Both prenatal undernutrition and early postnatal overnutrition are associated with reduced muscle stem cell number and reduced muscle regenerative capacity. These defects, while sustained during early life, persist into adulthood and may contribute to developmentally mediated reductions in muscle mass and altered body composition. Given that muscle mass is an important mediator of insulin-stimulated glucose uptake and systemic metabolism, reductions in stem cell number may contribute to associations between early life nutrition and developmental risk for adult disease.
Muscle growth and maintenance depend on adequate stem cell availability and repair. Muscle stem cells are normally quiescent, but upon muscle injury, can be activated to proliferate, self-renew, and differentiate into myoblasts. These myoblasts subsequently fuse with other myoblasts, as well as damaged muscle fibers, to form new functional muscle. This process may be particularly vulnerable to postnatal nutrition, as starvation reduces satellite cell proliferation and increases apoptosis in poultry [15
] and satellite cell number is reduced in malnourished human children [18
]. Reduced function of genes critical for postnatal satellite cell survival, such as Pax7, can cause accelerated muscle wasting soon after birth [14
]. Thus, the reductions in stem cell number observed in UN mice may contribute directly to the decreased muscle mass observed during postnatal growth and in adulthood—a key phenotype in both UN mice and LBW humans [4
]. Indeed, these experimental data confirm the hypothesis suggested by Cianfarani, who proposed that stem cells are critical for normal tissue maintenance and function and that prenatal malnutrition would decrease these populations leading to early tissue malfunction and contribute to LBW-associated disease phenotypes [30
Alterations in stem cell number or function can also affect the ability of an organism to repair [14
]. For example, mice with mutations in the dystrophin gene (mdx
mice) have poor integrity of muscle fibers, increased vulnerability to mechanical stress, and thus need for perpetual repair, but also have reduced frequency of SMPs [22
]. Together with the early observations of Schultz and colleagues that repeated stresses reduce the proliferative capacity of satellite cells [31
], it is also possible that reductions in regeneration in UN mice could reflect not only early life reductions in SMP number, but also subclinical increases in muscle damage accumulated during life, which further magnify age-related reductions in repair capacity.
Tissue stem cell number and function are highly dependent on features of the systemic and local microenvironment (niche), as demonstrated for aging-related dysfunction. For example, in muscle, repair responses may convert to favor fibrogenic, rather than myogenic, processes with age [32
]. Regeneration capacity can be restored in aging mice by exposure to the circulation of young mice, in part via reactivation of Notch signaling [27
]. In accord with this concept, the present data suggest that alterations in the intrauterine or early postnatal nutritional/metabolic environment also affect muscle regenerative function. Since functional impairment was detected in vivo, reflected by reduced regeneration after muscle injury, but not during ex vivo myogenic colony formation assays, it is likely that reduced availability of myogenic stem cells in UN mice contributes to this process.
Since undernutrition in mice is associated with early onset adiposity and progressive glucose intolerance with aging, it is possible that obesity per se or other features of a diabetogenic microenvironment might contribute to reductions in stem cell frequency or function. Interestingly, early life obesity (produced by high-fat feeding) was associated with a 27% reduction in SMP frequency and reduced regeneration after muscle injury. Moreover, the effects of HFD to reduce regeneration after muscle injury were additive with prenatal undernutrition. Thus, an adverse prenatal metabolic environment, early life onset of nutritional obesity (or both), and chronic obesity may all be detrimental for stem cell activity and repair.
While the specific mechanisms mediating the effects of both the prenatal and postnatal nutrient environment on stem cell number and/or function remain unclear at this time, stem cell-independent mechanisms may also contribute to our findings of decreased regeneration in UN and/or HFD-fed mice, including the size of the injury, extent of inflammation, and other aspects of the systemic or local tissue milieu. While larger injuries, whether in absolute size or as a percent of the muscle, could slow the regenerative process (fewer satellite cells to repair the injury), there were no differences in the area of injury (percentage of cross-sectional area) in this model. Similarly, either reduced or excessive inflammation could also impair muscle growth, regeneration, and injury responses [28
]. However, systemic or local inflammation was not altered in UN mice. While elevated glucose levels may alter differentiation of muscle stem cells [37
], circulating glucose levels are consistently normal in our models at the age when stem cell number was assessed [6
]. Whether nutritional or obesity-related alterations in amino acids, other metabolites, or nutritionally responsive growth factors critical for satellite cell development (e.g., insulin like growth factor 1 [IGF1]) could also contribute is an important question for future study [16
]. Additional developmental signals or components of the systemic or tissue microenvironment, for example, stem cell niche [39
], or growth factors produced locally by muscle fibers [16
] could also contribute to reductions in stem cell frequency, function, and/or differences in myogenic versus adipogenic lineage development during regeneration [43
], in the setting of UN exposure and obesity.
In summary, our studies demonstrate that low birth weight induced by prenatal undernutrition is associated with decreased frequency and in vivo functionality of muscle stem cells, as assessed by regeneration after injury, reduced muscle mass, and altered fiber type. Further, a HFD during early life is also associated with reduced stem cell number and decreased regenerative capacity, in both normal- and low-birth-weight mice. Thus, decreased number of stem cells and associated changes in regenerative capacity can result from adverse metabolic environments during both prenatal and early postnatal life. Such reductions in muscle stem cell number and function may contribute to alterations in muscle mass and body composition associated with developmentally mediated risk for adult disease.