Myosins: tails (and heads) of functional diversity

Identification, modeling, and characterization studies of Tetrahymena thermophila myosin FERM domains suggests a conserved core fold but functional differences

Myosins (MYO) define a superfamily of motor proteins which facilitate movement along cytoskeletal actin filaments in an ATP-dependent manner. To date, over 30 classes of myosin have been defined that vary in their roles and distribution across different taxa. The multidomain tail of myosin is responsible for the observed functional differences in different myosin classes facilitating differential binding to different cargos. One domain found in this region, the FERM domain, is found in several diverse proteins and is involved in many biological functions ranging from cell adhesion and actin-driven cytoskeleton assembly to cell signaling. Recently, new classes of unconventional myosin have been identified in Tetrahymena thermophila. In this study, we have identified, modeled, and characterized eight FERM domains from the unconventional T. thermophila myosins as their complete functional MyTH4-FERM cassettes. Our results reveal notable sequence, structural, and electrostatic differences between T. thermophila and other characterized FERM domains. Specifically, T. thermophila FERM domains contain helical inserts or extensions, which contribute to significant differences in surface electrostatic profiles of T. thermophila myosin FERMs when compared to the conventional FERM domains. Analyses of the modeled domains reveal differences in key functional residues as well as phosphoinositide-binding signatures and affinities. The work presented here broadens the scope of our understanding of myosin classes and their inherent functions, and provides a platform for experimentalists to design rational experimental studies to test the functional roles for T. thermophila myosins.

Myosin Id is required for planar cell polarity in ciliated tracheal and ependymal epithelial cells

In wild type (WT) tracheal epithelial cells, ciliary basal bodies are oriented such that all cilia on the cell surface beat in the same upward direction. This precise alignment of basal bodies and, as a result, the ciliary axoneme, is termed rotational planar cell polarity (PCP). Rotational PCP in the multi-ciliated epithelial cells of the trachea is perturbed in rats lacking myosin Id (Myo1d). Myo1d is localized in the F-actin and basal body rich subapical cortex of the ciliated tracheal epithelial cell. Scanning and transmission electron microscopy of Myo1d knock out (KO) trachea revealed that the unidirectional bending pattern is disrupted. Instead, cilia splay out in a disordered, often radial pattern. Measurement of the alignment axis of the central pair axonemal microtubules was much more variable in the KO, another indicator that rotational PCP is perturbed. The asymmetric localization of the PCP core protein Vangl1 is lost. Both the velocity and linearity of cilia-driven movement of beads above the tracheal mucosal surface was impaired in the Myo1d KO. Multi-ciliated brain ependymal epithelial cells exhibit a second form of PCP termed translational PCP in which basal bodies and attached cilia are clustered at the anterior side of the cell. The precise asymmetric clustering of cilia is disrupted in the ependymal cells of the Myo1d KO rat. While basal body clustering is maintained, left-right positioning of the clusters is lost.

Identification and Clinical Implications of Novel MYO15A Mutations in a Non-consanguineous Korean Family by Targeted Exome Sequencing

Mutations of MYO15A are generally known to cause severe to profound hearing loss throughout all frequencies. Here, we found two novel MYO15A mutations, c.3871C>T (p.L1291F) and c.5835T>G (p.Y1945X) in an affected individual carrying congenital profound sensorineural hearing loss (SNHL) through targeted resequencing of 134 known deafness genes. The variant, p.L1291F and p.Y1945X, resided in the myosin motor and IQ2 domains, respectively. The p.L1291F variant was predicted to affect the structure of the actin-binding site from three-dimensional protein modeling, thereby interfering with the correct interaction between actin and myosin. From the literature analysis, mutations in the N-terminal domain were more frequently associated with residual hearing at low frequencies than mutations in the other regions of this gene. Therefore we suggest a hypothetical genotype-phenotype correlation whereby MYO15A mutations that affect domains other than the N-terminal domain, lead to profound SNHL throughout all frequencies and mutations that affect the N-terminal domain, result in residual hearing at low frequencies. This genotype-phenotype correlation suggests that preservation of residual hearing during auditory rehabilitation like cochlear implantation should be intended for those who carry mutations in the N-terminal domain and that individuals with mutations elsewhere in MYO15A require early cochlear implantation to timely initiate speech development.

