An evolutionarily conserved pacemaker role for HCN ion channels in smooth muscle. (17th March 2023)

• Record Type: Journal Article
• Authors: Yang, Lei; Arbona, Rodolfo J. Ricart; Smith, Carl S.; Banks, Kelly M.; Thomas, V. Kaye; Palmer, Lawrence; Evans, Todd; Hurtado, Romulo
• Is Part Of: Journal of physiology. Volume 601:Number 7(2023)
• Page Start: 1225
• Page End: 1246
• Publication Date: 2023-03-17
• Subjects: HCN channels -- Ih funny current -- pacemakers -- PDGFR‐α -- peristalsis -- renal pelvis -- smooth muscle; Physiology -- Periodicals; 612.005
• DOI: 10.1113/JP283701
• Languages: English
• ISSNs: 0022-3751

Abstract

Abstract : Abstract: Although hyperpolarization‐activated cation (HCN) ion channels are well established to underlie cardiac pacemaker activity, their role in smooth muscle organs remains controversial. HCN‐expressing cells are localized to renal pelvic smooth muscle (RPSM) pacemaker tissues of the murine upper urinary tract and HCN channel conductance is required for peristalsis. To date, however, the I h pacemaker current conducted by HCN channels has never been detected in these cells, raising questions on the identity of RPSM pacemakers. Indeed, the RPSM pacemaker mechanisms of the unique multicalyceal upper urinary tract exhibited by humans remains unknown. Here, we developed immunopanning purification protocols and demonstrate that 96% of isolated HCN+ cells exhibit I h . Single‐molecule STORM to whole‐tissue imaging showed HCN+ cells express single HCN channels on their plasma membrane and integrate into the muscular syncytium. By contrast, PDGFR‐α+ cells exhibiting the morphology of ICC gut pacemakers were shown to be vascular mural cells. Translational studies in the homologous human and porcine multicalyceal upper urinary tracts showed that contractions and pacemaker depolarizations originate in proximal calyceal RPSM. Critically, HCN+ cells were shown to integrate into calyceal RPSM pacemaker tissues, and HCN channel block abolished electrical pacemaker activity and peristalsis of the multicalyceal upper urinary tract. Cumulatively, these studies demonstrate that HCN ion channels play a broad, evolutionarily conserved pacemaker role in both cardiac and smooth muscle organs and have implications for channelopathies as putative aetiologies of smooth muscle disorders. Key points: Pacemakers trigger contractions of involuntary muscles. Hyperpolarization‐activated cation (HCN) ion channels underpin cardiac pacemaker activity, but their role in smooth muscle organs remains controversial. Renal pelvic smooth muscle (RPSM) pacemakers trigger contractions that propel waste away from the kidney. HCN+ cells localize to murine RPSM pacemaker tissue and HCN channel conductance is required for peristalsis. The HCN ( I h ) current has never been detected in RPSM cells, raising doubt whether HCN+ cells are bona fide pacemakers. Moreover, the pacemaker mechanisms of the unique multicalyceal RPSM of higher order mammals remains unknown. In total, 97% of purified HCN+ RPSM cells exhibit I h . HCN+ cells integrate into the RPSM musculature, and pacemaker tissue peristalsis is dependent on HCN channels. Translational studies in human and swine demonstrate HCN channels are conserved in the multicalyceal RPSM and that HCN channels underlie pacemaker activity that drives peristalsis. These studies provide insight into putative channelopathies that can underlie smooth muscle dysfunction. Abstract : Abstract figure legend HCN channels play an evolutionarily conserved pacemaker role in renal pelvic smooth muscle (RPSM) of lower and higher order mammals. The function of hyperpolarization‐activated cation (HCN) channels in smooth muscle pacemakers remains controversial. Renal pelvic smooth muscle pacemakers trigger smooth muscle contractions that expel waste from the kidney, and HCN channels have been localized to these pacemaker tissues. To date, however, the mechanisms underlying RPSM pacemaker activity remain elusive. RPSM pacemaker activity was investigated in both lower (top left) and higher order (bottom left) mammalian models, which exhibit divergent upper urinary tract anatomies. We performed morphological and functional studies from the single‐molecule to the whole‐organ level and showed that HCN channels drive RPSM pacemaker activity. RPSM pacemakers (boxed regions) integrated into the muscular syncytium, expressed HCN channels on their plasmalemma and exhibited the I h 'funny' pacemaker current conducted by HCN channels. Critically, HCN channel block abolished electrical pacemaker activity and peristaltic smooth muscle contractions in both lower and higher order mammalian upper urinary tracts. Thus, HCN channels play an evolutionarily conserved pacemaker role in RPSM pacemakers.