Characterization of a Dmd EGFP reporter mouse as a tool to investigate dystrophin expression
© The Author(s). 2016
Received: 11 April 2016
Accepted: 8 June 2016
Published: 5 July 2016
Dystrophin is a rod-shaped cytoplasmic protein that provides sarcolemmal stability as a structural link between the cytoskeleton and the extracellular matrix via the dystrophin-associated protein complex (DAPC). Mutations in the dystrophin-encoding DMD gene cause X-linked dystrophinopathies with variable phenotypes, the most severe being Duchenne muscular dystrophy (DMD) characterized by progressive muscle wasting and fibrosis. However, dystrophin deficiency does not only impair the function of skeletal and heart muscle but may also affect other organ systems such as the brain, eye, and gastrointestinal tract. The generation of a dystrophin reporter mouse would facilitate research into dystrophin muscular and extramuscular pathophysiology without the need for immunostaining.
We generated a Dmd EGFP reporter mouse through the in-frame insertion of the EGFP coding sequence behind the last Dmd exon 79, which is known to be expressed in all major dystrophin isoforms. We analyzed EGFP and dystrophin expression in various tissues and at the single muscle fiber level. Immunostaining of various members of the DAPC was done to confirm the correct subsarcolemmal location of dystrophin-binding partners. We found strong natural EGFP fluorescence at all expected sites of dystrophin expression in the skeletal and smooth muscle, heart, brain, and retina. EGFP fluorescence exactly colocalized with dystrophin immunostaining. In the skeletal muscle, dystrophin and other proteins of the DAPC were expressed at their correct sarcolemmal/subsarcolemmal localization. Skeletal muscle maintained normal tissue architecture, suggesting the correct function of the dystrophin-EGFP fusion protein. EGFP expression could be easily verified in isolated myofibers as well as in satellite cell-derived myotubes.
The novel dystrophin reporter mouse provides a valuable tool for direct visualization of dystrophin expression and will allow the study of dystrophin expression in vivo and in vitro in various tissues by live cell imaging.
Duchenne muscular dystrophy (DMD; OMIM#310200), the most severe form of dystrophinopathies, is a lethal X-linked recessive disorder characterized by progressive muscle degeneration, loss of walking ability, decline of respiratory and cardiac function, and early death . Besides the skeletal and heart muscle , the disease affects other organs such as the central nervous system  and organs which are dependent on smooth muscle function such as the gastrointestinal  and the vascular system . DMD is caused by mutations in the dystrophin gene (DMD) that prevent the expression of functional dystrophin. The dystrophin gene is the largest human gene spanning 2.4 Mb at the Xp21 locus and comprises 79 primary exons [6–8].
The main dystrophin isoform has a molecular weight of 427 kDa and is expressed in muscle cells, where it is localized at the cytoplasmatic side of the sarcolemma [9, 10]. Dystrophin has four main functional domains: (i) the N-terminal actin-binding domain, (ii) the central rod domain, (iii) the cysteine-rich, and (iv) the C-terminal domain. The N-terminus and part of the dystrophin rod domain interact with cytoskeletal actin [11, 12]. The rod domain folds into α-helical coils composed of 24 spectrin-like repeats interrupted by four proline-rich hinges that lend flexibility to the protein [13, 14]. Dystrophin assembles several transmembrane and cytoplasmic proteins into the dystrophin-associated protein complex (DAPC), interactions provided by the cysteine-rich and C-terminal domains. The DAPC can be divided into (i) the dystroglycan complex, (ii) the sarcoglycan complex, and (iii) the cytoplasmic and extracellular matrix (ECM) components including sarcospan, α-laminin, syntrophins (β, γ2), dystrobrevin, and neuronal nitric oxide synthase (nNOS) [15, 16]. The DAPC connects the cytoskeleton with the ECM, thereby lending mechanical stability to the sarcolemma and protecting the muscle from contraction-related muscle damage [9, 10]. The importance of dystrophin in DAPC assembly is highlighted by the loss of DAPC components in DMD muscle.
