A TRPM4-dependent current in murine renal primary cilia

Area of Science:

• Cell Biology
• Ion Channel Physiology
• Renal Physiology

Background:

• Defects in primary cilia are linked to human diseases, including polycystic kidney disease.
• Primary cilia contain Ca²⁺-gated ion channels, such as TRPP2, implicated in cystogenesis.
• Altered apical Ca²⁺ homeostasis is suspected in ciliary disease development.

Purpose of the Study:

• To identify and characterize Ca²⁺-gated ion channels in the primary cilia of renal epithelial cells.
• To investigate the functional properties and regulation of newly identified ciliary channels.
• To explore the potential role of these channels in renal primary cilia function and disease.

Main Methods:

• Direct electrophysiological recordings from native primary cilia of mIMCD-3 cells.
• Characterization of channel properties including Ca²⁺ sensitivity, ion permeability, and modulation by MgATP and PIP2.
• Gene silencing using short hairpin RNA (shRNA) to reduce TRPM4 expression and assess its impact on cilia length and channel activity.

Main Results:

• Identified transient receptor potential cation channel, subfamily M, member 4 (TRPM4) as a Ca²⁺-gated channel in primary cilia.
• TRPM4 exhibits low Ca²⁺ sensitivity (EC₅₀ = 646 μM), is inhibited by MgATP, and is permeable to K⁺ over Na⁺ (PK/PNa = 1.42).
• TRPM4 knockdown significantly reduced TRPM4 current and primary cilia length, and its activity is modulated by phosphatidylinositol 4,5-bisphosphate (PIP2).

Conclusions:

• TRPM4 is a novel Ca²⁺-gated ion channel present in renal primary cilia.
• TRPM4 activity is regulated by intracellular Ca²⁺ levels and PIP2, with potential implications for apical Ca²⁺ dynamics.
• TRPM4 may contribute to renal primary cilia function, potentially in conjunction with TRPP2, and warrants further investigation in kidney diseases.