Mitochondria are a roving herd of power plants that exist in every cell, storing energy in chemical form for use in powering cellular processes. They are the remnants of symbiotic bacteria-like organisms, and so bear their own DNA that encodes much of the protein machinery needed for their operation. Unfortunately, this mitochondrial DNA (usually abbreviated as mtDNA) sits right next to processes that generate reactive byproducts, and is far less protected than the DNA in the cell nucleus. It becomes damaged over time in ways that spiral out to harm the cell, harm surrounding tissue, and ultimately cause some fraction of degenerative aging.
Thus fixing mitochondrial DNA damage is an important line item for any future rejuvenation toolkit. Seven years ago, there was an unofficial claim of the ability to replace mitochondrial DNA in a laboratory animal using a methodology known as protofection. There has not been a great deal of progress on this front since then however: to the best of my knowledge, no-one else has managed to replicate that result. Yet that research group and others continue to work on the mechanisms used: so what's going on here?
At its heart, protofection is a cargo delivery method, and the cargo consists of gene sequences to be inserted into mitochondria - such as extra copies of important genes that tend to get damaged, causing mitochondrial dysfunction. The delivery mechanism is a protein assembled of various parts that enable it to (a) cross cell membranes, (b) be transported into the interior of mitochondria within the cell, and (c) participate in the normal processes of DNA replication. Thus a cargo of mitochondrial genes attached to this basic assembly will be carried to mitochondria and then added to the mitochondrial DNA that is already present.
This delivery protein is known of late as recombinant human mitochondrial transcription factor A (rhTFAM), and in older materials such as those used for the unfinished Open Cures protofection protocol as PTD-SODMLS-Mature TFAM:
PTD stands for Protein Transduction Domain. This part of the protein facilitates its ability to cross lipid membranes.
SODMLS stands for Superoxide Dismutase Mitochondrial Localization Signal. This domain targets mitochondria specifically. In the literature, the term MTD (Mitochondrial Transduction Domain) is used in place of PTD + SODMLS indicating that the MTD is the part of the protein that causes it to enter cellular mitochondria.
TFAM refers to human mitochondrial transcription factor A. This protein plays several roles in the mitochondria. It participates in mtDNA transcription, replication and maintenance. It also non-specifically binds to mtDNA which is the property we want to exploit as we attempt to pull pristine mtDNA into mitochondria which contains damaged DNA.
In recent years, it seems that the core protofection research group has dropped the use of rhTFAM as a carrier and are instead focused on exploring how it might be used in and of itself, without any cargo, as a therapy for mitochondrial conditions. It so happens that rhTFAM boosts mitochondrial activity in some ways, and progressive mitochondrial dysfunction is implicated in a range of age-related conditions, especially in the brain. Even marginal therapies here could have meaningful market value.
You might look on this as one of the more subtle ways in which the present US regulatory structure for medical research and development distorts the undertaking of science. The FDA only permits clinical applications for specific named conditions, and thus anything other than the development of treatments for late-stage disease becomes either too expensive or outright forbidden. Treatment of aging falls into the latter category, as aging is not recognized as a medical condition that should be treated. So research efforts that might have some application to aging are sidelined into the development of marginal therapies for one specific disease, often type 2 diabetes, rather than what are arguably far more important applications.
But back to rhTFAM: here is a recent paper that illustrates the present research direction. It is an evaluation of rhTFAM as a therapeutic agent intended to galvanize mitochondria, rather than as a means of delivering mitochondrial DNA to fix damage.
To treat mitochondrial deficiencies, we have developed recombinant human mitochondrial transcription factor A (rhTFAM). TFAM is an essential component of the mitochondrial DNA replication and expression machinery ... RhTFAM enters the mitochondrial compartment of cells rapidly and can also transport mtDNA cargo into mitochondria.
RhTFAM stimulates mitochondrial biogenesis of human cells modeling sporadic Parkinson's disease or containing high abundance mtDNA mutations of Leber's hereditary optic neuropathy (LHON) or Leigh syndrome. RhTFAM treatment of cells exposed to parkinsonian neurotoxins restores ATP deficiencies and reduces oxidative stress. Systemic treatment of young adult mice with rhTFAM stimulates mitochondrial biogenesis, increases respiration in brain, heart and muscle, increases brain mitochondrial ATP synthesis and reduces oxidative stress damage to proteins.
These desirable properties of rhTFAM suggest that it might improve bioenergetic deficiencies produced as a consequence of aging. To test that possibility, we treated aged mice with rhTFAM in a manner similar to our prior study of treating young adult mice. We observed stimulation of mitochondrial biogenesis and mtDNA gene expression in the absence of any apparent systemic toxicity. Increases in mitochondrial oxidative metabolism were mirrored by improvements in Morris water maze performance in aged mice, including platform acquisition (learning) and platform location recall (memory), and increases in brain protein levels of BDNF and synapsin. These findings support beneficial use of rhTFAM in human aging and development for experimental use of rhTFAM in humans.
There are a lot of good references in the paper for those who want to look into other recent publications on this topic. For a broader context, you might compare the research discussed here with some other recent advances that might be of use in repairing or replacing mitochondria - this is far from the only line of research that might turn out to win the day: