Cell-Cell Interactions

Dr. Corl

BIOL 105

September 13, 2013

Cell-Cell Interactions

• The cell surface• The extracellular layer• Cell-cell connections

– Cell-cell attachments– Cell-cell gaps

• Cell-cell communication (long distance)

The Cell Surface

• Recall the structure of the plasma (cell) membrane:– Phospholipid bilayer w/ cholesterol molecules interspersed– Both integral proteins and peripheral proteins

• Many of which have carbohydrate groups covalently attached!

The Extracellular Layer

• Most organisms have an extracellular layer just exterior to the plasma membrane:– Provides an extra layer of protection / defense.– Helps define cell shape.– Helps attach one cell to a neighboring cell.

• Broad types of extracellular layers:– Cell wall:

• Surrounds plant, fungi, bacteria, and algal cells.

– Extracellular matrix:• Surrounds animal cells.

The Extracellular Layer

• Usually “fiber composites”:– Cross-linked network of long filaments (fibers)

surrounded by a stiff ground substance.– Protects cell from stretching (tension) and compression.

The Plant Primary Cell Wall

• Fibrous components = Cellulose microfibrils.• Ground substance = Pectins and other

gelatinous polysaccharides.

The Plant Cell Wall

• Primary cell wall: – Defines shape of plant cell.– Counteracts force of water entering the plant cell via osmosis: cell

wall exerts wall pressure.• Secondary cell wall:

– Secreted by certain plant cells. (e.g. xylem cells, above)– Secreted interior to the primary cell wall.– Can provide tough structural support (contains lignin).

The Plant Cell Wall

• Secondary cell wall:– Contains the durable polymer lignin.– Found primarily in the xylem (water conducting)

tissue of plants with a true vascular system:• e.g. Ferns, “evergreen plants,” and flowering plants.

– Adaptation that allows vascular plants to grow tall and resist the force of gravity:• Xylem system acts like an internal skeleton!

The Extracellular Matrix (ECM)

• Fiber composite secreted by animal cells.

• Fibrous component:– Cable-like collagen protein

• Ground substance: – Rich in proteoglycan

complexes:• Contain hundreds of

proteoglycan molecules:– Core protein with many

hydrophilic carbohydrate chains attached.

The Extracellular Matrix (ECM)

• Provides structural support.• More pliable (flexible) than the plant cell wall.• Helps cells adhere to each other.

The Extracellular Matrix (ECM)

• The cell’s internal cytoskeleton is physically connected to the ECM via protein-protein interactions.

The Extracellular Matrix (ECM)

• Specifically, actin filaments are linked to transmembrane proteins called integrins, which are linked to proteins (e.g. fibronectins and laminins) which are linked to collagen proteins.

Cell-Cell Interactions

• The cell surface• The extracellular layer• Cell-cell connections

– Cell-cell attachments– Cell-cell gaps

• Cell-cell communication (long distance)

Cell-Cell Connections

• Unicellular organisms may secrete polysaccharide-rich biofilms, connecting them to each other and to the substrate.– e.g. Dental plaque in your mouth!

Multicellularity Through Cell-Cell Connections

• In multicellular organisms (e.g. plants and animals), various types cell-cell attachments and cell-cell gaps help to connect neighboring cells within a given tissue.

Cell-Cell Attachments

• Middle lamella (plants)– Joins neighboring cell walls.

• Tight junctions (animals)• Desmosomes (animals)

Middle Lamella (Plants)

• Gelatinous polysaccharides (pectins) glue together neighboring plant cell walls.

Tight Junctions(Animals)

• Specialized proteins from adjacent cell membranes line up and bind to each other, “stitching” the cells together.

Tight Junctions

• Can form a watertight seal between cells.• Common in cells lining your skin, stomach,

intestines, and bladder.

Desmosomes(Animals)

• Anchoring and membrane proteins binding to each other and to intermediate filaments link the cytoskeletons of adjacent cells.

Desmosomes

• Made of proteins that link the cytoskeletons of adjacent cells.

• Common in epithelial and muscle cells.

Cadherins

• A major class of cell adhesion proteins.

