Biomedical Engineering Reference
In-Depth Information
about its environment. Receptors are particularly important in animals in facilitating cell-
to-cell communication. Animal cell surface receptors are important in transducing signals
for growth or cellular differentiation. These receptors are also prime targets for the devel-
opment of therapeutic drugs. Many viruses mimic certain chemicals (e.g. a growth factor)
and use cell surface receptors as a means to entering a cell.
Simpler examples exist with bacteria. Some motile bacteria have been observed to move
up concentration gradients for nutrients or down gradients of toxic compounds. This
response is called chemotaxis. Some microbes also respond to gradients in oxygen (aerotaxis)
or light (phototaxis). Such tactic phenomena are only partially understood. However, the
mechanism involves receptors binding to specific compounds, and this binding reaction
results in changes in the direction of movement of the flagella. Motile cells move in
a random-walk fashion; the binding of an attractant extends the length of time the cell moves
on a “run” toward the attractant. Similarly, repellents decrease the length of runs up the
concentration gradient.
Microbial communities can be highly structured (e.g. biofilms), and cell-to-cell
communication is important in the physical structure of the biofilm. Cell-to-cell commu-
nication is also important in microbial phenomena such as bioluminescence, exoenzyme
synthesis, and virulence factor production. Basically, these phenomena depend on local
cell concentration. How do bacteria count? They produce a chemical known as quorum
sensing molecule, whose accumulation is related to cell concentration. When the
quorum-sensing molecule reaches a critical concentration, it activates a response in all
of the cells present. A typical quorum-sensing molecule is an acylated homoserine
lactone. The mechanism of quorum sensing depends on an intracellular receptor protein,
while chemotaxis depends on surface receptor proteins. With higher cells, the timing of
events in cellular differentiation and development is associated with surface receptors.
With higher organisms, these receptors are highly evolved. Some receptors respond to
steroids (steroid hormone receptors). Steroids do not act by themselves in cells but rather
the hormone-receptor complex interacts with specific gene loci to activate the transcrip-
tion of a target gene.
A host of other animal receptors respond to a variety of small proteins that act as
hormones or growth factors. These growth factors are normally required for the cell to
initiate DNA synthesis and replication. Such factors are a critical component in the large
scale use of animal tissue cultures. Other cell surface receptors are important in the
attachment of cells to surfaces. Cell adhesion can lead to changes in cell morphology,
which are often critical to animal cell growth and normal physiological function. The
exact mechanism by which receptors work is only now starting to emerge. One possi-
bility for growth factors that stimulate cell division is that binding of the growth factor
tothereceptorcausesanalterationinthestructure of the receptor. This altered structure
possesses catalytic activity (e.g. tyrosine kinase activity), which begins a cascade of reac-
tions leading to cellular division. Surface receptors are continuously internalized,
complexes degraded, and receptors recycled to supplement newly formed receptors.
Thus, the ability of cells to respond to changes in environmental signals is continuously
renewed. Such receptors will be important in our later discussions on animal cell
culture.
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