Desmin stimulates differentiation of cardiomyocytes and up-regulation of brachyury and nkx2.5

Numb family proteins play roles in Desmin and Vimentin localization at the Z-disc

Desmin and Vimentin are major intermediate filaments at the Z-disc and play significant roles in sarcomere architecture and signaling transduction. Abnormal expression of sarcomeric Desmin and Vimentin (SDV) results in severe dysfunctions of striated muscles. In this study, it was found that paired Numb family proteins (NFPs), including Numb and its homolog Numblike, determined the range for the recruitment of SDV to the primitive Z-disc. Notably, NFPs were identified as SDV associated proteins and bound to the head, rod, and tail domains of SDV in a splicing-variant-dependent manner. Last, the construction and consolidation of the Z-disc was completed through the gradual adjustment of the position of SDV by clockwise/anticlockwise rotation of paired NFPs to 90° in the same direction. Conditional knockout of NFPs altered the arrangement and accumulated the expression level of SDV. This study further enriches the functions of NFPs in sarcomere assembly and maintenance through cooperation with SDV.

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.

Stem Cell Growth and Differentiation in Organ Culture: New Insights for Uterine Fibroid Treatment

Organ culture allows for the understanding of normal and tumor cell biology, and tissues generally remain viable for 5–7 days. Strikingly, we determined that myometrial and MED12 mutant leiomyoma cells repopulated cell-depleted tissue slices after 20 days of culture. Using immunofluorescence and quantitative PCR of stem cell and undifferentiated cell markers, we observed clusters of CD49b⁺ cells in tumor slices. CD49b⁺ cells, however, were sparsely detected in the myometrial slices. Almost all LM cells strongly expressed Ki67, while only a few myometrial cells were stained for this proliferation marker. The CD73 marker was expressed only in tumor cells, whereas the mesenchymal stem cell receptor KIT was detected only in normal cells. HMGA2 and CD24 showed broader expression patterns and higher signal intensity in leiomyoma than in myometrial cells. In this study, we propose that activating CD49b⁺ stem cells in myometrium leads to asymmetrical division, giving rise to transit-amplifying KIT⁺ cells that differentiate to smooth muscle cells. On the contrary, activated leiomyoma CD49b⁺ cells symmetrically divide to form clusters of stem cells that divide and differentiate to smooth muscle cells without losing proliferation ability. In conclusion, normal and mutant stem cells can proliferate and differentiate in long-term organ culture, constituting a helpful platform for novel therapeutic discovery.

What does desmin do: A bibliometric assessment of the functions of the muscle intermediate filament

Intermediate filaments were first described in muscle in 1968, and desmin was biochemically identified about 10 years afterwards. Its importance grew after the identification of desminopathies and desmin mutations that cause mostly cardiopathies. Since its characterization until recently, different functions have been attributed to desmin. Here, we use bibliometric tools to evaluate the articles published about desmin and to assess its several putative functions. We identified the most productive authors and the relationships between research groups. We studied the more frequent words among 9734 articles (September 2021) containing "desmin" on the title and abstract, to identify the major research focus. We generated an interactive spreadsheet with the 934 papers that contain "desmin" only on the title that can be used to search and quantify terms in the abstract. We further selected the articles that contained the terms "function" or "role" from the spreadsheet, which we then classified according to type of function, organelle, or tissue involved. Based on the bibliographic analysis, we assess comparatively the putative functions, and we propose an alternative explanation for the desmin function.

Physical Exercise and Cardiac Repair: The Potential Role of Nitric Oxide in Boosting Stem Cell Regenerative Biology

