Cell Bio Cell Communication

cell communication

the ability of cells to detect and respond to changes in their extracellular environment

conservation of cell communication

cells from all forms of life are dependent upon cell communication

Many signals that cells receive alter one or more of the following

1. cell growth and division

2. cell metabolism

3. cell defenses (immune function)

4. cell signaling with other cells (ex. neuronal signaling)

5. development (cell fate/differentiation, migration)

6. wound healing (migration)

7. cell/tissue coordination (collective tissue physiology)

8. mating/organism reproduction

9. quorum sensing

ligand

a chemical message used to mediate communication between cells; may be nearly any class of molecules

receptor

the cellular molecule (typically a protein) which binds to the ligand specifically; may be found on the cell surface or intracellularly

true or false: cells lacking receptors will respond to ligands

false

true or false: upon binding to receptors, ligands induce signal cascades within the cell

true

cascade

a sequential, multi-step chain reaction where molecules (usually proteins) activate each other in a series

uni- and multicellular organisms are under…

constant signaling from a wide variety of sources and over a diverse level of distances

types of cell communication in order of relative distance that a message is sent

autocrine, juxtacrine (contact dependent), paracrine, endocrine

Autocrine signaling

a cell produces a ligand and had a receptor to receive the same message it is transmitting

purpose of autocrine signaling

to amplify a cellular response or to promote a more specific or effective response to a stimulus

Common messages sent and received via autocrine signaling

cytokines and growth hormones

cytokines functions

wide range of functions, most play key roles in mediating cell immune responses

growth hormones function

promote growth and differentiation

Example of autocrine signaling

macrophage secretes signals for itself to induce a cascade of reactions inside it

another name of jusxtracrine signaling

codependent signaling

juxtacrine signaling

a cell in direct physical contact with an adjacent cell activates/induces a change directly through the shared contact (ex. through gap junctions and plasmodesmata)

3 common types of juxtacrine signaling

protein-protein contacts, ligand-receptor contacts, or cytoplasmic activation

protein-protein contacts

transmembrane proteins contact inducing a conformational change in one or both cells leading to intracellular signaling

Example of protein-protein contacts

delta-notch signaling

delta-notch signaling

conserves signaling pathway for development

Ligand-receptor contacts

surface bound ligands on one cell are recognized by receptors on an adjacent cell leading to activation; common with immune cells

cytoplasmic activation

chemical messages transmitted directly through shared cytoplasm induces activation

Another name for paracrine signaling

local signaling

paracrine signaling

a cell secretes a ligand which binds to nearby cells, which in turn elicits a response; there are many different and unique forms of this type of signaling

Examples of paracrine signaling

synaptic or neuronal signaling, quorum signaling

synaptic or neuronal signaling

a specialized variant of paracrine signaling that occurs in the nervous system where two cells meet as a synapse and an action potential triggers the release of neurotransmitters which bind and signal to a post-synaptic neuron

quorum sensing or signaling

the regulation of gene expression in response to fluctuations in cell population density

-it is a variant of paracrine signaling that occurs between prokaryotic organisms to mediate individual vs group gene expression dynamics in a population

how does quorum sensing work

bacteria each produce autoinducers (ligands) which bind to gene regulatory machinery in the cell; gene expression changes are concentration-dependent and drive independent or group-based population behaviors

quorum

a minimum number of members that must be present

key steps of quorum sensing

production, release, and sensing of the autoinducer molecules in an amount that is capable to stimulate a population behavior or phenotype

endocrine signaling

a cell secretes a ligand or chemical message via the blood stream to be received by a long-distance target

what are the steps of endocrine signaling

plants and animals produce molecules called hormones in glands, hormones are secreted into circulatory systems and are received distant from the initial site or release, the ability of a cell to respond is dependent upon whether it has a hormone receptor

classes of hormones by chemical nature

lipid or hydrophobic and water-soluble or hydrophilic

lipid or hydrophobic hormone class

many are steroids or fatty-acid derivatives

water-soluble or hydrophilic hormone class

typically peptide or protein based in humans

endocrine system

a complex network of glands and organs; it uses hormones to control and coordinate the body metabolism, energy level, reproduction, growth and development, and response to injury, stress, and mood. it works as a messenger system that sends hormones released by the internal glands of an organism directly into the circulatory system, regulating distant target organs. in vertebrates, the hypothalamus is the neural control center for all endocrine systems

Earl Sutherland

discovered how the hormone epinephrine acts on cells and proposed a three-stage mechanism for its activity, which now represents the three stages of cell signaling:

1. reception

2. transduction

3. response

three stages of cell signaling

1. reception: target cell detects a signaling molecule that binds to a receptor on or within the cell

2. transduction: binding of the signaling molecule alters the receptor and initiates signal transduction pathways through a series of steps

3. response: the transduced signal triggers a specific response in the target cell

Receptor stage: Ligand-receptor association

the first stage of cell signaling, reception, is associates with the binding of a signal molecule (ligand) to a receptor through a specific interactions (think lock and key)

Receptor stage: initiating a signaling cascade

upon binding, the receptor often undergoes a change in shape called a conformational change or is activated through protein modifications which initiates a signaling cascade through subsequent interactions

Receptors are found either…

on the cell surface or intracellularly based on the chemical nature of the ligand

-can be hydrophilic or hydrophobic

hydrophilic/water-soluble ligands

bind extracellularly to cell-surface receptors

Hydrophobic (very small ligands)

cross the plasma membrane and bind intracellular receptors

Signaling with reception at the cell surface

the majority of signaling molecules or ligands received by cells are hydrophilic and bind to signal receptors on the plasma membrane or cell surface since they cannot cross the plasma membrane

common classes of cell surface receptors

G-protein coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), ion channel receptors

G-protein coupled receptors (GPCRs)

also known as seven-transmembrane domain receptors; the largest family of cell-surface receptors; work through interactions with G proteins which bind energy-rich GTP; very diverse in ligands and functions

Receptor tyrosine kinases (RTKs)

membrane receptors that activate through dimerization by attaching phosphates to tyrosines (kinase activity); may trigger multiple pathways at once; may bind growth factors and often associated with cancer if not functioning properly

Ion channel receptors

undergo a conformational change to open a gate permitting specific ions to flow such as Na+ or Ca2+ based on their chemiosmotic gradients; play central roles in neural cell signaling

intracellular receptor proteins

found in the cytoplasm or nucleus of target cells. small or hydrophobic chemical messengers can readily cross the membrane and activate these receptors

-many steroids fit this category of chemical messenger'

—aldosterone is a cholesterol-derived hormone involved in regulating blood osmolarity and water balance

example of reception intracellularly

aldosterone crosses the plasma membrane by simple diffusion and binds an intracellular receptor in the cytoplasm. the complex then enters the nucleus where it activates gene transcription as a transcription factor