Non-muscle myosin heavy chain 9 maintains intestinal homeostasis by preventing epithelium necroptosis and colitis adenoma formation

Non-muscle myosin IIA plays an important role in cell adhesion, cell migration, and tissue architecture. We previously showed that low activity of the heavy chain of non-muscle myosin II Myh9 is beneficial to LGR5+ intestinal stem cell maintenance. However, the function of Myh9 in adult mouse intestinal epithelium is largely unclear. In this study, we used the inducible Villin-creERT2 knockout approach to delete Myh9 in adult mouse intestinal epithelium and observed that homozygous deletion of Myh9 causes colitis-like morphologic changes in intestine, leads to a high sensitivity to dextran sulfate sodium and promotes colitis-related adenoma formation in the colon. Myh9 deletion disturbs cell junctions and impairs intestinal lumen barrier integrity, promoting the necroptosis of epithelial cells. Consistently, these changes can be partially rescued by Ripk3 knockout. Our results indicate that Myh9 is required for the maintenance of intestinal epithelium integrity and the prevention of cell necroptosis.

Myh10 deficiency leads to defective extracellular matrix remodeling and pulmonary disease

Impaired alveolar formation and maintenance are features of many pulmonary diseases that are associated with significant morbidity and mortality. In a forward genetic screen for modulators of mouse lung development, we identified the non-muscle myosin II heavy chain gene, Myh10. Myh10 mutant pups exhibit cyanosis and respiratory distress, and die shortly after birth from differentiation defects in alveolar epithelium and mesenchyme. From omics analyses and follow up studies, we find decreased Thrombospondin expression accompanied with increased matrix metalloproteinase activity in both mutant lungs and cultured mutant fibroblasts, as well as disrupted extracellular matrix (ECM) remodeling. Loss of Myh10 specifically in mesenchymal cells results in ECM deposition defects and alveolar simplification. Notably, MYH10 expression is downregulated in the lung of emphysema patients. Altogether, our findings reveal critical roles for Myh10 in alveologenesis at least in part via the regulation of ECM remodeling, which may contribute to the pathogenesis of emphysema.

Ronin Governs Early Heart Development by Controlling Core Gene Expression Programs

Ronin (THAP11), a DNA-binding protein that evolved from a primordial DNA transposon by molecular domestication, recognizes a hyperconserved promoter sequence to control developmentally and metabolically essential genes in pluripotent stem cells. However, it remains unclear whether Ronin or related THAP proteins perform similar functions in development. Here, we present evidence that Ronin functions within the nascent heart as it arises from the mesoderm and forms a four-chambered organ. We show that Ronin is vital for cardiogenesis during midgestation by controlling a set of critical genes. The activity of Ronin coincided with the recruitment of its cofactor, Hcf-1, and the elevation of H3K4me₃ levels at specific target genes, suggesting the involvement of an epigenetic mechanism. On the strength of these findings, we propose that Ronin activity during cardiogenesis offers a template to understand how important gene programs are sustained across different cell types within a developing organ such as the heart.

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Ronin displays complex expression patterns during embryogenesis

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Ronin is critical for heart growth

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Ronin regulates genetic growth programs

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Ronin binding influences H3K4me₃ levels at target genes

Loss of smooth muscle myosin heavy chain results in the bladder and stomach developing lesion during foetal development in mice

Smooth muscle myosin heavy chain (SM-MHC) is exclusively expresses in smooth muscle, which takes part in smooth muscle cell contraction. Here, we used an insertional mutation mouse whose heavy polypeptide 11 (Myh11) gene has been disrupted and no SM-MHC protein has been detected. Compared to the wild-type and SM-MHC^+/- mice, the SM-MHC^-/- neonates had large round bellies, thin-walled giant bladders, and large stomachs with huge gas bubbles. Most of it died within 10 h and the rest within 20 h after birth. Further analysis of the developing foetuses from 16.5 days postcoitum (dpc) stage to newborn showed no significant (P<0.05) difference in the ratio of Mendelian inheritance and average body weight among SM-MHC^+/+ , SM-MHC^+/- and SM-MHC^-/- mice, whereas the abnormal exterior appearance was observed in each SM-MHC^-/- bladders from 16.5 dpc. Histological analysis showed no difference in stomach tissues but evidently thin-walled smooth muscle layer and a giant cavity in bladders of SM-MHC^-/- foetuses at various stages from 15.5 dpc to newborn. The results indicated that the defect of SM-MHC lead to the bladder developing lesions initially at 15.5 dpc stage in mouse and also implied that the SM-MHC loss might result in the gas bubbles in stomach. The study should facilitate further detailed analyses of the potential role of SM-MHC in bladder and stomach development.

