Mutations in the human mitochondrial genome are implicated in neuromuscular diseases, metabolic defects, and aging. An efficient and simple mechanism for neutralizing deleterious mitochondrial DNA (mtDNA) alterations has unfortunately remained elusive. Here, we report that a 20-ribonucleotide stem-loop sequence from the H1 RNA, the RNA component of the human RNase P enzyme, appended to a nonimported RNA directs the import of the resultant RNA fusion transcript into human mitochondria. The methodology is effective for both noncoding RNAs, such as tRNAs, and mRNAs. The RNA import component, polynucleotide phosphorylase (PNPASE), facilitates transfer of this hybrid RNA into the mitochondrial matrix. In addition, nucleus-encoded mRNAs for mitochondrial proteins, such as the mRNA of human mitochondrial ribosomal protein S12 (MRPS12), contain regulatory sequences in their 3′-untranslated region (UTR) that confers localization to the mitochondrial outer membrane, which is postulated to aid in protein translocation after translation. We show that for some mitochondrial-encoded transcripts, such as COX2, a 3′-UTR localization sequence is not required for mRNA import, whereas for corrective mitochondrial-encoded tRNAs, appending the 3′-UTR localization sequence was essential for efficient fusion-transcript translocation into mitochondria. In vivo, functional defects in mitochondrial RNA (mtRNA) translation and cell respiration were reversed in two human disease lines. Thus, this study indicates that a wide range of RNAs can be targeted to mitochondria by appending a targeting sequence that interacts with PNPASE, with or without a mitochondrial localization sequence, providing an exciting, general approach for overcoming mitochondrial genetic disorders.

The mitochondrial genome encodes a very small fraction of the macromolecular components that are required to generate functional mitochondria. Therefore, most components are encoded within the nuclear genome and are imported into mitochondria from the cytosol. Understanding how mitochondria are assembled, function, and dysfunction in diseases requires detailed knowledge of mitochondrial import mechanisms and pathways. The import of nucleus-encoded RNAs is required for mitochondrial biogenesis and function, but unlike pre-protein import, the pathways and cellular machineries of RNA import are poorly defined, especially in mammals. Recent studies have shown that mammalian polynucleotide phosporylase (PNPASE) localizes in the mitochondrial intermembrane space (IMS) to regulate the import of RNA. The identification of PNPASE as the first component of the RNA import pathway, along with a growing list of nucleus-encoded RNAs that are imported and newly developed assay systems for RNA import studies, suggest a unique opportunity is emerging to identify the factors and mechanisms that regulate RNA import into mammalian mitochondria. Here we summarize what is known in this fascinating area of mitochondrial biogenesis, identify areas that require further investigation, and speculate on the impact unraveling RNA import mechanisms and pathways will have for the field going forward.

Activation of a large multisubunit protein kinase, called the inhibitor kappaB kinase (IKK) complex, is central to the induction of the family of transcription factors nuclear factor kappaB. IKK is comprised of two catalytic subunits, IKKalpha and IKKbeta, and a regulatory IKKgamma subunit. It is known that the catalytic IKKbeta and regulatory IKKgamma subunits associate through interactions mediated by the N-terminal region of IKKgamma and an 11-mer peptide located near the C-terminus of IKKbeta. In this study, we have defined the minimal IKKgamma segment that binds IKKbeta and determined the binding affinity of the IKKbeta/IKKgamma complex. We identified that the N-terminal segment spanning residues 40-130 of IKKgamma binds the IKKbeta C-terminal domain (residues 665-756) with Kd approximately 25 nM. Several smaller N-terminal IKKgamma deletion mutants within the N-terminal 130 residues, although in some cases retained IKKbeta binding activity, showed a tendency to aggregate and formed covalently linked complexes. However, expansion of the C-terminus of these fragments to residue 210 completely changed the solution behavior of the IKKgamma N-terminus without affecting the IKKbeta binding affinity. We also found that the IKKbeta C-terminal domain formed a dimer in solution and the basic unit of the IKKbeta/IKKgamma complex was a dimer/dimer.

The congenital heart disease 5 (CHD5)/tryptophan rich basic protein (WRB) is a protein containing a tryptophanrich carboxy-terminal region, which was discovered in the human fetal heart. In humans, this CHD5/WRB is located between the markers ACTL5-D21S268 within the Down syndrome (DS) Region-2 at chromosome 21. Congenital heart disease is commonly linked to DS patients. The functions of this gene product are unknown. To identify the functions of CHD5/WRB in heart formation during embryogenesis, the medaka CHD5 cDNA (mCHD5) was isolated and its gene expression pattern and the localization of its gene product were investigated. The obtained mCHD5 belongs to the CHD5 superfamily, whose members include coiled-coil proteins. The mCHD5 gene was found to be expressed in the developing heart after stage 28 at which the chamber (ventricle and atrium) differentiation in the heart tube is initiated in the embryo. Its gene product was also detected in the developing heart at embryonic stage 28 and 35. Knocking-down of mCHD5 function caused severe cardiac disorder, including abnormal chamber differentiation, abnormal looping and ocular abnormality such as Cyclops. Our results provide the mCHD5 gene expression pattern as well as its physiological role during heart formation in a vertebrate model system.

Because the EL group is more likely to have an adverse outcome, elevated hs-cTnT level can be a prognostic marker in patients with CHD.