Muscle isoactin expression during in vitro differentiation of murine embryonic stem cells

Isolation of a mesenchymal cell population from murine dermis that contains progenitors of multiple cell lineages

The skin contains two known subpopulations of stem cells/epidermal progenitors: a basal keratinocyte population found in the interfollicular epithelium and cells residing in the bulge region of the hair follicle. The major role of the interfollicular basal keratinocyte population may be epidermal renewal, whereas the bulge population may only be activated and recruited to form a cutaneous epithelium in case of trauma. Using 3-dimensional cultures of murine skin under stress conditions in which only reserve epithelial cells would be expected to survive and expand, we demonstrate that a mesenchymal population resident in neonatal murine dermis has the unique potential to develop an epidermis in vitro. In monolayer culture, this dermal subpopulation has long-term survival capabilities in restricted serum and an inducible capacity to evolve into multiple cell lineages, both epithelial and mesenchymal, depending on culture conditions. When grafted subcutaneously, this dermal subpopulation gave rise to fusiform structures, reminiscent of disorganized muscle, that stained positive for smooth muscle actin and desmin; on typical epidermal grafts, abundant melanocytes appeared throughout the dermis that were not associated with hair follicles. The multipotential cells can be repeatedly isolated from neonatal murine dermis by a sequence of differential centrifugation and selective culture conditions. These results suggest that progenitors capable of epidermal differentiation exist in the mesenchymal compartment of an abundant tissue source and may have a function in mesenchymal-epithelial transition upon insult. Moreover, these cells could be available in sufficient quantities for lineage determination or tissue engineering applications.

Expression and function of alpha-smooth muscle actin during embryonic-stem-cell-derived cardiomyocyte differentiation

Three alpha-muscle actin isoforms are sequentially expressed during in vivo cardiac development. alpha-Smooth muscle actin is first and transiently expressed, followed by alpha-skeletal and finally alpha-cardiac actin. The significance of these transitions in actin gene expression during myogenesis remains to be determined. To understand whether actin isoforms have specific functions during cardiac development and cardiomyocyte contractility, we have hampered alpha-smooth muscle and alpha-skeletal actin expression and organization during embryonic stem cell differentiation towards cardiomyocyte. We show that the sequence of actin isoform expression displays similar pattern in the in vitro model and in mouse heart embryogenesis. Treatment with an interfering fusion peptide containing the N-terminal sequence of alpha-smooth muscle actin during a time window preceding spontaneous beating, prevents proper cardiac sarcomyogenesis, whereas alpha-skeletal actin-fusion peptide has no effect. Knockdown of alpha-smooth muscle actin in embryonic stem cells using RNA interference also affects cardiac differentiation. The application of both fusion peptides on beating embryoid bodies impairs frequency. These results suggest specific functional activities for actin isoforms in cardiogenesis and cardiomyocyte contractility.

Embryonic and adult stem cell-derived cardiomyocytes: lessons from in vitro models

For years, research has focused on how to treat heart failure by sustaining the overloaded remaining cardiomyocytes. Recently, the concept of cell replacement therapy as a treatment of heart diseases has opened a new area of investigation. In vitro-generated cardiomyocytes could be injected into the heart to rescue the function of a damaged myocardium. Embryonic and/or adult stem cells could provide cardiac cells for this purpose. Knowledge of fundamental cardiac differentiation mechanisms unraveled by studies on animal models has been improved using in vitro models of cardiogenesis such as mouse embryonal carcinoma cells, mouse embryonic stem cells and, recently, human embryonic stem cells. On the other hand, studies suggesting the existence of cardiac stem cells and the potential of adult stem cells from bone marrow or skeletal muscle to differentiate toward unexpected phenotypes raise hope and questions about their potential use for cardiac cell therapy. In this review, we compare the specificities of embryonic vs adult stem cell populations regarding their cardiac differentiation potential, and we give an overview of what in vitro models have taught us about cardiogenesis.

