Mammalian tissue size is maintained by slow replacement of de-differentiating and dying cells. For adipocytes, key regulators of glucose and lipid metabolism, the renewal rate is only 10% per year. We used computational modeling, quantitative mass spectrometry, and single-cell microscopy to show that cell-to-cell variability, or noise, in protein abundance acts within a network of more than six positive feedbacks to permit pre-adipocytes to differentiate at very low rates. This reconciles two fundamental opposing requirements: High cell-to-cell signal variability is needed to generate very low differentiation rates, whereas low signal variability is needed to prevent differentiated cells from de-differentiating. Higher eukaryotes can thus control low rates of near irreversible cell fate decisions through a balancing act between noise and ultrahigh feedback connectivity.

Abstract-Our objective in work presented here was to understand the mechanisms by which activated p38␣ MAPK depresses myocardial contractility. To test the hypothesis that activation of p38 MAPK directly influences sarcomeric function, we used transgenic mouse models with hearts in which p38 MAPK was constitutively turned on by an upstream activator (MKK6bE). These hearts demonstrated a significant depression in ejection fraction after induction of the transgene. We also studied hearts of mice expressing a dominant negative p38␣ MAPK. Simultaneous determination of tension and ATPase activity of detergent-skinned fiber bundles from left ventricular papillary muscle demonstrated a significant inhibition of both maximum tension and ATPase activity in the transgenic-MKK6bE hearts.Fibers from hearts expressing dominant negative p38␣ MAPK demonstrated no significant change in tension or ATPase activity. There were no significant changes in phosphorylation level of troponin-T3 and troponin-T4, or myosin light chain 2. However, compared with controls, there was a significant depression in levels of phosphorylation of ␣-tropomyosin and troponin I in fiber bundles from transgenic-MKK6bE hearts, but not from dominant negative p38␣ MAPK hearts. Our experiments also showed that p38␣ MAPK colocalizes with ␣-actinin at the Z-disc and complexes with protein phosphatases (PP2␣, PP2␤). These data are the first to indicate that chronic activation of p38␣ MAPK directly depresses sarcomeric function in association with decreased phosphorylation of ␣-tropomyosin. (Circ Res. 2007;100:408-415.)Key Words: heart failure Ⅲ myofilaments Ⅲ protein phosphatase Ⅲ tropomyosin kinase T he response of the myocardium to stressors that signal hypertrophy, dilation, and failure represents integrated effects of pathways regulating myocyte growth, death, and contractility. 1 A potentially important signaling cascade involves the mitogen activated protein kinase, p38 MAPK. The p38 MAPKs are activated by G-protein coupled receptors, as well as chemical, physical, osmotic, and radiation stresses. 2,3 In vivo studies demonstrated that p38 MAPK activity is increased during hypertrophy induced by pressure overload, and is potentially important in transduction of mechanical signals. 3 Further evidence for p38 MAPK as a stress mediating factor has come from the observation of induction of apoptosis, hypertrophy, and the hypertrophic marker ANF in neonatal cardiac myocytes over-expressing MKK6bE and MKK3bE, the upstream activators of p38 MAPKs. 4,5,6 Activation of p38 MAPK activity in intact hearts also resulted in early lethal cardiomyopathy characterized by impaired contractility. 7 Acute activation of p38 MAPK with arsenite has also been demonstrated to depress tension generated in detergent extracted cardiac myocytes. 8 Moreover, prolonged activation of p38 MAPK activity is also associated with a negative inotropic effect that occurs without alterations in cellular Ca 2ϩ fluxes. 9 Although these studies suggest a general linkage between p38 MAPK activa...

Rationale: p38 is an important stress activated protein kinase involved in gene regulation, proliferation, differentiation, and cell death regulation in heart. p38 kinase activity can be induced through canonical pathway via upstream kinases or by noncanonical autophosphorylation. The intracellular p38 kinase activity is tightly regulated and maintained at low level under basal condition. The underlying regulatory mechanism for canonical p38 kinase activation is well-studied, but the regulation of noncanonical p38 autophosphorylation remains poorly understood. Objective: We investigated the molecular basis for the regulation of noncanonical p38 autophosphorylation and its potential functional impact in cardiomyocytes. Methods and Results: Using both proteomic and biochemical tools, we established that heat shock protein (Hsp)90-Cdc37 chaperones are part of the p38␣ signaling complex in mammalian cells both in vitro and in vivo. The Hsp90-Cdc37 chaperone complex interacts with p38 via direct binding between p38 and Cdc37. Cdc37 expression is both sufficient and necessary to suppress noncanonical p38 activation via autophosphorylation at either basal state or under TAB1 (TAK1 binding protein-1) induction. In contrast, Cdc37 expression has no impact on p38 activation by canonical upstream kinase MKK3 or oxidative stress. Furthermore, Hsp90 inhibition results in p38 activation via autophosphorylation, and p38 activity contribute to apoptotic cell death induced by Hsp90 inhibition. Conclusion: Our study has revealed a so far uncharacterized function of Hsp90-Cdc37 as an endogenous regulator of noncanonical p38 activity. (Circ Res. 2010;106:1404-1412.)Key Words: p38 mitogen-activated protein kinase Ⅲ noncanonical pathway Ⅲ Hsp90 Ⅲ Cdc37 Ⅲ autophosphorylation O xidative stress, proinflammatory cytokines and many other extracellular stimuli activate intracellular stress responses through stress signaling pathways, including SAPK (stress-activated protein kinase) p38 as a member of the mitogen-activated protein kinase (MAPK) superfamily. [1][2][3][4][5] Although the role of p38 in heart remains controversial, activation of p38 in response to ischemia/reperfusion has been demonstrated repetitively. Several findings suggest p38␣ contributes to myocyte cell death by promoting production of proinflammatory cytokines including tumor necrosis factor (TNF)␣, interleukin (IL)-1, and IL-8. This is also supported by the protective effect of p38-specific inhibitor SB203580 in ischemia. On the other hand, protective effect of p38 through activation of heat shock protein (Hsp)27 has also been suggested. There are 2 modes of p38 activation in ischemia. One is the canonical p38 pathway, which is activated by a cascade of phosphorylation through upstream MEKKs (MAPK/extracellular-activated kinase kinases) such as Ask1, 6 and MAPK kinase (MKK)3, -4, or -6, leading to phosphorylation of the TGY motif within the p38 activation loop. 3,4,7 In addition, noncanonical MKK-independent activation of p38 has been demonstrated through intrinsic au...