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Broad-spectrum l-amino acid sensing by class 3 G-protein-coupled receptors

https://doi.org/10.1016/j.tem.2006.10.012Get rights and content

The sensing of nutrients is essential to the control of growth and metabolism. Although the sensing mechanisms responsible for the detection and coordination of metabolic responses to some nutrients, most notably glucose, are well understood, the molecular basis of amino acid sensing by cells and tissues is only now emerging. In this article, we consider evidence that some members of G-protein-coupled receptor class 3 are broad-spectrum amino acid sensors that couple changes in extracellular amino acid levels to the activation of intracellular signaling pathways. In particular, we consider both the molecular basis of specific and broad-spectrum amino acid sensing by different members of class 3 and the physiological significance of broad spectrum amino acid sensing by the extracellular calcium-sensing receptor, heterodimeric taste receptors and the recently ‘deorphanized’ receptor GPRC6A and its goldfish homolog, the 5.24 chemoreceptor.

Introduction

Changes in the abundance and types of proteins in the diet provide signals for growth and/or changes in metabolic programming 1, 2, 3, 4. Finding the molecular links between variations in protein intake and variations in growth or metabolic programming and demonstrating how variations in protein intake affects body composition and metabolism is a major challenge confronting current research. Because protein is broken down to small peptides and free amino acids in the small intestine [5], one set of biological signals of protein ingestion arises from changes in the concentrations of amino acids in the luminal fluid. After their absorption, with the attendant hydrolysis of small peptides to free amino acids in the cytoplasm of intestinal epithelial cells [6], other sets of biological signals arise from changes in the concentrations of amino acids in the portal and systemic blood. Amino acid sensors, the molecules that support amino acid sensing, are crucial for the endocrine control of metabolism and growth. This article examines the molecular requirements of amino acid sensing and considers the locations and molecular identities of recognized amino acid sensors, focusing on class 3 G-protein-coupled receptors (GPCRs) that exhibit broad-spectrum amino acid sensing properties.

Section snippets

Requirements for amino acid sensing

Levels of free amino acids in the plasma are perturbed in response to variations in protein intake via the gastrointestinal tract, portal circulation and liver, or in response to a change in body protein stores (e.g. arising from muscle protein breakdown). Although amino acids differ in the structures of their side chains, they otherwise conform to a generic structural plan in which a carboxylate-linked α carbon is also linked to an amino group, a hydrogen atom and a specific side chain. Thus,

Where are the amino acid sensors?

l-Amino acid sensors are widely distributed. The tissues affected include endocrine glands such as the anterior pituitary, pancreatic islets, parathyroid and endocrine and epithelial cells of the gastrointestinal tract, as well as the kidney, liver and muscle. In addition, the brain is equipped with amino acid sensing centers that control feeding behaviors. For example, an intracellular amino acid sensor in the mammalian piriform cortex and a leucine-sensitive sensor that couples to the

What are the amino acid sensors?

Which molecules act as the amino acid sensors and how do they contribute to the control of metabolism? The answers to these questions are complex because amino acid levels regulate growth and metabolism at multiple levels. These include the pathways involved in the control of amino acid synthesis, cellular amino acid uptake or release, and protein synthesis and breakdown, as well as the secretion of regulatory peptides into the blood and of digestive enzymes and fluids into the gut lumen.

Extracellular amino acid sensing by class 3 G-protein-coupled receptors

A group of extracellular amino acid sensors has recently been identified within class 3 of the G-protein-coupled receptor superfamily. These sensors control G-protein-regulated signalling pathways and are widely distributed. They seem to be designed for the detection of subgroups of amino acids or even specific amino acids including the acidic amino acid, l-glutamate (the metabotropic glutamate (mGlu) receptors). They also include several broad-spectrum amino acid sensors; the extracellular Ca2+

Amino acid activation of the calcium-sensing receptor

The cloned extracellular CaR is a class 3 GPCR that provides feedback control of extracellular calcium homeostasis by responding sensitively to acute fluctuations in extracellular ionized Ca2+ concentration. In humans, the physiological normal range for ionized Ca2+ concentration is 1.1–1.3 mM. The CaR is widely expressed and is active in tissues that are not directly involved in extracellular calcium homeostasis. It is resistant to desensitization. Analysis of the impact of CaR activators and

GPRC6A

GPRC6A was classified as an orphan class 3 GPCR until the recent demonstration that both the mouse and human orthologs are activated by amino acids 23, 24. GPRC6A shows the highest degree of amino acid sequence identity to the goldfish 5.24 chemosensory receptor (41%), which has a broad spectrum of amino acid sensing and an apparent preference for the basic amino acids l-Arg and l-Lys [22]. The 5.24 receptor is expressed in the olfactory epithelium and is thought to have a role in

Amino acid sensing by class 3 GPCR taste receptors

In addition to the four cardinal categories of taste - sweet, bitter, sour and salt - a fifth, more complex, protein-associated taste is referred to as umami. Umami is classically induced by millimolar concentrations of the sodium or potassium salts of l-glutamate and less potently by l-aspartate. An additional characteristic of umami is its potentiation by inosine- or guanosine 5′-monophosphates and broadening, under these circumstances, to include sensitivity to other amino acids. Recent

Venus Fly Trap domains as amino acid sensing modules in class 3

The VFT domains mediate amino acid binding in the class 3 GPCRs. These bilobed structural motifs have an ancient lineage providing the basis of nutrient sensing by the periplasmic binding proteins of Gram-negative bacteria [47]. The periplasmic space lies between the cytoplasmic membrane and the cell wall. Nutrients such as small sugars and amino acids cross the cell wall by free diffusion to enter the periplasmic space. In the periplasmic space, they bind to soluble nutrient-binding proteins

Molecular models of the amino acid binding sites in class 3 GPCRs

In all known cases, the amino acid binding site of class 3 GPCRs is located within the bilobed VFT domain. Approaches that have been used to analyze ligand binding include site-directed mutagenesis, molecular modeling using crystal structures of the mGlu1 VFT domain as the templates 49, 50 and, more recently, in silico ligand docking analysis [51]. The mGlu1 crystal structures indicate that as many as 12 residues in the binding pocket establish hydrogen bonds with the bound glutamate molecule.

Conclusions and future directions

Current research is actively pursuing the molecular identities and cellular mechanisms that underlie amino acid sensing and thus couple changes in protein ingestion and whole-body nitrogen metabolism to changes in growth and metabolic programming. GPCR class 3 includes the mGlu receptors and GABAB receptors that are highly selective for specific amino acids or amino acid analogs. It is now clear, however, that there is also a significant subgroup of class 3 GPCRs that are broad-spectrum l-amino

Acknowledgements

This work was supported by grants to A.D.C. from the National Health and Medical Research Council of Australia and to D.R.H. from the Canadian Institutes for Health Research and the Canadian Natural Sciences and Engineering Research Council.

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