Silencing SERCA1b in a few fibers stimulates growth in the entire regenerating soleus muscle

Follistatin treatment suppresses SERCA1b levels independently of other players of calcium homeostasis in C2C12 myotubes

Follistatin (FS) is a high affinity activin-binding protein, neutralizing the effects of the Transforming Growth Factor-beta (TGF-β) superfamily members, as myostatin (MSTN). Since MSTN emerged as a negative regulator, FS has been considered as a stimulator of skeletal muscle growth and differentiation. Here, we studied the effect of FS administration on the Ca²⁺-homeostasis of differentiating C2C12 skeletal muscle cells. FS-treatment increased the fusion index, the size of terminally differentiated myotubes, and transiently elevated the expression of the calcium-dependent protein phosphatase, calcineurin, at the beginning of differentiation. Functional experiments did not detect any alterations in the Ca²⁺ transients following the stimulation by KCl or caffeine in myotubes. On the other hand, decreased Ca²⁺-uptake capability was determined by calculating the maximal pump rate (332 ± 17 vs. 279 ± 11 µM/s, in control and FS-treated myotubes, respectively; p < 0.05). In the same way, the expression and ATPase activity of the neonatal sarcoplasmic/endoplasmic reticulum Ca²⁺ATPase (SERCA1b) were decreased (0.59 ± 0.01 vs. 0.19 ± 0.01 mM ATP/min, in control and FS-treated myotubes, respectively; p < 0.05). However, the expression level of other proteins involved in Ca²⁺-homeostasis and differentiation (calsequestrin, STIM1, MyoD) were not affected. Our results suggest that the FS controlled myotube growth is paralleled with the tight regulation of cytosolic calcium concentration, and the decline of SERCA1b appears to be one of the key components in this process.

The Effect of SERCA1b Silencing on the Differentiation and Calcium Homeostasis of C2C12 Skeletal Muscle Cells

The sarcoplasmic/endoplasmic reticulum Ca2+ATPases (SERCAs) are the main Ca2+ pumps which decrease the intracellular Ca2+ level by reaccumulating Ca2+ into the sarcoplasmic reticulum. The neonatal SERCA1b is the major Ca2+ pump in myotubes and young muscle fibers. To understand its role during skeletal muscle differentiation its synthesis has been interfered with specific shRNA sequence. Stably transfected clones showing significantly decreased SERCA1b expression (cloneC1) were selected for experiments. The expression of the regulatory proteins of skeletal muscle differentiation was examined either by Western-blot at the protein level for MyoD, STIM1, calsequestrin (CSQ), and calcineurin (CaN) or by RT-PCR for myostatin and MCIP1.4. Quantitative analysis revealed significant alterations in CSQ, STIM1, and CaN expression in cloneC1 as compared to control cells. To examine the functional consequences of the decreased expression of SERCA1b, repeated Ca2+-transients were evoked by applications of 120 mM KCl. The significantly higher [Ca2+]i measured at the 20th and 40th seconds after the beginning of KCl application (112±3 and 110±3 nM vs. 150±7 and 135±5 nM, in control and in cloneC1 cells, respectively) indicated a decreased Ca2+-uptake capability which was quantified by extracting the maximal pump rate (454±41 μM/s vs. 144±24 μM/s, in control and in cloneC1 cells). Furthermore, the rate of calcium release from the SR (610±60 vs. 377±64 μM/s) and the amount of calcium released (843±75 μM vs. 576±80 μM) were also significantly suppressed. These changes were also accompanied by a reduced activity of CaN in cells with decreased SERCA1b. In parallel, cloneC1 cells showed inhibited cell proliferation and decreased myotube nuclear numbers. Moreover, while cyclosporineA treatment suppressed the proliferation of parental cultures it had no effect on cloneC1 cells. SERCA1b is thus considered to play an essential role in the regulation of [Ca2+]i and its ab ovo gene silencing results in decreased skeletal muscle differentiation.

The neonatal sarcoplasmic reticulum Ca2+-ATPase gives a clue to development and pathology in human muscles

The sarcoplasmic/endoplasmic reticulum calcium ATPase 1 (SERCA1) has two muscle specific splice isoforms; SERCA1a in fast-type adult and SERCA1b in neonatal and regenerating skeletal muscles. At the protein level the only difference between these two isoforms is that SERCA1a has C-terminal glycine while SERCA1b has an octapeptide tail instead. This makes the generation of a SERCA1a specific antibody not feasible. The switch between the two isoforms is a hallmark of differentiation so we describe here a method based on the signal ratios of the SERCA1b specific and pan SERCA1 antibodies to estimate the SERCA1b/SERCA1a dominance on immunoblot of human muscles. Using this method we showed that unlike in mouse and rat, SERCA1b was only expressed in pre-matured infant leg and arm muscles; it was replaced by SERCA1a in more matured neonatal muscles and was completely absent in human foetal and neonatal diaphragms. Interestingly, only SERCA1a and no SERCA1b were detected in muscles of 7-12 years old boys with Duchenne, a degenerative-regenerative muscular dystrophy. However, in adult patients with myotonic dystrophy type 2 (DM2), the SERCA1b dominated over SERCA1a. Thus the human SERCA1b has a different expression pattern from that of rodents and it is associated with DM2.

Transfection efficiency along the regenerating soleus muscle of the rat

We investigated the efficiency of a single plasmid transfection along the longitudinal axis of the regenerating soleus of young rats. This also reflected transfection efficiency along the fibers because the soleus is a nearly fusiform muscle in young animals. The complete regeneration was induced by notexin and the transfection was made by intramuscular injection of enhanced green fluorescent protein- or Discosoma red-coding plasmids after 4 days. One week after transfection the number of transfected fibers was higher at the place of injection (i.e., in the muscle belly) and lower or absent at the ends of the muscle. The inspection of longitudinal sections and neuromuscular endplates indicated that one of the reasons of uneven transfection might be the shortness of transfected myotubes and the other reason might be the limit of diffusion of transgenic proteins from the expressing nuclei. As a result, the efficiency of transfection in the whole regenerating muscle was much lower than it could be estimated from the most successfully transfected part.

Recent progress in histochemistry and cell biology

Studies published in Histochemistry and Cell Biology in the year 2011 represent once more a manifest of established and newly sophisticated techniques being exploited to put tissue- and cell type-specific molecules into a functional context. The review is therefore the Histochemistry and Cell Biology's yearly intention to provide interested readers appropriate summaries of investigations touching the areas of tissue biology, developmental biology, the biology of the immune system, stem cell research, the biology of subcellular compartments, in order to put the message of such studies into natural scientific-/human- and also pathological-relevant correlations.