Phosphoinositide GPCR Signaling: Videos & Practice Problems

The Phosphoenositide GPCR signaling pathway (PSP) involves the activation of an alpha adrenergic GPCR by ligands like epinephrine, leading to G protein activation (Gq). This triggers phospholipase C (PLC) to hydrolyze PIP₂, generating secondary messengers IP₃ and DAG. IP₃ releases calcium ions from the endoplasmic reticulum, activating calmodulin and downstream kinases, while DAG activates protein kinase C (PKC). Together, these events culminate in various cellular responses, highlighting the intricate nature of signal transduction and the role of second messengers in cellular communication.

Phosphoinositide GPCR Signaling

Okay. So here we're going to revisit our map of the lesson on biosignalling pathways, which is down below right here. And, of course, we know that we've been exploring this map by following the leftmost branches first. And so we've covered G protein-coupled receptors or GPCRs, and we've talked all about the adenylate cyclase GPCR signaling system, including the stimulatory pathway that involves cAMP and PKA, and the inhibitory pathway that involves also drugs and toxins. And so now what we're going to do in this video is talk about another G protein-coupled receptor pathway, another GPCR pathway called the Phosphoinositide GPCR Signaling Pathway. And so let's get started talking about that.

So here we're going to introduce another classic example of a GPCR signal transduction pathway, which is phosphoinositide GPCR signaling. Now, depending on your textbook, sometimes they'll refer to this pathway as the phosphoinositide signaling pathway, but in other textbooks, they'll refer to it as the inositol phospholipid signaling pathway. But either way, you can see that the phospho prefix and the inositide prefix correspond with the inositol and the phospholipid prefixes that we see here. And so although these are 2 different words, you can see how they're related to one another and referring to the same exact pathway. However, moving forward in our Clutch Prep Biochemistry course, we're actually going to refer to this pathway as the Phosphoinositide pathway or the PSP pathway for short.

And so this phosphoinositide signaling pathway or PSP pathway, again is another classic example of a GPCR signaling pathway. And the PSP pathway actually mediates a wide variety of hormonal effects that include the hormones epinephrine, angiotensin, vasopressin, and much more that are not listed here. However, the main takeaway here is that depending on the exact hormone, the final function of this PSP pathway or the final cell response can actually vary. And so the PSP pathway actually has many, many different functions that could include blood platelet and injured cell signaling to stimulate growth and the injury repair process.

Now phosphoinositide signaling or the PSP pathway really requires these 5 components that you can see down below in our image right here. And so if we take a look down below at this image, what you'll notice is that we have these yellow numbers to label each of these 5 components in the PSP pathway. And we'll talk a lot more about each of these 5 components as we move forward in our course. But of course, the first major component in the PSP pathway, which of course we know is a GPCR signaling pathway, is going to be the GPCR itself. Now, rather than being a beta-adrenergic GPCR like what we saw in the adenylate cyclase GPCR signaling pathway, here in the PSP pathway we have an alpha-adrenergic GPCR, and so that is one major difference between the 2 GPCR pathways.

Now, of course, the second major component of a PSP pathway is going to be the heterotrimeric G protein here, which still has the alpha, beta, and gamma subunits just as we saw it before. And, of course, we know that the G proteins are going to be associated with G protein-coupled receptors or GPCRs. And I'm not exactly sure why they decided to use the Q here in this notation, but this is something that you'll commonly see throughout your textbooks and throughout your practice problems. And so you need to know that G_Q is the abbreviation that's referring to the G protein specifically in the PSP pathway.

Now the 3rd component that we have here is the effector enzyme called phospholipase C or PLC for short. So we can go ahead and label this as PLC. And of course, we know that the effector enzyme is responsible for generating secondary messengers, and we'll talk more about the secondary messengers later in our course. But the substrate for the PLC is this molecule that you see right here, which is actually our representation of a glycerophospholipid. More specifically, it's the glycerophospholipid Phosphatidylenositol 4,5-bisphosphate, or as you could see, PIP₂. And so, you can see these 2 phosphate groups here in position 4,5 and this phosphate group that you see here is actually representing the glycerophospholipid bond here. And so this is going to be our representation of PIP₂ moving forward in our course. And again, we'll talk a lot more about each of these components moving forward in our course as well.

