Kinetic signatures of myosin-5B, the motor involved in microvillus inclusion disease

Microvillus inclusion disease-causing MYO5B point mutations exert differential effects on motor function

Microvillus inclusion disease (MVID) is a rare congenital diarrheal disorder typically caused by loss of function mutations in the unconventional myosin, myosin 5b (MYO5B), which leads to the mistrafficking of apical components in enterocytes. MVID can manifest in two phenotypes: in both the intestine and liver or the liver alone. Although previous studies seeking to understand MVID disease pathology used MYO5B KO models, many patients have point mutations and thus express a dysfunctional MYO5B. How these point mutations lead to a broad spectrum of disease severity and the development of two distinct disease phenotypes is still not known. Here, we investigate the effect of MVID patient mutations on the function of the MYO5B motor domain, independent of cargo binding, using confocal imaging and fluorescence recovery after photobleaching. Patient mutations demonstrated a range of effects in these assays, from rigor-like behavior to loss of actin binding. Additionally, analysis of fluorescence recovery after photobleaching turnover kinetics suggests that some mutations negatively impact the ability of MYO5B to stay bound to actin. Collectively, our findings indicate that patient mutations affect the MYO5B motor domain in diverse ways, consistent with the spectrum of phenotypes observed in patients.

Motor properties of Myosin 5c are modulated by tropomyosin isoforms and inhibited by pentabromopseudilin

Myosin 5c (Myo5c) is a motor protein that is produced in epithelial and glandular tissues, where it plays an important role in secretory processes. Myo5c is composed of two heavy chains, each containing a generic motor domain, an elongated neck domain consisting of a single α-helix with six IQ motifs, each of which binds to a calmodulin (CaM) or a myosin light chain from the EF-hand protein family, a coiled-coil dimer-forming region and a carboxyl-terminal globular tail domain. Although Myo5c is a low duty cycle motor, when two or more Myo5c-heavy meromyosin (HMM) molecules are linked together, they move processively along actin filaments. We describe the purification and functional characterization of human Myo5c-HMM co-produced either with CaM alone or with CaM and the essential and regulatory light chains Myl6 and Myl12b. We describe the extent to which cofilaments of actin and Tpm1.6, Tpm1.8 or Tpm3.1 alter the maximum actin-activated ATPase and motile activity of the recombinant Myo5c constructs. The small allosteric effector pentabromopseudilin (PBP), which is predicted to bind in a groove close to the actin and nucleotide binding site with a calculated ΔG of -18.44 kcal/mol, inhibits the motor function of Myo5c with a half-maximal concentration of 280 nM. Using immunohistochemical staining, we determined the distribution and exact localization of Myo5c in endothelial and endocrine cells from rat and human tissue. Particular high levels of Myo5c were observed in insulin-producing β-cells located within the pancreatic islets of Langerhans.

Coherent light scattering from cellular dynamics in living tissues

This review examines the biological physics of intracellular transport probed by the coherent optics of dynamic light scattering from optically thick living tissues. Cells and their constituents are in constant motion, composed of a broad range of speeds spanning many orders of magnitude that reflect the wide array of functions and mechanisms that maintain cellular health. From the organelle scale of tens of nanometers and upward in size, the motion inside living tissue is actively driven rather than thermal, propelled by the hydrolysis of bioenergetic molecules and the forces of molecular motors. Active transport can mimic the random walks of thermal Brownian motion, but mean-squared displacements are far from thermal equilibrium and can display anomalous diffusion through Lévy or fractional Brownian walks. Despite the average isotropic three-dimensional environment of cells and tissues, active cellular or intracellular transport of single light-scattering objects is often pseudo-one-dimensional, for instance as organelle displacement persists along cytoskeletal tracks or as membranes displace along the normal to cell surfaces, albeit isotropically oriented in three dimensions. Coherent light scattering is a natural tool to characterize such tissue dynamics because persistent directed transport induces Doppler shifts in the scattered light. The many frequency-shifted partial waves from the complex and dynamic media interfere to produce dynamic speckle that reveals tissue-scale processes through speckle contrast imaging and fluctuation spectroscopy. Low-coherence interferometry, dynamic optical coherence tomography, diffusing-wave spectroscopy, diffuse-correlation spectroscopy, differential dynamic microscopy and digital holography offer coherent detection methods that shed light on intracellular processes. In health-care applications, altered states of cellular health and disease display altered cellular motions that imprint on the statistical fluctuations of the scattered light. For instance, the efficacy of medical therapeutics can be monitored by measuring the changes they induce in the Doppler spectra of livingex vivocancer biopsies.

