Rebuttal

Abstract

We would like to thank Järvinen et al. for their extensive and thoughtful reply to our letter. We basically agree with many of their arguments. We are especially interested in their new data indicating a synergy between (micro)injury-induced inflammation and mechanical stress in inducing tenascin-C (TN-C) expression. This is where our different views might meet: mechanical stress is perhaps not sufficient, but nevertheless essential, for TN-C induction both at normal and ectopic sites.Järvinen et al. provide autoradiographs of in situ hybridizations to document their new set of experiments. Unfortunately, as is the case in their former paper (Järvinen et al., 2003), these photographs are taken at very low magnification (5-10×). Under these conditions it is not possible to determine the cellular source of the hybridization signal. Moreover, if there was low to medium level expression of TN-C mRNA in endomysial fibroblasts (e.g. in uninjured but loaded parts of the muscle) it remained undetected. If we photographed our old samples of loaded chick muscle at a magnification this low we would barely see the signal, although its cellular specificity becomes very obvious at 40-300× (see Flück et al., 2000). Nevertheless, the synergy hypothesis by Järvinen et al. is indeed worth being tested further. Concerning the points raised regarding the experimental design, the high magnitude of mechanical factors acting on the muscle during the predominately eccentric, low-repetitive (unidirectional) ALD muscle stretch (Flück et al., 2000) compared with that acting on the muscle during the concentric, high-repetitive contractions with running on the inclined treadmill (Järvinen et al., 2003) may explain part of the difference in TNC mRNA expression in the two reports (Lindstedt et al., 2001).Lastly, Järvinen et al. seem to think that TN-C induction in muscle fibroblasts must involve a paracrine factor originating from the injured and/or loaded muscle fibers, and urge us to test this. In cultured fibroblasts, however, we have demonstrated that tensile stress can directly induce TN-C in the absence of other cells (for a review, see Chiquet et al., 2003); the same has been shown for heart myocytes (Yamamoto et al., 1999). Medium conditioned by stressed cells does not induce TN-C in resting fibroblasts. In our hands, serum and certain growth factors such as TGF-β act in an additive (rather than a synergistic) way with tensile stress to induce TN-C expression. These and other results obtained in vitro should be taken into account when designing experiments in a more complex in vivo environment.