* The Impact of Age on Skeletal Muscle Progenitor Cell Survival and Fate After Injury

Three-dimensional volume and shape of the infrapatellar fat pad during quasi-static knee extension from 30° to 0°: comparisons of patients with osteoarthritic knees and young, healthy individuals

[Purpose] Previous studies suggest that the infrapatellar-fat-pad is affected by aging or knee osteoarthritis, and that the infrapatellar-fat-pad in knee osteoarthritis cases may be associated with limited mobility during knee movement. This study aimed to determine changes in the shape and volume of the infrapatellar-fat-pad between 30° and 0° of knee extension in knee osteoarthritis cases and in young, healthy individuals, and to characterize differences in patellar mobility, patellar tendon mobility, and length between the groups. [Participants and Methods] We created 3D models of the infrapatellar-fat-pad, the patellar tendon, and bones using sagittal MRI with the knee at 30° and 0°. The following four parameters were determined: (1) movement of the infrapatellar-fat-pad; (2) infrapatellar-fat-pad volume; (3) angle and surface length of the patellar tendon; and (4) patellar movement. [Results] Compared with the knee osteoarthritis group, the healthy group showed (1) reduced anterior movement of the infrapatellar-fat-pad; (2) smaller volume changes only in the infero-postero-lateral portion; and (3) no changes in the angle of the patellar tendon to the tibial plateau between 30° to 0°. [Conclusion] In conclusion, between 30° and 0°, (1) the infrapatellar-fat-pad in patients with knee osteoarthritis exhibited less anterior movement, and (2) the patellar tendon angle was diminished in patients with knee osteoarthritis compared with those of young-healthy knees.

The role of the aging microenvironment on the fate of PDGFRβ lineage cells in skeletal muscle repair

Background

During aging, perturbation of muscle progenitor cell (MPC) constituents leads to progressive loss of muscle mass and accumulation of adipose and fibrotic tissue. Mesenchymal stem cells (MSCs) give rise to adipocytes and fibroblasts that accumulate in injured and pathological skeletal muscle through constitutive activation of platelet-derived growth factor receptors (PDGFRs). Although the role of the PDGFRα has been widely explored, there is a paucity of evidence demonstrating the role of PDGFRβ in aged skeletal muscle.

Methods

In this study, we investigated the role of PDGFRβ lineage cells in skeletal muscle during aging by using Cre/loxP lineage tracing technology. The PDGFR-Cre mice were crossed with global double-fluorescent Cre reporter mice (mTmG) that indelibly marks PDGFRβ lineage cells. Those cells were analyzed and compared at different ages in the skeletal muscle of the mice.

Results

Our results demonstrated that PDGFRβ lineage cells isolated from the muscles of young mice are MPC-like cells that exhibited satellite cell morphology, expressed Pax7, and undergo myogenic differentiation producing myosin heavy chain expressing myotubes. Conversely, the PDGFRβ lineage cells isolated from muscles of old mice displayed MSC morphology with a reduced myogenic differentiation potential while expressing adipogenic and fibrotic differentiation markers. PDGFRβ lineage cells also gave rise to newly regenerated muscle fibers in young mice after muscle injury, but their muscle regenerative process is reduced in old mice.

Conclusions

Our data suggest that PDGFRβ lineage cells function as MPCs in young mice, while the same PDGFRβ lineage cells from old mice undergo a fate switch participating in adipose and fibrotic tissue infiltration in aged muscle. The inhibition of fate-switching in PDGFRβ lineage cells may represent a potential approach to prevent fibrosis and fatty infiltration in skeletal muscle during the aging process.

Enhancement of myogenic potential of muscle progenitor cells and muscle healing during pregnancy

The decline of muscle regenerative potential with age has been attributed to a diminished responsiveness of muscle progenitor cells (MPCs). Heterochronic parabiosis has been used as a model to study the effects of aging on stem cells and their niches. These studies have demonstrated that, by exposing old mice to a young systemic environment, aged progenitor cells can be rejuvenated. One interesting idea is that pregnancy represents a unique biological model of a naturally shared circulatory system between developing and mature organisms. To test this hypothesis, we evaluated the muscle regeneration potential of pregnant mice using a cardiotoxin (CTX) injury mouse model. Our results indicate that the pregnant mice demonstrate accelerated muscle healing compared to nonpregnant control mice following muscle injury based on improved muscle histology, superior muscle regeneration, and a reduction in inflammation and necrosis. Additionally, we found that MPCs isolated from pregnant mice display a significant improvement of myogenic differentiation capacity in vitro and muscle regeneration in vivo when compared to the MPCs from nonpregnant mice. Furthermore, MPCs from nonpregnant mice display enhanced myogenic capacity when cultured in the presence of serum obtained from pregnant mice. Our proteomics data from these studies provides potential therapeutic targets to enhance the myogenic potential of progenitor cells and muscle repair.

Aged Skeletal Muscle Retains the Ability to Remodel Extracellular Matrix for Degradation of Collagen Deposition after Muscle Injury

Aging causes a decline in skeletal muscle function, resulting in a progressive loss of muscle mass, quality, and strength. A weak regenerative capacity is one of the critical causes of dysfunctional skeletal muscle in elderly individuals. The extracellular matrix (ECM) maintains the tissue framework structure in skeletal muscle. As shown by previous reports and our data, the gene expression of ECM components decreases with age, but the accumulation of collagen substantially increases in skeletal muscle. We examined the structural changes in ECM in aged skeletal muscle and found restricted ECM degradation. In aged skeletal muscles, several genes that maintain ECM structure, such as transforming growth factor β (TGF-β), tissue inhibitors of metalloproteinases (TIMPs), matrix metalloproteinases (MMPs), and cathepsins, were downregulated. Muscle injury can induce muscle repair and regeneration in young and adult skeletal muscles. Surprisingly, muscle injury could not only efficiently induce regeneration in aged skeletal muscle, but it could also activate ECM remodeling and the clearance of ECM deposition. These results will help elucidate the mechanisms of muscle fibrosis with age and develop innovative antifibrotic therapies to decrease excessive collagen deposition in aged muscle.