The dynamics of actin protrusions can be controlled by tip-localized myosin motors

Class III myosins localize to inner ear hair cell stereocilia and are thought to be crucial for stereocilia length regulation. Mutations within the motor domain of MYO3A that disrupt its intrinsic motor properties have been associated with non-syndromic hearing loss, suggesting that the motor properties of MYO3A are critical for its function within stereocilia. In this study, we investigated the impact of a MYO3A hearing loss mutation, H442N, using both in vitro motor assays and cell biological studies. Our results demonstrate the mutation causes a dramatic increase in intrinsic motor properties, actin-activated ATPase and in vitro actin gliding velocity, as well as an increase in actin protrusion extension velocity. We propose that both "gain of function" and "loss of function" mutations in MYO3A can impair stereocilia length regulation, which is crucial for stereocilia formation during development and normal hearing. Furthermore, we generated chimeric MYO3A constructs that replace the MYO3A motor and neck domain with the motor and neck domain of other myosins. We found that duty ratio, fraction of ATPase cycle myosin is strongly bound to actin, is a critical motor property that dictates the ability to tip localize within filopodia. In addition, in vitro actin gliding velocities correlated extremely well with filopodial extension velocities over a wide range of gliding and extension velocities. Taken together, our data suggest a model in which tip-localized myosin motors exert force that slides the membrane tip-ward, which can combat membrane tension and enhance the actin polymerization rate that ultimately drives protrusion elongation.

A mutation in switch I alters the load-dependent kinetics of myosin Va

Myosin Va is the molecular motor that drives intracellular vesicular transport, powered by the transduction of chemical energy from ATP into mechanical work. The coupling of the powerstroke and phosphate (Pi) release is key to understanding the transduction process, and crucial details of this process remain unclear. Therefore, we determined the effect of elevated Pi on the force-generating capacity of a mini-ensemble of myosin Va S1 (WT) in a laser trap assay. By increasing the stiffness of the laser trap we determined the effect of increasing resistive loads on the rate of Pi-induced detachment from actin, and quantified this effect using the Bell approximation. We observed that WT myosin generated higher forces and larger displacements at the higher laser trap stiffnesses in the presence of 30 mM Pi, but binding event lifetimes decreased dramatically, which is most consistent with the powerstroke preceding the release of Pi from the active site. Repeating these experiments using a construct with a mutation in switch I of the active site (S217A) caused a seven-fold increase in the load-dependence of the Pi-induced detachment rate, suggesting that the S217A region of switch I may help mediate the load-dependence of Pi-rebinding.

Analysis of Plasmodium falciparum myosin B ATPase activity and structure in complex with the calmodulin-like domain of its light chain MLC-B

Myosin B (MyoB) is a class 14 myosin expressed in all invasive stages of the malaria parasite, Plasmodium falciparum. It is not associated with the glideosome complex that drives motility and invasion of host cells. During red blood cell invasion, MyoB remains at the apical tip of the merozoite but is no longer observed once invasion is completed. MyoB is not essential for parasite survival, but when it is knocked out, merozoites are delayed in the initial stages of red blood cell invasion, giving rise to a growth defect that correlates with reduced invasion success. Therefore, further characterization is needed to understand how MyoB contributes to parasite invasion. Here, we have expressed and purified functional MyoB with the help of parasite-specific chaperones Hsp90 and Unc45, characterized its binding to actin and its known light chain MLC-B using biochemical and biophysical methods and determined its low-resolution structure in solution using small angle X-ray scattering. In addition to MLC-B, we found that four other putative regulatory light chains bind to the MyoB IQ2 motif in vitro. The purified recombinant MyoB adopted the overall shape of a myosin, exhibited actin-activated ATPase activity, and moved actin filaments in vitro. Additionally, we determined that the ADP release rate was faster than the ATP turnover number, and thus, does not appear to be rate limiting. This, together with the observed high affinity to actin and the specific localization of MyoB, may point toward a role in tethering and/or force sensing during early stages of invasion.

Photorhabdus luminescens TccC3 Toxin Targets the Dynamic Population of F-Actin and Impairs Cell Cortex Integrity

Due to its essential role in cellular processes, actin is a common target for bacterial toxins. One such toxin, TccC3, is an effector domain of the ABC-toxin produced by entomopathogenic bacteria of Photorhabdus spp. Unlike other actin-targeting toxins, TccC3 uniquely ADP-ribosylates actin at Thr-148, resulting in the formation of actin aggregates and inhibition of phagocytosis. It has been shown that the fully modified F-actin is resistant to depolymerization by cofilin and gelsolin, but their effects on partially modified actin were not explored. We found that only F-actin unprotected by tropomyosin is the physiological TccC3 substrate. Yet, ADP-ribosylated G-actin can be produced upon cofilin-accelerated F-actin depolymerization, which was only mildly inhibited in partially modified actin. The affinity of TccC3-ADP-ribosylated G-actin for profilin and thymosin-β4 was weakened moderately but sufficiently to potentiate spontaneous polymerization in their presence. Interestingly, the Arp2/3-mediated nucleation was also potentiated by T148-ADP-ribosylation. Notably, even partially modified actin showed reduced bundling by plastins and α-actinin. In agreement with the role of these and other tandem calponin-homology domain actin organizers in the assembly of the cortical actin network, TccC3 induced intense membrane blebbing in cultured cells. Overall, our data suggest that TccC3 imposes a complex action on the cytoskeleton by affecting F-actin nucleation, recycling, and interaction with actin-binding proteins involved in the integration of actin filaments with each other and cellular elements.