Arabidopsis ACT11 modifies actin turnover to promote pollen germination and maintain the normal rate of tube growth

Actin is an ancient conserved protein that is encoded by multiple isovariants in multicellular organisms. There are eight functional actin genes in the Arabidopsis genome, and the precise function and mechanism of action of each isovariant remain poorly understood. Here, we report the characterization of ACT11, a reproductive actin isovariant. Our studies reveal that loss of function of ACT11 causes a delay in pollen germination, but enhances pollen tube growth. Cytological analysis revealed that the amount of filamentous actin decreased, and the rate of actin turnover increased in act11 pollen. Convergence of actin filaments upon the germination aperture was impaired in act11 pollen, consistent with the observed delay of germination. Reduction of actin dynamics with jasplakinolide suppressed the germination and tube growth phenotypes in act11 pollen, suggesting that the underlying mechanisms involve an increase in actin dynamics. Thus, we demonstrate that ACT11 is required to maintain the rate of actin turnover in order to promote pollen germination and maintain the normal rate of pollen tube growth.

Down-regulated MYH11 expression correlates with poor prognosis in stage II and III colorectal cancer

The MYH11 gene may be related to cell migration and adhesion, intracellular transport, and signal transduction. However, its relationship with prognosis is still uncertain. The aim of this study was to investigate correlations between MYH11 gene expression and prognosis in 58 patients with stage II and III colorectal cancer. Quantitative real-time polymerase chain reaction was performed in fresh CRC tissues to examine mRNA expression, and immunohistochemistry was performed with paraffin-embedded specimens for protein expression. On univariate analysis, MYH11 expression at both mRNA and protein levels, perineural invasion and lymphovascular invasion were related to disease-free survival (p<0.05; log-rank test). Cancers with lower MYH11 expression were more likely to have a poor prognosis. Otherwise, MYH11 expression was unrelated to patient clinicopathological features. On multivariate analysis, low MYH11 expression proved to be an independent adverse prognosticator (p<0.05). These findings show that MYH11 can contribute to predicting prognosis in stage II and III colorectal cancers.

Bowel "dissection" in microvillus inclusion disease

A preterm male neonate was diagnosed as having microvillus inclusion disease based on the characteristics of histological and ultrastructural findings. The peripheral blood sample also revealed MYO5B mutation. He had been on long-term parenteral nutrition. However, a bowel segment was seen in the baby's diaper during hospitalization when he was 5 months old. Serial abdominal ultrasound demonstrated progressive dissection of the bowel wall with detached mucosa in real-time. Small intestinal epithelia were seen on the histology of the detached bowel segment. He died 2 weeks after the episode; postmortem autopsy showed diffuse detachment of mucosa of small bowels without perforation. This is the first report of an infant with microvillus inclusion disease that presented with bowel "dissection". Weakened adhesion and integrity of intestinal epithelial cells caused by MYO5B mutation was speculated to result in the dissection and detachment of the epithelia of the gastrointestinal tract.

Invertebrate and vertebrate class III myosins interact with MORN repeat-containing adaptor proteins

In Drosophila photoreceptors, the NINAC-encoded myosin III is found in a complex with a small, MORN-repeat containing, protein Retinophilin (RTP). Expression of these two proteins in other cell types showed NINAC myosin III behavior is altered by RTP. NINAC deletion constructs were used to map the RTP binding site within the proximal tail domain of NINAC. In vertebrates, the RTP ortholog is MORN4. Co-precipitation experiments demonstrated that human MORN4 binds to human myosin IIIA (MYO3A). In COS7 cells, MORN4 and MYO3A, but not MORN4 and MYO3B, co-localize to actin rich filopodia extensions. Deletion analysis mapped the MORN4 binding to the proximal region of the MYO3A tail domain. MYO3A dependent MORN4 tip localization suggests that MYO3A functions as a motor that transports MORN4 to the filopodia tips and MORN4 may enhance MYO3A tip localization by tethering it to the plasma membrane at the protrusion tips. These results establish conserved features of the RTP/MORN4 family: they bind within the tail domain of myosin IIIs to control their behavior.

Mutations in the MYO15A Gene Are a Significant Cause of Nonsyndromic Hearing Loss

Objectives:

Screening for MYO15A mutations was carried out using a large cohort to clarify the frequency and clinical characteristics of patients with MYO15A (DFNB3) mutations in a hearing loss population.

