to the editor: A recent paper by Cornachione et al. (1) claims refutation of the hypothesis that an interaction between titin and actin may be responsible for Ca2+-activated tension or residual force enhancement in muscle. Examination of the evidential basis for this claim reveals that it is premature. In these experiments, the authors used gelsolin to extract actin from single myofibrils. They used fluorescent phalloidin to visualize actin before and after gelsolin extraction [Fig. 1 in Cornachione et al. (1)]. Using these methods, the authors state, in the legend of Fig. 1, “that the A bands are larger and more visible before gelsolin treatment” (1).
There is a major problem with the methods used by Cornachione et al. (1) to visualize actin. Specifically, it is long known that phalloidin staining of muscle does not reflect the known disposition of sarcomeric thin filaments (4). Indeed, as shown by Cornachione et al. (1) in myofibrils from rabbit psoas muscle, phalloidin produces a large region of staining in the A-bands, alternating with a narrow region of staining at the Z-lines prior to treatment with gelsolin (Fig. 1A). However, there is no staining in the I-bands of sarcomeres, despite the known presence of actin in this region. Zhukarev et al. (4) have suggested that nebulin in the sarcomeric I-band may block the binding of phalloidin in this region.
Several recent papers have suggested that a titin-actin interaction could be responsible for titin activation upon Ca2+ influx in muscle (2, 3). These papers specifically hypothesize that I-band titin may interact with actin in active muscles. Therefore, because phalloidin does not bind to actin in the sarcomeric I-band, it is impossible to refute a titin-actin binding interaction as proposed in these recent works using the methods employed by Cornachione et al. (1).
Although Cornachione et al. (1) state that the A bands are larger and more visible before gelsolin treatment, the change in A-band phalloidin fluorescence was not measured quantitatively. Furthermore, it is clear from their Fig. 1B that the Z-lines are wider and the resting sarcomere lengths are longer in gelsolin-treated myofibrils, suggesting that sarcomere integrity may be compromised by the gelsolin treatment (1).
No conflicts of interest, financial or otherwise, are declared by the author.
K.C.N. drafted manuscript.
- Copyright © 2016 the American Physiological Society