Biallelic variants in WARS2 encoding mitochondrial tryptophanyl-tRNA synthase in six individuals with mitochondrial encephalopathy.

Mitochondrial protein synthesis involves an intricate interplay between mitochondrial DNA encoded RNAs and nuclear DNA encoded proteins, such as ribosomal proteins and aminoacyl-tRNA synthases. Eukaryotic cells contain 17 mitochondria-specific aminoacyl-tRNA synthases. WARS2 encodes mitochondrial tryptophanyl-tRNA synthase (mtTrpRS), a homodimeric class Ic enzyme (mitochondrial tryptophan-tRNA ligase; EC 6.1.1.2). Here, we report six individuals from five families presenting with either severe neonatal onset lactic acidosis, encephalomyopathy and early death or a later onset, more attenuated course of disease with predominating intellectual disability. Respiratory chain enzymes were usually normal in muscle and fibroblasts, while a severe combined respiratory chain deficiency was found in the liver of a severely affected individual. Exome sequencing revealed rare biallelic variants in WARS2 in all affected individuals. An increase of uncharged mitochondrial tRNATrp and a decrease of mtTrpRS protein content were found in fibroblasts of affected individuals. We hereby define the clinical, neuroradiological, and metabolic phenotype of WARS2 defects. This confidently implicates that mutations in WARS2 cause mitochondrial disease with a broad spectrum of clinical presentation.

Viperin mRNA is a novel target for the human RNase MRP/RNase P endoribonuclease.

RNase MRP is a conserved endoribonuclease, in humans consisting of a 267-nucleotide RNA associated with 7-10 proteins. Mutations in its RNA component lead to several autosomal recessive skeletal dysplasias, including cartilage-hair hypoplasia (CHH). Because the known substrates of mammalian RNase MRP, pre-ribosomal RNA, and RNA involved in mitochondrial DNA replication are not likely involved in CHH, we analyzed the effects of RNase MRP (and the structurally related RNase P) depletion on mRNAs using DNA microarrays. We confirmed the upregulation of the interferon-inducible viperin mRNA by RNAi experiments and this appeared to be independent of the interferon response. We detected two cleavage sites for RNase MRP/RNase P in the coding sequence of viperin mRNA. This is the first study providing direct evidence for the cleavage of a mRNA by RNase MRP/RNase P in human cells. Implications for the involvement in the pathophysiology of CHH are discussed.

Co-chaperones are limiting in a depleted chaperone network.

To probe the limiting nodes in the chaperoning network which maintains cellular proteostasis, we expressed a dominant negative mutant of heat shock factor 1 (dnHSF1), the regulator of the cytoplasmic proteotoxic stress response. Microarray analysis of non-stressed dnHSF1 cells showed a two- or more fold decrease in the transcript level of 10 genes, amongst which are the (co-)chaperone genes HSP90AA1, HSPA6, DNAJB1 and HSPB1. Glucocorticoid signaling, which requires the Hsp70 and the Hsp90 folding machines, was severely impaired by dnHSF1, but fully rescued by expression of DNAJA1 or DNAJB1, and partially by ST13. Expression of DNAJB6, DNAJB8, HSPA1A, HSPB1, HSPB8, or STIP1 had no effect while HSP90AA1 even inhibited. PTGES3 (p23) inhibited only in control cells. Our results suggest that the DNAJ co-chaperones in particular become limiting in a depleted chaperoning network. Our results also suggest a difference between the transcriptomes of cells lacking HSF1 and cells expressing dnHSF1.

Epitope spreading of the anti-citrullinated protein antibody response occurs before disease onset and is associated with the disease course of early arthritis.