Methods:

Genetic analysis of 63 previously reported deafness genes based on massively parallel DNA sequencing (MPS) in 1120 Japanese hearing loss patients from 53 otorhinolaryngology departments was performed. Detailed clinical features of the patients with MYO15A mutations were then collected and analyzed.

Results:

Eleven patients from 10 families were found to have compound heterozygosity for MYO15A. Audiograms showed profound or high frequency hearing loss, with some patients showing progressive hearing loss. Age at onset was found to vary from 0 to 14 years, which seemed to be associated with the mutation. Four children underwent bilateral cochlear implantation for congenital hearing loss, with all showing good results.

Conclusion:

Mutations in the MYO15A gene are a notable cause of nonsyndromic hearing loss. MPS technology successfully detected mutations in relatively rare deafness genes such as MYO15A.

Class I myosins have overlapping and specialized functions in left-right asymmetric development in Drosophila

The class I myosin genes are conserved in diverse organisms, and their gene products are involved in actin dynamics, endocytosis, and signal transduction. Drosophila melanogaster has three class I myosin genes, Myosin 31DF (Myo31DF), Myosin 61F (Myo61F), and Myosin 95E (Myo95E). Myo31DF, Myo61F, and Myo95E belong to the Myosin ID, Myosin IC, and Myosin IB families, respectively. Previous loss-of-function analyses of Myo31DF and Myo61F revealed important roles in left-right (LR) asymmetric development and enterocyte maintenance, respectively. However, it was difficult to elucidate their roles in vivo, because of potential redundant activities. Here we generated class I myosin double and triple mutants to address this issue. We found that the triple mutant was viable and fertile, indicating that all three class I myosins were dispensable for survival. A loss-of-function analysis revealed further that Myo31DF and Myo61F, but not Myo95E, had redundant functions in promoting the dextral LR asymmetric development of the male genitalia. Myo61F overexpression is known to antagonize the dextral activity of Myo31DF in various Drosophila organs. Thus, the LR-reversing activity of overexpressed Myo61F may not reflect its physiological function. The endogenous activity of Myo61F in promoting dextral LR asymmetric development was observed in the male genitalia, but not the embryonic gut, another LR asymmetric organ. Thus, Myo61F and Myo31DF, but not Myo95E, play tissue-specific, redundant roles in LR asymmetric development. Our studies also revealed differential colocalization of the class I myosins with filamentous (F)-actin in the brush border of intestinal enterocytes.

Trafficking of excitatory amino acid transporter 2-laden vesicles in cultured astrocytes: a comparison between approximate and exact determination of trajectory angles

A clear consensus concerning the mechanisms of intracellular secretory vesicle trafficking in astrocytes is lacking in the physiological literature. A good characterization of vesicle trafficking that may assist researchers in achieving that goal is the trajectory angle, defined as the angle between the trajectory of a vesicle and a line radial to the cell's nucleus. In this study, we provide a precise definition of the trajectory angle, describe and compare two methods for its calculation in terms of measureable trafficking parameters, and give recommendations for the appropriate use of each method. We investigated the trafficking of vesicles containing excitatory amino acid transporter 2 (EAAT2) fluorescently tagged with enhanced green fluorescent protein (EGFP) to quantify and validate the precision of each method. The motion of fluorescent puncta--taken to represent vesicles containing EAAT2-EGFP--was found to be typical of secretory vesicle trafficking. An exact method for calculating the trajectory angle of these puncta produced no error but required large computation time. An approximate method reduced the requisite computation time but produced an error depending on the inverse of the ratio of the punctum's initial distance from the nucleus centroid to its maximal displacement. Fitting this dependence to a power function allowed us to establish an exclusion distance from the centroid, beyond which the approximate method is less likely to produce an error above an acceptable 5%. We recommend that the exact method be used to calculate the trajectory angle for puncta closer to the nucleus centroid than this exclusion distance.