And so the 5th and final component here of PSP pathways is protein kinase C, which, of course, is going to be a kinase and is abbreviated as PKC. And so, protein kinase C is another component that we'll get to talk a lot more about, moving forward in our course. But these here are really the 5 main components that you guys should be familiar with as we move forward in our course, and we'll continue to talk more and more about their functions as we move forward in our course as well. And so for now, this here concludes our brief introduction to the phosphoinositide GPCR signaling pathway or the PSP pathway, and we'll continue to talk more about this moving forward. So I'll see you guys in our next lesson video.

Phosphoinositide GPCR Signaling

In this video, we're going to introduce the 3 Phosphoinositide Secondary Messengers. And so the PSP pathway or the phosphoinositide signaling pathway generally involves the production of 3 secondary messengers that we have numbered down below right here.

And so the very first secondary messenger is inositol 1,4,5-triphosphate or IP₃ for short. And so the I is going to be for the I in inositol, the P will be for the P in phosphate, and of course, the 3 here will be for the 3 phosphate groups that are on positions 1, 4, and 5. And so if we take a look at our image down below over here on the left-hand side, notice that we're showing you the structure of inositol 1,4,5-triphosphate right here. And so if we were to number these carbons, this would be carbon 1, 2, 3, this is carbon 4, and this is carbon 5, and so notice that it is indeed carbon 1, carbon 4, and carbon 5 that have these phosphate groups on them and that's why it's 1,4,5-triphosphate, or again, IP₃ for short.

Now moving forward in our course, we're going to show IP₃ in this format that you see over here, so it's important to start to recognize how we'll be showing this particular secondary messenger IP₃ moving forward in our course.

Now the second type of secondary messenger that is going to be produced is 1,2-diacylglycerol or DAG for short, coming from the diacylglycerol. And so if we take a look at this structure down below right here, what you'll notice is that this is actually a glycerol attached to 2 fatty acids. And so, notice that the glycerol is this structure right here, and the 2 fatty acid chains would be these R groups that are, essentially ester linked right here. And so, we're going to represent our diacylglycerol as this structure right here moving forward in our course, this red-looking structure. And so, the diacylglycerol again can be commonly abbreviated as just DAG.

Now the 3rd and final secondary messenger is calcium ions or Ca²⁺. And so over here on the far right, we're showing you guys some calcium ions. Now one thing to note is that this structure and this structure may already look somewhat familiar to you guys from our very last lesson video, and that's because notice that these two structures actually come from the phosphatidylinositol 4,5-bisphosphate or PIP₂ that we introduced in our last lesson video. And so you can see that the DAG molecule is right here, and the IP₃ molecule is right here. And so really these 2 secondary messengers, what we'll see moving forward in our course, is that they're coming directly from PIP₂, and so that's one thing to keep in mind as we move forward in our course.

But for now, this here concludes our introduction to the 3 phosphoinositide secondary messengers and we'll be able to see the exact roles of each of these three moving forward in our course. So I'll see you guys in our next video.

Which of the following are secondary messengers produced in the PSP pathway?

• A. cGMP & K⁺.
• B. IP₂ & IP₃.
• C. IP₃ & DAG.
• D. DAG & cAMP.

Answer: (C)

Phosphoinositide GPCR Signaling

In this video, we're going to introduce the steps in Phosphoenolpyruvate GPCR signaling. Now it turns out that there are actually many different variations of Phosphoenolpyruvate signaling pathways or PSP pathways. However, all of these different variations can be condensed into 8 general steps that we have listed and numbered down below in our image. So you can see the first three steps are right here, then steps 4 through 7 are right here in yellow, and step number 8 is way down below this large image right over here. And so what's important to note is that some of these steps are associated with a gray color like what you see here, and these steps are very very similar to what we've seen before in the adenylate cyclase GPCR signal transduction pathway. And then notice that other steps are numbered with these yellow numbers, and that's because these steps are really really specific and pretty much brand new in the PSP pathway. And so we've color-coordinated it like this to help you guys organize the information a little bit better. And so taking a look at this first set of 3 steps right here, which again are color-coded in gray, so they're pretty much the same as before, we're labeling this as the alpha-adrenergic pathway because ultimately these first three steps right here lead to the activation of the alpha-adrenergic GPCR.

