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Developmental biology - Mitochondrial DNA|
Reversing effects of an age-associated gene
The mutation that does this occurs in a nuclear gene affecting how mitochondria function. These tiny organelles are the powerhouses of cells. Ninety percent of the chemical energy cells need to survive comes from their numerous mitochondria.
In humans, a decline in mitochondrial function is seen during aging, and mitochondrial dysfunction can drive age-related diseases. Depleting DNA in mitochondria is implicated in human mitochondrial diseases, cardiovascular disease, diabetes, age-associated neurological disorders and cancers.
"This mouse model," Singh said, "should provide an unprecedented opportunity for the development of preventive and therapeutic drug development strategies to augment the mitochondrial functions for the treatment of aging-associated skin and hair pathology and other human diseases in which mitochondrial dysfunction plays a significant role."
The mutation in the mouse model is induced when the antibiotic doxycycline is added to the food or drinking water. This depletes mitochondrial DNA because the enzyme to replicate the DNA becomes inactive.
Dramatically, this hair loss and wrinkled skin could be reversed by turning off the gene mutation process. The photos to the right show the hair loss and wrinkled skin after two months of doxycycline induction, and the same mouse a month later after doxycycline was stopped, allowing restoration of the depleted mitochondrial DNA. The work is published in Cell Death and Disease, a Nature online journal.
Little change was seen in other organs when the mutation was induced, suggesting an important role for mitochondria in skin compared to other tissues.
The wrinkled skin showed changes similar to those seen in both intrinsic and extrinsic aging - intrinsic aging is the natural process of aging, and extrinsic aging is the effect of external factors that influence aging, such as skin wrinkles that develop from excess sun or long-term smoking. Among the details, the skin of induced-mutation mice showed increased numbers of skin cells, abnormal thickening of the outer layer, dysfunctional hair follicles and increased inflammation that appeared to contribute to skin pathology. These are similar to extrinsic aging of the skin in humans. The mice with depleted mitochondrial DNA also showed changed expression of four aging-associated markers in cells, similar to intrinsic aging.
The skin also showed disruption in the balance between matrix metalloproteinase enzymes [enzymes which play a significant role in the fusion of muscle cells during embryo development] and their tissue-specific inhibitor. A balance of these two is needed to maintain collagen fibers in the skin that prevent wrinkling.
The mitochondria of induced-mutation mice had reduced mitochondrial DNA content, altered mitochondrial gene expression and destabilized large complexes in mitochondria involved in oxidative phosphorylation which changes ADP into energized ATP. Reversal of the mutation restored mitochondrial function, as well as the normal function of mice skin and hair. This showed that mitochondria are reversible regulators of skin aging and loss of hair, an observation that Singh calls "surprising."
"It suggests that epigenetic mechanisms underlying mitochondria-to-nucleus cross-talk must play an important role in the restoration of normal skin and hair. Further experiments are required to determine whether phenotypic changes in other organs can also be reversed to wildtype [normal] level by restoration of mitrochondrial DNA."
Mitochondrial DNA (mtDNA) depletion is involved in mtDNA depletion syndromes, mitochondrial diseases, aging and aging-associated chronic diseases, and other human pathologies. To evaluate the consequences of depletion of mtDNA in the whole animal, we created an inducible mtDNA-depleter mouse expressing, in the polymerase domain of POLG1, a dominant-negative mutation to induce depletion of mtDNA in various tissues. These mice showed reduced mtDNA content, reduced mitochondrial gene expression, and instability of supercomplexes involved in oxidative phosphorylation (OXPHOS) resulting in reduced OXPHOS enzymatic activities. We demonstrate that ubiquitous depletion of mtDNA in mice leads to predominant and profound effects on the skin resulting in wrinkles and visual hair loss with an increased number of dysfunctional hair follicles and inflammatory responses. Development of skin wrinkle was associated with the significant epidermal hyperplasia, hyperkeratosis, increased expression of matrix metalloproteinases, and decreased expression of matrix metalloproteinase inhibitor TIMP1. We also discovered markedly increased skin inflammation that appears to be a contributing factor in skin pathology. Histopathologic analyses revealed dysfunctional hair follicles. mtDNA-depleter mice also show changes in expression of aging-associated markers including IGF1R, KLOTHO, VEGF, and MRPS5. mtDNA-repleter mice showed that, by turning off the mutant POLG1 transgene expression, mitochondrial function, as well as the skin and hair pathology, is reversed to wild-type level. To our knowledge that restoration of mitochondrial functions can reverse the skin and hair pathology is unprecedented.
Authors: Bhupendra Singh, Trenton R. Schoeb, Prachi Bajpai, Andrzej Slominski and Keshav K. Singh.
Acknowledgements: This work was supported by grants from Veterans Administration 1I01BX001716 and NIH R01 CA204430 to K.K.S., and partial support from NIH grants 1R01AR071189-01A1 and R01AR073004 to A.S.
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a. LEFT Normal (wild-type) mouse. MIDDLE Mouse after 2 months doxycycline (dox) exposure.
RIGHT same mouse following 1 month withdrawal from doxycycline.
b. Stained dorsal skin of wild-type mouse
(i) mtDNA-depleted mouse after 2 months of continuous dox exposure
(ii) mtDNA-depleted mouse after 1 month of dox withdrawal (mtDNA repletion)
(iii) stained skin sections showing presence of increased mast cells in skin
c. A lower number of mast cells - a type of white blood cell - are present in the skin of mtDNA-depleted mice after 1 month of dox withdrawal. RED ARROWS indicate presence of mast cells in skin sections.