Exocytosis
Abstract
Nuclear export of messenger RNAs (mRNAs) through nuclear pore complexes (NPCs) is crucial for gene regulation.
The role of chemical modifications in this process is not fully understood.
Discovery: N-methyladenosine (m6A), the most common internal mRNA modification, significantly enhances the efficiency and speed of mRNA nuclear export.
Method: Single-molecule imaging reveals that METTL3 (mRNA methyltransferase) interacts with the nucleoporin NUP93, which facilitates the connection between m6A-modified mRNPs and NPCs.
Significance: Disruption of the METTL3-NUP93 interaction, as seen with a variant NUP93 linked to steroid-resistant nephrotic syndrome, results in defective nuclear export of vital methylated mRNAs that are necessary for kidney function.
Conclusion: This research elucidates a previously unknown regulatory mechanism influencing mRNA export and offers insights into how its dysregulation could lead to diseases.
Transport Mechanisms Across Bio-membranes
Overview of Transport Modes
Passive Transport
Definition: Movement of substances across cell membranes without the use of energy by the cell.
Types of Passive Transport:
Simple Diffusion:
Involves nonpolar, hydrophobic molecules such as lipids.
Movement is from high to low concentration gradient.
Facilitated Transport:
Involves diffusion of polar, hydrophilic molecules through protein channels.
Also moves from high to low concentration gradient.
Active Transport
Definition: Movement of substances against their concentration gradient, requiring energy (ATP).
Characteristics:
Movement occurs from low to high concentration gradient.
Utilizes protein pumps to facilitate transport.
Resting Membrane Potential and Action Potential
Resting Membrane Potential
Definition: The established voltage difference across the plasma membrane due to variations in Na+ and K+ concentrations inside and outside the cell.
Action Potential
Stages:
Depolarization: Triggered by Na+ ions entering the cell.
Repolarization: At peak action potential, K+ channels open, allowing K+ to exit.
Hyperpolarization: Occurs as the cell becomes more negative after the action potential peak.
Bulk Transport Mechanisms
Definition of Bulk Transport
Movement of large amounts of molecules into and out of cells.
Requires energy from respiration.
Modes of Bulk Transport
Endocytosis
Process: Large external molecules enter the cell by forming an inward pocket in the cell membrane which pinches off forming a vesicle.
Requires ATP, hence is an active transport process.
Variants:
Phagocytosis: Engulfing of solid particles.
Pinocytosis: Engulfing of liquids.
Receptor-mediated Endocytosis: Specific entry of molecules via receptor binding.
Exocytosis
Process: Vesicles containing internal macromolecules fuse with the cell membrane to release their contents outside the cell.
Also an active process requiring energy.
Detailed Overview of Endocytosis
General Overview
Definition: Movement of large particles into cells by enclosing them in a vesicle made from the cell membrane.
Mechanism:
The cell membrane folds inwards to form a pocket around target particles.
The pocket pinches off, forming a vesicle or vacuole inside the cell.
Modes of Endocytosis
Phagocytosis
Definition: A form of endocytosis where large particles are engulfed, often termed "cell eating."
Mechanism:
Large particles (>0.5 micrometers) are ingested into membrane-bound vesicles called phagosomes.
Phagosomes target lysosomes for degradation of ingested material.
Pinocytosis
Definition: A type of endocytosis also known as "cell drinking," where small amounts of extracellular fluid are ingested continuously.
Characteristics:
Vesicles formed are much smaller compared to phagosomes.
Receptor-Mediated Endocytosis
Mechanism:
Receptors on the cell surface capture specific target molecules.
When binding occurs, vesicles engulfs the receptors and target molecules.
Significance: Allows uptake of specific large molecules present in low concentrations in the extracellular fluid.
Pathological Note: Pathogens, such as certain viruses and toxins, exploit this pathway for cell entry.
Phagocytosis Overview
Initial Steps of Phagocytosis
Initiated by receptors binding to target particles.
Actin polymerization occurs, inducing plasma membrane deformation, leading to formation of pseudopodia that engulf the target.
Once fully engulfed, actin depolymerization retracts, sealing the particle in a phagosome which later matures by fusion with lysosomes.
Phagocytosis in Immune Response
Role: Critical for protecting the body from pathogens.
Phagocytic Cells:
Neutrophils: Quickly respond to inflammation by phagocytizing pathogens.
Macrophages: Linked to monocytes, respond in chronic inflammation with additional paracrine factors.
Dendritic Cells: Important in eliciting specific immune responses beyond direct destruction of pathogens.
B Lymphocytes: Exhibit minimal phagocytic activity to support antibody production.
Induced Phagocytosis and Pathogen Interaction
Mechanisms of Induced Phagocytosis
Bacteria use the zipper mechanism, manipulating the membrane to induce phagocytosis.
Pathogen survival can occur, as listeria and shigella can escape the phagosome once engulfed.
Salmonellae Interaction with Host Processes
Entry via destruction of microvilli and subsequent vesicle formation within enterocytes.
Colonization involves extensive transcriptional changes to evade host defense mechanisms.
Type 3 Secretion System
Mechanism: Allows bacteria to inject effector proteins into host cells, facilitating membrane ruffling and endocytosis.
Cholesterol Transport and Receptor-Mediated Endocytosis
Cholesterol Transport Dynamics
Two Key Lipoproteins:
LDL (Low-Density Lipoprotein): Known as "bad cholesterol.”
High levels increase heart disease risk.
HDL (High-Density Lipoprotein): Known as "good cholesterol.”
Transports cholesterol back to the liver for elimination from the body, reducing heart disease risk.
Mechanism of LDL Endocytosis
Receptor-mediated endocytosis involves LDL binding to specific receptors on liver cells, leading to internalization and subsequent digestion, yielding useful lipids.
Summary and Critical Thinking Questions
Questions for Discussion
What constitutes bulk transport?
How is bulk transport critical to cellular function?
What differentiates various transport modes in biology?
Why are certain cells specialized for phagocytosis?
Implications of Phagocytosis in Disease Mechanisms
Understanding Infective Strategies
How various pathogens exploit the mechanisms of phagocytosis and maneuver around immune defenses should be further studied to enhance therapeutic strategies against infections caused by these pathogens.
Key Terms and Definitions
N-methyladenosine (m6A): The most common internal mRNA modification, enhancing nuclear export efficiency.
METTL3: An mRNA methyltransferase that interacts with NUP93 to facilitate m6A-modified mRNP connection to NPCs.
NUP93: A nucleoporin that interacts with METTL3 to facilitate the nuclear export of m6A-modified mRNPs.
Passive Transport: Movement of substances across cell membranes without cellular energy.
Simple Diffusion: Passive transport involving nonpolar, hydrophobic molecules moving from high to low concentration.
Facilitated Transport: Passive transport involving polar, hydrophilic molecules diffusing through protein channels.
Active Transport: Movement of substances against their concentration gradient, requiring ATP (energy).
Resting Membrane Potential: The voltage difference across the plasma membrane due to ion concentration variations.
Depolarization: Stage of action potential triggered by Na+ ions entering the cell.
Repolarization: Stage of action potential where K+ channels open, allowing K+ to exit, restoring negative potential.
Hyperpolarization: Stage where the cell becomes more negative than its resting potential after an action potential.
Bulk Transport: Movement of large amounts of molecules into and out of cells, requiring energy.
Endocytosis: Movement of large external molecules into the cell by forming an inward pocket in the cell membrane that pinches off.
Phagocytosis: A form of endocytosis