Cell Communication: Extra- and Intracellular

Extracellular Communication

• Occurs when a signal is received from outside the cell.
• Cells can communicate at variable distances, either through direct contact via gap junctions or through secretions when not touching.

Intracellular Communication

• External signals lead to changes within a cell.

Direct Cell-to-Cell Communication: Gap Junctions

• Communication method for cells in direct contact.
• Connexons form channels between cells, allowing chemical signals to pass directly.
• Only small particles like ions and small signaling molecules can pass; proteins and carbohydrates are too large.
• Excitable cells, such as those in cardiac muscle, can pass electrical signals through gap junctions.
• Gap junctions are regulated and can open/close, potentially as a defense mechanism.

Cell-to-Cell Communication: Secretions

• Autocrine: Substances released affect the same cell.
• Paracrine: Substances released affect nearby cells.
• Endocrine: Substances released affect distant cells.
• Neurotransmitters: Substances released from a nerve terminal into the synapse.

Secretions: Neurotransmitters

• Occur where a nerve cell axon terminates on a target cell.
• Neurotransmitters are released into the synapse upon an excitatory signal and can bind to receptors on the target cell, be taken back by the original nerve cell, or be degraded.
• Secretions must interact with a receptor to affect the target cell.

Components of a Signaling Pathway

• Intracellular communication converts extracellular information into a cellular response via signal transduction.
• Consists of a signal (membrane permeable or impermeable), receptors, signaling proteins, and second messengers.

Structure of a Signaling Pathway

• Membrane-permeable signals bind to receptor proteins in the cytosol.
• Membrane-impermeable signals bind to transmembrane cell surface receptor proteins, activating second messengers.
• Signaling proteins and second messengers amplify and distribute signals.
• Cytoplasmic effectors lead to fast, short-lived responses.
• Nuclear effectors (transcription factors) control gene expression, resulting in slower, prolonged responses.

Signal Transduction

• Linear: One receptor interacts with one signaling protein or second messenger.
• Convergent: Several receptors share common signaling proteins or second messengers.
• Divergent: A single receptor interacts with multiple signaling proteins or second messengers.
• Multi-Branched: Combination of convergence and divergence.

Signals (Ligands)

• Originate from the extracellular space and must bind to a receptor.
• Membrane-impermeable ligands bind to proteins on the cell surface.
• Membrane-permeable ligands (primarily steroids) can interact with cytosolic receptors.
• Physical signals include pressure, temperature, and light.

Receptors Overview

• Often found on the plasma membrane but can also be found in the cytoplasm of the cell.
• Include G-Protein coupled receptors (GPCR), Ion Channels, Guanylate cyclase, Protein kinase receptors, Transmembrane scaffolds, and Nuclear receptors.

Receptors: G-Protein Coupled Receptors (GPCR)

• Structure: Seven transmembrane domains (H1 to H7) and heterotrimeric G-protein with alpha, beta, and gamma subunits.
• Function: Ligand binding causes a conformational change leading to G-protein activation.

Receptors: Ion Channel Receptors

• Also known as ‘ligand-gated channels’ in the plasma membrane.
• Transmit signal information by permitting ions to flow across the membrane.
• Ligand binding opens pores for ion flow through conformational change.
• Responsible for voluntary muscle contraction and common in nerve cell communication.

Receptors: Guanylate Cyclase

• Found bound to the membrane and soluble within the cytosol.
• Structure: Externalized ligand binding domain, transmembrane domain, and internal catalytic domain.
• Function: Membrane-bound guanylate cyclase converts GTP to cGMP upon activation, while the soluble form mediates some intracellular processes.
• Plays an important role in vision, converting light signals into electrical signals in the eye.

Receptors: Protein Kinase

• General action: Phosphorylate other proteins on serine, threonine, or tyrosine residues.
• Dysfunction is associated with cancer development.
• Two classes: RTK (phosphorylates tyrosine) and S/TKR (phosphorylates serine/threonine).

Protein Tyrosine Kinase Receptor Ligand Binding

• Inactive receptors are separate polypeptides with inactive tyrosine kinase domains.
• Ligand binding causes dimerization, activating the kinase.
• Transaurophosphorylation: One subunit phosphorylates the tyrosine amino acids on the other.
• Resulting phosphotyrosine amino acids are binding sites for additional signaling proteins.
• Ligand release and dephosphorylation by phosphoprotein phosphatases reset the kinase to its inactive state.

