The bacterial Ras-like protein Era has been reported previously to bind 16S rRNA within the 30S ribosomal subunit and to play a crucial role in ribosome assembly. An orthologue of this essential GTPase ERAL1 (Era G-protein-like 1) exists in higher eukaryotes and although its exact molecular function and cellular localization is unknown, its absence has been linked to apoptosis. In the present study we show that human ERAL1 is a mitochondrial protein important for the formation of the 28S small mitoribosomal subunit. We also show that ERAL1 binds in vivo to the rRNA component of the small subunit [12S mt (mitochondrial)-rRNA]. Bacterial Era associates with a 3′ unstructured nonanucleotide immediately downstream of the terminal stem–loop (helix 45) of 16S rRNA. This site contains an AUCA sequence highly conserved across all domains of life, immediately upstream of the anti-Shine–Dalgarno sequence, which is conserved in bacteria. Strikingly, this entire region is absent from 12S mt-rRNA. We have mapped the ERAL1-binding site to a 33 nucleotide section delineating the 3′ terminal stem–loop region of 12S mt-rRNA. This loop contains two adenine residues that are reported to be dimethylated on mitoribosome maturation. Furthermore, and also in contrast with the bacterial orthologue, loss of ERAL1 leads to rapid decay of nascent 12S mt-rRNA, consistent with a role as a mitochondrial RNA chaperone. Finally, whereas depletion of ERAL1 leads to apoptosis, cell death occurs prior to any appreciable loss of mitochondrial protein synthesis or reduction in the stability of mitochondrial mRNA.

Purpose: High-risk neuroblastoma is characterized by poor survival rates, and the development of improved therapeutic approaches is a priority. Temozolomide and topotecan show promising clinical activity against neuroblastoma. Poly(ADP-ribose) polymerase-1 (PARP-1) promotes DNA repair and cell survival following genotoxic insult; we postulated that its inhibition may enhance the efficacy of these DNA-damaging drugs in pediatric cancers. Experimental Design: We evaluated the chemosensitizing properties of the PARP inhibitor AG014699 (Pfizer, Inc.) in combination with temozolomide and topotecan, against human neuroblastoma cells and xenografts, alongside associated pharmacologic and toxicologic indices. Results: Addition of PARP-inhibitory concentrations of AG014699 significantly potentiated growth inhibition by both topotecan (1.5-to 2.3-fold) and temozolomide (3-to 10-fold) in vitro, with equivalent effects confirmed in clonogenic assays. In two independent in vivo models (NB1691 and SHSY5Y xenografts), temozolomide caused a xenograft growth delay, which was enhanced by co-administration of AG014699, and resulted in complete and sustained tumor regression in the majority (6 of 10; 60%) of cases. Evidence of enhanced growth delay by topotecan/AG014699 co-administration was observed in NB1691 xenografts. AG014699 metabolites distributed rapidly into the plasma (C max , 1.2-1.9 nmol/L at 30 min) and accumulated in xenograft tissues (C max ,1-2 Amol/L at 120 min), associated with a sustained suppression of PARP-1enzyme activity. Doses of AG014699 required for potentiation were not toxic per se. Conclusions: These data show enhancement of temozolomide and topotecan efficacy by PARP inhibition in neuroblastoma. Coupled with the acceptable pharmacokinetic, pharmacodynamic, and toxicity profiles of AG014699, our findings provide strong rationale for investigation of PARP inhibitors in pediatric early clinical studies.

Mitochondrial gene expression is a fundamental process that is largely dependent on nuclear-encoded proteins. Several steps of mitochondrial RNA processing and maturation, including RNA post-transcriptional modification, appear to be spatially organized into distinct foci, which we have previously termed mitochondrial RNA granules (MRGs). Although an increasing number of proteins have been localized to MRGs, a comprehensive analysis of the proteome of these structures is still lacking. Here, we have applied a microscopy-based approach that has allowed us to identify novel components of the MRG proteome. Among these, we have focused our attention on RPUSD4, an uncharacterized mitochondrial putative pseudouridine synthase. We show that RPUSD4 depletion leads to a severe reduction of the steady-state level of the 16S mitochondrial (mt) rRNA with defects in the biogenesis of the mitoribosome large subunit and consequently in mitochondrial translation. We report that RPUSD4 binds 16S mt-rRNA, mt-tRNAMet, and mt-tRNAPhe, and we demonstrate that it is responsible for pseudouridylation of the latter. These data provide new insights into the relevance of RNA pseudouridylation in mitochondrial gene expression.

Multiple respiratory chain deficiencies represent a common cause of mitochondrial diseases and are associated with a wide range of clinical symptoms. We report a subject, born to consanguineous parents, with growth retardation and neurological deterioration. Multiple respiratory chain deficiency was found in muscle and fibroblasts of the subject as well as abnormal assembly of complexes I and IV. A microsatellite genotyping of the family members detected only one region of homozygosity on chromosome 17q24.2–q25.3 in which we focused our attention to genes involved in mitochondrial translation. We sequenced MRPL12, encoding the mitochondrial ribosomal protein L12 and identified a c.542C>T transition in exon 5 changing a highly conserved alanine into a valine (p.Ala181Val). This mutation resulted in a decreased steady-state level of MRPL12 protein, with altered integration into the large ribosomal subunit. Moreover, an overall mitochondrial translation defect was observed in the subject's fibroblasts with a significant reduction of synthesis of COXI, COXII and COXIII subunits. Modeling of MRPL12 shows Ala181 positioned in a helix potentially involved in an interface of interaction suggesting that the p.Ala181Val change might be predicted to alter interactions with the elongation factors. These results contrast with the eubacterial orthologues of human MRPL12, where L7/L12 proteins do not appear to have a selective effect on translation. Therefore, analysis of the mutated version found in the subject presented here suggests that the mammalian protein does not function in an entirely analogous manner to the eubacterial L7/L12 equivalent.