Absence of platelet phenotype in mice lacking the motor protein myosin Va

Background

The motor protein myosin Va plays an important role in the trafficking of intracellular vesicles. Mutation of the Myo5a gene causes Griscelli syndrome type 1 in humans and the dilute phenotype in mice, which are both characterised by pigment dilution and neurological defects as a result of impaired vesicle transport in melanocytes and neuroendocrine cells. The role of myosin Va in platelets is currently unknown. Rab27 has been shown to be associated with myosin Va cargo vesicles and is known to be important in platelet dense granule biogenesis and secretion, a crucial event in thrombus formation. Therefore, we hypothesised that myosin Va may regulate granule secretion or formation in platelets.

Methodology/Principal Findings

Platelet function was studied in vitro using a novel Myo5a gene deletion mouse model. Myo5a^−/− platelets were devoid of myosin Va, as determined by immunoblotting, and exhibited normal expression of surface markers. We assessed dense granule, α-granule and lysosomal secretion, integrin α_IIbβ₃ activation, Ca²⁺ signalling, and spreading on fibrinogen in response to collagen-related peptide or the PAR4 agonist, AYPGKF in washed mouse platelets lacking myosin Va or wild-type platelets. Surprisingly, Myo5a^−/− platelets showed no significant functional defects in these responses, or in the numbers of dense and α-granules expressed.

Conclusion

Despite the importance of myosin Va in vesicle transport in other cells, our data demonstrate this motor protein has no non-redundant role in the secretion of dense and α-granules or other functional responses in platelets.

The UNC-45 chaperone is critical for establishing myosin-based myofibrillar organization and cardiac contractility in the Drosophila heart model

UNC-45 is a UCS (UNC-45/CRO1/She4P) class chaperone necessary for myosin folding and/or accumulation, but its requirement for maintaining cardiac contractility has not been explored. Given the prevalence of myosin mutations in eliciting cardiomyopathy, chaperones like UNC-45 are likely to be equally critical in provoking or modulating myosin-associated cardiomyopathy. Here, we used the Drosophila heart model to examine its role in cardiac physiology, in conjunction with RNAi-mediated gene silencing specifically in the heart in vivo. Analysis of cardiac physiology was carried out using high-speed video recording in conjunction with movement analysis algorithms. unc-45 knockdown resulted in severely compromised cardiac function in adults as evidenced by prolonged diastolic and systolic intervals, and increased incidence of arrhythmias and extreme dilation; the latter was accompanied by a significant reduction in muscle contractility. Structural analysis showed reduced myofibrils, myofibrillar disarray, and greatly decreased cardiac myosin accumulation. Cardiac unc-45 silencing also dramatically reduced life-span. In contrast, third instar larval and young pupal hearts showed mild cardiac abnormalities, as severe cardiac defects only developed during metamorphosis. Furthermore, cardiac unc-45 silencing in the adult heart (after metamorphosis) led to less severe phenotypes. This suggests that UNC-45 is mostly required for myosin accumulation/folding during remodeling of the forming adult heart. The cardiac defects, myosin deficit and decreased life-span in flies upon heart-specific unc-45 knockdown were significantly rescued by UNC-45 over-expression. Our results are the first to demonstrate a cardiac-specific requirement of a chaperone in Drosophila, suggestive of a critical role of UNC-45 in cardiomyopathies, including those associated with unfolded proteins in the failing human heart. The dilated cardiomyopathy phenotype associated with UNC-45 deficiency is mimicked by myosin knockdown suggesting that UNC-45 plays a crucial role in stabilizing myosin and possibly preventing human cardiomyopathies associated with functional deficiencies of myosin.

Fast-twitch sarcomeric and glycolytic enzyme protein loss in inclusion body myositis

Inclusion body myositis (IBM) is an inflammatory disease of skeletal muscle of unknown cause. To further understand the nature of the tissue injury in this disease, we developed methods for large-scale detection and quantitation of proteins in muscle biopsy samples and analyzed proteomic data produced by these methods together with histochemical, immunohistochemical, and microarray data. Twenty muscle biopsy samples from patients with inflammatory myopathies (n = 17) or elderly subjects without neuromuscular disease (n = 3) were profiled by proteomic studies using liquid chromatographic separation of peptides followed by mass spectrometry. Thirteen of the diseased samples additionally underwent microarray studies. Seventy muscle specimens from patients with a range of neuromuscular disorders were examined by ATPase histochemical methods. Smaller numbers of samples underwent immunohistochemical and immunoblot studies. Mass spectrometric studies identified and quantified approximately 300 total distinct proteins in each muscle sample. In IBM and to a lesser extent in polymyositis, proteomic studies confirmed by histochemical, immunohistochemical, and immunoblot studies showed loss of many fast-twitch specific structural proteins and glycolytic enzymes despite relative preservation of transcript levels. Increased abundance of a nuclear membrane protein, immunoglobulins, and two calpain-3 substrates were present. The atrophy present in IBM muscle is accompanied by preferential loss of fast-twitch structural proteins and glycolytic enzymes, particularly glycogen debranching enzyme, with relative preservation of the abundance of their respective transcripts. Although muscle atrophy has long been recognized in IBM, these studies are the first to report specific proteins which are reduced in quantity in IBM muscle.