Experimental Studies on the Impact of the Projected Ocean Acidification on Fish Survival, Health, Growth, and Meat Quality; Black Sea Bream (Acanthopagrus schlegelii), Physiological and Histological Studies

Simple Summary

This study’s data suggest that under the projected scenarios of ocean acidification by 2100 and beyond, significant negative impacts on growth, health, and meat quality are expected, particularly on black sea bream, and will be susceptible to the scientifically approved fish having a weaker resistance to diseases and environmental changes if CO₂ emissions in the atmosphere are not curbed. Knowing the expected consequences, mitigation measures are urgently needed.

Abstract

Acidification (OA), a global threat to the world’s oceans, is projected to significantly grow if CO₂ continues to be emitted into the atmosphere at high levels. This will result in a slight decrease in pH. Since the latter is a logarithmic scale of acidity, the higher acidic seawater is expected to have a tremendous impact on marine living resources in the long-term. An 8-week laboratory experiment was designed to assess the impact of the projected pH in 2100 and beyond on fish survival, health, growth, and fish meat quality. Two projected scenarios were simulated with the control treatment, in triplicates. The control treatment had a pH of 8.10, corresponding to a pCO₂ of 321.37 ± 11.48 µatm. The two projected scenarios, named Predict_A and Predict_B, had pH values of 7.80-pCO₂ = 749.12 ± 27.03 and 7.40-pCO₂ = 321.37 ± 11.48 µatm, respectively. The experiment was preceded by 2 weeks of acclimation. After the acclimation, 20 juvenile black sea breams (Acanthopagrus schlegelii) of 2.72 ± 0.01 g were used per tank. This species has been selected mainly due to its very high resistance to diseases and environmental changes, assuming that a weaker fish resistance will also be susceptibly affected. In all tanks, the fish were fed with the same commercial diet. The seawater’s physicochemical parameters were measured daily. Fish samples were subjected to physiological, histological, and biochemical analyses. Fish growth, feeding efficiency, protein efficiency ratio, and crude protein content were significantly decreased with a lower pH. Scanning electron microscopy revealed multiple atrophies of microvilli throughout the small intestine’s brush border in samples from Predict_A and Predict_B. This significantly reduced nutrient absorption, resulting in significantly lower feed efficiency, lower fish growth, and lower meat quality. As a result of an elevated pCO₂ in seawater, the fish eat more than normal but grow less than normal. Liver observation showed blood congestion, hemorrhage, necrosis, vacuolation of hepatocytes, and an increased number of Kupffer cells, which characterize liver damage. Transmission electron microscopy revealed an elongated and angular shape of the mitochondrion in the liver cell, with an abundance of peroxisomes, symptomatic of metabolic acidosis.

Roles and regulation of myosin V interaction with cargo

A major question in cell biology is, how are organelles and large macromolecular complexes transported within a cell? Myosin V molecular motors play critical roles in the distribution of organelles, vesicles, and mRNA. Mis-localization of organelles that depend on myosin V motors underlie diseases in the skin, gut, and brain. Thus, the delivery of organelles to their proper destination is important for animal physiology and cellular function. Cargoes attach to myosin V motors via cargo specific adaptor proteins, which transiently bridge motors to their cargoes. Regulation of these adaptor proteins play key roles in the regulation of cargo transport. Emerging studies reveal that cargo adaptors play additional essential roles in the activation of myosin V, and the regulation of actin filaments. Here, we review how motor-adaptor interactions are controlled to regulate the proper loading and unloading of cargoes, as well as roles of adaptor proteins in the regulation of myosin V activity and the dynamics of actin filaments.

Myosins and Disease

Myosins constitute a superfamily of actin-based molecular motor proteins that mediates a variety of cellular activities including muscle contraction, cell migration, intracellular transport, the formation of membrane projections, cell adhesion, and cell signaling. The 12 myosin classes that are expressed in humans share sequence similarities especially in the N-terminal motor domain; however, their enzymatic activities, regulation, ability to dimerize, binding partners, and cellular functions differ. It is becoming increasingly apparent that defects in myosins are associated with diseases including cardiomyopathies, colitis, glomerulosclerosis, neurological defects, cancer, blindness, and deafness. Here, we review the current state of knowledge regarding myosins and disease.

