Abstract
G protein-coupled receptor kinase 2 (GRK2) is an important serine/threonine-kinase regulating different membrane receptors and intracellular proteins. Attenuation of Drosophila Gprk2 in embryos or adult flies induced a defective differentiation of somatic muscles, loss of fibers, and a flightless phenotype. In vertebrates, GRK2 hemizygous mice contained less but more hypertrophied skeletal muscle fibers than wild-type littermates. In C2C12 myoblasts, overexpression of a GRK2 kinase-deficient mutant (K220R) caused precocious differentiation of cells into immature myotubes, which were wider in size and contained more fused nuclei, while GRK2 overexpression blunted differentiation. Moreover, p38MAPK and Akt pathways were activated at an earlier stage and to a greater extent in K220R-expressing cells or upon kinase downregulation, while the activation of both kinases was impaired in GRK2-overexpressing cells. The impaired differentiation and fewer fusion events promoted by enhanced GRK2 levels were recapitulated by a p38MAPK mutant, which was able to mimic the inhibitory phosphorylation of p38MAPK by GRK2, whereas the blunted differentiation observed in GRK2-expressing clones was rescued in the presence of a constitutively active upstream stimulator of the p38MAPK pathway. These results suggest that balanced GRK2 function is necessary for a timely and complete myogenic process.
Introduction
Skeletal muscle mass is maintained by signaling pathways that control cell proliferation, differentiation, protein synthesis and degradation. Loss of muscle mass occurs in multiple settings including cancer, AIDS, cachexia, and neuromuscular disorders, as well as during aging, and is an important factor contributing to morbidity (Glass, 2010b). On the other hand, skeletal muscle hypertrophy, the increase in muscle mass that occurs as a result of an increase in load, is characterized by increased fiber size and protein synthesis (Serrano et al., 2008). Understanding the molecular pathways that regulate gain or loss of muscle mass is therefore crucial for treating muscle wasting-associated disorders. In Drosophila melanogaster, fusion between two different types of cells leads to the formation of a muscle precursor cell/myotube, which further fusion events transform into a multinucleated myotube. In vertebrates, skeletal myogenesis is initiated by the generation of myoblasts followed by their differentiation to myocytes and the formation of myofibers (Buckingham et al., 2003). In the mouse (Mus musculus), differentiated myoblasts elongate and migrate toward other myoblasts thus giving rise to a nascent and, later, to a mature myotube through fusion events among equivalent cells (Simionescu and Pavlath, 2011). In all cases, the determination of myoblasts and their differentiation are controlled by muscle regulatory factors that are activated at specific stages during myogenesis (Abmayr and Pavlath, 2012).
In vertebrates, at least two important signaling pathways are strictly required for the success of this multistep complex program of myogenesis: those of phosphatidylinositol 3-kinase (PI3-K) and p38 mitogen activated protein kinase (p38MAPK), which regulate the activation of key muscle transcription factors as MyoD. PI3-Ks are important mediators of tyrosine kinase receptor signal transduction. Their inhibition blocks myogenesis (Glass, 2010a) as so happens with Akt, an important downstream target of PI3-K (Fujio et al., 1999; Jiang et al., 1999; Conejo and Lorenzo, 2001). p38MAPK is also a central player in this process (Conejo et al., 2002; Perdiguero et al., 2007a) acting through several well described mechanisms (Lluis et al., 2006). p38MAPK are a family of Ser/Thr kinases, including p38αMAPK, p38βMAPK, p38γMAPK, and p38δMAPK, that regulate important cellular processes such as stress responses, differentiation, and cell cycle control (Nebreda and Porras, 2000; Zarubin and Han, 2005). p38αMAPK is the isoform involved in the induction of myogenesis (De Alvaro et al., 2008) and, consistently, treatment with p38α/βMAPK inhibitors or experimental depletion of the p38MAPKα isoform prevent the fusion of immortalized myoblasts into myotubes, and also the induction of specific muscle differentiation genes (Cuenda and Cohen, 1999; Zetser et al., 1999; Li et al., 2000; Perdiguero et al., 2007a, b; Alonso-Chamorro et al., 2011), thus demonstrating the requirement of p38MAPKα/β during myogenesis (de Angelis et al., 2005; Keren et al., 2005). The p38MAPK pathway seems also to be involved in the modulation of skeletal muscle differentiation by cell-to-cell contact and adhesion (Krauss, 2010) or mechanical stimulation (Niu et al., 2013) and in the epigenetic control of muscle regeneration (Palacios et al., 2010). In marked contrast to the PI3-K/Akt pathway, little is known about the upstream modulators of the p38MAPK pathway in myogenesis other than MAPK kinase (MAPKKs) (Nebreda and Porras, 2000). Accordingly, it has been reported that forced activation of p38MAPK by ectopic expression of a constitutively active mutant of MKK6 is sufficient to override the inhibitory factors present in proliferating cells and to induce both the expression of differentiation markers and the appearance of multinucleated myotubes, reinforcing the idea that p38MAPK activity plays an essential role in muscle differentiation (Li et al., 2000; Wu et al., 2000; Perdiguero et al., 2007a). However, the mechanisms that regulate p38MAPK activity during muscle cell differentiation remain unidentified, although they appear to be different from those involved in the response to stress or cytokines.
