biology 1107 presentation number nine cell communication cell to cell communication is a feature of most multicellular organisms plants and animals the actual process of communicating a message from one cell to another cell oftentimes is referred to as signal transduction a signal is a chemical message and that chemical message is generally produced by some cell or maybe even some location in the cell that's destined to receive the message so along with signal initiation is the process of actually receiving that signal which could be a signal reception event inside the cell or at the surface of the cell and then communicating that signal within the cell process called transduction and delivering some kind of response sometimes that response is initiated in the cytoplasm and sometimes that response might actually be the control of a gene perhaps turning it on perhaps turning it off now you'll notice i've used the word transduction twice on this slide just make a real quick comment the word transduction can be used in a more general sense and that is a process from which point the signal is initiated to the point where the signal actually generates some kind of response then again there's a more restrictive concept of the word transduction generally commencing with the reception of the signal either at the surface of the cell or with some receptor within the cell and then leading to the response so there's a more general sense of the word transduction and a more restrictive sense of the word transduction either way is appropriate just want to make sure that you are aware the fact that the process of cell to cell communication is a process of signal transduction where we initiate a signal receive a signal and carry that signal to some point where we are going to deliver a response there are several forms of cell communication and we can actually classify it on the basis of proximity and the idea of course is how close the signal initiation event is to the actual signal reception and response sometimes that can happen within the cell itself you can create a signal within a cell and receive that signal and deliver a response within the very same cell that is commonly referred to as autocrine signaling i have it listed here as intracellular signaling because that's where the signal initiation and reception and response occurs in the very same cell the second item down here talks about local signaling now sometimes this is a case where you'll create a signal and receive that signal and deliver response in two adjoining cells there are three general ways this can happen first in animal cells we have these things called gap junctions and we mentioned this in a lecture not too long ago a gap junction is an actual cytoplasmic connection between two animal cells in plant cells we identified the existence of plasma desmond same thing cytoplasmic connections between two adjoining cells idea of course is if we create a signal molecule in one cell it can travel through that cytoplasmic connection via the gap junction in an animal cell or via the plasma desmonder in a plant cell finally we can actually mediate a signal transition from one cell to another through cell surface macromolecules cell surface macromolecules are proteins embedded in the cell membrane this is generally a situation that we find in animal cells because plant cells have cell walls this is generally not the case take a look at an illustration bottom right you'll see cell a on the left cell b on the right you see two big fat red protein molecules which are our surface macromolecules could happen is a signal molecule generated in cell a could bind to that cell surface macromolecule in cell a and then transmit its information via perhaps a phosphorylation event to the second protein in the membrane of cell b so the actual signal molecule is not being carried directly but it's being mediated through these two cell surface molecules now it is conceivable that a signal molecule might have a little bit more far reaching influence than just two adjoining cells we're going to talk about another type of local signaling and this is where we actually produce what we're going to call regulators molecules that regulate to have an influence on other cells technically they're hormones but they're really not traveling anywhere near as far as we would consider hormone molecules that's why we call them regulators and they technically are going to diffuse from the cell of origin and not necessarily are they limited to the adjoining cells but actually can diffuse to a few or far ranging cells in the general region locally produced regulators there's two different classifications of how they operate one is called paracrine signaling and that's a much more generalized view of things you will see a central initiator cell and then you'll see a bunch of arrows indicating that those regulators produced by that initiating cell will have an influence on cells in that general region now a much more specialized version of local signaling is a form of signaling called synaptic signaling and we see that illustrated below synaptic signaling is specific to nerve cells nerve cells are also known as neurons now below you will see neuron a and you'll see neuron b neurons are very strange cells in terms of their appearance they consist of a cell body they consist of small branch-like structures called dendrites and they have long unbranched structures called axons neurons transmit information and as a rule they will transact information in an orderly fashion from dendrites to cell body to axon so if we take a look at neuron a let's suppose neuron a is going to generate a signal now in neurons the far majority of the signal is going to be electrical in nature because it can be traveling along the length of that axon it's really nothing more than a wave of charge and that wave of charge reaches the end of the axon and it's about to be transmitted from the end of axon a to the dendrite of neuron b close-up view you will see the dendrite and you'll see the axon and you'll see a gap between the two this is a common situation in neurons neurons very rarely if ever connect directly to anything else there's always this teeny tiny little gap and that teeny tiny little gap is officially referred to as a synapse unfortunately i have not illustrated it there as far as label goes but you will see a gap between axon and dendrite that is a synapse what's going to happen is once my electrical signal reaches that axon i will release substances called neurotransmitters those neurotransmitters are illustrated as those little dots the arrow indicates the direction of those