BACKGROUND: Anti-citrullinated protein antibodies (ACPA) are the most predictive factor for the development of rheumatoid arthritis (RA). OBJECTIVE: To investigate whether the recognition of citrullinated epitopes changes during disease onset or progression, by studying the fine specificity of ACPA in serum samples collected throughout the disease course, from before the onset of arthritis to longstanding RA. METHODS: Antibodies recognising five distinct citrullinated antigens were determined by enzyme-linked immunosorbent assay. Serum samples from 36 individuals who had donated blood before and after disease manifestation were used to investigate the development of citrullinated antigen recognition before disease onset. The association of ACPA reactivities with disease outcome was studied using sera from anti-cyclic citrullinated peptide-2 (CCP2)-positive patients with undifferentiated arthritis (UA) who did or did not progress to RA (UA-RA n=81, or UA-UA n=35). To investigate the ACPA recognition profile in patients with RA over a prolonged period of time, baseline serum samples from 68 patients were compared with samples obtained 7 years later. RESULTS: The number of recognised citrullinated peptides increased in the period preceding disease onset. At the time of disease manifestation, patients with UA who later developed RA recognised significantly more peptides than UA-UA patients. At later stages of the disease course, the ACPA fine specificity did not change. CONCLUSION: Epitope spreading with an increase in the recognition of citrullinated antigens occurs before the onset of RA. Immunological differences in ACPA fine specificity between UA-UA patients and UA-RA patients are present at baseline and are associated with the future disease course.

Low molecular weight silicones induce cell death in cultured cells.

Women with silicone gel-filled breast implants are exposed to organosilicon compounds, in particular methylsiloxanes, as a result of 'gel bleed' and implant rupture. Although these silicones were originally considered to be inert, increasing evidence indicates that they can cause serious health problems. Here, we have analyzed the effects of microdroplets of the methylcyclosiloxanes, in particular D4, on the viability of cultured human cells. The exposure of Jurkat suspension and HeLa monolayer cells to D4 resulted in morphological changes of the cells. The analysis of molecular markers for apoptotic and necrotic processes not only demonstrated that caspases were activated and DNA was fragmented in Jurkat cells exposed to D4, but that also the permeability of the plasma membrane was altered. The induction of apoptotic pathways by D4 was substantiated by the inhibition of caspase activation in cells overexpressing Bcl-2. Cleavage of the caspase-3 substrate U1-70K appeared to be dependent on the D4 content and the efficiency of cleavage decreased with increasing size of the methylcyclosiloxanes (D4, D5 and D6). In addition to Jurkat cells, D4-induced U1-70K cleavage was also observed in HeLa cells, but not in HEp-2 cells. Taken together, these results indicate that D4 and, to a lesser extent, D5 can activate cell-death-related pathways in a cell type-specific fashion and suggest that this phenomenon may contribute to the development of Breast Implant Illness.

Hsp27 enhances recovery of splicing as well as rephosphorylation of SRp38 after heat shock.

A heat stress causes a rapid inhibition of splicing. Exogenous expression of Hsp27 did not prevent that inhibition but enhanced the recovery of splicing afterward. Another small heat shock protein, alphaB-crystallin, had no effect. Hsp27, but not alphaB-crystallin, also hastened rephosphorylation of SRp38-dephosphorylated a potent inhibitor of splicing-after a heat shock, although it did not prevent dephosphorylation by a heat shock. The effect of Hsp27 on rephosphorylation of SRp38 required phosphorylatable Hsp27. A Hsp90 client protein was required for the effect of Hsp27 on recovery of spicing and on rephosphorylation of SRp38. Raising the Hsp70 level by either a pre-heat shock or by exogenous expression had no effect on either dephosphorylation of SRp38 during heat shock or rephosphorylation after heat shock. The phosphatase inhibitor calyculin A prevented dephosphorylation of SRp38 during a heat shock and caused complete rephosphorylation of SRp38 after a heat shock, indicating that cells recovering from a heat shock are not deficient in kinase activity. Together our data show that the activity of Hsp27 in restoring splicing is not due to a general thermoprotective effect of Hsp27, but that Hsp27 is an active participant in the (de)phosphorylation cascade controlling the activity of the splicing regulator SRp38.

