Pain management by chemogenetic control of sensory neurons
Authors: Yize Li, Xin Ge, Ru-Rong Ji
Journal: Cell Reports Medicine (2023)

In this study, Perez-Sanchez et al.1Perez-Sanchez J. Middleton S.J. Pattison L.A. Hilton H. Ali Awadelkareem M. Zuberi S.R. Renke M.B. Hu H. Yang X. Clark A.J. et al.A humanized chemogenetic system inhibits murine pain-related behavior and hyperactivity in human sensory neurons.Sci. Transl. Med. 2023; 15eadh3839Crossref Scopus (1) Google Scholar developed a chemogenetic method aimed at alleviating pain in mouse models while dampening excitability in human sensory neurons. This analgesic effect was attained through the introduction of human α7 nicotinic acetylcholine receptor and glycine receptor pore domain via virus-mediated expression in sensory neurons, forming a chloride channel. The activation of this channel was made possible by specific agonists. This study highlights the potential for treating clinical pain by gene therapy. In this study, Perez-Sanchez et al.1Perez-Sanchez J. Middleton S.J. Pattison L.A. Hilton H. Ali Awadelkareem M. Zuberi S.R. Renke M.B. Hu H. Yang X. Clark A.J. et al.A humanized chemogenetic system inhibits murine pain-related behavior and hyperactivity in human sensory neurons.Sci. Transl. Med. 2023; 15eadh3839Crossref Scopus (1) Google Scholar developed a chemogenetic method aimed at alleviating pain in mouse models while dampening excitability in human sensory neurons. This analgesic effect was attained through the introduction of human α7 nicotinic acetylcholine receptor and glycine receptor pore domain via virus-mediated expression in sensory neurons, forming a chloride channel. The activation of this channel was made possible by specific agonists. This study highlights the potential for treating clinical pain by gene therapy. Chronic pain represents a formidable clinical challenge because of the lack of effective treatments, necessitating the urgent development of new non-addictive strategies.2Cohen S.P. Vase L. Hooten W.M. Chronic pain: an update on burden, best practices, and new advances.Lancet. 2021; 397: 2082-2097Abstract Full Text Full Text PDF PubMed Scopus (542) Google Scholar The pathophysiology of chronic pain involves tissue damage in cases of arthritic pain or nerve damage in neuropathic pain. This damage triggers inflammation and neuroinflammation (localized inflammation in the peripheral and central nervous system),3Matsuda M. Huh Y. Ji R.R. Roles of inflammation, neurogenic inflammation, and neuroinflammation in pain.J. Anesth. 2019; 33: 131-139Crossref PubMed Scopus (229) Google Scholar potentially contributing to disease progression and persistence of pain.4Colloca L. Ludman T. Bouhassira D. Baron R. Dickenson A.H. Yarnitsky D. Freeman R. Truini A. Attal N. Finnerup N.B. et al.Neuropathic pain.Nat. Rev. Dis. Primers. 2017; 317002Crossref PubMed Scopus (1256) Google Scholar,5Walsh D.A. McWilliams D.F. Mechanisms, impact and management of pain in rheumatoid arthritis.Nat. Rev. Rheumatol. 2014; 10: 581-592Crossref PubMed Scopus (182) Google Scholar Chronic pain is characterized by hyperexcitability of primary sensory neurons, including nociceptors that are responsible for transmitting pain signals from the periphery to the central nervous system.6Gold M.S. Gebhart G.F. Nociceptor sensitization in pain pathogenesis.Nat. Med. 2010; 16: 1248-1257Crossref PubMed Scopus (568) Google Scholar Despite significant progress in understanding the molecular and cellular mechanisms underlying pain pathogenesis, translating these insights into successful treatments has been largely unsuccessful. In this study, Perez-Sanchez et al.1Perez-Sanchez J. Middleton S.J. Pattison L.A. Hilton H. Ali Awadelkareem M. Zuberi S.R. Renke M.B. Hu H. Yang X. Clark A.J. et al.A humanized chemogenetic system inhibits murine pain-related behavior and hyperactivity in human sensory neurons.Sci. Transl. Med. 2023; 15eadh3839Crossref Scopus (1) Google Scholar present