Gheyas Rifah, Menko A Sue
Department of Pathology and Genomic Medicine, Sidney Kimmel Medical College, Thomas Jefferson University, Philadelphia, PA, US.
Department of Ophthalmology, Sidney Kimmel Medical College, Thomas Jefferson University, Philadelphia, PA, US.
Cell Death Discov. 2023 Oct 21;9(1):386. doi: 10.1038/s41420-023-01680-y.
The terminal differentiation of lens fiber cells involves elimination of their organelles, which must occur while still maintaining their functionality throughout a lifetime. Removal of non-nuclear organelles is accomplished through induction of autophagy following the spatiotemporal suppression of the PI3K/Akt signaling axis. However, blocking this pathway is not alone sufficient to induce removal of fiber cell nuclei. While the final steps in fiber cell nuclear elimination are highlighted by the appearance of TUNEL-positive nuclei, which are associated with activation of the lens-specific DNaseIIβ, there are many steps in the process that precede the appearance of double stranded DNA breaks. We showed that this carefully regulated process, including the early changes in nuclear morphology resulting in nuclear condensation, cleavage of lamin B, and labeling by pH2AX, is reminiscent of the apoptotic process associated with caspase activation. Multiple caspases are known to be expressed and activated during lens cell differentiation. In this study, we investigated the link between two caspase downstream targets associated with apoptosis, ICAD, whose cleavage by caspase-3 leads to activation of CAD, a DNase that can create both single- and double-stranded DNA cleavages, and lamin B, a primary component of the nuclear lamina. We discovered that the specific inhibition of caspase-3 activation prevents both lamin B and DNA cleavage. Inhibiting caspase-3 did not prevent nuclear condensation or removal of the nuclear membrane. In contrast, a pan-caspase inhibitor effectively suppressed condensation of fiber cell nuclei during differentiation. These studies provide evidence that caspases play an important role in the process of removing fiber cell nuclei during lens differentiation.
晶状体纤维细胞的终末分化涉及细胞器的清除，这一过程必须在其终身维持功能的同时发生。非核细胞器的清除是通过PI3K/Akt信号轴的时空抑制后诱导自噬来完成的。然而，阻断这一途径并不足以单独诱导纤维细胞核的清除。虽然纤维细胞核清除的最后步骤以TUNEL阳性细胞核的出现为特征，这与晶状体特异性DNaseIIβ的激活有关，但在双链DNA断裂出现之前，该过程有许多步骤。我们表明，这个精心调控的过程，包括导致核浓缩的核形态早期变化、核纤层蛋白B的裂解以及pH2AX标记，让人联想到与半胱天冬酶激活相关的凋亡过程。已知多种半胱天冬酶在晶状体细胞分化过程中表达并被激活。在本研究中，我们研究了与凋亡相关的两个半胱天冬酶下游靶点之间的联系，即ICAD，其被半胱天冬酶-3裂解会导致CAD激活，CAD是一种能产生单链和双链DNA裂解的脱氧核糖核酸酶，以及核纤层的主要成分核纤层蛋白B。我们发现，特异性抑制半胱天冬酶-3的激活可防止核纤层蛋白B和DNA的裂解。抑制半胱天冬酶-3并不能阻止核浓缩或核膜的去除。相反，一种泛半胱天冬酶抑制剂有效地抑制了分化过程中纤维细胞核的浓缩。这些研究提供了证据，表明半胱天冬酶在晶状体分化过程中纤维细胞核的清除过程中起重要作用。