Over the years strong evidence has been accumulated showing that aerobic physical exercise exerts beneficial effects on the prevention and reduction of cardiovascular risk. Exercise in healthy subjects fosters physiological remodeling of the adult heart. Concurrently, physical training can significantly slow-down or even reverse the maladaptive pathologic cardiac remodeling in cardiac diseases, improving heart function. The underlying cellular and molecular mechanisms of the beneficial effects of physical exercise on the heart are still a subject of intensive study. Aerobic activity increases cardiovascular nitric oxide (NO) released mainly through nitric oxidase synthase 3 activity, promoting endothelium-dependent vasodilation, reducing vascular resistance, and lowering blood pressure. On the reverse, an imbalance between increasing free radical production and decreased NO generation characterizes pathologic remodeling, which has been termed the "nitroso-redox imbalance". Besides these classical evidence on the role of NO in cardiac physiology and pathology, accumulating data show that NO regulate different aspects of stem cell biology, including survival, proliferation, migration, differentiation, and secretion of pro-regenerative factors. Concurrently, it has been shown that physical exercise generates physiological remodeling while antagonizes pathologic remodeling also by fostering cardiac regeneration, including new cardiomyocyte formation. This review is therefore focused on the possible link between physical exercise, NO, and stem cell biology in the cardiac regenerative/reparative response to physiological or pathological load. Cellular and molecular mechanisms that generate an exercise-induced cardioprotective phenotype are discussed in regards with myocardial repair and regeneration. Aerobic training can benefit cells implicated in cardiovascular homeostasis and response to damage by NO-mediated pathways that protect stem cells in the hostile environment, enhance their activation and differentiation and, in turn, translate to more efficient myocardial tissue regeneration. Moreover, stem cell preconditioning by and/or local potentiation of NO signaling can be envisioned as promising approaches to improve the post-transplantation stem cell survival and the efficacy of cardiac stem cell therapy.

Intermediate filaments in cardiomyopathy

Intermediate filament (IF) proteins are critical regulators in health and disease. The discovery of hundreds of mutations in IF genes and posttranslational modifications has been linked to a plethora of human diseases, including, among others, cardiomyopathies, muscular dystrophies, progeria, blistering diseases of the epidermis, and neurodegenerative diseases. The major IF proteins that have been linked to cardiomyopathies and heart failure are the muscle-specific cytoskeletal IF protein desmin and the nuclear IF protein lamin, as a subgroup of the known desminopathies and laminopathies, respectively. The studies so far, both with healthy and diseased heart, have demonstrated the importance of these IF protein networks in intracellular and intercellular integration of structure and function, mechanotransduction and gene activation, cardiomyocyte differentiation and survival, mitochondrial homeostasis, and regulation of metabolism. The high coordination of all these processes is obviously of great importance for the maintenance of proper, life-lasting, and continuous contraction of this highly organized cardiac striated muscle and consequently a healthy heart. In this review, we will cover most known information on the role of IFs in the above processes and how their deficiency or disruption leads to cardiomyopathy and heart failure.

ATP Binding Cassette Sub-family Member 2 (ABCG2) and Xenobiotic Exposure During Early Mouse Embryonic Stem Cell Differentiation

BACKGROUND

ATP binding cassette sub-family member 2 (ABCG2) is a well-defined efflux transporter found in a variety of tissues. The role of ABCG2 during early embryonic development, however, is not established. Previous work which compared data from the ToxCast screening program with that from in-house studies suggested an association exists between exposure to xenobiotics that regulate Abcg2 transcription and differentiation of mouse embryonic stem cells (mESC), a relationship potentially related to redox homeostasis.

METHODS

mESC were grown for up to 9 days. Pharmacological inhibitors were used to assess transporter function with and without xenobiotic exposure. Proliferation and differentiation were evaluated using RedDot1 and quantiative reverse transcriptase-polymerase chain reaction, respectively. ABCG2 activity was assessed using a Pheophorbide a-based fluorescent assay. Protein expression was measured by capillary-based immunoassay.

RESULTS

ABCG2 activity increased in differentiating mESC. Treatment with K0143, an inhibitor of ABCG2, had no effect on proliferation or differentiation. As expected, mitoxantrone and topotecan, two chemotherapeutics, displayed increased toxicity in the presence of K0143. Exposure to K0143 in combination with chemicals predicted by ToxCast to regulate ABCG2 expression did not alter xenobiotic-induced toxicity. Moreover, inhibition of ABCG2 did not shift the toxicity of either tert-Butyl hydroperoxide or paraquat, two oxidative stressors.

CONCLUSION

As previously reported, ABCG2 serves a protective role in mESC. The role of ABCG2 in regulating redox status, however, was unclear. The hypothesis that ABCG2 plays a fundamental role during mESC differentiation or that regulation of the receptor by xenobiotics may be associated with altered mESC differentiation could not be supported. Birth Defects Research, 110:35-47, 2018. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

Type III Intermediate Filaments Desmin, Glial Fibrillary Acidic Protein (GFAP), Vimentin, and Peripherin

SummaryType III intermediate filament (IF) proteins assemble into cytoplasmic homopolymeric and heteropolymeric filaments with other type III and some type IV IFs. These highly dynamic structures form an integral component of the cytoskeleton of muscle, brain, and mesenchymal cells. Here, we review the current ideas on the role of type III IFs in health and disease. It turns out that they not only offer resilience to mechanical strains, but, most importantly, they facilitate very efficiently the integration of cell structure and function, thus providing the necessary scaffolds for optimal cellular responses upon biochemical stresses and protecting against cell death, disease, and aging.

