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FROM
STIMULUS TO MESSAGE IN BIOLOGICAL SYSTEMS,
AN ILLUSTRATED SURVEY
A
review
C. BONNE
Université
Montpellier I, Faculté de Pharmacie
34060 Montpellier,
France
1 - Introduction
2 - Physical stimuli
3 - Chemical Stimuli and the Key/Lock Image
4 - Stimuli and the 4th D
5 - Transduction from Receptor to Effector
6 - Integration of Primary Messages
7 - High Level Integration
For a century, a major part of physiology and
pharmacology has been devoted to the investigation of the mechanisms which
transform a stimulus into a message.
All the present notions are still based on the revolutionary concept of
Paul Ehrlich: "Corpora nun agunt
nisi fixata". Substances
cannot act without binding, and, as far as the physical stimuli are concerned,
one can consider that the stimulus modifies the binding of pre-existing
biochemical components. The aim of this
survey, is to illustrate the various mechanisms which transform external
stimuli into cellular messages.
A
stimulus is an agent that produces a response in a living organism. One
stimulus has no significance per se but
acquires one particular meaning, becoming a message, when applied to one
particular biological system. Stimuli can be of very different natures; they can be physical, chemical, static or
dynamic. Ultimately, the absence of stimulus can be a stimulus by itself if we
consider the environment of the living organisms.
As an illustration of this paradox, darkness is a stimulus
as far as the retinal photoreceptors are concerned, since sensory cells are
very active in the absence of light in terms of metabolism and synaptic
transmission ! Actually, under this condition, rhodopsin, the photosensitive
pigment located in the outer segment disks of the cell, does not interact with
cation channels of the membrane. These
channels are maintained in the open confirguration by cyclic GMP and
consequently, the membrane is depolarized and the neurotransmitter, glutamate,
is continuously released (Figure 1).
Obviously,
light is also a stimulus for these cells.
When a photon interacts with rhodopsine, the conformation of retinol, a
component of the pigment, changes from eleven-cis to all trans.
This
event allows rhodopsin to bind another protein (Gt), a G-Protein also called
transducin, which activates an enzyme (PDE) able to metabolize cGMP. Without
cGMP, cation channels are closed, the cell becomes hyperpolarized and the
synaptic transmission is stopped (Pugh and Lamb, 1993).
This
is a good illustration of a stimulus transformation into a message, through the
activation of a receptor, a transduction and an effector system.
Figure 1. Physical stimuli,
darkness and light, modulate the release of the photoreceptor neurotransmitter.
Another
example of physical stimulus is mechanical force. Mechanosensitive membrane ion
channels provide a means of transducing membrane deformation or stretch into an
electrical signal. These
mechanoreceptors are ubiquitous, they are expressed in a wide variety of cell
types including both sensory and non-sensory cells.They are implicated in such
diverse functions as cell volume regulation, stretch-activated reflexes in
vascular cells, but also in cell growth and embryogenesis.
Among
the physical stimuli, a particular one, magnetic field, has to be considered
with interest but caution. There is a lot of contradictory data reported in the
medical and scientific litterature, which concern the effect of magnetic fields
on biological systems.
To
the present knowledge, it seems however quite clear that biochemical parameters
can be modified by exposure to 50 Hz magnetic fields at very low flux densities
(Loscher and Mevissen, 1994; Sandyk, 1992). For example, animal studies have
shown that magnetic field exposure reduces the nocturnal plasma level of
melatonin, a hormon secreted by the pineal gland. Another example is given by
racing pigeons which are believed to use a magnetic map for homing. This
question raises a great deal of controversies but, if some reports are very
impressive, no magneto-receptor has been yet identified in terms of molecular
entity.
3. Chemical
Stimuli and the Key/Lock Image
Molecules are the key information vectors in
biological systems. They are the extracellular and intracellular stimuli which
regulate cell functions, metabolism as well as genetic expression. As
previously quoted, molecules become informative by binding to their receptors.
The classical concept of ligand/receptor interaction is illustrated by the
key/lock image, where the dimensional characteristics of the ligand are
complementary to the structure of the binding site. Recent developments of
molecular biology, in particular experiments based on site-directed mutagenesis
have established the role of specific residues of the receptor proteins and
clearly identified the binding pocket of an increasing number of hormon and
neurotransmitter specific receptors.
In
fact, specificity of binding sites for ligands is only relative (fortunately
for the pharmacist, otherwise there were no possible drugs !). The figure 2 shows the functional structure
of a corticosteroid hormon receptor, which is a soluble protein of the
cytoplasm. The glucocorticoid receptor has now been cloned and we know that the
primary structure consists of approximately 800 amino-acid residues. Using
deletion mutagenesis, several distinct domains within the structure have been
defined.
Figure 2. Structure and homology of steroid hormon receptors.
The
steroid-binding domain is at the C-terminal end of the molecule. The other domains are implicated in
interactions with other structures such as DNA. As far as binding specificity is concerned, it is interesting to
note that there is a high percentage of homology of the binding domain in the
various steroid hormon receptors (Barnes and Adcock; 1993). This can explain that some synthetic
steroids present a low specificity for one particular receptor, can bind to
several classes of hormon receptors and consequently have dual hormonal activities
(e.g. the anti-aldosterone drug, spironolactone, presents anti-androgenic side
effects). These data recall to pharmacologists that the lock is not really a
safety one !
