Murtuza Mohammad I, Isokawa Masako
Department of Health and Biomedical Sciences, University of Texas Rio Grande Valley, Brownsville, Texas, USA.
J Neurochem. 2018 Jan;144(1):58-67. doi: 10.1111/jnc.14244. Epub 2017 Nov 20.
Ghrelin is an appetite-stimulating peptide. Serine 3 on ghrelin must be acylated by octanoate via the enzyme ghrelin-O-acyltransferase (GOAT) for the peptide to bind and activate the cognate receptor, growth hormone secretagogue receptor type 1a (GHSR1a). Interest in GHSR1a increased dramatically when GHSR1a mRNA was demonstrated to be widespread in the brain, including the cortex and hippocampus, indicating that it has multifaceted functions beyond the regulation of metabolism. However, the source of octanoylated ghrelin for GHSR1a in the brain, outside of the hypothalamus, is not well understood. Here, we report the presence of GOAT and its ability to acylate non-octanoylated ghrelin in the hippocampus. GOAT immunoreactivity is aggregated at the base of the dentate granule cell layer in the rat and wild-type mouse. This immunoreactivity was not affected by the pharmacological inhibition of GHSR1a or the metabolic state-dependent fluctuation of systemic ghrelin levels. However, it was absent in the GHSR1a knockout mouse hippocampus, pointing the possibility that the expression of GHSR1a may be a prerequisite for the production of GOAT. Application of fluorescein isothiocyanate (FITC)-conjugated non-octanoylated ghrelin in live hippocampal slice culture (but not in fixed culture or in the presence of GOAT inhibitors) mimicked the binding profile of FITC-conjugated octanoylated ghrelin, suggesting that extracellularly applied non-octanoylated ghrelin was acylated by endogenous GOAT in the live hippocampus while GOAT being mobilized out of neurons. Our results will advance the understanding for the role of endogenous GOAT in the hippocampus and facilitate the search for the source of ghrelin that is intrinsic to the brain.
胃饥饿素是一种刺激食欲的肽。胃饥饿素上的丝氨酸3必须通过胃饥饿素 - O - 酰基转移酶（GOAT）被辛酸酰化，该肽才能结合并激活同源受体，即1a型生长激素促分泌素受体（GHSR1a）。当GHSR1a mRNA被证明广泛存在于大脑中，包括皮质和海马体时，人们对GHSR1a的兴趣急剧增加，这表明它在代谢调节之外具有多方面的功能。然而，下丘脑以外的大脑中GHSR1a的辛酰化胃饥饿素的来源尚不清楚。在这里，我们报告了GOAT在海马体中的存在及其酰化非辛酰化胃饥饿素的能力。在大鼠和野生型小鼠中，GOAT免疫反应性聚集在齿状颗粒细胞层的基部。这种免疫反应性不受GHSR1a的药理学抑制或全身胃饥饿素水平的代谢状态依赖性波动的影响。然而，在GHSR1a基因敲除小鼠的海马体中不存在这种免疫反应性，这表明GHSR1a的表达可能是GOAT产生的先决条件。在活的海马切片培养物中（但不在固定培养物中或在存在GOAT抑制剂的情况下）应用异硫氰酸荧光素（FITC）偶联的非辛酰化胃饥饿素模仿了FITC偶联的辛酰化胃饥饿素的结合模式，这表明在活的海马体中，细胞外应用的非辛酰化胃饥饿素被内源性GOAT酰化，同时GOAT从神经元中被动员出来。我们的结果将推进对海马体内源性GOAT作用的理解，并有助于寻找大脑固有的胃饥饿素来源。
