Biochemical Activities of the Wiskott-Aldrich Syndrome Homology Region 2 Domains of Sarcomere Length Short (SALS) Protein

Peripheral thickening of the sarcomeres and pointed end elongation of the thin filaments are both promoted by SALS and its formin interaction partners

During striated muscle development the first periodically repeated units appear in the premyofibrils, consisting of immature sarcomeres that must undergo a substantial growth both in length and width, to reach their final size. Here we report that, beyond its well established role in sarcomere elongation, the Sarcomere length short (SALS) protein is involved in Z-disc formation and peripheral growth of the sarcomeres. Our protein localization data and loss-of-function studies in the Drosophila indirect flight muscle strongly suggest that radial growth of the sarcomeres is initiated at the Z-disc. As to thin filament elongation, we used a powerful nanoscopy approach to reveal that SALS is subject to a major conformational change during sarcomere development, which might be critical to stop pointed end elongation in the adult muscles. In addition, we demonstrate that the roles of SALS in sarcomere elongation and radial growth are both dependent on formin type of actin assembly factors. Unexpectedly, when SALS is present in excess amounts, it promotes the formation of actin aggregates highly resembling the ones described in nemaline myopathy patients. Collectively, these findings helped to shed light on the complex mechanisms of SALS during the coordinated elongation and thickening of the sarcomeres, and resulted in the discovery of a potential nemaline myopathy model, suitable for the identification of genetic and small molecule inhibitors.

Drosophila Tropomodulin is required for multiple actin-dependent processes within developing myofibers

Proper muscle contraction requires the assembly and maintenance of sarcomeres and myofibrils. Although the protein components of myofibrils are generally known, less is known about the mechanisms by which they individually function and together synergize for myofibril assembly and maintenance. For example, it is unclear how the disruption of actin filament (F-actin) regulatory proteins leads to the muscle weakness observed in myopathies. Here, we show that knockdown of Drosophila Tropomodulin (Tmod), results in several myopathy-related phenotypes, including reduction of muscle cell (myofiber) size, increased sarcomere length, disorganization and misorientation of myofibrils, ectopic F-actin accumulation, loss of tension-mediating proteins at the myotendinous junction, and misshaped and internalized nuclei. Our findings support and extend the tension-driven self-organizing myofibrillogenesis model. We show that, like its mammalian counterpart, Drosophila Tmod caps F-actin pointed-ends, and we propose that this activity is crucial for cellular processes in different locations within the myofiber that directly and indirectly contribute to the maintenance of muscle function. Our findings provide significant insights to the role of Tmod in muscle development, maintenance and disease.

The Mechanisms of Thin Filament Assembly and Length Regulation in Muscles

The actin containing tropomyosin and troponin decorated thin filaments form one of the crucial components of the contractile apparatus in muscles. The thin filaments are organized into densely packed lattices interdigitated with myosin-based thick filaments. The crossbridge interactions between these myofilaments drive muscle contraction, and the degree of myofilament overlap is a key factor of contractile force determination. As such, the optimal length of the thin filaments is critical for efficient activity, therefore, this parameter is precisely controlled according to the workload of a given muscle. Thin filament length is thought to be regulated by two major, but only partially understood mechanisms: it is set by (i) factors that mediate the assembly of filaments from monomers and catalyze their elongation, and (ii) by factors that specify their length and uniformity. Mutations affecting these factors can alter the length of thin filaments, and in human cases, many of them are linked to debilitating diseases such as nemaline myopathy and dilated cardiomyopathy.

Celebrating 20 years of live single-actin-filament studies with five golden rules

The precise assembly and disassembly of actin filaments is required for several cellular processes, and their regulation has been scrutinized for decades. Twenty years ago, a handful of studies marked the advent of a new type of experiment to study actin dynamics: using optical microscopy to look at individual events, taking place on individual filaments in real time. Here, we summarize the main characteristics of this approach and how it has changed our ability to understand actin assembly dynamics. We also highlight some of its caveats and reflect on what we have learned over the past 20 years, leading us to propose a set of guidelines, which we hope will contribute to a better exploitation of this powerful tool.

