Protein phosphatase 1 regulatory subunit 18 suppresses the transcriptional activity of NFATc1 via regulation of c-fos

The transcription factor NFATc1 and its binding partner AP-1 (a complex containing c-fos and c-Jun) play a central role in osteoclast differentiation. NFATc1 and AP-1 promote the expression of target genes such as Acp5, Ctsk and also auto-regulate NFATc1 expression as well. We previously reported that protein phosphatase 1 regulatory subunit 18 (PPP1r18) is a negative regulator of osteoclast bone resorption by inhibiting cell attachment to bone matrix. We also reported that PPP1r18 potentially regulates NFATc1 expression during osteoclast differentiation. To further explore this, in this study we have examined the effect of PPP1r18 on NFATc1 expression and activity by overexpressing PPP1r18 during the early stage of osteoclast differentiation. We found that PPP1r18 suppressed NFATc1 expression through inhibition of the transcriptional activity of NFATc1. Since PPP1r18 does not regulate NFATc1 directly, we next explored the involvement of AP-1. Our data showed that c-fos phosphorylation and nuclear localization were reduced by PPP1r18 overexpression. Further experiments showed that overexpression of c-fos together with PPP1r18 rescued NFATc1 expression and transcriptional activity. Moreover, c-fos activity inhibition by PPP1r18 was canceled by mutation of the phosphatase binding site of PPP1r18. Taken together, PPP1r18-regulated phosphatase activity targets c-fos phosphorylation and suppresses subsequent NFATc1 expression and activity.

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PPP1r18 suppresses osteoclast differentiation.

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PPP1r18 suppresses c-fos phosphorylation and nuclear localization.

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PPP1r18 suppresses NFAT via c-fos.

Endoplasmic reticulum stress and eIF2α phosphorylation: The Achilles heel of pancreatic β cells

BACKGROUND

Pancreatic β cell dysfunction and death are central in the pathogenesis of most if not all forms of diabetes. Understanding the molecular mechanisms underlying β cell failure is important to develop β cell protective approaches.

SCOPE OF REVIEW

Here we review the role of endoplasmic reticulum stress and dysregulated endoplasmic reticulum stress signaling in β cell failure in monogenic and polygenic forms of diabetes. There is substantial evidence for the presence of endoplasmic reticulum stress in β cells in type 1 and type 2 diabetes. Direct evidence for the importance of this stress response is provided by an increasing number of monogenic forms of diabetes. In particular, mutations in the PERK branch of the unfolded protein response provide insight into its importance for human β cell function and survival. The knowledge gained from different rodent models is reviewed. More disease- and patient-relevant models, using human induced pluripotent stem cells differentiated into β cells, will further advance our understanding of pathogenic mechanisms. Finally, we review the therapeutic modulation of endoplasmic reticulum stress and signaling in β cells.

MAJOR CONCLUSIONS

Pancreatic β cells are sensitive to excessive endoplasmic reticulum stress and dysregulated eIF2α phosphorylation, as indicated by transcriptome data, monogenic forms of diabetes and pharmacological studies. This should be taken into consideration when devising new therapeutic approaches for diabetes.

Incorporation of a tag helps to overcome expression variability in a recombinant host

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Reason for the lack of recombinant protein expression in E. coli is indefinite.

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Recombinant histone expression does not correlate with rare codon content.

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Translational variability may lead to lack of expression or degradation of protein.

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Expression variability could be averted by incorporating a tag.

Epigenetics have witnessed a renewed interest over the past decade and assays with recombinant histones has become an important tool for uncovering various aspects of histone biology. However, at times absence of recombinant histone accumulation in bacteria is encountered which is also commonly observed for many eukaryotic proteins in general. In this study, we have investigated the effect of multiple parameters on heterologous expression of proteins. We show that there is marked variability in the accumulation of H2A.2, H2B.1, H3.2 and H4 in the recombinant host, possibly owing to translational variability and degradation by the host proteases. We found that the variability could be overcome by incorporation of the commonly used purification tags, like GST or MBP, of appropriate size and position. Our results provide compelling evidence that transcript parameters like rare codon and GC content, mRNA secondary structure etc. together modulate translation kinetics and govern recombinant protein accumulation.