Chpt 9 Comprehensive Study Notes on Cellular Communication and Signal Transduction

Introduction and Reference Material

• Total Lesson Duration: 90 minutes.
• Subject: Biochemistry & Cell Biology (CP2318).
• Reference Text: Mason K, Duncan T and Losos J. Understanding Biology, 3rd Edition (2021) McGraw-Hill. Chapter 9: Cell Communication.
• Core Components:
  • Part 1: Fundamentals of Cellular Communication.
  • Part 2: Cell Surface Receptors.
  • Part 3: Intracellular Receptors.

Basic Mechanisms of Cellular Communication

Communication between cells is a universal biological requirement in multicellular organisms, providing the indispensable mechanism by which cells influence one another. There are four basic mechanisms of communication, classified primarily by the distance between signaling and responding cells:

• Direct Contact:
  • Mechanism: Molecules (proteins, carbohydrates, lipids) on the surface of one cell membrane are recognized by receptors on the plasma membrane of an adjacent cell.
  • Structure: Adjacent cells may communicate through gap junctions.
  • Surface Composition: Eukaryotic cell surfaces are richly populated with molecules extending outward from the plasma membrane for this purpose.
• Paracrine Signaling:
  • Characteristics: Signals have short-lived, local effects.
  • Mechanism: Signal molecules diffuse through extracellular fluid. They may be quickly taken up by neighboring cells, destroyed by extracellular enzymes, or otherwise removed.
  • Example: Signaling between immune cells in vertebrates.
• Endocrine Signaling:
  • Characteristics: Involves longer-lived signal molecules (hormones).
  • Mechanism: Molecules enter the circulatory system and travel widely throughout the body, affecting cells very distant from the releasing cell.
• Synaptic Signaling:
  • Mechanism: Specific to the nervous system in animals. Nerve cells provide rapid communication with distant cells via long, fiber-like extensions.
  • Process: Neurotransmitters are released from the tips of these fibers very close to the target cells.
  • Structure: The association between a neuron and its target cell is called a chemical synapse.
• Autocrine Signaling:
  • Definition: Some cells send signals to themselves. The cell secretes a signal that binds to specific receptors on its own plasma membrane.

Fundamentals of Ligands and Receptors

Cellular communication requires two primary components to initiate signal transduction:

1. Ligand: The signaling molecule.
2. Receptor Protein: The molecule to which the ligand binds.

• Signal Transduction: This refers to the series of events within a cell occurring in response to a signal, converting external information into a specific cellular response.

• Signal Chemistry: Cells use various molecules as signals, including:

  • Peptides and large proteins.
  • Individual amino acids.
  • Nucleotides.
  • Steroids and other lipids.
  • Dissolved gases, such as nitric oxide ($\text{NO}$).

• Receptor Locations:

  1. Intracellular Receptors: Located inside the cell; they bind to hydrophobic ligands that can easily cross the plasma membrane.
  2. Plasma Membrane (Cell Surface) Receptors: Located on the cell surface; they bind to hydrophilic ligands that cannot cross the membrane.

Protein Phosphorylation

A cell's response to a signal frequently involves the activation or inactivation of proteins through phosphorylation (the addition of a phosphate group).

• Enzymatic Control:
  • Protein Kinase: An enzyme that adds a phosphate group to a protein.
  • Phosphatase: An enzyme that removes a phosphate group from a protein.
• Phosphorylation State: Many proteins are controlled by whether they are phosphorylated. Some are activated by phosphorylation and deactivated by dephosphorylation, while others may be the reverse.
• Classes of Kinases (based on the amino acid receiving the phosphate):
  1. Serine-threonine Kinase: Adds phosphate to serine or threonine residues.
  2. Tyrosine Kinase: Adds phosphate to tyrosine residues.

Channel-Linked Receptors

• Mechanism: These are chemically gated ion channels that allow the passage of ions only when a specific chemical (ligand) binds to the receptor.
• Ion Types: Includes gated channels for $Na^+$, $K^+$, $Ca^{2+}$, and $Cl^-$.
• Classic Example: The Acetylcholine receptor on muscle cell membranes.
  • When acetylcholine binds, the channel opens to allow $Na^+$ to flow into the muscle cell.
  • This link is critical for translating a signal from a motor neuron into muscle cell contraction.

Receptor Tyrosine Kinases (RTK)

RTKs are a major class of membrane receptors that recognize hydrophilic ligands and possess intrinsic enzymatic activity.

• Structure: Composed of three domains:
  1. Extracellular ligand-binding domain.
  2. Single transmembrane domain.
  3. Intracellular kinase domain.
• Activation Process:
  • Step 1: Ligands bind to the extracellular domains of two RTK proteins.
  • Step 2 (Dimerization and Autophosphorylation): Two receptor-ligand complexes associate (dimerize) and phosphorylate each other at their tyrosine residues.
  • Step 3 (Signal Propagation): The phosphorylated RTK acts as a docking site. Response proteins bind to the phosphotyrosine or are directly phosphorylated by the receptor's kinase domain.
• Functions: RTKs influence the cell cycle, cell migration, metabolism, and proliferation. Response variation between cells depends on which response proteins are present in the cytoplasm.

The Insulin Receptor: An RTK Example

Insulin is a hormone vital for maintaining constant blood glucose levels. It utilizes an RTK to lower blood glucose through several mechanisms:

1. Activation: Insulin binds to the extracellular domain ($\text{α subunit}$), causing the $\text{β subunit}$ of one receptor to phosphorylate the other.
2. Glycogen Synthesis: Phosphorylated insulin response proteins activate glycogen synthase, which converts glucose into glycogen for storage.
3. Glucose Transport: Other response proteins increase the number of glucose transporter proteins in the plasma membrane, moving more glucose from the bloodstream into the cell.
4. Inhibition: It inhibits the production of enzymes involved in synthesizing new glucose.