G protein-coupled receptor kinase 2 (GRK2) is a ubiquitous, essential protein kinase that is emerging as an integrative node in many signaling networks. Moreover, changes in GRK2 abundance and activity have been identified in several inflammatory, cardiovascular disease, and tumor contexts, suggesting that those alterations may contribute to the initiation or development of certain pathologies (Penela et al., 2003; Salcedo et al., 2006; Garcia-Guerra et al., 2010). GRKs were initially identified as key players in the desensitization and internalization of multiple GPCRs, but GRK2 also phosphorylates several non-GPCR substrates and dynamically associates with a variety of proteins related to signal transduction (Peregrin et al., 2006; Ribas et al., 2007; Penela et al., 2010) including, among others, certain p38MAPK isoforms (Peregrin et al., 2006). However the implication of GRK2 in the mechanisms controlling the regulation of skeletal muscle development and function as well as how this regulation could affect the functionality of this tissue remain to be investigated. In this study, we report the importance of GRK2 in the regulation of skeletal muscle myogenesis and elucidate possible downstream effectors.
Results
Grpk2 is required for myogenesis in Drosophila melanogaster
Gprk2 is required for Drosophila myogenesis. (A–C) Lateral views of a stage 16 Dmef2Gal4>UAS-GPF control embryo (A) showing the normal pattern of lateral and ventral somatic muscles (schematically represented in D), and the same Dmef2Gal4>UAS-GPF embryo visualized at 20 h (B) and 24 h (C) after injection of Gprk2 dsRNA. Note the aberrant morphology of the LT muscles in the anterior region of injected embryo (corresponding to the site of injection, arrow in B) and their detachment 4 h later (arrow in C) and compare to LT muscles in the posterior region of the embryos (arrowheads in A–C and arrow in the scheme shown in D). (E) Flightless phenotypes obtained after manipulation of Gprk2 expression using the UAS/Gal4 system at 29°C. UAS-Ri-Gprk2, RNAi silencing construct for Gprk2. (F–I) Transverse sections of pupal (F and G) and adult thoraces (H and I). (F and H) w/+; UAS-Flp/+; FRT82 Df(3R)Gprk2/FRT82 M control thoraces, showing the wild-type arrangement of the six pairs of DLM muscles. (G and I) 1151Gal4/+; UAS-Flp/+; FRT82 Df(3R)Gprk2/FRT82 M P8 pupa (G) and 3 days old fly (I) displaying a reduction in the number of DLM muscles (45%, n = 11 in pupae, 47%, n = 15 in 3 days old adults; arrow in G points at the left hemithorax containing 5 DLM muscles).
Due to strong maternal contribution for Grpk2 and its early requirement during oogenesis, the study of its function during embryonic myogenesis is not possible, so an in depth analysis was performed during Drosophila adult myogenesis. We focused on the development of a subset of the indirect flight muscles (IFMs), the dorsal longitudinal muscles (DLMs) that consist in six muscles per hemithorax (Fernandes et al., 1991). We first attenuated Gprk2 function using the 1151Gal4 line to drive the expression of an RNAi silencing construct, the UAS-Ri-Gprk2, at 29°C in all adult muscle precursors and found a significant increase in the number of flies with a flightless phenotype (31%) compared with controls (9%) (Figure 1E). However, the overexpression with the same driver of the full-length protein had low effect in the flying ability of the adults (Figure 1E). When we made use of a combination of the Gal4/UAS and the Flp-mediated recombination systems for generating stronger loss-of-function conditions (see Materials and methods, and Duffy, 2002), the penetrance of the flightless phenotype drastically increased to 100%. This phenotype was associated with a reduction in the number of DLMs already visible at 48 h after puparium formation (APF) (Figure 1F–I), suggestive of a failure in the splitting of the DLM larval templates (Roy and VijayRaghavan, 1998). We also observed motor defects in a test for climbing ability in these flies: 3-day-old flies deprived of Gprk2 function in adult muscles, showed a reduction in climbing speed compared with their control siblings (Supplementary Data), demonstrating that in the absence of Gprk2 the development of DLM muscles is compromised. However, due to the late expression of the 1151Gal4 driver in adult muscle tendons (Soler et al., 2004), we could not discard a contribution of Gprk2 tendon-requirement to the late phenotype. To investigate whether there is a late requirement of Gprk2 function in DLM muscles we attenuated Gprk2 in differentiating DLM muscles using the IFM-specific UH3Gal4 driver (Katzemich et al., 2012). Besides an absence of any apparent defect in DLM splitting UH3Gal4>UAS-Ri-Gprk2 flies were flightless (100%, Figure 1E), indicating a late requirement of Gprk2 for muscle differentiation.