Conditional mutagenesis of the murine serum response factor gene blocks cardiogenesis and the transcription of downstream gene targets

Serum response factor (SRF) homozygous-null embryos from our backcross of SRF(LacZ/)(+) "knock-in" mice failed to gastrulate and form mesoderm, similar to the findings of an earlier study (Arsenian, S., Weinhold, B., Oelgeschlager, M., Ruther, U., and Nordheim, A. (1998) EMBO J. 17, 6289-6299). Our use of embryonic stem cells provided a model system that could be used to investigate the specification of multiple embryonic lineages, including cardiac myocytes. We observed the absence of myogenic alpha-actins, SM22alpha, and myocardin expression and the failure to form beating cardiac myocytes in aggregated SRF null embryonic stem cells, whereas the appearance of transcription factors Nkx2-5 and GATA4 were unaffected. To study the role of SRF during heart organogenesis, we then performed cardiac-specific ablation of SRF by crossing the transgenic alpha-myosin heavy chain Cre recombinase line with SRF LoxP-engineered mice. Cardiac-specific ablation of SRF resulted in embryonic lethality due to cardiac insufficiency during chamber maturation. Conditional ablation of SRF also reduced cell survival concomitant with increased apoptosis and reduced cellularity. Significant reductions in SRF (> or =95%), atrial naturetic factor (> or =80%), and cardiac (> or =60%), skeletal (> or =90%), and smooth muscle (> or =75%) alpha-actin transcripts were also observed in the cardiac-conditional knock-out heart. This was consistent with the idea that SRF directs de novo cardiac and smooth muscle gene activities. Finally, quantitation of the knock-in LacZ reporter gene transcripts in the hearts of cardiac-conditional knock-out embryos revealed an approximately 30% reduction in gene activity, indicating SRF gene autoregulation during cardiogenesis.

High-ratio differentiation of embryonic stem cells into hepatocytes in vitro

BACKGROUND/PURPOSE

To date, in differentiating system of embryonic stem (ES) cells into hepatocytes, hepatic differentiation ratio was still not shown. Here, after investigating hepatic differentiation from ES cells, we determined the differentiation ratios of hepatocytes and studied how to improve the ratios in ES cell differentiating system.

METHODS

Embryonic bodies (EBs) formed from ES cells for 5 days were plated onto culture dishes and some growth factors were added into medium for hepatic differentiation. Expressions of hepatic genes and proteins were analysed using reverse transcriptase-polymerase chain reaction, immunocytochemistry (ICC) and radioimmunoassay. The relative counts of hepatocyte-like cells among all EBs cells were analysed by flow cytometry by which hepatic differentiation ratios were determined. Then, we observed the spatial distribution of ICC-positive cells in EB cells cluster and isolated the cells of positive areas in other EBs clusters without ICC examined. At last, isolated cells were re-cultured with previous condition and hepatic differentiation ratios were also determined.

RESULTS

The hepatic genes and proteins were, respectively, expressed in cytoplasm. Hepatic differentiation ratio was first determined at day 11 to be 12.1% and the level reached maximum to be 33.4% at day 21. In isolated cells culture system, hepatic genes and proteins expressed stronger than that expressed in EBs cluster and hepatic differentiation ratio was got to 72.6% at day 21.

CONCLUSIONS

Isolating hepatocyte-like cells from EBs cell cluster and re-culturing them could produce hepatocytes with high differentiation ratio. This culture system may produce a new source of cell types for hepatocytes replacement therapies in hepatic failure.

Osteogenic Cells Derived From Embryonic Stem Cells Produced Bone Nodules in Three-Dimensional Scaffolds

An approach for 3D bone tissue generation from embryonic stem (ES) cells was investigated. The ES cells were induced to differentiate into osteogenic precursors, capable of proliferating and subsequently differentiating into bone-forming cells. The differentiated cells and the seeded scaffolds were characterized using von Kossa and Alizarin Red staining, electron microscopy, and RT-PCR analysis. The results demonstrated that ES-derived bone-forming cells attached to and colonized the biocompatible and biodegradable scaffolds. Furthermore, these cells produced bone nodules when grown for 3-4 weeks in mineralization medium containing ascorbic acid and beta-glycerophosphate both in tissue culture plates and in scaffolds. The differentiated cells also expressed osteospecific markers when grown both in the culture plates and in 3D scaffolds. Osteogenic cells expressed alkaline phosphatase, osteocalcin, and osteopontin, but not an ES cell-specific marker, oct-4. These findings suggest that ES cell can be used for in vitro tissue engineering and cultivation of graftable skeletal structures.