In step number 1, the hormone signaling ligand, again, epinephrine, or some other hormone potentially as well, could bind to the GPCR more specifically in this PSP pathway, again, the alpha-adrenergic GPCR or receptor, and that's going to cause a conformational shift in the structure of the GPCR, just as we've seen before in the adenylate cyclase GPCR signaling pathway. Then in step number 2, this GPCR conformational shift is going to activate the g protein. The g protein in the PSP pathway is abbreviated as gq. The gPCR conformational shift activates gq by promoting the alpha subunit to exchange GDP with GTP. Then in step number 3, with the bound GTP, the alpha subunit of gq is going to dissociate from the beta-gamma subunits and diffuse towards a nearby effector enzyme, and of course, the effector enzyme is going to be different in the PSP pathway. It will not be adenylate cyclase; instead, it will be phospholipase c as we'll see here very shortly. But let's take a look down below at our image just to take a look at the first three steps, which again is pretty much review from our previous lesson videos.

And so notice that steps 1, 2, and 3 are labeled right here with these gray numbers that we see. Then in step number 4, the activated GTP-bound alpha subunit of GQ not only is going to dissociate towards the effector enzyme but is also going to bind and activate that effector enzyme as well. The effector enzyme in the PSP pathway is not adenylate cyclase. Instead, it's going to be phospholipase c, or PLC for short. So if we take a look at our image down below right here in step number 4, notice that the alpha subunit of the G protein Gq with the bound GTP is going to activate the effector enzyme right here, which again is going to be, phospholipase c, PLC.

This leads us to step number 5, which we can see down below right here. The activated PLC is going to hydrolyze its substrate, and its substrate is going to be phosphatidylinositol 4,5-bisphosphate, which we know is going to be PIP₂. PIP₂, when it is hydrolyzed, is going to generate 2 secondary messengers. This active PLC is going to cleave PIP₂ to generate 2 additional secondary messengers. Those 2 secondary messengers are going to be IP₃ and DAG. What you'll notice is that the active PLC is right here in the membrane and PIP₂ is right here. Again, PLC is going to cleave PIP₂ or hydrolyze PIP₂ and essentially generate these 2 secondary messengers. Down below, what we have is IP₃, which we can label right here, IP₃. Then up above, still embedded within the membrane, what we have is DAG, diacylglycerol here.

IP₃ is a cytosolic molecule that can diffuse through the cytoplasm making its way downwards into the cytosol of the cell, whereas DAG on the other hand is still embedded within the cell's plasma membrane and is not actually able to diffuse into the cytoplasm. It's only able to diffuse laterally, left and right here. This is going to make a big difference, and you can also see that this step number 5 is going to cause a split in our pathway where we're going to need to follow IP₃ and see what it does, but then we're also going to need to come back and follow DAG and see what it does in the membrane.

So first we're going to start off in step number 6, as you can see here, following IP₃ and its diffusion into the cytoplasm. What you can see is that IP₃, the secondary messenger, is going to diffuse further into the cytoplasm towards the endoplasmic reticulum in order to bind and open calcium, Ca²⁺, transport channels that are embedded in the endoplasmic reticulum. When it opens these transport channels, it's going to allow all of the calcium that's stored in the endoplasmic reticulum to be released into the cytoplasm. This is going to cause a drastic increase in cytosolic calcium concentration. The increased calcium in the cytoplasm is going to be used to activate another protein that's known as calmodulin, abbreviated as CAM. The calcium-calmodulin complex is going to move on to activate other cytosolic proteins and that ultimately, cytosolic protein kinases or enzymes that phosphorylate things, and ultimately this is going to lead to the cell response.