a promising chemogenetic approach capable of suppressing the activities of mouse sensory neurons. Through the introduction of designer receptors into specific sensory neuron populations, chemogenetics allows precise control of neuronal firing, thereby mitigating pain in animal models without affecting other physiological processes. Moreover, the study demonstrates the compatibility of this humanized chemogenetic system and its potential in gene therapy by employing human sensory neurons from induced pluripotent stem cells (iPSCs).1Perez-Sanchez J. Middleton S.J. Pattison L.A. Hilton H. Ali Awadelkareem M. Zuberi S.R. Renke M.B. Hu H. Yang X. Clark A.J. et al.A humanized chemogenetic system inhibits murine pain-related behavior and hyperactivity in human sensory neurons.Sci. Transl. Med. 2023; 15eadh3839Crossref Scopus (1) Google Scholar Perez-Sanchez et al. investigated the pharmacologically selective actuator module (PSAM) system, namely PSAM4-GlyR. This system involves a modified human α7 nicotinic acetylcholine receptor fused with a glycine receptor pore domain, creating a chloride channel responsive to specific agonists of α7 nicotinic acetylcholine receptors—uPSEM792 and the FDA-approved drug varenicline—at single nanomolar concentrations.1Perez-Sanchez J. Middleton S.J. Pattison L.A. Hilton H. Ali Awadelkareem M. Zuberi S.R. Renke M.B. Hu H. Yang X. Clark A.J. et al.A humanized chemogenetic system inhibits murine pain-related behavior and hyperactivity in human sensory neurons.Sci. Transl. Med. 2023; 15eadh3839Crossref Scopus (1) Google Scholar Such a design ensures high selectivity for sensory neurons compared to existing chemogenetic tools. Results from this study demonstrate stable expression of PSAM4-GlyR in approximately 50% of sensory neurons, even 11 months after intrathecal injection of the viral vector. Additionally, transfection of PSAM4-GlyR effectively silenced neuronal activity and suppressed behavioral hyperactivity in mouse models of tissue and nerve injuries associated with pain (Figure 1). In this study, Perez-Sanchez et al. meticulously documented the creation of the PSAM4-GlyR construct and its successful applications in silencing mouse sensory neuron activity, suppressing synaptic transmission in nociceptive dorsal horn neurons, and providing analgesia in various murine pain models, including intraplantar complete Freund’s adjuvant (CFA)-induced acute inflammatory pain, intra-articular CFA-induced arthritis pain, and tibial spared-nerve injury-induced neuropathic pain (Figure 1). Activation of PSAM4-GlyR by uPSEM792 or varenicline robustly and reversibly suppressed sensory neuron activities by shunting chloride currents. Particularly, targeting the dorsal root ganglia (DRG) with the PSAM4-GlyR construct effectively inhibited electrical activity in the cell bodies and central endings of sensory neurons, blocking the transmission of nociceptive signals to the spinal cord by inhibiting excitatory synaptic transmission in dorsal horn neurons. Additionally, intra-articular administration of the PSAM4-GlyR construct specifically targeting pain-inducing nerve fibers in the damaged joint reduced reflexive pain behavior and spontaneous pain behaviors associated with arthritis.1Perez-Sanchez J. Middleton S.J. Pattison L.A. Hilton H. Ali Awadelkareem M. Zuberi S.R. Renke M.B. Hu H. Yang X. Clark A.J. et al.A humanized chemogenetic system inhibits murine pain-related behavior and hyperactivity in human sensory neurons.Sci. Transl. Med. 2023; 15eadh3839Crossref Scopus (1) Google Scholar Importantly, the study validated the use of PSAM4-GlyR in a fully humanized system. Given significant species differences between mouse and human sensory neurons, human sensory neurons are often used to study pain mechanisms, test analgesics, and improve translation validity.7Chang W. Berta T. Kim Y.H. Lee S. Lee S.Y. Ji R.R. Expression and Role of Voltage-Gated Sodium Channels in Human Dorsal Root Ganglion Neurons