Tanycytes As Regulators of Seasonal Cycles in Neuroendocrine Function.
Tanycytes As Regulators of Seasonal Cycles in Neuroendocrine Function.
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DOI：
10.3389/fneur.2017.00079
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发表时间：
2017
影响因子：
3.4
通讯作者：
Ebling FJ
中科院分区：
文献类型：
--
作者：
Lewis JE;Ebling FJ
生理和行为的年周期在温带和极地的生物中非常普遍。哺乳动物的例子包括食欲和身体脂肪组成的变化，冬眠和麻木，鹿角，皮毛和角的生长以及季节性繁殖。这些季节性周期的时间反映了变化的环境信号（如白昼长度）和内在节律过程（年周期时钟）的相互作用。由于神经内分泌信号是这些节律过程的基础，大多数机制研究的焦点都集中在下丘脑的神经系统上。最近的研究也暗示垂体柄（结节部）和下丘脑伸长细胞作为关键途径的季节性时间。结节部表达高密度的褪黑激素受体，因此对松果体夜间分泌的褪黑激素的变化高度敏感，因为全年的光周期变化。结节部又通过旁分泌信号调节邻近下丘脑中的伸展细胞功能。伸展细胞是放射状胶质细胞，持续到成年期，并作为干细胞龛发挥作用。它们的细胞索马包埋在第三脑室的室管膜内层，它们也通过弓状核发出精细的投射，其中许多终止于正中隆起的毛细血管。这种解剖结构是它们作为循环中营养物传感器以及作为激素和代谢物运输到下丘脑的调节器的功能的基础。原位杂交研究揭示了伸展细胞中基因表达的强烈季节性变化，例如，那些控制甲状腺激素和视黄酸的运输和代谢的基因。这些激素信号在大脑的初始发育中起着关键作用，对成年下丘脑中甲状腺激素可用性的实验操作可以加速或阻断绵羊和西伯利亚仓鼠的季节性周期。我们假设季节性节律依赖于成年人下丘脑的发育机制的重复使用，而伸长细胞是这些过程的关键。
Annual cycles of physiology and behavior are highly prevalent in organisms inhabiting temperate and polar regions. Examples in mammals include changes in appetite and body fat composition, hibernation and torpor, growth of antlers, pelage and horns, and seasonal reproduction. The timing of these seasonal cycles reflects an interaction of changing environmental signals, such as daylength, and intrinsic rhythmic processes: circannual clocks. As neuroendocrine signals underlie these rhythmic processes, the focus of most mechanistic studies has been on neuronal systems in the hypothalamus. Recent studies also implicate the pituitary stalk (pars tuberalis) and hypothalamic tanycytes as key pathways in seasonal timing. The pars tuberalis expresses a high density of melatonin receptors, so is highly responsive to changes in the nocturnal secretion of melatonin from the pineal gland as photoperiod changes across the year. The pars tuberalis in turn regulates tanycyte function in the adjacent hypothalamus via paracrine signals. Tanycytes are radial glial cells that persist into adulthood and function as a stem cell niche. Their cell soma are embedded in the ependymal lining of the third ventricle, and they also send elaborate projections through the arcuate nucleus, many of which terminate on capillaries in the median eminence. This anatomy underlies their function as sensors of nutrients in the circulation, and as regulators of transport of hormones and metabolites into the hypothalamus. In situ hybridization studies reveal robust seasonal changes in gene expression in tanycytes, for example, those controlling transport and metabolism of thyroid hormone and retinoic acid. These hormonal signals play a key role in the initial development of the brain, and experimental manipulation of thyroid hormone availability in the adult hypothalamus can accelerate or block seasonal cyclicity in sheep and Siberian hamsters. We hypothesize that seasonal rhythms depends upon reuse of developmental mechanisms in the adult hypothalamus and that tanycytes are key orchestrators of these processes.