• An important component of desmosomes.

• Different types of cells express different types of cadherins on their plasma membranes.– Selective adhesion: adjacent cells of the same

cell type often adhere to one another due to interactions of their cell-type specific cadherins.

Cell-Cell Interactions

• The cell surface• The extracellular layer• Cell-cell connections

– Cell-cell attachments– Cell-cell gaps

• Cell-cell communication (long distance)

Cell-Cell Gaps

• Create a direct connection between the cytoplasm of adjacent cells.

• Allows neighboring cells to communicate directly through membrane “holes” and channels.

• Two major types:– Plasmodesmata (plants)– Gap junctions (animals)

Plasmodesmata (Plants)

• Cell-cell gaps connecting adjacent plant cells.• Lined with plasma membrane.• Allows a plant cell to directly share cytoplasm with

neighboring plant cells.

Plasmodesmata (Plants)

• Function in movement of water:– Speeds the movement of water from the root

exterior to the root interior (location of xylem).• Function in movement of sugars:

– Speeds the movement of sugars between adjacent phloem cells.

• Water, sugars, and other molecules can travel through plant tissues via the:

– Symplastic route:• Traveling via the symplast (continuous network of shared

cytoplasm between plant cells connected by plasmodesmata)– Apoplastic route:

• Traveling around plant cells (e.g. through porous cell walls and the middle lamella) without actually entering the cytoplasm of individual cells.

• Apoplast: Extracellular space around cells.

Plasmodesmata (Plants)

Gap Junctions (Animals)

• Each gap junction consists of many channels (made of _______) that connect adjacent ______ cells.

• Allow water, ions, and small molecules to move between adjacent cells.

Gap Junctions (Animals)

• Extensively found within _____ muscle tissue:– Speeds conduction of electrical impulses throughout the heart,

coordinating heart muscle contraction (your heartbeat!).• Also found (to a limited extent) within ________ tissue:

– Allow electrical impulses to directly flow from neuron to neuron.

Cell-Cell Connections: Summary

______ junctions

__________

_____ junction

Cell-Cell Interactions

• The cell surface• The extracellular layer• Cell-cell connections

– Cell-cell attachments– Cell-cell gaps

• Cell-cell communication (long distance)

Long Distance Communication

• Distant cells communicate with each other via ________:– Information carrying molecules that:

• Are secreted by a cell, • ________ in the body, and • Act on target cells far from the original cell.

– ____ concentrations of hormones can have a large impact on target cells!

– Hormone function and structure vary widely.• Lipid soluble (steroids) vs. non lipid soluble.

Hormone Signal Receptors

• Signal receptors are ________ that change conformation (shape) upon hormone binding.

• Each hormone binds to a specific type of signal receptor:– Steroid receptors: Located in ______.– Other hormone receptors: Located in cell ________.

• To _______ to a particular hormone, a cell must express the appropriate signal receptor!

Steroid Hormone Receptors

• ______ diffuses across plasma membrane and binds to receptor in cytosol.

• Hormone-receptor complex can enter ______ and change gene activity.

Steroid Hormone: Estradiol

• Estradiol, for example:– Is released by follicle cells in the _______ of females.– Binds to ________ within the ______ of various cell

types, ultimately causing target cells to:• Differentiate (mammary gland cells during puberty).• Proliferate (endometrial cells lining the uterine wall).• Produce and secrete its own hormones (hypothalamic neurons).

Other Hormone Receptors

• Non-lipid soluble (non-steroid) hormones bind to receptors on plasma ________.

• Signal ____________: – Conversion of an extracellular signal (hormone) to

an intracellular signal.

Signal Transduction

Pathways

• Involve several steps.• Message is _________ as it changes form.

Non-Steroid Hormone:

Epinephrine

• Epinephrine is a non-steroid hormone:– Produced and released by the _______

glands in response to short-term stress.– Binds to epinephrine ________ embedded

in the cell membranes of liver cells:• Triggers a signal transduction cascade that

ultimately activates phosphorylase:– Enzyme that helps convert glycogen to ________.