Identification of yeast IQGAP (Iqg1p) as an anaphase-promoting-complex substrate and its role in actomyosin-ring-independent cytokinesis

In the yeast Saccharomyces cerevisiae, a ring of myosin II forms in a septin-dependent manner at the budding site in late G1. This ring remains at the bud neck until the onset of cytokinesis, when actin is recruited to it. The actomyosin ring then contracts, septum formation occurs concurrently, and cytokinesis is soon completed. Deletion of MYO1 (the only myosin II gene) is lethal on rich medium in the W303 strain background and causes slow-growth and delayed-cell-separation phenotypes in the S288C strain background. These phenotypes can be suppressed by deletions of genes encoding nonessential components of the anaphase-promoting complex (APC/C). This suppression does not seem to result simply from a delay in mitotic exit, because overexpression of a nondegradable mitotic cyclin does not suppress the same phenotypes. Overexpression of either IQG1 or CYK3 also suppresses the myo1Delta phenotypes, and Iqg1p (an IQGAP protein) is increased in abundance and abnormally persistent after cytokinesis in APC/C mutants. In vitro assays showed that Iqg1p is ubiquitinated directly by APC/C(Cdh1) via a novel recognition sequence. A nondegradable Iqg1p (lacking this recognition sequence) can suppress the myo1Delta phenotypes even when expressed at relatively low levels. Together, the data suggest that compromise of APC/C function allows the accumulation of Iqg1p, which then promotes actomyosin-ring-independent cytokinesis at least in part by activation of Cyk3p.

Defective CFTR apical endocytosis and enterocyte brush border in myosin VI-deficient mice

In polarized epithelial cells such as those that line the inner ear, kidney and gut, myosin VI has been localized to the intermicrovillar domains where it is proposed to regulate clathrin-dependent endocytosis; however, a direct role for myosin VI in apical endocytosis has not been shown. We examined the apical membrane distribution and endocytosis of cystic fibrosis transmembrane conductance regulator (CFTR) in myosin VI-deficient Snell's Waltzer Myo6((sv/sv)) mice. Confocal microscopy and cell-surface biotinylation confirmed that surface levels of CFTR in the intestine of Myo6((sv/sv)) mice were markedly higher, and CFTR internalization from the apical plasma membrane was reduced compared with heterozygous controls. Consistent with a defect in CFTR endocytosis and accumulation at the cell surface, exaggerated CFTR-mediated fluid secretion was observed in Myo6((sv/sv)) mice following treatment of isolated jejunum with the cyclic GMP-activated heat stable enterotoxin. These data establish that myosin VI modulates apical endocytosis and may be an important physiological modulator of CFTR function and CFTR-associated secretory diarrhea in the gut.

Binding of internalized receptors to the PDZ domain of GIPC/synectin recruits myosin VI to endocytic vesicles

Myosin VI (myo6) is the only actin-based molecular motor that translocates along actin filaments toward the minus end. Myo6 participates in two steps of endocytic trafficking; it is recruited to both clathrin-coated pits and to ensuing uncoated endocytic vesicles (UCV). Although there is evidence suggesting that the PDZ adaptor protein GIPC/synectin is involved in the association of myo6 with UCV, the recruitment mechanism is unknown. We show that GIPC/synectin is required for both internalization of cell surface receptors and for coupling of myo6 to UCV. This coupling occurs via a mechanism wherein engagement of the GIPC/synectin PDZ domain by C termini of internalized receptors facilitates in trans myo6 binding to the GIPC/synectin C terminus located outside of the PDZ domain. Analysis of megalin, a prototypical GIPC/synectin-binding receptor, revealed that deletion of its PDZ-binding motif drastically reduced GIPC/synectin and myo6 recruitment to UCV. Furthermore, interaction with GIPC/synectin was required for megalin's function, as megalin was mistargeted in the renal proximal tubules of GIPC/synectin-null mice and these mice exhibited proteinuria, a condition consistent with defective megalin trafficking.

