Two cell lineages, myf5 and myf5-independent, participate in mouse skeletal myogenesis

Abstract

In skeletal muscle development, the myogenic regulatory factors myf5 and myoD play redundant roles in the specification and maintenance of myoblasts, whereas myf6 has a downstream role in differentiating myocytes and myofibers. It is not clear whether the redundancy between myf5 and myoD is within the same cell lineage or between distinct lineages. Using lineage tracing and conditional cell ablation in mice, we demonstrate the existence of two distinct lineages in myogenesis: a myf5 lineage and a myf5-independent lineage. Ablating the myf5 lineage is compatible with myogenesis sustained by myf5-independent, myoD-expressing myoblasts, whereas ablation of the myf6 lineage leads to an absence of all differentiated myofibers, although early myogenesis appears to be unaffected. We also demonstrate here the existence of a significant myf5 lineage within ribs that has an important role in rib development, suggested by severe rib defects upon ablating the myf5 lineage.

Figure 1. Alleles of myf5 and myf6 Cre Drivers

(A) myf5 and myf6 genes are linked on chromosome 10. The 3′UTR of myf6 (in myf6-ICN)or myf5 (in myf5-ICN) harbors an encephalomyocarditis Internal Ribosomal Entry Site (IRES) that is linked to the Cre coding sequence and is followed by the Neomycin (neo)-resistance selection cassette with MC1 promoter. The selection cassette with MC1 promoter is flanked by two FRT sequences. (B and C) Breeding with FLP-e mice leads to recombination between FRT sites, generating the “NN” alleles. (D–G) Semiquantitative RT-PCR on total RNA from E12.5 embryos shows (D) reduced expression of myf6 in myf6-ICN^+/− mice (star) but (E) normal expression in myf6-NN^+/− mice compared to wild-type. Similarly, myf5 expression is (F) reduced in myf5-ICN^+/− mice (star) but (G) normal in myf5-NN^+/− mice compared to wild-type. (H and I) β-gal staining revealed a reduced myf6 lineage in E15.5 (H) myf6-ICN/R-LZ embryos compared to (I) myf6-NN/R-LZ embryos. (J and K) Similarly, the myf5 lineage in E11.5 (J) myf5-ICN/R-LZ embryos is reduced compared to (K) myf5-NN/R-LZ embryos. (L) A control wild-type E11.5 embryo shows an absence of any β-gal staining.

Figure 2. The myf5 Lineage in Myogenesis

(A) (Aa) The myf5 lineage (blue) in myf5-NN/R-LZ embryos comprises developing embryonic musculature at E10.5, which is predominantly in the (Ab and Ad) myotome but absent in the (Ac and Ad) neural tube. Migratory myogenic populations in the (Ab) limbs (arrow) and (Ad) nonmyogenic dermal precursors (arrows) are shown. (Ae) myf5 expression (red, anti-Myf5) occurs exclusively within the β-gal lineage marker (green, anti-β-gal) in myf5-NN/R-LZ embryos. (B) The myf5 lineage (red, anti-β-gal) is present within (Ba and Bb) fast-type (green, anti-MyHC) myofibers and (Bc) slow-type (green, anti-MS) myofibers of adult myf5-NN/nLacZ intercostal muscle. (Bd and Be) Enzymatic detection of β-gal activity (red nuclei) reveals a similar distribution of the myf5 lineage, with hematoxylin as counterstain (blue nuclei). Arrows show myonuclei not derived from Myf5. (C) The distribution of the myf5 lineage (with anti-β-gal) and MyoD expression (using anti-MyoD) was analyzed in epaxial (left, blue box) and hypaxial (right, gray box) regions by cell counting in E12.5 ZEG/myf5-NN embryos. The numbers in boxes are the numbers of nuclei counted. +, presence; −, absence. (D) (Da) Fast-type (green, anti-MyHC) and (Db) slow-type (green, anti-MS) fibers seen in E18.5 myf5-NN/R-DTA intercostal musculature (D, diaphragm; IC, intercostals; R, ribs) that show normal morphology based on (Dc) H&E staining and (Dd) electron micrograph. (E) The myf5 lineage (green) demonstrated in (Ea) whole mount and (Eb) sections (green, anti-GFP) in E12.5 ZEG/myf5-NN embryos. (Ec) MyoD (gree,n anti-MyoD) and (Ed) myogenin (green, anti-MyoG) are expressed within and in proximity to the myf5 lineage (red, anti-GFP). Absence of the myf5 lineage in ZEG/myf5-NN/R-DTA embryos is demonstrated in (Ee) whole mount (absence of green signal) and (Ef) sections (absence of green anti-GFP staining). (Eg) MyoD and (Eh) MyoG expression occurs independent of the myf5 lineage in ZEG/myf5-NN/R-DTA. (F) Semiquantitative RT-PCR for myoD at E10.5, E11.5, E12.5, and E13.5 reveals increased numbers of MyoD transcripts in myf5-NN/R-DTA embryos compared to wild-type by E13.5.