The C-terminal tail extension of myosin 16 acts as a molten globule, including intrinsically disordered regions, and interacts with the N-terminal ankyrin

The lesser-known unconventional myosin 16 protein is essential in proper neuronal functioning and has been implicated in cell cycle regulation. Its longer Myo16b isoform contains a C-terminal tail extension (Myo16Tail), which has been shown to play a role in the neuronal phosphoinositide 3-kinase signaling pathway. Myo16Tail mediates the actin cytoskeleton remodeling, downregulates the actin dynamics at the postsynaptic site of dendritic spines, and is involved in the organization of the presynaptic axon terminals. However, the functional and structural features of this C-terminal tail extension are not well known. Here, we report the purification and biophysical characterization of the Myo16Tail by bioinformatics, fluorescence spectroscopy, and CD. Our results revealed that the Myo16Tail is functionally active and interacts with the N-terminal ankyrin domain of myosin 16, suggesting an intramolecular binding between the C and N termini of Myo16 as an autoregulatory mechanism involving backfolding of the motor domain. In addition, the Myo16Tail possesses high structural flexibility and a solvent-exposed hydrophobic core, indicating the largely unstructured, intrinsically disordered nature of this protein region. Some secondary structure elements were also observed, indicating that the Myo16Tail likely adopts a molten globule-like structure. These structural features imply that the Myo16Tail may function as a flexible display site particularly relevant in post-translational modifications, regulatory functions such as backfolding, and phosphoinositide 3-kinase signaling.

The Activities of the Gelsolin Homology Domains of Flightless-I in Actin Dynamics

Flightless-I is a unique member of the gelsolin superfamily alloying six gelsolin homology domains and leucine-rich repeats. Flightless-I is an established regulator of the actin cytoskeleton, however, its biochemical activities in actin dynamics are still largely elusive. To better understand the biological functioning of Flightless-I we studied the actin activities of Drosophila Flightless-I by in vitro bulk fluorescence spectroscopy and single filament fluorescence microscopy, as well as in vivo genetic approaches. Flightless-I was found to interact with actin and affects actin dynamics in a calcium-independent fashion in vitro. Our work identifies the first three gelsolin homology domains (1–3) of Flightless-I as the main actin-binding site; neither the other three gelsolin homology domains (4–6) nor the leucine-rich repeats bind actin. Flightless-I inhibits polymerization by high-affinity (∼nM) filament barbed end capping, moderately facilitates nucleation by low-affinity (∼μM) monomer binding, and does not sever actin filaments. Our work reveals that in the presence of profilin Flightless-I is only able to cap actin filament barbed ends but fails to promote actin assembly. In line with the in vitro data, while gelsolin homology domains 4–6 have no effect on in vivo actin polymerization, overexpression of gelsolin homology domains 1–3 prevents the formation of various types of actin cables in the developing Drosophila egg chambers. We also show that the gelsolin homology domains 4–6 of Flightless-I interact with the C-terminus of Drosophila Disheveled-associated activator of morphogenesis formin and negatively regulates its actin assembly activity.

Myosin IIA and formin dependent mechanosensitivity of filopodia adhesion

Filopodia, dynamic membrane protrusions driven by polymerization of an actin filament core, can adhere to the extracellular matrix and experience both external and cell-generated pulling forces. The role of such forces in filopodia adhesion is however insufficiently understood. Here, we study filopodia induced by overexpression of myosin X, typical for cancer cells. The lifetime of such filopodia positively correlates with the presence of myosin IIA filaments at the filopodia bases. Application of pulling forces to the filopodia tips through attached fibronectin-coated laser-trapped beads results in sustained growth of the filopodia. Pharmacological inhibition or knockdown of myosin IIA abolishes the filopodia adhesion to the beads. Formin inhibitor SMIFH2, which causes detachment of actin filaments from formin molecules, produces similar effect. Thus, centripetal force generated by myosin IIA filaments at the base of filopodium and transmitted to the tip through actin core in a formin-dependent fashion is required for filopodia adhesion.

Tropomyosins Regulate the Severing Activity of Gelsolin in Isoform-Dependent and Independent Manners

The actin cytoskeleton fulfills numerous key cellular functions, which are tightly regulated in activity, localization, and temporal patterning by actin binding proteins. Tropomyosins and gelsolin are two such filament-regulating proteins. Here, we investigate how the effects of tropomyosins are coupled to the binding and activity of gelsolin. We show that the three investigated tropomyosin isoforms (Tpm1.1, Tpm1.12, and Tpm3.1) bind to gelsolin with micromolar or submicromolar affinities. Tropomyosin binding enhances the activity of gelsolin in actin polymerization and depolymerization assays. However, the effects of the three tropomyosin isoforms varied. The tropomyosin isoforms studied also differed in their ability to protect pre-existing actin filaments from severing by gelsolin. Based on the observed specificity of the interactions between tropomyosins, actin filaments, and gelsolin, we propose that tropomyosin isoforms specify which populations of actin filaments should be targeted by, or protected from, gelsolin-mediated depolymerization in living cells.