GRK2 hemizygous mice present less skeletal muscle fibers and increased fiber size
GRK2 hemizygous mice show decreased fiber number and increased skeletal muscle fiber size. Determination of GRK2 protein and gene expression in Wt and GRK2+/− mice by immunoblotting (A) and qPCR (B). Wt and GRK2+/− muscles were isolated and weighted (C), embedded in paraffin, cut in sections (5 µM) and stained with hematoxylin and eosin (D). Representative images are shown in 20× magnification. Scale bar 100 µm. The cross-sectional area of myofibers of the muscle from each individual mouse was measured to determine relative fiber size (Wt 3 m = 18.08 ± 0.7 a.u.; Wt 9 m = 11.81 ± 0.05 a.u.) (E) and the number of fibers (Wt 3 m = 48.33 ± 4.86; Wt 9 m = 50.91 ± 2.36) (F) per field. Values are expressed as percentage over Wt animals. Muscle sections from neonates were stained with MHC (G), and relative fiber size (Wt = 3.01 ± 0.08 a.u.) (H) and number of fibers (Wt = 187.8 ± 9.0) (I) per field were determined. Scale bar: 50 µm (20×) and 20 µm (60×). Values are expressed as percentage over Wt animals. *P < 0.01.
We observed no differences in muscle weight (Figure 2C) between Wt and GRK2+/− mice at 3 months of age in the absence of significant differences in either food or water intake (Supplementary Data). Morphological analysis of muscle sections revealed a decreased number of fibers with wider cross sections in GRK2+/− mice at both 3 and 9 months of age (Figure 2D–F). This decrease in fiber number in GRK2+/− animals was not apparent in newborn mice (Figure 2G–I) although they already showed a larger fiber size (Figure 2H), suggesting that fiber loss was taking place after birth and before 3 months of age while fiber growth is an earlier phenotype. These results suggest that GRK2 plays an important role in regulating myofiber growth and integrity also in vertebrates.
GRK2 overexpression impairs muscle differentiation in C2C12 myoblasts while a kinase-deficient mutant accelerates this process, giving rise to immature myotubes
GRK2 impairs and GRK2-K220R accelerates myoblast differentiation. (A) C2C12 myoblast were differentiated and lysed in successive days along the culture to analyze proteins by SDS–PAGE. Cell lines with increased expression of wild type GRK2 (pc-GRK2), catalytically inactive GRK2 (pc-K22R) or empty vector (pcDNA3) were used. (B) GRK2 protein expression was immunodetected by western blot in control cells. (C) Phase-contrast images were taken along the differentiation process. (D) Representative western blot results using antibodies against MyoD, Myogenin, Caveolin-3, MHC and β-actin, and densitometric analysis after standardization using β-actin content. Both are results from three independent experiments. Values are expressed as percentage over 0 days of differentiation. Different symbols show P < 0.05; Asterisks depict differences between pcDNA3 and K220R, triangles between pcDNA3 and GRK2.
C2C12 cells proliferate in culture as undifferentiated cells in high serum; upon confluence and serum withdrawal (differentiation medium), myoblasts differentiate into myocytes, which subsequently fuse into multinucleated myotubes (Buckingham, 2001; Jansen and Pavlath, 2008). As expected, differentiated C2C12 myoblasts display a multinucleated tubular morphology, which was similarly observed in control cells after 4 days in culture (Figure 3C and Supplementary Data). Interestingly, pc-K220R myoblasts fully differentiated to an extent that appeared similar to that of pcDNA3 cells. We observed multinucleated myotubes in pc-K220R myoblasts whereas pc-GRK2 myocytes were primarily mononuclear, exhibiting a severe defect in their ability to form multinucleated cells, even after 4 or more days in low serum (Figure 3C and Supplementary Data).
