Alternative Oxidase Gene from Sea Squirts is Used to Partially Bypass a Form of Mitochondrial Dysfunction in Mice

Researchers recently demonstrated that they could rescue a form of mitochondrial dysfunction in mice by importing a gene from a sea squirt species. This is particularly interesting in the context of aging, as it appears to be possible to use this approach to work around any sort of damage to complexes III and IV in the mitochondrial electron transport chain (ETC). Every cell is equipped with a herd of mitochondria that act as generators, packaging the chemical energy store molecules used to power the cell. The ETC is central to this function.

The protein complexes that make up the ETC are made up of a mix of proteins encoded in both nuclear DNA and mitochondrial DNA. Dramatic mutations, such as deletions, can lead to mitochondria that function poorly or not at all. When this occurs during embryonic development, the result is either death or a much shortened and more uncomfortable life. When a mutation in mitochondrial DNA occurs in a single cell in an adult, on the other hand, as the result of the sort of random damage that takes place constantly in cells, it is usually either promptly repaired or the damaged mitochondrion is recycled.

Some forms of damage can lead to a more insidious result, however, producing a mitochondrion that is both dysfunctional and able to evade quality control mechanisms. Since mitochondria replicate like bacteria, on the rare occasions on which this happens, a cell is quickly overtaken by broken mitochondria. The cell becomes broken itself, exporting harmful oxidative molecules into the surrounding tissue and bloodstream. This has a range of undesirable downstream consequences, one of which is the creation of oxidized lipids that contribute to atherosclerosis.

The SENS Research Foundation's approach to this problem is gene therapy to place backup copies of mitochondrial genes into the better protected cell nucleus. Thus even given damage to mitochondrial DNA, there is still a supply of proteins to ensure that the ETC functions correctly. The paper here represents an alternative but conceptually similar approach, adding novel protein machinery from other species that can do some of the work of ETC protein complexes. It only fixes a portion of the lost functionality in this case, but is nonetheless most intriguing. The researchers are focused on mitochondrial disease, but it would be very interesting to repeat their approach in the context of aging and mitochondrial function.

Alternative oxidase-mediated respiration prevents lethal mitochondrial cardiomyopathy

Mitochondrial disorders are the most common class of inherited errors of metabolism. However, effective treatments are lacking, and their clinical management remains largely supportive. In patients with electron transport chain complex III (cIII) deficiency, mutations in several genes encoding either cIII subunits or assembly factors have been identified. These compromise cIII enzymatic activity and result in a wide variety of clinical manifestations.

BCS1L mutations are the most common cause of cIII deficiency, with various neonatal and adult phenotypes described worldwide, the most severe and prevalent of them being GRACILE syndrome. BCS1L is a mitochondrial inner membrane translocase required for correct function of cIII. Homozygous Bcs1lc.A232G (Bcs1lp.S78G) knock-in mice bearing the GRACILE syndrome-analogous mutation recapitulate many of the clinical manifestations, and a short survival of 35 days. In the slightly different C57BL/6JCrl substrain, the mice develop the same early manifestations but do not succumb to the early metabolic crisis. This extends their survival to over 150 days and brings additional later-onset phenotypes.

Under physiological conditions, quinols that transport electrons in the mitochondrial inner membrane are efficiently oxidized by cIII, with electron transfer via cytochrome c and cytochrome c oxidase (complex IV, cIV) to oxygen. However, plants and some lower organisms, but not mammals, express alternative oxidases (AOXs) that transfer electrons directly from quinols to oxygen. Their main role is to maintain electron flow when the cIII-cIV segment of the electron transport chain is impaired, limiting production of ROS and supporting redox and metabolic homeostasis.

Ciona intestinalis AOX has been cloned and expressed in human cultured cells, fruit flies, and mice. In these models, AOX is inert under non-stressed conditions, most likely because it accepts electrons only when the quinone pool is highly reduced, such as under inhibition or overload of cIII or cIV. Accordingly, upon inhibition of cIII or cIV by mutations or chemical inhibitors, ectopic AOX can maintain respiration and prevent cell death.

We set out to test whether AOX expression could prevent the detrimental effects of cIII deficiency in a mammalian model, by restoring electron flow upstream of cIII. To this end, we crossed mice carrying a broadly expressed AOX transgene with the Bcs1lc.A232G mice and assessed disease progression, organ manifestations, and metabolism in the homozygotes with and without AOX expression.

The mice expressing AOX were viable, and their median survival was extended from 210 to 590 days due to permanent prevention of lethal cardiomyopathy. AOX also prevented renal tubular atrophy and cerebral astrogliosis, but not liver disease, growth restriction, or lipodystrophy, suggesting distinct tissue-specific mechanisms. Assessment of reactive oxygen species (ROS) production and damage suggested that ROS were not instrumental in the rescue. Cardiac mitochondrial ultrastructure, mitochondrial respiration, and pathological transcriptome and metabolome alterations were essentially normalized by AOX, showing that the restored electron flow upstream of cIII was sufficient to prevent cardiac energetic crisis. These findings demonstrate the value of AOX, both as a mechanistic tool and a potential therapeutic strategy, for cIII deficiencies.

Comments

Does this approach have the potential to be superior or inferior compared to the SENSRF approach for complex III?

Also it is curious that it didn't work in the mice's livers. Is this just due to differential gene expression in the liver, or is it something else?

Posted by: Jim at December 26th, 2018 11:58 PM

@Jim: It seems inferior. That may be because there are other functions for the damaged proteins, or for other reasons. The allotopic expression route bypasses all such uncertainties.

Posted by: Reason at December 27th, 2018 5:53 AM

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