Cell signalling I - ion gradient

What is Signalling?

  • Definition: A cascade of processes through which an extracellular stimulus (e.g., neurotransmitter or hormone) effects a change in cell function.

  • Components of signalling:

    • Stimulus: External trigger that initiates the signalling process.

    • Response: The outcome of the signalling process within the cell.

    • Nerve communication (neurotransmitter action)

    • Fight or flight response (stress signalling)

Types of Signalling

  1. Signalling through Ion Channels

    • Involves the flow of ions across membranes, typically through selective membrane proteins (such as carrier proteins and channel proteins)

    • Key ions include:

      • K^+ (Potassium)

      • Na^+ (Sodium)

      • Ca^{2+} (Calcium)

  2. Signalling through Receptors

    • External signal molecules bind to receptor proteins, initiating intracellular signalling pathways.

    • Involved Proteins:

      • Intracellular signalling proteins

      • Target proteins (e.g., metabolic enzymes, gene regulatory proteins, cytoskeletal proteins)

    • Effects:

      • Altered metabolism

      • Changes in gene expression

      • Modification of cell shape or movement

Signalling through Ion Channels

  • Ion channels are selective for specific ions and play a crucial role in electrical signalling.

  • Ca^{2+} is a universal signalling ion that plays important roles in various cellular activities.

Cell Membrane Permeability:

  • Ions (charged) are normally not permeable across cell membranes due to their charge, unlike gases.

  • Mechanisms to Transport Ions:

    • Carrier Proteins: Undergo large conformational changes to move ions across membranes.

    • Ion Channels: No conformational changes. Allows for ion passage, crucial for signalling.

Calcium Signalling

  • Concentration changes of Ca^{2+} induce various cellular responses:

    • Exocytosis (microseconds)

    • Muscle contraction (milliseconds)

    • Gene transcription (minutes)

    • Fertilization (hours)

Ion Gradients

  • Membranes are typically impermeable to ions, but protein channels allowed ions to across and establish concentration gradients.

  • Differences in ion concentrations between intracellular and extracellular environments:

    • Sodium (Na+): High concentration outside the cell, low inside, critical for depolarization during action potentials.

    • Potassium (K+): High concentration inside the cell, low outside, essential for repolarization and maintaining resting membrane potential.

    • Calcium (Ca2+): Low concentration inside the cell, high outside, plays a vital role in signaling pathways and muscle contractions.

    • Chloride (Cl-): Varies across different cell types but generally moves with other ions to maintain electrochemical balance.

  • Ions exist in gradients across both the plasma membrane and organelles.

  • Organelles and Gradients:

    • Calcium concentrations differ in organelles like the endoplasmic reticulum (0.5 mM).

    • Proton gradients exist across organelle membranes, crucial for cellular processes.

Ion Transport Mechanisms

  • Types of Ion Transport:

    • Passive Transport:

    • Simple diffusion and channel-mediated or carrier-mediated diffusion.

      • Carrier proteins can both do passive AND active transport.

      • Down the concentration gradient.

    • Active Transport: Requires energy (ATP).

    • Electrochemical Gradient:

      • Considers both concentration and electrical gradients affecting ion movement.

Distribution of ions across the membrane in gradients by P-type ATPase to establish ion and electrical gradients and other coupled secondary transport

  • Ion gradients are generated by active transport via P-type ATPases.

  • Electrochemical gradient is the combined effect of both the ion gradient and the electrical potential difference. It is important to distinguish these two because of cations and anions. For example, Cl- commonly has a higher concentration outside of the cell, so the exterior of the cell is relatively negative. While for Na2+ which also has a higher concentration outside the cell, it is the interior of the cell that is relatively negative.

  • Passive transport is established by electrochemical gradient.

  • Gradients exist not only between the inside and outside of cell, but also between the cytoplasm and the inside of organelles (e.g. endoplasmic reticulum, lysosome, mitochondria).

  • Coupled transport is enabled once an electrochemical gradient is established. This is an example of secondary transport. Symport and antiport are examples of coupled transporters also called exchangers.

