Treadmill running exercise results in the presence of numerous myofibroblasts in mouse patellar tendons

Abstract

Mechanical loading is known to alter tendon structure, but its cellular mechanisms are unclear. This study aimed to determine the effect of mechanical loading on tendon cells in vivo. C57BL/6J female mice were used in a treadmill running study. The treadmill running protocol consisted of treadmill training for 1 week, followed by sustained moderate running at 13 m/min for 50 min/day, 5 days/week, for 3 weeks. Immunohistochemical staining of tendon sections of mice after treadmill running revealed that numerous cells in the tendon section expressed alpha-SMA, whereas in the tendon sections of control mice, only a few cells exhibited weak alpha-SMA signals. Furthermore, mouse patellar tendon cells (MPTCs) derived from treadmill running mice were generally larger in culture, proliferated faster, expressed a higher level of alpha-SMA, and formed more abundant stress fibers compared to MPTCs from control mice. In addition, MPTCs from treadmill running mice generated larger traction forces (169 +/- 66.1 Pa) than those from control mice (102 +/- 34.2 Pa). Finally, cells from treadmill running mice produced higher levels of total collagen (516.4 +/- 92.7 microg/10,000 cells) than their counterparts (303.9 +/- 34.8 microg/10,000 cells). Thus, mechanical loading via treadmill running increased the presence of myofibroblasts in mouse patellar tendons. As myofibroblasts are activated fibroblasts, their presence in the tendon following treadmill running indicates that they actively repair and remodel tendon tissue under strenuous mechanical loading, leading to known changes in tendon structure.

Fig. 1

Confocal microscopy of representative tendon sections stained by immunohistochemistry for α-SMA expression of MPTCs from treadmill running mouse (A) and cage control mouse (B). (Scale bars: 20 µm).

Fig. 2

The morphology of MPTCs from treadmill running mouse (A) and cage control mouse (B). MPTCs from treadmill running group show a typical myofibroblast morphology with larger and more spread out cells compared to those from the cage control group. (Scale bars: 100 µm).

Fig. 3

Western blot analysis of α-SMA expression in MPTCs from treadmill running and control mice (A). MPTCs from treadmill running mice had much higher levels of α-SMA expression compared to those cells from cage control mice. Immunocytochemistry for α-SMA (green) and actin filaments (red) of MPTCs from treadmill running mouse (B) and control mouse (C). More a-SMA green signals (arrows) were seen in MPTCs from treadmill running mice than in cells from cage control mice. (Scale bars: 20 µm.)

Fig. 4

Population doubling time (PDT) of MPTCs. After treadmill running, myofibroblasts from exercised mice proliferated much faster than those cells from cage control mice.

Fig. 5

Traction forces of MPTCs from treadmill running and control mice. After treadmill running, MPTCs generated larger traction forces than cells from cage control mice.

Fig. 6

Total collagen production by MPTCs from treadmill running and control mice. Treadmill running significantly increased cellular collagen production compared to cage control mice.