Factors secreted by muscle-derived stem cells boost regenerative capacity of stem cells in old animals.
Researchers have found that transplanting a particular type of muscle-derived stem cell into either naturally aged mice or animals with a genetic form of accelerated aging can prolong health and lifespan. Researchers at the Stem Cell Research Center in Pittsburgh, and the University of Pittsburgh School of Medicine, demonstrated that injecting muscle-derived stem/progenitor cells (MDSPCs) taken from normal young mice into aging animals led to tissue regeneration in multiple organ systems. Interestingly, the benefits didn’t appear to relate directly to proliferation and differentiation of the donor cells, but rather was due to factors secreted by them boosting the regenerative capacity of the recipient’s own aged MDSPCs.
Johnny Huard, Ph.D., and colleagues report their findings in Nature Communications in a paper titled “Muscle-derived stem/progenitor cell dysfunction limits healthspan and lifespan in a murine progeria model.” They suggest the results indicate that MDSPCs, and potentially other forms of stem cells, could have therapeutic potential in holding back age-related physiological decline, and as a treatment for progeria.
Reduced regenerative capacity that comes with aging is linked with the loss of number, or impaired function or proliferative/differentiation capacity of adult stem cells in multiple tissues. To explore the impact of aging on stem cells more closely in the musculoskeletal system, the Pittsburgh team focused on examining murine MDSPCs both during the course of natural aging and in a mouse model of the accelerated aging disorder progeria. Human XFE syndrome is caused by mutations in XPF (xeroderma pigmentosum complementation group F). In the Ercc−/− murine model of the disease, animals with undetectable levels of ERCC–XPF have a lifespan of just one month, while Ercc1−/Δ mice that express 10% of the normal level of ERCC1–XPF have a lifespan of seven months. Both these models mimic the systemic accelerated aging of XFE progeroid syndrome in humans, the authors note.
To determine whether there is a loss of stem cell function with aging and progeria, MDSPCs were isolated from old wild-type, progeroid Ercc−/− (2 – 3-weks old) and Ercc1−/Δ animals (five months of age). Analysis of growth kinetics showed that in comparison with MDSPCs from young wild-type mice, those from the old wild-type animals, and the Ercc−/− and Ercc1− /Δ animals, proliferation was significantly reduced, and cell doubling time longer.
Cultured old and progeroid MDSPCs that were switched to a differentiation medium also exhibited reduced myogenic, osteogenic, and chondrogenic differentiation capacity. Interestingly, when MDSPCs isolated either from young wild-type mice or from progeroid mice were injected intramuscularly into a mouse model of Duchenne muscular dystrophy (DMD), those from the progeroid animals had an impaired ability to regenerate muscle fibers.
To confirm that MDSPC impairments in progeroid and old mice weren’t an artefact resulting from extensive ex vivo passaging, the team measured the number of cells with stem-progenitor surface markers and capacity for myogenic differentiation, within a day after isolation from skeletal muscle. These evaluations supported the notion that it was indeed aging, and not cell passaging, which was associated with a loss of MDSPC number as well as their capacity for myogenic differentiation.
The effects of impaired MDSPC number and differentiation capacity in old and progeroid mice could be visualized in terms of the regeneration of muscle in a cardiotoxin injury-based model. While the total area of muscle regeneration was not significantly different in young wild-type, old wild-type, and progeroid mice, the average cross-sectional area of the regenerated myofibers was much smaller, and there was increased fibrosis.
Encouragingly, the lifespan of Ercc−/− animals could be significantly prolonged by administering intraperitoneal injections of MDSPCs taken from young wild-type animals. In contrast, injections of mouse embryonic fibroblasts or MDSPCs taken from old or progeroid animals had no beneficial effects. Ercc1−/Δ animals receiving young wild-type MDSPCs similarly exhibited a delayed onset of range of progeroid-related symptoms. “These data provide evidence that loss of stem cell function has a direct causal role in aging-related degeneration,” the authors state.
Interestingly, transplanted MDSPCs were found to engraft only in a limited number of tissues in recipient animals, and weren’t found in the heart, brain, skeletal muscles, or lymph nodes. This observation, combined with the fact that the therapeutic effects of transplanted MDSPCs were evident in recipient progeroid mice within three weeks, indicated it was unlikely that the benefits were due to substantial MDSPC engraftment, proliferation, differentiation, and tissue regeneration.
Indeed, subsequent co-culturing experiments showed that exposing MDSPCs from progeroid Ercc−/− mice to proliferation media from young wild-type MDSPCs was sufficient to significantly boost their proliferation and myogenic differentiation capacity. Similarly, the impaired differentiation of old wild-type MDSPCs was rescued by culturing them in the presence of conditioned media from their young counterparts.
The indication that secreted factors were responsible for rejuvenating native MDSPCs in old wild-type mice and progeroid animals were supported by the finding that intraperitoneal injection of young wild-type MDSPCs promoted neovascularization in the brain and led to increased muscle regeneration, even though the transplanted MDSPCs weren’t themselves found in these tissues. Moreover, transplanting young wild-type MDSPCs directly into progeroid muscle led to muscle regeneration as well as neovascularization, which demonstrates a paracrine mechanism requiring secreted factors, the researchers note.
“These observations demonstrate that old, dysfunctional MDSPCs can be rescued by exogenous factors, rather than the cells being inherently defective. This is in contrast to aged hematopoietic stem cells, which appear to have an intrinsic defect.”
The team concludes that MDSPCs may have therapeutic use for delaying aging-related functional decline or treating human progerias. “It will be of critical importance to test other adult stem cell types for a similar therapeutic effect.”