Dynamics of actin in cardiac myofibrils and fibroblast stress fibers

Knockout of Wdr1 results in cardiac hypertrophy and impaired cardiac function in adult mouse heart

WDR1 is a major cofactor of the actin depolymerizing factor (ADF)/cofilin, accelerating ADF/cofilin-mediated actin disassembly. We had previously showed that WDR1-mediated actin dynamics is required for postnatal myocardial growth and adult myocardial maintenance in mice, in which the detailed phenotypes of adult cardiomyocyte-specific Wdr1 deletion mice had not been analyzed. In this study, we systematically analyzed the role of Wdr1 in adult mouse heart. Adult cardiomyocyte-specific Wdr1 deletion mice (cKO) exhibited cardiac hypertrophy and myocardial fibrosis. Echocardiographic study and electrocardiography revealed impaired contractile function, prolonged QT interval and Tpeak-Tend interval, and abnormal T-wave amplitude in cKO mice. Increased levels of sarcomeric proteins, adherens junction proteins and cofilin, and severe actin filament (F-actin) accumulations were observed in cKO mice heart. Taken together, this finding demonstrates that WDR1 is a critical factor for normal structure and function of adult mouse heart.

A cardiomyocyte-specific Wdr1 knockout demonstrates essential functional roles for actin disassembly during myocardial growth and maintenance in mice

Actin dynamics are critical for muscle development and function, and mutations leading to deregulation of actin dynamics cause various forms of heritable muscle diseases. AIP1 is a major cofactor of the actin depolymerizing factor/cofilin in eukaryotes, promoting actin depolymerizing factor/cofilin-mediated actin disassembly. Its function in vertebrate muscle has been unknown. To investigate functional roles of AIP1 in myocardium, we generated conditional knockout (cKO) mice with cardiomyocyte-specific deletion of Wdr1, the mammalian homolog of yeast AIP1. Wdr1 cKO mice began to die at postnatal day 13 (P13), and none survived past P24. At P12, cKO mice exhibited cardiac hypertrophy and impaired contraction of the left ventricle. Electrocardiography revealed reduced heart rate, abnormal P wave, and abnormal T wave at P10 and prolonged QT interval at P12. Actin filament (F-actin) accumulations began at P10 and became prominent at P12 in the myocardium of cKO mice. Within regions of F-actin accumulation in myofibrils, the sarcomeric components α-actinin and tropomodulin-1 exhibited disrupted patterns, indicating that F-actin accumulations caused by Wdr1 deletion result in disruption of sarcomeric structure. Ectopic cofilin colocalized with F-actin aggregates. In adult mice, Wdr1 deletion resulted in similar but much milder phenotypes of heart hypertrophy, F-actin accumulations within myofibrils, and lethality. Taken together, these results demonstrate that AIP1-regulated actin dynamics play essential roles in heart function in mice.

Dynamic regulation of sarcomeric actin filaments in striated muscle

In striated muscle, the actin cytoskeleton is differentiated into myofibrils. Actin and myosin filaments are organized in sarcomeres and specialized for producing contractile forces. Regular arrangement of actin filaments with uniform length and polarity is critical for the contractile function. However, the mechanisms of assembly and maintenance of sarcomeric actin filaments in striated muscle are not completely understood. Live imaging of actin in striated muscle has revealed that actin subunits within sarcomeric actin filaments are dynamically exchanged without altering overall sarcomeric structures. A number of regulators for actin dynamics have been identified, and malfunction of these regulators often result in disorganization of myofibril structures or muscle diseases. Therefore, proper regulation of actin dynamics in striated muscle is critical for assembly and maintenance of functional myofibrils. Recent studies have suggested that both enhancers of actin dynamics and stabilizers of actin filaments are important for sarcomeric actin organization. Further investigation of the regulatory mechanism of actin dynamics in striated muscle should be a key to understanding how myofibrils develop and operate.

