G protein-coupled receptor 68 (GPR68) responds to extracellular protons, thus called the proton-sensing G protein-coupled receptor (GPCR), leading to activation of the phospholipase C-β (PLCβ)/calcium (Ca²⁺) pathway or the adenylyl cyclase (AC)/cyclic AMP (cAMP) pathway. We recently found that whole body deletion of Gpr68 (Gpr68^−/− mice) reduced the number of B lymphocytes with age and during hematopoietic regeneration, such as in response to fluorouracil (5-FU) administration. This prompted us to characterize the hematopoietic stem cell (HSC) phenotype in Gpr68^−/− mice. Despite high level of Gpr68 protein expression on HSC in bone marrow (BM), the pool size of HSC was unaltered in Gpr68^−/− mice either under steady state or upon stress, including aging and 5-FU treatment. HSC from Gpr68^−/− mice exhibited comparable cellular features, such as cell cycle quiescence and cell survival. HSC from Gpr68^−/− mice also exhibited comparable competitiveness after serial transplantation. Surprisingly, cytosolic Ca²⁺ accumulation was increased in HSC from Gpr68^−/− mice. In contrast, cAMP levels were reduced in hematopoietic stem and progenitor cells (HSPC) from Gpr68^−/− mice. Intriguingly, we found high level of Gpr68 protein expression on non-hematopoietic cells in BM, especially endothelial cells that function as HSC niche. In addition, expression of other proton-sensing GPCR was upregulated in HSPC from Gpr68^−/− mice. Our studies suggest that Gpr68^−/− mice display insignificant phenotype on HSC biology, possibly due to the function of Gpr68 in non-hematopoietic cells and/or the compensatory effects from other proton-sensing GPCR.

G 蛋白偶联受体 68 (GPR68) 响应细胞外质子，因此称为质子感应 G 蛋白偶联受体 (GPCR)，导致磷脂酶 C-β (PLCβ)/钙 (Ca ²⁺ ) 通路的激活或腺苷酸环化酶 (AC)/环 AMP (cAMP) 途径。我们最近发现 Gpr68 的全身缺失（Gpr68 ^-/-小鼠）会随着年龄和造血再生过程中的 B 淋巴细胞数量减少，例如对氟尿嘧啶 (5-FU) 给药的反应。这促使我们表征 Gpr68 ^-/-小鼠中的造血干细胞 (HSC) 表型。尽管骨髓 (BM) 中 HSC 上的 Gpr68 蛋白表达水平高，但 Gpr68 中 HSC 的池大小未改变^-/-小鼠处于稳态或处于应激状态，包括衰老和 5-FU 治疗。来自 Gpr68 ^-/-小鼠的HSC表现出类似的细胞特征，例如细胞周期静止和细胞存活。来自 Gpr68 ^-/-小鼠的HSC在连续移植后也表现出相当的竞争力。令人惊讶的是，来自 Gpr68 ^-/-小鼠的HSC 中的胞质 Ca ²⁺积累增加。相反，来自 Gpr68 ^{-/- 的}造血干细胞和祖细胞 (HSPC) 中的 cAMP 水平降低老鼠。有趣的是，我们在 BM 的非造血细胞上发现了高水平的 Gpr68 蛋白表达，尤其是作为 HSC 生态位的内皮细胞。此外，其他质子感应 GPCR 的表达在来自 Gpr68 ^-/-小鼠的HSPC 中上调。我们的研究表明 Gpr68 ^-/-小鼠在 HSC 生物学上表现出微不足道的表型，这可能是由于 Gpr68 在非造血细胞中的功能和/或其他质子感应 GPCR 的代偿作用。