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.

Strategies to Study Desmin in Cardiac Muscle and Culture Systems

Intermediate filament (IF) cytoskeleton comprises the fine-tuning cellular machinery regulating critical homeostatic mechanisms. In skeletal and cardiac muscle, deficiency or disturbance of the IF network leads to severe pathology, particularly in the latter. The three-dimensional scaffold of the muscle-specific IF protein desmin interconnects key features of the cardiac muscle cells, including the Z-disks, intercalated disks, plasma membrane, nucleus, mitochondria, lysosomes, and potentially sarcoplasmic reticulum. This is crucial for the highly organized striated muscle, in which effective energy production and transmission as well as mechanochemical signaling are tightly coordinated among the organelles and the contractile apparatus. The role of desmin and desmin-associated proteins in the biogenesis, trafficking, and organelle function, as well as the development, differentiation, and survival of the cardiac muscle begins to be enlightened, but the precise mechanisms remain elusive. We propose a set of experimental tools that can be used, in vivo and in vitro, to unravel crucial new pathways by which the IF cytoskeleton facilitates proper organelle function, homeostasis, and cytoprotection and further understand how its disturbance and deficiency lead to disease.

Desmin related disease: a matter of cell survival failure

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network that through interactions with all vital cell structures, provides an effective mechanochemical integrator of morphology and function, absolutely necessary for intra-cellular and intercellular coordination of all muscle functions. A good candidate for such a system is the desmin intermediate filament cytoskeletal network. Human desmin mutations and post-translational modifications cause disturbance of this network, thus leading to loss of function of both desmin and its binding partners, as well as potential toxic effects of the formed aggregates. Both loss of normal function and gain of toxic function are linked to mitochondrial defects, cardiomyocyte death, muscle degeneration and development of skeletal myopathy and cardiomyopathy.

VUT-MK142 : a new cardiomyogenic small molecule promoting the differentiation of pre-cardiac mesoderm into cardiomyocytes

Intra-cardiac cell transplantation is a new therapy after myocardial infarction. Its success, however, is impeded by the limited capacity of donor cells to differentiate into functional cardiomyocytes in the heart. A strategy to overcome this problem is the induction of cardiomyogenic function in cells prior to transplantation. Among other approaches, recently, synthetic small molecules were identified, which promote differentiation of stem cells of various origins into cardiac-like cells or cardiomyocytes. The aim of this study was to develop and characterise new promising cardiomyogenic synthetic low-molecular weight compounds. Therefore, the structure of the known cardiomyogenic molecule cardiogenol C was selectively modified, and the effects of the resulting compounds were tested on various cell types. From this study, VUT-MK142 was identified as the most promising candidate with respect to cardiomyogenic activity. Treatment using this novel agent induced the strongest up-regulation of expression of the cardiac marker ANF in both P19 embryonic carcinoma cells and C2C12 skeletal myoblasts. The activity of VUT-MK142 on this marker superseded CgC; moreover, the novel compound significantly up-regulated the expression of other cardiac markers, and promoted the development of beating cardiomyocytes from cardiovascular progenitor cells. We conclude that VUT-MK142 is a potent new cardiomyogenic synthetic agent promoting the differentiation of pre-cardiac mesoderm into cardiomyocytes, which may be useful to differentiate stem cells into cardiomyocytes for cardiac repair. Additionally, an efficient synthesis of VUT-MK142 is reported taking advantage of continuous flow techniques superior to classical batch reactions both in yield and reaction time.

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.

Effects of increased preload on the force-frequency response and contractile kinetics in early stages of cardiac muscle hypertrophy