Hypertrophic stimulation increases beta-actin dynamics in adult feline cardiomyocytes

The myocardium responds to hemodynamic stress through cellular growth and organ hypertrophy. The impact of cytoskeletal elements on this process, however, is not fully understood. While α-actin in cardiomyocytes governs muscle contraction in combination with the myosin motor, the exact role of β-actin has not been established. We hypothesized that in adult cardiomyocytes, as in non-myocytes, β-actin can facilitate cytoskeletal rearrangement within cytoskeletal structures such as Z-discs. Using a feline right ventricular pressure overload (RVPO) model, we measured the level and distribution of β-actin in normal and pressure overloaded myocardium. Resulting data demonstrated enriched levels of β-actin and enhanced translocation to the Triton-insoluble cytoskeletal and membrane skeletal complexes. In addition, RVPO in vivo and in vitro hypertrophic stimulation with endothelin (ET) or insulin in isolated adult cardiomyocytes enhanced the content of polymerized fraction (F-actin) of β-actin. To determine the localization and dynamics of β-actin, we adenovirally expressed GFP-tagged β-actin in isolated adult cardiomyocytes. The ectopically expressed β-actin-GFP localized to the Z-discs, costameres, and cell termini. Fluorescence recovery after photobleaching (FRAP) measurements of β-actin dynamics revealed that β-actin at the Z-discs is constantly being exchanged with β-actin from cytoplasmic pools and that this exchange is faster upon hypertrophic stimulation with ET or insulin. In addition, in electrically stimulated isolated adult cardiomyocytes, while β-actin overexpression improved cardiomyocyte contractility, immunoneutralization of β-actin resulted in a reduced contractility suggesting that β-actin could be important for the contractile function of adult cardiomyocytes. These studies demonstrate the presence and dynamics of β-actin in the adult cardiomyocyte and reinforce its usefulness in measuring cardiac cytoskeletal rearrangement during hypertrophic stimulation.

A potential yeast actin allosteric conduit dependent on hydrophobic core residues val-76 and trp-79

Intramolecular allosteric interactions responsible for actin conformational regulation are largely unknown. Previous work demonstrated that replacing yeast actin Val-76 with muscle actin Ile caused decreased nucleotide exchange. Residue 76 abuts Trp-79 in a six-residue linear array beginning with Lys-118 on the surface and ending with His-73 in the nucleotide cleft. To test if altering the degree of packing of these two residues would affect actin dynamics, we constructed V76I, W79F, and W79Y single mutants as well as the Ile-76/Phe-79 and Ile-76/Tyr-79 double mutants. Tyr or Phe should decrease crowding and increase protein flexibility. Subsequent introduction of Ile should restore packing and dampen changes. All mutants showed decreased growth in liquid medium. W79Y alone was severely osmosensitive and exhibited vacuole abnormalities. Both properties were rescued by Ile-76. Phe-79 or Tyr decreased the thermostability of actin and increased its nucleotide exchange rate. These effects, generally greater for Tyr than for Phe, were reversed by introduction of Ile-76. HD exchange showed that the mutations caused propagated conformational changes to all four subdomains. Based on results from phosphate release and light-scattering assays, single mutations affected polymerization in the order of Ile, Phe, and Tyr from least to most. Introduction of Ile-76 partially rescued the polymerization defects caused by either Tyr-79 or Phe-79. Thus, alterations in crowding of the 76-79 residue pair can strongly affect actin conformation and behavior, and these results support the theory that the amino acid array in which they are located may play a central role in actin regulation.

