Cell Membrane Proteins: Receptors, Channels, Signalling

Membrane Permeability and Intracellular Homeostasis

• This topic covers membrane permeability and intracellular homeostasis over two lectures by Prof Arwyn T Jones.
• The overall effect on the body starts at the cellular level.
• Aims include revisiting intrinsic membrane proteins, understanding their impact on membrane permeability, and how they transmit information into cells.
• Also covered are why they are drug targets, consequences of dysfunction, mechanisms of cell death (apoptosis vs. necrosis), and integration of material.

Cells and Tissues

• Cells are highly differentiated and specialized.
• Epithelia and endothelia are distinct types of tissues with different functions.

Membrane Permeability - Phospholipid Bilayer

• Polar materials that can permeate include ions, charged drugs, and peptides.
• Apolar materials include glucose, fats, and uncharged drugs.

Membrane Permeability

• The movement of water and solutes across a membrane depends on the solute concentration, leading to hypotonic, isotonic, or hypertonic conditions.

Cellular Environment and Membrane Permeability

• Resting membrane potential is -20 to -200 mV, typically written as -70 mV.
• The plasma membrane acts as a barrier to solute movement.
• Cells maintain their intracellular environment through various mechanisms to maintain appropriate concentrations of ions.
• Examples of intracellular and extracellular concentrations of ions: K+ (139 in/4 out), Na+ (12 in/145 out), Cl- (4 in/116 out).

Membrane Proteins

• Intrinsic membrane proteins include ion channels (e.g., Na+, Ca2+, K+ channels), receptors for neurotransmitters (e.g., acetylcholine), and pumps/transporters (e.g., Na+/K+ pump).
• Extrinsic membrane proteins include recognition or adhesion molecules.

Ion Channels vs Ion Pumps

• Ion channels allow ions to flow down concentration or electrical potential gradients (passive transport).
• Ion pumps actively push ions against a gradient using energy (ATP).
• Both types need to be highly selective for specific ions.

Sodium-Potassium Pump (Na+/K+ ATPase)

• The Na+/K+ ATPase pump uses 20% of a cell's energy.

Membrane Ion Channels

• Examples of membrane ion channels: Na+, Ca2+, K+, and Cl- channels.
• Ion channels are water-filled pores allowing ions to cross the membrane down their concentration gradient.

Channelopathies

• Dysfunction in ion channels can be direct or indirect, affecting various systems, including respiratory (e.g., cystic fibrosis), nervous, cardiovascular, endocrine, and urinary systems.

Ligand-Gated Ion Channels

• Ionotropic receptors (ligand-gated ion channels) include the nicotinic acetylcholine receptor.

Receptors as Pharmaceutical Targets

• Receptors are the largest target group for pharmaceutical intervention, with roughly 25% of licensed pharmaceuticals targeting receptors.
• Beta 2 adrenoceptor agonists (e.g., salbutamol, salmeterol) cause smooth muscle relaxation and opening of the airways.

Plasma Membrane Receptors - Ligand Gated

• Direct receptors are ionotropic, and may consist of more than one protein.
• Indirect receptors involve intracellular signalling molecules.
• Indirect signalling: the signal is not membrane-limited, transduction is slower and subject to more regulation.

Cell Signalling

• Many proteins involved in cell signalling are kinases or are affected by kinases.
• Kinases are enzymes that phosphorylate proteins and lipids.

Bronchodilators

• Cell signalling inside cells has effects at the tissue level.
• Salbutamol is an agonist - Beta-2 adrenergic receptor 3, Ipratroprium Bromide is an antagonist-M2 Muscarinic Receptor

Breathing and Cell Signalling

• A cellular process fundamentally underlies breathing and tissue action.

G-Protein Coupled Receptor Mediated Process

• Intracellular switch proteins bind GTP rather than ATP and are termed G-proteins or GTPases
• They are Active when bond to GTP and inactive when bound to GDP.
• Can hydrolyse GTP to GDP

G-Coupled Receptors and G-Proteins

• Useful youtube links:
• https://www.youtube.com/watch?v=NB7YfAvez3o
• https://www.youtube.com/watch?v=xT0mAQ4726s

Signal Transduction and Cell signalling

• Signal transduction describes the mechanisms by which a command is executed within a cell and how an external signal generates an intracellular message.
• Examples of signal transduction: an action potential causes transmitter release, acetylcholine phosphorylates ion channels, hormone release is controlled, agonists modify cell growth.

Signal Transduction

• Signal transduction employs different "schemes" that include command molecules, various effectors, and G proteins.

Transporters/Pumps

• Membrane transporters/pumps move solutes against their concentration gradients, requiring an energy source like ATP.
• They are present in the plasma membrane and organelles like mitochondria.

Ion channels vs Ion pumps - Reminder

• Ion channels allow ions to flow down gradients of concentration or eletrical potential via passive transport.
• Ion Pumps Actively push ions AGAINST a gradient and therefore build up the gradient and uses ATP

Types of Transporters

• Uniport transports one substance (e.g. Ca2+ pump).
• Symport transports two substances in the same direction (e.g., glucose-Na+ in epithelia).
• Antiport transports two substances in opposite directions (e.g., Na+/K+ pump in all cells).

Na+/K+ Pump

• The Na+/K+ pump, a P type ATPase, is present on the plasma membrane of all cells.
• It transports ions uphill, against electrochemical gradients, and uses energy from ATP hydrolysis (active transport).
• The stoichiometry is 3Na+:2K+.
• Digoxin inhibits ATPase activity.

Symport

• Na+-glucose coupled transport occurs across the apical membrane of the enterocyte

Intracellular Calcium Regulation

• Calcium regulation uses both antiport and uniport mechanisms.
• Intracellular Ca2+ is kept very low and extracellular Ca2+ is 1.5-2.0mM

Cell Death

• Apoptosis is programmed cell death, while necrosis is unprogrammed.

Apoptosis vs Necrosis

• Apoptosis, or "cellular suicide," allows the removal of specific cells and involves intrinsic and extrinsic pathways.
• Apoptosis results in membrane blebbing and the formation of apoptotic bodies cleared by immune cells, linked to intracellular Ca2+ and mitochondrial Ca2+ levels.

Apoptosis Pathways

• Intrinsic pathway is initiated by cellular stress,
• Extrinsic pathway is initiated by death receptors.
• Regulation of BCL-2 proteins, MOMP, caspase activation.
• Important players include cytochrome c, Bcl-2, caspases, and p53.

Apoptosis - Key Players

• The key players in apoptosis include activators, receptors, mitochondria, Cytochrome C and Caspases.

Necrosis

• Necrosis is traumatic cell death caused by acute cellular injury and can be induced by ischaemia/hypoxia, poisons, and lytic viruses.
• It results in complete cellular lysis, releasing cytotoxic components into the extracellular space, causing inflammation and an immune response.

Apoptosis vs Necrosis key differences

• Apoptosis involves membrane blebbing without loss of integrity and shrinking of the cytoplasm + requires energy in the form of ATP
• Necrosis involves loss of plasma membrane integrity and swelling of the mitochondria and cytoplasm + does not require energy in the form of ATP

Summary: Key facts

• The plasma membrane is impermeable to many solutes.
• Membrane proteins aid transport.
• Maintaining the cellular environment is a complex and dynamic process.

Learning Objectives

• Aim to define the membrane potential, osmosis, identify channels and mechanisms, identify the purposes and types of cell death.
• Read up on main players in apoptosis, p53, Bcl-2, caspases, cytochrome c