A homozygous loss-of-function variant in MYH11 in a case with megacystis-microcolon-intestinal hypoperistalsis syndrome

Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS) is characterized by marked dilatation of the bladder and microcolon and decreased intestinal peristalsis. Recent studies indicate that heterozygous variants in ACTG2, which codes for a smooth muscle actin, cause MMIHS. However, such variants do not explain MMIHS cases that show an autosomal recessive mode of inheritance. We performed exome sequencing in a newborn with MMIHS and prune belly phenotype whose parents are consanguineous and identified a homozygous variant (c.3598A>T: p.Lys1200Ter) in MYH11, which codes for the smooth muscle myosin heavy chain. Previous studies showed that loss of Myh11 function in mice causes a bladder and intestinal phenotype that is highly reminiscent of MMIHS. All together, these observations strongly suggest that loss-of-function variants in MYH11 cause MMIHS. The documentation of variants in ACTG2 and MYH11 thus points to the involvement of the contractile apparatus of the smooth muscle in MMIHS. Interestingly, dominant-negative variants in MYH11 have previously been shown to cause thoracic aortic aneurism and dilatation. Different mechanisms of MYH11 disruption may thus lead to distinct patterns of smooth muscle dysfunction.

Block the function of nonmuscle myosin II by blebbistatin induces zebrafish embryo cardia bifida

Nonmuscle myosin II (NM II) is the name given to the multi-subunit protein product of three genes encoding different nonmuscle myosin heavy chains including NM II-A, NM II-B, and NM II-C. Blebbistatin is a small molecule that has been shown to be a relatively specific inhibitor of NM II. Blocking the function of NM II by blebbistatin induces zebrafish embryo cardia bifida at a dose-dependent manner. In situ hybridization analysis with ventricular marker ventricular myosin heavy chain (vmhc) and atrial marker atrial myosin heavy chain (amhc) showed each of the heart contained both distinct atria and ventricle. However, the cardia bifida embryos had highly variable distance between two separate ventricles. We also provided evidence that time window from 12 to 20 h post fertilization (hpf) is necessary and sufficient for cardia bifida formation caused by blebbistatin treatment. Expression of spinster homolog 2 (spns2) was decreased in blebbistatin-treated embryos, suggesting the cardia bifida phenotype caused by NM II inhibition was relevant to precardiac mesoderm migration defects. Through in situ hybridization analysis, we showed that foxa1 was expressed in endoderm of blebbistatin-treated embryos at 24-hpf stage, suggesting the endoderm formation is normal in cardia bifida embryos caused by blebbistatin treatment. In addition, we demonstrated that blebbistatin treatment resulted in morphology alteration of zebrafish cardiomyocytes in vivo and neonatal mouse cardiomyocytes in vitro.

Class I myosin Myo1e regulates TLR4-triggered macrophage spreading, chemokine release, and antigen presentation via MHC class II

TLR-mediated recognition of microbial danger induces substantial changes in macrophage migration, adherence, and phagocytosis. Recently, we described the LPS-regulated phosphorylation of many cytoskeleton-associated proteins by phosphoproteomics. The functional role of these cytoskeletal and motor proteins in innate immune cell responses is largely unexplored. Here, we first identified both long-tailed class I myosins Myo1e and Myo1f as important contributors to LPS-triggered macrophage spreading. Mouse bone marrow-derived macrophages and DCs deficient in Myo1e selectively secreted increased amounts of the chemokine CCL2. In addition, the cell surface expression of MHC class II (MHC-II) on both cell types was reduced in the absence of Myo1e. However, transcriptional changes in CCL2 and MHC-II were not observed in the absence of Myo1e, indicating that Myo1e regulates specific intracellular transport processes. The capacity of macrophages and DCs lacking Myo1e to stimulate antigen-specific CD4(+) T-cell proliferation was impaired, consistent with the reduced MHC-II surface protein levels. Surprisingly, in Myo1e-deficient DCs, the proteolytic cleavage of endocytosed antigen was also increased. Together, our results provide evidence for a non-redundant function of the motor protein Myo1e in the regulation of TLR4-controlled, cytoskeleton-associated functional properties of macrophages and DCs, and in induction of a full MHC-II-restricted adaptive immune response.