Myofibril assembly visualized by imaging N-RAP, alpha-actinin, and actin in living cardiomyocytes

N-RAP is a striated muscle-specific scaffolding protein that organizes alpha-actinin and actin into symmetrical I-Z-I structures in developing myofibrils. Here we determined the order of events during myofibril assembly through time-lapse confocal microscopy of cultured embryonic chick cardiomyocytes coexpressing fluorescently tagged N-RAP and either alpha-actinin or actin. During de novo myofibril assembly, N-RAP assembled in fibrillar structures within the cell, with dots of alpha-actinin subsequently organizing along these structures. The initial fibrillar structures were reminiscent of actin fibrils, and coassembly of N-RAP and actin into newly formed fibrils supported this. The alpha-actinin dots subsequently broadened to Z-lines that were wider than the underlying N-RAP fibril, and N-RAP fluorescence intensity decreased. FRAP experiments showed that most of the alpha-actinin dynamically exchanged during all stages of myofibril assembly. In contrast, less than 20% of the N-RAP in premyofibrils was exchanged during 10-20 min after photobleaching, but this value increased to 70% during myofibril maturation. The results show that N-RAP assembles into an actin containing scaffold before alpha-actinin recruitment; that the N-RAP scaffold is much more stable than the assembling structural components; that N-RAP dynamics increase as assembly progresses; and that N-RAP leaves the structure after assembly is complete.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Nebulin regulates the assembly and lengths of the thin filaments in striated muscle

In many tissues, actin monomers polymerize into actin (thin) filaments of precise lengths. Although the exact mechanisms involved remain unresolved, it is proposed that "molecular rulers" dictate the lengths of the actin filaments. The giant nebulin molecule is a prime candidate for specifying thin filament lengths in striated muscle, but this idea has never been proven. To test this hypothesis, we used RNA interference technology in rat cardiac myocytes. Live cell imaging and triple staining revealed a dramatic elongation of the preexisting thin filaments from their pointed ends upon nebulin knockdown, demonstrating its role in length maintenance; the barbed ends were unaffected. When the thin filaments were depolymerized with latrunculin B, myocytes with decreased nebulin levels reassembled them to unrestricted lengths, demonstrating its importance in length specification. Finally, knockdown of nebulin in skeletal myotubes revealed its involvement in myofibrillogenesis. These data are consistent with nebulin functioning as a thin filament ruler and provide insight into mechanisms dictating macromolecular assembly.

Dynamics of actin and alpha-actinin in nascent myofibrils and stress fibers

Actin labeled with fluorescein isothiocyanate (FITC) and alpha-actinin labeled with rhodamine (rh) were co-injected into chick embryonic cardiac myocytes and fibroblasts. In cardiomyocytes, FITC-actin was distributed in nonstriated lines, linearly arranged punctate structures with short intervals, and cross-striated bands with regular sarcomeric intervals. rh-alpha-Actinin was seen to be distributed in the same pattern in the former two portions, and in the center of each striation in the latter portion. Photobleaching of structures incorporated with these fluorescent analogs revealed that the fluorescent recovery rate of actin decreased in the order of nonstriated > punctated > striated portions, while that of alpha-actinin was low and stable at all portions. During the transition phase from punctate to regular sarcomere structures of these proteins, short spaced alpha-actinin dots adjoined each other and aligned with Z bands of neighboring myofibrils. It appears that both the difference in exchangeability between actin and alpha-actinin molecules and the movement of alpha-actinin dots during this phase of myofibrillogenesis are related to sarcomere lengthening and I-Z-I brush formation; adjoining dots of low-exchangeable alpha-actinin may provide favorable situations for exchangeable actin molecules in filaments to elongate and/or rearrange. In fibroblasts, both FITC-actin and rh-alpha-actinin formed nonstriated lines. In these cells, exchangeabilities of both proteins were high and similar in rate. This seems to indicate that stress fibers are constantly exchanging their components for motile and other vital functions of these cells. The high exchangeabilities of both proteins in stress fibers showthat these fibers are clearly different from nonstriated, stress-fiber like structures of nascent myofibrils.

