Cell signaling principles and mechanisms pdf
File Name: cell signaling principles and mechanisms .zip
- Properties of cell signaling pathways and gene expression systems operating far from steady-state
- Cell signaling
- General principles of cellular communication
Cell Signaling presents the principles and components that underlie all known signaling processes. It provides undergraduate and graduate students the conceptual tools needed to make sense of the dizzying array of pathways used by the cell to communicate.
Properties of cell signaling pathways and gene expression systems operating far from steady-state
All organisms, whether unicellular or multicellular, need to respond to their ever-changing environment in order to survive and flourish. Such responses are governed by the ability of cells to sense physical changes and chemical cues occurring around them.
Cells respond to a wide range of extrinsic signals that include chemical messengers e. In this free course, General principles of cellular communication , you will explore the most common paradigm for cellular communication, which is the detection of extrinsic stimuli by receptors on the surface of cells. Particular emphasis is placed on how the interaction between an extrinsic stimulus and its receptor on the cell surface subsequently causes cellular responses through the activation of specific intracellular signalling pathways.
You will explore this chain of events using well-characterised examples in prokaryotic and eukaryotic cells. This OpenLearn course is an adapted extract from the Open University course S Biological science: from genes to species. Cellular communication encompasses a vast range of extrinsic signals, intracellular signalling pathways and cellular responses.
In fact, no two cell types express exactly the same repertoire of signalling components. Rather, cells have signalling systems that suit their physiological function. The main focus of this topic is cellular communication that occurs when extrinsic stimuli bind to receptors on their target cells. Although not all cellular communication relies on the activation of receptors, it is the most common mechanism by which cells sense their environment or communicate with each other. By activating receptors, extrinsic signals trigger events that relay information within cells and ultimately cause cells to change their behaviour.
It is important to remember that receptors are highly selective for their specific cognate extrinsic stimuli. In most cases, a particular kind of receptor will only be activated by one type of extrinsic stimulus. Activated receptors often have pleiotropic actions. That is, they alter the activity of numerous cellular processes simultaneously. These processes could include DNA transcription, protein synthesis or changes in metabolic activity. The overall effect of switching various processes on or off determines the consequent change in cellular behaviour.
The fidelity, accuracy and appropriateness of these cellular communication processes are critically important for the cell and for the organism. It is well known that aberrant cellular communication leads to conditions such as cancer, diabetes, heart failure and neurological diseases. Cells are simultaneously bombarded with numerous extrinsic signals and they make sense of these incoming signals through the activation of specific signalling pathways.
The key point that you should take from this illustration is that cellular communication triggers specific responses by recruiting particular signalling pathways.
The activation of receptors following ligand binding is conveyed into a cell by a cascade of signalling proteins or messengers. Note that each pathway can elicit a variety of responses, although only one example is shown for each of the pathways in Figure 2.
Exactly how a cell will respond to extrinsic stimuli is sometimes hard to predict, even for scientists with considerable experience of studying cellular communication. In part, the response of a cell is determined by the input signals it receives, but there are also many intrinsic factors that determine how cells respond. For example, the age of a cell, its position within the cell cycle and its metabolic status could impact on how it responds to particular extrinsic stimuli.
Under some conditions, such as in a nutrient-rich environment, a cell could receive an extrinsic signal that activates anabolic processes and cell division. At another time, when nutrients are depleted, the same signal may trigger catabolism and cell death. Although cellular signalling pathways are numerous and complex, there are in fact relatively few pathways in comparison to the diversity of cell types and their intricate molecular processes.
These seven pathways are used during development to define the size, shape and other characteristics of an animal. How can so few pathways achieve so much? The answer is that signalling pathways can act together to produce outcomes that are different from the outcome of single pathways acting alone.
This combinatorial action of pathways may actually allow hundreds of different signalling—response combinations. In general terms, the number of genes involved in signalling increases with complexity of an organism, with vertebrates e. Whole genome duplication events, such as those that occurred in the evolution of vertebrates between and million years ago, were responsible for these marked differences.
However, the correlation between complexity and number of genes involved in signalling is not a strict one. Not only have genes encoding signalling components been acquired during evolution, but such genes have also been lost. For example, the nematode C.
For many of the cellular communication mechanisms found in animals there are analogous systems in plants, fungi and protists. It is likely that ancestral eukaryotic cells developed complex signalling systems well over two billion years ago, long before multicellular organisms emerged.
Indeed, the acquisition of multiple signalling pathways was certainly an essential prerequisite for multicellularity. Prokaryotes also express complex cellular communication systems, as will be discussed below. It is generally believed that cellular communication systems evolved very early in the history of life on Earth. The ability to sense their environment and to communicate with each other would have provided primordial cells with distinct adaptive and replicative advantages.
Quorum sensing coordinates the behaviour of bacteria within a colony. In particular, quorum sensing is important for controlling bacterial gene expression and maintaining the viability of a bacterial colony. Under certain conditions, bacteria secrete peptides known as autoinducers into their surroundings; if the bacterial population density increases so will the concentration of autoinducer.
Quorum sensing therefore allows planktonic single-celled, free-swimming bacteria to adopt group behaviours when they reach a certain density. The production of autoinducer by bacteria does not have a simple proportional relationship to bacterial population density.