Modulation of formin processivity by profilin and mechanical tension

Formins are major regulators of actin networks. They enhance actin filament dynamics by remaining processively bound to filament barbed ends. How biochemical and mechanical factors affect formin processivity are open questions. Monitoring individual actin filaments in a microfluidic flow, we report that formins mDia1 and mDia2 dissociate faster under higher ionic strength and when actin concentration is increased. Profilin, known to increase the elongation rate of formin-associated filaments, surprisingly decreases the formin dissociation rate, by bringing formin FH1 domains in transient contact with the barbed end. In contrast, piconewton tensile forces applied to actin filaments accelerate formin dissociation by orders of magnitude, largely overcoming profilin-mediated stabilization. We developed a model of formin conformations showing that our data indicates the existence of two different dissociation pathways, with force favoring one over the other. How cells limit formin dissociation under tension is now a key question for future studies.

Cardiac leiomodin2 binds to the sides of actin filaments and regulates the ATPase activity of myosin

Leiomodin proteins are vertebrate homologues of tropomodulin, having a role in the assembly and maintenance of muscle thin filaments. Leiomodin2 contains an N-terminal tropomodulin homolog fragment including tropomyosin-, and actin-binding sites, and a C-terminal Wiskott-Aldrich syndrome homology 2 actin-binding domain. The cardiac leiomodin2 isoform associates to the pointed end of actin filaments, where it supports the lengthening of thin filaments and competes with tropomodulin. It was recently found that cardiac leiomodin2 can localise also along the length of sarcomeric actin filaments. While the activities of leiomodin2 related to pointed end binding are relatively well described, the potential side binding activity and its functional consequences are less well understood. To better understand the biological functions of leiomodin2, in the present work we analysed the structural features and the activities of Rattus norvegicus cardiac leiomodin2 in actin dynamics by spectroscopic and high-speed sedimentation approaches. By monitoring the fluorescence parameters of leiomodin2 tryptophan residues we found that it possesses flexible, intrinsically disordered regions. Leiomodin2 accelerates the polymerisation of actin in an ionic strength dependent manner, which relies on its N-terminal regions. Importantly, we demonstrate that leiomodin2 binds to the sides of actin filaments and induces structural alterations in actin filaments. Upon its interaction with the filaments leiomodin2 decreases the actin-activated Mg²⁺-ATPase activity of skeletal muscle myosin. These observations suggest that through its binding to side of actin filaments and its effect on myosin activity leiomodin2 has more functions in muscle cells than it was indicated in previous studies.

Covalent and non-covalent chemical engineering of actin for biotechnological applications

The cytoskeletal filaments are self-assembled protein polymers with 8-25nm diameters and up to several tens of micrometres length. They have a range of pivotal roles in eukaryotic cells, including transportation of intracellular cargoes (primarily microtubules with dynein and kinesin motors) and cell motility (primarily actin and myosin) where muscle contraction is one example. For two decades, the cytoskeletal filaments and their associated motor systems have been explored for nanotechnological applications including miniaturized sensor systems and lab-on-a-chip devices. Several developments have also revolved around possible exploitation of the filaments alone without their motor partners. Efforts to use the cytoskeletal filaments for applications often require chemical or genetic engineering of the filaments such as specific conjugation with fluorophores, antibodies, oligonucleotides or various macromolecular complexes e.g. nanoparticles. Similar conjugation methods are also instrumental for a range of fundamental biophysical studies. Here we review methods for non-covalent and covalent chemical modifications of actin filaments with focus on critical advantages and challenges of different methods as well as critical steps in the conjugation procedures. We also review potential uses of the engineered actin filaments in nanotechnological applications and in some key fundamental studies of actin and myosin function. Finally, we consider possible future lines of investigation that may be addressed by applying chemical conjugation of actin in new ways.