neurotransmitters as they diffuse from the axon to the dendrite it's a very very small amount of space and it takes virtually ain't hardly any time at all but once those neurotransmitters reach the dendrite they will bind to receptor molecules at which point will create a new electrical impulse that will travel along the length of neuron b so when neurons we have a unique form of local signaling it will be called synaptic signaling now not too long ago i mentioned that signal molecules can travel long distances and those are generally referred to as hormones whereas regulators had an influence much more locally hormones have an influence long distance we're going to take a look at hormones in plants compare that to hormones in animals in plants hormones are commonly referred to as growth substances a classic example is a compound called auxin thing is is that these hormones are not produced by specific glands as they are in animals most of us realize that hormones are produced by glands like the pituitary gland or the thyroid gland and plants are produced by plant tissues or plant parts but not in specific glands and when they move they move not through a circulatory system but they will diffuse they will diffuse from the point at which they're made to wherever they're gonna have an influence if you take a look the illustration at right you'll see a tip of a plant and you'll see an arrow the auxin is being produced at the very tip of the plant and it's being diffused backwards along the length of that plant that auxin in the picture there is going to inhibit lateral branch formation and you will notice that there are no lateral branches being formed near the growing tip that's where the oxygen is being made and it's being diffused from that tip backwards now as a rule plant growth substances have a tremendous variety of effects in animals generally a hormone is very specific in terms of what sorts of effects it delivers in the case of plants plant hormones have a tremendous variety of effects now for example auxin as we're looking at it here yes can inhibit lateral branch formation but auxin can have an influence on cell wall formation oxygen can have an influence on how the plant reacts to light it can have an effect on how the plant reacts to gravity tremendous variety of effects from one single growth substance and those individual responses a lot of times are dependent upon what cell receives that signal where that cell is located maybe the influence of other growth substances okay so the situation in animals is going to be a little bit different the hormones in animals fall into two general classes there are peptide hormones which are ultimately derived from chains of amino acids and there are steroid hormones that are ultimately derived from cholesterol and they actually behave a little bit differently we're going to see this in a minute we'll find out that the peptide hormones must bind to cell surface receptors whereas the steroid hormones can actually enter the cell directly the source of animal hormones are from specific glands we're going to call them endocrine glands and those hormones diffuse from the gland surface to enter the blood circulatory system so they are going to be transported via the blood circulatory system and they're going to influence on a very limited specific type of cell we're going to call those target tissue cells so whereas in plants the hormone diffused and had a kind of a more general response in animals the hormones circulate have an influence on very specific target tissue cells and will result in very specific effects the whole cell communication process begins with signal production now we're not going to go in tremendous amount of detail but we are going to understand that the signal itself needs to be produced by some cell that could be a cell in a region of the plant it could be the cell in an individual gland in an animal it involves a number of things that will trigger the exocytic release of these signal molecules they might be local regulators they might be hormones so technically these compounds are generated within the cell probably in the endoplasmic reticulum or then secondarily in the golgi regardless these materials are going to be released from the exterior of the cell and they're going to wind up being transported now the basic difference of course is that in plants the transport is a process of diffusion so it's slow and it's very general and in the case of animals it's blood circulation which is much more rapid in the case of blood circulation even though you're circulating systemically all throughout the body your signal molecules are going to have a very specific effect on certain target tissue cells so in animal cells the process is rapid and the response is specific and in plant cells it's much more gradual and much more generalized let's talk about signal reception and what we're going to do is going to be a bit more specific you remember not too long ago i mentioned that in animal cells and we will be focusing primarily on animal cells here i said that in animal cells we've got two general types of signal molecules which we'll call hormones those are steroid hormones and those are peptide hormones now we're going to take a look at steroid hormones first and we will understand that steroid hormones the signal reception is actually an intracellular event that means the signal molecule actually enters the target cell at that point it's going to bind with the receptor protein in the target cell and then will subsequently trigger some changes in the nucleus so let's take our bullet points and then i'll explain why this is an intracellular event the signal molecule the steroid which ultimately is derived from cholesterol could be testosterone could be estrogen could be cortisol enters the target cell at that point it binds with an inactive intracellular receptor protein smack dab in the cytoplasm and at that point you've got an activated signal receptor complex that is a brand spanking new molecule that is a composition of both entities both the signal molecule and receptor at which point it actually physically enters the nucleus and controls gene operation now what we can do is we can actually be a little more specific and it's something we haven't talked about yet but i am going to use the word transcription transcription is a process by which the dna sequence is copied we're in the nucleus the dna cannot leave the nucleus but what i can do is make a copy of that sequence then i can export that sequence to the cytoplasm and make the protein well if i turn off transcription i will not make the copy and therefore