SoS, Dept. of Biology, Lautoka Campus BIO508 Cell Biology Slide Design: Copyright © McGraw-Hill Global Education Holdings, LLC. Lecturer: Dr.Ramesh Subramani Topic 10: Cells in development (Cell junctions, adhesions and ECM)

Extracellular components and connections between cells help coordinate cellular activities Most cells synthesize and secrete materials that are external to the plasma membrane These extracellular structures include Cell walls of plants The extracellular matrix (ECM) of animal cells Intercellular junctions

Cell Walls of Plants The cell wall is an extracellular structure that distinguishes plant cells from animal cells Prokaryotes, fungi, and some protists also have cell walls The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein

Plant cell walls may have multiple layers Primary cell wall: relatively thin and flexible Middle lamella: thin layer between primary walls of adjacent cells Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall Plasmodesmata are channels between adjacent plant cells

Plant cell walls Secondary cell wall Primary cell wall Middle lamella Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata

The Extracellular Matrix (ECM) of Animal Cells Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM) The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin ECM proteins bind to receptor proteins in the plasma membrane called integrins

Extracellular matrix (ECM) of an animal cell Collagen EXTRACELLULAR FLUID Polysaccharide molecule Proteoglycan complex Carbo- hydrates Fibronectin Core protein Integrins Proteoglycan molecule Plasma membrane Proteoglycan complex Micro- filaments CYTOPLASM

Functions of the ECM Support Adhesion Movement Regulation

Collagen provides structural support to tissues The principal function of collagens is to provide structural support to tissues. Collagens are a family of over 20 different extracellular matrix proteins. Together they are the most abundant proteins in the animal kingdom.

Collagen subunits are: All collagens are organized into triple helical, coiled-coil “collagen subunits.” They are composed of three separate collagen polypeptides. Collagen subunits are: secreted from cells then assembled into larger fibrils and fibers in the extracellular space

Mutations of collagen genes can lead to a wide range of diseases, from mild wrinkling to brittle bones to fatal blistering of the skin.

Proteoglycans provide hydration to tissues Proteoglycans consist of a central protein “core” to which long, linear chains of disaccharides, called glycosaminoglycans (GAGs), are attached. GAG chains on proteoglycans are negatively charged. This gives the proteoglycans a rodlike, bristly shape due to charge repulsion.

Proteoglycans attract water to form gels that: The GAG bristles act as filters to limit the diffusion of viruses and bacteria in tissues. Proteoglycans attract water to form gels that: keep cells hydrated cushion tissues against hydrostatic pressure

Expression of proteoglycans is: Proteoglycans can bind to a variety of extracellular matrix components, including: growth factors structural proteins cell surface receptors Expression of proteoglycans is: cell type specific developmentally regulated

Fibronectins connect cells to collagenous matrices The principal function of the extracellular matrix protein fibronectin is to connect cells to matrices that contain fibrillar collagen. At least 20 different forms of fibronectin have been identified. All of them arise from alternative splicing of a single fibronectin gene.

The soluble forms of fibronectin are found in tissue fluids. The insoluble forms are organized into fibers in the extracellular matrix.

Fibronectin proteins contain six structural regions. Fibronectin fibers consist of crosslinked polymers of fibronectin homodimers. Fibronectin proteins contain six structural regions. Each has a series of repeating units.

Fibrin, heparan sulfate proteoglycan, and collagen: bind to distinct regions in fibronectin integrate fibronectin fibers into the extracellular matrix network Some cells express integrin receptors that bind to the Arg-Gly-Asp (RGD) sequence of fibronectin.

Most integrins are receptors for extracellular matrix proteins Virtually all animal cells express integrins. They are the most abundant and widely expressed class of extracellular matrix protein receptors. Some integrins associate with other transmembrane proteins.

Integrins are composed of two distinct subunits, known as α and β chains. The extracellular portions of both chains bind to extracellular matrix proteins The cytoplasmic portions bind to cytoskeletal and signaling proteins.

In vertebrates, there are many α and β integrin subunits. These combine to form at least 24 different αβ heterodimeric receptors. Most cells express more than one type of integrin receptor. The types of receptor expressed by a cell can change: over time or in response to different environmental conditions

Integrin receptors bind to specific amino acid sequences in a variety of extracellular matrix proteins. All of the known sequences contain at least one acidic amino acid.

Integrin receptors participate in cell signaling Integrins are signaling receptors that control both: cell binding to extracellular matrix proteins intracellular responses following adhesion Integrins have no enzymatic activity of their own. Instead, they interact with adaptor proteins that link them to signaling proteins.

Cell Junctions Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact Intercellular junctions facilitate this contact There are several types of intercellular junctions Plasmodesmata Tight junctions Desmosomes Gap junctions

Plasmodesmata in Plant Cells Plasmodesmata are channels that perforate plant cell walls Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell

Plasmodesmata between plant cells Cell walls Interior of cell Interior of cell Figure 6.31. 0.5 m Plasmodesmata Plasma membranes

Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid Desmosomes (anchoring junctions) fasten cells together into strong sheets Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells

Cell Junctions in Animal Tissues Tight junctions prevent fluid from moving across a layer of cells Tight junction TEM 0.5 m Tight junction Intermediate filaments Cell Junctions in Animal Tissues Desmosome TEM 1 m Gap junction Ions or small molecules Space between cells TEM Extracellular matrix Plasma membranes of adjacent cells 0.1 m

Malignant cancer cells from the breast (See the ABNORMAL “crab” shape of the cells.)

Mutation and Cancer Mutation Hyperactive Ras protein Growth factor Mutation Hyperactive Ras protein (product of oncogene) issues signals on its own Nucleus G protein Cell cycle-stimulating Pathway Cell cycle inhibiting pathway Receptor Protein kinases (phosphorylation cascade) Transcription factor (activator DNA Gene expression Protein that Stimulates the cell cycle Protein kinases UV Active Light form of P53 Damaged DNA DNA Mutation: missing transcription factor, such as p53, cannot activate Protein that inhibits the cell cycle

Multiple signal breakdowns Cell growth and division is such an important process that it is under tight control with many checks and balances. But even so, cell communication can break down. The result is uncontrolled cell growth, often leading to cancer. Cancer can occur in many ways, but it always requires multiple signaling breakdowns.

Often, cancer begins when a cell gains the ability to grow and divide even in the absence of a signal. Ordinarily this unregulated growth triggers a signal for self-destruction. But when the cell also loses the ability to respond to death signals, it divides out of control, forming a tumor. Later cell communication events cause blood vessels to grow into the tumor, enabling it to grow larger. Additional signals allow the cancer to spread to other parts of the body.

Acknowledgements… Any Questions?? The teaching material used in this lecture is taken from: JB Reece, LA Urry, ML Cain, SA Wasserman, PV Minorsky and RB Jackson. 2011. Campbell Biology (9th Edition), Publisher Pearson is gratefully acknowledged. Some information presented in this power point lecture presentation is collected from various sources including Google, Wikipedia, research articles and some book chapters from various biology books. Material and figures used in this presentation are gratefully acknowledged. This material is collected and presented only for teaching purpose. Any Questions?? Dr.Ramesh Subramani, Assistant Professor in Biology