with Special Focus on Nav1.7, Species Differences, and Regulation by Paclitaxel.Neurosci. Bull. 2018; 34: 4-12Crossref PubMed Scopus (0) Google Scholar,8Shiers S. Klein R.M. Price T.J. Quantitative differences in neuronal subpopulations between mouse and human dorsal root ganglia demonstrated with RNAscope in situ hybridization.Pain. 2020; 161: 2410-2424Crossref PubMed Scopus (89) Google Scholar To evaluate the effectiveness of the human-adapted PSAM4-GlyR channel in a model of “human neuropathic pain in a dish,” the authors examined human iPSC-derived sensory neurons obtained from a patient with hereditary erythromelalgia (IEM). This condition is characterized by extremity pain and redness due to a gain-of-function mutation (F1449V) in SCN9A, encoding the NaV1.7 channel. The study revealed a greater change in resting membrane potentials in human sensory neurons compared to mouse sensory neurons following agonist treatment. This observation aligns with the PSAM4-GlyR system being designed based on human protein sequences. Findings in human iPSC-derived sensory neurons emphasize the potential application of PSAM4-GlyR or similar chemogenetic constructs in mitigating ectopic neural activity in human sensory neurons, a primary cause of clinical neuropathic pain. Although human genetic studies have associated SCN9A mutations—both loss of function and gain of function—with various human pain states, clinical development of NaV1.7-selective inhibitors has been disappointing. Challenges in developing subtype-specific inhibitors arise from high sequence and structural similarity between voltage-gated sodium channel (NaV) subtypes. However, epigenetic repression of NaV1.7 in primary afferents via CRISPR-dCas9-based epigenome engineering led to long-term pain inhibition in murine models.9Moreno A.M. Alemán F. Catroli G.F. Hunt M. Hu M. Dailamy A. Pla A. Woller S.A. Palmer N. Parekh U. et al.Long-lasting analgesia via targeted in situ repression of Na(V)1.7 in mice.Sci. Transl. Med. 2021; 13eaay9056Crossref Scopus (42) Google Scholar Recently, a selective NaV1.8 inhibitor, VX-548, demonstrated reduced acute postoperative pain in patients,10Jones J. Correll D.J. Lechner S.M. Jazic I. Miao X. Shaw D. Simard C. Osteen J.D. Hare B. Beaton A. et al.Selective Inhibition of Na(V)1.8 with VX-548 for Acute Pain.N. Engl. J. Med. 2023; 389: 393-405Crossref PubMed Scopus (0) Google Scholar yet its efficacy in treating chronic pain, like neuropathic pain, remains to be investigated. Exploring the analgesic efficacy of a dual inhibitor targeting NaV1.7 and NaV1.8 could be of interest. The scientific community increasingly acknowledges that using a single analgesic may not provide sufficient pain relief, especially for chronic pain sufferers. Therefore, multimodal analgesia combining drugs acting on multiple targets has been proposed for pain management, albeit resulting in complexities, arbitrariness, and uneven pain relief effects. Polypharmacy can also elevate clinical trial costs. The present study offers a novel perspective: directly reducing the excitability of injury-affected peripheral nerves or diseased sensory neurons via chemogenetics may offer a more effective approach to pain control. The justified use of AAVPHP.S as a transfection tool specifically targeting peripheral sensory neurons is encouraging. Notably, the demonstration that intra-articular knee injection of the AAVPHP.S vector led to sensory neuron expression of PSAM4-GlyR for murine arthritic pain relief is significant, as this route of drug administration is common in clinical practice for treating arthritic pain. For the next stage of translation, validating this chemogenetic approach in large animals before proceeding to clinical trials in humans will be crucial. This study is partially supported by DoD grants W81XWH2110755 (X.G.) and W81XWH2110756 (R.-R.J). BioRender was used to prepare the figure with permission. The authors declare no competing interests.