will not make the protein so when these steroid hormones work what they do is they either turn on or turn off the process of transcription if i turn off turn off the process of transcription technically i'm turning on or off the gene and i will either get the protein i need or i won't so let me talk a little bit about the practical reasons is why this is an intracellular event you might remember not too long ago we were talking about membranes and we understand there are some molecules can actually pass through the phospholipid bilayer of a membrane c membranes are a little bit discriminatory i mean they discriminate between some molecules that can pass through and some molecules that can't well one of the points we made is that nonpolar molecules generally small molecules have no problem passing through the phospholipid bilayer steroids are relatively small steroids are definitely non-polar steroids as molecules have no problem passing through the cell membrane to actually bind with an intracellular receptor now the situation with peptide hormones is quite a bit different we'll see that in a moment if my reception event is extracellular that's going to necessitate some cell surface macromolecule that will actually bind to the signal molecule and the other thing is we'll remember here in this case because it's an extracellular event the logic is is that the signal molecule for some reason cannot pass through phospholipid bilayer so let's take a look at the process as illustrated here you'll see the cell surface macromolecule the protein receptor in purple you'll see our signal molecule in red you will see peptide hormones over there on the left indicating to me that these are probably pretty big probably polar molecules and they cannot pass through the phospholipid bilayer hence i need the cell surface receptor molecule now it's a little bit detailed here because what happens is when that signal molecule binds to the cell surface receptor that creates an activated molecule that then passes its message onto an intermediate and that intermediate is called a g protein you'll see that illustrated there in the picture it's a protein that appears to be found associated with the membrane and that g protein is going to be activated you see the little starburst around it that starburst indicates a phosphorylation event without going a whole lot of detail the g protein is a modified guanine it picks up a high-energy phosphate when it picks up the high-energy phosphate it was going to in turn pass its message on to a variety of intracellular molecules which will turn will result in some intracellular event that event being the desired result of the signal molecule in the first place on these next two slides i'm going to focus a bit more on the g protein because g protein is consistently found in all cases of extracellular signal reception the g protein is key to getting that message from the outside of the cell into the inside of the cell and it's found in all eukaryotic cells so i'm going to series of illustrations to show you kind of in detail what i just mentioned on the previous slide i see my cell surface receptor i see my signal molecule in green my signal molecule is obviously some kind of peptide hormone it is not able to pass through the phospholipid bilayer it in turn combines with the cell surface receptor activates the g protein the starburst indicates the high energy phosphate from atp at that point g protein is activated and it's going to shift physically shift from the receptor to an enzyme some other molecule that's going to bring that message into the cell the g protein in turn passes its high energy phosphate to the enzyme which in turn is going to trigger other intracellular events resulting in the change in the target tissue cell that's initially triggered by the signal molecule to begin with what we're going to do now is identify two major ways we can actually get this message from the g protein into the cell so i'm going to pick up with where i left off my activated relay molecule has shifted from the receptor protein to a molecule in my membrane now in this case we're going to call this enzyme a protein kinase protein kinases are proteins that are easily activated and deactivated and you'll see that they actually work in a series and so a high energy phosphate will be passed from the g protein to the first protein kinase the reason why i've got gdp attached to the g protein at this point is it has lost its high energy phosphate the high energy phosphate is in turn passed to a second protein kinase which is in turn passed to a third which is in turn passed to a fourth this is commonly referred to as a phosphorylation cascade because the transition of phosphates are nothing more than a series of phosphorylations activating these protein kinases in a series it's kind of like passing a message from person to person to person to person until some point we get to the recipient and in this case the recipient are those intracellular events that are going to occur within the target tissue cell so one major way of getting a message from the cell surface into the cell is through a phosphorylation cascade involving a series of protein kinase molecules now second method to get this message into the cell is to not necessarily rely on a series of protein kinases but to rely on a single molecule that's going to function as our messenger and it's commonly referred to as a second messenger because the first messenger is the signal molecule to begin with so let's pick up where we left off before we understand we got a signal molecule it's bonding to our receptor the g protein has been activated i see the star burst indicating the high energy phosphate which is going to in turn activate a specific enzyme in the membrane called adenylyl cyclase and adenylcyclise in turn is going to activate something called cyclic amp now that might seem familiar and it is it's a cousin of atp it's a cousin of adp it's basically adenosine with one phosphate attached to it but it plays important role because it's going to function as what we call our second messenger so the process is going to involve carrying my signal from the cell surface into the cell using our second messenger cyclic amp which in turn is going to activate some protein kinase which in turn is going to trigger our intracellular event so make sure you're able to distinguish what is the second messenger mechanism versus what is a phosphorylation cascade and in the case of the second messenger mechanism make darn sure you know that cyclic amp is our second messenger and it is derived from adenylyl cyclase in the cell membrane