Development of Bipotent Cardiac/Skeletal Myogenic Progenitors from MESP1+ Mesoderm

Abstract

The branchiomeric skeletal muscles co-evolved with new chambers of the heart to enable predatory feeding in chordates. These co-evolved tissues develop from a common population in anterior splanchnic mesoderm, referred to as cardiopharyngeal mesoderm (CPM). The regulation and development of CPM are poorly understood. We describe an embryonic stem cell-based system in which MESP1 drives a PDGFRA+ population with dual cardiac and skeletal muscle differentiation potential, and gene expression resembling CPM. Using this system, we investigate the regulation of these bipotent progenitors, and find that cardiac specification is governed by an antagonistic TGFβ-BMP axis, while skeletal muscle specification is enhanced by Rho kinase inhibition. We define transcriptional signatures of the first committed CPM-derived cardiac and skeletal myogenic progenitors, and discover surface markers to distinguish cardiac (PODXL+) from the skeletal muscle (CDH4+) CPM derivatives. These tools open an accessible window on this developmentally and evolutionarily important population.

Keywords: Mesp1; cardiac development; cardiopharyngeal mesoderm; mesoderm; skeletal myogenesis.

Graphical abstract

Figure 1

MESP1 Promotes Cardiac or Skeletal Myogenic Differentiation Depending on Serum-Factor Signaling (A) MESP1 induced a KDR− PDGFRA+ population in day-5 EBs cultured in serum-free condition. (B) These putative paraxial mesoderm cells gradually acquired a skeletal myogenic fate that was tubular, MYOGENIN+, α-actinin+, and myosin heavy chain (MHC)+ by day 12 (top row). Serum supplement from day 5 produced cardiac cells that were planar, MYOGENIN−, α-actinin+, and MHC+ (bottom row). Images are representative of five independent experiments. Scale bar represents 100 μm. (C) Scheme depicting the protocol used to evaluate the effects of various treatments on MESP1-induced cardiac versus skeletal myogenic differentiation. (D) Quantitative RT-PCR analysis showing that the addition of serum downregulated skeletal myogenic genes (Tcf15, Myf5, Myod1, Myog) and upregulated cardiac genes (Isl1, Nkx2-5, Gata4, Tbx5, Tnnt2) as early as 24 hr (left), and also after 7 days (right) (n = 12, from four independent experiments). Mean ± SEM is shown. See also Figure S1 and Tables S1 and S2.

Figure 2

MESP1-Induced PDGFRA+ Progenitors Give Rise to Cardiac or Skeletal Myogenic Derivatives at a Single-Cell Level (A) Paraxial mesoderm genes (Meox1, Tcf15) and CPM genes (Tbx1, Pitx2, Ebf1, Lhx2) were enriched in MESP1-induced day-5 PDGFRA+ cells cultured in serum-free condition (n = 6, from two independent experiments). Mean ± SEM is shown. (B) Immunostaining shows that day-5 PDGFRA+ cells predominantly acquire a skeletal myogenic fate (MHC+ MYOGENIN+) in serum-free conditions or a cardiac fate (MHC+ MYOGENIN−) in serum-containing condition by day 12, but PDGFRA− cells do not acquire either fate. Images are representative of three independent experiments. Scale bar represents 100 μm. (C) Quantitative RT-PCR shows that the default skeletal myogenic differentiation of day-5 PDGFRA+ cells under serum-free conditions switches to cardiac in the presence of serum (n = 6, from two independent experiments). Mean ± SEM is shown. (D and E) Clonal analysis of MESP1-induced PDGFRA+ cells. (D) Single EGFP-labeled MESP1-induced PDGFRA+ cells gave rise to cardiac (CM) or skeletal myogenic (SKM) derivatives. Images are representative of four independent experiments. (E) A single MESP1-induced PDGFRA+ cell differentiated into both cardiac (arrowhead) and skeletal myogenic (arrow) lineages. Cells were grown in serum-free conditions supplemented with Y27632, conditions under which skeletal myogenic differentiation predominates. Scale bar represents 50 μm. (F) Sarcomeric structures were observed in cardiac (left) and skeletal myogenic derivatives (right). Areas depicted by the white dotted rectangle are magnified for clarity. Scale bar represents 20 μm. See also Figure S2 and Tables S1 and S2.

