Beginning today and continuing for about two weeks we will be discussing the ways in which cells sense and respond to changes in their environment. By way of introduction to this subject, let me ask what you know about signal transduction and second messenger systems in cells.
One can easily get lost in the details of signal transduction, because there are many different ways that cells use to respond to signals and because there is a bunch of details to learn about each way. Thus, it’s easy to get confused by the specifics and to lose sight of the general principles, which are the most important thing to keep in mind. The book attempts to begin with a survey of general principles, but I feel that this is only partly successful, because I learn better when I first get the specifics, then step back to get the big picture. When they try to sketch in the big picture, it’s hard for me to follow without specific examples, but some of you may find this a useful approach.
Let’s start by considering the problem that the cell faces. Many cells are parts of multicellular organism. For the organisms to be viable, the cells must function as a unit; that is, they must be able to react to, and adapt to, things that other cells are doing. For example, if you eat a meal, the levels of insulin in your blood rise, which increases the rate of synthesis of glycogen and other energy storage molecules. The functional value of this is clear--if you eat, food is temporarily plentiful and it makes sense for cells to take it up and store it for times in between meals when they will need to break it down for energy. But how do the insulin-secreting cells recognize and respond to changes in the levels of sugar in the blood? How do the storage cells, such as the liver, recognize the presence of insulin and shift their metabolism toward storage? Can you think of other examples where cells need to respond to changing conditions in or around the body? The general point is clear; cells in a complex organism need to communicate with each other, sometimes locally and sometimes over long distances. Even some free living, single-celled organisms like yeast or bacteria sometimes need to interact and to coordinate their activities, so they also need systems to detect and respond to the environment. These cell-cell communication processes can occur locally, such as in the nervous system, or over very long distances, but the principles of how they work is very similar in all cases (See Fig. 20-1 of Lodish et al. for an overview of the classes of cell-cell signaling.)
Of course, there’s a problem for cells in managing this process. It is this. The enzymes and genes that are the business part of the cell are inside it, the conditions to which the cell must respond are outside it, and in between is a plasma membrane that is very highly selective in what it will allow to pass. So how does information get from outside the cell to inside?
In general, when I speak of "Information" outside the cell, what I mean is the concentration of one or more chemicals in the extracellular fluid. In the example I mentioned earlier, the blood surrounding the storage cells contains a higher concentration of insulin after a meal than before. How can the cell detect and react to that change in insulin concentration?
Well, in principle there are two ways that a change in concentration outside the cell can influence the inside of the cell--both involve a change in the concentration of one or more molecules inside the cell. That is, the essence of this signaling and response process is that when the concentration of some molecule outside the cell changes, the concentration of some molecule inside the cell also changes. The molecule whose concentration inside the cell changes could be either the same as, or different from, the molecule that is outside the cell.
The easiest way for this to work is simply for the concentration to rise or fall outside the cell, and for the molecule to cross the membrane so that its concentration inside is proportional to its concentration outside. For instance, if insulin concentration outside the cell goes up, it could cross the membrane and raise the concentration of insulin inside the cell. However, as you know there’s a problem with that. Molecules that are soluble in water, which is mostly what blood or sap is, are not good at crossing a hydrophobic membrane. So a moderate-sized protein like insulin will not be able to diffuse through the membrane readily, nor will any other protein or polar molecule. So the chemical properties of membranes make it unlikely that the extracellular messenger molecule will be able to get into the cell by diffusion. However, there is an exception to this rule, and that is that messengers that are hydrophobic would be able to cross the cell membrane and get into the cell. And as you probably know, a certain class of hormones, called steroid hormones, are quite hydrophobic and they can cross the membrane relatively easily. Chemically, these hormones are all derivatives of cholesterol, which as you know is a common constituent of animal cell membranes, so they are able to pass the membrane relatively readily (Fig. 20-2.). The hormones in this class include the sex hormones-estrogen and testosterone, vitamin D, Vitamin A, corticosterone, aldosterone, etc. In addition it has recently been found that certain gases, such as NO and CO, can apparently function as cell-cell signal molecules; these diffuse through membranes fairly readily. So in these relatively uncommon cases the intracellular signal is the same as the extracellular signal. That is, the same molecule carries information on both sides of the membrane, and an increase in the levels of the hormone in the blood leads directly to an increase inside the cell.
