G protein coupled receptor 132, also termed G2A, is classified as a member of the proton sensing G protein coupled receptor (GPR) subfamily. Like other members of this subfamily, i.e. GPR4, GPR68 (OGR1), and GPR65 (TDAG8), G2A is a G protein coupled receptor that resides in the cell surface membrane, senses changes in extracellular pH, and can alter cellular function as a consequence of these changes. Subsequently, G2A was suggested to be a receptor for lysophosphatidylcholine (LPC). However, the roles of G2A as a pH-sensor or LPC receptor are disputed. Rather, current studies suggest that it is a receptor for certain metabolites of the polyunsaturated fatty acid, linoleic acid.
The G2A gene G2A in humans is encoded by the GPR132 gene. The G2A gene is located on chromosome 14q32.3 codes for two alternative splice variants, the original one, G2A-a, and G2A-b, that consist of 380 and 371 amino acids, respectively; the two receptor variants, when expressed in Chinese hamster ovary cells, gave very similar results when analyzed for functionality. G2A-a and G2A-b mRNA are expressed at similar levels in blood leukocytes ( macrophages, dendritic cells, neutrophils [PMN], mast cells, T lymphocytes and B lymphocytes at the highest levels followed by lower levels in spleen, lung and heart tissues; both variants are expressed at similar levels, and are almost equally induced by DNA synthesis inhibitors (hydroxyurea and cytosine arabinoside) or a differentiation inducer (all-trans retinoic acid) in HL-60 human leukemic cells. The mouse G2A receptor, encoded by Gpr132, has 67% amino acid identity to human G2A but does not sense pH and does not respond to certain presumptive ligands (i.e. linoleic acid metabolites) that activate the human G2A.
G2A deficiency in mice Targeted disruption of G2A in mice causes the development of a late onset (> 1 year) slowly progressive wasting and autoimmune disease characterized by lymphoid organ enlargement, lymphocytic infiltration into various tissues, glomerular immune complex deposition, and anti-nuclear autoantibodies. Mice transplanted with bone marrow cells containing the BCR-ABL leukemia-inducing fusion gene but deficient in G2A exhibit expanded populations of leukemic cells compared to recipients of BCR-ABL-containing, G2A-sufficient bone marrow cells. BCR-ABL is the oncogene of the Philadelphia chromosome that causes human Chronic myelogenous leukemia and is sometimes found associated with human acute lymphocytic leukemia and acute myelocytic leukemia; furthermore, the forced expression of BCR-ABL in cultured rodent cells induces the expression of G2A and the overexpression of G2A inhibits the malignant growth to these cells. Thus, the G2A deficiency studies suggest that G2A functions in mice to suppress certain immune dysfunctions and BCR-ABL-related leukemic cell growth.
G2A function
pH sensor G2A was initially defined as one of the gene products whose production was stimulated in mouse pre-B lymphocytes (see Immunoglobulin heavy chain) by transfecting the cells with the human oncogene (i.e., cancer causing) BCR-ABL or by treating the cells with DNA damaging agents; its expression in these cells blocked their progression through the cell cycle specifically at the G2-M DNA damage checkpoint. These studies allow that G2A limits the potentially malignant growth of certain cells in mice and possible could do so in humans. In addition, Gene knockout studies in mice find G2A to be necessary for suppressing an autoimmune syndrome (see G2A deficiency in mice). These results allow that G2A may function in blocking certain aspects of autoimmunity, particularly those involving the proliferation and tissue trafficking of lymphocytes. Early studies first classified G2A as a proton-sensing receptor and suggested that G2A contributed to the regulation of proliferation in certain cells and the regulation of lymphocytes' contributions to certain immune functions by being activated by changes in extracellular pH. Tissues suffering malignant cell growth, autoimmune reactions, poor blood flow ischemia, inflammation and allergy reactions, and tissue injury develop extracellular acidification due to the stimulation of anaerobic glycolysis; The proton-sensing function of G2A could be involved in combating or, in certain cases promoting these conditions. An example implicating G2A's pH sensitivity in physiological responses involves pain perception. In rats, G2A, similar to other pH sensing GPCRs, is located in dorsal root ganglia neurons, small diameter neurons responsible for nociception, and other nerve tissues responsible for sensing pain; it is suggested that G2A in these nerve tissues detects the acid changes that occur in the extracellular media of injured tissues and signal for the perception of pain However, the activity of the human G2A receptor and its mouse homolog are significantly less sensitive to pH fluctuations than other pH sensing GPCRs; indeed, in studies of thymocytes and splenocytes taken from mice deficient in either the G2A or another pH-sensing GPCR, TDAG8, TDAG8 was found critical while G2A was found dispensable for sensing pH changes. Thus, the cited functions of G2A presumed due to its pH sensing ability could reflect other means for this receptor's activation.
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