Cargo recognition and cargo-mediated regulation of unconventional myosins

Organized motions are hallmarks of living organisms. Such motions range from collective cell movements during development and muscle contractions at the macroscopic scale all the way down to cellular cargo (e.g., various biomolecules and organelles) transportation and mechanoforce sensing at more microscopic scales. Energy required for these biological motions is almost invariably provided by cellular chemical fuels in the form of nucleotide triphosphate. Biological systems have designed a group of nanoscale engines, known as molecular motors, to convert cellular chemical fuels into mechanical energy. Molecular motors come in various forms including cytoskeleton motors (myosin, kinesin, and dynein), nucleic-acid-based motors, cellular membrane-based rotary motors, and so on. The main focus of this Account is one subfamily of actin filament-based motors called unconventional myosins (other than muscle myosin II, the remaining myosins are collectively referred to as unconventional myosins). In general, myosins can use ATP to fuel two types of mechanomotions: dynamic tethering actin filaments with various cellular compartments or structures and actin filament-based intracellular transport. In contrast to rich knowledge accumulated over many decades on ATP hydrolyzing motor heads and their interactions with actin filaments, how various myosins recognize their specific cargoes and whether and how cargoes can in return regulate functions of motors are less understood. Nonetheless, a series of biochemical and structural investigations in the past few years, including works from our own laboratory, begin to shed lights on these latter questions. Some myosins (e.g., myosin-VI) can function both as cellular transporters and as mechanical tethers. To function as a processive transporter, myosins need to form dimers or multimers. To be a mechanical tether, a monomeric myosin is sufficient. It has been shown for myosin-VI that its cellular cargo proteins can play critical roles in determining the motor properties. Dab2, an adaptor protein linking endocytic vesicles with actin-filament-bound myosin-VI, can induce the motor to form a transport competent dimer. Such a cargo-mediated dimerization mechanism has also been observed in other myosins including myosin-V and myosin-VIIa. The tail domains of myosins are very diverse both in their lengths and protein domain compositions and thus enable motors to engage a broad range of different cellular cargoes. Remarkably, the cargo binding tail of one myosin alone often can bind to multiple distinct target proteins. A series of atomic structures of myosin-V/cargo complexes solved recently reveals that the globular cargo binding tail of the motor contains a number of nonoverlapping target recognition sites for binding to its cargoes including melanophilin, vesicle adaptors RILPL2, and vesicle-bound GTPase Rab11. The structures of the MyTH4-FERM tandems from myosin-VIIa and myosin-X in complex with their respective targets reveal that MyTH4 and FERM domains extensively interact with each other forming structural and functional supramodules in both motors and demonstrate that the structurally similar MyTH4-FERM tandems of the two motors display totally different target binding modes. These structural studies have also shed light on why numerous mutations found in these myosins can cause devastating human diseases such as deafness and blindness, intellectual disabilities, immune disorders, and diabetes.

Myosin VIIA regulates microvillus morphogenesis and interacts with cadherin Cad99C in Drosophila oogenesis

Microvilli and related actin-based protrusions permit multiple interactions between cells and their environment. How the shape, length and arrangement of microvilli are determined remains largely unclear. To address this issue and explore the cooperation of the two main components of a microvillus, the central F-actin bundle and the enveloping plasma membrane, we investigated the expression and function of Myosin VIIA (Myo7A), which is encoded by crinkled (ck), and its interaction with cadherin Cad99C in the microvilli of the Drosophila follicular epithelium. Myo7A is present in the microvilli and terminal web of follicle cells, and associates with several other F-actin-rich structures in the ovary. Loss of Myo7A caused brush border defects and a reduction in the amount of the microvillus regulator Cad99C. We show that Myo7A and Cad99C form a molecular complex and that the cytoplasmic tail of Cad99C recruits Myo7A to microvilli. Our data indicate that Myo7A regulates the structure and spacing of microvilli, and interacts with Cad99C in vivo. A comparison of the mutant phenotypes suggests that Myo7A and Cad99C have co-dependent and independent functions in microvilli.

Diversity and convergence in the mechanisms establishing L/R asymmetry in metazoa

Differentiating left and right hand sides during embryogenesis represents a major event in body patterning. Left-Right (L/R) asymmetry in bilateria is essential for handed positioning, morphogenesis and ultimately the function of organs (including the brain), with defective L/R asymmetry leading to severe pathologies in human. How and when symmetry is initially broken during embryogenesis remains debated and is a major focus in the field. Work done over the past 20 years, in both vertebrate and invertebrate models, has revealed a number of distinct pathways and mechanisms important for establishing L/R asymmetry and for spreading it to tissues and organs. In this review, we summarize our current knowledge and discuss the diversity of L/R patterning from cells to organs during evolution.

