These data suggest that redistribution of the proximal tubule Na/K-ATPase in response to endogenous cardiotonic steroids plays an important role in renal adaptation to salt loading.

Pannexins (PANX1, 2, 3) are channel-forming glycoproteins that are expressed throughout the cardiovascular and musculoskeletal system. The canonical function of these proteins is to release nucleotides that act as purinergic signalling at the cell membrane or Ca²⁺ channels at the endoplasmic reticulum membrane. These two forms of signalling are essential for autocrine and paracrine signalling in health, and alterations in this signalling have been implicated in the pathogenesis of many diseases. Many musculoskeletal and cardiovascular diseases are largely the result of a lack of physical activity which causes altered gene expression. Considering exercise training has been shown to alter a wide array of gene expression in musculoskeletal tissues, understanding the interaction between exercise training, gene function and expression in relevant diseases is warranted. With regards to pannexins, multiple publications have shown that exercise training can influence pannexin expression and may influence the significance of its function in certain diseases. This review further discusses the potential interaction between exercise training and pannexin biology in relevant tissues and disease models. We propose that exercise training in relevant animal and human models will provide a more comprehensive understanding of the implications of pannexin biology in disease.

Background: Osteoarthritis (OA) is a multi-factorial disease that is strongly associated with aging. As the molecular mechanisms underpinning the pathogenesis of this disease are partially unclear, there are no disease-modifying drugs to combat OA. The mechanosensitive channel Pannexin 3 (PANX3) has been shown to promote cartilage loss during posttraumatic OA. In contrast, the ablation of Panx3 in male mice results in spontaneous full-thickness cartilage lesions at 24 months of age. Additionally, while protected from traumatic intervertebral disc (IVD) degeneration, Panx3 knockout (KO) mice show signs of IVD disease with altered disc mechanics. Whether the deleterious effects of ablating Panx3 in aging is the result from accumulated mechanical damage is unknown. Methods: Male and female wildtype (WT) and global Panx3 KO C57Bl6 mice were aged to 18 months of age. Mice were then randomized to sedentary (SED) or forced treadmill running (FEX) for 6 weeks (N = 5-14). Knee joint tissues including patellar tendon, quadriceps and distal patellar enthesis, and synovium were analyzed histologically, along with lumbar spine IVDs. Results: Approximately half of male and female Panx3 KO mice developed full-thickness cartilage lesions, severe synovitis, and ectopic fibrocartilage deposition and calcification of the knee joints. Additionally, Panx3 KO mice with severe OA show signs of quadriceps and patellar enthesitis, characterized by bone and marrow formation. Forced treadmill running did not seem to exacerbate these phenotypes in male or female Panx3 KO mice; however, it may have contributed to the development of lateral compartment OA. The IVDs of aged Panx3 KO mice displayed no apparent differences to control mice, and forced treadmill running had no overt effects in either genotype. Conclusion: Aged Panx3 KO mice show histological features of late-stage primary OA including full-thickness cartilage erosion, subchondral bone thickening, and severe synovitis. This data suggests the deletion of Panx3 is deleterious to synovial joint health in aging.

Pannexin 3 (Panx3) is a glycoprotein that forms mechanosensitive channels expressed in chondrocytes and annulus fibrosus cells of the intervertebral disc (IVD). Evidence suggests Panx3 plays contrasting roles in traumatic versus aging osteoarthritis (OA) and intervertebral disc degeneration (IDD). However, whether its deletion influences the response of joint tissue to mechanical stress is unknown. The purpose of this study was to determine if Panx3 deletion in mice causes increased knee joint OA and IDD after forced treadmill running. Male and female wildtype (WT) and Panx3 knockout (KO) mice were randomized to either a no exercise group (sedentary; SED) or daily forced treadmill running (forced exercise; FEX) from 24 to 30 weeks of age. Knee cartilage, tibial subchondral bone and IVD histopathology were evaluated by histology. Both male and female Panx3 KO mice developed larger superficial defects of the tibial cartilage after forced treadmill running compared to SED WT mice. Additionally, both male and female Panx3 KO mice developed a sclerotic secondary ossification center of the tibia with running. In the lower lumbar spine, both male and female Panx3 KO mice developed histopathological features of IDD after running compared to SED WT mice. Clinical Significance: These findings suggest that the combination of deleting Panx3 and forced treadmill running induces OA and causes histopathological changes associated with degeneration of the IVDs in mice.

Pannexin channel isoforms (Panx1‐3) are thought to release nucleotides into the extracellular milieu and have been shown to effect vascular hemodynamics. For this reason, we examined their mRNA and protein expression in hypertensive humans and genetically‐inbred hypertensive mice. In both mouse and humans, we found a significant reduction in Panx3 expression in resistance artery endothelium. Thus, we hypothesized Panx3 may be a regulator of vascular function. In en face endothelial preparations from 3rd order mesenteric arteries, we localized Panx3 to the Golgi Apparatus as opposed to Panx1 which localized to the plasma membrane. Next, we generated an inducible, endothelial cell Panx3 knockout mouse (Panx3ECKO). Radiotelemetry revealed a renin‐independent spontaneous hypertension, with unremarkable immune infiltration in the kidney. There was no change in cytoplasmic or released ATP. To understand how Panx3 may regulate blood pressure, we examined whether Panx3 interacted with B Cell Lymphoma 6 (BCL6), a potential binding partner. En face proximity ligation assays demonstrated an interaction between Panx3 and BCL6 in the Golgi. Panx3ECKO mice exhibited significantly decreased BCL6 protein, hinting that Panx3 may stabilize BCL6 by binding at the BCL6 ubiquitin sites. In silico “threading” of the Panx3 sequence onto the cryo‐EM structure of Panx1 confirmed this site of interaction. BCL6 is a NFκB repressor, thus its degradation in Panx3ECKO mice caused an increase in NFκB activity with IκBα and p100 significantly upregulated. A novel mimetic peptide designed to block Panx3‐BCL6 interactions was administered into C57Bl/6J mice, which recapitulated these results. In addition, Panx3ECKO mice had increased endothelial NOX4 (but not NOX1 or NOX2), likely due to increased NFκB activity—this correlated with a significant increase in plasma H2O2, nitrotyrosine (NT3), and 4‐hydroxynonenal (4‐HNE). In line with this observation, 3rd order mesenteric arteries from Panx3ECKO mice constricted (not dilated) to acetylcholine, which was rescued with the H2O2‐scavenger catalase (1000U/mL), suggesting that vascular oxidative stress drives hypertension in Panx3ECKO mice. Interestingly, Panx3ECKO mice also exhibit a significant increase in IL‐4 receptors on endothelium, and increased IL‐4 cytokines in bone marrow lysates. Because IL‐4 can drive BCL6 expression in other cell types, we suggest a possible homeostatic immune‐endothelial signaling axis. These data elucidate a novel Golgi‐localized oxidative signaling pathway in endothelium with a potential immune‐derived negative feedback loop.