    • The Na gradient is used to drive Ca2+ efflux via the Na+/Ca2+ antiporter (NCX Na+/Ca2+ exchange). Because Na+ concentration is higher outside the cell, movement of Na+ into the cell is favoured. Antiporter utilises the energy generated by this strong inward movement down the concentration gradient to drive outward movement of Ca2+ ions against the concentration gradient. The entry of Na+ is said to be coupled to the efflux of Ca2+. This is enabled by transport mechanisms such as the Na+/Ca2+ antiporter.

  • The interior of the cell is negatively charged relative to the outside due to the distribution of ions. As a result, positively charged Na+ are attracted to the negatively charged interior of the cell.

Active Transport and examples of P-type ATPase

  • The Na+-K+ pump is a P-type ATPase (gets phosphorylated as it pumps ion against the concentration gradient using energy from ATP hydrolysis) that establishes gradients of Na^+ and K^+ across the plasma membrane.

    • Mechanism:

      • Pumps 3 Na^+ out and 2 K^+ into the cell per ATP used.

  • A second example is PMCA (plasma membrane calcium ATPase) establishes Ca2+ gradient across the plasma membrane.

  • A third example is SERCA (Sarco/Endoplasmic Reticulum Calcium ATPase).

  • In addition to P-type ATPases, there are several other types of ATPases, each with distinct functions and characteristics:

    1. F-type ATPases:

      • These are involved in the synthesis of ATP using a proton gradient generated by electron transport chains.

      • They are commonly found in the inner mitochondrial membrane and bacterial plasma membranes.

      • The example includes ATP synthase, which synthesizes ATP during oxidative phosphorylation.

    2. V-type ATPases:

      • Primarily found in the membranes of organelles such as lysosomes.

      • They transport protons (H+) into organelles, creating an acidic environment essential for various cellular processes.

      • V-type ATPases do not phosphorylate themselves like P-type ATPases. They use energy derived from ATP hydrolysis.

    3. ABC transporters (ATP-binding cassette transporters) - involved in multidrug resistance.

Ion Channel Proteins

  • Classification:

    • Voltage-Gated Ion Channels:

      • Open in response to changes in membrane potential.

      • Voltage-gated ion channels are evolutionarily related (S4). (see figure in Anki) It is composed of a voltage-sensing domain and a pore-domain. The S4 region is conserved and is responsible for triggering conformation change upon changes in potential difference.

      • Voltage-gated ion channels are crucial for signal propagation in neurons.

    • Ligand-Gated Ion Channels: Open when specific molecules bind.

    • Mechanically-Gated Ion Channels: Respond to mechanical forces.

Voltage-gated channel composed of voltage-sensing domain and pore-domain - special case of ion Selectivity in K+ channels

  • Ion selectivity mediated by structures such as the selectivity filter that allow passage of specific ions (e.g., K^+ over Na^+).

  • The selectivity filter stabilises the larger dehydrated K+ ion over Na+ ion. Hydrated just means ion not surrounded by water but instead by the channel. It is energetically stable and favourable.

  • Positively charged residues (arginine and lysine) within the S4 domain mediate the response to potential changes, allowing conformational changes in channel structure.

  • These channels share a common tetrameric structure across different ion types (Na+, K+, Ca2+).

Further info: How does the electrochemical gradient of Na+ help its own influx?

The electrochemical gradient of Na+Na+ plays a critical role in facilitating its influx into the cell through the following mechanisms:

  1. Concentration Gradient: There is a high concentration of Na+Na+ ions outside the cell compared to the inside. This difference in concentration creates a natural tendency for Na+Na+ ions to move into the cell, where their concentration is lower.

  2. Electrical Gradient: The interior of the cell is negatively charged relative to the outside due to the distribution of ions, particularly potassium (K+K+) and other anions. As a result, positively charged Na+Na+ ions are attracted to the negatively charged interior of the cell.

  3. Combined Effect: Together, the concentration gradient (which encourages Na+Na+ influx due to a higher outside concentration) and the electrical gradient (which attracts Na+Na+ ions due to the negative interior) create a strong electrochemical gradient driving Na+Na+ ions into the cell.

  4. Resulting Action: This influx of Na+Na+ ions is essential for processes such as depolarization during action potentials in neurons and muscle cells, allowing the propagation of electrical signals and muscle contraction, respectively.