Tuning local calcium availability: cell-type-specific immobile calcium buffer capacity in hippocampal neurons

Area of Science:

• Neuroscience
• Cellular Biology
• Biophysics

Background:

• Intracellular calcium buffers are vital for neuronal function, but the role of immobile buffers remains unclear due to unknown quantities.
• Quantifying fixed calcium buffers is essential for understanding their contribution to cellular calcium signaling.

Purpose of the Study:

• To quantify the buffering capacity of fixed intracellular calcium buffers in central neurons.
• To investigate the functional impact of fixed buffers on calcium signal propagation and neuronal signaling.

Main Methods:

• Utilized prolonged whole-cell patch-clamp dialysis to isolate fixed buffer fractions in murine hippocampal slices.
• Employed confocal calcium imaging and the "added-buffer" approach to measure buffering capacity.
• Performed theoretical analysis of calcium spread to assess the impact of fixed buffers on calcium mobility.

Main Results:

• Quantified a substantial fixed buffer capacity (κfixed ≈ 100) in dentate granule cells and a lower capacity (κfixed ≈ 30) in CA1 OLM interneurons.
• Demonstrated that fixed buffers significantly reduce intracellular calcium mobility (100-fold in granule cells, 30-fold in OLM cells), slowing calcium signals.
• Showed that high fixed buffer capacity optimizes the effects of mobile buffers like calbindin D28k, sharpening calcium gradients without altering peak concentrations.

Conclusions:

• The amount of fixed intracellular calcium buffers critically influences calcium signal dynamics and temporal availability for binding partners.
• Fixed buffers play a pivotal role in determining the diversity of cellular calcium signaling patterns.
• Understanding fixed buffer capacity is key to deciphering neuronal calcium signaling complexity.