Apocalmodulin itself promotes ion channel opening and Ca(2+) regulation

Area of Science:

• Molecular and Cellular Neuroscience
• Biophysics
• Ion Channel Physiology

Background:

• Calmodulin (CaM) typically modulates target proteins upon binding calcium (Ca2+).
• The calcium-free form (apoCaM) was considered a dormant Ca2+ sensor for voltage-gated Ca2+ channels, awaiting Ca2+ to inhibit channel opening.
• This established model suggested apoCaM passively associates with channels before Ca2+ triggers inhibition.

Purpose of the Study:

• To investigate the direct modulatory role of apoCaM on ion channel function.
• To challenge the prevailing view of apoCaM as an inert Ca2+ sensor.
• To explore the implications of apoCaM's interaction with channel variants.

Main Methods:

• Single-molecule measurements of ion channels.
• Chemical dimerization techniques to elevate apoCaM levels.
• Analysis of RNA-edited and spliced channel variants' affinities for apoCaM.

Main Results:

• ApoCaM binding alone significantly upregulates ion channel opening, demonstrating potent activation.
• Ca2+ binding to CaM reverses this initial upregulation, suggesting a dual regulatory mechanism.
• Differential affinities of apoCaM for channel variants correlate with functional diversity and neuronal action potential elongation.

Conclusions:

• ApoCaM is an active and potent regulator of ion channels, not merely a dormant sensor.
• The apoCaM-mediated upregulation mechanism is a fundamental principle in ion channel modulation, also observed in voltage-gated Na+ channels.
• This finding redefines the role of apoCaM in cellular signaling and neuronal excitability.