Vast progress has been made in the last two decades to comprehend the role of dystrophin in the muscle. Prominent contributions to the advancement of this field have been obtained through identification and later also the generation of animal models (e.g., mdx mice, GRMD dogs) [17, 18].
However, despite the insight into the pathophysiology of dystrophinopathies, DMD cannot be cured at present and the extramuscular functions of dystrophin are not well understood. This calls for further investigations into the (patho)physiology of dystrophin, not only in the muscle but also in other organs and tissues where it is expressed. The character of DMD as a multisystem disorder is reflected by a large number of highly conserved isoforms and splicing variants that differ in their cellular and subcellular localization. The high diversity of dystrophin isoforms is achieved through the use of tissue-specific promoters (Additional file 1: Figure S1). The full-length dystrophin isoform, Dp427, is generated from three different tissue-specific promoters: (i) muscle type (M), which drives the expression of skeletal, cardiac, and smooth muscle dystrophin, (ii) brain type (B), which is active in cortical and cerebellar neurons and heart, and (iii) Purkinje cell type (P), which regulates the cerebellar dystrophin expression [19–22]. Shorter dystrophin isoforms such as the retinal (Dp260), the cerebellar and renal (Dp140) , as well as the Schwann cell (Dp116) isoforms  are transcribed from internal promoters. The Dp260, Dp116, and Dp140 isoforms include parts of the rod domain and express the cysteine-rich and C-terminal domains, but lack the N-terminal actin-binding domain [23–25]. The short Dp71 isoform is detected in most non-muscle tissues including the brain, liver, kidney, and lung [26–29]. Further dystrophin diversification is achieved by alternative splicing throughout the coding sequence of dystrophin [20, 30]. Notably, two alternative splicing sites exist at the 3′-end of the DMD gene . Their usage has been well characterized in the Dp71 isoform. In the Dp71d isoform, excision of exon 71 does not alter the reading frame of the transcript and still generates the 13-amino-acid-long C-terminus common to most dystrophin isoforms including exon 79. The Dp71f (Dp71b) isoform, however, is generated by alternative splicing of exon 78 which shifts the reading frame and produces a C-terminus that contains 31 new amino acids with hydrophobic properties (Additional file 1: Figure S1). Another isoform is Dp71c lacking exon 71-74 that encode the 110-amino-acid sequence of the syntrophin-binding domain. Moreover, exon 78 can be additionally skipped in the latter isoform creating the Dp71Δ110 variant. The expression of these isoforms is differentially regulated during human embryonic development and adulthood [20, 27, 31, 32]. Similar expression patterns have been observed in animal models on the mRNA and protein level [20, 33]. The shortest dystrophin isoform, Dp40, has the same promoter as Dp71 but lacks the normal C-terminal end of Dp427. Although less abundant Dp40 shows a similar expression pattern as Dp71 .
In the nervous system, all the different dystrophin isoforms have been identified. They are expressed not only in the adult but also during neural development. DMD patients may suffer from CNS dysfunction including cognitive [15, 35] and visual impairment [25, 36–38]. However, the exact role of dystrophin in the CNS as well as its contribution to the CNS phenotype of DMD patients is still a matter of debate and hampered by the complexity and high variety of the dystrophin isoforms and their DAPC components: Dp427 is expressed at the postsynapse of neurons in the hippocampus, cerebellum, and cerebral cortex [39–43]. An involvement of this isoform in neuronal activity as well as in the formation and maintenance of the blood-brain barrier has been suggested [44–47]. The Dp140 isoform is mainly expressed during development . Dp71 is expressed in glial cells, notably in perivascular astrocytes and the Bergmann glia of the cerebellum and in the Müller glia of the retina that surrounds the endothelial cells [23, 48]. In the brain and retina, several authors proposed a role of Dp71 in the maintenance of potassium and water homeostasis as well as in the regulation of vascular permeability [49–53]. In the retina, Dp427 and Dp260 isoforms are associated with photoreceptor terminals suggesting the involvement of dystrophin in synaptic transmission, but the precise mechanism is still unclear [38, 54, 55].
