Calreticulin is a highly conserved, ubiquitous Ca2+-buffering protein in the endoplasmic reticulum that controls transcriptional activity of various developmental programs and also of embryonic stem cell (ESC) differentiation. Calreticulin activates calcineurin, which dephosphorylates and induces the nuclear import of the osteogenic transcription regulator nuclear factor of activated T cells 1 (NFATC1). We investigated whether calreticulin controls a switch between osteogenesis and chondrogenesis in mouse ESCs through NFATC1. We found that in the absence of calreticulin, intranuclear transport of NFATC1 is blocked and that differentiation switches from osteogenic to chondrogenic, a process that could be mimicked by chemical inhibition of NFAT translocation. Glycogen synthase kinase 3β (GSK3β) deactivation and nuclear localization of β-catenin critical to osteogenesis were abrogated by calreticulin deficiency or NFAT blockade. Chemically induced GSK3β inhibition bypassed the calreticulin/calcineurin axis and increased osteoblast output from both control and calreticulin-deficient ESCs, while suppressing chondrogenesis. Calreticulin-deficient ESCs or cells treated with an NFAT blocker had enhanced expression of dickkopf WNT-signaling pathway inhibitor 1 (Dkk1), a canonical Wnt pathway antagonist that blocks GSK3β deactivation. The addition of recombinant mDKK1 switched osteogenic ESC differentiation toward chondrogenic differentiation. The results of our study indicate a role for endoplasmic reticulum calcium signaling via calreticulin in the differentiation of ESCs to closely associated osteoblast or chondrocyte lineages.

中文翻译：

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钙网蛋白调节成骨细胞和源自鼠胚胎干细胞的软骨细胞谱系之间的转换。

钙网蛋白是内质网中高度保守的，普遍存在的Ca2 +缓冲蛋白，可控制各种发育程序以及胚胎干细胞（ESC）分化的转录活性。钙网蛋白激活钙调神经磷酸酶，钙磷酸酶磷酸化并诱导活化T细胞1（NFATC1）的成骨转录调节因子核因子的核输入。我们调查了钙网蛋白是否通过NFATC1控制小鼠ESC中成骨和软骨形成之间的转换。我们发现，在没有钙网蛋白的情况下，NFATC1的核内运输受到阻滞，并且分化从成骨性转变为成软骨性，这一过程可以通过化学抑制NFAT转运来模仿。钙网蛋白缺乏或NFAT阻断废除了糖原合酶激酶3β（GSK3β）的失活和对成骨至关重要的β-catenin的核定位。化学诱导的GSK3β抑制作用绕过了钙网蛋白/钙调神经磷酸酶轴，并增加了来自对照和钙网蛋白缺陷的ESC的成骨细胞输出，同时抑制了软骨形成。钙网蛋白缺陷的ESC或经NFAT阻断剂处理的细胞具有增强的Dckkopf WNT信号通路抑制剂1（Dkk1）的表达，Dkk1是阻断GSK3β失活的典型Wnt通路拮抗剂。重组mDKK1的添加将成骨性ESC分化转变为软骨分化。