Differentiation of skeletal muscle cells involves the sequential expression of specific transcription factors such as MyoD, and also early and terminal differentiation markers such as myogenin and myosin heavy chain (MHC), respectively (Weintraub, 1993). As expected, pcDNA3 myotubes showed a temporal expression pattern of differentiation markers similar to that reported for C2C12 cells (De Alvaro et al., 2008). Interestingly, the elevation of MyoD and myogenin levels started 1 day earlier in pc-K220R-expressing clones than in pcDNA3 myotubes (Figure 3D and Supplementary Data) and remained significantly higher than in control cells all along the process even when terminal differentiation was achieved. On the contrary, late differentiation markers such as Caveolin-3 and MHC, indicative of mature myotubes, were expressed at lower levels in pc-K220R myotubes along the process (Figure 3D and data not shown) and Caveolin-3 remained below control levels even at Day 4. On the other hand, pc-GRK2 cells did not express any of these skeletal muscle differentiation markers after 4 or more days in low serum (Figure 3D and Supplementary Data). This cannot be ascribed to increased cell death occurring in GRK-overexpressing clones (Supplementary Data and data not shown). These results clearly demonstrate that enhanced GRK2 expression abrogates differentiation, whereas interfering with GRK2 function triggers an untimely differentiation process with increased early differentiation markers and a decreased expression of proteins characteristic of fully mature myotubes.
GRK2 regulates the formation of membrane protrusions in myocytes
Overexpression of GRK2 abrogates myoblasts fusion and membrane protrusions in low serum. Myoblasts from the cell lines pcDNA3, pc-GRK2, and pc-K220R were grown in 10% FS-DMEM until confluence and then further cultured in 2% HS-DMEM for 4 days to induce differentiation. (A) Cells were fixed and actin cytoskeleton was visualized by fluorescence microscopy of cells stained with rhodamine-phalloidin. Nuclei were stained with DAPI. (B) Quantification of phalloidin–stained cells was conducted by scoring individual cells from 10 different fields for the presence or absence of membrane ruffles. Membrane protrusions are indicated by white arrows. The percentage of cells with fused nuclei (C), the number of nuclei per myotube (D), and the diameter of myotubes (a.u.) (E) were determined. *P < 0.05.
Myoblast differentiation requires growth arrest and cell cycle exit, and thus we analyzed the expression of several cell cycle regulators such as p21 (Shen et al., 2003); retinoblastoma (Rb), (Poznic, 2009); and proliferating cell nuclear antigen (PCNA) (Shen et al., 2003; Essers et al., 2005). We detected Rb in the hyperphosphorylated state and PCNA protein in proliferating myoblasts of the transfectant cells. Four days of differentiation in low serum produced sequential growth arrest with upregulation of the expression of p21, reduction of Rb phosphorylation and downregulation of PCNA (Supplementary Data). The percentage of G0/G1 cells after 4 days in low serum was significantly increased to the same extent in all three transfectants, thus indicating that all cells exited the cell cycle in a similar manner (Supplementary Data). Thus, GRK2 does not play a role in the growth arrest associated to skeletal muscle differentiation.
GRK2 kinase activity regulates the Akt, p38MAPK, and ERK routes during myogenic differentiation
GRK2 kinase activity regulates the Akt, p38MAPK, and ERK1/2 pathways during myogenic differentiation. Cells were lysed at different days of culture and total protein (30 μg) was subjected to SDS–PAGE and immunodetected with total and phosphorylated forms of Akt, ERK 1/2 (A) and p38MAPK (B) antibodies. Graphs in A and B show the densitometric analysis of P-Akt (Ser473), P-p38MAPK (Thr180/Tyr182), and P-ERK1/2 after standardization using total Akt, p38MAPK, and ERK1/2, respectively. Values are expressed as percentage over 0 days of differentiation. Results show representative blots of four independent experiments. Asterisks represent differences between pcDNA3 and K220R, triangles between pcDNA3 and GRK2. *P < 0.01. (C) C2C12 cells were infected with either a control adenovirus carrying GFP (Ad-CTRL) or an sh-GRK2 adenovirus (Ad-sh-GRK2), differentiated and lysed at different days along the differentiation process. Representative western blot results using antibodies against phosphorylated and total p38MAPK (Thr180/Tyr182) and AKT (Ser473) are shown. Graphs show densitometric analysis after standardization. *P < 0.05, n = 3.
We also observed a clear phosphorylation on the activation loop residues Thr180/Tyr182 of p38MAPK, an important pathway required for muscle differentiation (Lluis et al., 2006; Perdiguero et al., 2007a), after Day 2 in pc-DNA3 myotubes, whereas this phosphorylation was blunted in pc-GRK2 cells (Figure 5B). This activation of p38MAPK during the differentiation process was accelerated in pc-K220R cells (Figure 5B). Thus, upon interference with GRK2 kinase activity, there is a prompt and sustained activation of p38MAPK that probably helps establish the precocious but immature differentiation program detected in pc-K220R cells (see Discussion).
