Cell Communication
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Cell Communication and Interactions between organisms can range from cooperative to antagonistic. This is true for single-celled organisms and even the individual cells that make up multicellular organisms. As a general rule, cooperation among individuals (or cells) is more likely (though by no means guaranteed) the more genetically similar the cells or individuals are, with nearly complete cooperation occurring particularly when organisms (or cells) are genetically identical plus dependent upon one another for their replication (thereby one cell making babies in a [[genet sense is making babies for every genetically identical cell).
Such is the case among the cells that make up most multicellular organisms, and cell-to-cell communication represents how such cells coordinate their physiological behaviors so as to create a cooperative whole, one that is greater than the sum of their cellular parts. This chapter combines bits of endocrinology (the study of hormones), cell biology, and biochemistry to introduce the complexities of the cooperative molecular interactions between cells. When cell-to-cell communication is unsuccessful, a result can be a harmful absence of cooperation, a.k.a., defection, which between cells within a multicellular organism we might recognize as tumors or cancer, as adult-onset diabetes, as developmental abnormalities, etc.
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Chemical signaling (communication by direct contact)
- Most cell-to-cell communication involves some kind of chemical signaling, including
- Chemicals that are allowed to freely diffuse between cells,
- Chemicals that are received by a cell only given cell-to-cell contact, and
- Chemicals that freely diffuse from one cell’s cytoplasm to another’s via junctions directly linking the cytoplasms of adjacent cells.
- Signals can be purposeful in the sense that one cell is sending off a signal meant to be received and interpreted in a certain way by another cell (e.g., a hormone)
- Alternatively, signals can be byproducts of cellular metabolism that one cell releases essentially as waste or without intending (in an evolutionary algorithm sense) to initiate a signal to another cell, but nevertheless other cells are capable of interpreting those signals and acting on them (e.g., the release of lactic acid from an anaerobically exercising muscle cell)
- This chapter considers particularly signaling that involves chemicals that are purposefully released from one cell and allowed to freely diffuse to a second (or more) recipient cell(s) in an act of communication that is deliberately initiated, received, and interpreted in order to increase the physiological coordination of the cells of a multicellular organism
- In addition, this chapter’s emphasis is particularly on those events that occur following reception of a chemical signal rather than on the purpose of the signal or why and how a given cell released the signal
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Local regulator
- A local regulator is a chemical signal that influences only neighboring cells
- Within an animal this would imply a lack of systemic diffusion which, in turn, suggests that the local regular is not released into the blood or the lymph (which are the routes to systemic diffusion) but instead into the intercellular space/extracellular matrix
- Among the localities in which local regulators are active is within the small gaps (called synapses) that occur between nerve cells and between nerve and muscle cells
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Hormones
- Unlike local regulators, hormones are chemical signals that diffuse systemically (e.g., diffusion through and are carried by blood and lymph as well as the intercellular space/extracellular matrix)
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Three stages of cell signaling (signal-transduction pathway)
- Local regulators or hormones are released by cells, received, and then acted upon by other cells
- We can biochemically differentiate the reception, etc. of these chemical signals into three stages:
- Reception (by a cell)
- Transduction (from outside of the cell to inside the cell, etc.)