Functional specificity of actin isoforms

Actin, one of the main proteins of muscle and cytoskeleton, exists as a variety of highly conserved isoforms whose distribution in vertebrates is tissue-specific. Synthesis of specific actin isoforms is accompanied by their subcellular compartmentalization, with both processes being regulated by factors of cell proliferation and differentiation. Actin isoforms cannot substitute for each other, and the high-level synthesis of exogenous actins leads to alterations in cell organization and morphology. This indicates that the highly conserved actins are functionally specialized for the tissues in which they predominate. The first goal of this review is to analyze the data on the polymerizability of actin isoforms to show that cytoskeleton isoactins form less stable polymers than skeletal muscle actin. This difference correlates with the dynamics of actin microfilaments versus the stability of myofibrillar systems. The three-dimensional actin structure as well as progress in the analysis of conformational changes in both the actin monomer and the filament allows us to view the data on the structure and polymerization of isoactins in terms of structure-function relationships within the actin molecule. Most of the amino acid substitutions that distinguish actin isoforms are located apart from actin-actin contact sites in the polymer. We suggest that these substitutions can modulate the ability of actin monomers to form more or less stable polymers by long-range (allosteric) regulation of the contact sites.

Microfilament stabilization by jasplakinolide arrests oocyte maturation, cortical granule exocytosis, sperm incorporation cone resorption, and cell-cycle progression, but not DNA replication, during fertilization in mice

Jasplakinolide (JAS), which induces microfilament polymerization and stabilization, inhibits microfilament-mediated events in murine oocyte maturation and fertilization in a fashion unlike the effects of cytochalasin B (CCB) and latranculin A (LAT A). JAS prevents egg polar body emission at a much lower concentration than either CCB or LAT A. Microfilament bundles were detected on the entire egg cortex after JAS exposure. Conversely, microfilament patterns did not change after exposure to CCB, and few microfilaments were observed after exposure to LAT A. Eggs that were allowed to recover from JAS were unable to recover normal microfilament organization. During oocyte maturation, JAS prevented both spindle migration to the oocyte cortex and first polar body emission. During in vitro fertilization, sperm head entered the eggs and formed pronuclei, but sperm tail entry, pronuclear centration, and second polar body emission were not detected. DNA synthesis occurs in these JAS-treated zygotes. JAS inhibited not only the formation, but also the disassembly, of incorporation cones. JAS was also found to prevent cortical granule exocytosis following artificial activation, and cortical granules were still beneath the plasma membrane even after activation. Finally, incorporation of microinjected nonmuscle actin into the microfilament network of mice eggs was delayed by JAS. We conclude that JAS acts as a microfilament inhibitor during maturation and fertilization and is more powerful than other inhibitors. Its mechanism differs in that it promotes assembly and stabilization of microfilaments. JAS is a novel cell permeable tool for the investigation of microfilament-dependent events in early mammalian development.

Relation of nebulin and connectin (titin) to dynamics of actin in nascent myofibrils of cultured skeletal muscle cells

Cultured embryonic chicken skeletal muscle cells microinjected with rhodamine (rh)-labeled actin were stained with antibodies against nebulin and connectin (titin). In premyofibril areas, nebulin was observed as dotted structures, many of which were arranged in a linear fashion. These structures were associated with injected rh-actin. Among these linearly arranged dots of nebulin and rh-actin, numerous small nebulin dots without rh-actin incorporation were scattered. It is probable that the dots of nebulin and/or its associated protein(s) represent a preformed scaffold upon which actin monomers accumulate; exogenously introduced actin associates initially with small nebulin dots, which in turn coalesce to form rh-actin dots and are arranged linearly. In developing myofibrils, two patterns of nebulin distribution were found: "singlets" and "doublets." Recovery of rh-actin's fluorescence after photobleaching was slowest in the nonstriated dotted portions, followed by the striated myofibrillar portions with nebulin singlets and those with doublets, in that order. Thus, the distribution patterns of nebulin seem to be related to the accessibility/exchangeability of actin into nascent myofibrils. It is possible that early nebulin filaments exhibiting singlets are not tightly associated with actin filaments and that this loose association allows myofibrils to exchange nonadult isoforms of actin and other proteins into adult types. Connectin formed a striated pattern before the formation of rh-actin/nebulin striations. It appears that connectin does not have any significant role in the accessibility of actin into nascent myofibrils.