Overall, there are still many questions to be answered about the role of dystrophin in health and disease, especially in non-muscle tissues. Interestingly, the recent discovery of a role for dystrophin in satellite cells points to the fact that there are still unknown functions to be found that might open new leads in DMD research . Therefore, it would be beneficial to have a model organism to facilitate the study of dystrophin expression from its natural promoter(s) in various tissues without the need for immunostaining. Hence, we set out to generate a Dmd EGFP reporter mouse, tagging the C-terminus of the protein with EGFP and verifying its proper subcellular expression in various mouse organs.
Generation of transgenic mice
Isolation of EDL myofibers, culture, and differentiation of single fiber-derived satellite cells
Extensor digitorum longus (EDL) muscles were dissected from the hind limbs of 8-month-old Dmd EGFP mice and their wild-type littermates and digested in 0.2 % collagenase type I (Sigma-Aldrich, Germany) in DMEM . Muscles were then transferred to DMEM-filled horse serum (HS)-rinsed dishes using a heat-polished Pasteur glass pipette. Muscles were triturated until the fibers had separated, transferred to a fresh dish, and incubated at 37 °C, 5 % CO2. Intact fibers were transferred into fresh DMEM-filled and HS-rinsed dishes using a thinly bored Pasteur pipette and under a stereomicroscope to exclude any cell debris. The isolated myofibers with attached satellite cells (SC) were either fixed for immunofluorescence (IF) staining or cultured in DMEM supplemented with 20 % FBS (Sigma-Aldrich), 10 % HS (Sigma-Aldrich), and 1 % chicken embryo extract (CEE Ultrafiltrate, VWR International, Germany) in dishes that had been coated with 10 % Matrigel® (BDBiosciences, Germany) . After 2 days, SC-derived myogenic progenitors started to migrate off the myofibers and formed myogenic colonies composed of proliferating myoblasts that differentiated into myotubes by days 6–8. This process was enhanced by changing the growth medium to low serum medium (4 % HS) after 5 days. Differentiating myoblasts were monitored daily under the microscope and fixed after 8 days for subsequent IF staining.
Tibialis anterior (TA), EDL, quadriceps (QUAD), gastrocnemius (GAS), and soleus (SOL) muscles, as well as the diaphragm (DIA), heart, stomach, colon, ileum, duodenum, brain, and eyes were dissected from 8–12-week-old (adult) or 10-month-old (aged) male Dmd EGFP mice and from their age- and sex-matched wild-type littermates. The tissue was mounted on Tissue-Tek® O.C.T. (Hartenstein, Germany), frozen in fluid nitrogen cooled isopentane and processed for cryostat sectioning; 8-μm cross sections were collected from the mid-belly of the muscles and either stored at −80 °C or directly fixed and stained. Hematoxylin/eosin staining was done according to standard procedures.
Immunohistochemistry was performed on 8-μm cryosections using the iVIEW-Ventana ABC Kit (Ventana, AZ, USA) with primary antibodies and corresponding biotinylated secondary antibodies using the diaminobenzidine (DAB) visualization method. In order to suppress unspecific background staining, we used the MOM kit (Vector Laboratories, Burlingame, CA, USA) for all primary mouse antibodies. All primary and secondary antibodies used in this study are described in Additional file 2: Tables S1 and S2.