- Response (how the cell responds to having received the signal)
- Most of this chapter is devoted to discussing the complexity of these processes, with particular emphasis on transduction
- Note that this signal transduction is simply one of the many highly complex processes one studies when considering the cell biology (and biochemistry) of the cells of multicellular organisms
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Reception
- Reception of a chemical signal literally involves the attachment (or association) of the chemical signal to some aspect of the recipient cell’s plasma membrane
- The means of reception, typically involving a membrane protein, may be intimately linked to the existence of an intact plasma membrane
- A membrane is thus a requirement for the occurrence of subsequent signal transduction and response (i.e., cell-to-cell signaling typically requires that recipient cells are intact)
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Transduction (signal transduction)
- There exist three stages of cell signaling, a beginning, a middle, and an end
- Signal reception represents the beginning while transduction represents the middle
- Transduction is the conversion of the reception signal, typically found at the surface of the cell, to a signal that directly facilitates a response
- Very often signal transduction involves a number of steps that, taken as a whole, can be somewhat complex (perhaps overwhelmingly so)
- Though not explaining the complexity, nevertheless a basic purpose of the need for signal transduction — linking reception and response — is that the plasma membrane receptor and the molecules involved in formulating a response are not always (rarely?) located in the same region of the cell; thus intracellular signals (often chemical) serve to physically connect reception and response
- For example, a signal-transduction pathway may involve the following:
- Reception (at the plasma membrane) à
- Transduction (through the cytoplasm) à
- Response (in the nucleus, e.g., transcription)
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Response
- The response to cell signaling varies enormously, depending on the signal as well as the receiving cell
- Suffice it to say that responses typically involve either the turning on of a specific (often enzymatic) activity (including the synthesis of new enzymes) or a reduction in (or turning off of) a specific enzymatic activity
- In addition, a response can involve the turning on or off of more than one activity
- “Explanation for the specificity exhibited in cellular responses to signals is the same as the basic explanation for virtually all differences between cells: Different kinds of cells have different collections of proteins. The response of a particular cell to a signal depends on its particular collection of signal receptor proteins, relay proteins, and proteins needed to carry out the response.” (p. 202, Campbell et al., 1999)
- For the sake of discussion throughout this chapter, consider response to be simply some end point of a signal-transduction pathway
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Signal receptor (ligand)
- “Most signal molecules are water-soluble and too large to pass freely through the plasma membrane… A cell targeted by a particular chemical signal has molecules of a receptor protein that recognizes the signal molecule. The signal molecule is complementary in shape to a specific site on the receptor and attaches there, like a key in a lock—or like a substrate in a catalytic site of an enzyme. The signal molecule behaves as a ligand, the term for a small molecule that specifically binds to a larger one. Ligand binding generally causes a receptor protein to undergo a change in conformation—that is, to change shape. For many receptors, this shape change directly activates the receptor so that it can interact with another cellular molecule. For other kinds of receptors… the immediate effect of ligand binding is more limited, mainly causing the aggregation of two or more receptor molecules.” (p. 192, Campbell et al., 1999)
- Membrane-protein signal receptors come in a variety of types including:
- G-protein-linked receptors
- Tyrosine-kinase receptors
- Ion-channel receptors
- Some chemical signal molecules, such as steroid hormones, are able to pass through the plasma membrane without the aid of a membrane-protein receptor, allowing reception and transduction to be carried out by the same (intracellular) protein
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G-protein-linked receptors (G protein)
- G-protein-mediated pathways involve at least three components:
- A G-protein-linked receptor (responsible for reception of the chemical signal)
- A G protein (responsible for signal transduction)
- And the protein the G protein activates (responsible either for signal transduction or directly effecting the response)
- G proteins are signal-transduction proteins that are in an active state when they are bound to a molecule of GTP (similar to ATP except possessing the guanine purine rather than the adenine purine; see Kreb’s cycle for another example of the use of GTP in a metabolic pathway)
- G-protein-linked receptors are membrane proteins that interact with a G protein upon reception of a chemical signal
- The G protein interacts with the receptor on the receptor’s cytoplasmic side, and conformational changes in the receptor (induced by ligand attachment) results in the activation of the G protein
