Gene Expr Patterns
October 1, 2006;
Xtn3 is a developmentally expressed cardiac and skeletal muscle-specific novex-3 titin isoform.
proteins play an essential role in maintaining muscle
function and structure. Recent work has implicated the involvement of the novex-3 titin
isoform in sarcomere
restructuring and disease. Unlike avian and mammalian systems, Xenopus laevis myogenesis is characterized by a wave of primary myogenesis followed by apoptosis of the primary muscles and formation of new muscles by secondary myogenesis. We show here that the Xenopus laevis novex-3 titin
) is developmentally expressed throughout the somites
, and primary muscles of the developing embryo
. Downregulation of Xtn3
expression at tadpole
stages appears to coincide with the change in myofiber composition from solely embryonic "fast" fiber types to myofibers containing both "fast" and "slow" fiber types. We demonstrate that Xtn3
is expressed early in the presomitic mesoderm
and remains expressed in the somites
myoblasts, and developing jaw muscles through late tailbud stage
. Furthermore, we show that Xtn3
is expressed in the cardiac primordia
prior to linear heart tube
formation and remains expressed in the heart
until tadpole stage
, at which point it is downregulated in the heart
except in discrete patches of cardiac cells. Finally, we demonstrate that Xtn3
transcripts are detectable in adult heart
Gene Expr Patterns
[+] show captions
Fig. 2. Xtn3 is developmentally expressed throughout the somites, heart, and jaw myoblasts from early neurula to late tailbud stages. (A–P) In situ hybridization showing expression of the Xtn3 at the indicated stages. (A–C) Neurula stage embryos. Anterior is to the left. (A and C) Dorsal views. (B) Lateral view. (D–M) Tailbud stage embryos. (E, G, I, K, and M) Ventral anterior views. Anterior is up. (H) Inset shows higher magnification of somite region. Bracket indicates one individual somite. (N) Tadpole stage embryo. Note the lack of staining in trunk and heart. Some light staining remains in facial muscles. (O and P) Adult heart lacking global Xtn3 expression. (P) A magnification of (O). Black arrowhead indicates isolated group of Xtn3-expressing cells. h, heart; hp, heart primordia; jm, jaw myoblasts; psm, presomitic mesoderm; pc, pigment cells; vm, ventral myoblasts; s, somites; t, trunk.
Fig. 3. Histology of Xtn3 expression. Paraffin embedded embryos were sectioned at a depth of 16 μm after in situ hybridization. (A, C, E, and G) Transverse sections through developing somites. (B, D, F, and H) Transverse sections through developing cardiac region. (I) Transverse section showing jaw myoblasts in the head. a, atrium; ec, endocardium; hp, heart primordia; jm, jaw myoblasts; mc, myocardium; nc, notochord; nt, neural tube; s, somites; v, ventricle; vm, ventral myoblasts.
tn3 (titin novex-3) gene expression in Xenopus laevis embryos, NF stage 40, as assayed by in situ hybridization, lateral view, anterior left, dorsal up.
Key: jm=jaw muscle, h=heart; vm=ventral myoblasts, also called hypaxial muscles.
Fig. 4. Xtn3 is expressed in adult heart and skeletal muscle. RNA from St.13 embryos, St.38 embryos, adult heart, and adult muscle tissue was isolated and 100 ng of RNA subjected to RT-PCR. Xtn3 RT-PCR shows expression in St.38, adult heart and adult muscle tissues. “−RT” controls lack reverse transcriptase to show lack of genomic contamination. Tbx20 primers serve as controls for heart tissue, MHCα and muscle actin serve as controls for muscle tissue, and EF1α serves as loading control.
tn3 (titin novex-3) gene expression in Xenopus laevis embryos, NF stage 40, as assayed by in situ hybridization, transverse section through head at eye level, dorsal up. Key: jm= jaw muscle.