Figure 3

Signaling Environment Governs Cardiac versus Skeletal Myogenic Differentiation of MESP1+ Mesoderm (A) Screening of modulators of signaling pathways involved in myogenesis revealed that BMP4 decreased skeletal myogenic gene expression and increased cardiac gene expression by day 12 (n = 3, technical replicates). (B) BMP4 promoted cardiac at the expense of skeletal myogenic differentiation as early as 24 hr (left) and after 7 days (right) (n = 3, technical replicates). (C) Immunostaining using pan-troponin I (for both skeletal and cardiac) and cardiac-specific troponin I verified that MESP1 induced pan-TnI+ cTnI− skeletal myogenic cells in serum-free conditions (control), whereas BMP4 produced pan-TnI+ cTnI+ cardiac cells instead. (D and E) ALK4/5/7 inhibition by A83-01 promoted cardiac differentiation, as shown by immunostaining (D) and quantitative RT-PCR (E) (n = 3, technical replicates). Efficiency of differentiation was determined by staining for MHC and DAPI, and analyzing images with G-Tool (Ippolito et al., 2012) (n = 3). Images are representative of three independent experiments. (F) TGFβ/activin-BMP antagonism, as shown by TGFβ1 and activin A diminishing the pro-cardiac effect of BMP4 (n = 3, technical replicates). (G and H) ROCK inhibition by Y27632 promoted skeletal myogenic differentiation, as shown by immunostaining (G) and quantitative RT-PCR (H) (n = 3, technical replicates). Efficiency of skeletal myogenic differentiation was determined, as above, using G-Tool (n = 3). Images are representative of three independent experiments. (I) Y27632 improved the survival of MESP1-induced PDGFRA+ cells by inhibiting apoptosis (Annexin V+ 7-AAD+/−). Mean ± SEM is shown in (A), (B), and (D)–(H). Scale bar represents 100 μm in (C), (D), and (G). See also Figure S3 and Tables S1, S2, and S3.

Figure 4

PODXL and CDH4 Distinguish MESP1-Induced Early Cardiac and Skeletal Myogenic Progenitors, Respectively (A) Principal-component analysis of RNA-seq data generated from early (day 6) versus late (day 12) cardiac (CM) versus skeletal myogenic (SKM) populations (n = 3 independent experiments). (B and C) RNA-seq analysis of early cardiac versus skeletal myogenic populations revealed a total of 669 genes whose expression was significantly altered during the early fate-specification process (B), wherein existed a membrane proteins subset (C). (D) PODXL (podocalyxin) marked a portion of early cardiac progenitors (left column), and CDH4 (cadherin-4) marked most early skeletal myogenic progenitors (right column). FACS analysis was performed on MESP1-induced day-6 early cardiac (BMP4-treated, top row) and skeletal myogenic (untreated, bottom row) cells. (E) PODXL+ sorted cells were highly enriched for cardiac transcripts (left two panels), and similarly, skeletal myogenic transcripts were abundantly found in CDH4+ sorted cells (right two panels) (n = 3, technical replicates). Mean ± SEM is shown. (F) Immunostaining of day-12 cells previously sorted on day 6 based on PODXL and CDH4 expression. In BMP4-treated cells, cardiac progenitors were enriched in the PODXL+ fraction (left); whereas in untreated cells, skeletal myogenic progenitors were only observed in the CDH4+ fraction (right). Images are representative of three independent experiments. Scale bar represents 100 μm. See also Figure S4 and Tables S1 and S2.