Molecular and functional analyses of the contractile apparatus in lymphatic muscle

Lymphatics are necessary for the generation and regulation of lymph flow. Lymphatics use phasic contractions and extrinsic compressions to generate flow; tonic contractions alter resistance. Lymphatic muscle exhibits important differences from typical vascular smooth muscle. In this study, the thoracic duct exhibited significant functional differences from mesenteric lymphatics. To understand the molecular basis for these differences, we examined the profiles of contractile proteins and their messages in mesenteric lymphatics, thoracic duct, and arterioles. Results demonstrated that mesenteric lymphatics express only SMB smooth muscle myosin heavy chain (SM-MHC), whereas thoracic duct and arterioles expressed both SMA and SMB isoforms. Both SM1 and SM2 isoforms of SM-MHC were detected in arterioles and mesenteric and thoracic lymphatics. In addition, the fetal cardiac/skeletal slow-twitch muscle-specific beta-MHC message was detected only in mesenteric lymphatics. All four actin messages, cardiac alpha-actin, vascular alpha-actin, enteric gamma-actin, and skeletal alpha-actin, were present in both mesenteric lymphatics and arterioles. However, in thoracic duct, predominantly cardiac alpha-actin and vascular alpha-actin were found. Western blot and immunohistochemical analyses corroborated the mRNA studies. However, in arterioles only vascular alpha-actin protein was detected. These data indicate that lymphatics display genotypic and phenotypic characteristics of vascular, cardiac, and visceral myocytes, which are needed to fulfill the unique roles of the lymphatic system.

Identification of insulin-producing cells derived from embryonic stem cells by zinc-chelating dithizone

BACKGROUND AND AIMS

Embryonic stem (ES) cells have a pluripotent ability to differentiate into a variety of cell lineages in vitro. We have recently identified the emergence of cellular clusters within differentiated ES cell cultures by staining with dithizone (DTZ). DTZ is a zinc-chelating agent known to selectively stain pancreatic beta cells because of their high zinc content. The aim of the present study was to investigate the characteristics of DTZ-stained cellular clusters originating from ES cells.

METHODS

Embryoid bodies (EBs), formed by a 5-day hanging drop culture of ES cells, were allowed to form outgrowths in the culture. The outgrowths were incubated in DTZ solution (final concentration, 100 microg/ml ) for 15 minutes before being examined microscopically. The gene expression of endocrine pancreatic markers was also analyzed by reverse transcriptase-polymerase chain reaction. In addition, insulin production was examined immunohistochemically, and its secretion was examined using enzyme-linked immunosorbent assay.

RESULTS

DTZ-stained cellular clusters appeared after approximately 16 days in the EB culture and became more apparent by day 23. They were found to be immunoreactive to insulin and expressed pancreatic-duodenal homeobox 1 (PDX1), proinsulin 1, proinsulin 2, glucagon, pancreatic polypeptide, glucose transporter-2 (GLUT2), and islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) mRNA. They were also able to secrete detectable amounts of insulin.

CONCLUSIONS

ES cell-derived DTZ-positive cellular clusters possess characteristics of the endocrine pancreas, including insulin secretion. Further, DTZ staining is a useful method for the identification of differentiated pancreatic islets developed from EBs in vitro.

In vitro differentiation of embryonic stem cells into hepatocyte-like cells identified by cellular uptake of indocyanine green

BACKGROUND AND AIMS

Embryonic stem (ES) cells have a pluripotent ability to differentiate into a variety of cell lineages in vitro. We have recently found the emergence of cell clusters that show the cellular uptake of indocyanine green (ICG) in the culture of differentiated ES cells. ICG is clinically used as a test substance to evaluate liver function because it is eliminated exclusively by hepatocytes. The aim of the present study was to investigate the hepatic characteristics of ICG-stained cells.

