Chapter 11Chapter 11

Cell-Cell Cell-Cell InteractionsInteractions

Why do cells interact with one another?

How do cells interact with one another?

Cell-Cell InteractionsCell-Cell Interactions

Importance of Cell-Cell Interactions

Interactions between… Adjacent cells Distant cells

Interactions between… Adjacent cells Distant cells

Adjacent Cells:Connections & Communication

extracellular structures cell walls (plants) extracellular matrix (animals)

cell-cell attachments middle lamella (plants) tight junctions and

desmosomes (animals)

cell-cell gaps plasmodesmata (plants) gap junctions (animals)

Cells constantly interact with other cells

Strong ConnectionsStrong Connections

What are these What are these bars for?bars for?

Fiber Composites: Plant Cell WallsFiber Composites: Plant Cell Walls

Extracellular Structures

Extracellular Matrix (animal cells)

Functions Helps to define cell shape May aid in adhesion to other cells May serve protective role

Composition Fibrous “rods” – collagen (protein) Ground substance/Matrix – gel-like polysaccharides

How do these materials get outside of the cell?

The Cell Surface:Cytoskeleton, Plasma Membrane and Extracellular Layers

ExtracellularMatrix (ECM)

Gel-formingpolysaccharides

Integrin

Fibronectin

Collagen

PlasmaMembrane

Actin filament

Cytoskeleton

Extracellular Matrix (ECM):Extracellular Layers in Animal Cells

The structure of a cell’s ECM correlates with that cell’s function

Epithelia Cartilage

ECM quantity and composition vary by cell type

The ECM is directly linked to cytoskeleton via integrins and fibronectins

Holds cells in place

Helps adjacent cells adhere to one another

Extracellular Matrix (ECM):Extracellular Layers in Animal Cells

Conditions resulting from defective ECM: Scurvy – insufficient hydroxyproline, a component of collagen

(due to Vitamin C deficiency) Photoaging – loss of collagen from dermis

Extracellular Matrix (ECM):Importance

Cell-Cell Attachments

Cell-Cell Attachments – structures that hold cells together within multicellular organisms

Tight Junctions Desmosomes

specialized proteins in plasma membranes of adjacent cells that line up and bind to one another

form watertight seal between cells – important in epithelial tissues; prevent passage of molecules/ions between cells

Tight Junctions:Cell-Cell Attachments in Animal Cells

Membrane proteins thatform a tight

junction

Plasma membranesof adjacent

cells

Where would you expect to find tight junctions?

cell-cell attachments that bind cytoskeletons of adjacent cells

composed of membrane proteins that link cells (cadherins) and anchoring proteins

Provide strength in smooth muscle and epithelial tissue

Desmosomes:Cell-Cell Attachments in Animal Cells

Plasma membranesof adjacent

cells

Anchoringproteins

inside cell

Membraneproteins that

link cells(cadherins)

Intermediatefilaments

Where would you expect to find desmosomes?

Cell-Cell Attachments

Cells of the same tissue type aggregate. How do they ‘find’ and ‘stick to’ one another?

Cadherins and Selective Adhesion

Selective adhesion – tendency of cells of the same tissue type to adhere to one another CADHERINS

Loss of N-cadherin in endothelial cells leads to embryonic lethality.

Luo Y , Radice G L J Cell Biol 2005;169:29-34

© 2005 Rockefeller University Press

Question 7

Colchicine is a drug that inhibits polymerization of the subunits that form microtubules. Would colchicine be effective in stopping bacterial flagella? _____

yes no

Test Question 9

Unicellular organisms live in a wide range of habitats, from the hot springs in Yellowstone National Park to the freezing temperatures of the Antarctic. Make a prediction about the saturation status of membrane phospholipids in organisms that live in extremely cold environments versus those that live in extremely hot environments. Organisms that live in extremely cold environments are likely to have the

same number of saturated and unsaturated phospholipids. Organisms that live in extremely cold environments are likely to have highly

unsaturated phospholipids versus those that live in extremely hot environments.

Organisms that live in extremely cold environments are likely to have phospholipids with saturated tails versus those that live in extremely hot environments.

The saturation status of membrane phospholipids does not depend on the environmental conditions.

Adjacent Cells:Connections & Communication

extracellular structures cell walls (plants) extracellular matrix (animals)

cell-cell attachments middle lamella (plants) tight junctions and

desmosomes (animals)

cell-cell gaps plasmodesmata (plants) gap junctions (animals)

Cells constantly interact with other cells

Cell-Cell Gaps

Cell-Cell Gaps – direct connections between the cytoplasm of adjacent cells

Enables efficient communication between cells

Enables cells within a tissue to act in concert (as an integrated whole)

Plasmodesmata:Cell-Cell Gaps in Plant Cells

Notice: cell membranes/cytoplasm are connected (continuous)

Gap Junctions:Cell-Cell Gaps in Animal Cells

Notice: cytoplasm is connected (continuous)

Group Question 1

What characteristics do tight junctions bestow on tissues that use these adhesions to connect adjacent cells? They allow communication between adjacent cells. They provide strong connections to resist pulling

forces. They use the extracellular matrix to indirectly

connect adjacent cells. They form a watertight barrier between the cells.

