Depletion "skraps" and dynamic buffering inside the cellular calcium store.

Area of Science:

• Cellular Physiology
• Biochemistry
• Molecular Biology

Background:

• Calcium (Ca2+) signals regulate cellular functions, including metabolic responses.
• In muscle cells, Ca2+ sparks initiate rapid, cell-wide signaling events.
• Understanding the precise source and dynamics of Ca2+ release is critical.

Purpose of the Study:

• To investigate the spatial and temporal dynamics of Ca2+ release from cellular stores.
• To identify the functional compartment responsible for Ca2+ release in muscle cells.
• To elucidate the role of calsequestrin in Ca2+ signaling and store dynamics.

Main Methods:

• Imaging intracellular Ca2+ concentration within the store using shifted excitation and emission ratioing of fluorescence.
• Analyzing Ca2+ depletion kinetics during localized sparks and voltage-induced cell-wide release.
• Evaluating the proposed role of calsequestrin as a dynamic Ca2+ delivery system.

Main Results:

• Ca2+ depletion kinetics indicate release occurs from a compartment functionally distinct from the store lumen and cytosol.
• This 'proximate' compartment explains the delayed depletion observed even after Ca2+ release channels close.
• A paradoxical surge in intrastore Ca2+ during prolonged release is explained by the collapse of calsequestrin polymers.

Conclusions:

• Ca2+ is released from a proximate compartment, likely composed of calsequestrin polymers, located between the store lumen and cytosol.
• Calsequestrin acts as a dynamic store, efficiently delivering Ca2+ to release channels and sustaining the release gradient.
• This model refines the understanding of Ca2+ signaling and cellular metabolic regulation.