Download ch08_Cell-Cell Communication

Cell-Cell Interactions
8.1 The Cell Surface
The Structure and Function of an Extracellular Layer
The Plant Cell Wall
The Extracellular Matrix of Animals
8.2 How Do Adjacent Cells Connect and Communicate?
Cell-Cell Attachments
– Tight Junctions; Desmosones
– Selective Adhesion; The Discovery of Cadherins
Cell-Cell Gaps
8.3 How Do Distant Cells Communicate?
Signal Reception
Signal Processing
– G Proteins
– Receptor Tyrosine Kinases
Signal Response
Signal Deactivation
Plant cell, with chloroplasts and large vacuole
Most cells secrete extracellular material that generally consists of long
crosslinked filaments surrounded by a stiff ground substance. The rods or
filaments protect against stretching forces, and the ground substance protects
against compression. This is especially obvious in plants.
The Plant Cell Wall
The extracellular material secreted by plant cells first builds
a primary cell wall of long strands of cellulose bundled into
microfibril filaments that form a crisscrossed network. This
network becomes filled with a gelatinous polysaccharide
such as pectin, which is hydrophilic and keeps the cell wall
moist.
•In young cells, enzymes called expansins allow microfibrils
to slide past each other, and turgor pressure then forces the
cell wall to elongate, allowing cell growth.
•Animal cells secrete an extracellular matrix (ECM) that
consists of a gelatinous polysaccharide as ground substance and
protein fibers like collagen instead of polysaccharide filaments.
The ECM provides structural support for the cell and also helps cells stick
together. Cytoskeletal actin filaments connect to transmembrane integrins that
bind to ECM fibronectins, which bind to collagen. These protein-protein
attachments link the cytoskeletons within cells directly to the ECM
In multicellular organisms, cells can form tissues, which may
combine to form an organ specialized for one biological function.
A middle-lamella-like layer exists between cells in many animal tissues.
Some tissues, such as epithelia, have additional proteins that connect
neighboring cells more strongly. In tight junctions, this seal is water-tight.
Example: epithelial cells
Each major cell type has its own cell adhesion proteins. The cadherins are a group
of cell adhesion proteins found in plasma membranes that bind only to other
cadherins of the same type, causing cells of the same tissue type to bind together.
In most animal tissues, gap
junctions connect adjacent
cells by forming channels
that allow the flow of small
molecules between cells to
coordinate their activities
Chemical signals (hormones, etc) travel throughout animals
and plants to target cells and convey information from one
tissue or organ to another. This intercellular signaling
involves four steps: signal reception, signal processing,
signal response, and signal deactivation.
The function and chemical structure of plant and animal
hormones vary widely (Table 8.1). However, all hormones
play a role in coordinating cell activity in response to
information from outside or inside the body.
• Signal receptors are proteins that change their
conformation or activity when a hormone binds to them.
•There are many types of receptors, each of which is found
only in certain cell types.
•In all cases, a change in receptor structure indicates that a
signal has been received.
•Hormones that cannot diffuse across the plasma membrane
rely on a signal-transduction pathway to convert the
extracellular hormone signal to a new intracellular signal.
Many of these involve transmembrane so-called G proteins
or else receptor protein kinases.
• Common second messengers are normal cell constituents
such as Ca2+ ions, cyclic adenosine monophosphate (cAMP),
cyclic GMP (cGMP), or one of two molecules made from
membrane lipids: diacylglycerol (DAG) or inositol
triphosphate (IP3)
•Cells can produce large quantities of these second messenger
molecules in a short time after each hormone-receptor
binding event, so this signal transduction amplifies the
original signal.
•Signal transduction converts an easily transmitted
extracellular message into a greatly amplified intracellular
message that carries information throughout the cell and
induces a pre-programmed cell response.
Receptor Tyrosine Kinases
• Receptor tyrosine kinases are transmembrane
proteins that bind a hormone signal and
phosphorylate one another. The phosphorylated
form activates other enzymes by phosphorylating
them.
•These enzymes phosphorylate yet more enzymes
and so on, creating a phosphorylation cascade
that activates many enzymes.
•Example: insulin receptor
Signal Response and Deactivation
• Signal responses include changes in gene expression
and changes in the activity of specific proteins.
•Turning off cell signals is just as important as
turning them on. Cells have automatic and rapid
mechanisms for signal deactivation. These mechanisms
allow the cell to remain sensitive to future hormone
signals.
• The end result of cell sensitivity to
hormonal signaling is an integrated
whole-organism response to changing
conditions both inside and outside the
multicellular organism.