The activities of the C-terminal regions of the formin protein disheveled-associated activator of morphogenesis (DAAM) in actin dynamics

Disheveled-associated activator of morphogenesis (DAAM) is a diaphanous-related formin protein essential for the regulation of actin cytoskeleton dynamics in diverse biological processes. The conserved formin homology 1 and 2 (FH1-FH2) domains of DAAM catalyze actin nucleation and processively mediate filament elongation. These activities are indirectly regulated by the N- and C-terminal regions flanking the FH1-FH2 domains. Recently, the C-terminal diaphanous-autoregulatory domain (DAD) and the C terminus (CT) of formins have also been shown to regulate actin assembly by directly interacting with actin. Here, to better understand the biological activities of DAAM, we studied the role of DAD-CT regions of Drosophila DAAM in its interaction with actin with in vitro biochemical and in vivo genetic approaches. We found that the DAD-CT region binds actin in vitro and that its main actin-binding element is the CT region, which does not influence actin dynamics on its own. However, we also found that it can tune the nucleating activity and the filament end-interaction properties of DAAM in an FH2 domain-dependent manner. We also demonstrate that DAD-CT makes the FH2 domain more efficient in antagonizing with capping protein. Consistently, in vivo data suggested that the CT region contributes to DAAM-mediated filopodia formation and dynamics in primary neurons. In conclusion, our results demonstrate that the CT region of DAAM plays an important role in actin assembly regulation in a biological context.

The formin DAAM is required for coordination of the actin and microtubule cytoskeleton in axonal growth cones

Directed axonal growth depends on correct coordination of the actin and microtubule cytoskeleton in the growth cone. However, despite the relatively large number of proteins implicated in actin-microtubule crosstalk, the mechanisms whereby actin polymerization is coupled to microtubule stabilization and advancement in the peripheral growth cone remained largely unclear. Here, we identified the formin Dishevelled-associated activator of morphogenesis (DAAM) as a novel factor playing a role in concerted regulation of actin and microtubule remodeling in Drosophilamelanogaster primary neurons. In vitro, DAAM binds to F-actin as well as to microtubules and has the ability to crosslink the two filament systems. Accordingly, DAAM associates with the neuronal cytoskeleton, and a significant fraction of DAAM accumulates at places where the actin filaments overlap with that of microtubules. Loss of DAAM affects growth cone and microtubule morphology, and several aspects of microtubule dynamics; and biochemical and cellular assays revealed a microtubule stabilization activity and binding to the microtubule tip protein EB1. Together, these data suggest that, besides operating as an actin assembly factor, DAAM is involved in linking actin remodeling in filopodia to microtubule stabilization during axonal growth.

Polarization sensitive localization based super-resolution microscopy with a birefringent wedge

A practical method has been presented for polarization sensitive localization based super-resolution microscopy using a birefringent dual wedge. The measurement of the polarization degree at the single molecule level can reveal the chemical and physical properties of the local environment of the fluorescent dye molecule and can hence provide information about the sub-diffraction sized structure of biological samples. Polarization sensitive STORM imaging of the F-Actins proved correlation between the orientation of fluorescent dipoles and the axis of the fibril.

Intrinsic, Functional, and Structural Properties of β-Thymosins and β-Thymosin/WH2 Domains in the Regulation and Coordination of Actin Self-Assembly Dynamics and Cytoskeleton Remodeling

β-Thymosins are a family of heat-stable multifunctional polypeptides that are expressed as small proteins of about 5kDa (~45 amino acids) almost exclusively in multicellular animals. They were first isolated from the thymus. As full-length or truncated polypeptides, they appear to stimulate a broad range of extracellular activities in various signaling pathways, including tissue repair and regeneration, inflammation, cell migration, and immune defense. However, their cell surface receptors and structural mechanisms of regulations in these multiple pathways remain still poorly understood. Besides their extracellular activities, they belong to a larger family of small, intrinsically disordered actin-binding domains called WH2/β-thymosin domains that have been identified in more than 1800 multidomain proteins found in different taxonomic domains of life and involved in various actin-based motile processes including cell morphogenesis, motility, adhesions, tissue development, intracellular trafficking, or pathogen infections. This review briefly surveys the main recent findings to understand how these small, intrinsically disordered but functional domains can interact with many unrelated partners and can thus integrate and coordinate various intracellular activities in actin self-assembly dynamics and cell signaling pathways linked to their cytoskeleton remodeling.