Department of Anatomy, University of Bern, Switzerland
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
(5-10x). 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 40-300x (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-ß 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.
References
Chiquet, M., Renedo, A. S., Huber, F. and Flück, M. (2003). How do fibroblasts translate mechanical signals into changes in extracellular matrix production? Matrix Biol. 22, 73-80.[CrossRef][Medline]
Flück, M., Tunc-Civelek, V. and Chiquet, M.
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