Figure 3. The myf6 Lineage in Myogenesis

(A) The myf6 lineage is undetectable at (Aa) E9.5 and is detectable at (Ab) E10.5 (arrows) based on β-gal staining (blue) of myf6-NN/R-LZ embryos. (Ac) The myf6 lineage (red nuclei) in the intercostal musculature of an adult myf6-NN/nLacZ mouse is detected by β-gal staining and hematoxylin counterstain (blue nuclei). (Ad) shows the same based on immunohistochemistry (red, anti-β-gal; green, anti-MyHC). (Ae) shows Myf6 expression (red, anti-Myf6) in adult intercostal muscles (green, anti-MyHC). (B) Transverse sections through the epaxial region of myf6-NN/R-YFP embryos demonstrate that the myf6 lineage (red, anti-GFP) comprises only a fraction of (Ba) MyoD- (green, anti-MyoD), (Bb) MyoG- (green, anti-MyoG), and (Bc) MyHC-expressing (green, anti-MyHC) cells at E14.5. (C) (Ca and Cb) E18.5 myf6-NN/R-DTA embryos show basophilic cellular debris in place of musculature (back muscles). (D) Robust myogenesis within an E12.5 myf6-NN/R-DTA embryo demonstrated by expression of (Da) myf5 (green, anti-Myf5), (Db) myoD (green, anti-MyoD), and (Dc) myoG (green, anti-MyoG) in sections through the myotome. (E) Apoptosis detected by expression of activated caspase-3 (red) within differentiated myofibers (green) at (Ea) E14.5 and (Eb) E18.5.

Figure 4. Unique Role of the myf5 Lineage in Rib Morphogenesis

(A) The myf5 lineage is detected by blue β-gal staining at E18.5 in ribs of myf5-NN/R-LZ, demonstrated in (Aa) whole mount (arrows) and (Ab) sections. (Aa) IC, intercostal muscles; D, diaphragm. The counterstain is nuclear red in (Ab). Control wild-type littermates show an absence of β-gal staining in (Ac) whole mount and in (Ad) sections. Rib lineages of Myf5 appear to be prominent at the (Ae and Ag) sternal end (arrows) and the (Af and Ah) costochondral region (arrows) and appear less often in the (Ah) “ICN” allele compared to the (Af) “NN” allele in costochondral regions. (B) Abnormal rib cage of newborn myf5-NN/R-DTA mice characterized by (Ba) fusions (arrow) and (Bb) staggered asymmetric attachment to the sternum (arrow) at the cartilaginous part and by (Bc) abnormal protrusions in the bony part (arrow). (Bd)–(Bf) show comparative regions in a wild-type littermate. (C) myf5-ICN/R-DTA mice have (Ca) normal rib cages and occasional mild anomalies such as (Cb) waviness of the floating ribs (arrow) and (Cc) occasional knobs in the costal regions (arrow). (D) (Da–Dc) Ablating the pax7 lineage does not perturb rib morphogenesis. (Dd) The pax7 lineage is absent within ribs of E15.5 pax7-cre/R-LZ embryos (arrow). (E) (Ea)–(Ec) Ablating the myf6 lineage does not perturb normal rib morphogenesis, except for (Ea) occasional mild waviness (arrow). (Ed) The myf6 lineage is absent within ribs of E17.5 myf6-NN/R-LZ embryos (arrow).