Calcineurin and NFAT4 induce chondrogenesis

Nuclear factor of activated T-cells (NFAT) and calcineurin are essential regulators of immune cell and mesenchymal cell differentiation. Here we show that elevated intracellular calcium induces chondrogenesis through a calcineurin/NFAT signaling axis that activates bone morphogenetic protein (BMP) expression. The calcium ionophore, ionomycin, induced chondrogenesis through activation of calcineurin. The calcineurin substrate, NFAT4, also induced chondrogenesis and chondrocyte gene expression. Significantly, the BMP antagonist, noggin, or dominant negative BMP receptors blocked the effects of elevated intracellular calcium on chondrogenesis. This suggested that calcineurin/NFAT4 activates BMP expression. Consistent with this, BMP2 gene expression was increased by ionomycin and suppressed by the calcineurin inhibitor, cyclosporine A. Furthermore, activated NFAT4 induced BMP2 gene expression. These results have important implications for the effects of NFATs during development and adaptive responses.     

Osmotic stabilizer-coupled suppression of NDR defects is dependent on the calcium-calcineurin signaling cascade in Aspergillus nidulans

Establishment and maintenance of cell polarity are coordinated by signaling pathways such as NDR (nuclear Dbf2-related) protein-kinase signaling and calcium signaling pathway. The NDR family of kinase is structurally related to the human myotonic dystrophy kinase, which, when impaired, confers a disease that involves changes in cytoarchitecture and ion homeostasis. CotA kinase, a member of the NDR protein kinase family, forms a complex with MobB to regulate cell polarized growth in Aspergillus nidulans. Our previous study demonstrated that mobB/cotA defects could be suppressed by the osmotic stress in the presence of external calcium. In this study, via the genetic and molecular approach, we further demonstrated that Ca²⁺-permeable stretch-activated nonselective cation channel-MidA, calcium/calmodulin-dependent protein phosphatase catalatic subunit-CnaA and external calcium, but not voltage-gated calcium channel homolog-CchA, were required for the osmotic stabilizer-coupled suppression. The up-regulation of calcium/calcineurin signaling pathway induced by osmotic stress might be the reason for bypassing the requirements of NDR kinase complex, which is otherwise necessary for polar morphogenesis. Our results suggest that calcium-calcineurin signaling pathway coordinates with MobB/CotA kinase complex in regulating polarity growth via maintaining cellular calcium homeostasis. However, CchA may act differently as other components in calcium signaling pathway in Aspergillus nidulans. These findings provide an excellent opportunity to identify the potential pathway linking NDR protein-kinase network to calcium signaling pathway.     

Stretch-modulation of second messengers: effects on cardiomyocyte ion transport

In cardiomyocytes, mechanical stress induces a variety of hypertrophic responses including an increase in protein synthesis and a reprogramming of gene expression. Recently, the calcium signaling has been reported to play an important role in the development of cardiac hypertrophy. In this article, we report on the role of the calcium signaling in stretch-induced gene expression in cardiomyocytes. Stretching of cultured cardiomyocytes up-regulates the expression of brain natriuretic peptide (BNP). Intracellular calcium-elevating agents such as the calcium ionophore A23187, the calcium channel agonist BayK8644 and the sarcoplasmic reticulum calcium-ATPase inhibitor thapsigargin up-regulate BNP gene expression. Conversely, stretch-induced BNP gene expression is suppressed by EGTA, stretch-activated ion channel inhibitors, voltage-dependent calcium channel antagonists, and long-time exposure to thapsigargin. Furthermore, stretch increases the activity of calcium-dependent effectors such as calcineurin and calmodulin-dependent kinase II, and inhibitors of calcineurin and calmodulin-dependent kinase II significantly attenuated stretch-induced hypertrophy and BNP expression. These results suggest that calcineurin and calmodulin-dependent kinase II are activated by calcium influx and subsequent calcium-induced calcium release, and play an important role in stretch-induced gene expression during the development of cardiac hypertrophy.     

IGF-1 increases laminin, cyclin D1, and p21Cip1 expression in glomerular mesangial cells: an investigation of the intracellular signaling pathway and cell-cycle progression

Insulin-like growth factor (IGF)-1 is accumulated in the diabetic kidney and is considered to be involved in the development of glomerular sclerosis. Here, we investigate IGF-1 regulation of laminin, an extracellular matrix (ECM) component, and cyclin D1 and p21Cip1, cell-cycle progression factor, expressions in glomerular mesangial cells. We show that IGF-1 increases the level of laminin gamma1 and beta1 subunits approximately 1.5- and 2.5-fold, respectively, in a time-dependent manner. IGF-1 also stimulates protein kinase Akt/PKB phosphorylation at Thr 308, which correlates with its activity, up to 24 h. The Akt activation is coupled with Ser 9 phosphorylation of its downstream target, glycogen synthase kinase-3beta (GSK-3beta), which inhibits its kinase activity. Laminin beta1 is reduced significantly (P < 0.03) by inhibitors of Akt and p38MAPK whereas laminin gamma1 is not affected. Surprisingly, IGF-1 activates the expression of both cyclin D1 and cell-cycle arrest factor, p21Cip1 parallely. Pharmacological inhibition of calcineurin by cyclosporin A blocks IGF-1-induced cyclin D1 and p21Cip1expression significantly (P < 0.05). IGF-1 enhances cellular metabolic activity and viability of rat mesangial cells; however, they are arrested at the G1 phase of cell cycle as revealed by the FACS analysis. These results indicate that IGF-1 mediates mesangial cell-cycle progression, hypertrophy, and ECM protein synthesis. The Akt/GSK-3beta, p38MAPK, and calcineurin pathways may play an important role in IGF-1 signaling, cell-cycle regulation, and matrix gene expression in mesangial cells leading to the development of diabetic glomerulopathy.