The Mrj co-chaperone mediates keratin turnover and prevents the formation of toxic inclusion bodies in trophoblast cells of the placenta

Abstract

DEVELOPMENT 1809 DEVELOPMENT AND DISEASE RESEARCH ARTICLE INTRODUCTIONProtein aggregation and the formation of intracellular protein inclusion bodies are associated with many degenerative disorders, including Parkinson's and Huntington's diseases, and with cirrhosis of the liver (Carrell, 2005). In many cases, the role of protein aggregates in disease pathogenesis is unclear. Efforts to understand aggregation disorders have focused mainly on mutations within the disease-associated protein. However, mounting evidence suggests that a major cause of protein inclusion body formation is failure of the protein quality-control system (McClellan et al., 2005). This surveillance mechanism recruits both molecular chaperones and proteases to safeguard against protein aggregation by maintaining the equilibrium between protein folding and degradation (Hohfeld et al., 2001). A malfunction in the ubiquitin-proteasome degradation pathway is hypothesized to be one cause of Mallory body formation in the hepatocytes of patients with chronic liver disorders (Bardag-Gorce et al., 2003;Bardag-Gorce et al., 2004;Denk et al., 2000). Mallory bodies are inclusions composed of the keratin intermediate filaments keratin 8 (K8; also known as Krt8) and keratin 18 (K18; also known as Krt18), as well as ubiquitin, the proteasome complex and molecular chaperones, including heat shock protein 70 (HSP70; HSPA1B) and heat shock protein 90 (HSP90) (Coulombe and Omary, 2002). Cells that have been treated with a chemical inhibitor of proteasome function (Bardag-Gorce et al., 2004) or that contain the UBB +1 ubiquitin mutation (Bardag-Gorce et al., 2003) form Mallory bodies due to an accumulation of non-degraded keratin.Misfolded proteins also tend to aggregate. Proper protein folding within a cell often does not occur spontaneously and, thus, molecular chaperones are required for some proteins to reach its native state efficiently (Hartl and Hayer-Hartl, 2002). Hsp70 is a ubiquitous chaperone that, together with diverse co-chaperone binding partners, facilitates protein folding and the presentation of proteins to the proteasome for degradation (Esser et al., 2004). Co-chaperones of the DnaJ (Hsp40) protein family are characterized by a conserved Jdomain that regulates substrate binding and release by activating the ATPase activity of Hsp70 (Fan et al., 2003). These co-chaperones differ in their tissue and subcellular patterns of expression, and convey substrate specificity to Hsp70 (Fan et al., 2003). However, little is known about the precise substrates that are regulated in this manner.The Mrj (Mammalian relative of DnaJ; also known as Dnajb6 -Mouse Genome Informatics) gene encodes a co-chaperone that is widely expressed throughout the adult mouse and during development of the embryo and placenta (Chuang et al., 2002;Dai et al., 2005;Hunter et al., 1999;Izawa et al., 2000;Seki et al., 1999). Mrj -/-embryos die at mid-gestation due to a failure of chorioallantoic attachment during placental development (Hunter et al., 1999). Recently, a few diverse ...

Neural stem cell self‐renewal requires the Mrj co‐chaperone

“…However, we did not observe intermediate filament protein aggregates, similar to those seen in Mrj À/À trophoblast cells (Watson et al, 2007), in Mrj À/À neural tubes. Notably, other studies have shown that Mrj associates with neural disease-related protein aggregates, for example, polyglutamine-expanded Huntington's disease protein (Chuang et al, 2002;Fayazi et al, 2006) and Lewy bodies of Parkinson's disease (Durrenberger et al, 2009).…”

“…To determine whether the neural tube of Mrj À/À embryos remained open as a result of the placenta phenotype, we re-analyzed tetraploid aggregation data in which the Mrj chorioallantoic attachment defect was rescued (Watson et al, 2007). In these experiments, chimeric embryos were derived from aggregating tetraploid wild-type embryos with diploid embryos from Mrj þ/À intercrosses (Watson et al, 2007). Of the two chimeric Mrj þ/þ /!Mrj À/À conceptuses in which chorioallantoic attachment was rescued, both displayed exencephaly at E9.5 (data not shown), phenocopying Mrj À/À embryos without chorioallantoic attachment.…”

“…Bmi-1, a component of the Polycomb PRC1 complex that suppresses gene expression, is a critical regulator of stem cell self-renewal (Molofsky et al, 2003). Mrj, itself, has been implicated in transcriptional regulation (Dai et al, 2005;Hurst et al, 2006;Pan et al, 2008) and, at least in trophoblast cells, has a ''speckled'' nuclear expression pattern (Watson et al, 2007) reminiscent of Bmi-1 expression in other cell types (Stauffer et al, 2001). As a result, we hypothesized that Bmi-1 may be a substrate for Mrj.…”

“…2F) (Izawa et al, 2000). We previously determined that Mrj deficiency leads to the formation of toxic aggregates of K8/K18 filaments in trophoblast cells which prevents normal placental development (Watson et al, 2007). While keratin filaments are not expressed within neural cells (Thorey et al, 1993), other intermediate filament proteins are, such as nestin (neural stem cells) and neurofilaments (neurons; Dahlstrand et al, 1995;Lardelli et al, 1996).…”

“…Mrj À/À embryos were produced by mating Mrj þ/À mice. PCR genotyping was performed using DNA from yolk sac tissue as previously described (Watson et al, 2007). Experiments were performed in accordance with the Canadian Counsel on Animal Care and the University of Calgary Committee on Animal Care (Protocol no.…”