Abstract
Injury to skeletal muscle affects millions of people worldwide. The underlying regenerative process however, is a very complex mechanism, time-wise highly coordinated, and subdivided in an initial inflammatory, a regenerative and a remodeling phase. Muscle regeneration can be impaired by several factors, among them diet-induced obesity (DIO). In order to evaluate if obesity negatively affects healing processes after trauma, we utilized a blunt injury approach to damage the extensor iliotibialis anticus muscle on the left hind limb of obese and normal weight C57BL/6J without showing any significant differences in force input between normal weight and obese mice. Magnetic resonance imaging (MRI) of the injury and regeneration process revealed edema formation and hemorrhage exudate in muscle tissue of normal weight and obese mice. In addition, morphological analysis of physiological changes revealed tissue necrosis, immune cell infiltration, extracellular matrix (ECM) remodeling, and fibrosis formation in the damaged muscle tissue. Regeneration was delayed in muscles of obese mice, with a higher incidence of fibrosis formation due to hampered expression levels of genes involved in ECM organization. Furthermore, a detailed molecular fingerprint in different stages of muscle regeneration underlined a delay or even lack of a regenerative response to injury in obese mice. A time-lapse heatmap determined 81 differentially expressed genes (DEG) with at least three hits in our model at all-time points, suggesting key candidates with a high impact on muscle regeneration. Pathway analysis of the DEG revealed five pathways with a high confidence level: myeloid leukocyte migration, regulation of tumor necrosis factor production, CD4-positive, alpha-beta T cell differentiation, ECM organization, and toll-like receptor (TLR) signaling. Moreover, changes in complement-, Wnt-, and satellite cell-related genes were found to be impaired in obese animals after trauma. Furthermore, histological satellite cell evaluation showed lower satellite cell numbers in the obese model upon injury. Ankrd1, C3ar1, Ccl8, Mpeg1, and Myog expression levels were also verified by qPCR. In summary, increased fibrosis formation, the reduction of Pax7+ satellite cells as well as specific changes in gene expression and signaling pathways could explain the delay of tissue regeneration in obese mice post trauma.
Highlights
The skeletal muscle exhibits a high potential to regenerate after injury, the total turnover rate of the muscle is very low
In order to assess the influence of obesity on blunt muscle injuries, an established drop tower device was used for trauma induction on the muscle extensor iliotibialis anticus on the left hind leg of 16-weeks old C57BL/6J mice fed either with a 10 or 60% kcal diet (Claes et al, 2006; Weckbach et al, 2013; Werner et al, 2018)
We used a drop tower unit to induce an indirect blunt muscle injury to extensor iliotibialis anticus in female normal weight and obese C57BL/6J mice, thereby not finding any significant differences in force input into the muscle between both groups
Summary
The skeletal muscle exhibits a high potential to regenerate after injury, the total turnover rate of the muscle is very low. The complement system as major fluid phase of the innate immune system is crucially involved in the initiation of the inflammatory phase. It is rapidly activated in injured muscle tissue leading to immune cell infiltration in the lesion site (Frenette et al, 2000). Resident M are activated to remove tissue debris, attract satellite cells to the injured site, and stimulate their proliferation (Tidball and Villalta, 2010; Rigamonti et al, 2014). Satellite cells (Pax7+ progenitor cells), which can be found in a niche between sarcolemma and basal lamina, are activated (as documented by the expression of the myogenic regulatory factors Myf and Myod), transiently proliferate and upregulate genes necessary for terminal differentiation (myogenin and MRF4) (Chargé and Rudnicki, 2004). Research has focused on understanding the processes within the microenvironment that modulate satellite cell renewal and differentiation, including the composition and milieu secreted by both inflammatory and non-inflammatory cells (Boppart et al, 2013; Yin et al, 2013)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