Immunofluorescent staining was performed on single fibers, differentiated myoblasts, and on 8-μm cryosections from different tissues. Fibers and myoblasts were fixed in 4 % PFA, washed, incubated 1 h at room temperature in blocking solution containing 5 % normal goat serum (NGS), 0.5 % BSA, and 0.2 % Triton X-100, followed by overnight incubation with primary antibodies. The cryosections were fixed in ice-cold acetone or 4 % PFA, washed in PBS, blocked, and incubated with primary antibodies for 1 h at room temperature or at 4 °C overnight. After primary antibody incubation, the samples were washed and incubated for 1 h with fluorescently labeled secondary antibodies (AlexaFluor® 488 and 568, Life Technologies, CA, USA, dilution 1:400), washed in PBS, and mounted in DAPI-containing mounting medium (Vectashield, Vector Laboratories, USA).
Primary antibodies in this study were directed against the following proteins or protein subdomains: laminin, β-spectrin, dystrophin, Dys2, MANDYS19, H4, GFP, FLAG, GFAP, CD31, α-, β-, and γ-sarcoglycan, α-dystroglycan, nNOS, and vinculin. Acetylcholine receptors were stained using AlexaFluor568 coupled α-bungarotoxin. Images were recorded with an inverted fluorescent microscope (Leica DMI4000).
Serum CPK analysis
Blood was collected from the facial vein of 8-week-old wild-type, Dmd EGFP and mdx-mice (n = 3 of each). Serum creatine phosphokinase (CPK) activity was measured with the Creatine Kinase Activity Assay Kit (Sigma-Aldrich, Munich, Germany) according to the manufacturer’s instructions.
8 μm cryosections of the EDL and soleus muscles from 8-week-old wild-type and Dmd EGFP mice were stained with an anti-laminin antibody using the DAB visualization method to delineate the muscle fibers. Bright field color images were taken at ×200 magnification and saved as TIF files. The minimal Feret diameter was calculated for 500–820 fibers per muscle using the Image-Pro Plus v.7.0 software (Media Cybernetics, Rockville, MD, USA). Fibers at the border of the image were excluded from analysis. Three animals per group were analyzed, and the fiber diameters for EDL and soleus were plotted as a histogram (Fig. 3D).
Western blot analysis
Protein was extracted from the tibialis anterior muscle or from the brain of 8-week-old male Dmd EGFP mice and wild-type littermates using a combination of following two buffers (1:1) containing proteinase inhibitor cocktail: (i) 250 mM sucrose, 10 mM Tris-HCl, pH 8.2 and (ii) 20 % SDS, 20 % glycerol, 10 % β-mercaptoethanol, 12.5 % Western blot running buffer in bidest water; 50, 5, 0.5, or 0.05 μg protein were loaded on 3–8 % Tris-acetate gradient gels (Novex, Life Technologies), electrophoresed, and wet-blotted onto a nitrocellulose membrane. NuPage® LDS sample buffer (Invitrogen) was used for gel electrophoresis. The blots were probed using the iBind Flex® Western System (Invitrogen, LifeTechnologies) with mouse antibodies directed against the dystrophin rod domain (Dys1 or MANDYS19) and a rabbit polyclonal antibody against the dystrophin C-terminus (clone H4, self-made, gift from Cyrille Vaillend) together with secondary fluorescently labeled antibodies (AlexaFluor® 700 and 800). A second blot was probed with a monoclonal anti-GFP antibody together with a secondary fluorescent antibody (AlexaFluor® 700). As a loading control, a mouse monoclonal anti-vinculin antibody was used. Bands were visualized using the Odyssey CLx system (Li-Cor) (Fig. 1c). All used antibodies are listed in Additional file 2: Tables S1 and S2.