Disorders of the mitochondrial genome cause a wide spectrum of disease, these present mainly as neurological and/or muscle related pathologies. Due to the intractability of the human mitochondrial genome there are currently no effective treatments for these disorders. The majority of the pathogenic mutations lie in the genes encoding mitochondrial tRNAs. Consequently, the biochemical deficiency is due to mitochondrial protein synthesis defects, which manifest as aberrant cellular respiration and ATP synthesis. It has previously been reported that overexpression of mitochondrial aminoacyl tRNA synthetases has been effective, in cell lines, at partially suppressing the defects resulting from mutations in their cognate mt-tRNAs. We now show that leucyl tRNA synthetase is able to partially rescue defects caused by mutations in non-cognate mt-tRNAs. Further, a C terminal peptide alone can enter mitochondria and interact with the same spectrum of mt-tRNAs as the entire synthetase, in intact cells. These data support the possibility that a small peptide could correct at least the biochemical defect associated with many mt-tRNA mutations, inferring a novel therapy for these disorders.

LRPPRC is a protein that has attracted interest both for its role in post-transcriptional regulation of mitochondrial gene expression and more recently because numerous mutated variants have been characterized as causing severe infantile mitochondrial neurodegeneration. LRPPRC belongs to the pentatricopeptide repeat (PPR) protein family, originally defined by their RNA binding capacity, and forms a complex with SLIRP that harbours an RNA recognition motif (RRM) domain. We show here that LRPPRC displays a broad and strong RNA binding capacity in vitro in contrast to SLIRP that associates only weakly with RNA. The LRPPRC–SLIRP complex comprises a hetero-dimer via interactions by polar amino acids in the single RRM domain of SLIRP and three neighbouring PPR motifs in the second quarter of LRPPRC, which critically contribute to the LRPPRC–SLIRP binding interface to enhance its stability. Unexpectedly, specific amino acids at this interface are located within the PPRs of LRPPRC at positions predicted to interact with RNA and within the RNP1 motif of SLIRP's RRM domain. Our findings thus unexpectedly establish that despite the prediction that these residues in LRPPRC and SLIRP should bind RNA, they are instead used to facilitate protein–protein interactions, enabling the formation of a stable complex between these two proteins.

Tra2β regulates a number of splicing switches including activation of the human testis-specific exon HIPK3-T in the Homeodomain Interacting Protein Kinase 3 gene. By testing HIPK3-T exons of different intrinsic strengths, we found Tra2β most efficiently activated splicing inclusion of intrinsically weak exons, although these were spliced at a lower overall level. Both the RRM and N-terminal RS-rich region of Tra2β were required for splicing activation. Bioinformatic searches for splicing enhancers and repressors mapped four physically distinct exonic splicing enhancers (ESEs) within HIPK3-T, each containing the known Tra2β AGAA-rich binding site. Surprisingly disruption of each single ESE prevented Tra2β-mediated activation, although single mutated exons could still bind Tra2β protein by gel shifts and functional splicing analyses. Titration experiments indicate an additive model of HIPK3-T splicing activation, requiring availability of an array of four distinct ESEs to enable splicing activation. To enable this efficient Tra2β-mediated splicing switch to operate, a closely adjacent downstream and potentially competitive stronger 5′-splice site is actively repressed. Our data indicate that a novel arrangement of multiple mono-specific AGAA-rich ESEs coupled to a weak 5′-splice site functions as a responsive gauge. This gauge monitors changes in the specific nuclear concentration of the RNA binding protein Tra2β, and co-ordinately regulates HIPK3-T exon splicing inclusion.

Background:Temozolomide shows activity against medulloblastoma, the most common malignant paediatric brain tumour. Poly(ADP-ribose) polymerase (PARP) inhibitors enhance temozolomide activity in extracranial adult and paediatric human malignancies.Methods:We assessed the effect of AG-014699, a clinically active PARP inhibitor, on temozolomide-induced growth inhibition in human medulloblastoma models. Pharmacokinetic, pharmacodynamic and toxicity assays were performed in tumour-bearing mice.Results:Sensitivity to temozolomide in vitro was consistent with methylguanine methyltransferase (MGMT) and DNA mismatch repair (MMR) status; MGMT+ MMR+ D384Med cells (temozolomide GI50=220 μ), representative of most primary medulloblastomas, were sensitised fourfold by AG-014699; MGMT− MMR+ D425Med cells were hypersensitive (GI50=9 μ) and not sensitised by AG-014699, whereas MGMT+ MMR− temozolomide-resistant D283Med cells (GI50=807 μ) were sensitised 20-fold. In xenograft models, co-administration of AG-014699 produced an increase in temozolomide-induced tumour growth delay in D384Med xenografts. Consistent with the in vitro data, temozolomide caused complete tumour regressions of D425Med xenografts, whereas D283Med xenografts were relatively resistant. AG-014699 was not toxic, accumulated and reduced PARP activity ⩾75% in xenograft and brain tissues.Conclusion:We show for the first time central nervous system penetration and inhibition of brain PARP activity by AG-014699. Taken together with our in vitro chemosensitisation and toxicity data, these findings support further evaluation of the clinical potential of AG-014699–temozolomide combinations in intra-cranial malignancies.