Now in step number 7, notice that here DAG, diacylglycerol, is going to diffuse in the membrane, and along with that released calcium from step number 6, it's going to activate a membrane-bound kinase c or PKC for short. DAG is membrane-bound, so it can only diffuse in the membrane laterally. Here we have lateral diffusion where DAG can diffuse towards this other enzyme over here, called PKC, protein kinase c. And protein kinase c is another kinase that can go on to phosphorylate its targets and help lead to the cell response. And notice that the calcium that was also released by step number 6 can also be used here to help activate the PKC along with DAG, and together they help activate PKC to generate the cell response.

This leads us to step number 8, which is way down below right here, and notice step number 8 is color-coded in gray, so it's very similar to a previous step from the adenylate cyclase GPCR pathway. In step number 8, the g protein alpha subunit of gq, again is going to have GTPase activity, so it's going to slowly inactivate itself along with the effector enzyme PLC. Again, by hydrolyzing its GTP, the high energy active form, into the low energy inactive form GDP. And so this will allow the alpha subunit to reassemble its original inactive form with the beta gamma subunits, and then the ligand can dissociate from the GPCR to reset the pathway.

These are the 8 steps to the PSP pathway. After you guys get some practice going through these steps, it will come more naturally. There is quite a lot here to remember. In our next video, we're going to talk about a memory tool and a story that you can use to help you effectively remember the essential components and events in the correct order. I'll see you guys in our next video.

Phosphoinositide GPCR Signaling

So as you guys have probably noticed from our last lesson video, when it comes to phosphoinositide GPCR signaling, there's quite a lot for you guys to remember. In this video, we're going to talk about how to remember the most important components and the most important events in phosphoinositide GPCR signaling with the most important components and the most important events in the correct order.

Of course, we know that all GPCR signaling pathways start with a ligand binding to a GPCR, more specifically, the alpha adrenergic GPCR in the phosphoinositide GPCR signaling pathway. This causes a conformational shift in the GPCR that ultimately activates the G protein. By activating the G protein, we mean that it allows the G protein to exchange its GDP, the low energy inactive form, with GTP, the high energy active form. Then the G Protein Alpha subunit will be able to dissociate from the Beta Gamma subunits and make its way towards the effector enzyme, which in the PSP pathway is phospholipase C or PLC. PLC is able to act on its substrate, which is PIP₂, and it can hydrolyze or cleave PIP₂ into 2 secondary messengers, IP₃ and DAG. IP₃ will diffuse through the cytoplasm towards calcium ion channels in the endoplasmic reticulum membrane and it will bind and open those calcium channels so that calcium is released into the cytoplasm, increasing the concentration of calcium in the cytoplasm. The released calcium is able to activate calmodulin, which we have abbreviated here as CAM. The activated calmodulin will activate other protein kinases, which will ultimately lead to the cell response.

Regarding DAG, it will remain embedded in the membrane and will diffuse towards another enzyme called Protein Kinase C or PKC, and it will activate Protein Kinase C along with calcium that's released from the endoplasmic reticulum. Protein Kinase C is a kinase that's able to phosphorylate other targets, which also helps lead to the cell response. This is quite a lot to remember, but really everything that you the most important features and components and stuff that you need to know is all embedded right here.

Down below, what we have is a really interesting and unique story that we've come up with here at Clutch Prep to help you guys remember not all of the details, but again, just the most important components and the most important events in the correct order. The plot of our story is that the cell wants to be able to save the day, which is going to represent the cell response. So, Pip and his two weapons represent PIP₂, the substrate of the effector enzyme phospholipase C. We are numbering the steps of our stories steps number 1 through 7, and the numbers in the steps of our story correspond with the numbers on the image. In the very first step of our story, we need to have the ligand binding to the GPCR, represented by a lightning strike. The ligand binding will provide a warning of danger that this evil villain Pip and his two weapons are around causing havoc.

When the ligand binds to the GPCR, it will cause a conformational shift that activates the G protein. This activation requires the GTP exchange. The alpha hero or the G protein alpha subunit is going to perform the GTP exchange, exchanging the empty siren battery with the full siren battery, so that it can sound the alarm and alert the police, the effector enzyme phospholipase C.