METHODS

Embryoid bodies (EBs), formed by a 5-day hanging drop culture of ES cells, were allowed to outgrow in the placed culture. Gene expression of hepatocyte markers was analyzed by reverse transcriptase-polymerase chain reaction, and albumin production was examined immunohistochemically. Morphology and cellular components were investigated by electron microscopy. ICG-stained cells were further transplanted into the portal vein of mice.

RESULTS

ICG-stained cells appeared around 14 days of the EB culture and formed distinct three-dimensional structures. They were immunoreactive to albumin and expressed mRNAs such as albumin, alpha-fetoprotein, transthyretin, hepatocyte nuclear factor 3 beta, alpha-1-antitrypsin, tryptophan-2,3-dioxygenase, urea cycle enzyme, gluconeogenic enzyme, and liver-specific organic anion transporter-1. An ultrastructural analysis revealed a well-developed system of organelles such as mitochondria, lysosomes, Golgi apparatus, and rough and smooth endoplasmic reticulum. The transplantation of ICG-positive cells into the portal vein resulted in the incorporation into mice livers, where they were morphologically indistinguishable from neighboring hepatocytes.

CONCLUSIONS

ES cell-derived ICG-positive cells possess characteristics of hepatocytes, and ICG-staining is a useful marker to identify differentiated hepatocytes from EBs in vitro.

In vitro functional gut-like organ formation from mouse embryonic stem cells

BACKGROUND AND AIMS

Embryonic stem (ES) cells have a pluripotent ability to differentiate into a variety of cell lineages in vitro. We have recently found that ES cells can give rise to a functional gut-like unit, which forms a three-dimensional dome-like structure with lumen and exhibits mechanical activity, such as spontaneous contraction and peristalsis. The aim of the present study was to investigate the electrophysiological and morphological properties of ES cell-derived contracting clusters.

METHODS

Electrical activity was examined by an extracellular recording. Morphology and cellular components were investigated by immunohistochemistry and electron microscopy.

RESULTS

Clusters with rhythmic contractions displayed electrical slow waves at a regular rhythm, and clusters with highly coordinated peristalsis showed regular slow waves and spontaneous spike action potentials. Immunoreactivity for c-Kit, a marker of interstitial cells of Cajal (ICC), was observed in dense network structures. Neuronal marker PGP9.5 immunoreactivity was observed only in clusters with peristalsis. The topographical structure of the wall was organized by an inner epithelial layer and outer smooth muscle layer. The smooth muscle layer was provided with an ICC network and innervated with enteric neurons.

CONCLUSIONS

ES cells can differentiate into a functional gut-like organ in vitro that exhibits physiological and morphological properties characteristic of the gastrointestinal (GI) tract. This ES cell-derived gut provides a powerful tool for studying GI motility and gut development in vitro, and has potential for elucidating and treating a variety of motility disorders.

Probing the functional roles of titin ligands in cardiac myofibril assembly and maintenance

Sarcomeres of cardiac muscle are comprised of numerous proteins organized in an elegantly precise order. The exact mechanism of how these proteins are assembled into myofibrils during heart development is not yet understood, although existing in vitro and in vivo model systems have provided great insight into this complex process. It has been proposed by several groups that the giant elastic protein titin acts as a "molecular template" to orchestrate sarcomeric organization during myofibrillogenesis. Titin's highly modular structure, composed of both repeating and unique domains that interact with a wide spectrum of contractile and regulatory ligands, supports this hypothesis. Recent functional studies have provided clues to the physiological significance of the interaction of titin with several titin-binding proteins in the context of live cardiac cells. Improved models of cardiac myofibril assembly, along with the application of powerful functional studies in live cells, as well as the characterization of additional titin ligands, is likely to reveal surprising new functions for the titin third filament system.

