INCREASED SATELLITE CELL APOPTOSIS IN VASTUS LATERALIS MUSCLE AFTER ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTION

ObjectiveRecovery of the quadriceps femoris muscle after anterior ligament reconstruction is impaired. The aim of this study was to investigate satellite cell content and function of the vastus lateralis muscle after anterior ligament reconstruction.MethodsBiopsies were obtained from the vastus lateralis muscle of 16 recreational athletes immediately before and again 12 weeks after anterior ligament reconstruction. Total satellite cell number (Pax7⁺), activated (Pax7⁺/MyoD⁺), differentiating (Pax7^–/MyoD⁺), and apoptotic (Pax7⁺/TUNEL⁺) satellite cells, myofibers expressing myosin heavy chain (MHC) I and II, and neonatal MHC (MHCneo) were determined immunohistochemically.ResultsAfter anterior ligament reconstruction, the number of apoptotic satellite cells was significantly (p = 0.019) increased, concomitant with a significant (p < 0.001) decrease in total satellite cell number, with no change in activated and differentiating satellite cell number. MHCneo⁺ myofibers tended towards an increase.CONCLUSIONSatellite cell apoptosis and the reduction in the satellite cell pool might provide an explanation for prolonged quadriceps muscle atrophy after anterior ligament reconstruction.LAY ABSTRACTProtracted muscle atrophy is common after anterior ligament reconstruction, even if athletes adhere to a structured rehabilitation programme. Satellite cells, the stem cells of skeletal muscle, play an important role in recovery of an atrophied muscle. Exercise can activate satellite cells, induce their proliferation, and probably also differentiation of these stem cells. The current study evaluated satellite cell content and function in biopsies from the vastus lateralis muscle of 16 recreational athletes immediately before and 12 weeks after anterior ligament reconstruction. After anterior ligament reconstruction, an increased number of satellite cells showed signs of apoptosis (cell death). Furthermore, total satellite cell number was decreased, with no change in the numbers of activated and differentiating satellite cells. The number of regenerating myofibers expressing neonatal myosin tended to increase. In conclusion, satellite cell apoptosis and the reduced satellite cell number might provide an explanation for the impaired muscle recovery after anterior ligament reconstruction.Key words: satellite cells, apoptosis, muscle regeneration, developmental myosin heavy chain, muscular atrophy, quadriceps muscle

Protracted atrophy and weakness of the quadriceps muscle are common after anterior cruciate ligament (ACL) injury and/or anterior cruciate ligament reconstruction (ACL-R), even if the patients undergo guided rehabilitation programmes (1–4). Muscle recovery is compromised due to negative changes in the knee extensor muscles, most likely due to impaired neuromuscular function (5), post-surgery inflammation (6) and immobilization (7). After ACL injury, fibrogenic alterations were observed in biopsies obtained from the vastus lateralis muscle of the injured leg (8, 9). Satellite cell (SC) abundance was also reduced compared with biopsies taken from the vastus lateralis muscle of the uninjured leg (8, 10). Furthermore, there was a surprising lack of increase in SC number after regular rehabilitation training (10), as well as after 12 weeks of supervised quadriceps strength training during rehabilitation after ACL-R (3).SCs play an important role in skeletal muscle growth and regeneration (11, 12). Increases in SC number occur after 11–12 weeks of quadriceps strength training in healthy subjects (13–15). The role of SCs in atrophy of human skeletal muscle, however, has scarcely been investigated. In the very few studies on the role of SCs in atrophy of human skeletal muscle (8, 10, 16), loss of SCs with atrophy is not a consistent finding. However, there is some evidence that a particularly severe atrophic environment, as is found, for example, after severe burn injury, has a negative impact on SC number and SC function and can induce SC apoptosis (17). With regard to findings in animal studies, it has been hypothesized that muscle wasting in old age (sarcopaenia) could at least partly be explained by SC dysfunction with increased SC apoptosis due to chronic low-grade systemic inflammation (18). The significantly reduced SC number in biopsies from the vastus lateralis muscle after ACL injury (8, 10) and the lack of increase in SC number after resumption of muscular training after ACL-R (3, 10) suggest that ACL injury and/or ACL-R with quadriceps tendon or semitendinosus tendon autografts, respectively, might generate a severe atrophic environment with negative effects on SC number and function.The primary aim of this study was to further investigate the effects of ACL-R on SCs and to determine whether the previously described reduction in SC number might be due to SC apoptosis. The study analysed muscle biopsies from the vastus lateralis muscle of recreational athletes immediately before ACL surgery and again after 12 weeks of early rehabilitation.