Most hormones, however, are hydrophilic or large or both, so they don’t cross the plasma membrane well. But in some sense the way that they work is the same as for the steroid hormones; their presence outside the cell causes the increase in the level of (another) molecule inside the cell. In this case, however, the change is accomplished indirectly, rather than directly. That is, the messenger molecule, whatever it is, must first bind to a receptor protein that is an integral membrane protein. The binding of the messenger-hormone, neurotransmitter, growth factor, etc. to these cell surface receptor--causes a change in the conformation of the receptor protein, which in turn activates an enzyme associated with the cytoplasmic face of the membrane. Once it is activated, this enzyme can catalyze a reaction that raises the intracellular concentration of the product of its reaction, and this product reflects the internal form of information that was delivered to the cell by the extracellular messenger molecule. This internal molecule is therefore sometimes called the "second messenger".
How do the signaling molecules (variously called "hormones", "neurotransmitters", "cytokines", "growth factors", etc.) get out of the cells that produce them so they can travel to act on other cells? Well, mostly they're secreted via the vesicular release pathways that we've already discussed. That is, as secretory vesicles fuse with the plasma membrane, they release their contents to the exterior of the cell and those contents include signaling molecules. Some signaling molecules are small organic compounds like acetylcholine, glutamate, norepinephrine, etc. that are made in the cytosol and then carried into secretory vesicles by active transport by proteins in the vesicle membrane. Other signaling molecules are either proteins or short bits of linked amino acids called peptides. These are made on ribosomes in the rough ER, for the most part, and packaged into secretory vesicles in the Golgi apparatus. Often these proteins are packaged along with proteases that chop them up into smaller, more active molecules inside the secretory vesicles. This is what happens to insulin, for example, (Fig. 17-42), as well as many other signaling molecules like adrenocorticotropin (ACTH). They are made a part of a larger protein, then that precursor protein is partly nibbled away by proteases to produce the active hormone. The secretion of these signaling molecules can be constitutive (i.e., continuous), but is most often regulated. That is, the cell must be stimulated to release the vesicle containing the secreted signaling molecule, as is true for most hormones, like insulin, neurotransmitters, etc.
To return to considering the cells that are on the receiving end of the signal, the general scheme is that the external messenger substance released by another cell acts to increase the intracellular concentration of some molecule inside the responding cell. So what? That is, how does the increase of this internal molecule affect the cell? It does so in one of two ways. Either it changes the number of specific proteins that are present in the cell, or it changes the activity of existing proteins (and sometimes both). To change the activity of existing proteins, the molecule either binds directly to the existing protein and acts as an allosteric effector, or it causes the covalent modification of the existing protein, which changes its activity. In particular, all known cell surface receptors directly or indirectly activate a class of enzymes that adds phosphate groups to proteins inside the cell. The "protein kinases" alter the shape and activity of intracellular proteins, and in so doing alter the processes that are occurring in the cell.
To change the amount of proteins that are in the cell, the messenger molecule must affect that rate at which the proteins are made. That is, it must alter the rate at which the gene for the protein is copied into RNA or the rate that the RNA is copied into protein or both. Most of these effector molecules act at the level of gene expression--that is, they alter the rate of transcription.
Let’s return to the example of the steroid hormones, for an example of hormones that exert their influence at the level of transcription.. Steroid hormones almost all act by changing the rate of synthesis of some proteins in the cell. The way it’s thought to occur is this. Steroids in the blood are bound to carrier proteins (which protect them from the aqueous environment in the blood). Somehow the hormones can unbind from the carrier protein and diffuse through the plasma membrane (Fig. 20-2a). When they are inside the cell, they bind to a specific protein, which is also called the hormone receptor. Only some cells in the body will make this receptor, and therefore only they will respond to the hormone--for example, testosterone receptors are present in some, but not all, cells, and only those cells that contain the receptor can respond to testosterone.