Non-muscle myosins in tumor progression, cancer cell invasion, and metastasis

The actin cytoskeleton, which regulates cell polarity, adhesion, and migration, can influence cancer progression, including initial acquisition of malignant properties by normal cells, invasion of adjacent tissues, and metastasis to distant sites. Actin-dependent molecular motors, myosins, play key roles in regulating tumor progression and metastasis. In this review, we examine how non-muscle myosins regulate neoplastic transformation and cancer cell migration and invasion. Members of the myosin superfamily can act as either enhancers or suppressors of tumor progression. This review summarizes the current state of knowledge on how mutations or epigenetic changes in myosin genes and changes in myosin expression may affect tumor progression and patient outcomes and discusses the proposed mechanisms linking myosin inactivation or upregulation to malignant phenotype, cancer cell migration, and metastasis.

Class I myosins in B-cell physiology: functions in spreading, immune synapses, motility, and vesicular traffic

Myosins comprise a family of motor proteins whose role in muscle contraction and motility in a large range of eukaryotic cells has been widely studied. Although these proteins have been characterized extensively and much is known about their function in different cellular compartments, little is known about these molecules in hematopoietic cells. Myosins expressed by cells from the immune response are involved in maintaining plasma membrane tension, moving and secreting vesicles, endo- and exocytotic processes, and promoting the adhesion and motility of cells. Herein, we summarize our current understanding of class I myosins in B cells, with an emphasis on the emerging roles of these molecular motors in immune functions.

The novel proteins Rng8 and Rng9 regulate the myosin-V Myo51 during fission yeast cytokinesis

The myosin-V family of molecular motors is known to be under sophisticated regulation, but our knowledge of the roles and regulation of myosin-Vs in cytokinesis is limited. Here, we report that the myosin-V Myo51 affects contractile ring assembly and stability during fission yeast cytokinesis, and is regulated by two novel coiled-coil proteins, Rng8 and Rng9. Both rng8Δ and rng9Δ cells display similar defects as myo51Δ in cytokinesis. Rng8 and Rng9 are required for Myo51's localizations to cytoplasmic puncta, actin cables, and the contractile ring. Myo51 puncta contain multiple Myo51 molecules and walk continuously on actin filaments in rng8(+) cells, whereas Myo51 forms speckles containing only one dimer and does not move efficiently on actin tracks in rng8Δ. Consistently, Myo51 transports artificial cargos efficiently in vivo, and this activity is regulated by Rng8. Purified Rng8 and Rng9 form stable higher-order complexes. Collectively, we propose that Rng8 and Rng9 form oligomers and cluster multiple Myo51 dimers to regulate Myo51 localization and functions.

Cytoskeleton and Adhesion in Myogenesis

The function of muscle is to contract, which means to exert force on a substrate. The adaptations required for skeletal muscle differentiation, from a prototypic cell, involve specialization of housekeeping cytoskeletal contracting and supporting systems into crystalline arrays of proteins. Here I discuss the changes that all three cytoskeletal systems (microfilaments, intermediate filaments, and microtubules) undergo through myogenesis. I also discuss their interaction, through the membrane, to extracellular matrix and to other cells, where force will be exerted during contraction. The three cytoskeletal systems are necessary for the muscle cell and must exert complementary roles in the cell. Muscle is a responsive system, where structure and function are integrated: the structural adaptations it undergoes depend on force production. In this way, the muscle cytoskeleton is a portrait of its physiology. I review the cytoskeletal proteins and structures involved in muscle function and focus particularly on their role in myogenesis, the process by which this incredible muscle machine is made. Although the focus is on skeletal muscle, some of the discussion is applicable to cardiac and smooth muscle.

To understand muscle you must take it apart

Striated muscle is an elegant system for study at many levels. Much has been learned about the mechanism of contraction from studying the mechanical properties of intact and permeabilized (or skinned) muscle fibers. Structural studies using electron microscopy, X-ray diffraction or spectroscopic probes attached to various contractile proteins were possible because of the highly ordered sarcomeric arrangement of actin and myosin. However, to understand the mechanism of force generation at a molecular level, it is necessary to take the system apart and study the interaction of myosin with actin using in vitro assays. This reductionist approach has lead to many fundamental insights into how myosin powers muscle contraction. In addition, nature has provided scientists with an array of muscles with different mechanical properties and with a superfamily of myosin molecules. Taking advantage of this diversity in myosin structure and function has lead to additional insights into common properties of force generation. This review will highlight the development of the major assays and methods that have allowed this combined reductionist and comparative approach to be so fruitful. This review highlights the history of biochemical and biophysical studies of myosin and demonstrates how a broad comparative approach combined with reductionist studies have led to a detailed understanding of how myosin interacts with actin and uses chemical energy to generate force and movement in muscle contraction and motility in general.