Results and discussion
Generation and validation of the Dmd EGFP reporter mice
For generation of the Dmd EGFP mice, the murine dystrophin locus on the X-chromosome was modified using a targeting vector (Fig. 1a). This resulted in a modification of dystrophin exon 79 whose natural termination codon was removed and a C-terminal fusion protein generated with a FLAG- and an EGFP-L221K coding sequence. The 3′UTR further contained a loxP flanked neomycin (neo) cassette for ES-cell selection (Fig. 1a). After germline transmission, the neo cassette was successfully removed by transient crossbreeding with ubiquitous Cre-deleter mice. Dmd EGFP transgenic animals could be identified by allele-specific PCR (Fig. 1b). Hemizygous males, as well as hetero- and homozygous females were viable and fertile and were analyzed in comparison with their wild-type littermates.
An important consideration for our targeting strategy was the site for EGFP insertion into the endogenous dystrophin locus. The aim was (i) to target the major dystrophin isoforms that are expressed in most of the relevant tissues and (ii) to find out whether the natural endogenous dystrophin-EGFP expression would be strong enough for detection without further amplification by immunofluorescence. Since the FLAG-EGFP tag was appended to the C-terminus of dystrophin, to a protein domain where several interacting proteins bind, we had to investigate (iii) whether the genetic manipulation might cause a dystrophic phenotype per se and whether (iv) dystrophin and its binding partners from the DAPC would have the correct subcellular localization. (v) Finally, we wanted to investigate whether the published data in the literature on tissue-specific dystrophin expression patterns would correspond to those seen in our Dmd EGFP mice.
The aim of our study was to generate a reporter mouse, in which dystrophin expression can be visualized and tracked in different tissues by means of EGFP fluorescence. Dystrophin and its numerous isoforms and splice variants share the C-terminal domain, which is very important for the interaction of the protein with its binding partners and might thus interfere with proper function; however, previous studies had already shown that C-terminally EGFP-tagged mini- or micro-dystrophins in transgenic mice or cells would be functional [64–66]. Therefore, we inserted the FLAG-EGFP sequence downstream of exon 79, the last exon present in the major dystrophin isoforms (Additional file 1: Figure S1). Insertion of the FLAG-EGFP downstream of exon 79, would theoretically tag the C-terminus and express the following proteins as fusions: Dp427 (B, M, P), Dp260, Dp140, Dp116, Dp71, Dp71d, and Dp71c. The alternative hydrophobic C-terminus of Dp71f and Dp71Δ110 would not be targeted in our model since skipping of exon 78 shifts the reading frame of the last exon 79 due to alternative splicing (Additional file 1: Figure S1) [31, 67]. In addition, Dp40 expressing another alternative C-terminus would not be tagged as well.
Dystrophin-EGFP expression in skeletal muscle and normal muscle morphology in Dmd EGFP reporter mice
The correct localization of native EGFP fluorescence was further confirmed in TA, soleus, and gastrocnemius muscles by co-immunostaining with anti-dystrophin antibodies against the rod (MANDYS19) and C-terminal (Dys2) domains (Fig. 2, Additional file 1: Figure S2), which showed an exact superposition of the signals. MANDYS19 binds to the amino acid sequence encoded by dystrophin exons 21-22 and thus recognizes only the full-length protein . All the alternative dystrophin promoters that control the expression of shorter dystrophin isoforms are located downstream of intron 29, and dystrophin isoforms generated from these promoters would not be picked up by the MANDYS19 antibody. Hence, the signal from the MANDYS19 antibody, which was similar to the signal of the Dys2 C-terminal antibody, confirmed the correct tagging and subcellular localization of the skeletal muscle Dp427 isoform. Correct expression of the ECM component laminin as well as of the subsarcolemmal β-spectrin was also confirmed on skeletal muscle sections (Additional file 1: Figure S3), where β-spectrin colocalized with dystrophin-EGFP in the myofibers.
Importantly, a comparison between the EGFP and laminin signals exhibited, as expected, distinct differences, with EGFP being present at the cytoplasmatic side of the sarcolemma and excluded from the capillaries. Notably, the endothelial cells of the endomysial capillaries were also stained by the anti-β-spectrin antibody where dystrophin-EGFP expression was absent. This suggests that dystrophin expression in the vascular system is confined to larger vessels that possess a tunica media (e.g., arteries and larger veins) .