Signal Transduction Pathways

• G-protein cascades:– Binding of hormone to receptor activates a _____

inside the cell, which then in turn activates other proteins inside the cell.

– e.g. Epinephrine binding to epinephrine receptor on liver cell membranes.

• Enzyme-linked receptor cascades:– Binding of hormone to receptor triggers a cascade of

phosphorylation events inside cell.• Usually, the hormone-bound receptor is the first target to be

phosphorylated. (Autophosphorylation)– e.g. _______ binding to insulin receptor on liver cell

membranes.

G-Protein Cascades

• G-protein initially in “___” conformation.• Signal (hormone) binds to _______.

G-Protein Cascades

• Receptor changes _____ and activates.• G-protein activates (turns on) and ____.

G-Protein Cascades

• Activated G-protein binds to and activates an _____.• Enzyme catalyzes formation of a ______ messenger.• Second messenger triggers a ______.

Second Messengers

• ________ intracellular signaling molecules.• May open ion ______ or activate protein kinases.• Protein _______:

– Enzymes that activate/inactivate other proteins by adding phosphate groups to them (phosphorylation).

Epinephrine Action

• 1.) Epinephrine binds to and activates the epinephrine _______ on liver cell membranes.

• 2.) Receptor activates an intracellular _______:– G-protein activates an

enzyme, adenylyl cyclase.• 3.) Adenylyl cyclase

catalyzes the formation of a second messenger, cyclic AMP (_______).

Epinephrine Action

• 4.) cAMP activates the enzyme protein _____ A.

• 5.) Protein kinase A activates phosphorylase kinase.

• 6.) Phosphorylase kinase activates phosphorylase.

• 7.) Activated phosphorylase catalyzes the cleavage of _______ into _______ monomers!

Enzyme-linked Receptors

• Hormone binding to receptor results in autophosphorylation and __________ of receptor.

• Activated receptors then induce phosphorylation of many other _______ in the cell: a phosphorylation cascade.

• Cascade causes _________ of signal.

• Best understood subgroup:– Receptor tyrosine kinases (RTKs)

Enzyme-linked Receptors

Enzyme-linked Receptors

Enzyme-linked Receptors

Signal amplification!

Insulin Action

• Insulin is a non-steroid hormone:– Released by the _______ in response to

elevated blood glucose levels.– Binds to insulin _______ on the cell

membrane of ______ cells:• Enzyme-linked receptors that initiate a

“phosphorylation” cascade within the liver cell.

Insulin Action• 1.) Insulin binds to insulin

receptor on liver cells.• 2.) Insulin ______

becomes phosphorylated.• 3.) _____ protein

becomes activated.• 4.) Ras activates an

______ called MAPKKK.• 5.) MAPKKK activates

another enzyme: MAPKK.

Insulin Action• 6.) MAPKK activates

another enzyme: MAPK.• 7.) MAPK activates a

transcription factor, which enters the ______.

• 8.) Transcription factor increases the the expression of _______ involved in glycogen synthesis.

• 9.) Liver synthesizes more _________ from glucose monomers.

Signal Transduction

Pathways• Convert an

extracellular signal to an intracellular signal.

• Original message is __________ as it changes form.

• May ultimately lead to the activation of:– Intracellular _______– _________ factors– Membrane channels

Signal Deactivation

• How are cell signals turned off?

– Hormone ______ away from receptor.– G-proteins turn back “____” - deactivate.– Second messengers are degraded.– Phosphatases remove _______ groups

from proteins.

Signal Transduction Pathways

• As a biologist, you will encounter signal transduction pathways often, especially when studying:– The _______

system– The _______

system– The nervous

system

Cell-Cell Interactions

• The cell surface• The extracellular layer• Cell-cell connections

– Cell-cell attachments– Cell-cell gaps

• Cell-cell communication (long distance)

Review Questions

• Contrast the extracellular matrix in animals versus the plant cell wall.

• What are some different ways that neighboring cells can be joined to one another?

• How do plasmodesmata differ from gap junctions?

Review Questions

• How do steroid hormones differ from non-steroid hormones?

• Draw out a G-protein signaling cascade.

• Draw out an enzyme-linked receptor signaling cascade.