The Alpha Hero alarms the police but forgets his Beta Blaster and his Gamma Gadget behind, reminding us that the G protein alpha subunit is the only subunit to dissociate towards the phospholipase C effector enzyme. In step number 4, the Alpha Hero directs the police to target Pip's two weapons, signifying the activation of phospholipase C. The police, taking advice from the Alpha Hero, taser Pip's two weapons, separating them into the two secondary messengers: the dagger and the 3 ice picks, representing DAG and IP₃, respectively. IP₃ will drop to the membrane of the endoplasmic reticulum and trigger the release of Captain Marvelous, representing calcium release and calmodulin activation which leads to the cell response. Meanwhile, the DAG will move towards a crowd representing the enzyme PKC, leading to further cell response activation.

This creative story, although not foolproof, is devised to help with remembering the key components and events in the phosphoinositide GPCR signaling pathway. I hope revisiting this story will aid in understanding and applying these important concepts. I'll see you all in our next video.

Phosphoinositide GPCR Signaling Example 1

Alright. So here we have an example problem that wants us to place the following steps of the PSP signaling pathway down below right here in the correct order from step 1 to step 7. And so, of course, what we need to recall is that the PSP pathway or the phosphoinositide signaling pathway is a GPCR signaling pathway. And, of course, GPCR signaling pathways begin with the ligand binding to the GPCR itself. And so we can go ahead and put this as our step number 1. And, of course, we know that the ligand binding to the GPCR is going to cause a conformational shift in the GPCR, and the conformational shift in the GPCR ends up causing a conformational shift of the G protein, allowing it to exchange its GDP for GTP, more specifically, the alpha subunit of the G protein. And so this will be our step number 2.

And then, of course, at that point, once the G protein is activated, the alpha subunit of the G protein is going to dissociate from the beta gamma subunits of the G protein, and so this will be step number 3. And then, of course, we know that the alpha subunit of the G protein that has dissociated is going to diffuse towards an effector enzyme, more specifically, phospholipase C, and it's going to activate phospholipase C. And so that is going to be step number 4.

And then, of course, we know that the substrate of phospholipase C is PIP2. And so what will happen next is that phospholipase C is going to hydrolyze its substrate PIP2 and convert it into the 2 secondary messengers, IP3 and DAG. So this will be our step number 5 here. And then, of course, at that point, we know that we need to follow IP3 and DAG separately since IP3 is going to be cytosolic and diffuse into the cytosol, whereas DAG is going to remain membrane bound and can only diffuse laterally within the membrane.

And so if we follow IP3, then the next step is that IP3 is going to diffuse through the cytoplasm to the endoplasmic reticulum membrane and bind to calcium channels where cytoplasmic calcium concentration. So this will be step number 6 here. And then, of course, last but not least, our step number 7 will be right here. And that is that the diacylglycerol, or DAG here, along with the calcium ions that were released, are going to activate protein kinase C, or PKC. And, again, this will be step number 7.

And so these are really the answers to this practice problem, and that concludes this practice. So I'll see you guys in our next video.

When epinephrine binds to a(n) ________________ receptor, _______ is activated, causing an increase in the cellular concentration of ________________.

• A. Beta ; G_αi ; Inositol triphosphate.
• B. Alpha ; G_αq ; Inositol triphosphate.
• C. Beta ; G_αi ; Diacylglycerol.
• D. Alpha ; G_αs ; cAMP.
• E. Beta ; G_αq ; Diacylglycerol & Inositol triphosphate.

Answer: (B)

In cells, epinephrine binding to the α-adrenergic receptor activates G _αq. All of the following happen EXCEPT:

• A. Protein Kinase C activity will be decreased.
• B. Inositol triphosphate will cause the release of Ca²⁺ from the endoplasmic reticulum.
• C. Increased binding of Ca²⁺ to the calmodulin will occur.
• D. Increased Ca²⁺-Calmodulin complex will bind to and activate calmodulin-binding proteins.
• E. Protein Kinase C will be activated by increased levels of diacylglycerol.

Answer: (A)

All G proteins are activated by GDP/GTP exchange (binding to GTP), but lose their activity when they hydrolyze GTP to GDP. However, different G proteins can have different roles. Which of the following is INCORRECT?