Finally, we screened different skeletal muscles for the expression of representative members of the various DAPC sub-complexes. We confirmed normal sarcolemmal/subsarcolemmal expression patterns for α-, β-, γ-sarcoglycan, α-dystroglycan, as well as for nNOS suggesting proper function of the fusion protein (Fig. 3b, Additional file 1: Figures S5-S9). We conclude that no deleterious changes of the dystrophin protein structure might have occurred that would disturb those interactions. However, for some components, it appeared as if the expression levels would differ between Dmd EGFP and wild-type mice. However, the DAB-mediated visualization of immune signals is not really quantitative and subject to too many confounding factors in order to pick up subtle differences. Here, it would be preferable in the future to employ mass spectrometric techniques to identify and quantify proteins that bind to the dystrophin-EGFP, which could be easily immunoprecipitated with an anti-FLAG or anti-GFP antibody .
Dystrophin-EGFP expression in cardiac and smooth muscle
The localization of dystrophin near the plasma membrane of smooth muscle cells via its natural EGFP fluorescence could be readily observed in cryosections of other intestinal organs as well. In cross sections of stomach, ileum, and duodenum of Dmd EGFP mice, we observed bundles of longitudinal and circular layers of smooth muscle cells with dystrophin located at their periphery (Additional file 1: Figure S10A). The distribution of the EGFP signal at the membrane of smooth muscle cells was discontinuous, which is in line with previous studies using dystrophin antibodies . This phenomenon is due to the exclusion of dystrophin from the adherens junctions between smooth muscles .
Dystrophin-EGFP expression in non-muscle tissue
In the brain and retina of Dmd EGFP mice, we observed natural dystrophin-EGFP expression around blood vessels that were stained with the anti-CD31 endothelial cell marker (Figs. 5 and 6b). Moreover, staining with an antibody against the glial fibrillary acidic protein (GFAP) revealed partial colocalization of the dystrophin-EGFP with the Bergmann glia in the cerebellum (Fig. 5), the glial cells of the hippocampus (Additional file 1: Figure S10B), and the Müller cells at the inner limiting membrane of the retina (Fig. 6c). In these regions, the EGFP signal only colocalized with the glial endfeet, which confirms a characteristic expression pattern of the Dp71 isoform . On the other hand, dystrophin is also expressed in the blood vessels, mainly in the wall surrounding the endothelial cells. The EGFP signal was at some locations exactly encircling the CD31 immunosignal (Fig. 5), suggesting expression of the dystrophin in the outer walls of the vessels, but not in the vascular endothelium. However, from our immunofluorescent images, it did not become entirely clear, whether the green fluorescence really derived from the vessel wall or from the glial endfeet. Here, high-resolution imaging techniques like STED microscopy could resolve the issue.
It was reported in the literature that the full-length dystrophin isoform in the brain was expressed in neurons such as in Purkinje cells of the cerebellum or in neurons of the hippocampus [21, 74, 75]. Unfortunately, we were unable to detect any natural EGFP fluorescence in the aforementioned regions using our standard fluorescence EGFP-imaging techniques. However, via immunostaining using an anti-GFP antibody, we detected neuronal dystrophin-EGFP expression in CA1/CA2 regions of the hippocampus and in Purkinje cells (Additional file 1: Figure S11). The apparent absence of natural EGFP fluorescence in the neurons of Dmd EGFP mice could be explained by the much lower expression levels of the full-length Dp427 (B) dystrophin in the brain in contrast to the Dp71 isoform, which gave much stronger signals in the Western blot analysis (Additional file 1: Figure S13). Moreover, immunofluorescence staining with an anti-GFP antibody shows large differences in the GFP signal intensities between Dp71-expressing blood vessels and Dp427-expressing neurons. For the detection of dystrophin-EGFP in the neurons, we had to use high exposure times that led to massive overexposure of the EGFP-positive blood vessels (Additional file 1: Figure S11). Even after immunostaining with an anti-GFP antibody, using lower exposure times to image the blood vessels would fail to visualize the neurons.