Rather, the release of autoinducer substantially increases at a critical population level dashed vertical line in Figure 3. This surge in autoinducer release underlies the coordinated change from planktonic to group behaviour. Cellular signalling systems often show rapid changes in activity beyond a threshold point.
This allows for cellular processes to be either switched on or off with only a relatively small change in stimulation. Through changes in the expression of specific genes, autoinducers help bacterial colonies to survive. Typically, autoinducers increase the transcription of genes that encode for antibiotic resistance, in addition to genes that lead to the development of biofilms Figure 4. A biofilm is a bacterial niche that arises from the aggregation of bacteria within a polysaccharide matrix that they secrete.
Biofilms are found in many places including water distribution pipes and on teeth, lung and intestines. The formation of the protective biofilm niches through quorum sensing is believed to be a major contributor to antibiotic resistance.
The bacterial cells release a signalling molecule into their environment the autoinducer. The signalling molecule binds to specific receptors that are located on the surface of the bacterial cells. The binding triggers a transduction process involving relay molecules , which activates a response gene transcription.
It is believed that the evolution of quorum sensing provided bacteria with a mechanism to coordinate their behaviour. In addition, the effectiveness of autoinducers can tell bacteria something about the environment they live in, such as its viscosity or chemical composition. These bacterial niches allow interspecies cooperation that is similar to intercellular communication in multicellular organisms.
Before considering cellular communication systems in detail, it is necessary to discuss some of the general principles by which signalling pathways function.
Cellular communication processes can be described in terms of a series of four fundamental steps. Drag and drop the descriptions below into the correct sequence to describe a generalised cellular communication process. You can also click or tap through the descriptions in each empty field to make your choices.
An example of this is calcium, an ion that cells use to control many cellular processes. Calcium ions are released into the cytosol of cells in response to certain extrinsic stimuli. In that context, they can be considered as a second messenger.
However, to terminate signalling, the calcium ions within a cell are extruded across the cell membrane. When released into the extracellular space, these calcium ions can bind to calcium-sensing receptors on neighbouring cells and thereby become an extrinsic stimulus. When this happens they could also be considered a first messenger too. We will avoid use of terms such as first or second messengers, and instead consider cellular communication pathways as sequential chains of interacting components.
To understand information flow through signalling pathways, the components are usually considered to be upstream or downstream of other constituents in the pathway. Ultimately, signalling molecules affect the activity of target effector proteins resulting in the cellular response s.
Once a cellular response has taken place, mechanisms that lead to termination of signalling take over. Alternatively, cells can display adaptation or desensitisation; situations where cellular communication is still active, but it causes a lesser effect. Many signalling pathways, though by no means all, are composed entirely of proteins. To help you make sense of how signalling pathways work, some general principles governing proteins involved in signalling, their functions and their regulation, are reviewed here.
Proteins are capable of interacting in a highly specific manner with various ligands and also other proteins. These features are required to ensure fidelity in a signalling pathway. Also, the activity of proteins can be acutely modulated by altering their conformation for example, by allosteric regulation and by covalent modification , thereby tuning the flow of information via signalling pathways.
If you were thinking along the lines of the structure or conformation of the protein and its ligand, you would be on the right track.
Protein—ligand interactions depend on the chemical and physical compatibility of the protein and its ligand and involve the formation of a variety of non-covalent interactions.
All of these are types of non-covalent interaction and they are all involved in protein—ligand interactions. One of the most important aspects of how proteins function in signalling pathways is that they can act as molecular switches. Most signalling proteins exist in interchangeable active or inactive states.
Such acute changes in activity are essential for the transmission of signals within cells. Altered protein expression often accompanies activation of cellular communication for minutes or hours, but changes in protein levels are usually not sufficiently rapid to convey dynamic signals. Rather, the activity of signalling proteins is acutely modulated over a much shorter time frame seconds to minutes.
The most rapid responses are seen with ion channels, where the binding of a ligand causes the channels to open within fractions of a second. The rapid activation of ion channels is critical for the transmission of electrical signals in neurons and muscle cells. How does a protein actually convey a signal? The answer lies in the conformation of the protein, which is related to its activity.
One of the most commonly encountered forms of covalent protein modification in signalling pathways is phosphorylation.
All organisms, whether unicellular or multicellular, need to respond to their ever-changing environment in order to survive and flourish. Such responses are governed by the ability of cells to sense physical changes and chemical cues occurring around them. Cells respond to a wide range of extrinsic signals that include chemical messengers e. In this free course, General principles of cellular communication , you will explore the most common paradigm for cellular communication, which is the detection of extrinsic stimuli by receptors on the surface of cells. Particular emphasis is placed on how the interaction between an extrinsic stimulus and its receptor on the cell surface subsequently causes cellular responses through the activation of specific intracellular signalling pathways. You will explore this chain of events using well-characterised examples in prokaryotic and eukaryotic cells.
General principles of cellular communication
Each pathway is used repeatedly during the development of a given organism, activating different subsets of target genes in different developmental contexts. These seven pathways are strikingly diverse in both their complexity and the biochemical mechanism of signal transduction, ranging from direct transcriptional regulation by the nuclear receptor proteins to the extended protein phosphorylation cascades characteristic of RTK pathways. Nevertheless, the primary consequence of signaling is the same: activation of specific target genes by signal-regulated transcription factors. Recent work has revealed several surprising and fundamental commonalities in the transcriptional mechanisms by which these pathways control the expression of their target genes.