Cell Signaling: Signal Reception

00:01 With cell-to-cell signaling, it doesn’t work if you just send that signal out and don’t have anyone to receive it.

00:08 You need to be able to receive the signal.

00:11 And this doesn’t matter if it is an endocrine signal, if it is neural signal, if it’s paracrine or autocrine signal, you need to be able to receive it.

00:21 And how does the body and various cells receive these signals? It could be that that particular signal that you’re trying to sense is from outside the body and you need to look at a light wave or a sound wave or maybe a smell or a taste.

00:39 But within the body, the signals are usually more subtle, like a signaling molecule that gets delivered to your front door, or maybe a wire transmits that to a certain spot of the body.

00:52 So let’s now go through a number of these informational type of gathering sensors.

00:59 So in the neural system, we’ll go through things like light receptors or photo receptors in the eye.

01:06 We’ll go through hair cells in the ear that sense vibrations and sound waves.

01:12 We’ll have taste odorant receptors, as well as ones on your tongue to sense different types of tastes.

01:21 So, all of these types of receptors will garner information about the external environment around you.

01:29 Other cells will utilize various receptors that will look for what we’re going to call a ligand.

01:37 A ligand is simply a signaling molecule and a molecule that will bind only to a specific receptor.

01:45 This is very handy way to signal.

01:48 This allows you to put out a number of signals and they only get received by the cells that you want to.

01:57 And how do you know? Based upon the receptor.

02:01 So let’s go through a number of these receptors.

02:05 The ligand itself will vary depending upon what kind of signaling molecule it is.

02:11 It might be an ion, it might be a neurotransmitter.

02:15 Again, it might be a hormone.

02:18 When it binds to a particular receptor, it will take many forms.

02:23 Sometimes it makes the receptor come together to form a dimer, sometimes it just bind to a very small part of a receptor.

02:34 The types of receptors we have could be catalytic.

02:38 And what we mean by a catalytic receptor is as soon as the ligand binds, an enzymatic reaction happens and there is either some phosphorylation or some action that takes place.

02:51 We just provided you a couple of examples here of an ANP or atrial natriuretic peptide receptor or a growth hormone receptor.

03:00 So you can see here, even though these two signals look somewhat the same in terms of their receptors, they’re going to cause dramatically different responses if it’s an ANP receptor versus a growth hormone receptor.

03:14 Another type of receptor is a G protein-coupled receptor.

03:20 There are numerous types of G protein-coupled receptors and what these are are ways to sense a signal and then to cause an enzymatic reaction and cascade process to happen.

03:33 There are a number of different types of these.

03:35 And usually, not only is which ligand binds to it is important, but also which of these G proteins are activated will depend upon what kind of response you’ll get within the cell.

03:50 This utilizes a secondary messenger system usually to start the cascade process.

04:00 Another example here of a G protein-coupled receptor is this one, where you have a ligand binding to a receptor.

04:11 You can see there are three G proteins around that receptor.

04:15 We have them labeled here as an alpha, a beta, and a gamma.

04:20 In this case, you’re going to see the alpha component move over once the ligand binds to bind to adenylate cyclase.

04:30 Adenylate cyclase is an enzyme.

04:33 It will convert ATP into cyclic AMP, and then cyclic AMP will start a cascade of protein phosphorylation to other proteins such as protein kinase A.

04:47 This type of transduction of the signal is important.

04:52 It allows for many things anywhere from amplification to specificity of the particular cell response.

05:01 This is another example of a G protein-coupled receptor.

05:05 Here, when the ligand binds to the receptor, it activates G proteins.

05:11 In this case, the G protein that’s activated is a G alpha Q.

05:17 G alpha Qs then migrate over to activate phospholipase C, and phospholipase C converts a molecule PIP2 into two things that are going to be active throughout the cell, and that is DAG and IP3.

05:37 IP3 then can move throughout the cell and do other tasks such as bind to IP3 receptors within the cell and release substances from the endoplasmic reticulum.

05:50 The DAG molecule can activate other proteins and phosphorylate them such as protein kinase C.

05:59 So you can see here how you can have a multitude of responses simply by signaling one enzyme, a lot of different things can happen within the cell.

06:11 Other receptors usually just move in ions and these are ion channel-linked receptors.

06:20 You might have the ligand bind to a specific part of the channel and that then allows for the ions to travel through.

06:29 So it might be closed initially, the ligand binds, and it opens up.

06:36 Another example is you may need to have multiple ligands bind to a particular receptor to have it open up.

06:46 The example that we’re looking at here is an acetylcholine nicotinic receptor in which you have a neurotransmitter, acetylcholine in this case, binding to this acetylcholine receptor to allow for ions to travel through.

07:07 Not all receptors though are located on the cell surface.

07:12 If your signaling molecule is a steroid hormone or another lipophilic type of signaling molecule, it can sometimes make it through the cell membrane.

07:24 In this case, the receptor is now located in the cytosol.

07:31 Other times, you might even have a receptor that’s located in the nucleus itself.

07:37 But in this case, we have the steroid hormone binding to a cytosolic receptor, this is then translocated into the nucleus, and will bind to a response element.

07:48 You’ll get an initial response in which you will then produce various proteins.

07:56 And as you produce proteins, then, you will have a response via a nuclear signal.

08:04 It’s interesting though when you look at cytosolic and nuclear receptors that you can break them down into their quick effects, which are their primary effects, and then their secondary effects which are more long-lasting because these involve making a new protein or making a new channel.

08:23 And so, sometimes you’ll have quick effects and then you’ll have secondary or longer-term effects.

08:30 The last type of cell-to-cell communication that we need to deal with here doesn’t involve a ligand and that is you can have direct contact between two cells and usually it’s sharing electricity or a voltage change.

08:47 These kind of voltage changes allow for then tissues to work in coordination with each other.

08:54 So a good example of that are smooth muscle cells.

08:58 You need to have smooth muscle cells contract in a coordinated manner to do useful work for you.

09:07 So to summarize this cell-to-cell signaling, remember that you might have various receptors to sense external environments, such as light and sound, but you also need to have the cells within the body be able to communicate with each other and respond to ligands or voltages so that there can be an active and coordinated response to whatever is necessary in the environment.