G Protein-Coupled Receptors (GPCR)

GPCRs represent the largest category of receptor types in animal cells, with approximately 800 genes in humans (half of which encode odorant receptors for taste and smell).

• Structure: Featuring seven transmembrane domains that anchor the receptor in the membrane.
• Mechanism: They act indirectly on effector proteins (enzymes or ion channels) via a G protein.
• Heterotrimeric G Protein: Made of three subunits: $G\text{α}$, $G\text{β}$, and $G\text{γ}$.
  • Active State: Bound to GTP (guanosine triphosphate).
  • Inactive State: Bound to GDP (guanosine diphosphate).
• Signal Pathway:
  1. Ligand binds to GPCR.
  2. G protein exchanges GDP for GTP and dissociates into $G\text{α}$ and a $G\text{βγ}$ complex.
  3. The active components (usually $G\text{α}$) activate an effector protein.
  4. The $G\text{α}$ subunit eventually hydrolyzes GTP back to GDP, causing the subunits to reassociate and return to the "off" state.

Effector Proteins and Second Messengers

Effector proteins are usually enzymes that propagate the signal further, often by creating second messengers—signaling molecules produced within the cell.

• Adenylyl Cyclase:
  • Second Messenger: Cyclic AMP (cAMP).
  • Process: Adenylyl cyclase converts ATP into cAMP and pyrophosphate ($PP_i$). cAMP then activates Protein Kinase A (PKA).
• Phospholipase C:
  • Second Messengers: Inositol triphosphate ($IP_3$) and diacylglycerol (DAG).

The Glucagon Receptor: A GPCR Example

Glucagon is a hormone that stimulates liver cells to mobilize glucose, increasing blood glucose levels. Its pathway involves:

1. G Protein Activation: Glucagon binds to the GPCR, activating the G protein (GTP replaces GDP).
2. Effector Activation: The active G protein activates adenylyl cyclase.
3. cAMP Production: Adenylyl cyclase produces cAMP, which activates PKA.
4. Phosphorylation Cascade:
   • PKA activates phosphorylase kinase.
   • Phosphorylate kinase activates glycogen phosphorylase.
   • Glycogen phosphorylase cleaves $glucose-6-phosphate$ from glycogen.
5. Simultaneous Inhibition: PKA phosphorylates glycogen synthase to inhibit it, preventing the storage of glucose as glycogen.
6. Gene Expression: PKA activates the expression of genes encoding enzymes needed for glucose synthesis.

Intracellular Steroid Hormone Receptors

Steroid hormones (cortisol, oestrogen, progesterone, testosterone) are non-polar and diffuse readily across the plasma membrane.

• Receptor Structure: Three functional domains:
  1. Hormone-binding domain.
  2. DNA-binding domain.
  3. Coactivator-interaction domain (affects level of gene transcription).
• Activation Mechanism:
  • Prior to hormone binding, the receptor is inactive and unable to bind DNA due to inhibitor proteins.
  • Binding of the hormone releases inhibitor proteins.
  • The steroid-receptor complex shifts from the cytoplasm to the nucleus.
  • The DNA-binding domain attaches to specific sequences near target genes to regulate gene expression.

Nitric Oxide (NO) Receptor

Nitric Oxide is a small gas molecule that diffuses easily into cells. Its receptor is an intracellular enzyme called Guanylyl Cyclase.

• Mechanism: NO binds to guanylyl cyclase, which converts GTP to cyclic guanosine monophosphate (cGMP).
• Biological Effect: Relaxes smooth muscle cells, leading to blood vessel dilation and increased blood flow.
• Pharmacological Applications:
  • Nitroglycerin: Used to treat angina. It is converted by cells into NO, which relaxes blood vessels to relieve symptoms.
  • Viagra (Sildenafil): Inhibits the enzyme cGMP phosphodiesterase (which breaks down cGMP). By maintaining high cGMP levels, it ensures prolonged smooth muscle relaxation and increased blood flow. Note: Viagra does not bind to the receptor itself but maintains the signal produced by it.

Questions & Discussion

1. The action of hormone is a type of _____ signaling.

• Answer: Endocrine.

2. Which statement about protein phosphorylation is true?

• Answer: It is carried out by a protein kinase. (Note: Phosphorylation does not always activate or always deactivate; it depends on the specific protein).

3. The second messenger of the effector protein adenylyl cyclase is:

• Answer: cAMP.

4. The process by which two RTK phosphorylate each other upon binding of ligand is known as _____.

• Answer: Autophosphorylation.

5. Insulin receptor is a type of _____.

• Answer: Receptor tyrosine kinase.

6. Which of the followings is/are not intracellular receptor?

• Answer: RTK and GPCR (both are plasma membrane receptors).

7. G protein become active when bound to:

• Answer: GTP.

8. The primary cellular response of activated steroid hormone receptor is the regulation of _____ expression.

• Answer: Gene.

9. In cell communication, the signaling molecule is also known as the:

• Answer: Ligand.

10. Nitric oxide receptor characteristics:

• Answer: It is an intracellular receptor; it is an enzyme known as guanylyl cyclase; when activated, it leads to relaxation of smooth muscle cells; it is the target (via ligand conversion) of the drug nitroglycerin.