In vivo heteromer formation. Expression of soluble betaA4-crystallin requires coexpression of a heteromeric partner.

The beta-crystallins are a family of long-lived, abundant structural proteins that are coexpressed in the vertebrate lens. As beta-crystallins form heteromers, a process that involves transient exposure of hydrophobic interfaces, we have examined whether in vivobeta-crystallin assembly is enhanced by protein chaperones, either small heat shock proteins, Hsp27 or alphaB-crystallin, or Hsp70. We show here that betaA4-crystallin is abundantly expressed in HeLa cells, but rapidly degraded, irrespective of the presence of Hsp27, alphaB-crystallin or Hsp70. Degradation is even enhanced by Hsp70. Coexpression of betaA4-crystallin with betaB2-crystallin yielded abundant soluble betaA4-betaB2-crystallin heteromers; betaB1-crystallin was much less effective in solubilizing betaA4-crystallin. As betaB2-crystallin competed for betaA4-crystallin with Hsp70 and the proteasomal degradation pathway, betaB2-crystallin probably captures an unstable betaA4-crystallin intermediate. We suggest that the proper folding of betaA4-crystallin is not mediated by general chaperones but requires a heteromeric partner, which then also acts as a dedicated chaperone towards betaA4-crystallin.

An atypical unfolded protein response in heat shocked cells.

BACKGROUND: The heat shock response (HSR) and the unfolded protein response (UPR) are both activated by proteotoxic stress, although in different compartments, and share cellular resources. How these resources are allocated when both responses are active is not known. Insight in possible crosstalk will help understanding the consequences of failure of these systems in (age-related) disease. RESULTS: In heat stressed HEK293 cells synthesis of the canonical UPR transcription factors XBP1s and ATF4 was detected as well as HSF1 independent activation of the promoters of the ER resident chaperones HSPA5 (BiP) and DNAJB9 (ERdj4). However, the heat stress activation of the DNAJB9 promoter, a XBP1s target, was not blocked in cells expressing a dominant negative IRE1α mutant, and thus did not require XBP1s. Furthermore, the DNA element required for heat stress activation of the DNAJB9 promoter is distinct from the ATF4 and ATF6 target elements; even though inhibition of eIF2α phosphorylation resulted in a decreased activation of the DNAJB9 promoter upon heat stress, suggesting a role for an eIF2α phosphorylation dependent product. CONCLUSIONS: The initial step in the UPR, synthesis of transcription factors, is activated by heat stress but the second step, transcriptional transactivation by these factors, is blocked and these pathways of the UPR are thus not productive. Expression of canonical ER chaperones is part of the response of heat stressed cells but another set of transcription factors has been recruited to regulate expression of these ER chaperones.

The anti-cyclic citrullinated peptide response in tuberculosis patients is not citrulline-dependent and sensitive to treatment.

INTRODUCTION: Patients with tuberculosis (TB) frequently produce anti-citrullinated protein antibodies (ACPA). The objective of this study is to characterize the citrulline-dependence of the ACPA reactivity in sera of patients with mycobacterium infections. METHODS: Serum samples of 134 patients with untreated mycobacterium infections (122 TB, 12 nontuberculous mycobacterium) were tested for antibodies against both the citrullinated (Cit) and the non-citrullinated (Arg) form of 2 cyclic synthetic peptides. In 33 patients, a follow-up sample was tested six months after starting anti-mycobacterial drugs. RESULTS: A substantial proportion of patients with mycobacterial infections demonstrated antibodies against 0401Cit, 0401Arg, 0722Cit and 0722Arg. Fourteen patients demonstrated anti-0401Cit, 83 anti-0401Arg, 22 anti-0722Cit and 61 anti-0722Arg, while none of these antibodies were detected in the 20 healthy controls. All the patients but one, who were anti-0401Cit and anti-0722Cit positive, demonstrated reactivity against the respective Arg peptide. In the subset of 33 patients with a follow-up test six months after starting treatment, the mean levels of antibodies to 0401Cit, 0401Arg, 0722Cit and 0722Arg significantly decreased after treatment. All the patients who were anti-0401Cit and anti-0722Cit positive turned negative after treatment. The presence of anti-0401Cit/Arg and anti-0722Cit/Arg was found to be significantly correlated with the presence of HIV. CONCLUSIONS: ACPA may be found in patients with TB. In most of the cases, the reactivity is citrulline independent. A positive cyclic citrullinated peptide (CCP) test in these patients should therefore be interpreted with care, and preferably followed by a control ELISA with a non-citrullinated antigen.