Receptors: Transmembrane Scaffolds

• Form large clusters to regulate signal transduction. Functions:
• Bring signaling proteins together.
• Regulate signal transduction.
• Localize signaling proteins to specific cellular areas.
• Isolate specific signalling pathways

Receptors: Nuclear Receptors (Transcription Factors)

• Found in the cytosol of cells.
• Ligand bound receptors move into the nucleus and bind to specific DNA sequences (steroid response elements) to control gene expression.
• Important in the body’s response to toxic substances.

Signaling Proteins

• Transmit and amplify signal information.
• Highly mobile in the cytosol or plasma membrane.
• Can be enzymes catalyzing chemical reactions for signal amplification or capable of binding to enzymes.

Signaling Proteins: G-Proteins

• Bind GTP and propagate signals.
• Monomeric G-Proteins: Single polypeptides with binding sites for GTP/GDP and target protein and a GTPase domain. Not coupled to GPCRs.
• Activated by GTP binding, can bind to its target protein.
• Heterotrimeric G-Proteins: Contain alpha, beta, and gamma subunits.
• Anchored to the plasma membrane and are activated by GPCRs.
• The alpha subunit binds GTP/GDP and a target protein.
• Beta/gamma subunits stabilize the inactive (GDP bound) form of the alpha subunit.

Signaling Proteins: Activity of G-Proteins

• Inactive form: Heterotrimer (alpha, beta/gamma) bound to GDP.
• Ligand binding to receptor: Changes conformation to interact with G-protein.
• GDP exchanged for GTP on alpha subunit: Heterotrimer separates into alpha and beta/gamma subunits, activating G-proteins.
• Separated subunits bind downstream targets: Propagating signal pathway.
• Alpha subunit cleaves GTP to GDP: Alpha and beta/gamma subunits reform inactive heterotrimer.

Signaling Proteins: Protein Kinases

• Most are non-receptor cytosolic signaling proteins.
• Act as intermediaries, activating other protein kinases or directly phosphorylating effector proteins.
• Phosphorylation generally activates target proteins, but some are inactivated.

Signaling Proteins: Calcium-Binding Proteins

• Intracellular calcium levels are typically low.
• When calcium levels rise, it interacts with proteins causing downstream events.
• Calmodulin: Upon calcium binding, it undergoes a conformational change, allowing it to bind to its target protein.

Signaling Proteins: Adenylyl Cyclase

• Converts ATP into cAMP perpetuating the signal.
• Binds to the alpha subunit of heterotrimeric G-proteins.
• As (alpha s): Stimulates adenylyl cyclase.
• Ai (alpha i): Inhibits adenylyl cyclase.

Signaling Proteins: Lipid Kinases

• Phosphorylate phospholipids in the cytoplasmic leaflet of the membrane.
• The added phosphate results in a conformational change, allowing the phospholipid to bind to its target protein.

Signaling Proteins: Adaptor Proteins

• Contain binding domains that recognize phosphorylated amino acids or activated structures on signaling proteins.
• These domains "glue" elements of signaling networks together.
• Important to allow cascades to be associated in the right space and time.

Features of Second Messengers

• cAMP and cGMP are degraded by phosphodiesterases, while ionic messengers like Ca2+ are sequestered into cellular organelles.
• Small in size.
• Diffuse rapidly in the cytosol or membrane.
• Amplify signals.
• Do not remain in the cytosol for long.

Heterotrimeric G-Protein Signaling Cascade

• GPCRs: Initiated by ligand binding, allowing receptor interaction with the heterotrimeric G-protein.
• cAMP: Ligand-bound receptor stimulates GDP replacement with GTP in Galpha subunit, causing dissociation into G (beta/gamma) and Galpha-GTP.
• PKA: cAMP binds to protein kinase A (PKA), releasing active catalytic subunit to phosphorylate cellular proteins.
• CREB: Common nuclear target is CREB, which, once phosphorylated by PKA, binds CBP and interacts with DNA to initiate transcription.

Phospholipid Kinase Signaling Cascade

• GPCR: Initiated by ligand binding, allowing receptor interaction with the heterotrimeric G-protein.
• The ligand-bound receptor stimulates the replacement of GDP for GTP in the Galpha subunit causing the dissociation into G (beta/gamma) and Galpha-GTP
• PLC: G Alpha-GTP binds to phospholipid kinase signaling protein phospholipase C (PLC).
• PIP2/IP3: Activated PLC breaks down phosphatidylinositol 4,5-bisphosphate to release diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3).
• CA2+: IP3 activates its receptor on the endoplasmic reticulum, opening a calcium channel. Ca2+ then activates calcium-binding proteins.
• PKC: Diacylglycerol and Ca2+ bind to protein kinase C (PKC), activating it. Activated PKC has numerous cellular targets that it can phosphorylate.

