Seminario 6 Dinámica del citoesqueleto en interfase y division celular - Tomas Falzone

Understanding the Cytoskeleton Dynamics

Overview of Cytoskeleton Functions

• The seminar focuses on the dynamics of the cytoskeleton and its role in various cellular processes and functions.

• Key topics include cellular polarity, migration, and division processes, emphasizing how cytoskeletal structure enables these functions.

Genetic Basis of Cellular Function

• Understanding cell biology involves exploring genetic mechanisms that dictate how cells operate, particularly through gene expression from DNA.

• Gene expression defines cellular destiny, allowing for specialization into different cell types with distinct functions. This is crucial for understanding processes like mitosis and meiosis.

Impact of Genetic Modifications

• Changes at the genetic level (expression variations or mutations) can significantly alter cellular processes such as proliferation and organization. These changes may lead to diseases like cancer or muscular dystrophies.

• The seminar will explore how genetic modifications affect protein subunits in the cytoskeleton, using keratin as a primary example. Variants can influence tissue physiology and function.

Cytoskeletal Components

Types of Cytoskeletal Elements

• The cytoskeleton consists of three main components: microtubules, microfilaments (actin), and intermediate filaments, each contributing to cell shape and polarity.

• Microtubules: Essential for maintaining cell morphology; they provide intracellular transport pathways and are involved in mitosis and flagella formation.

• Microfilaments: Actin filaments support structural integrity while facilitating cell movement and shape changes during migration.

• Intermediate Filaments: Provide mechanical resistance; they are specific to different cell types and play roles in nuclear structure during division processes.

Functional Implications of Cytoskeletal Organization

• Proper organization of microtubules is critical for establishing connections between cellular structures, influencing overall functionality within tissues such as skin where keratin expression varies by region.

Understanding Microtubules and Actin Filaments

Microtubule Structure and Function

• Microtubules are organized from a nucleating center composed of gamma-tubulin, with centrioles serving as the organizing centers.

• Proteins associated with microtubules play various roles, including stabilizing them and facilitating their polymerization or depolymerization.

• Some proteins promote microtubule growth at the positive end while others induce catastrophe (depolymerization), crucial during cell division.

• The stabilization of microtubules is essential for cellular shape changes and polarity, especially in cells that extend structures like axons or dendrites.

• Motor proteins such as kinesin (transporting towards the positive end) and dynein (transporting towards the negative end) facilitate organelle transport along microtubules.

Role of Microtubules in Cellular Transport

• Kinesin transports materials to the plasma membrane, while dynein moves materials back to the organizing center or around the nucleus.

• The cytoskeleton's organization aids in positioning organelles and distributing vesicles throughout the cell using molecular motors associated with microtubules.

• Epithelial cells exhibit specific polarization with distinct apical and basolateral membrane functions, influenced by microtube dynamics.

• Endocytosis occurs at the apical side of epithelial cells, where vesicles are transported across to maintain cellular function.

Actin Filaments: Dynamics and Functions

• Actin filaments are more dynamic than microtubules, forming various structures such as gels and parallel bundles through polymerization processes.

• These filaments contribute to cellular protrusions like lamellipodia and filopodia, which are critical for movement and sensing environments.

• Stress fibers formed by antiparallel actin filaments enable tension generation within cells, facilitating movement.

Cytoskeleton Components and Their Functions

Actin Filaments and Myosins

• The structure of actin filaments includes motor proteins called myosins, which differ from those that bind to microtubules.

• Myosins generate force along the fibers, facilitating movement in muscle contraction and organelle transport.

• Different types of myosins exist; for example, type I is associated with membrane anchoring while type II is primarily involved in contraction events.

Intermediate Filaments

• Intermediate filaments are abundant and specific to cell types, providing mechanical resistance and cellular anchorage.

• Unlike microtubules or actin filaments, intermediate filaments do not undergo dynamic polymerization but assemble from monomers into dimers and then higher-order structures.