In line with these results, silencing of GRK2 by adenovirus-driven expression of an sh-construct specific for this kinase (Lafarga et al., 2012) led to a differentiation dynamics most reminiscent to that observed in K220R-expressing cells (Supplementary Data), including the upregulated phosphorylation profile of both p38MAPK and Akt proteins (Figure 5C). These data indicate that the phenotypic changes detected when GRK2 levels vary depend mostly on its kinase activity and are related to an altered p38MAPK activation status. Also, Western blot analysis of GRK2-expressing cells (Figure 5B and Supplementary DataA) showed a decreased amount of phospho-p38MAPK (Thr180/Tyr182) in pc-GRK2, whereas phospho-p38MAPK (Thr123), an inhibitory residue target for GRK2 located at the docking groove of the protein (Peregrin et al., 2006), was enhanced. In contrast, in pc-K220R cells we observed that the increase in phospho-p38MAPK (Thr180/Tyr182) paralleled a decrease in the phospho-p38MAPK (Thr123) signal of endogenous p38MAPK. These results may explain how lowering GRK2 activity enhances p38MAPK activation and myogenesis, were consistent with what has been already described during the adipocyte differentiation process (Peregrin et al., 2006), and suggest a possible role for Thr123-p38 phosphorylation in downregulating p38MAPK activation in myogenesis.
Modulation of the p38MAPK pathway correlates with the inhibitory effect of GRK2 on myogenesis
Effects of a p38MAPK mutant that mimics its phosphorylation by GRK2 and of constitutive p38MAPK activation on a GRK2-overexpressing background. (A) Phase-contrast images of the terminally differentiated cells for detection of multinucleated myotubes. Magnification, 10×. (B) C2C12 cells transfected with Flag-p38αWT or Flag-p38α T123D were differentiated, fixed, and immunostained with rhodamine-phalloidin and DAPI to visualize actin cytoskeleton or nuclei, respectively. The percentage of membrane protrusions (C), percentage of cells with fused nuclei (D), number of nuclei per myotube (E), and myotube diameter (F) were determined. Four independent experiments were performed in triplicates. The images shown are representative from at least 10 different fields from independent coverslips. *P < 0.05. (G) Both C2C12 and pcGRK2 stable cell lines were infected with a retrovirus carrying a constitutively active MKK6 mutant (MKK6-E) using an empty vector as a control. Cells were differentiated to take phase-contrast images and then lysed at Day 0 and 4 of the process. (H) Lysates were subjected to SDS–PAGE and immunodetected with antibodies against MHC, GRK2, and actin.
Interestingly, when differentiation-incompetent GRK2-overexpressing cells were infected with a constitutively active mutant of the MKK6 protein (termed MKK6-E), a potent and direct activator of p38MAPK, a reversion of the GRK2-induced phenotype was detected, and cells started to differentiate in a manner more reminiscent of controls (Figure 6G), and the expression of key myogenic markers such as MHC (almost undetectable in control GRK2 cells 4 days after differentiation) was rescued along with stimulatory p38-phosphorylation in MKK6-E-infected GRK2-expressing cells (Figure 6H and Supplementary DataC). These results demonstrate that overexpression of GRK2 negatively regulates the activation of signaling cascades implicated in skeletal muscle differentiation and that this inhibition can be overcome by activation of the p38MAPK pathway.
Discussion
Skeletal myogenesis is a highly orchestrated multistep process that involves the determination of multipotential mesoderm cells to give rise to myoblasts. Exit of myoblasts from the cell cycle is followed by an increase in the expression of myogenic transcription factors and formation of multinucleated myotubes and by the expression of myofibrillar proteins (Buckingham et al., 2003; Jansen and Pavlath, 2008). A timely regulation of skeletal muscle formation is essential for normal development. Its dysregulation is the basis for certain pathological conditions such as muscular dystrophies and inflammatory myopathies in which prominent muscle loss and degeneration take place (Glass, 2010b). In the present study, we characterize the mechanisms underlying the effects of GRK2 on muscle differentiation, and reveal the important role of this kinase in the regulation of the kinetics of the myogenic process.