Rho, nuclear actin, and actin-binding proteins in the regulation of transcription and gene expression

Actin cytoskeleton is one of the main targets of Rho GTPases, which act as molecular switches on many signaling pathways. During the past decade, actin has emerged as an important regulator of gene expression. Nuclear actin plays a key role in transcription, chromatin remodeling, and pre-mRNA processing. In addition, the "status" of the actin cytoskeleton is used as a signaling intermediate by at least the MKL1-SRF and Hippo-pathways, which culminate in the transcriptional regulation of cytoskeletal and growth-promoting genes, respectively. Rho GTPases may therefore regulate gene expression by controlling either cytoplasmic or nuclear actin dynamics. Although the regulation of nuclear actin polymerization is still poorly understood, many actin-binding proteins, which are downstream effectors of Rho, are found in the nuclear compartment. In this review, we discuss the possible mechanisms and key proteins that may mediate the transcriptional regulation by Rho GTPases through actin.

Myosin light chain kinase regulates hearing in mice by influencing the F-actin cytoskeleton of outer hair cells and cochleae

Myosin light chain kinase (MLCK) phosphorylates myosin regulatory light chains to facilitate its interaction with actin filaments and produce contractile activity. The outer hair cells (OHCs) in the ear contain large amounts of actin and a variety myosins. The stereociliary and somatic motility of OHCs are closely related to hearing. It appears likely that MLCK may play an important role in acoustic trans-duction. In this study, we analyzed, both in vivo and in vitro, the OHCs of mice bearing a specific deletion of the MLCK gene and the OHCs of control mice. The phenotype was assessed by auditory function [acoustic brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs)], inner ear morphology and histology. MLCK-deficient mice aged 6-7 months showed impaired hearing, a 5- to 10-dB sound pressure level (SPL) increase in the ABR thresholds, when responding to clicks and tones of different frequencies (8 and 16 kHz) (P<0.05). The DPOAE amplitudes of 3-month-old MLCK-deficient mice decreased significantly (>10 dB SPL) at low frequencies (4, 5 and 6 kHz). The OHCs in the MLCK-deficient mice increased with abnormal stereocilia. The staining of F-actin and the phosphorylation of the regulatory light chain in MLCK-deficient OHCs was weak. Our results indicate that MLCK may regulate the structure and the motility of stereocilia through F-actin polymerization.

The RhoGAP activity of myosin IXB is critical for osteoclast podosome patterning, motility, and resorptive capacity

Osteoclasts are large, multinucleated cells of the monocyte-macrophage lineage that generate specialized substrate adhesion complexes to facilitate their function as bone-degrading cells. The patterning and function of these actin-based complexes, podosomes and sealing zones, are regulated by the small GTPase Rho. Myosin IXB (Myo9b) is a unique actin-based motor protein that contains a RhoGAP domain, which, like other RhoGAPs, is inhibitory to Rho signaling. In this study, Myo9b is shown to be expressed in osteoclasts and act as a critical regulator of podosome patterning and osteoclast function. SiRNA-mediated knockdown of Myo9b results in increased activity of Rho but not Rac in osteoclasts. Knockdown in osteoclasts on glass results in altered podosome patterning and decreased motility, and this effect is reversed by addition of a Rho inhibitor. SiRNA-mediated suppression of Myo9b expression in osteoclasts on bone results in a dramatic loss of resorptive capacity even though sealing zones appear normal. This loss of resorption is also reversible with addition of a Rho inhibitor. Cells with diminished Myo9b levels display mislocalization and suppressed activation of Src, a tyrosine kinase with critical effects on osteoclast actin cytoskeletal rearrangement and function. In addition, siRNA-treated cells display poorly formed microtubule networks and a lack of tubulin acetylation, a marker of microtubule stability. However, short-term addition of TNFα to cells with suppressed Myo9b levels overcomes or circumvents these defects and causes increased sealing zone size and resorptive capacity. These results indicate that the RhoGAP activity of Myo9b plays a key role in regulating the actin-based structures necessary for osteoclast motility and resorption, and confirms that Myo9b can act as a motorized signaling molecule that links Rho signaling to the dynamic actin cytoskeleton.

