The signal and the trap: Targeted delivery and retention of proteins in the mitochondrion
Authors: Terence R. Flotte
Journal: Molecular Therapy (2003)

Protein augmentation represents one of the oldest and most effective therapeutic uses of biological agents, dating back at least to Banting and Best's breakthrough introduction of insulin injections for patients with diabetes. More than 80 years later, our concept of protein replacement has largely remained focused on the delivery of proteins and polypeptides to the extracellular compartment. Now available, however, are several technologies that have the potential to allow partition of infused proteins to much more specific compartments, including specific organs, cells, or even subcellular organelles. In this issue of Molecular Therapy, Del Gaizo and Payne [1Del Gaizo V. Payne R.M. A novel TAT-mitochondrial signal sequence fusion protein is processed, stays in mitochondria and crosses the placenta.Mol. Ther. 2003; (this issue.)PubMed Google Scholar] describe a new combination of such technologies to achieve remarkably efficient delivery and retention of exogenous proteins in the mitochondrial matrix. The value of delivering proteins to the mitochondrial matrix by cellular manipulation, gene therapy, or protein replacement has been recognized and could have therapeutic value in a wide variety of inherited and acquired conditions. Rizzuto et al. have demonstrated that efficient cellular pathways exist for targeting proteins to mitochondria [2Rizzuto R. et al.Chimeric green fluorescent protein as a tool for visualizing subcellular organelles in living cells.Curr. Biol. 1995; 5: 635-642Abstract Full Text Full Text PDF PubMed Scopus (424) Google Scholar]. These pathways are necessary, because only 13 of the hundreds of polypeptides required for mitochondrial function are produced endogenously from mitochondrial DNA genes. They further demonstrated that these signals could be fused to heterologous proteins, allowing their retargeting to mitochon dria by a process currently known as allo topic expression. However, the Rizzuto method requires that the protein in question be produced on cytosolic ribosomes within the intended cell target and thus is reliant upon efficient gene transfer [3Owen R. Flotte T.R. Approaches and limitations to gene therapy for mitochondrial diseases.Antioxid. Redox Signal. 2001; 3: 451-460Crossref PubMed Scopus (8) Google Scholar]. In contrast, the current report combines peptides targeted to the mitochondria with the protein transfer domain of the HIV-tat protein to accomplish efficient delivery of the protein across the membrane from the extracellular space. The incorporation of HIV-tat alone, however, has the potential to counteract the specificity of mitochondrial pre-greatly enhances the efficiency of targeting, even in vivo and transplacentally. This last aspect of the study indicates the potential for very early prenatal or perinatal use of therapeutic protein augmentation. Many infants afflicted with genetic and metabolic defects of mitochondrial metabolism suffer severe brain injury at a very early stage. The widespread availability of expanded newborn screening and prenatal genetic testing for diseases of mitochondrial metabolism can only be truly useful if there is an effective therapeutic intervention available as well. The potential for protein replacement in the mitochondrial compartment could provide such an intervention, if only as a bridge to a subsequent more longlasting gene therapy. The potential value of such a system is very broad [4Stacpoole P.W. Lactic acidosis and other mitochondrial disorders.Metabolism. 1997; 46: 306-321Abstract Full Text PDF PubMed Scopus (70) Google Scholar]. Mitochondrial pathology is of primary importance in a number of prototypic metabolic diseases states, including urea cycle defects, disorders of fatty acid oxidation, and congenital lactic acidosis. Secondary mitochondrial pathology is also common in a wide range of age-related illnesses, including Parkinson disease. Even more broadly, oxidant injury and caspase activation within mitochondria are common themes involved in apoptotic cell death from a wide variety of causes. The possibility of modulating these processes with targeted protein delivery has the potential to be both a very useful tool for studies of disease pathobiology and perhaps even a new paradigm in therapeutic protein delivery.