Targeting proteins to mitochondria using TAT

Transactivating transcriptional activator-mediated drug delivery

Cell-penetrating peptides (CPPs) are peptide vectors that can traverse through the plasma membrane barrier without breaching the integrity of the cell, and deliver various cargoes inside cell. The range of cargoes that can be delivered intracellularly by CPPs encompasses a broad variety of hydrophilic molecules, such as peptides, proteins, antibodies, imaging agents, DNA and even nanosized entities, including polymer-based systems, solid nanoparticles and liposomes. Multiple studies have focused on CPPs such as transactivating transcriptional activator peptide (TATp), penetratin, VP22, transportan and synthetic oligoarginines because of their high inherent potential as intracellular delivery vectors. However, the TATp remains the most popular CPP used for a variety of purposes. This review article attempts to bring together the available data on TAT-mediated intracellular uptake of a broad range of molecules and nanoparticles. It also considers potential practical applications of this approach.

Cell-penetrating peptides do not cross mitochondrial membranes even when conjugated to a lipophilic cation: evidence against direct passage through phospholipid bilayers

CPPs (cell-penetrating peptides) facilitate the cellular uptake of covalently attached oligonucleotides, proteins and other macromolecules, but the mechanism of their uptake is disputed. Two models are proposed: direct movement through the phospholipid bilayer and endocytic uptake. Mitochondria are a good model system to distinguish between these possibilities, since they have no vesicular transport systems. Furthermore, CPP-mediated delivery of macromolecules to the mitochondrial matrix would be a significant breakthrough in the study of mitochondrial function and dysfunction, and could also lead to new therapies for diseases caused by mitochondrial damage. Therefore we investigated whether two CPPs, penetratin and Tat, could act as mitochondrial delivery vectors. We also determined whether conjugation of the lipophilic cation TPP (triphenylphosphonium) to penetratin or Tat facilitated their uptake into mitochondria, since TPP leads to uptake of attached molecules into mitochondria driven by the membrane potential. Neither penetratin nor Tat, nor their TPP conjugates, are internalized by isolated mitochondria, indicating that these CPPs cannot cross mitochondrial phospholipid bilayers. Tat and TPP-Tat are taken up by cells, but they accumulate in endosomes and do not reach mitochondria. We conclude that CPPs cannot cross mitochondrial phospholipid bilayers, and therefore cannot deliver macromolecules directly to mitochondria. Our findings shed light on the mechanism of uptake of CPPs by cells. The lack of direct movement of CPPs through mitochondrial phospholipid bilayers, along with the observed endosomal accumulation of Tat and TPP-Tat in cells, makes it unlikely that CPPs enter cells by direct membrane passage, and instead favours cellular uptake via an endocytic pathway.

Targeted drug delivery to mammalian mitochondria in living cells

Mitochondrial dysfunction causes or contributes to a large number of human disorders including neuromuscular and neurodegenerative diseases, diabetes, ischaemia-reperfusion injury and cancer. Increasing efforts are being made towards mitochondria-directed pharmacological intervention, leading to the emergence of 'mitochondrial medicine' as a new field of biomedical research. The identification of new molecular mitochondrial drug targets in combination with the development of methods for selectively delivering biologically active molecules to the site of mitochondria will eventually launch new therapies for the treatment of mitochondria-related diseases, based either on the selective protection, repair or eradication of cells. This review discusses the need for the development of mitochondria-specific drug and DNA delivery systems, and evaluates the currently employed strategies for mitochondrial drug targeting, including some of their potential therapeutic applications.

Development of mitochondrial gene replacement therapy

Many "classic" mitochondrial diseases have been described that arise from single homoplasmic mutations in mitochondrial DNA (mtDNA). These diseases typically affect nonmitotic tissues (brain, retina, muscle), present with variable phenotypes, can appear sporadically, and are untreatable. Evolving evidence implicates mtDNA abnormalities in diseases such as Alzheimer's, Parkinson's, and type II diabetes, but specific causal mutations for these conditions remain to be defined. Understanding the mtDNA genotype-phenotype relationships and developing specific treatment for mtDNA-based diseases is hampered by inability to manipulate the mitochondrial genome. We present a novel protein transduction technology ("protofection") that allows insertion and expression of the human mitochondrial genome into mitochondria of living cells. With protofection, the mitochondrial genotype can be altered, or exogenous genes can be introduced to be expressed and either retained in mitochondria or be directed to other organelles. Protofection also delivers mtDNA in vivo, opening the way to rational development of mitochondrial gene replacement therapy of mtDNA-based diseases.

Transactivator of transcription fusion protein transduction causes membrane inversion

The transactivator of transcription (TAT) protein transduction domain is an 11-amino acid positively charged peptide that has been shown to pull diverse molecules across cell membranes in vitro and in vivo. Fusion proteins constructed with TAT rapidly enter and exit cells and have been shown to cross intracellular membranes as well. Electrostatic interactions between TAT and the cell membrane have been implicated as a part of the mechanism of transduction. Here, we report that TAT transduction causes membrane phospholipid rearrangement as evidenced by detection of phosphatidylserine on the outer surface of the cell membrane. Furthermore, these rearrangements can be blocked by positively charged polylysine, further implicating electrostatic interactions as a part of the mechanism. Neither apoptosis nor necrosis is induced in these cells after exposure to TAT. We conclude that the process of TAT.GFP transduction causes phosphatidylserine to translocate from the inner to the outer leaflet of the plasma membrane. These results provide insight into the mechanism of TAT protein transduction domain transduction.