Mutations and Diseases of G Protein Coupled Receptors

Title: Mutations and Diseases of G Protein Coupled Receptors
Author: Birnbaumer, Mariel
Publisher: Journal of Receptor and Signal Transduction Research
Date Published: January 01, 1995
Reference Number: 240
Currently known disease-causing mutations in G_protein coupled receptors are reviewed and discussed in conjunction with other naturally occurring receptor mutations. Special emphasis is made on opsin, vasopressin and MSH receptor mutations and what they tell are beginning to tell us about the inner workings of this superfamily of signaling molecules.

This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)

Receptors are molecular structures in a cell or on the surface of the cell that form a weak, reversible chemical bond with a specific extracellular agent such as a hormone, neurotransmitter or antigen. Once the bond is formed, the receptor passes the agent's signal along a molecular sequence to affect a particular physiologic result.

G-protein coupled receptors (GPCRs) are a huge family of receptors that are anchored in a cell membrane and couple with a G protein located immediately beneath the membrane. The GPCR must be able to couple with its G protein once it has bound with its agent. For example, the vasopressin-2 receptor (V2R) is a GPCR located in the basolateral membranes of the principal cells of the kidney collecting duct. It binds with the extracellular agent, arginine vasopressin (AVP), a hormone that carries a signal for the kidneys to concentrate urine and reabsorb body water. Once V2R and AVP bind, they activate the Gs protein (stimulatory G protein) to which V2R couples. The activated Gs protein passes AVP's signal to the next step in the molecular sequence which results in body water reabsorption and urine concentration.

Think of a GPCR, which is a protein, as a beaded string. The beads are amino acids. The number and sequence of amino acids differs with each different type of GPCR, but all GPCRs share a common structure. A significant position of the GPCR lies coiled in seven distinct clumps within the cell membrane, that thin strip of tissue that encircles the cell, separating it from its environment. These coiled clumps are called transmembrane domains 1 - 7. Part of the GPCR snakes outside the membrane into the extracellular environment, forming three curves called extracellular loops 1 - 3. Part of it snakes inside the cell forming three curves called intracellular loops 1 - 3. One end, called the amino-terminus, sits outside the cell with the extracellular loops. The other end, called the carboxy-terminus, sits inside the cell with the intracellular loops. The transmembrane domains, extracellular loops and first and second intracellular loops of most of the different types of GPCR are similar in size. However, most types of GPCRs have different sized amino-termini, third intracellular loops and carboxy-termini. (Please refer to V2R for a visual example of a GPCR.)

Researchers have studied GPCRs to determine the different structural elements that make it up (e.g., the transmembrane domains), what function each element performs and how the different elements interact with one another. For example, the transmembrane domains are perpendicular to the membrane in which they reside. This depends not only on the intra and extracellular loops that connect the transmembrane domains, but on interactions with the transmembrane domains themselves. For example, mutations that interfere with the interaction between transmembranes 1 and 7 prevent the receptor from folding itself into its proper shape.

Each GPCR has a specific intracellular agent that it binds with. Small agents often interact directly with the transmembrane domains within the cell membrane to trigger a response in the GPCR. Larger extracellular agents interact with the transmembrane regions, but primarily bind with the extracellular loops. Researchers are discovering so much about the function of the elements of different GPCRs that they are often able to define which amino acid performs what function in what segment of the GPCR in question. For example, most GPCRs have a cysteine amino acid in each of the first and second extracellular loops, which may form a disulfide bridge that helps make the seven transmembrane structure more stable.

GPCRs are synthesized within the cell and must assume a proper shape before they are allowed to travel to and insert themselves into the cell membrane. Little is known about the GPCRs' folding and membrane insertion process except that the two halves of the GPCR (one consisting of the amino-terminus to the first half of the third intracellular loop; the other consisting of the second half of the third intracellular loop to the carboxy-terminus) can fold independently and come together to form the complete unit.

When a GPCR's binding site is occupied by its extracellular agent (e.g. a hormone) the binding site must change shape in a manner that can be sensed by a G protein. Further, the GPCR must be able to form a complex with a specific G protein, and it must be able to be desensitized by desensitization machinery that senses the GPCR's occupation by its agent.

G proteins have three subunits called alpha, beta and gamma. When the G protein activates the next step in the molecular sequence, it splits into two units, the alpha subunit and the beta/gamma subunit. GPCRs must have elements that recognize the alpha unit and the beta/gamma unit. The alpha unit will not interact with the GPCR in the absence of the beta/gamma unit. Whereas, the latter unit will interact with the GPCR in the absence of the alpha subunit.

Researchers are currently attempting to locate those regions of the GPCR responsible for G protein interaction. The third intracellular loop varies widely in length and amino acid composition among the GPCRs. Therefore, researchers believe it determines which G protein the GPCR can couple with. The third intracellular loop may actually physically contact the G protein and thereby also be the means by which GPCR and G proteins couple.

Naturally occurring mutations in GPCRs have enabled researchers to study the relation between GPCR structure and function. This is so because when there is a mutation in a GPCR, it changes the structure of the GPCR in identifiable ways. The structural change often results in functional defects. By identifying the structural alteration and observing the functional result, then comparing this to a normal example of the same type of GPCR, researchers are able to determine which function the defective element in the GPCR should be performing.

For example, the vasopressin-2 receptor (V2R) gene, when mutated, is responsible for the X-linked form of inherited nephrogenic diabetes insipidus (NDI). So far, 38 distinct mutations of the V2R have been identified. Some of the mutations result in V2Rs so far from being fully developed as to render them biologically inactive. Others result in a single amino acid change. For example, one mutation of the V2R gene, the Q2 mutation, results in a histidine amino acid substituting for an arginine amino acid in the V2R's second intracellular loop just beneath the third transmembrane domain. This mutation completely uncouples the V2R from the Gs protein, while leaving the rest of its functional abilities unaltered. This identifies the section where the mutation takes place as essential for the V2R's ability to couple with its G protein.