Cell-Cell Interactions-BIO1113

OSU Dr.Ball/Mackey BIO 1113

Stepping Outside the Cell

Cells are not self-contained

• Must interact with other cells and environment

What’s outside the membrane

• Extracellular matrix (ECM)

• Cell Walls

Extracellular Matrix

Found in animal tissues

• The “stuff” found outside of the cell

Main components:

• Collagen

• Proteoglycans

Functions

• Support

• Segregate tissues

• Regulation of intercellular communication

Cell Walls

Found in plants (and bacteria, fungi and some protists)

Major functions:

• Protection

• Maintain Shape

• Prevent excess water uptake
  Main component

• Middle lamella: like peanut butter, remains sticky and when pulled apart has resistance

Connection and Communication between adjacent cells

Tight junctions

• seal cells together

Desmosomes

• connect the cytoskeletons of cells

Gap junctions

• act as channels between cells

Cell-Cell Attachments

Tight Junction

• Form water tight seal between cells

• Restricts passive movement

     - Can vary between tissues (amount of material that needs to pass depends)

     - Dynamic (tight junctions can change based on conditions)

Desmosomes (Tight + Strong)

• Bind the cytoskeletons of adjacent cells together

• Cell-cell adhesions are selective

     - Cadherins= proteins in desmosomes that link cells (type varies between tissues) ( calcium dependent, if you want to separate need to take calcium out)

Gap junctions (animals)

• Membrane proteins line up to form channels between cells

• Allows passage of ions and small molecules

Plasmodesmata (plants)

• Membrane-lined channels through cell wall connect adjacent cells

Long-distance signaling

• Uses hormones (endocrine signaling)

• Travel through bloodstream to reach target cells

• Ability of cell to respond depends on whether it has receptors that can bind the signaling molecule

3 Stages of Cell Signaling

1. Reception

• The signal is detected by the cell

2. Transduction

• The signal is converted into a form that can cause some type of cellular response

3. Response

• The cells responds in some way to the signal

Step 1: Reception

A receptor protein on the surface of the cell (or inside the cell) binds the signaling molecule (ligand) and transmits the signal

Binding is very specific and causes the receptor to change its shape

3 main types of membrane receptors

• G protein-coupled receptors

• Tyrosine kinase receptors

• Ion channel receptors

G protein- coupled receptors (GPCRs)

Plasma membrane receptors that are associated with G proteins

• G protein acts as on/off switch

    - When GTP is bound=ON

    - When GDP is bound=OFF

• Contains GTPase domain

    - Shuts itself off

    - Often activate the production of second messengers

Receptor Tyrosine Kinases (RTKs)

Membrane receptors with enzymatic activity which attach phosphates to tyrosine residues

• Activates Ras (G protein)

• Abnormally activated RTKs are associated with many types of cancer

Ion Channel Receptors

• Binding of ligand causes conformation change

• When gate opens specific ions can pass through

Intracellular Receptors

Signaling molecule must be able to pass through the membrane

• Steroid hormones

• Thyroid hormones

• Nitric oxide

Binds to receptor (in cytoplasm)

• Transported to nucleus

• Trigger change in gene expression

Transduction

The relay of signals from receptors to target molecules in the cell

• Activates enzyme to produce 2nd messengers

• Activate phosphorylation cascade

Often involves multiple steps

• Allows the signal to be amplified

• Also allows for more regulation

Message is often passed along through a change in protein conformation

• Phosphorylation=protein kinases

• Dephosphorylation=protein phosphatases

Second Messengers

Small non-protein water-soluble molecules

• Can easily spread message throughout cell

2 most common second messengers

• Cyclic AMP (cAMP)

• Calcium ions (Ca²⁺)

cAMP

• Cyclic adenosine monophosphate

• Adenylyl cyclase ATP → cAMP

Vibrio cholerae, toxin, causes G protein to be unable to hydrolyze GTP to GDP, remains stuck in “ON” position, activating cAMP

• cAMP levels cause salt to be excreted into intestines and water follows = diarrhea

Ca²⁺

Used as a second messenger in both G-protein and tyrosine kinase receptor pathways and cell division

Increasing Ca²⁺ concentrations cause a variety of responses

• Muscle contraction

• Secretion

• Cell division

Level of Ca²⁺ is 10,000x higher outside cell

• Ca²⁺ is actively transported out of the cell or into ER or mitochondria

Step 3: Response

Whole point of the signal is to tell the cell to do something

• Change (turning on and off) in gene expression (protein synthesis)

• Regulate the activity of a protein (i.e. open an ion channel)

Nuclear Response

• Response happening at nucleus at level of gene, less energy needed

• Gene expression

Cytoplasmic Response

• Activating proteins, quicker as protein is already made just need to flip switch 

• Activity, requires more energy + resources

• Requires a bigger investment as you need a ready protein

Specificity and Coordination of Response

All the cells in your body contain the SAME DNA so why is a heart cell different from a liver cell

• Different genes are turned on in different cells

• Different cells therefore have different proteins = different responses

Crosstalk

• Multiple pathways interact to modify response

• Weighing different signals to see the response

Termination of the Signal

In order to respond to new signal, old signals must be shut off

Binding of signaling molecules to receptors is reversible

The messengers return to their inactive form

• GTP hydrolyzes to GDP

• cAMP is converted to AMP 

• Phosphatases remove phosphates to deactivate proteins

Signaling in Unicellular Organisms

Process of reception and signal transduction very similar between unicellular and multicellular organisms

Often relay information about environment

• Quorum sensing

- Response to population density

- Biofilms, slime molds