Protein Kinase Signaling Cascade

• FGFs (Fibroblast growth factors): Stimulate mammalian cell growth.
• Bind to FGF receptors (FGFRs): Homoedimer receptor kinase. Ligand binding causes dimerization and tyrosine transautophosphorylation.
• Grb2: Example of a phosphotyrosine-binding protein. Binding causes it to undergo a conformational change to bind Sos.
• Sos: Activates Ras, leading to the replacement of GDP with GTP.
• Raf: Active Ras binds to a serine/threonine kinase called.
• MEK: Activated Raf phosphorylates MEK.
• Erk: MEK phosphorylates Erk. Dimerized Erk phosphorylates other signaling proteins in the cytosol or nucleus.

How the Cell Handles Dangerous Cargo

• Lysosomes: Break down misfolded and damaged organelles, nucleic acids, and lipids.
• Proteasomes: Break down damaged and misfolded proteins in the nucleus and cytosol.
• Peroxisomes: Handle dangerous free radicals, including reactive oxygen species.

Getting Cargo to the Lysosome

• Cargo is delivered to the lysosome in an endosome through the endomembrane system after being tagged with M6P.
• Enzymes that degrade damaged proteins (proteases) are also directed to the lysosome with the M6P tag.
• Vesicles deliver engulfed proteins and soluble proteins that are fused and digested by the proteases.
• Proteases are synthesized in the ER, tagged with M6P, and delivered to the lysosome in vesicles.

Digestion in the Lysosome

• Contains high protease concentrations for cleaving membrane and contained proteins.
• Can engulf other organelles or bacteria.
• Large molecules are broken down into basic parts (proteins → amino acids) and transported to the cytosol for reuse.
• A special coat (glycocalyx) prevents it from digesting itself.

Protein Degradation by the Lysosome

• Cytosolic Proteins: Tagged with a polyubiquitin chain for recognition by the proteasome and degradation.
• Nuclear Proteins: Degraded by nuclear proteasomes without export to the cytosol.

The Function of Peroxisomes

• Keep and use reactive oxygen species (peroxides, ions, and free radicals) safely, using enzymes including catalase.
• Contain enzymes that catalyze a variety of metabolic reactions.
• Contain essential peroxisome proteins (peroxins) synthesized in the cytosol and targeted to the peroxisome by specific signals.
• Decompose cargo such as uric acid, amino acids, and long-chain fatty acids.

Apoptosis: Programmed Cell Death

• An energy-consuming process that ends the life of a cell.
• Protects the body from damaged cells and is used in development.

Mechanism of Apoptosis

• Initiation:
• Intrinsic: From the outer membrane of the mitochondria triggered by intracellular signals like massive DNA damage, ROS, toxins, or other trauma.
• Extrinsic: Uses a plasma membrane death receptor. Neighboring cells release death ligands that bind to the death receptors of the damaged cell.
• Membrane Blebbing and Enzyme Activation:
• Cell shrinks and forms blebs
• Initiator caspases are activated by the intrinsic or extrinsic pathway, which then activate executioner caspases.
• Cell Structure Changes:
• DNA repair halts, the nuclear membrane breaks down, and the nucleus disappears.
• The cytoskeleton is disassembled and the plasma membrane phospholipid content changed with scramblases, with PS benign exposed on the exoplasmic leaflet on the plasma membrane.
• Engulfment: Phagocytes endocytose the apoptotic bodies for digestion in lysosomes, minimizing disturbance to surrounding tissues.

Necrosis: “Accidental” Cell Death

• Results from cellular injury and is the major pathway of cell death from severe damage.

Mechanism of Necrosis

• Damage: Causes include toxins, extreme heat or radiation, freezing, ischemia, pathogens, mechanical trauma.
• Swelling: Organelles lose structure and swell, vacuoles form, and DNA may be degraded.
• Destruction: The cell membrane and organelles lose integrity, spilling cellular components and triggering inflammatory signals.

Apoptosis vs Necrosis

• Causes:
• Apoptosis: DNA damage, withdrawal of growth factors or nutrients, detachment, cytotoxic lymphocytes.
• Necrosis: Trauma.
• Key Features:
• Apoptosis: Nucleus fragments, cell shrinks, apoptotic body forms.
• Necrosis: Cell swells, cell bursts, organelles break down.
• End Results:
• Apoptosis: Engulfment of fragments, no inflammation.
• Necrosis: Inflammatory response.