Drosophila melanogaster constitutes one of the key model systems to study myogenesis since it recapitulates the process observed in vertebrates but simplified to one precursor cell giving rise to one myotube and one adult fiber. Using this model to target Gprk2, a Drosophila ortholog of vertebrate GRKs (Molnar et al., 2007), we have detected both in Drosophila embryos and in newborn flies severe muscular dysfunction accompanied by fiber loss upon Gprk2 silencing or depletion. Gprk2 silencing in adult flies impaired motor activities such as climbing and flight behavior, and this phenotype further deteriorated with age. A reduction in the number of DLMs present in Gprk2-silenced or depleted flies, which was more evident with age, demonstrated an essential role for Gprk2 both on the differentiation and on the stability of muscles in this system. Importantly, GRK2 hemizygous mice show a significantly reduced fiber number in muscles both at 3 and at 9 months of age accompanied by an increased fiber size. This skeletal muscle hypertrophy can be regarded as a compensatory response of the tissue to try to counteract the decreased fiber number, or, instead, as a phenomenon triggered by GRK2 downregulation in parallel to fiber loss. This second hypothesis seems to better fit the results observed in vertebrates, where the same amount of fibers that are, however, already larger in size is detected in newborn mice. The contribution to the observed muscle phenotype of other cell types that also present lower levels of GRK2 in hemizygous mice cannot be ruled out. However, the hypertrophic phenotype seems not to be specific for skeletal muscle, since we also detect a hypertrophy of cardiomyocytes of GRK2+/− mice with aging (unpublished data) with no differences in basal blood pressure in both genotypes (Avendaño et al., 2014).
The observed phenotypes in Drosophila and Mus musculus are reminiscent of those observed for the attenuation of the inhibitor of muscle differentiation gene him (Soler and Taylor, 2009) and of the reduction of p38MAPK activity both in Drosophila and in vertebrates (Perdiguero et al., 2007b; Vrailas-Mortimer et al., 2011) (discussed below). However, it cannot be ruled out that, as so happens with the Notch ligand delta 1 (Schuster-Gossler et al., 2007) (a known inhibitor of the differentiation of myoblasts to myocytes), GRK2 loss can provoke a premature and excessive differentiation that would eventually lead to a loss of muscle fibers.
In cell lines overexpressing GRK2, the expression of early and late key myogenic markers, such as MyoD and myogenin, or Caveolin-3 and MHC respectively, was strongly reduced in GRK2-overexpressing cells. Accordingly, GRK2-expressing myoblasts did not differentiate properly. This is consistent with the phenotype observed for mice deficient for these marker proteins that show severe defects during myogenesis (Sabourin et al., 1999; Galbiati et al., 2001). In K220R-expressing cells, a kinase-deficient mutant of GRK2 able to recapitulate events dependent on GRK2 scaffolding properties, but not those dependent on its kinase activity, the expression of early myogenic markers was not blunted, but rather accelerated what was indicative of a premature differentiation. This early maturation process was apparently successful in giving rise to multinucleated myotubes since, at Day 4, K220R cells were morphologically indistinguishable from control cells. However, K220R myotubes did not reach the same degree of maturity as control cells, as indicated by the expression pattern of Caveolin-3 or MHC.
In the absence of apparent differences in cell death or cell cycle exit between control and GRK2-expressing cells, other important early events are also decompensated. The activation of key protein kinases such as Akt and p38MAPK is impaired in GRK2 myoblasts and accelerated in both K220R-expressing C2C12 cells and upon GRK2 silencing. This effect was observed in the phosphorylation of the Ser473 of Akt, which has been described to correlate with Akt kinase activity along myoblast differentiation (Gonzalez et al., 2004). More importantly, p38MAPK activation was more prominent all along the differentiation process in K220R-expressing or GRK2-silenced myoblasts, and blunted when GRK2 was elevated.
This event can have important consequences in the development of myogenesis (Gonzalez et al., 2004). Treatment with p38MAPKα/β inhibitor SB203580 prevented the fusion of myoblasts into myotubes, and the induction of muscle-specific genes (Cuenda and Cohen, 1999; Li et al., 2000; Wu et al., 2000). Myoblasts obtained from mice lacking p38MAPKα showed impaired myoblast differentiation and fusion, with reduced expression of myogenin and MHC and blunted myotube formation (Perdiguero et al., 2007b). So, the kinase activity of p38MAPK is essential along the differentiation process, but it also serves to ensure the timely activation of different subsets of differentiation factors that guarantee a successful terminal differentiation (Penn et al., 2004). For instance, inhibition of p38MAPK activity during late stages of C2C12 cell differentiation resulted in increased expression of certain skeletal muscle genes (Suelves et al., 2004). In particular, p38MAPK phosphorylated in vitro and in vivo certain residues in Nt-MRF4, leading to a repressive but selective effect during terminal differentiation. Therefore, activation of p38MAPK at early time points triggers premature differentiation, but when that activation is maintained (as so happens with K220R overexpression or GRK2 silencing), it activates a repressive role of MRF4 over late myogenic markers and impairs terminal differentiation (Suelves et al., 2004). This effect might explain why in K220R-overexpressing cells differentiation is detected earlier but does not reach the degree of maturation observed in control cells. These cells, even when they differentiate well into long multinuclear myotubes containing more fused nuclei than control cells (data not shown), conserve the biochemical characteristics of immature myocytes, since they express significantly larger amounts of early differentiation markers such as MyoD and myogenin relative to control cells even when terminal differentiation is reached.