Importance of Cytoskeletal Integrity

• Alterations in cytoskeletal components can lead to significant changes in tissue stability due to mutations affecting filament formation.

• Mechanical support provided by the cytoskeleton is crucial for maintaining cell junction integrity; disruptions can lead to tissue damage.

Disease Implications

• Conditions like simple epidermolysis bullosa arise from keratin mutations leading to skin fragility due to loss of structural integrity.

• Patients experience blistering from minor trauma as a result of compromised connections between dermal layers.

Cellular Polarization Mechanisms

• Understanding cytoskeletal components aids in grasping how cells acquire polarity during division or anchoring processes.

• The segregation of internal factors within a cell initiates polarization, allowing for specialized functions at different cellular regions.

Establishment of Cell Polarity

• The cytoskeleton's role extends to recognizing environmental cues that help establish cellular polarity through interactions with the extracellular matrix.

• This process involves regionalizing components within the cell, enabling distinct apical-basal differentiation essential for function.

Cellular Interactions and Structures

Interaction of Cells with Their Environment

• The interaction of cells with their environment facilitates the establishment of various structures, allowing for internal segregation of proteins to different regions, leading to regionalization and specific functions.

• In epithelial cells, lateral membranes exhibit different types of junctions that provide functionality; tight junctions prevent passage from apical to basal sides, while adhesion junctions enhance resistance.

• Adhesion junctions are associated with cytoskeletal components to generate resistance. Communicating junctions allow cells to understand functional relationships between different cell types.

Types of Cell Junctions

• Junctional complexes include adherens junctions formed by cadherin proteins (homophilic interactions), occluding junctions (tight junctions) involving claudins and occludins, and desmosomes connecting cells or anchoring them to the extracellular matrix.

• Gap junctions consist mainly of connexin proteins that facilitate communication through the passage of small ions or uncharged molecules between adjacent cells.

Cytoskeleton's Role in Cell Functionality

• The establishment of membrane polarity and specialization relies on a dynamic cytoskeleton that enables communication and formation of specific structures like microvilli or connections with the extracellular matrix.

• Certain cells can migrate through epithelia or move within the extracellular matrix under defined conditions, highlighting the importance of cytoskeletal remodeling in cellular migration processes.

Dynamics of Cytoskeletal Structures

• The cytoskeleton is not a static structure; it can change dynamically in response to signals. This adaptability allows for significant alterations in its structure based on external stimuli.

• Neutrophils exemplify this dynamic behavior as they exit blood circulation in response to inflammatory signals. Understanding how cytoskeletal components interact during this process is crucial for comprehending cell migration mechanisms.

Signaling Pathways Affecting Migration

• Specific signaling pathways activate receptors that trigger intracellular cascades, enabling cell polarization necessary for migration characterized by an advancing front and a retracting rear.

• Actin filaments play a critical role at the leading edge (lamellipodia and filopodia), where selective signaling promotes polymerization into parallel fibers or gel-like structures aiding movement towards migratory regions.

Contrasting Effects on Cell Movement

• At the rear end, opposing effects mediated by Rho protein lead to depolymerization of actin filaments and activation of myosin for stress fiber mobility, essential for effective cell retraction during migration.

Mechanisms of Cell Movement and Adhesion

Understanding Cell Mobility in Blood Flow

• The discussion begins with the concept that signals can modulate cellular mechanisms, particularly how cells interact with filament structures while moving through blood flow.

• A specific signaling event can cause a cell, such as a neutrophil, to slow down its movement in the bloodstream and exit into tissues to respond to inflammatory signals.

• Neutrophils rolling along blood vessels encounter specific signals (e.g., selectins) that allow them to decelerate and eventually anchor themselves due to the presence of adhesion proteins.

Types of Adhesion Molecules

• Adhesion molecules are primarily membrane proteins that facilitate homophilic interactions (e.g., cadherins) or heterophilic interactions (e.g., selectins), which are crucial for cell adhesion during inflammation.

