UCLA Researchers have exploited a recently-discovered mammalian system for the mitochondrial import of nuclear-encoded RNA to import, express, and demonstrate functional protein translation from engineered mRNA and tRNA constructs. They used this system, with modifications for mitochondrial targeting and orthotopic translation, to rescue respiration in human mitochondriopathy cells. While further characterization and extension is clearly needed, this approach appears offer great promise for the correction of age-related mitochondrial DNA mutations.
SENS describes the promise for life extension
The UCLA group’s approach is highly promising. Their work builds upon and may potentially supersede several previous approaches to the problem of mitochondrial mutations that occur as a result of the degenerative aging process, including allotopic protein expression,(6) its optimization using an MTS,(3-5) and the exploitation of the multiprotein RNA import complex (RIC) of the protozoal parasite Leishmania tropica(7) (which the investigators characterize as “requir[ing] the introduction of nonnative tRNAs with foreign protein factors or the transfer of a large multisubunit aggregate into cells, which is of low efficiency and difficult to reproduce in desirable disease-relevant settings”(2)).
As compared to allotopic protein expression, an RNA-based approach has the theoretical advantage of abrogating the difficulties encountered thus far with the mitochondrial import of large and hydrophobic proteins. But as we suggested in discussion of their earlier, more discovery-phase research, allotopic protein and RNA approaches are not mutually exclusive: different mitochondrially-encoded proteins could be either allotopically expressed, or their mRNAs generated allopically and imported for in situ translation, depending on the ease or efficiency of each approach for the protein in question. The use of a dual-track approach might be speculated to have an additional advantage, in avoiding any hypothetical “saturation” of the relevant mitochondrial import machinery (PNPase or TIM/TOM complex) if only one approach is used for all 13 mitochondrially-encoded proteins.
There is clearly more work to do to probe the limits of this approach. Thus far, even the largest RNA that Teitell’s group have imported (that for human COX2) is relatively small, and the efficiency relatively low, so larger proteins’ mRNA must be trialed. Individual mRNA constructs should be evaluated for the dispensibility, and for the relative efficiency, of including (or not) of the modification of the aminoacyl stem of the RP import sequence, and of the MTS. The complete translation of “allotopic” mRNA for COX2 and for future candidates into their encoded proteins, and their structural integrity, should be more rigorously tested, and a more ironclad ascertainment be provided of their functional integration into the ETS.
For their part, Dr. Teitell’s group is evidently optimistic, and have clearly moved beyond the basic science focus of their earlier report(1):
this approach may generalize to mtDNA mutations in mt-tRNAs, mt-rRNAs, and protein-encoding mtRNAs as well as to heteroplasmic mtDNA populations, where ribozymes can be targeted. Thus, this rational transcript engineering approach may represent a unique therapeutic opportunity for a wide range of diseases caused by mutations in the mitochondrial genome for which current effective therapies are lacking(2)
… a range which, as they note elsewhere in their report, extends to “muscular and neuronal diseases and … decline of organ function with aging.” They will doubtless be exploring many of the questions we have poised above — and are hereby put on notice that others are now considering the theft of their thunder. In response to this report, SENS Foundation CSO Dr. Aubrey de Grey has said that “If this is as good as it looks, I think it could be a real game-changer”, and Dr. O’Connor and his team at the SENS Foundation RC are considering testing a construct based on Teitell’s methods in a system that the RC has already generated and used for testing of allotopic expression of cytochrome B.
The race is on — as it should be, for the stakes are large. Large, age-related deletions in mtDNA are likely responsible for the systemic rise in oxidative stress with aging, and for localized but terrible pathologies of skeletal muscle and substantia nigra dopaminergic neurons in aging bodies. The obviation of these mutations is a desperate medical need, and biomedicine is shamed for every day that a solution is delayed. This new method must be tested and exploited to its limits, and all approaches must be trialed, until the fires of life are once again burning in rejuvenated cells, in bodies restored to their youthful prime.