High-throughput site-directed mutagenesis in ES cells

An autocrine synergistic desmin-SPARC network promotes cardiomyogenesis in cardiac stem cells

The mammalian heart contains cardiac stem cells throughout life, but it has not been possible to harness or stimulate these cells to repair damaged myocardium in vivo. Assuming physiological relevance of these cells, which have evolved and have been maintained throughout mammalian evolution, we hypothesize that cardiac stem cells may contribute to cardiomyogenesis in an unorthodox manner. Since the intermediate filament protein desmin and the matricellular Secreted Protein Acidic and Rich in Cysteine (SPARC) promote cardiomyogenic differentiation during embryogenesis in a cell-autonomous and paracrine manner, respectively, we focus on their genes and employ mouse embryonic and cardiac stem cell lines as in vitro models to ask whether desmin and SPARC cooperatively influence cardiomyogenesis in cardiac stem and progenitor cells. We show that desmin also promotes cardiomyogenesis in a non-cell autonomous manner by increasing the expression and secretion of SPARC in differentiating embryonic stem cells. SPARC is also secreted by cardiac stem cells where it promotes cardiomyogenesis in an autocrine and concentration-dependent manner by upregulating the expression of myocardial transcription factors and its elicitor desmin. Desmin and SPARC interact genetically, forming a positive feedback loop and secreted autocrine and paracrine SPARC negatively affects sparc mRNA expression. Paracrine SPARC rescues cardiomyogenic desmin-haploinsufficiency in cardiac stem cells in a glycosylation-dependent manner, increases desmin expression, the phosphorylation of Smad2 and induces the expression of gata4, nkx2.5 and mef2C. Demonstration that desmin-induced autocrine secretion of SPARC in cardiac stem cells promotes cardiomyogenesis raises the possibility that a physiological function of cardiac stem cells in the adult and aging heart may be the gland-like secretion of factors such as SPARC that modulate age-related and adverse environmental influences and thereby contribute to cardiac homeostasis throughout life.

Desmin enters the nucleus of cardiac stem cells and modulates Nkx2.5 expression by participating in transcription factor complexes that interact with the nkx2.5 gene

The transcription factor Nkx2.5 and the intermediate filament protein desmin are simultaneously expressed in cardiac progenitor cells during commitment of primitive mesoderm to the cardiomyogenic lineage. Up-regulation of Nkx2.5 expression by desmin suggests that desmin may contribute to cardiogenic commitment and myocardial differentiation by directly influencing the transcription of the nkx2.5 gene in cardiac progenitor cells. Here, we demonstrate that desmin activates transcription of nkx2.5 reporter genes, rescues nkx2.5 haploinsufficiency in cardiac progenitor cells, and is responsible for the proper expression of Nkx2.5 in adult cardiac side population stem cells. These effects are consistent with the temporary presence of desmin in the nuclei of differentiating cardiac progenitor cells and its physical interaction with transcription factor complexes bound to the enhancer and promoter elements of the nkx2.5 gene. These findings introduce desmin as a newly discovered and unexpected player in the regulatory network guiding cardiomyogenesis in cardiac stem cells.

Embryonic stem cells facilitate the isolation of persistent clonal cardiovascular progenitor cell lines and leukemia inhibitor factor maintains their self-renewal and myocardial differentiation potential in vitro

Compelling evidence for the existence of somatic stem cells in the heart of different mammalian species has been provided by numerous groups; however, so far it has not been possible to maintain these cells as self-renewing and phenotypically stable clonal cell lines in vitro. Thus, we sought to identify a surrogate stem cell niche for the isolation and persistent maintenance of stable clonal cardiovascular progenitor cell lines, enabling us to study the mechanism of self-renewal and differentiation in these cells. Using postnatal murine hearts with a selectable marker as the stem cell source and embryonic stem cells and leukemia inhibitory factor (LIF)-secreting fibroblasts as a surrogate niche, we succeeded in the isolation of stable clonal cardiovascular progenitor cell lines. These cell lines self-renew in an LIF-dependent manner. They express both stemness transcription factors Oct4, Sox2, and Nanog and early myocardial transcription factors Nkx2.5, GATA4, and Isl-1 at the same time. Upon LIF deprivation, they exclusively differentiate to functional cardiomyocytes and endothelial and smooth muscle cells, suggesting that these cells are mesodermal intermediates already committed to the cardiogenic lineage. Cardiovascular progenitor cell lines can be maintained for at least 149 passages over 7 years without phenotypic changes, in the presence of LIF-secreting fibroblasts. Isolation of wild-type cardiovascular progenitor cell lines from adolescent and old mice has finally demonstrated the general feasibility of this strategy for the isolation of phenotypically stable somatic stem cell lines.

Differentiation of cardiomyocytes requires functional serine residues within the amino-terminal domain of desmin

Desmin contributes to the stability of the myocardium and its amino-terminal domain influences intermediate filament formation and interacts with a variety of proteins and DNAs. Specific serine residues located in this domain are reversibly phosphorylated in a cell cycle and developmental stage-dependent manner as has been demonstrated also for other cytoplasmic type III intermediate filament proteins. Although absence of desmin apparently does not affect cardiomyogenesis, homozygous deletion of the amino-terminal domain of desmin severely inhibited in vitro cardiomyogenesis. To demonstrate the significance of phosphorylation of this domain in cardiomyogenic commitment and differentiation, we inhibited phosphorylation of serine residues 6, 7, and 8 by mutation to alanine, and investigated early cardiomyogenesis in heterozygous embryoid bodies. As control, serine residues 31 and 32, which are not phosphorylated by kinases mutating serine residues 6, 7, and 8, were mutated to alanine in a second set. Desmin(S6,7,8A) interfered with cardiomyogenesis and myofibrillogenesis in a dominant negative fashion, whereas desmin(S31,32A) produced only a mild phenotype. Desmin(S6,7,8A) led to the down-regulation of the transcription factor genes brachyury, goosecoid, nkx2.5, and mef2C and increased apoptosis of presumptive mesoderm and differentiating cardiomyocytes. Surviving cardiomyocytes which were few in number had no myofibrils. Demonstration that some but not any mutant desmin interfered with the very beginning of cardiomyogenesis suggests an important function of temporarily phosphorylated serine residues 6, 7, and 8 in the amino-terminal domain of desmin in cardiomyogenic commitment and differentiation.

Amino-terminally truncated desmin rescues fusion of des(-/-) myoblasts but negatively affects cardiomyogenesis and smooth muscle development

Desmin fulfils important functions in maintenance of muscle cells and mutations in the desmin gene have been linked to a variety of myopathies. To ascertain the role of desmin's amino-terminal domain in muscle cells we generated embryonic stem cells constitutively expressing desmin(Delta1-48) in a null background and investigated muscle cell development in vitro. Desmin(Delta1-48) lacking the first 48 amino acid residues promotes fusion of myoblasts, rescues myogenesis and down-regulates vimentin expression in embryoid bodies, but hampers cardiomyogenesis and blocks smooth muscle development. These results demonstrate that desmin's amino-terminus has different roles in skeletal, cardiac, and smooth muscle cell development and function.