Myosin VI stabilizes an actin network during Drosophila spermatid individualization

Here, we demonstrate a new function of myosin VI using observations of Drosophila spermatid individualization in vivo. We find that myosin VI stabilizes a branched actin network in actin structures (cones) that mediate the separation of the syncytial spermatids. In a myosin VI mutant, the cones do not accumulate F-actin during cone movement, whereas overexpression of myosin VI leads to bigger cones with more F-actin. Myosin subfragment 1-fragment decoration demonstrated that the actin cone is made up of two regions: a dense meshwork at the front and parallel bundles at the rear. The majority of the actin filaments were oriented with their pointed ends facing in the direction of cone movement. Our data also demonstrate that myosin VI binds to the cone front using its motor domain. Fluorescence recovery after photobleach experiments using green fluorescent protein-myosin VI revealed that myosin VI remains bound to F-actin for minutes, suggesting its role is tethering, rather than transporting cargo. We hypothesize that myosin VI protects the actin cone structure either by cross-linking actin filaments or anchoring regulatory molecules at the cone front. These observations uncover a novel mechanism mediated by myosin VI for stabilizing long-lived actin structures in cells.

Molecular and cellular defects of skeletal muscle in an animal model of acute quadriplegic myopathy

Muscle denervation and concomitant high-dose dexamethasone treatment in rodents produces characteristic pathologic features of severe muscle atrophy and selective myosin heavy filament (MyHC) depletion, identical to those seen in acute quadriplegic myopathy (AQM), also known as critical illness myopathy. We tested the hypothesis that defective pre-translational processes contribute to the atrophy and selective MyHC depletion in this model. We examined the effects of combined glucocorticoid-denervation treatment on MyHC and actin mRNA populations; we also studied mRNA expression of the myogenic regulatory factors (MRFs), primary transcription factors for MyHC. Adult female rats were subjected to proximal sciatic denervation followed by high-dose dexamethasone (DD) treatment (5 mg/kg body weight daily) for 7 days. Disease controls included rats treated with denervation alone (DN) or dexamethasone alone (DX). At 1 week the plantaris atrophied by approximately 42% in DD muscles. DD treatment resulted in selective MyHC protein depletion; actin protein concentration was not significantly changed. Despite an increase in total RNA concentration in DN and DD muscles, MyHC and actin mRNA concentrations were significantly decreased in these muscles. MyHC mRNA showed a significantly more extensive depletion relative to actin mRNA in DD muscles. Glucocorticoid treatment did not influence a denervation-induced increase in the mRNA expression of the MRFs. We conclude that a deleterious interaction between glucocorticoid and denervation treatments in skeletal muscle is responsible for pre-translational defects that reduce actin and MyHC mRNA substrates in a disproportionate fashion. The resultant selective MyHC depletion contributes to the severe muscle atrophy.

Myosin-1a is critical for normal brush border structure and composition

To develop our understanding of myosin-1a function in vivo, we have created a mouse line null for the myosin-1a gene. Myosin-1a knockout mice demonstrate no overt phenotypes at the whole animal level but exhibit significant perturbations and signs of stress at the cellular level. Among these are defects in microvillar membrane morphology, distinct changes in brush-border organization, loss of numerous cytoskeletal and membrane components from the brush border, and redistribution of intermediate filament proteins into the brush border. We also observed significant ectopic recruitment of another short-tailed class I motor, myosin-1c, into the brush border of knockout enterocytes. This latter finding, a clear demonstration of functional redundancy among vertebrate myosins-I, may account for the lack of a whole animal phenotype. Nevertheless, these results indicate that myosin-1a is a critical multifunctional component of the enterocyte, required for maintaining the normal composition and highly ordered structure of the brush border.

Targeted disruption of mouse ortholog of the human MYH9 responsible for macrothrombocytopenia with different organ involvement: hematological, nephrological, and otological studies of heterozygous KO mice

Among three different isoforms of non-muscle myosin heavy chains (NMMHCs), only NMMHCA is associated with inherited human disease, called MYH9 disorders, characterized by macrothrombocytopenia and characteristic granulocyte inclusions. Here targeted gene disruption was performed to understand fundamental as well as pathological role of the gene for NMMHCA, MYH9. Heterozygous intercrosses yielded no homozygous animals among 552 births, suggesting that MYH9 expression is required for embryonic development. In contrast, MYH9+/- mice were viable and fertile without gross anatomical, hematological, and nephrological abnormalities. Immunofluorescence analysis also showed the normal cytoplasmic distribution of NMMHCA. We further measured the auditory brainstem response and found two of six MYH9+/- mice had hearing losses, whereas the remaining four were comparable to wild-type mice. Such observation may parallel the diverse expression of Alport's manifestations of human individuals with MYH9 disorders and suggest the limited requirement of the gene for maintenance and function of specific organs.