Overall, our data suggest that the phenotypic changes on the kinetics and degree of the myogenic process detected when GRK2 levels vary depend mostly on its kinase activity and are related to an altered p38MAPK activation status. This is in full accordance with the described effect that GRK2 protein has on the process of adipocyte differentiation in 3T3-L1 preadipocytes. GRK2-mediated phosphorylation of the p38MAPK docking domain at the Thr123 residue prevents its interaction with substrates and activators and impedes progress of the differentiation process in a kinase-dependent manner (Peregrin et al., 2006). GRK2 might regulate skeletal muscle differentiation through a similar mechanism, since a phospho-mimetic mutant of p38MAPK (T123D) is able to recapitulate the effects of GRK2 overexpression on the myogenic process and to prevent formation of membrane protrusions, a process that should precede the formation of fused myotubes. On the other hand, the blunted differentiation observed in GRK2-expressing clones can be rescued by the activation of the p38MAPK route upon MKK6-E expression, strongly supporting the notion that modulation of the p38MAPK pathway is key to the effects of GRK2 in muscle differentiation. The fact that K220R-expressing cells do not reach full differentiation maturity could be ascribed to an untimely p38MAPK activation pattern as discussed above, although we cannot rule out the possibility that at later stages of the normal differentiation process GRK2 activity toward alternative targets would contribute to some aspects of the differentiation process.
The very early event of nuclei fusion is also altered by GRK2 expression. Myoblast fusion is initiated by finger-like membrane protrusions that search and engage neighboring fusion-competent myoblasts to initiate the fusion process. Thus, the formation of filopodia/lamelipodia-like structures represents one of the earliest detectable events in the myocyte fusion process. Such actin-labeled protrusions are almost absent in GRK2-overexpressing cells while K220R cells are fully able to generate membrane protrusions as control cells do. At Day 0, before differentiation is initiated, K220R cells appear at least as competent as control cells to fuse to neighboring myoblasts, while GRK2 cells would be less able to extend and fuse their membranes. At Day 4, even when control cells and K220R cells had the same amount of membrane protrusions, K220R cells had wider myotubes indicating more fusion events per myotube had occurred earlier on along the process. These results indicate that K220R cells are more competent or efficient in terms of promoting successful fusion events. So, it could be inferred that K220R cells possess a small but very precocious advantage during the first stages of fusion what eventually translates into larger myotubes with more fused nuclei. These myotubes would be, however immature, since early differentiation markers are still upregulated (and therefore would be causing differentiation to prolong), and late markers do not reach the levels achieved in control cells. Thus, these larger and multinucleated myotubes would remain immature in nature and, therefore, more prone to deterioration and detachment. GRK2 downregulation would cause fiber retraction due to hypercontraction of immature or dysfunctional myotubes, or by mechanical loss upon fiber detachment when the first contractile movements take place, what would explain the fiber loss that we detect in flies soon after birth. This phenomenon would also explain the fiber hypertrophy detected in adult mice, since the remaining fibers would grow larger in size and competency in an effort to compensate for fiber loss. Finally, it would also explain the lack of differences in muscle mass detected in GRK2-deficient animals, since deteriorated immature fibers can be replaced by hypertrophied larger ones, thus compensating for muscle mass loss.
Given the connection between GRK2 activity and p38 pathways described in this work, investigating the extracellular stimuli regulating muscle GRK2 expression/activity during myogenesis or in muscle pathologies constitutes a very interesting avenue for future research. These extracellular signals may act through GPCR such as muscarinic or adrenergic receptors, leading to subsequent release of activating Gβγ subunits. A variety of GPCR and growth factors can also promote activation of kinases known to modulate GRK2 levels and/or activity, such as PKC, c-Src, or MAPK (Ribas et al., 2007; Penela et al., 2010). β-adrenoceptors (βARs) play an important role in myogenesis and skeletal muscle regeneration after myotoxic injury cells (reviewed in Lynch and Ryall, 2008), and are also upregulated during differentiation of C2C12. On the other hand, the IGF1 pathway, which plays an important role in muscle growth, maintenance and repair, has been shown to upregulate GRK2 expression in other cell types (Penela et al., 2010). Interestingly, GRK2 levels increase in muscle during aging and upon development of insulin resistance (Garcia-Guerra et al., 2010). Whether this increase contributes to age-related muscle wasting is however not known, Interestingly, βAR agonists have therapeutic potential for attenuating muscle wasting associated with age-, cancer cachexia, and neuromuscular diseases such as the muscular dystrophies (reviewed in Koopman et al., 2009). Also, a very recent report demonstrates that the loss in self-renewal detected in satellite cells from aged mice can be overcome by p38α/β inhibition (Bernet and Rudnicki, 2014). Whether GRK2 is found mutated or dysregulated in muscle wasting contexts or in human patients of muscular diseases is an interesting question for future studies.