• A. When active, G_αs activates adenylate cyclase.
• B. When active, G_αs results in increased production of cAMP.
• C. When active, G_αi results in the inhibition of adenylate cyclase.
• D. When active, G_αq results in the activation of phospholipase C.
• E. When active, G_αs results in increased production of inositol triphosphate.

Answer: (E)

The reaction catalyzed by Phospholipase C can use all of the following EXCEPT:

• A. Phosphatidylinositol bisphosphate as a substrate.
• B. Diacylglycerol as a product.
• C. Diacylglycerol phosphate as a product.
• D. Inositol triphosphate as a product.
• E. Water as a substrate.

Answer: (C)

Here’s what students ask on this topic:

What is the Phosphoinositide GPCR signaling pathway?

The Phosphoinositide GPCR signaling pathway (PSP) is a type of G protein-coupled receptor (GPCR) pathway. It involves the activation of an alpha adrenergic GPCR by ligands such as epinephrine. This activation leads to the stimulation of the G protein G_q, which in turn activates phospholipase C (PLC). PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂) to generate two secondary messengers: inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ releases calcium ions from the endoplasmic reticulum, activating calmodulin and downstream kinases, while DAG activates protein kinase C (PKC). These events lead to various cellular responses, demonstrating the complexity of signal transduction and the role of second messengers in cellular communication.

What are the main components of the Phosphoinositide GPCR signaling pathway?

The main components of the Phosphoinositide GPCR signaling pathway (PSP) include:

• GPCR: Specifically, an alpha adrenergic GPCR.
• G protein (G_q): A heterotrimeric G protein with alpha, beta, and gamma subunits.
• Phospholipase C (PLC): The effector enzyme that hydrolyzes PIP₂.
• PIP₂: Phosphatidylinositol 4,5-bisphosphate, the substrate for PLC.
• Secondary Messengers: Inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG).
• Protein Kinase C (PKC): A kinase activated by DAG and calcium ions.

These components work together to mediate various cellular responses through signal transduction.

How does the Phosphoinositide GPCR signaling pathway lead to calcium release?

In the Phosphoinositide GPCR signaling pathway, the activation of the alpha adrenergic GPCR by a ligand like epinephrine leads to the activation of the G protein G_q. The G_q protein then activates phospholipase C (PLC), which hydrolyzes PIP₂ into two secondary messengers: IP₃ and DAG. IP₃ diffuses through the cytoplasm and binds to calcium channels on the endoplasmic reticulum (ER). This binding opens the channels, allowing calcium ions (Ca²⁺) to be released from the ER into the cytoplasm. The increase in cytosolic calcium concentration activates various downstream proteins, including calmodulin, leading to further cellular responses.

What are the secondary messengers involved in the Phosphoinositide GPCR signaling pathway?

The Phosphoinositide GPCR signaling pathway involves three main secondary messengers:

• Inositol 1,4,5-trisphosphate (IP₃): This molecule diffuses through the cytoplasm to the endoplasmic reticulum, where it binds to and opens calcium channels, releasing Ca²⁺ ions into the cytoplasm.
• Diacylglycerol (DAG): This lipid remains in the plasma membrane and, along with Ca²⁺, activates protein kinase C (PKC), which phosphorylates various target proteins.
• Calcium ions (Ca²⁺): Released from the endoplasmic reticulum by IP₃, these ions activate calmodulin and other calcium-dependent proteins, leading to various cellular responses.

These secondary messengers play crucial roles in propagating the signal initiated by the GPCR activation.

What is the role of phospholipase C (PLC) in the Phosphoinositide GPCR signaling pathway?

Phospholipase C (PLC) plays a critical role in the Phosphoinositide GPCR signaling pathway. Upon activation by the G_q protein, PLC hydrolyzes its substrate, phosphatidylinositol 4,5-bisphosphate (PIP₂), which is located in the plasma membrane. This hydrolysis reaction produces two important secondary messengers: inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ diffuses into the cytoplasm and triggers the release of calcium ions from the endoplasmic reticulum, while DAG remains in the membrane and, along with calcium, activates protein kinase C (PKC). These events lead to various downstream cellular responses, highlighting PLC's essential role in signal transduction.