As
everybody knows, a majority of informative molecules interact with their
receptor according to the action of mass law. It is important to remark that
Kd, the dissociation constants at equilibrium, are always in the range of
physiological concentrations of ligands, such as hormons, neurotransmitters and
other informative molecules (Kelly and Beaulieu, 1989). That is to say that the message can be
quantitatively modulated as a function of stimulus concentration. Kd values are
most frequently in the range of micro- to nanomolar, but can also be lower, for
instance at picomolar for several growth factors. In fact, possible lower
values could occur but there is a technical limit for their measurement even
with radioligands. Moreover, in some cases, chemicals can be detected at
extremely diluted concentrations by receptors of relatively low affinity. This
is the case for odorous molecules which are recognized by receptors of
olfactory cells due to prior dissolving and accumulation in nasal mucus. This
process concentrates the ligand up to the Kd value of the receptor.
If a
lot of chemical stimulus/receptor interactions are governed by this sample low,
some other receptor activation processes are more complex : cooperativity of
binding has been reported for various ligands either positive or negative. In
these cases, the binding kinetic parameters depend on the concentration of the
unbound ligand. In a similar way, it can be noted that the Kd of some receptors
for their ligand is modulated by the interaction of other molecules with
allosteric binding sites. For example, the sedative benzodiazepins bind to the
GABA-A receptor on a specific site and increase the affinity of the receptor
for this inhibitory neurotransmitter. (This mechanism explains why
benzodiazepins are not active in GABA-depleted organisms).
Another aspect of the stimulus/receptor interaction
had to be considered from the point of view of the 4th D, i.e. time.
Indeed, the transformation of a stimulus into a message depends on the previous
state of the reception machinery which is not a frozen apparatus but a plastic
one.
One
illustration of the dynamic nature of the conversion of a stimulus into message
is given by chemotaxis. This is a general phenomenon in biology which is
responsible, for example, for sexual attraction of butterflies by pheromons, or
for the directional migration of leukocytes toward an inflammation site. This
mechanism requires recognition of a molecular gradient, in other words,
recognition of a molecular concentration as a function of time. Such a
phenomenon implicates a dynamic participation of the receptive system to give
significance to the stimulus.
This
notion can also be illustrated by tachyphylaxis : a first stimulus can
inactivate the receptor/transduction machinery which become desensitized to a
second stimulus. For example, the inflammatory mediator, leukotriene B4,
down-regulates its own receptor by producing a low-affinity phosphorylated form
of this protein.
5. Transduction
from Receptor to Effector
It is fascinating to discover both the great
homogeneity and the large variety of the receptor-tranduction systems in
biology. If we consider only the membrane receptors, we can distinguish four
general systems. Ionotropic receptors which are linked to ion channels and
metabotroptic receptors which are linked to enzymes. They can also be
classified according to their dependence or independence on G-Proteins (Figure
3).
Figure 3 : Membrane receptor transduction systems.
5.1. Metabotropic receptors
Recently, Alfred Gilman and Martin Rodbell have
received the Nobel Price for their discovery of the G-Proteins (Hanoune, 1994).
G-Proteins are a superfamily of proteins which are formed by 3 subunits : the
hydrophylic a and b and the lipophylic g subunits.
These
proteins are called G-Proteins because they bind GDP or GTP and hydrolyse GTP.
Finally these protein complexes provide a link between activated receptors and
effector proteins. There are several G-Proteins which can either activate or
inhibit various enzymes and ion channels.All the receptors linked to G-Proteins
present the same general structure with seven helix transmembrane domains. This
is the case for b adrenoreceptors, the muscarinic acetylcholine
cerebral receptor, the numerous olfactory receptors recently identified in the
nasal mucosa as well as rhodopsin.
So,
a chemical stimulus and a physical stimulus as light, use a similar machinery
to be converted into a specific message in a particular cell. Metabotropic
receptors, when stimulated by their ligand, trigger the formation of a
metabolite called second messenger. The first discovered second messenger is
cyclic adenosine monophosphate (cAMP). This messenger becomes an intracellular
stimulus capable to activate a protein kinase then a cascade of enzymes which
lead to cellular responses.
This
transduction system is also a very efficient amplifier : it can be calculated
that one molecule of b1-Receptor
agonist is able to trigger the synthesis of 2000 molecules of cAMP/min. Other
enzymes than adenylate cyclase are linked to receptors via G-Proteins. An
ubiquitous one is phospholipase C which produces two second messengers,
inositol triphosphate and diacylglycerol. The first one triggers the release of
calcium from intracellular stores then the activation of Ca++-dependent enzymes. The second one, diacylglycerol,
activates protein kinase C. This transduction mechanism is shared by a lot of
chemical stimuli including hormones and neurotransmitters.
