Distinct functions of soluble guanylyl cyclase isoforms NO-GC1 and NO-GC2 in inflammatory and neuropathic pain processing

Discovery of the first isoform-specific sGC activator: Selective activation of GC-1 by runcaciguat

Drug research and development programmes targeting soluble guanylyl cyclase (sGC) have been highly successful, leading to the launch of the sGC stimulators riociguat for pulmonary hypertension (2013) and vericiguat for chronic heart failure (2021). As the main receptor for nitric oxide, sGC plays a vital role in various physiological processes. It consists of an alpha and a beta subunit, with two distinct isoforms identified in humans: GC-1 (α1/β1) and GC-2 (α2/β1). Growing evidence indicates that these isoforms engage in different downstream signalling pathways, indicating that isoform-specific approaches could lead to novel therapeutic opportunities and reduce potential side effects. In this study, we performed concentration-response measurements with the sGC activators BAY 60-2770, BI 703704 and runcaciguat (BAY 1101042) in cell systems expressing each isoform and in purified enzymes. We found that runcaciguat selectively activated GC-1, while acting as a competitive antagonist to other activators in GC-2, without interfering with nitric oxide. BAY 60-2770 and BI 703704 activated both isoforms, albeit with varying efficacy. Our findings challenge the historical view that the two sGC isoforms are functionally indistinguishable. In fact, we demonstrate that the isoforms can be activated independently, highlighting their distinct functional profiles. Notably, runcaciguat is the first sGC activator identified to selectively target GC-1 at therapeutic concentrations. This selective targeting of isoforms not only opens avenues for new therapeutic strategies but also provides an alternative to knockout animal models for investigating isoform-specific functions.

The role of nitric oxide and neuroendocrine system in pain generation

Previous studies have indicated a complex interplay between the nitric oxide (NO) pain signaling pathways and hormonal signaling pathways in the body. This article delineates the role of nitric oxide signaling in neuropathic and inflammatory pain generation and subsequently discusses how the neuroendocrine system is involved in pain generation. Hormonal systems including the hypothalamic-pituitary axis (HPA) generation of cortisol, the renin-angiotensin-aldosterone system, calcitonin, melatonin, and sex hormones could potentially contribute to the generation of nitric oxide involved in the sensation of pain. Further research is necessary to clarify this relationship and may reveal therapeutic targets involving NO signaling that alleviate neuropathic and inflammatory pain.

The 10th International Conference on cGMP 2022: recent trends in cGMP research and development-meeting report

Increasing cGMP is a unique therapeutic principle, and drugs inhibiting cGMP-degrading enzymes or stimulating cGMP production are approved for the treatment of various diseases such as erectile dysfunction, coronary artery disease, pulmonary hypertension, chronic heart failure, irritable bowel syndrome, or achondroplasia. In addition, cGMP-increasing therapies are preclinically profiled or in clinical development for quite a broad set of additional indications, e.g., neurodegenerative diseases or different forms of dementias, bone formation disorders, underlining the pivotal role of cGMP signaling pathways. The fundamental understanding of the signaling mediated by nitric oxide-sensitive (soluble) guanylyl cyclase and membrane-associated receptor (particulate) guanylyl cyclase at the molecular and cellular levels, as well as in vivo, especially in disease models, is a key prerequisite to fully exploit treatment opportunities and potential risks that could be associated with an excessive increase in cGMP. Furthermore, human genetic data and the clinical effects of cGMP-increasing drugs allow back-translation into basic research to further learn about signaling and treatment opportunities. The biannual international cGMP conference, launched nearly 20 years ago, brings all these aspects together as an established and important forum for all topics from basic science to clinical research and pivotal clinical trials. This review summarizes the contributions to the "10th cGMP Conference on cGMP Generators, Effectors and Therapeutic Implications," which was held in Augsburg in 2022 but will also provide an overview of recent key achievements and activities in the field of cGMP research.

Targeting the nitric oxide/cGMP signaling pathway to treat chronic pain

Nitric oxide (NO)/cyclic guanosine 3',5'-monophosphate (cGMP) signaling has been shown to act as a mediator involved in pain transmission and processing. In this review, we summarize and discuss the mechanisms of the NO/cGMP signaling pathway involved in chronic pain, including neuropathic pain, bone cancer pain, inflammatory pain, and morphine tolerance. The main process in the NO/cGMP signaling pathway in cells involves NO activating soluble guanylate cyclase, which leads to subsequent production of cGMP. cGMP then activates cGMP-dependent protein kinase (PKG), resulting in the activation of multiple targets such as the opening of ATP-sensitive K+ channels. The activation of NO/cGMP signaling in the spinal cord evidently induces upregulation of downstream molecules, as well as reactive astrogliosis and microglial polarization which participate in the process of chronic pain. In dorsal root ganglion neurons, natriuretic peptide binds to particulate guanylyl cyclase, generating and further activating the cGMP/PKG pathway, and it also contributes to the development of chronic pain. Upregulation of multiple receptors is involved in activation of the NO/cGMP signaling pathway in various pain models. Notably the NO/cGMP signaling pathway induces expression of downstream effectors, exerting both algesic and analgesic effects in neuropathic pain and inflammatory pain. These findings suggest that activation of NO/cGMP signaling plays a constituent role in the development of chronic pain, and this signaling pathway with dual effects is an interesting and promising target for chronic pain therapy.