CELLS COMMUNICATE OVER CELLS COMMUNICATE OVER LONG DISTANCES VIA LONG DISTANCES VIA HORMONES AND SIGNAL HORMONES AND SIGNAL TRANSDUCTION PATHWAYS.TRANSDUCTION PATHWAYS.

How do they How do they communicate?communicate?

What do all of these have in common?What do all of these have in common?

Relay a message (signal) from outside the cell into the cell.

Way of transmitting information Way of transmitting information (signal) from outside a cell to the (signal) from outside a cell to the

inside of a cell, resulting in a inside of a cell, resulting in a response.response.

= Signal Transduction:

Cell Signaling Pathway

Cell Signaling Pathways (General)

Signal Sent – intercellular signal (hormone) released

1. Signal Reception – hormone binds to signal receptor

2. Signal Transduction – transmission/conversion of a signal from outside the cell into the cell.

3. Signal Amplification – signal magnified as it is “transported” through cell to its final destination.

4. Signal Response – signal reaches intracellular target(s) and triggers a cellular response (activation/deactivation)

5. Signal Deactivation – intracellular signal is turned off

Stages of Signal Stages of Signal TransductionTransduction

(5. Signal deactivation)

Signal Sent: Hormones

Hormones – chemical signals produced by one set of cells that affect a different set of cells Most are distributed by the bloodstream to target cells Only affect cells that have a receptor for that specific

hormone (target cells)

Hormones

Steroid Hormones – hormones that are composed of four carbon rings (steroid structure)

Peptide (Non-Steroid) Hormones – hormones that are peptides, proteins, glycoproteins or modified amino acids

Hormones

There are others too!

Signal Reception

Peptide hormones bind to receptors on the cell surface.

Steroid hormones bind to receptors inside the cell.

Why?

Upon hormone binding, signal receptors change conformation (shape) resulting in a change in activity

Signal Reception

Signal receptors… Are specific (bind specific hormones) Vary between cells (different cell types have different sets of

signal receptors) Are dynamic (numbers may increase/decrease) Can be blocked (often used as drug target);

Example: Tamoxifen

Signal Transduction Pathways

Cell activity changes

Receptorproteinin membrane

Inside of cell

Intracellularsignal

Outside of cell

Intercellularsignal

Signal Reception

Signal Transduction• extraintracellular signal

Signal Response

Signal Amplification

Signal Transduction Pathways

G ProteinsEnzyme-

Linked Receptors

Signal Amplification:

Second Messengers

Signal Amplification:

Phosphorylation Cascade

Second Messengers

Second Messengers – nonprotein intracellular signaling molecules

Signal Responses

Intracellular Targets:

DNA – activate or deactivate gene expression (making a protein using DNA “instructions”)

Pre-existing proteins – activate or deactivate enzymes, membrane channels, etc.

Result… change in cell activities!

Signal Deactivation

Deactivation allows signal to be turned off quickly Hydrolysis of GTP by G proteins deactivates them Deactivation or sequestration of second messengers Dephosphorylation

Enables cells to respond rapidly to changing conditions

Group Question 2

What does it mean to say that a signal is transduced? The signaling molecule enters the cell directly and

binds to a receptor inside. The physical form of the signal changes between the

outside of the cell and the inside. The signal is amplified, such that even a single

molecule evokes a large response. The signal triggers a sequence of phosphorylation

events inside the cell.

Steroid Hormone Signaling: Example

http://web.virginia.edu/Heidi/chapter34/Images/8883n34_42.jpg

Estrogen – triggers production of proteins that promote cell proliferation (multiplication)

Estrogen – triggers production of proteins that promote cell proliferation (multiplication)

Steroid Hormone Signaling: Example

http://web.virginia.edu/Heidi/chapter34/Images/8883n34_42.jpg

Tamoxifen binds to the estrogen receptor (prevents estrogen binding)

Tamoxifen binds to the estrogen receptor (prevents estrogen binding)

Steroid Hormone Signaling: Example

http://web.virginia.edu/Heidi/chapter34/Images/8883n34_42.jpg

Tamoxifen binds to the estrogen receptor (prevents estrogen binding)

Inhibits cell proliferationInhibits tumor growth!

Tamoxifen binds to the estrogen receptor (prevents estrogen binding)

Inhibits cell proliferationInhibits tumor growth!