How Myosin 5 Walks Deduced from Single-Molecule Biophysical Approaches

Myosin 5a is a two-headed myosin that functions as a cargo transporter in cells. To accomplish this task it has evolved several unique structural and kinetic features that allow it to move processively as a single molecule along actin filaments. A plethora of biophysical techniques have been used to elucidate the detailed mechanism of its movement along actin filaments in vitro. This chapter describes how this mechanism was deduced.

Myosin V regulates synaptopodin clustering and localization in the dendrites of hippocampal neurons

The spine apparatus (SA) is an endoplasmic reticulum-related organelle that is present in a subset of dendritic spines in cortical and pyramidal neurons, and plays an important role in Ca²⁺ homeostasis and dendritic spine plasticity. The protein synaptopodin is essential for the formation of the SA and is widely used as a maker for this organelle. However, it is still unclear which factors contribute to its localization at selected synapses, and how it triggers local SA formation. In this study, we characterized development, localization and mobility of synaptopodin clusters in hippocampal primary neurons, as well as the molecular dynamics within these clusters. Interestingly, synaptopodin at the shaft-associated clusters is less dynamic than at spinous clusters. We identify the actin-based motor proteins myosin V (herein referring to both the myosin Va and Vb forms) and VI as novel interaction partners of synaptopodin, and demonstrate that myosin V is important for the formation and/or maintenance of the SA. We found no evidence of active microtubule-based transport of synaptopodin. Instead, new clusters emerge inside spines, which we interpret as the SA being assembled on-site.

F-actin patches associated with glutamatergic synapses control positioning of dendritic lysosomes

Organelle positioning within neurites is required for proper neuronal function. In dendrites, with their complex cytoskeletal organization, transport of organelles is guided by local specializations of the microtubule and actin cytoskeleton, and by coordinated activity of different motor proteins. Here, we focus on the actin cytoskeleton in the dendritic shaft and describe dense structures consisting of longitudinal and branched actin filaments. These actin patches are devoid of microtubules and are frequently located at the base of spines, or form an actin mesh around excitatory shaft synapses. Using lysosomes as an example, we demonstrate that the presence of actin patches has a strong impact on dendritic organelle transport, as lysosomes frequently stall at these locations. We provide mechanistic insights on this pausing behavior, demonstrating that actin patches form a physical barrier for kinesin-driven cargo. In addition, we identify myosin Va as an active tether which mediates long-term stalling. This correlation between the presence of actin meshes and halting of organelles could be a generalized principle by which synapses control organelle trafficking.

Dissecting myosin-5B mechanosensitivity and calcium regulation at the single molecule level

Myosin-5B is one of three members of the myosin-5 family of actin-based molecular motors. Despite its fundamental role in recycling endosome trafficking and in collective actin network dynamics, the molecular mechanisms underlying its motility are inherently unknown. Here we combine single-molecule imaging and high-speed laser tweezers to dissect the mechanoenzymatic properties of myosin-5B. We show that a single myosin-5B moves processively in 36-nm steps, stalls at ~2 pN resistive forces, and reverses its directionality at forces >2 pN. Interestingly, myosin-5B mechanosensitivity differs from that of myosin-5A, while it is strikingly similar to kinesin-1. In particular, myosin-5B run length is markedly and asymmetrically sensitive to force, a property that might be central to motor ensemble coordination. Furthermore, we show that Ca2+ does not affect the enzymatic activity of the motor unit, but abolishes myosin-5B processivity through calmodulin dissociation, providing important insights into the regulation of postsynaptic cargoes trafficking in neuronal cells.

Affimer proteins for F-actin: novel affinity reagents that label F-actin in live and fixed cells

Imaging the actin cytoskeleton in cells uses a wide range of approaches. Typically, a fluorescent derivative of the small cyclic peptide phalloidin is used to image F-actin in fixed cells. Lifeact and F-tractin are popular for imaging the cytoskeleton in live cells. Here we characterised novel affinity reagents called Affimers that specifically bind to F-actin in vitro to determine if they are suitable alternatives as eGFP-fusion proteins, to label actin in live cells, or for labeling F-actin in fixed cells. In vitro experiments showed that 3 out of the 4 Affimers (Affimers 6, 14 and 24) tested bind tightly to purified F-actin, and appear to have overlapping binding sites. As eGFP-fusion proteins, the same 3 Affimers label F-actin in live cells. FRAP experiments suggest that eGFP-Affimer 6 behaves most similarly to F-tractin and Lifeact. However, it does not colocalise with mCherry-actin in dynamic ruffles, and may preferentially bind stable actin filaments. All 4 Affimers label F-actin in methanol fixed cells, while only Affimer 14 labels F-actin after paraformaldehyde fixation. eGFP-Affimer 6 has potential for use in selectively imaging the stable actin cytoskeleton in live cells, while all 4 Affimers are strong alternatives to phalloidin for labelling F-actin in fixed cells.