Lack of efficacy of a partial adenosine A1 receptor agonist in neuropathic pain models in mice

Previous studies suggest that adenosine A₁ receptors (A₁R) modulate the processing of pain. The aim of this study was to characterize the distribution of A₁R in nociceptive tissues and to evaluate whether targeting A₁R with the partial agonist capadenoson may reduce neuropathic pain in mice. The cellular distribution of A₁R in dorsal root ganglia (DRG) and the spinal cord was analyzed using fluorescent in situ hybridization. In behavioral experiments, neuropathic pain was induced by spared nerve injury or intraperitoneal injection of paclitaxel, and tactile hypersensitivities were determined using a dynamic plantar aesthesiometer. Whole-cell patch-clamp recordings were performed to assess electrophysiological properties of dissociated DRG neurons. We found A₁R to be expressed in populations of DRG neurons and dorsal horn neurons involved in the processing of pain. However, administration of capadenoson at established in vivo doses (0.03–1.0 mg/kg) did not alter mechanical hypersensitivity in the spared nerve injury and paclitaxel models of neuropathic pain, whereas the standard analgesic pregabalin significantly inhibited the pain behavior. Moreover, capadenoson failed to affect potassium currents in DRG neurons, in contrast to a full A₁R agonist. Despite expression of A₁R in nociceptive neurons, our data do not support the hypothesis that pharmacological intervention with partial A₁R agonists might be a valuable approach for the treatment of neuropathic pain.

cGMP signalling in dorsal root ganglia and the spinal cord: Various functions in development and adulthood

Cyclic GMP (cGMP) is a second messenger that regulates numerous physiological and pathophysiological processes. In recent years, more and more studies have uncovered multiple roles of cGMP signalling pathways in the somatosensory system. Accumulating evidence suggests that cGMP regulates different cellular processes from embryonic development through to adulthood. During embryonic development, a cGMP-dependent signalling cascade in the trunk sensory system is essential for axon bifurcation, a specific form of branching of somatosensory axons. In adulthood, various cGMP signalling pathways in distinct cell populations of sensory neurons and dorsal horn neurons in the spinal cord play an important role in the processing of pain and itch. Some of the involved enzymes might serve as a target for future therapies. In this review, we summarise the knowledge regarding cGMP-dependent signalling pathways in dorsal root ganglia and the spinal cord during embryonic development and adulthood, and the potential of targeting these pathways.

Functional Coupling of Slack Channels and P2X3 Receptors Contributes to Neuropathic Pain Processing

The sodium-activated potassium channel Slack (K_Na1.1, Slo2.2, or Kcnt1) is highly expressed in populations of sensory neurons, where it mediates the sodium-activated potassium current (I_KNa) and modulates neuronal activity. Previous studies suggest that Slack is involved in the processing of neuropathic pain. However, mechanisms underlying the regulation of Slack activity in this context are poorly understood. Using whole-cell patch-clamp recordings we found that Slack-mediated I_KNa in sensory neurons of mice is reduced after peripheral nerve injury, thereby contributing to neuropathic pain hypersensitivity. Interestingly, Slack is closely associated with ATP-sensitive P2X3 receptors in a population of sensory neurons. In vitro experiments revealed that Slack-mediated I_KNa may be bidirectionally modulated in response to P2X3 activation. Moreover, mice lacking Slack show altered nocifensive responses to P2X3 stimulation. Our study identifies P2X3/Slack signaling as a mechanism contributing to hypersensitivity after peripheral nerve injury and proposes a potential novel strategy for treatment of neuropathic pain.

Genetic manipulation of cyclic nucleotide signaling during hippocampal neuroplasticity and memory formation

Decades of research have underscored the importance of cyclic nucleotide signaling in memory formation and synaptic plasticity. In recent years, several new genetic techniques have expanded the neuroscience toolbox, allowing researchers to measure and modulate cyclic nucleotide gradients with high spatiotemporal resolution. Here, we will provide an overview of studies using genetic approaches to interrogate the role cyclic nucleotide signaling plays in hippocampus-dependent memory processes and synaptic plasticity. Particular attention is given to genetic techniques that measure real-time changes in cyclic nucleotide levels as well as newly-developed genetic strategies to transiently manipulate cyclic nucleotide signaling in a subcellular compartment-specific manner with high temporal resolution.

Redox regulation of soluble epoxide hydrolase does not affect pain behavior in mice

Signaling mediated by soluble epoxide hydrolase (sEH) has been reported to play an important role in pain processing. Previous studies revealed that sEH activity is inhibited by specific binding of electrophiles to a redox-sensitive thiol (Cys521) adjacent to the catalytic center of the hydrolase. Here, we investigated if this redox-dependent modification of sEH is involved in pain processing using "redox-dead" knockin-mice (sEH-KI), in which the redox-sensitive cysteine is replaced by serine. However, behavioral characterization of sEH-KI mice in various animal models revealed that acute nociceptive, inflammatory, neuropathic, and visceral pain processing is not altered in sEH-KI mice. Thus, our results suggest that redox-dependent modifications of sEH are not critically involved in endogenous pain signaling in mice.