Lecture notes, Cell Bio PDF

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Cell Bio Lecture 19 December 1st, 2015 Cell communication and signaling III Ca2+ Signaling - 3 types we’ve looked at 1. depolarizaitoninduced Ca2+ release 2. IP3-mediated Ca2+ release – DAG, PK3 3. Calcium induced calcium release (today) - These processes are important for muscle contraction, secretion from vesicles, other intercellular mechanisms Ca2+ induced Ca2+ release - Activation of plasma membrane Ca2+ channels induces influx - If it was a neuron, depolarization could be from influx of Na+ - Ca2+ ions bind to ryanodine receptors (RyRs). They’re named like this because of their pharmacological characterization - Calcium will bind to these receptors which will release more calcium into the ER or SR, which will lead to more calcium coming out - In the retina, this is an important mechanism - Low cytosolic Ca2+ is otherwise maintained by Ca2+ pump - Calcium is always kept in low concentrations, because otherwise there would be a flooding of signals - Calcium is released and eventually there is a slight increase in intercellular calcium which could result in activation of something (ex. Calmodulin) - Always more calcium inside the ER and outside the Cell than there is in the cytosol Ca2+ signaling - Generally, calcium signaling can produce local changes (in microdomains or microunits) in the intracellular calcium, this regulating activity of a small number of other cells - Or calcium may involve global elevation of cytosolic calcium Calcium binding via calmodulin (CaM) - Calcium can bind directly to some proteins - In other cases, there are calcium binding proteins which can relay a signal by… - Can also act through calmodulin - Proteins like calmodulin which are sensitive to calcium concentration become transducers of Ca2+ signals - Calmodulin binds to calcium when cytosolic calcium reaches 10^-6M but releases when Ca2+ fgalls to 10^-7M – Very sensitive calcium binding proteins - Calmodulin will go on to modify the activity of other enzymes and other target proteins and membrane transport proteins

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Calcium pumps can be activity when there is a lot of calcium, and they will be activated by calmodulin often in order to get rid of calcium into the extracellular space In that sense, calmodulin can be self-regulating because it can inactivate itself

Calcium binding via calmodulin - 3 basic states of calmodulin 1. Inactive and binding calcium (calcium binding sites are always available) 2. Conformational change that occurs when calcium binds, and this exposes sites that will bind to target proteins/enzymes etc. There’s a binding site that is suitable for the protein on calmodulin, and that’s where/when these target protein ‘hands’ will bind 3. Globular hands bind to calmodulin binding sites of target protein to modify it’s activity CaM-kinase II (CaMKII) - CaM-kinase II can encode calcium signals - Strucutre of this protein is such that it can expose a varying number of kinase domains, can go through various levels of activity - Can remain active even when calmodulin is no longer bound to it - Calcium and calmodulin are necessary to activate activation, but then CaMKinase II can become calcium iindependent CaM-Kinase II - Large enzyme of 12 catalytic subunits/regions - Initially dependent on calcium/calmodulin binding - CaM-KinaseII has autophosphorylation activity - Kinase domain that kind of randomly pops out and comes back in - When calmodulin binds, there’s migration of the kinase domain away from the regulatory segment/element and it can move to a state of activity - Eventually for full activation, there will be auto- or cross- phosphorylation (they’re the same thing) - The idea is that there is another kinase domain that will wiggle a bit, bound with calmodulin, and then open up and then there is two active domains - This moves the whole complex to a fully active state (?, check slides 8 and 9) - Calmodulin is cacium sensitive. Calcium dissociates from calmodulin which will cause calmodulin to dissociate away from CaMKII, but this takes a few seconds, so this means that the CaMKII can technically stay active in the absence of calcium - Deactivation can occur by a protein phosphatase CaMKII - Calcium signals can be encoded physiologically, this is one of the big things about CaMKII - There can be high frequency and low frequency calcium oscillations

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This will change the CaMKII activity, at low frequency oscillations, there will be a quicker increase in response of CaMKII and this will eventually plateau/reach its maximal amount of activity.

CaMKII - Important in LTP - Becomes activated by calcium but has a ‘memory’, it has encoded these oscillations in frequency even in the absence of calcium - Phosphorylation on Non-NMDA receptors and increased activity - Mobilization of more non-NMDA receptors to the plasma membrane Gap Junctions - Atypical or unconventional way of cells communicating with each other - Pathway by which two or more cells can communicate directly - Essentially there are bridges between cells through which man ions and large molecules can flow - Cell 1 and Cell 2 membranes are close - Full gap junction – each cell produces half of the gap junction complex, and it integrates with the other cells gap junction complex - As a hemi channel (only one membrane has the gap junction protein) is useful as well - Span an intercellular gap of 2-4 nanometers - Connexon is formed by 6 integral membrane proteins/transmembrane proteins, they form a pore across the membrane of one cell - Each of these subunits is called a connexin - Forms a cytosolic link between cells Gap Junctions - Mediate communication between adjacent cells. Intercellular communication - These cells are electrically coupled to each other. If one cell becomes electrically excited, it is a matter of time before that excitation is passed to other cells - Allows also for the movement of second messengers (cAMP, ATP) and any molecules less than one kDa can move between cells - These channels are also regulated by calcium, pH and dopamine - If you have one cell connected to other cells, and one cell has way too much calcium and is dying, then it would only be a matter of time before this calcium leaks to other cells and has a negative impact on it. - However, these gap junctions are closed in the presence of high calcium, in order to stop this from occurring...