious abnormality in the morphology and distribution of specialized cutaneous Schwann cells.The distribution of specialized cutaneous Schwann cells and the density of Schwann cell processes in the epidermis were significantly reduced,and the behavioral responses induced by optogenetic activation of Schwann cells were significantly decreased.These results indicate that specialized cutaneous Schwann cells are not the key regulators of the initiation and maintenance of chronic pain.We also found that there were specialized Schwann cells in the hairy skin on the back of neck.The cell bodies were mainly distributed at the junction of dermis and epidermis.They emitted processes into the epidermis and accompanied with A-fiber and C-fiber nerve terminals.Optogenetic activation or inhibition of specialized cutaneous Plp1⁺Schwann cells did not affect the basal spontaneous and mechanical itching behaviors in mice.In the experiment of acute chemical itch behavioral testing,optogenetic activation of specialized cutaneous Schwann cells can enhance the acute itch behavior induced by H₂O₂,while optogenetic inhibition of these cells can produce the opposite effect.In addition,optogenetic inhibition of specialized cutaneous Schwann cells can significantly alleviate the acute itch behavior induced by compound48/80 and chloroquine,and significantly inhibit the activation of spinal dorsal horn neurons related to itching.However,in the AEW（Acetone-Ether-Water）-induced chronic itching model,the morphology and distribution of specialized cutaneous Schwann cells did not change significantly,and optogenetic activation of specialized cutaneous Schwann cells did not change the spontaneous and mechanical itching behavior of AEW model mice.In summary,the main conclusions of this study are as follows:（1）there are specialized cutaneous Schwann cells in the skin,and the cell bodies are mainly located in the dermis-epidermis junction area.They emit processes from each other and form a three-dimensional network structure.Many processes extend to the epidermis and have a close connection with the nerve terminals of A-fiber or C-fiber primary sensory neurons;（2）Activation of specialized cutaneous Schwann cells can induce nociception-related reflex behavior,which depends on the activation of A-fibers and C-fibers;（3）Specialized cutaneous Schwann cells do not play a key role in the occurrence and maintenance of chronic inflammatory pain and nerve injury-,chemotherapy-,diabetes-induced neuropathic pain;（4）Specialized cutaneous Schwann cells in hairy skin are involved in histamine-dependent or histamine-independent acute pruritus,but they are not the key regulators in the initiation and maintenance of AEW-induced chronic pruritus.