- The G protein diffuses to and then activates a subsequent protein in the signal-transduction pathway (or the protein that is directly responsible for the response) by binding to that protein while in its own (the G protein’s) active state
- Subsequently, the G protein hydrolyzes the GTP (to GDP) which inactivates the G protein and whatever the active G protein had activated (these activation-after-activation-after-activation pathways can get complicated)
- The important function of G-protein inactivation is that they allow a reversibility to the G-protein mediated activation of a protein, thus contributing to the dynamic nature of a cell
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Tyrosine-kinase receptors
- Tyrosine-kinase receptors differ from G-protein-linked receptors in three crucial ways
- Rather than activating G proteins following their conformational change (that follows ligand binding), tyrosine-kinase receptors instead activate their own enzymatic activity, the tyrosine-kinase activity and then phosphorylate themselves—the phosphorylated receptor is then recognized by cytoplasmic proteins which effect the transduction event through the cytoplasm
- Part of the process of activation of tyrosine-kinase activity involves a dimerization (linking together of two subunits) of the tyrosine-kinase receptor
- Individual tyrosine-kinase receptors are often capable of activating multiple transduction pathways
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Protein kinase
- A kinase is an enzyme that phosphorylates another protein (or, in the case of a tyrosine-kinase receptor, also themselves)
- ATP supplies the phosphate group
- A tyrosine kinase is thus an enzyme that phosphorylates tyrosine amino acids found on target proteins
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Protein phosphatase
- A protein phosphatase catalyzes the reverse reaction of that catalyzed by a protein kinase, i.e., the hydrolytic removal of a phosphate added to a protein
- The important function of protein phosphatases is that they allow a reversibility to the protein-kinase-mediated phosphorylation of a protein, thus contributing to the dynamic nature of a cell
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Ion-channel receptors
- With ion-channel receptors, the molecules responsible for transduction are ions (e.g., Na+ or Ca2+) that are normally found outside of cells
- Here binding of a ligand to the receptor (no, the external ions themselves are not the ligands) results in an opening of a gate through the plasma membrane that allows entrance of the ions (both gate and receptor are proteins, likely one in the same protein)
- The increased ion concentration in the cytoplasm either propagates signal transduction or results in a direct stimulation of a response
- Thus,
- Ligand binding (reception) à
- Channel opening à Ion inflow (transdution) à
- Further transduction or response
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Signal amplification
- During signal transduction, signals may be amplified
- Amplification is accomplished by transduction involving a series of enzymes (typically protein kinases)
- Recall that enzymes may catalyze a chemical reaction without being used up in the process; thus, one activated protein kinase may phosphorylate many more than one individual target protein (or instead may activate more than one of the same kind of target protein)
- Such activation allows an exponential increase in the number of activated proteins (e.g., one protein activates two, which together activate four, which together activated eight, etc.) which means that the reception of few ligands at the cell surface can lead to dramatic changes in enzyme activity within the cell
- “Keep in mind that the original signal molecule is not physically passed along a signaling pathway; in most cases, it never even enters the cell. When we say that the signal is relayed along a pathway, we mean that certain information is passed on. At each step the signal is transduced into a different form, commonly a conformational change in a protein. Very often, the conformational change is brought about by phosphorylation.” (p. 195, Campbell et al., 1999)specific gene by a growth factor
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Second messengers
- A second messenger is a non-protein molecule that participates in the intracellular transduction of a signal
- “The extracellular signal molecule that binds to the membrane receptor is a pathway’s ‘first messenger’.” (p. 197, Campbell et al., 1999)
- A major advantage of second messengers is their small size and water solubility which allows rapid diffusion throughout a cell’s cytoplasm
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Cyclic AMP (cAMP)
- Cyclic AMP is an important second messenger
- Cyclic AMP is formed from ATP and resembles (though is different from) adensosine monophosphate (cAMP contains a ring that is the result of a dehydration synthesis reaction)
- G-proteins, in certain signal-transduction pathways, can stimulate the production of cAMP, and cAMP can activate protein kinases also in certain signal-transduction pathways
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See Also
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References
- Chapter 11, Campbell & Reece, 2002 (1/29/2005)
- by Stephen T. Abedon (abedon.1@osu.edu) for Biology 113 at the Ohio State University
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External links
- http://www.beyondbooks.com/lif71/4i.asp
- http://www.biosignaling.com
- http://www.erc.montana.edu/Res-Lib99-SW/Image_Library/Cell-Cell_Communication/default.htm
