Department of Arctic and Marine Biology, UiT – The Arctic University of Norway, Tromsø, Norway
Faculty of Biology, Medicine and Health, School of Medical Sciences, University of Manchester, Manchester, UK
J Endocrinol. 2018 Oct 1;239(1):R13–R25. doi: 10.1530/JOE-18-0177.
Life in seasonally changing environments is challenging. Biological systems have to not only respond directly to the environment, but also schedule life history events in anticipation of seasonal changes. The cellular and molecular basis of how these events are scheduled is unknown. Cellular decision-making processes in response to signals above certain thresholds regularly occur i.e. cellular fate determination, apoptosis and firing of action potentials. Binary switches, the result of cellular decision-making processes, are defined as a change in phenotype between two stable states. A recent study presents evidence of a binary switch operating in the pars tuberalis (PT) of the pituitary, seemingly timing seasonal reproduction in sheep. Though, how a binary switch would allow for anticipation of seasonal environmental changes, not just direct responsiveness, is unclear. The purpose of this review is to assess the evidence for a binary switching mechanism timing seasonal reproduction and to hypothesize how a binary switch would allow biological processes to be timed over weeks to years. I draw parallels with mechanisms used in development, cell fate determination and seasonal timing in plants. I propose that the adult PT is a plastic tissue, showing a seasonal cycle of cellular differentiation, and that the underlying processes are likely to be epigenetic. Therefore, considering the mechanisms behind adult cellular plasticity offers a framework to hypothesize how a long-term timer functions within the PT.
生活在季节变化的环境中具有挑战性。生物系统不仅必须直接对环境做出反应，还必须预测季节变化来安排生活史事件。这些事件如何被安排的细胞和分子基础尚不清楚。细胞对超过一定阈值的信号做出反应的决策过程经常发生，例如细胞命运决定、细胞凋亡和动作电位的产生。二进制开关是细胞决策过程的结果，定义为两个稳定状态之间表型的变化。最近的一项研究提出了在垂体的垂体柄（PT）中存在二进制开关的证据，似乎在绵羊中定时季节性繁殖。然而，二进制开关如何允许对季节性环境变化进行预测，而不仅仅是直接响应，尚不清楚。本综述的目的是评估用于定时季节性繁殖的二进制开关机制的证据，并假设二进制开关如何允许生物过程在数周到数年的时间内定时。我将其与发育、细胞命运决定和植物季节性定时中使用的机制进行类比。我提出成年 PT 是一种具有可塑性的组织，表现出细胞分化的季节性循环，并且潜在的过程可能是表观遗传的。因此，考虑成年细胞可塑性背后的机制为假设 PT 内的长期定时器如何发挥作用提供了一个框架。