Tubedown-1, a novel acetyltransferase associated with blood vessel development

We have used an embryonic endothelial cell line (IEM cells) as an experimental system for identifying and characterizing new molecules which are regulated during blood vessel development. A novel gene isolated from IEM cells, tubedown-1 (tbdn-1), is expressed at high levels in unstimulated IEM cells and is downregulated during formation of capillary tube structures by the IEM cells induced by basic fibroblast growth factor (bFGF) and leukemia inhibitory factor (LIF) in vitro. Tbdn-1 is also downregulated in M1 myeloid leukemia cells after differentiation in response to LIF in vitro. Tbdn-1 is homologous to the yeast NAT-1 N-terminal acetyltransferases and encodes a novel protein of approximately 69 kDa associated with an acetyltransferase activity. Levels and distribution of tbdn-1 expression are regulated in both endothelial and hematopoietic cells during development in tissues such as the yolk sac blood islands, heart, and liver blood vessels. In the adult, tbdn-1 expression is low or undetected in most organs examined with the exception of the atrial endocardium, the endothelial and myeloid compartments of bone marrow, and the remodeling vascular bed of atretic ovarian follicles. The distribution and regulation of expression of tbdn-1 suggest that this novel acetyltransferase may be involved in regulating vascular and hematopoietic development and physiologic angiogenesis.

Analysis of ferrochelatase expression during hematopoietic development of embryonic stem cells

Ferrochelatase, the last enzyme in the heme pathway, chelates protoporphyrin IX and iron to form heme and is mutated in protoporphyria. The ferrochelatase gene is expressed in all tissues at low levels to provide heme for essential heme-containing proteins and is up-regulated during erythropoiesis for the synthesis of hemoglobin. The human ferrochelatase promoter contains 2 Sp1 cis-elements and GATA and NF–E2 sites, all of which bind their cognatetrans-acting factors in vitro. To investigate the role of these elements during erythropoiesis, we introduced expression of the green fluorescent protein (EGFP) transgenes driven by various ferrochelatase promoter fragments into a single locus in mouse embryonic stem cells. EGFP expression was monitored during hematopoietic differentiation in vitro using flow cytometry. We show that a promoter fragment containing the Sp1 sites, the NF–E2 and GATA elements, was sufficient to confer developmental-specific expression of the EGFP transgene, with an expression profile identical to that of the endogenous gene. In this system the −0.275 kb NF–E2 cis-element is required for erythroid-enhanced expression, the GATA cis-element functions as a stage-specific repressor and enhancer, and elements located between −0.375kb and −1.1kb are necessary for optimal levels of expression. Ferrochelatase mRNA increased before the primitive erythroid-cell stage without a concomitant increase in ferrochelatase protein, suggesting the presence of a translational control mechanism. Because of the sensitivity of this system, we were able to assess the effect of an A-to-G polymorphism identified in the promoters of patients with protoporphyria. There was no effect of the G haplotype on transcriptional activity of the −1.1 kb transgene.

Analysis of ferrochelatase expression during hematopoietic development of embryonic stem cells

Ferrochelatase, the last enzyme in the heme pathway, chelates protoporphyrin IX and iron to form heme and is mutated in protoporphyria. The ferrochelatase gene is expressed in all tissues at low levels to provide heme for essential heme-containing proteins and is up-regulated during erythropoiesis for the synthesis of hemoglobin. The human ferrochelatase promoter contains 2 Sp1 cis-elements and GATA and NF–E2 sites, all of which bind their cognatetrans-acting factors in vitro. To investigate the role of these elements during erythropoiesis, we introduced expression of the green fluorescent protein (EGFP) transgenes driven by various ferrochelatase promoter fragments into a single locus in mouse embryonic stem cells. EGFP expression was monitored during hematopoietic differentiation in vitro using flow cytometry. We show that a promoter fragment containing the Sp1 sites, the NF–E2 and GATA elements, was sufficient to confer developmental-specific expression of the EGFP transgene, with an expression profile identical to that of the endogenous gene. In this system the −0.275 kb NF–E2 cis-element is required for erythroid-enhanced expression, the GATA cis-element functions as a stage-specific repressor and enhancer, and elements located between −0.375kb and −1.1kb are necessary for optimal levels of expression. Ferrochelatase mRNA increased before the primitive erythroid-cell stage without a concomitant increase in ferrochelatase protein, suggesting the presence of a translational control mechanism. Because of the sensitivity of this system, we were able to assess the effect of an A-to-G polymorphism identified in the promoters of patients with protoporphyria. There was no effect of the G haplotype on transcriptional activity of the −1.1 kb transgene.