• Heterophilic interactions occur between different types of proteins, such as integrins binding to extracellular matrix components, allowing stronger anchoring of neutrophils.

Activation and Structural Changes

• The activation of integrins is essential for strengthening the adhesion between neutrophils and endothelial cells, enabling them to halt their movement in the bloodstream effectively.

• Integrin activation triggers intracellular signaling cascades that modify the cytoskeleton structure, particularly actin filaments, facilitating processes like extravasation where cells move out of blood vessels into tissues.

Cytoskeletal Dynamics During Migration

• Actin structures must generate forces necessary for cell migration; this involves significant remodeling of the cytoskeleton as cells adapt their shape for movement through epithelial layers.

• Once in tissue matrices, microtubules also undergo modulation alongside actin filaments as cells respond to chemotactic signals guiding their movement towards areas needing immune response.

Signaling Mechanisms and Lipid Rafts

• Cells detect chemotactic gradients through differential expression of receptors that sense these signals; this leads to changes in cytoskeletal dynamics critical for effective migration.

• Understanding cellular responses requires deep insight into signaling mechanisms mediated by soluble factors and extracellular matrix components interacting with membrane receptors.

Role of Lipid Rafts in Signal Transduction

• Lipid rafts are specialized membrane regions enriched with receptor proteins that play a vital role in signal transduction during cellular responses.

• These lipid rafts consist of saturated phospholipids and cholesterol, contributing to their rigidity and functionality in organizing signaling pathways within the cell membrane.

Cellular Signaling and Cytoskeletal Dynamics

Mechanisms of Cell Migration

• The adhesion in signaling allows association with intracellular molecules, linking to G-protein receptors and various kinase signaling cascades that mediate internal cellular signaling for structural modulation.

• During cell migration, membrane refilling occurs at the leading edge through exocytosis, where vesicles fuse with the plasma membrane to incorporate new membranes.

• Endocytosis events at the trailing edge create dynamic structures that facilitate movement from the rear to the front of migrating cells, essential for effective migration.

• The secretory and endocytic pathways are crucial for vesicle transport along microtubules and actin filaments, enabling membrane dynamics necessary for cell movement.

• Specific vesicles must be endocytosed and exocytosed at the leading edge; they possess unique membrane-associated proteins (RAVs) that direct them to specific regions within the cell.

Vesicular Transport Mechanisms

• RAV proteins on vesicles guide their directionality in both endocytic and exocytic pathways, allowing interaction with motor proteins for targeted delivery.

• SNARE proteins on vesicles interact with target membrane SNAREs to facilitate specific docking between vesicular and plasma membranes during exocytosis.

• Understanding how vesicles are directed from Golgi apparatus or plasma membrane towards endocytic pathways will be elaborated in future discussions about cellular transport mechanisms.

• Proteins forming basket-like structures play a critical role in membrane invagination and subsequent vesicle formation necessary for cellular transport systems.

Cytoskeleton's Role in Cell Division

• The cytoskeleton's structure is vital not only for determining polarity but also plays significant roles in migratory processes as well as cell division mechanics.

• Different components of the cytoskeleton enable various cellular functions essential for successful cell division, which is closely linked to checkpoint processes involving cyclin-dependent kinases (CDKs).

• Forces generated by cytoskeletal structures are crucial for dividing a single cell into two identical daughter cells during mitosis or meiosis.

Cell Cycle Regulation

• Cells must sense their environment and have internal cues to pass restriction points before expressing genes required for DNA synthesis and compaction during cell cycle progression.

• Key checkpoints ensure DNA integrity before entering mitosis, where duplicated genomic components are evenly distributed into two daughter cells alongside organelles like mitochondria.

Mitosis Control Mechanisms

• Specific regulators associated with cyclin-dependent kinases control mitotic processes; these include master regulators like mitotic promoting factor (MPF), which initiates early mitotic events.