Normal Lytic Granule Secretion by Cytotoxic T Lymphocytes Deficient in BLOC-1, -2 and -3 and Myosins Va, VIIa and XV

Melanocytes and cells of the immune system share an unusual secretory mechanism which uses the lysosome as a regulated secretory organelle. Recently, a number of the proteins required for these 'secretory lysosomes' to undergo exocytosis have been identified. These include Rab27a, Lyst, Rab geranyl geranyl transferase and the adapter protein complex AP-3. Patients lacking any of these proteins are characterized by the rare combination of albinism and immunodeficiency, revealing roles for these proteins in both melanocyte and immune cell secretion. In order to ask how far the link between albinism and immunodeficiency extends we have examined cytotoxic T-lymphocyte (CTL) secretion from two BLOC-3-deficient patients and seven different mouse models of Hermansky-Pudlak syndrome, all of which display defects in pigmentation and platelet function. We find that CTL function is normal in HPS patients and pale-ear mice deficient in BLOC-3, pallid, muted and sandy mice deficient in BLOC-1, ruby-eye mice deficient in BLOC-2 and buff mice deficient in Vps33a. Similarly, the unconventional myosins, Va, VIIa and XV, which can act as effectors for Rab27a in some cell types, are not required in CTL. These results reveal differences in the protein machinery required for biogenesis and/or secretion of lysosome-related organelles in CTL and melanocytes.

Defects in cell adhesion and the visceral endoderm following ablation of nonmuscle myosin heavy chain II-A in mice

Previous work has shown that ablation or mutation of nonmuscle myosin heavy chain II-B (NMHC II-B) in mice results in defects in the heart and brain with death occurring between embryonic day 14.5 (E14.5) and birth (Tullio, A. N., Accili, D., Ferrans, V. J., Yu, Z. X., Takeda, K., Grinberg, A., Westphal, H., Preston, Y. A., and Adelstein, R. S. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 12407-12412). Here we show that mice ablated for NMHC II-A fail to develop a normal patterned embryo with a polarized visceral endoderm by E6.5 and die by E7.5. Moreover, A(-)/A(-) embryoid bodies grown in suspension culture constantly shed cells. These defects in cell adhesion and tissue organization are explained by loss of E-cadherin and beta-catenin localization to cell adhesion sites in both cell culture and in the intact embryos. The defects can be reproduced by introducing siRNA directed against NMHC II-A into wild-type embryonic stem cells. Our results suggest an essential role for a single, specific nonmuscle myosin isoform in maintaining cell-cell adhesions in the early mammalian embryo.

Nonmuscle Myosin motor of smooth muscle

Nonmuscle myosin can generate force and shortening in smooth muscle, as revealed by studies of the urinary bladder from mice lacking smooth muscle myosin heavy chain (SM-MHC) but expressing the nonmuscle myosin heavy chains A and B (NM-MHC A and B; Morano, I., G.X. Chai, L.G. Baltas, V. Lamounier-Zepter, G. Lutsch, M. Kott, H. Haase, and M. Bader. 2000. Nat. Cell Biol. 2:371-375). Intracellular calcium was measured in urinary bladders from SM-MHC-deficient and SM-MHC-expressing mice in relaxed and contracted states. Similar intracellular [Ca2+] transients were observed in the two types of preparations, although the contraction of SM-MHC-deficient bladders was slow and lacked an initial peak in force. The difference in contraction kinetics thus do not reflect differences in calcium handling. Thick filaments were identified with electron microscopy in smooth muscle cells of SM-MHC-deficient bladders, showing that NM-MHC can form filaments in smooth muscle cells. Maximal shortening velocity of maximally activated, skinned smooth muscle preparations from SM-MHC-deficient mice was significantly lower and more sensitive to increased MgADP compared with velocity of SM-MHC-expressing preparations. Active force was significantly lower and less inhibited by increased inorganic phosphate. In conclusion, large differences in nucleotide and phosphate binding exist between smooth and nonmuscle myosins. High ADP binding and low phosphate dependence of nonmuscle myosin would influence both velocity of actin translocation and force generation to promote slow motility and economical force maintenance of the cell.

Cross-bridge mechanisms of muscle weakness in multiple sclerosis

Vastus lateralis muscle biopsies were obtained from six individuals with multiple sclerosis (MS) having an Expanded Disability Status Score of 4.75 +/- 0.28, and from six age- and gender-matched individuals without MS. Biopsies from the MS group showed fewer fibers (31 +/- 4 vs. 46 +/- 4%) containing the type IIa myosin heavy chain (MHC) isoform exclusively. However, the percentage of fibers coexpressing type IIa and IIx MHC increased in direct proportion with MS disability status. The average unloaded shortening velocity of skinned fibers containing type I or IIa MHC did not differ between subject groups. Peak Ca(2+)-activated force was 11-13% lower in fibers from the MS group due to atrophy (type I and IIa fibers) and reduced specific force (type I fibers). Increasing intracellular inorganic phosphate (0-30 mM) or hydrogen ion (pH 7.0-6.2) reduced Ca(2+)-activated force in a manner that was independent of MS status. Thus, fibers from the MS group showed a subtle shift in fast MHC isoform coexpression and a modest reduction in cross-bridge number, density, or average force, with no change in maximal cross-bridge cycling rate or susceptibility to intracellular metabolites. These changes explain part of the muscle weakness and fatigue experienced by individuals with MS.

