Mitochondrial DNA mutations attenuate Bleomycin-induced dermal fibrosis by inhibiting differentiation into myofibroblasts.

Post-mitotic, non-proliferative dermal fibroblasts have crucial functions in maintenance and restoration of tissue homeostasis. They are involved in essential processes such as wound healing, pigmentation and hair growth, but also tumor development and aging-associated diseases. These processes are energetically highly demanding and error prone when mitochondrial damage occurs. However, mitochondrial function in fibroblasts and the influence of mitochondrial dysfunction on fibroblast-specific demands are still unclear. To address these questions, we created a mouse model in which accelerated cell-specific mitochondrial DNA (mtDNA) damage accumulates. We crossed mice carrying a dominant-negative mutant of the mitochondrial replicative helicase Twinkle (RosaSTOP system) with mice that express fibroblast-specific Cre Recombinase (Collagen1A2 CreERT) which can be activated by Tamoxifen (TwinkleFIBRO). Thus, we are able to induce mtDNA deletions and duplications in specific cells, a process which resembles the physiological aging process in humans, where this damage accumulates in all tissues. Upon proliferation in vitro, Tamoxifen induced Twinkle fibroblasts deplete most of their mitochondrial DNA which, although not disturbing the stoichiometry of the respiratory chain complexes, leads to reduced ROS production and mitochondrial membrane potential as well as an anti-inflammatory and anti-fibrotic profile of the cells. In Sodium Azide treated wildtype fibroblasts, without a functioning respiratory chain, we observe the opposite, a rather pro-inflammatory and pro-fibrotic signature. Upon accumulation of mitochondrial DNA mutations in vivo the TwinkleFIBRO mice are protected from fibrosis development induced by intradermal Bleomycin injections. This is due to dampened differentiation of the dermal fibroblasts into α-smooth-muscle-actin positive myofibroblasts in TwinkleFIBRO mice. We thus provide evidence for striking differences of the impact that mtDNA mutations have in contrast to blunted mitochondrial function in dermal fibroblasts and skin homeostasis. These data contribute to improved understanding of mitochondrial function and dysfunction in skin and provide mechanistic insight into potential targets to treat skin fibrosis in the future.