Modulation of mitochondrial respiratory capacity by carrier-mediated transfer of RNA in vivo

Mitochondrial transport of catalytic RNAs and targeting of the organellar transcriptome in human cells

Mutations in the small genome present in mitochondria often result in severe pathologies. Different genetic strategies have been explored, aiming to rescue such mutations. A number of these strategies were based on the capacity of human mitochondria to import RNAs from the cytosol and designed to repress the replication of the mutated genomes or to provide the organelles with wild-type versions of mutant transcripts. However, the mutant RNAs present in mitochondria turned out to be an obstacle to therapy and little attention has been devoted so far to their elimination. Here, we present the development of a strategy to knockdown mitochondrial RNAs in human cells using the transfer RNA-like structure of Brome mosaic virus or Tobacco mosaic virus as a shuttle to drive trans-cleaving ribozymes into the organelles in human cell lines. We obtained a specific knockdown of the targeted mitochondrial ATP6 mRNA, followed by a deep drop in ATP6 protein and a functional impairment of the oxidative phosphorylation chain. Our strategy provides a powerful approach to eliminate mutant organellar transcripts and to analyse the control and communication of the human organellar genetic system.

Extrahepatic targeting of lipid nanoparticles in vivo with intracellular targeting for future nanomedicines

A new era of nanomedicines that involve nucleic acids/gene therapy has been opened after two decades in 21^st century and new types of more efficient drug delivery systems (DDS) are highly expected and will include extrahepatic delivery. In this review, we summarize the possibility and expectations for the extrahepatic delivery of small interfering RNA/messenger RNA/plasmid DNA/genome editing to the spleen, lung, tumor, lymph nodes as well as the liver based on our studies as well as reported information. Passive targeting and active targeting are discussed in in vivo delivery and the importance of controlled intracellular trafficking for successful therapeutic results are also discussed. In addition, mitochondrial delivery as a novel strategy for nucleic acids/gene therapy is introduced to expand the therapeutic dimension of nucleic acids/gene therapy in the liver as well as the heart, kidney and brain.

Recent advances in delivering RNA-based therapeutics to mitochondria

INTRODUCTION

After the emergence of lipid nanoparticles (LNP) containing therapeutic mRNA as vaccines for use against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the clinical usefulness of nucleic acid-encapsulated LNPs is now a fact. In addition to the nucleus and cytoplasm, mitochondria, which have their own genome, are a site where nucleic acids function in the cell. Gene therapies targeting mitochondria are expected to pave the way for the next generation of therapies.

AREAS COVERED

Methods for delivering nucleic acids to mitochondria are needed in order to realize such innovative therapies. However, only a few reports on delivery systems targeting mitochondria have appeared. In this review, we summarize the current state of research on RNA-based therapeutics targeted to mitochondria, with emphasis on mitochondrial RNA delivery therapies and on therapies that involve the use of mitochondrial genome editing devices.

EXPERT OPINION

We hope that this review article will focus our attention to this area of research, stimulate more interest in this field of research, and lead to the development of mitochondria-targeted nucleic acid medicine. It has the potential to become a major weapon against urgent and unknown diseases, including SARS-CoV-2 infections.

Effects of Transient Hypoxia versus Prolonged Hypoxia on Satellite Cell Proliferation and Differentiation In Vivo

The microenvironment of the injury site can have profound effects on wound healing. Muscle injury results in ischemia leading to short-term local hypoxia, but there are conflicting reports on the role of hypoxia on the myogenic program in vivo and in vitro. In our rat model of mitochondrial restoration (MR), temporary upregulation of mitochondrial activity by a cocktail of organelle-encoded RNAs results in satellite cell proliferation and initiation of myogenesis. We now report that MR leads to a transient hypoxic response in situ. Inhibition of hypoxia by lowering mitochondrial O₂ consumption, either by respiratory electron transport inhibitors, or by NO-mediated inhibition of O₂ binding to cytochrome c oxidase, resulted in exacerbation of inflammation. Lentivirus-mediated knockdown of hypoxia-inducible factor 1α (HIF1α) or of Notch signaling components had a similar effect, and pharmacologic inhibition of HIF or Notch reduced the number of proliferating Pax7⁺ cells. In contrast, a prolonged hypoxic response induced either by uncoupling of respiration from oxidative phosphorylation or through HIF stabilization by dimethyloxalylglycine (DMOG) had an immediate anti-inflammatory effect. Although significant satellite cell proliferation occurred in presence of DMOG, expression of differentiation markers was affected. These results emphasize the importance of transient hypoxia as opposed to prolonged hypoxia for myogenesis.

Mitochondrial disorders: aetiologies, models systems, and candidate therapies

It has become evident that many human disorders are characterised by mitochondrial dysfunction either at a primary level, due to mutations in genes whose encoded products are involved in oxidative phosphorylation, or at a secondary level, due to the accumulation of mitochondrial DNA (mtDNA) mutations. This has prompted keen interest in the development of cell and animal models and in exploring innovative therapeutic strategies to modulate the mitochondrial deficiencies observed in these diseases. Key advances in these areas are outlined in this review, with a focus on Leber hereditary optic neuropathy (LHON). This exciting field is set to grow exponentially and yield many candidate therapies to treat this class of disease.

Induction of muscle regeneration by RNA-mediated mitochondrial restoration

Skeletal muscle injury is associated with general down-regulation of mitochondrial function. Postinjury regeneration of skeletal muscle occurs through activation, proliferation, and differentiation of resident stem cells, including satellite cells and endothelial precursor cells. We wanted to determine the role of mitochondrial function in the regeneration process. Using a previously described method for complex-mediated delivery to intracellular mitochondria, a combination of polycistronic RNAs encoding the H strand of the rat mitochondrial genome was administered to injured rat quadriceps muscle, resulting in restoration of mitochondrial mRNA levels, organellar translation, and respiratory capacity. Intramuscular ATP levels were elevated on pcRNA treatment of injured muscle; concomitantly, levels of reactive oxygen species in the injured muscle were reduced. These effects combined to produce a notable increase in the rate of wound resolution, accompanied by reduction of fibrosis and acceleration of myogenesis, vasculogenesis, and resumption of muscle contractile function. There was evidence of proliferation of Pax7+ satellite cells, expression of muscle-specific regulatory factors in a specific time sequence, and formation of new myofibers in the regenerating muscle. RNA-induced wound resolution and satellite cell proliferation were sensitive to mitochondrial inhibitors, indicating the importance of oxidative phosphorylation. These results highlight the activation of endogenous stem cells through mitochondrial restoration as a possible alternative to implantation of cultured stem cells.

Targeting nucleic acids into mitochondria: progress and prospects

Given the essential functions of these organelles in cell homeostasis, their involvement in incurable diseases and their potential in biotechnological applications, genetic transformation of mitochondria has been a long pursued goal that has only been reached in a couple of unicellular organisms. The challenge led scientists to explore a wealth of different strategies for mitochondrial delivery of DNA or RNA in living cells. These are the subject of the present review. Targeting DNA into the organelles currently shows promise but remarkably a number of alternative approaches based on RNA trafficking were also established and will bring as well major contributions.