Myosin II contributes to the posterior contraction and the anterior extension during the retraction phase in migrating Dictyostelium cells

Cells must exert force against the substrate to migrate. We examined the vectors (both the direction and the magnitude) of the traction force generated by Dictyostelium cells using an improved non-wrinkling silicone substrate. During migration, the cells showed two 'alternate' phases of locomotory behavior, an extension phase and a retraction phase. In accordance with these phases, two alternate patterns were identified in the traction force. During the extension phase, the cell exerted a 'pulling force' toward the cell body in the anterior and the posterior regions and a 'pushing force' in the side of the cell (pattern 1). During the retraction phase, the cell exerted a 'pushing force' in the anterior region, although the force disappeared in the side and the posterior regions of the cell (pattern 2). Myosin II heavy chain null cells showed a single pattern in their traction force comparable to 'pattern 1', although they still had the alternate biphasic locomotory behavior similar to the wild-type cells. Therefore, the generation of 'pushing force' in the anterior and the cancellation of the traction force in the side and the posterior during the retraction phase were deficient in myosin knock-out mutant cells, suggesting that these activities depend on myosin II via the posterior contraction. Considering all these results, we hypothesized that there is a highly coordinated, biphasic mechanism of cell migration in Dictyostelium.

Inherited thrombocytopenias: from genes to therapy

BACKGROUND AND OBJECTIVES

Inherited thrombocytopenias are a heterogeneous group of rare diseases characterized by a reduced number of blood platelets. Some of these diseases are exclusive to megakaryocytes and platelets, while in others the pathology extends to other cell types. Although the defective genes, coding for membrane glyoproteins, cytoskeleton components and intracellular signaling pathways, as well as transcription factors, have been identified in most cases, the pathophysiology of these disorders is often unknown. This review describes recent contributions to clinical and diagnostic aspects, biology and treatments of familial thrombocytopenias.

EVIDENCE AND INFORMATION SOURCES

The information presented here derives from literature and the experience of the authors. The most relevant studies are critically analyzed and discussed.

STATE OF ART

The clinical and laboratory features of most of the inherited thrombocytopenias have been reviewed. The different forms have been classified into 3 groups depending on platelet volume. Although this criterion is not completely satisfactory, it is one of the most useful in diagnostic algorithms. We report on recent advances in Wiskott-Aldrich and Bernard-Soulier syndromes, as well as in MYH9-related diseases, a new nosological entity that groups old distinct forms known as May-Hegglin anomaly, Sebastian, Fetchner, and Epstein syndromes. Other, less frequent forms are also discussed, including non-syndromic forms of mild thrombocytopenia that are genetically heterogeneous.

PERSPECTIVES

In the past, inherited thrombocytopenias were considered exceedingly rare and the number of well-defined forms was very small. In the last few years, the widespread diffusion of electronic cell counters has allowed these conditions to be detected more frequently and several new entities have been identified through the co-ordinated efforts of physicians, biologists and geneticists. The pathogenesis of many new and old forms is being unraveled, thus providing insights on the molecular basis of platelet production and function. This knowledge will be a valuable resource for clinicians in the diagnostic approaches to such disorders.

Myosin IIB is required for growth cone motility

Growth cones are required for the forward advancement and navigation of growing axons. Modulation of growth cone shape and reorientation of the neurite are responsible for the change of outgrowth direction that underlies navigation. Change of shape involves the reordering of the cytoskeleton. Reorientation of the neurite requires the generation of tension, which is supplied by the ability of the growth cone to crawl on a substrate. The specific molecular mechanisms responsible for these activities are unknown but are thought to involve actomyosin-generated force combined with linkage to the cell surface receptors that are responsible for adhesion (Heidemann and Buxbaum, 1998). To test whether myosin IIB is responsible for the force generation, we quantified shape dynamics and filopodial-mediated traction force in growth cones from myosin IIB knock-out (KO) mice and compared them with neurons from normal littermates. Growth cones from the KO mice spread less, showed alterations in shape dynamics and actin organization, and had reduced filopodial-mediated traction force. Although peak traction forces produced by filopodia of KO cones were decreased significantly, KO filopodia occasionally developed forces equivalent to those in the wild type. This indicates that other myosins participate in filopodial-dependent traction force. Therefore, myosin IIB is necessary for normal growth cone spreading and the modulation of shape and traction force but acts in combination with other myosins for some or all of these activities. These activities are essential for growth cone forward advancement, which is necessary for outgrowth. Thus outgrowth is slowed, but not eliminated, in neurons from the myosin IIB KO mice.

