An evolutionarily conserved pacemaker role for HCN ion channels in smooth muscle

Area of Science:

• Physiology
• Ion Channel Biology
• Smooth Muscle Physiology

Background:

• Hyperpolarization-activated cation (HCN) ion channels are established cardiac pacemakers, but their function in smooth muscle remains debated.
• Renal pelvic smooth muscle (RPSM) cells in the murine upper urinary tract contain HCN channels and are crucial for peristalsis.
• Previous studies failed to detect the HCN (I_h) current in RPSM cells, questioning their pacemaker identity and the mechanisms in multicalyceal upper urinary tracts.

Purpose of the Study:

• To investigate the role of HCN channels in renal pelvic smooth muscle (RPSM) pacemaker activity.
• To identify the specific cell types responsible for pacemaker activity in the upper urinary tract.
• To determine if HCN channels are conserved in the multicalyceal upper urinary tracts of humans and pigs.

Main Methods:

• Immunopanning purification of HCN-expressing cells from murine renal pelvic smooth muscle.
• Single-molecule super-resolution microscopy (STORM) and whole-tissue imaging.
• Translational studies using homologous human and porcine upper urinary tracts.

Main Results:

• 96% of purified HCN+ cells exhibited the I_h current, confirming their pacemaker potential.
• HCN+ cells were found to integrate into the smooth muscle syncytium and express HCN channels on their plasma membrane.
• Translational studies in humans and pigs confirmed HCN+ cells in calyceal RPSM, with HCN channel block abolishing pacemaker activity and peristalsis.

Conclusions:

• HCN ion channels play a conserved pacemaker role in both cardiac and smooth muscle organs.
• HCN+ cells are the bona fide pacemakers in the renal pelvic smooth muscle of the upper urinary tract.
• Channelopathies affecting HCN channels are potential causes of smooth muscle dysfunction.