Biosignaling 3: Study with Video Lessons, Practice Problems & Examples

G proteins play a crucial role in cell signaling by activating enzymes like phospholipase C, which cleaves phosphatidylinositol (PIP2) into inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 facilitates calcium release, while DAG activates protein kinase C. Receptor tyrosine kinases (RTKs), such as the insulin receptor, undergo autophosphorylation upon ligand binding, triggering cascades that enhance glucose transport and glycogen synthesis. This signaling pathway illustrates the interconnectedness of cellular responses to maintain homeostasis, particularly in regulating blood sugar levels through GLUT4 transporters and glycogen synthase activation.

Biosignaling 3

Just like cyclic AMP has a bunch of functions in the cell, G also carries out a wide variety of functions in cell signaling. In addition to activating adenylyl cyclase, G protein with GTP bound will also activate phospholipase C, which is an enzyme you might remember from the last unit. And this enzyme cleaves lipids. Hopefully, you remember specifically where it cleaves lipids, as that's something that we covered for the last exam. Now phospholipase C, in the case of cell signaling, will cleave the bond between the inositol phosphate and the glycerol of the molecule phosphatidylinositol or abbreviated as PIP₂. And you can see that happening right here. We have our G protein-coupled receptor, with ligand bound, and that this little light thing saying it's that G is activating phospholipase C PLC, what we see right there. And it's going to cleave this molecule, PIP₂, Phosphatidyl Inositol, and it's going to cleave it into what's abbreviated as IP₃ or Inositol Triphosphate. What you see right here And it's also the other part of the molecule after it's cleaved is called diacylglycerol or DAG. So, just to recap, Phospholipase C gets activated by G and it cleaves PIP₂, into DAG and IP₃. Done. Just to note quickly that DAG will stay in the membrane, whereas IP₃ will exit the membrane, as it is able to interact with water much better. The other part of the molecule is very hydrophobic lipid, but IP₃ has those phosphate groups meaning, it's going to have a lot of charge so it can interact with water very nicely. So, inositol triphosphate will actually go diffuse into the cell and it will open up these calcium channels. So here's some sort of internal cell membrane. It's going to activate these calcium channels so that calcium gets released. And diacylglycerol and calcium, these guys will actually lead to the activation of protein kinase C. Additionally, inositol triphosphate and calcium will act as secondary messengers in a variety of other signaling pathways. But just know that calcium and diacylglycerol will activate protein kinase C. And know how the cleavage of phosphatidylinositol leads to DAG and IP₃. Moving on, we've spent a lot of time on G protein-coupled receptors. Now, let's actually take a look at a different type of receptor, called receptor tyrosine kinase or RTK. And these are receptors that are capable of autophosphorylating, meaning they can phosphorylate themselves and they'll do so at tyrosine residues. And they'll do this in response to ligand binding. So we're going to look at insulin as our example of an RTK or the insulin receptor rather as an example of an RTK. And the insulin receptor actually works best as a dimer. And you can see that we have a nice protein dimer right there meaning that we have these 2 halves basically coming together to, or dimerizing, and forming this nice protein dimer that we see there acting as a receptor. And here, we have insulin. That's our ligand. And insulin is going to bind into our receptor phosphorylation cascade. So let me hop out of the image here. So what you can see happening is this is autophosphorylate. You can see here that it is autophosphorylate. You can see here that it is going to have a bunch of phosphate groups attached to it. And specifically with insulin, the insulin receptor after it is phosphorylated, it's going to phosphorylate a protein called IRS1, and IRS1 is going to turn on this RAS protein complex. So it's a complex of proteins, kind of like all bound together. IRS1 is going to turn on RAS, the RAS protein complex. The RAS protein complex phosphorylates MEK. MEK phosphorylates ERK, and ERK enters the nucleus and activates genes. So basically, insulin binds, the receptor tyrosine kinase does its autophosphorylation, and this generates a phosphorylation cascade which ultimately leads to this protein ERK entering the nucleus and activating certain genes leading to the expression of certain genes. Anyways, IRS1 also activates protein kinase B, right? So hopefully you're starting to see a pattern here that all these components of signaling pathways tend to do multiple things, right? They don't just have one simple job. So anyways, IRS1 also activates protein kinase B, and that ultimately leads to more glucose transporters, specifically GLUT4 transporter in the membrane. So let's back up and think about this for a second. So insulin is released in response to high sugar levels in your blood. So when insulin is released, it's going to bind to this receptor tyrosine kinase and activate a bunch of genes related to processing this sugar basically. Additionally, it's going to activate protein kinase B and protein kinase B is going to put these glucose receptors on the membrane to transport into the cell this high concentration of glucose that is, that is present outside the cell, right? Like in the blood. So thinking about it on a more medical level, we often say that insulin lowers blood sugar levels. Well, this is partially how it's doing that by increasing the number of glucose transporters in the membrane thereby taking sugar out of the blood and putting it in the cell. Additionally, insulin causes glycogen formation because you have this excess of sugar; you want to store some of it for later because you don't need to burn all of it right away for nutrients. So protein kinase B, in addition to causing those GLUT4 receptors to be integrated into the membrane, also leads to the activation of glycogen synthase. And the way it does that is, it actually inactivates GSK3 or glycogen synthase kinase. That's what the GSK is. So it inactivates GSK3, and that allows glycogen synthase to become active because it's now, this is where things maybe get a little confusing. So GSK3 is normally an active protein, and its job is to inactivate glycogen synthase. So normally, GSK3 is on and it's keeping glycogen synthase off. And it does this by phosphorylating it. Here's a great example of where phosphorylating something actually inactivates it. So GSK3 keeps glycogen synthase off by phosphorylating it. But when protein kinase B comes around, it phosphorylates GSK3 which inactivates GSK3. And that allows glycogen synthase to actually become active and synthesize glycogen with all that extra glucose. And remember, a lot of glucose is getting transported into the cell now because of those extra glucose transporters. So you can see how this one ligand, right? Insulin is doing a wide variety of things in the cell but they're all kind of related to this central theme of trying to do something about all that sugar. So, hopefully, that kind of helps illustrate a little bit how expansive these signaling pathways are and how they can integrate into one another. Alright. Let's flip the page.