Postnatal myosin heavy chain isoform expression in normal mice and mice null for IIb or IId myosin heavy chains

The patterns of myosin heavy chain (MyHC) isoform expression in the embryo and in the adult mouse are reasonably well characterized and quite distinct. However, little is known about the transition between these two states, which involves major decreases and increases in the expression of several MyHC genes. In the present study, the expression of seven sarcomeric MyHCs was analyzed in the hindlimb muscles of wild-type mice and in mice null for the MyHC IIb or IId/x genes at several time points from 1 day of postnatal life (dpn) to 20 dpn. In early postnatal life, the developmental isoforms (embryonic and perinatal) comprise >90% of the total MyHC expression, while three adult fast isoforms (IIa, IIb, and IId) comprise <1% of the total MyHC protein. However, between 5 and 20 dpn their expression increases to comprise >90% of the total MyHC. Expression of each of the three adult fast isoforms occurs in a spatially and temporally distinct manner. We also show that alpha MyHC, which is almost exclusively expressed in the heart, is expressed in scattered fibers in all hindlimb muscles during postnatal development. Surprisingly, the timing and localization of expression of the MyHC isoforms is unchanged in IIb and IId/x null mice, although the magnitude of expression is altered for some isoforms. Together these data provide a comprehensive overview of the postnatal expression pattern of the sarcomeric MyHC isoforms in the mouse hindlimb.

Manipulating the contractile apparatus: genetically defined animal models of cardiovascular disease

Within the last 10 years via gene targeting and transgenesis, numerous models of cardiovascular disease have been established and used to determine if a protein's presence or absence causes cardiovascular disease. By affecting the heart's protein complement in a defined manner, the function of the different mutated proteins or protein isoforms present in the contractile apparatus can be determined and pathogenic mechanism(s) explored. We can now remodel the cardiac protein profile and effect replacement of even the most abundant contractile proteins. Precise genetic manipulation allows exploration of the structure-function relationships which underlie cardiac function, and the consequences of defined mutations at the molecular, biochemical, cytological and physiologic levels can be determined.

Smooth-muscle contraction without smooth-muscle myosin

Here we have used gene-targeting to eliminate expression of smooth-muscle myosin heavy chain. Elimination of this gene does not affect expression of non-muscle myosin heavy chain, and knockout individuals typically survive for three days. Prolonged activation, by KCl depolarisation, of intact bladder preparations from wild-type neonatal mice produces an initial transient state (phase 1) of high force generation and maximal shortening velocity, which is followed by a sustained state (phase 2) characterized by low force generation and maximal shortening velocity. Similar preparations from knockout neonatal mice do not undergo phase 1, but exhibit a normal phase 2. We propose that, in neonatal smooth muscle phase 1 is generated by recruitment of smooth-muscle myosin heavy chain, whereas phase 2 can be generated by activation of non-muscle myosin heavy chain. We conclude that phase 1 becomes indispensable for survival and normal growth soon after birth, particularly for functions such as homeostasis and circulation.

Gene dosage affects the cardiac and brain phenotype in nonmuscle myosin II-B-depleted mice

Complete ablation of nonmuscle myosin heavy chain II-B (NMHC-B) in mice resulted in cardiac and brain defects that were lethal during embryonic development or on the day of birth. In this paper, we report on the generation of mice with decreased amounts of NMHC-B. First, we generated B(DeltaI)/B(DeltaI) mice by replacing a neural-specific alternative exon with the PGK-Neo cassette. This resulted in decreased amounts of NMHC-B in all tissues, including a decrease of 88% in the heart and 65% in the brain compared with B(+)/B(+) tissues. B(DeltaI)/B(DeltaI) mice developed cardiac myocyte hypertrophy between 7 months and 11 months of age, at which time they reexpressed the cardiac beta-MHC. Serial sections of B(DeltaI)/B(DeltaI) brains showed abnormalities in neural cell migration and adhesion in the ventricular wall. Crossing B(DeltaI)/B(DeltaI) with B(+)/B(-) mice generated B(DeltaI)/B(-) mice, which showed a further decrease of approximately 55% in NMHC-B in the heart and brain compared with B(DeltaI)/B(DeltaI) mice. Five of 8 B(DeltaI)/B(-) mice were born with a membranous ventricular septal defect. Moreover, 5 of 5 B(DeltaI)/B(-) mice developed myocyte hypertrophy by 1 month; B(DeltaI)/B(-) mice also reexpressed the cardiac beta-MHC. More than 60% of B(DeltaI)/B(-) mice developed overt hydrocephalus and showed more severe defects in neural cell migration and adhesion than did B(DeltaI)/B(DeltaI) mice. These data on B(DeltaI)/B(DeltaI) and B(DeltaI)/B(-) mice demonstrate a gene dosage effect of the amount of NMHC-B on the severity and time of onset of the defects in the heart and brain.