Translational thermotolerance provided by small heat shock proteins is limited to cap-dependent initiation and inhibited by 2-aminopurine.

Heat shock results in inhibition of general protein synthesis. In thermotolerant cells, protein synthesis is still rapidly inhibited by heat stress, but protein synthesis recovers faster than in naive heat-shocked cells, a phenomenon known as translational thermotolerance. Here we investigate the effect of overexpressing a single heat shock protein on cap-dependent and cap-independent initiation of translation during recovery from a heat shock. When overexpressing alphaB-crystallin or Hsp27, cap-dependent initiation of translation was protected but no effect was seen on cap-independent initiation of translation. When Hsp70 was overexpressed however, both cap-dependent and -independent translation were protected. This finding indicates a difference in the mechanism of protection mediated by small or large heat shock proteins. Phosphorylation of alphaB-crystallin and Hsp27 is known to significantly decrease their chaperone activity; therefore, we tested phosphorylation mutants of these proteins in this system. AlphaB-crystallin needs to be in its non-phosphorylated state to give protection, whereas phosphorylated Hsp27 is more potent in protection than the unphosphorylatable form. This indicates that chaperone activity is not a prerequisite for protection of translation by small heat shock proteins after heat shock. Furthermore, we show that in the presence of 2-aminopurine, an inhibitor of kinases, among which is double-stranded RNA-activated kinase, the protective effect of overexpressing alphaB-crystallin is abolished. The synthesis of the endogenous Hsps induced by the heat shock to test for thermotolerance is also blocked by 2-aminopurine. Most likely the protective effect of alphaB-crystallin requires synthesis of the endogenous heat shock proteins. Translational thermotolerance would then be a co-operative effect of different heat shock proteins.

Mimicking phosphorylation of the small heat-shock protein alphaB-crystallin recruits the F-box protein FBX4 to nuclear SC35 speckles.

The mammalian small heat shock protein alphaB-crystallin can be phosphorylated at three different sites, Ser19, Ser45 and Ser59. We compared the intracellular distribution of wild-type, nonphosphorylatable and all possible pseudophosphorylation mutants of alphaB-crystallin by immunoblot and immunocytochemical analyses of stable and transiently transfected cells. We observed that pseudophosphorylation at two (especially S19D/S45D) or all three (S19D/S45D/S59D) sites induced the partial translocation of alphaB-crystallin from the detergent-soluble to the detergent-insoluble fraction. Double immunofluorescence studies showed that the pseudophosphorylation mutants localized in nuclear speckles containing the splicing factor SC35. The alphaB-crystallin mutants in these speckles were resistant to mild detergent treatment, and also to DNase I or RNase A digestion, indicating a stable interaction with one or more speckle proteins, not dependent on intact DNA or RNA. We further found that FBX4, an adaptor protein of the ubiquitin-protein isopeptide ligase SKP1/CUL1/F-box known to interact with pseudophosphorylated alphaB-crystallin, was also recruited to SC35 speckles when cotransfected with the pseudophosphorylation mutants. Because SC35 speckles also react with an antibody against alphaB-crystallin endogenously phosphorylated at Ser45, our findings suggest that alphaB-crystallin has a phosphorylation-dependent role in the ubiquitination of a component of SC35 speckles.