Postnatal Changes in the Expression Pattern of the Imprinted Signalling Protein XLαs Underlie the Changing Phenotype of Deficient Mice

The alternatively spliced trimeric G-protein subunit XLαs, which is involved in cAMP signalling, is encoded by the Gnasxl transcript of the imprinted Gnas locus. XLαs deficient mice show neonatal feeding problems, leanness, inertia and a high mortality rate. Mutants that survive to weaning age develop into healthy and fertile adults, which remain lean despite elevated food intake. The adult metabolic phenotype can be attributed to increased energy expenditure, which appears to be caused by elevated sympathetic nervous system activity. To better understand the changing phenotype of Gnasxl deficient mice, we compared XLαs expression in neonatal versus adult tissues, analysed its co-localisation with neural markers and characterised changes in the nutrient-sensing mTOR1-S6K pathway in the hypothalamus. Using a newly generated conditional Gnasxl lacZ gene trap line and immunohistochemistry we identified various types of muscle, including smooth muscle cells of blood vessels, as the major peripheral sites of expression in neonates. Expression in all muscle tissues was silenced in adults. While Gnasxl expression in the central nervous system was also developmentally silenced in some midbrain nuclei, it was upregulated in the preoptic area, the medial amygdala, several hypothalamic nuclei (e.g. arcuate, dorsomedial, lateral and paraventricular nuclei) and the nucleus of the solitary tract. Furthermore, expression was detected in the ventral medulla as well as in motoneurons and a subset of sympathetic preganglionic neurons of the spinal cord. In the arcuate nucleus of Gnasxl-deficient mice we found reduced activity of the nutrient sensing mTOR1-S6K signalling pathway, which concurs with their metabolic status. The expression in these brain regions and the hypermetabolic phenotype of adult Gnasxl-deficient mice imply an inhibitory function of XLαs in energy expenditure and sympathetic outflow. By contrast, the neonatal phenotype of mutant mice appears to be due to a transient role of XLαs in muscle tissues.

参与cAMP信号通路的可变剪接三聚体G蛋白亚基XLαs，由印记基因Gnas位点的Gnasxl转录本编码。XLαs缺陷型小鼠会出现新生期摄食障碍、体瘦、活动迟滞及高死亡率。存活至断奶龄的突变体可发育为健康可育的成年个体，尽管摄食量升高仍保持体瘦状态。该成年代谢表型可归因于能量消耗增加，而这似乎由交感神经系统活性升高所介导。为更好地阐明Gnasxl缺陷型小鼠的表型变化特征，我们对比了XLαs在新生期与成年组织中的表达谱，分析了其与神经标志物的共定位情况，并对下丘脑中营养感知mTOR1-S6K信号通路的变化进行了表征。利用新构建的条件性Gnasxl lacZ基因捕获品系结合免疫组织化学技术，我们鉴定出新生期小鼠的主要外周表达位点为各类肌肉组织，包括血管平滑肌细胞。成年小鼠的所有肌肉组织中，Gnasxl的表达均已被沉默。中枢神经系统中的Gnasxl表达也在部分中脑核团中发生了发育性沉默，但在视前区、内侧杏仁核、数个下丘脑核团（如弓状核、背内侧核、外侧下丘脑核及室旁核）以及孤束核中出现了表达上调。此外，在延髓腹侧区、运动神经元以及脊髓内的部分交感节前神经元中也检测到了Gnasxl的表达。在Gnasxl缺陷型小鼠的弓状核中，我们发现营养感知mTOR1-S6K信号通路的活性降低，这与其代谢状态相符。上述脑区的表达特征以及成年Gnasxl缺陷型小鼠的高代谢表型提示，XLαs在能量消耗与交感传出活动中发挥抑制性功能。与之相反，突变体小鼠的新生期表型似乎源于XLαs在肌肉组织中的一过性作用。