Calcium signalling

Calcium Signalling

Calcium signalling is a crucial cellular process that involves the use of calcium ions (Ca²⁺) as a signal to regulate a variety of cellular functions. This process is fundamental to many physiological activities, including muscle contraction, neurotransmitter release, and gene expression.

Overview

Calcium ions serve as a universal signal transduction element in cells. The concentration of calcium ions within the cytoplasm is tightly regulated, and changes in this concentration can trigger a wide range of cellular responses. Calcium signalling is initiated by the influx of calcium ions into the cytoplasm from either the extracellular space or intracellular stores such as the endoplasmic reticulum.

Mechanisms of Calcium Signalling

Calcium Entry

Calcium ions can enter the cell through various channels located on the plasma membrane. These include:

• Voltage-gated calcium channels (VGCCs): Activated by changes in membrane potential, these channels are crucial in excitable cells such as neurons and muscle cells.
• Ligand-gated calcium channels: These channels open in response to the binding of specific ligands, such as neurotransmitters.
• Store-operated calcium channels (SOCs): Activated when calcium stores in the endoplasmic reticulum are depleted.

Intracellular Calcium Release

Calcium can also be released from intracellular stores through:

• Inositol trisphosphate receptors (IP₃Rs): Activated by inositol trisphosphate (IP₃), a second messenger produced in response to various extracellular signals.
• Ryanodine receptors (RyRs): Primarily found in muscle cells, these receptors release calcium from the sarcoplasmic reticulum to trigger muscle contraction.

Calcium Removal

To restore basal calcium levels, cells employ mechanisms such as:

• Calcium pumps (e.g., SERCA): These pumps transport calcium back into the endoplasmic reticulum.
• Calcium exchangers (e.g., NCX): These transport calcium out of the cell in exchange for sodium ions.

Role in Cellular Functions

Calcium signalling is involved in numerous cellular processes, including:

• Muscle contraction: Calcium binds to troponin, facilitating the interaction between actin and myosin filaments.
• Neurotransmitter release: Calcium influx into presynaptic terminals triggers the release of neurotransmitters into the synaptic cleft.
• Gene expression: Calcium can activate transcription factors that regulate gene expression.

Pathophysiology

Dysregulation of calcium signalling can lead to various diseases, such as:

• Cardiovascular diseases: Abnormal calcium handling can result in arrhythmias and heart failure.
• Neurodegenerative diseases: Impaired calcium homeostasis is implicated in conditions like Alzheimer's disease.
• Muscle disorders: Defects in calcium signalling can cause muscle weakness and fatigue.

Also see

• Signal transduction
• Endoplasmic reticulum
• Neurotransmitter
• Muscle contraction

Cell biology