EGFP fluorescence is detectable on the level of single muscle fibers and in satellite cell-derived myotubes
This study describes and validates a Dmd EGFP reporter mouse that was generated through a C-terminal dystrophin-EGFP fusion protein, which is expressed from its natural promoters. The addition of the tag did not have any influence on the structure and function of the protein and the assembly of the dystrophin-associated protein complex. The natural EGFP expression is strong enough in the skeletal, heart, and smooth muscles, brain, and retina allowing detection of dystrophin even on the single fiber level without the need for immunofluorescent staining. We confirmed the successful targeting of the major dystrophin isoforms that express the last exon 79. The mouse model will provide better insight into the normal dystrophin expression in the muscle as well as in the non-muscle tissue and will facilitate studies on the dynamics of dystrophin expression. Furthermore, the crossing of the Dmd EGFP allele into the mdx background in cis (e.g., the Dmd exon 23 nonsense mutation lies on the same allele as the FLAG-EGFP tag) will provide an excellent model to investigate dystrophin re-expression in vivo or ex vivo after various gene therapy protocols that are aimed at the re-establishment of the dystrophin open reading frame or in naturally occurring revertant fibers.
bp, base pair; BTX, bungarotoxin; CNS, central nervous system; CPK, creatine phosphokinase; DAPC, dystrophin-associated protein complex; DAPI, 4′,6-diamidino-2-phenylindole dihydrochloride; DG, dentate gyrus; DIA, diaphragm; DMD, Duchenne muscular dystrophy; ECM, extracellular matrix; EDL, extensor digitorum longus muscle; EGFP, enhanced green fluorescent protein; ES cells, embryonic stem cells; Ex, exon; GAS, gastrocnemius; GFAP, glial fibrillary acidic protein; H&E, hematoxylin and eosin staining; HS, horse serum; IF, immunofluorescence; ILM, inner limiting membrane (of the retina); INL, inner nuclear layer (of the retina); kbp, kilobase pair, kDa, kilo-Dalton; neo, neomycin; NMJ, neuromuscular junction; nNOS, neuronal nitric oxide synthase; OPL, outer plexiform layer (of the retina); QUAD, quadriceps muscle; SOL, soleus muscle; TA, tibialis anterior muscle
The authors thank the animal technician Claudia Pallasch at the FEM of the Charité for her help and expertise towards our mouse breeding program and Cyrille Vaillend for the gift of the H4 anti-dystrophin antibody.
This work was supported by the Université Franco-Allemande (as part of the MyoGrad International Graduate School for Myology GK 1631/1 of the DFG and CDFA-06-11) to MVP, KR, LG,HA, MS, the Agence Nationale de la Recherche (ANR-DYSther, ANR-14-CE13-0037-02), and the BIH-Charité Clinical Scientist Program towards JR. MS is a member of and funded by the NeuroCure Center of Excellence (Exc 257) at the Charité Berlin, Germany.
Availability of supporting data
Supplementary online material is available as PDF.
MVP, LG, HA, and MS designed the study. MVP, SMG, KR, EG, and FS performed the genetic engineering and the molecular genetic experiments. MVP and MS supervised the breeding program. MVP, JR, WS, and MS performed the histological analysis and morphometry. MVP and MS wrote the first draft of the manuscript. LG, HA, and MS provided funding. All authors read the final version of the manuscript and gave their permission for publication.
The authors declare that they have no competing interests.
Consent for publication
All authors have seen the final version of the manuscript and consented to its submission to Skeletal Muscle.
Ethical approval and consent to participate
The animal (mouse) experiments were approved by the LaGeSo Berlin (Registration number T 0222/13).
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