Hyperosmolality activates polycystin-2 and TRPM4 in renal primary cilium

Area of Science:

• Molecular Biology
• Nephrology
• Ion Channel Physiology

Background:

• Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disorder causing renal cysts, linked to mutations in proteins like polycystin-2 (PC2) in primary cilia.
• Primary cilia are crucial for renal function, but their ion channels have not been studied under physiological osmolality conditions.
• The osmolality of renal filtrate varies significantly, suggesting a role for osmosensing in ciliary function.

Purpose of the Study:

• To investigate the activation of ion currents in primary cilia by physiological osmolality, particularly urine.
• To identify the ion channels responsible for these osmolality-activated currents.
• To explore the role of these currents and their activators in ADPKD pathogenesis and ciliary protein function.

Main Methods:

• Electrophysiological recordings of ion currents in primary cilia.
• Utilized urine and defined solutions with varying osmolality (urea, NaCl, D-mannitol) to stimulate cilia.
• Employed pharmacological inhibitors (MgATP, 9-phenanthrol) and shRNA knockdown to identify specific ion channels.

Main Results:

• Urine and hyperosmolar solutions activate a large cation current in primary cilia.
• This current is predominantly conducted by TRPM4 channels, as evidenced by inhibition and knockdown studies.
• A portion of the urea-activated current involves Ca2+ influx through unidentified channels, and external hyperosmolality enhances PC2 channel activity.

Conclusions:

• Hyperosmolality is a physiological stimulus for ciliary cation currents, primarily mediated by TRPM4 channels.
• The identified currents and PC2 channel activation by hyperosmolality may play roles in cellular volume regulation and ADPKD cystogenesis.
• This study reveals novel mechanisms of ciliary ion channel regulation relevant to kidney disease.