• A critical checkpoint exists during metaphase-anaphase transition governed by anaphase-promoting complex (APC), ensuring proper progression through mitosis while silencing MPF activity.

Cell Division and Mitotic Spindle Formation

Initiation of Centriole Duplication

• The duplication of centrioles begins, leading to their initial separation. This process is crucial for organizing the cytoskeletal structure necessary for mitotic spindle formation.

Microtubule Organization

• Fluorescence images show microtubules (green) and DNA (blue), illustrating the organization of microtubule organizing centers and their orientation towards various locations on the plasma membrane.

Chromosome Segregation Mechanism

• The reorganization of microtubule organizing centers is essential for creating two poles during cell division, allowing well-condensed chromosomes to be segregated effectively.

Control of Mitotic Spindle Structures

• The formation of specific mitotic spindle structures involves three components that generate selective forces necessary for chromosome separation at metaphase.

Cytokinesis Process

• Following anaphase, cytokinesis relies on actin filaments to facilitate the physical division of cells after chromosome segregation has occurred.

Microtubule Dynamics in Mitosis

Prophase Events

• In prophase, centriole duplication occurs alongside the generation of gamma-tubulin structures that assemble into microtubules, which are positioned at opposite ends of the cell.

Interaction with Condensed Chromosomes

• Microtubules must first disassemble the nuclear membrane structure to interact with condensed chromosomes during late G2 phase.

Nuclear Membrane Disassembly

• Controlled disassembly of nuclear membranes allows microtubules from the cytoplasm to access chromosomes; this requires a regulated breakdown facilitated by intermediate filament support beneath the nuclear membrane.

Role of Kinases in Nuclear Structure

Phosphorylation Mechanism

• Nuclear lamina phosphorylation by cyclin-dependent kinases initiates controlled disassembly, enabling microtubules to reach chromosomes once nuclear integrity is compromised.

Microtube Association with Chromosomes

• Once disassembled, microtube plus ends can associate strongly with centromeric DNA structures and kinetochores through polymerization and depolymerization processes during metaphase alignment.

Transition from Metaphase to Anaphase

Orientation at Equator

• During metaphase, forces generated by polymerization/depolymerization help orient chromosomes along the equatorial plane for proper separation later in anaphase.

Activation of Anaphase Promoting Complex (APC)

• Transitioning from metaphase to anaphase requires activation of APC through phosphorylation by cyclin-dependent kinase; this complex facilitates chromosome separation while inhibiting further mitosis-promoting actions.

Post-Separation Reorganization

Role in Cell Cycle Regulation

Mechanisms of Chromosome Separation During Cell Division

Role of APC in Chromosome Segregation

• The Anaphase Promoting Complex (APC) inactivates cyclin-dependent kinase, facilitating the transition from anaphase to telophase and eventually to interphase.

• APC functions as a ubiquitin ligase, specifically degrading securin, which releases separase. This allows for the cleavage of cohesins that hold sister chromatids together.

Microtubule Dynamics and Chromatid Movement

• The movement and segregation of chromosomes are driven by polar microtubules that slide apart due to motor proteins, enabling centriole separation.

• Depolymerization of microtubules mediated by kinesin 13 assists in transporting separated chromatids toward opposite poles.

Cytoskeletal Structures in Cell Division

• Astral microtubules anchor at the plasma membrane through complexes like dynein, pulling each pole towards the membrane while segregating genetic material and organelles.

• Inactivation of mitotic promoting factors leads to dephosphorylation of intermediate filaments, allowing nuclear lamina reformation around each pole.

Actin Filament Functionality During Cytokinesis

• Actin filaments play a crucial role during cytokinesis by forming a contractile ring that facilitates membrane separation between daughter cells.

• Myosin motors interact with actin filaments to generate contraction forces necessary for dividing the cell membranes effectively.

Summary and Future Directions

• The seminar concluded with insights into cytoskeletal functions across different cellular phases, emphasizing their roles in migration and division.