Defining actin filament length in striated muscle: rulers and caps or dynamic stability?

Actin filaments (thin filaments) are polymerized to strikingly uniform lengths in striated muscle sarcomeres. Yet, actin monomers can exchange dynamically into thin filaments in vivo, indicating that actin monomer association and dissociation at filament ends must be highly regulated to maintain the uniformity of filament lengths. We propose several hypothetical mechanisms that could generate uniform actin filament length distributions and discuss their application to the determination of thin filament length in vivo. At the Z line, titin may determine the minimum extent and tropomyosin the maximum extent of thin filament overlap by regulating alpha-actinin binding to actin, while a unique Z filament may bind to capZ and regulate barbed end capping. For the free portion of the thin filament, we evaluate possibilities that thin filament components (e.g. nebulin or the tropomyosin/troponin polymer) determine thin filament lengths by binding directly to tropomodulin and regulating pointed end capping, or alternatively, that myosin thick filaments, together with titin, determine filament length by indirectly regulating tropomodulin's capping activity.

Subcutaneous tissue fibroblasts transfected with muscle and nonmuscle actins: A good in vitro model to study fibroblastic cell plasticity

Cultured fibroblasts develop several biochemical and morphological properties of smooth muscle cells, particularly the expression of alpha-smooth muscle actin, the actin isoform typical of vascular smooth muscle cells. They resemble modified fibroblasts or myofibroblasts observed in granulation tissue during wound repair and in fibrotic situations. We have analysed by immunolabeling the fate of exogenous epitope-tagged actin isoforms by transfection of the corresponding cDNAs into fibroblasts cultured from rat subcutaneous tissue. Tagged muscle actins were efficiently integrated into stress fibers and did not produce obvious changes in cell shape of transfected cells. Transfected nonmuscle actins in contrast changed the morphology and were not or poorly incorporated into stress fibers. These cultured subcutaneous fibroblasts behave similarly to smooth muscle cells when transfected with the same actin encoding cDNAs, indicating another common characteristic of these two cell types in sorting and targeting actin isoforms. Subcutaneous fibroblasts transfected with muscle and nonmuscle actin isoforms provide a good in vitro model to analyze the intracellular sorting of isoactins and to improve our knowledge of myofibroblast characterization and differentiation during tissue repair as well as to understand the relationships between modifications of actin cytoskeleton, adhesion and extracellular matrix proteins.

Exchangeability of actin in cardiac myocytes and fibroblasts as determined by fluorescence photobleaching recovery

Rhodamine (Rho)-labeled muscle and non-muscle actins were microinjected into cultured embryonic chicken cardiac myocytes and fibroblasts. After incorporation of the fluorescent actin analog into cellular structures, small areas of labeled structures were photobleached with a laser pulse, and fluorescence recovery (FR) was measured to determine the exchangeability of isoactins in these structures. With both Rho-muscle and Rho-non-muscle actins, the FR rate in any part of stress fibers was consistently faster than that observed in any part of myofibrils. Thus, although non-striated (proximal and terminal) portions of nascent myofibrils are similar in appearance and composition to stress fibers, our data clearly revealed differences in actin stability between these two structures. Further, although cardiomyocytes were incapable of discriminating between the incorporation of muscle and non-muscle actin isoforms into myofibrils, FR after photobleaching of Rho-muscle actin was faster than that of Rho-non-muscle actin in immature non-striated portions. This indicates that actin molecules in cardiac myofibrils cannot be readily exchanged by heterotypic non-muscle actin. Fluorescently labeled actin incorporated into non-striated (proximal and terminal) portions of myofibrils and terminal portions of stress fibers was found to be more stable than alpha-actinin. The relative stability of actin could facilitate the formation of nascent Z-bands of myofibrils and the reorganization of stress fibers at these portions.