Flow cytometry analysis of cell cycle in myosin II-deficient yeast

OBJECTIVE

To determine whether cell cycle changes can be detected in myosin II-deficient cells using flow cytometry techniques.

BACKGROUND

Although the primary role of myosin II (Myo1p) in the yeast Saccharomyces cerevisiae is in cytokinesis we have reported that this conventional myosin also appears to influence the regulation of cell wall metabolism as indicated by increases in the expression of chitin metabolizing enzymes in a null mutant of the MYO1 gene. The expression of these enzymes is known to be regulated in the cell cycle suggesting that cell cycle changes may alter their expression.

METHODS

Flow cytometry was employed to assess the nuclear DNA content of logarithmic yeast cell cultures as a means of determining changes in the cell cycle of Myo1p-deficient cells.

RESULTS

Significant changes were observed in the Myo1p-deficient strain suggesting that these cells are arrested in G2/M-phase of the cell cycle.

CONCLUSIONS

Based on the results of this preliminary study, we propose a model in which the increased activity of chitin metabolizing enzymes may be explained by a mitotic arrest in these cells.

Elevated expression of chitinase 1 and chitin synthesis in myosin II-deficient Saccharomyces cerevisiae

To determine if the attached cells formed in Myosin II-deficient Saccharomyces cerevisiae result from deficient chitinase 1 (CTS1) expression, the activity of chitinase 1 was assayed. Secretion of this enzyme was not prevented by a MYO1 gene deficiency, and soluble and cell wall-associated Cts1p activity were increased approximately 5-fold and 20-fold, respectively, in these cells. The increase in soluble activity was correlated with an increase in enzyme levels. Likewise, intracellular chitinase activity was increased approximately 22-fold, and the chitin content of cell walls was elevated 2-fold. These data suggest that the origin of myo1-associated phenotypes is not due to deficient chitinase expression and may instead be due to a deregulation of cell wall metabolism in these cells.

Myosin heavy chains IIa and IId are functionally distinct in the mouse

Myosin in adult murine skeletal muscle is composed primarily of three adult fast myosin heavy chain (MyHC) isoforms. These isoforms, MyHC-IIa, -IId, and -IIb, are >93% identical at the amino acid level and are broadly expressed in numerous muscles, and their genes are tightly linked. Mice with a null mutation in the MyHC-IId gene have phenotypes that include growth inhibition, muscle weakness, histological abnormalities, kyphosis (spinal curvature), and aberrant kinetics of muscle contraction and relaxation. Despite the lack of MyHC-IId, IId null mice have normal amounts of myosin in their muscles because of compensation by the MyHC-IIa gene. In each muscle examined from IId null mice, there was an increase in MyHC-IIa- containing fibers. MyHC-IIb content was unaffected in all muscles except the masseter, where its expression was extinguished in the IId null mice. Cross-sectional fiber areas, total muscle cross-sectional area, and total fiber number were affected in ways particular to each muscle. Developmental expression of adult MyHC genes remained unchanged in IId null mice. Despite this universal compensation of MyHC-IIa expression, IId null mice have severe phenotypes. We conclude that despite the similarity in sequence, MyHC-IIa and -IId have unique roles in the development and function of skeletal muscle.

Growth and muscle defects in mice lacking adult myosin heavy chain genes

The three adult fast myosin heavy chains (MyHCs) constitute the vast majority of the myosin in adult skeletal musculature, and are >92% identical. We describe mice carrying null mutations in each of two predominant adult fast MyHC genes, IIb and IId/x. Both null strains exhibit growth and muscle defects, but the defects are different between the two strains and do not correlate with the abundance or distribution of each gene product. For example, despite the fact that MyHC-IIb accounts for >70% of the myosin in skeletal muscle and shows the broadest distribution of expression, the phenotypes of IIb null mutants are generally milder than in the MyHC-IId/x null strain. In addition, in a muscle which expresses both IIb and IId/x MyHC in wild-type mice, the histological defects are completely different for null expression of the two genes. Most striking is that while both null strains exhibit physiological defects in isolated muscles, the defects are distinct. Muscle from IIb null mice has significantly reduced ability to generate force while IId null mouse muscle generates normal amounts of force, but has altered kinetic properties. Many of the phenotypes demonstrated by these mice are typical in human muscle disease and should provide insight into their etiology.