slide 2 Flashcards
Cell to Cell Communication A critical component of a cell’s environment is the presence of other cells
Cell to cell communication is important in both unicellular and multicellular organisms
The yeast, Saccharomyces cerevisiae, has two mating types, a and a1
Cells of different mating types locate each other via secreted factors specific to each type.
Some soil-dwelling bacteria and amoebas aggregate when food becomes scarce and form coordinated fruiting bodies to disperse spores
Release of signaling molecules
into the environment initiates
and regulates aggregation
Multicellular organisms
can have trillions of
cells of hundreds of
different types
To maintain a functional organism these cells need to remain coordinated through communication
Cell communication is important in the study of
many diseases and detrimental conditions:
– Cancer – Allergies – Heart disease – Diabetes – Alzheimer’s
A signal transduction pathway
is a series of
steps by which a received signal is converted
into a specific cellular response.
The molecular details of signal transduction
pathways are
very similar among unicellular
and multicellular organisms.
Cells in a multicellular organism
communicate by chemical messengers
Communication is by either:
Local signaling
Long-distance signaling
Local Signaling
Both animal and plant cells have cell junctions that
directly connect the cytoplasm of adjacent cells
Animal cells can sometimes communicate via
direct contact in cell-cell recognition
(e.g. immune
response and embryonic development)
local regulators
In many other cases, animal cells communicate using local regulators, messenger molecules that travel only short distances
paracrine
In paracrine signaling
cells release molecules
that act on nearby cells
Signals are passed
between nerve cells via
neurotransmitter
released into the
synapse (gap) between
the cells
Long-Distance
Signaling
Animals and plants use
chemicals called
hormones for longdistance
signaling
Endocrine signaling
involves release of
hormones into the
circulatory system
The plant hormone
ethylene
is released to
coordinate fruit ripening
3 stages of Cell Signaling
1) reception
2) transduction
3) response
- Reception
In signal reception a signal molecule binds to a receptor protein causing a shape change Binding between signal (ligand) and receptor is highly specific.
- Reception
Receptors can be located
A. in the plasma membrane
B. intracellularly (within cells)
Plasma Membrane Receptors
There are three main types of membrane receptors
– G-protein-coupled receptors
– Receptor tyrosine kinases
– Ion channel receptors
G protein-coupled receptors (GPCRs)
are the largest family of human cell-surface receptors. G Protein Coupled Receptors Up to 60% of all medicines act by influencing G protein pathways.
G Protein Coupled Receptors
Pathway involves:
- G protein-coupled receptor
- G-protein
- Enzyme
G Protein Coupled Receptors Binding a signal molecule causes the receptor to change shape and bind the G-protein; GTP displaces GDP
Activated G-protein diffuses
along membrane and
activates enzyme
Bound GTP is hydrolyzed to GDP, G-protein dissociates from enzyme and becomes available for re-use
Receptor tyrosine kinases (RTKs)
are membrane receptors that attach phosphates to tyrosines. A receptor tyrosine kinase can trigger multiple signal transduction pathways simultaneously. Abnormal functioning of RTKs is associated with many types of cancers.
Ion Channel Receptors
When a signal molecule binds to the receptor, the gate allows specific ions through. Change in ion concentration (e.g. Na+ , Ca2+) can have effects throughout the cell.
Intracellular
Receptors
Signal molecules that are small and hydrophobic can readily cross the plasma membrane (e.g. steroids) Intracellular receptors are found in the cytoplasm or nucleus
- Transduction
Cascades of molecular interactions (multi-step
pathways) relay signals from receptors to target
molecules in the cell
- Transduction
Multistep pathways have some advantages:
- They can amplify a signal
- They provide more opportunities for
coordination and regulation
Phosphorylation Cascade
Many signal transduction pathways include “phosphorylation cascades” – A series of enzymes add a phosphate to the next one in line, activating it – different enzymes then remove the phosphates
- Cellular Response
Cell signaling leads to regulation of cytoplasmic
activities or transcription of mRNA
- Cellular Response
In the cytoplasm
– Signals can be amplified through a multi-step
pathway (e.g. phosphorylation cascades)
– Signaling pathways regulate a variety of cellular
activities – e.g. a change in cell metabolism or shape
- Cellular Response
In the nucleus
– Regulate genes by
activating “transcription
factors” that turn genes
on or off
Termination of Signals
Inactivation mechanisms are
also essential to cell functioning
Termination of Signals
Signal response must be
terminated quickly
– By the reversal of ligand binding
– Receptor reverts to inactive form
Example of cell signaling
Apoptosis
Apoptosis –
“programmed cell death”
Apoptosis is crucial in embryonic development, also occurs
in infected, damaged, or old cells
It is triggered by signals that activate “suicide proteins”