Here’s what students ask on this topic:

What role does phospholipase C play in G protein-coupled receptor signaling?

Phospholipase C (PLC) is an enzyme activated by G proteins in G protein-coupled receptor (GPCR) signaling. When a ligand binds to a GPCR, the G protein activates PLC. PLC then cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) into two secondary messengers: inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 diffuses into the cytoplasm and binds to IP3 receptors on the endoplasmic reticulum, causing the release of calcium ions (Ca²⁺) into the cytoplasm. DAG remains in the membrane and, along with Ca²⁺, activates protein kinase C (PKC). This cascade of events leads to various cellular responses, such as changes in gene expression, metabolism, and cell growth.

How does the insulin receptor function as a receptor tyrosine kinase (RTK)?

The insulin receptor is a type of receptor tyrosine kinase (RTK) that functions by autophosphorylation. When insulin binds to the extracellular domain of the receptor, the receptor dimerizes and undergoes autophosphorylation at specific tyrosine residues. This phosphorylation activates the receptor's intrinsic kinase activity, leading to the phosphorylation of downstream signaling proteins such as insulin receptor substrate 1 (IRS1). IRS1 activates a cascade involving the RAS protein complex, MEK, and ERK, which ultimately leads to gene expression changes. Additionally, IRS1 activates protein kinase B (PKB), which increases glucose transporter (GLUT4) translocation to the cell membrane and promotes glycogen synthesis by inactivating glycogen synthase kinase 3 (GSK3).

What are the secondary messengers produced by phospholipase C, and what are their functions?

Phospholipase C (PLC) produces two secondary messengers: inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 is water-soluble and diffuses through the cytoplasm to bind to IP3 receptors on the endoplasmic reticulum, triggering the release of calcium ions (Ca²⁺) into the cytoplasm. This increase in Ca²⁺ concentration can activate various calcium-dependent processes, including muscle contraction and enzyme activation. DAG remains in the plasma membrane and, together with Ca²⁺, activates protein kinase C (PKC). PKC phosphorylates target proteins, leading to diverse cellular responses such as changes in gene expression, cell growth, and metabolism.

How does insulin signaling lead to increased glucose uptake in cells?

Insulin signaling increases glucose uptake in cells through a series of steps involving the insulin receptor, a receptor tyrosine kinase (RTK). When insulin binds to its receptor, the receptor undergoes autophosphorylation and activates insulin receptor substrate 1 (IRS1). IRS1 then activates protein kinase B (PKB), also known as Akt. Activated PKB promotes the translocation of glucose transporter 4 (GLUT4) vesicles to the cell membrane. GLUT4 transporters facilitate the uptake of glucose from the bloodstream into the cell. Additionally, PKB inactivates glycogen synthase kinase 3 (GSK3), leading to the activation of glycogen synthase and promoting glycogen storage. These processes collectively lower blood glucose levels.

What is the role of protein kinase C in cell signaling?

Protein kinase C (PKC) plays a crucial role in cell signaling by phosphorylating various target proteins, leading to diverse cellular responses. PKC is activated by diacylglycerol (DAG) and calcium ions (Ca²⁺), which are produced during the activation of phospholipase C (PLC) in G protein-coupled receptor (GPCR) signaling. Once activated, PKC phosphorylates specific serine and threonine residues on target proteins, modulating their activity. This can result in changes in gene expression, cell growth, differentiation, and metabolism. PKC is involved in various physiological processes, including immune responses, neuronal signaling, and regulation of cell cycle and apoptosis.