In sum, we can conclude that GRK2 provides important signals required to prevent uncontrolled early differentiation and to ensure sustained muscle differentiation during development. Modulation of GRK2 activity is of essential importance for skeletal muscle formation during development, and also determines fiber size and characteristics in adult mammalian tissue, probably by regulating the timely differentiation program and the development of fusion events at very early time points during the myogenic process.
Materials and methods
Drosophila stocks
The following stocks were used: Dmef2Gal4>UAS-GPF, 1151Gal4 (Dutta et al., 2004), UH3Gal4 (from Upendra Nongthomba, IISc, Bangalore, India) kettin-GFP (Kelso et al., 2004), Df(3R)Gprk2 and UAS-Gprk2i (Molnar et al., 2007), UAS-Gprk2, (a gift from C. Molnar and J.F. de Celis, CBMSO, Madrid).
Cell line generation
Mouse C2C12 myoblasts (American Type Culture Collection) (Blau et al., 1985) were maintained in DMEM supplemented with 10% fetal serum (FS) and antibiotics, at 37°C and 5% CO2. C2C12 cells were stably transfected according to the LipofectAMINE™ protocol (Invitrogen), with 4 μg of pcDNA3-GRK2-WT-neo (pc-GRK2) or pcDNA3-GRK2-K220R-hygro (pc-K220R) or with the empty vector pcDNA3-hygro (pcDNA3) as a control. Afterwards, transfected cell lines were selected by neomycin (0.5 mg/ml) or hygromycin (0.5 mg/ml) for 3 weeks. Four clones were obtained from each construct using this procedure, which were analyzed for GRK2 overexpression and differentiation pattern. Cells showed stable GRK2 expression after several passages and no significant phenotypical differences among the four clones were observed in any group of cell lines. Accordingly, a representative clone from each group, pcDNA3, pc-GRK2, and pc-K220R, was used in this study. All cell lines were grown to confluence in 10% FS-DMEM before differentiation in low serum medium, DMEM supplemented with 2% horse serum (HS) for up to 4 days. Furthermore, C2C12 myoblasts were also subjected to transient transfection Flag-p38αWT or Flag-p38αT123D. These cells were also differentiated in 2% HS-DMEM for 4 days.
Animal protocols
Experiments were performed on Wt and hemizygous-GRK2 (GRK2+/−) mice maintained on the hybrid 129/SvJ C57BL/6 background. The animals were bred and housed on a 12-h light/dark cycle with free access to food and water. Muscle samples were collected at the corresponding ages and immediately weighted and frozen in liquid nitrogen and stored at −80°C. All animal experimentation described in this study was conducted in accordance with accepted standards of human animal care and approved by the University Committee.
Further information can be found in Supplementary Data.
Supplementary material
Funding
This work was supported by Grants BFU2008-04043 (to M.L. and S.F.-V.), SAF2012-3618 (to S.F.-V.), and SAF2011-23800 (to F.M.) from Ministerio de Economía y Competitividad, Spain; S2010/BMD-2332 (INDISNET) from Comunidad de Madrid, Spain (to F.M.); CIBER de Diabetes y Enfermedades Metabólicas Asociadas and The Cardiovascular Network (RD06-0014/0037 and RD12/0042/0012) from Ministerio Sanidad y Consumo-Instituto Carlos III, Spain (to F.M.); UAM-Banco de Santander (to C.M.); BFU2010-14884 (to M.R.-G.). S.F.-V. is recipient of a ‘Miguel Servet’ tenure track program (CP10/00438) co-financed by the European Regional Development Fund (ERDF). We also acknowledge the support of COST Action BM0602 from the European Commission (to M.L.) and institutional support from Fundación Ramón Areces.
Conflict of interest: none declared.
Acknowledgements
This work is dedicated to the memory of Prof. Margarita Lorenzo, who passed away on April 7, 2010, at the age of 51.
References
Author notes
These authors contributed equally to this work.
Deceased.