Steroid hormone receptors seem to be normally present in the cytoplasm, but when the cell contains the ligand that binds the receptor, receptor molecules bind the ligand and often bind another receptor molecule, forming a dimer, and then translocate into the nucleus (Fig. 10-67). When the hormone-receptor complex enters the nucleus it can bind to particular sequences in DNA which are always associated with the regulatory region of certain genes. These sequences are relatively constant for all the genes activated by a particular receptor, and they are called steroid hormone response elements. The hormone-binding domain of the receptor is necessary for this translocation into the nucleus (Fig. 10-66) because attaching it to a cytosolic protein can induce nuclear localization when a steroid is added to the cells.
The current picture is that binding of the hormone to the response element of the gene greatly increases the affinity of RNA polymerase for the promoter region of the gene, and therefore greatly increases the rate at which the gene is transcribed, which eventually leads to a much greater number of the protein molecules that are encoded by the genes (see Fig. 10-61 for a model). Thus for steroid hormones and their receptors (about 30 of which have been discovered), the primary mode by which they influence their "target" (define) cells is to activate transcription of particular genes, and the increase in the levels of the respective proteins alters the cell’s metabolism. Apparently the steroid hormone receptor family of proteins is quite similar in general structure--each contains three distinct regions, or domains (Fig. 10-63). The C terminal domain contains a region that binds specifically to one or the other of these steroid hormones; this region differs from receptor to receptor. The middle of the molecule contains a domain that binds to a particular sequence of bases in DNA; the DNA sequence recognized by the receptors differs slightly for different hormone receptors (Fig. 10-65) so this DNA-binding region also is somewhat different between the various receptors. And the N terminal domain is the part that binds to RNA polymerase or other transcription activating factors. So these receptors are sort of modular mix and match proteins where you can shuffle around the different domains with genetic engineering to confer different properties on the protein--which hormone it binds, which DNA sequence it binds, which transcription factor it binds.
I now want to consider the ways in which hormones or other signaling molecules can affect cells when they can’t cross the membrane. These signaling molecules activate second messenger systems and their effects on the target cell are more complicated than is the case for steroids because they both alter the activity of existing enzymes and cause the production of new enzyme molecules. Thus they act at both the protein level and the gene level
To do this they must first bind to a cell surface receptor. This kind of interaction is governed by the same thermodynamic rules as the interactions between proteins and other molecules inside cells. The signaling molecule that binds the receptor is called a "ligand", and the interaction between ligand and receptor is made up of non-covalent bonds. Thus the binding is reversible, and has an equilibrium constant. For the binding of a Ligand (L) to a receptor (R), the equilibrium is just L + R LR, and the dissociation equilibrium constant, KD, is [L][R]/[LR] (equation 20-1 in Lodish). Typical concentrations of [L] are in the range of 10-9 M in blood for many signaling molecules, and the values of KD are typically in the same range. That is, ligand sticks to the receptor very well and small changes in ligand concentration in the blood (sap, hemolymph, etc.) can cause large changes in the cellular response (see Fig. 20-8).
Before we begin to discuss the specifics of these second messenger systems, I want to make one important point, which is that for the signaling systems to work effectively, there must be a way to turn off the signal as quickly as it is turned on. To use a simple example, you probably know that muscle contraction is initiated by the release of acetylcholine (ACh) from motor nerve endings. The ACh binds to a receptor protein on the muscles, causes an action potential, which in turn raises the level of Ca2+ in the muscle cell, causing contraction. The muscle cell must be able quickly to relax again in order for the organism to move quickly, and in order for this to happen, not only must the ACh be removed from its receptor, but the effects of the ACh, such as increased Ca2+, must be quickly reversible. So not only are there elaborate systems in cells that rapidly increase the levels of messenger molecules in cells in response to extracellular signals, there are elaborate systems to reverse the process when the extracellular signal goes away. (How do you suppose that response to steroid hormones get turned off?)
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