Numerous studies have aimed to elucidate markers for the onset of decompensatory hypertrophy and heart failure in vivo and in vitro. Alterations in the force-frequency relationship are commonly used as markers for heart failure with a negative staircase being a hallmark of decompensated cardiac function. Here we aim to determine the functional and molecular alterations in the very early stages of compensatory hypertrophy through analysis of the force-frequency relationship, using a novel isolated muscle culture system that allows assessment of force-frequency relationship during the development of hypertrophy. New Zealand white male rabbit trabeculae excised from the right ventricular free wall were utilized for all experiments. Briefly, muscles held at constant preload and contracting isometrically were stimulated to contract in culture for 24 h, and in a subset up to 48 h. We found that, upon an increase in the preload and maintaining the muscles in culture for up to 24 h, there was an increase in baseline force produced by isolated trabeculae over time. This suggests a gradual compensatory response to the impact of increased preload. Temporal analysis of the force-frequency response during this progression revealed a significant blunting (at 12 h) and then reversal of the positive staircase as culture time increased (at 24 h). Phosphorylation analysis revealed a significant decrease in desmin and troponin (Tn)I phosphorylation from 12 to 24 h in culture. These results show that even very early on in the compensatory hypertrophy state, the force-frequency relationship is already affected. This effect on force-frequency relationship may, in addition to protein expression changes, be partially attributed to the alterations in myofilament protein phosphorylation.

Mechanisms of cardiogenesis in cardiovascular progenitor cells

Self-renewing cells of the vertebrate heart have become a major subject of interest in the past decade. However, many researchers had a hard time to argue against the orthodox textbook view that defines the heart as a postmitotic organ. Once the scientific community agreed on the existence of self-renewing cells in the vertebrate heart, their origin was again put on trial when transdifferentiation, dedifferentiation, and reprogramming could no longer be excluded as potential sources of self-renewal in the adult organ. Additionally, the presence of self-renewing pluripotent cells in the peripheral blood challenges the concept of tissue-specific stem and progenitor cells. Leaving these unsolved problems aside, it seems very desirable to learn about the basic biology of this unique cell type. Thus, we shall here paint a picture of cardiovascular progenitor cells including the current knowledge about their origin, basic nature, and the molecular mechanisms guiding proliferation and differentiation into somatic cells of the heart.

Modulation of the migration and differentiation potential of adult bone marrow stromal stem cells by nitric oxide

Nitric oxide (NO) is a diffusible free radical, which serves as a pluripotent intracellular messenger in numerous cell systems. NO has been demonstrated to regulate actin dependent cellular functions and functions as a putative inductive agent in directing stem cells differentiation. In this study, we investigated the effect of exogenous NO on the kinetics of movement and morphological changes in adult bone marrow stromal cells (BMSCs) in a wound healing model of cellular migration. Cellular migration and morphological changes were determined by measurement of changes in the area and fractal dimension of BMSCs monolayer as a function of time in the presence of an NO donor (S-Nitroso-N-Acetyl-D,L-Penicillamine, SNAP) compared to untreated BMSCs. Response of the BMSCs' actin cytoskeleton and desmin to NO was assessed by determining changes in their integrated optical density (IOD) and fractal dimension at 24 h and 7 days. NO suppressed BMSCs' migration accompanied by a reduction in cell size, with maintenance of their stellate to polygonal morphology. In response to NO, the actin cytoskeleton expressed an increase in randomness but maintained a constant amount of F-actin relative to the cell size. The presence of NO also induced an increase in randomly organized cytoplasmic desmin. These data suggest that NO has an apparent inductive effect on adult BMSCs and is capable of initiating phenotypic change at the gross cellular, cytoskeletal and molecular levels. It is apparent, however, that additional factors or conditions are required to further drive the differentiation of adult BMSCs into specific phenotypes, such as cardiomyocytes.

Self-organization phenomena in embryonic stem cell-derived embryoid bodies: axis formation and breaking of symmetry during cardiomyogenesis

Aggregation of embryonic stem cells gives rise to embryoid bodies (EBs) which undergo developmental processes reminiscent of early eutherian embryonic development. Development of the three germ layers suggests that gastrulation takes place. In vivo, gastrulation is a highly ordered process but in EBs only few data support the hypothesis that self-organization of differentiating cells leads to morphology, reminiscent of the early gastrula. Here we demonstrate that a timely implantation-like process is a prerequisite for the breaking of the radial symmetry of suspended EBs. Attached to a surface, EBs develop a bilateral symmetry and presumptive mesodermal cells emerge between the center of the EBs and a horseshoe-shaped ridge of cells. The development of an epithelial sheet of cells on one side of the EBs allows us to define an 'anterior' and a 'posterior' end of the EBs. In the mesodermal area, first cardiomyocytes (CMCs) develop mainly next to this epithelial sheet of cells. Development of twice as many CMCs at the 'left' side of the EBs breaks the bilateral symmetry and suggests that cardiomyogenesis reflects a local or temporal asymmetry in EBs. The asymmetric appearance of CMCs but not the development of mesoderm can be disturbed by ectopic expression of the muscle-specific protein Desmin. Later, the bilateral morphology becomes blurred by an apparently chaotic differentiation of many cell types. The absence of comparable structures in aggregates of cardiovascular progenitor cells isolated from the heart demonstrates that the self-organization of cells during a gastrulation-like process is a unique feature of embryonic stem cells.