Since
the last few years, it appears that receptor coupling is frequently
multifunctional. For example, epinephrine, via the activation of a2 receptors in
platelets for instance, can simultaneously inhibit adenylate cyclase and
stimulate phospholipase C, involving the different subunits of a G-Protein. At
last, we have previously shown (Figure 1) that second messengers can directly
operate ion channels, in addition to protein kinases.
5.2. Ionotropic receptors
In this class of receptors which operate ion-channels
two sub-classes are distinguished according to the involvement of G-Proteins in
the tranduction mechanism. This difference can be illustrated with two examples
: i) the cardiac M2 muscarinic receptor is a G-Protein-coupled receptor,
the activation of which directly triggers K+-channel opening,
responsible for cell hyperpolarization and bradycardia ; ii) The skeletal
muscle nicotinic receptor which is an hetero-oligomeric protein complex which
binds acetylcholine and becomes permeable to Na+/K+, leading to
depolarization and muscle contraction.
5.3. Receptor-enzymes and
enzymes as receptors
The fourth membrane receptor class quoted in Figure
3, is a class of transmembrane proteins which present both a receptor site and
an enzymatic moity. This kind of receptors is the model of growth factor
receptors, which exhibit protein kinase activity. To this mechanism which
basically regulates cell growth, we can also link those of integrins,
transmembrane proteins which allow cell adhesion and participate in information
transfer between cells in contact. These integrins do not present enzymatic
activity but directly activate some intracellular protein kinases.
In
addition to receptor-enzymes, one can also consider the case where an enzyme is
a target for an informative molecule. An example is given by nitric oxide
(Feldman et al., 1993). This very
simple molecule is known to be an ubiquitous informative entity with many
biological effects, responsible for vasodilation for instance, when released
from vascular endothelium. This labile mediator can cross the membranes and
reach the cytoplasmic enzyme of smooth muscle cells, the guanylate
cyclase. This heme enzyme is then
activated and cyclic GMP concentration increases leading to muscle relaxation.
5.4. Cytosolic receptors
When the chemical stimuli are able to enter the cell,
for instance due to their lipophilicity, they can have specific receptor sites
within the cell. This is the case for steroid hormons.
Their
receptors are cytosolic proteins associated with other molecules such as heat
shock proteins (HSP). When the steroid
binds this complex, HSP dissociate and the receptor-hormon complex is then
translocated to the nucleus where it modulates mRNA transcription through
interaction with responsive elements. In the case of glucocorticoid hormones,
about one hundred genes are either repressed or activated.
6. Integration
of Primary Messages
In the last part of this survey, I would like to move
on from this quite simplistic description of biological cybernetics to a more
integrated view of the transformation of stimuli into messages. Even at the
cell level the response to a stimulus depends on the state of the cell and on
the concomitant stimuli which are integrated by a system of an enormous
combinatory complexity. The combinatory complexity can be illustrated by the
communication networks summarized in figure 4.
Figure 4 : Communication networks. In A : Several stimuli acting on
a same target cell, through the same transduction mechanism, can induce the
same response. This is the case of glucagon, calcitonine and PTH on
electrolytic transport in the kidney. In B : this example shows that receptor
multiplicity allows a single molecule to be used in a great variety of
functions in different circuiteries or development stages. In other words, one
particular chemical stimulus can lead to various messages according to the
target cell. In C : there is a
combination of A + B. In D : at last,
two informative molecules acting on the same target cell, but through different
transduction mechanisms, can induce different responses of the same effector.
An example in the kidney again : PTH, via adenylate cyclase activation,
inhibits the Na+/H+ antiport, when angiotensin,
via phospholipase C activation, stimulates this antiport.
In addition to antagonism and synergy, permissive
action and all-or-none action from thresholds are also mechanisms involved in
the integration of messages. One example can be given with the "gate
control" of pain (Figure 5).
Figure 5 : The gate control theory
This theory has been proposed by Melzack and Wall in
the sixties, to explain how thermal, mechanical or chemical stimuli are,
fortunately, not always interpreted as painful. This neuronal circuitery is
present in the posterior roots of the spinal cord : Ab and C
fibers coming from the skin for instance, stimulate the neuron N implicated in
nociception, but this stimulation cannot occur when the peripheral stimulus is
weak because enkephalinergic interneurons (E) stimulated by sensory
somesthetic fibers Ab inhibit
nociceptive transmission. This is only when the stimulus is strong that the
nociceptor C lowers the efficacy of this inhibitory control. From such an
example, we can imagine the complexity of the circuitery of the central nervous
system which is a prodigious machine to transform stimuli into significant
messages.
The last example could illustrate the highest level of stimulus integration. A positron emission tomography (PET) of the brain of a British subject showed a small activity in the occipital and temporal cortex when a visual stimulus was a series of letters without any significance in the language of William Shakespeare. By contrast the PET picture showed a bright illumination of these area when the stimulus was an English word ! It appears that only the meaningful stimulus is able to activate neuron metabolism in visual and temporal cortical areas, because neuronal networks have been selected by education (Edelman, 1992). This is again an illustration that a stimulus has no significance by itself, and that a message depends on the reception machinery.
8.
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