2 - Ca2+ signalling

why is Ca2+ important?

used in many cells to trasmit signals.

describe the concentration gradient of calsium?

it is very extreme:

• Inside cells: ~100 nM (very low).

• Outside cells: 1.5–2 mM (much higher).

Calcium affects many affector proteins, give the 2 types + examples?

• Enzymes: kinases, phosphatases and phospholipases.

• Ion channels: potassium, chloride and TRP channels.

Give some examples of processes that calcium regulates?

muscle contraction, hormone release and neurotransmitter release.

what is the Ca²⁺ signalling toolkit?

the systems cells use to control calcium levels, including Ca²⁺ entry/release, buffering, removal and activation of target proteins.

describe calcium entry/release?

ca2+ enters from either outside the cell OR released from intracellular stores.

give some examples of how calcium enters?

• voltage-gated calcium channels (VGCCs): open when the cell membrane depolarises.

• transient receptor potential (TRP channels): allow Ca²⁺ to enter from outside the cell.

• IP3 receptors (IP3R): release Ca²⁺ from the ER when IP₃ binds.

• Ryanoid receptors (RyR): release Ca²⁺ from the ER in response to Ca²⁺.

what do calcium buffers do?

Bind excess Ca²⁺ in the cytoplasm to prevent sudden changes in its concentration.

why are calcium buffers important?

They help control the strength and duration of Ca²⁺ signals and protect cells from Ca²⁺ toxicity.

give examples of calcium buffers?

• Parvalbumin: slow Ca²⁺ buffer.

• Calbindin-28k: rapid Ca²⁺ buffer.

why is removing calcium from the cytoplasm important?

To restore resting levels and prevent toxicity.

give some examples of some mechanism that remove calcium?

• Mitochondria: take up Ca²⁺ and help regulate its concentration.

• PMCA: uses ATP to pump Ca²⁺ out of the cell.

• SERCA: uses ATP to pump Ca²⁺ into the ER for storage.

how can scientists measure changes in Ca²⁺ levels inside cells?

they use fluorescent indicators which bind Ca²⁺ and change their fluorescence, which makes them visible.

There are many pathways for calcium to enter/be released, for each describe the mechanism. 1. ligand-gated ion channels?

Open when a specific ligand binds to them, allowing Ca²⁺ to enter the cell.

TRP and store-operated channels?

allow Ca²⁺ to enter cells.

Voltage-gated calcium channels (VGCCs)?

open when the membrane depolarises.

Conformational couplin?

L-type VGCCs activate ryanoid receptors (RyRs) through physical interaction, triggering Ca²⁺ release from the ER.

Calcium-induced calcium release?

Incoming Ca²⁺ triggers further Ca²⁺ release from intracellular stores.

IP₃ receptor-mediated release?

IP₃ binds to IP₃Rs on the ER, causing Ca²⁺ release.

what are Rapid Ca²⁺ signals?

quick rises in intracellular Ca2+ that trigger fast cellular responses.

what channels are involved in rapid calcium signals?

VGCCs allow Ca²⁺ entry; RyRs release Ca²⁺ from the ER.

What is calmodulin (CaM)?

A protein that binds Ca²⁺ and activates other proteins.

What happens when Ca²⁺ binds to calmodulin?

It activates proteins that regulate ion channels and enzymes.

What are CaMKII and PKC?

Enzymes activated by Ca²⁺ signalling that regulate cellular activities.

How can Ca²⁺ signalling affect gene transcription?

It activates signalling proteins that influence which genes are expressed. (so can switch genes on and off).

What are slow Ca²⁺ signals?

Repeated rises and falls in intracellular Ca²⁺ called oscillations.

what are oscillations?

Repeated increases and decreases in intracellular Ca²⁺ concentration.

What can slow Ca²⁺ signals regulate?

Longer-lasting cellular responses, including changes in gene expression.

What is the key difference between rapid and slow Ca²⁺ signals?

Rapid signals act quickly; slow oscillations can regulate longer-lasting responses.

what do Calcium-binding proteins (buffers) do?

bind ca2+ to reduce fluctuations in intracellular conc. + help protect cells from calcium toxicity.

what is Ca2+ sequestration and extrusion?

• Sequestration: Moving Ca²⁺ into storage compartments, such as the ER or mitochondria.

• Extrusion: Pumping Ca²⁺ out of the cell.

Purpose: Both reduce cytoplasmic Ca²⁺ levels and restore resting concentrations.

what is a Mitochondrial Ca²⁺ uniporter?

transports Ca²⁺ into mitochondria, driven by the mitochondrial membrane potential.

what is a Na⁺/Ca²⁺ exchanger?

exchanges Na⁺ and Ca²⁺ across the plasma membrane.

what is SERCA (sarco-endoplasmic reticulum Ca2+-ATPase)?

Pumps Ca²⁺ into the sarcoplasmic/endoplasmic reticulum using ATP.

WHAT IS PMCA (Plasma Membrane Ca²⁺-ATPase)?

pumps Ca²⁺ out of the cell using ATP.

what does Phospholipase C do?

breaks down PIP₂ → DAG + IP₃.

what does IP3 do?

binds to IP₃ receptors on the endoplasmic reticulum (ER) → Ca²⁺ is released into the cytoplasm.

what are oscillations affected by?

the concentration of the signalling molecule, so a stronger stimulus → more frequent Ca²⁺ oscillations.

how do Ca²⁺ signals spread between neighbouring cells?

through gap junctions, allowing cells to communicate.

what is Calmodulin?

a calcium-binding protein that acts as an intracellular Ca²⁺ sensor.

because calmodulin is such an important protein, how is it’s presence insured in the body?

Encoded by 3 different genes, which produce identical calmodulin proteins.

how does calmodulin work?

It binds Ca²⁺, changes shape + interacts with target proteins to control their activity.

what are some examples of things that Ca2+/calmodulin complex interacts with + controls their activity?

• CaMKII: a Ca²⁺/calmodulin-dependent protein kinase that phosphorylates target proteins.

• Calcineurin: a Ca²⁺/calmodulin-dependent phosphatase that removes phosphate groups from proteins.

• Myosin Light Chain Kinase (MLCK): phosphorylates myosin light chains → promotes smooth muscle contraction.

• Ion channels: calmodulin regulates their activity → affects ion movement across cell membranes.

explain the process of Ca²⁺/Calmodulin-dependent protein kinase II (CaMKII)?

Ca²⁺ binds to calmodulin, which then binds to CaMKII. CaMKII phosphorylates itself + becomes active.

Even when Ca2+ levels in the cell decrease because it has autophosphorylated. - some sort of molecular memory as it remm=embers the presence of calcium and it’s still active.

how does CaMKII become inactive?

Protein phosphatases remove the phosphate groups from CaMKII.

what is CamKII dependent on?

frequency of Ca²⁺ oscillations, not just the intracellular Ca²⁺ concentration.

so why is CaMKII usefull?

because it can can produce longer-lasting changes in cell activity through autophosphorylation.

what is a cell process that CamKII can help regulate?

Cell division.