Embryonic stem cell transplantation: promise and progress in the treatment of heart disease

Cardiovascular diseases remain the leading cause of death worldwide, and the burden is equally shared between men and women around the globe. Cardiomyocytes that die in response to disease processes or aging are replaced by scar tissue instead of new muscle cells. Although recent reports suggest an intrinsic capacity for the mammalian myocardium to regenerate via endogenous stem/progenitor cells, the magnitude of such a response appears to be minimal and has yet to be realized fully in cardiovascular patients. Despite the advances in pharmacotherapy and new biomedical technologies, the prognosis for patients diagnosed with end-stage heart failure appears to be grave. While heart transplantation is a viable option, this life-saving intervention suffers from an acute shortage of cardiac organ donors. In view of these existing issues, donor cell transplantation is emerging as a promising strategy to regenerate diseased myocardium. Studies from multiple laboratories have shown that transplantation of donor cells (e.g. fetal cardiomyocytes, skeletal myoblasts, smooth muscle cells, and adult stem cells) can improve the function of diseased hearts over a short period of time (1-4 weeks). While long-term follow-up studies are warranted, it is generally perceived that the beneficial effects of transplanted cells are mainly due to increased angiogenesis or favorable scar remodeling in the engrafted myocardium. Although skeletal myoblasts and bone marrow stem cells hold the highest potential for implementation of autologous therapies, initial results from phase I trials are not promising. In contrast, transplantation of fetal cardiomyocytes has been shown to confer protection against the induction of ventricular tachycardia in experimental myocardial injury models. Furthermore, results from multiple laboratories suggest that fetal cardiomyocytes can couple functionally with host myocytes, stimulate formation of new blood vessels, and improve myocardial function. While it is neither practical nor ethical to test the potential of fetal cardiomyocytes in clinical trials, embryonic stem (ES) cells serve as a novel source for generation of unlimited quantities of cardiomyocytes for myocardial repair. The initial success in the application of ES cells to partially repair and improve myocardial function in experimental models of heart disease has been quite promising. However, multiple hurdles need to be crossed before the potential benefits of ES cells can be translated to the clinic. In this review, we summarize the current knowledge of cardiomyocyte derivation and enrichment from ES-cell cultures and provide a brief survey of factors increasing cardiomyogenic induction in both mouse and human ES cultures. Subsequently, we summarize the current state of research using mouse and human ES cells for the treatment of heart disease in various experimental models. Furthermore, we discuss the challenges that need to be overcome prior to the successful clinical utilization of ES-derived cardiomyocytes for the treatment of end-stage heart disease. While we are optimistic that the researchers in this field will sail across the hurdles, we also suggest that a more cautious approach to the validation of ES cardiomyocytes in experimental models would certainly prevent future disappointments, as seen with skeletal myoblast studies.

A missense mutation in desmin tail domain linked to human dilated cardiomyopathy promotes cleavage of the head domain and abolishes its Z-disc localization

A missense mutation (Ile 451 to Met) at the tail domain of the muscle-specific intermediate filament protein desmin has been suggested to be a genetic cause of dilated cardiomyopathy. The Ile451Met mutation is located inside a conserved motif in the desmin tail domain, believed to have a potential role in the lateral packing of type III intermediate filaments. Nevertheless, the role of the type III intermediate filament tail domain remains elusive. To further study the role of this domain in the function of cardiomyocytes and in the development of cardiomyopathy, we generated transgenic mice expressing the mutant desmin(I451M) in the cardiac tissue. Analysis of hearts from transgenic animals revealed that mutant desmin loses its Z-disc localization but it can still associate with the intercalated discs, which, however, have an altered architecture, resembling other examples of dilated cardiomyopathy. This is the first report demonstrating a critical role of the desmin head and tail domains in the formation of the IF scaffold around Z discs. It is further suggested that in cardiomyocytes, an interplay between desmin tail and head domains is taking place, which potentially protects the amino terminus of desmin from specific proteases. The fact that the association with intercalated discs seems unchanged suggests that this association must take place through the desmin tail, in contrast to the head domain that is most possibly involved in the Z-disc binding.

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.