Distant upstream regulatory domains direct high levels of beta -myosin heavy chain gene expression in differentiated embryonic stem cells

Eukaryotic gene transcription takes place in the context of chromatin. In order to study the expression of the beta -myosin heavy chain (MyHC) gene in its appropriate cardiac environment in vitro, embryonic stem cell lines were generated and induced to differentiate into the cardiac lineage. We show that the upstream region of the beta -MyHC gene (-5518 to -2490 relative to the transcriptional start site) directed high levels of transcriptional activity only when stably integrated, but not when expressed extrachromosomally in transient assays. These results are consistent with earlier findings using an in vivo transgenic approach. The expression of beta -MyHC reporter gene constructs was strictly correlated to differentiation status and coincided with the expression of endogenous cardiac marker genes and with morphological differentiation of embryoid bodies in vitro. Using populations of stably transfected cell clones, two domains important for high level expression were identified. The analysis of individual cell clones suggested that the positive regulatory domains act according to the graded model of enhancement. These results show that chromosomal integration is necessary for the appropriate function of the beta -MyHC gene's upstream regulatory region.

Establishment and characterization of a conditionally immortalized smooth muscle/myometrial-like cell line

A novel smooth muscle/myometrial-like cell line, SMU1-10, has been generated from the uterus of a H-2Kb-tsA58 transgenic mouse carrying a thermolabile SV40 large T-antigen gene. These cells grow continuously when maintained at the permissive temperature (33 degrees C) for the SV40 large T-antigen but stop dividing when placed at the non-permissive temperature (39 degrees C) and ultimately die within 3 weeks. All of the SMU1-10 cells produce smooth muscle alpha-actin (SMAA) at both 33 degrees C and 39 degrees C. A subset of the cells also contain smooth muscle gamma-actin (SMGA), a hallmark of smooth muscle differentiation, and the fraction of cells staining for this actin increases from about 1% when maintained for three days at 33 degrees C to as much as 30% at 39 degrees C over the same length of time. However, the appearance of SMGA in SMU1-10 cells appears to be regulated mainly at a post-transcriptional level since in situ hybridization indicates that all cells contain SMGA mRNA at both 33 degrees C and 39 degrees C. SMU1-10 cultures also contain smooth muscle myosin heavy chain (SM-MHC) and SM22 alpha, both of which are only found in smooth muscle of the adult mouse. Three additional smooth muscle (myometrium)-related markers, connexin 43, the thromboxane A2 receptor, and the progesterone receptor also are present in these cells. At the nonpermissive temperature for SV40 large T-antigen, the both level of SMGA mRNA and the number of cells staining for this actin are significantly increased in the presence of progesterone, a process that is similar to the upregulation of SMGA in the myometrium late in pregnancy. Overall, SMU1-10 cells provides a potentially useful in vitro model system to study smooth muscle/myometrial differentiation.

Rescue of cardiac alpha-actin-deficient mice by enteric smooth muscle gamma-actin

The muscle actins in higher vertebrates display highly conserved amino acid sequences, yet they show distinct expression patterns. Thus, cardiac alpha-actin, skeletal alpha-actin, vascular smooth muscle alpha-actin, and enteric smooth muscle gamma-actin comprise the major actins in their respective tissues. To assess the functional and developmental significance of cardiac alpha-actin, the murine (129/SvJ) cardiac alpha-actin gene was disrupted by homologous recombination. The majority ( approximately 56%) of the mice lacking cardiac alpha-actin do not survive to term, and the remainder generally die within 2 weeks of birth. Increased expression of vascular smooth muscle and skeletal alpha-actins is observed in the hearts of newborn homozygous mutants and also heterozygotes but apparently is insufficient to maintain myofibrillar integrity in the homozygous mutants. Mice lacking cardiac alpha-actin can be rescued to adulthood by the ectopic expression of enteric smooth muscle gamma-actin using the cardiac alpha-myosin heavy chain promoter. However, the hearts of such rescued cardiac alpha-actin-deficient mice are extremely hypodynamic, considerably enlarged, and hypertrophied. Furthermore, the transgenically expressed enteric smooth muscle gamma-actin reduces cardiac contractility in wild-type and heterozygous mice. These results demonstrate that alterations in actin composition in the fetal and adult heart are associated with severe structural and functional perturbations.