Calmodulin regulates dimerization, motility, and lipid binding of Leishmania myosin XXI

Myosin XXI is the only myosin expressed in Leishmania parasites. Although it is assumed that it performs a variety of motile functions, the motor's oligomerization states, cargo-binding, and motility are unknown. Here we show that binding of a single calmodulin causes the motor to adopt a monomeric state and to move actin filaments. In the absence of calmodulin, nonmotile dimers that cross-linked actin filaments were formed. Unexpectedly, structural analysis revealed that the dimerization domains include the calmodulin-binding neck region, essential for the generation of force and movement in myosins. Furthermore, monomeric myosin XXI bound to mixed liposomes, whereas the dimers did not. Lipid-binding sections overlapped with the dimerization domains, but also included a phox-homology domain in the converter region. We propose a mechanism of myosin regulation where dimerization, motility, and lipid binding are regulated by calmodulin. Although myosin-XXI dimers might act as nonmotile actin cross-linkers, the calmodulin-binding monomers might transport lipid cargo in the parasite.

Structural insights into functional overlapping and differentiation among myosin V motors

Myosin V (MyoV) motors have been implicated in the intracellular transport of diverse cargoes including vesicles, organelles, RNA-protein complexes, and regulatory proteins. Here, we have solved the cargo-binding domain (CBD) structures of the three human MyoV paralogs (Va, Vb, and Vc), revealing subtle structural changes that drive functional differentiation and a novel redox mechanism controlling the CBD dimerization process, which is unique for the MyoVc subclass. Moreover, the cargo- and motor-binding sites were structurally assigned, indicating the conservation of residues involved in the recognition of adaptors for peroxisome transport and providing high resolution insights into motor domain inhibition by CBD. These results contribute to understanding the structural requirements for cargo transport, autoinhibition, and regulatory mechanisms in myosin V motors.

Myocardin-related transcription factor A is up-regulated by 17β-estradiol and promotes migration of MCF-7 breast cancer cells via transactivation of MYL9 and CYR61

Many lines of evidence have suggested that estrogen plays important roles not only in the initiation and proliferation of breast cancer, but also in cancer metastasis. However, the mechanistic basis of the latter events is poorly understood. In addition, recent studies have suggested that myocardin-related transcription factor A (MRTF-A) might be related to cancer metastasis. However, as reports are contradictory, certain of its roles still remain confusing. In the present study, we showed that excessive 17β-estradiol could promote the migration of MCF-7 breast cancer cells and up-regulate the expression of MRTF-A, myosin regulatory light chain 9 (MYL9), and cysteine-rich angiogenic inducer 61 (CYR61). Overexpression of MRTF-A significantly promoted the migration of MCF-7 cells through its transactivation effects on MYL9 and CYR61 genes, while RNA interference-mediated knockdown of MRTF-A strongly inhibited transcription and expression of the target genes and reduced the migration ability of MCF-7 cells. These results provided novel evidence supporting the metastasis-promoting functions of MRTF-A, and implied that MRTF-A might be a switch for the estrogen pathway to change its proliferation-promoting roles into migration-stimulating roles in breast cancer.

Histone methyltransferase SMYD3 promotes MRTF-A-mediated transactivation of MYL9 and migration of MCF-7 breast cancer cells

Myocardin-related transcription factor-A (MRTF-A) is a Rho signal-responsive transcriptional coactivator of serum response factor (SRF). Recent studies indicated that MRTF-A might be an important regulator of mammary gland and be involved in cancer metastasis. However, the roles of histone modification in the MRTF-A-dependent signal pathway and tumor migration are still not very clear. Here, we report that histone methylation is required for the MRTF-A-mediated upregulation of myosin regulatory light chain 9 (MYL9), an important cytoskeletal component which is implicated in cell migration. Furthermore, we demonstrate that SET and MYND domain containing protein 3 (SMYD3), a hitone methyltransferase (HMT) associated with carcinogenesis, might be the one which is responsible for the histone methylation occurred in the MRTF-A-mediated- transactivation of MYL9 and migration of breast cancer cells. Overexpression of SMYD3 promotes MRTF-A-mediated upregulation of MYL9 and migration of MCF-7 breast cancer cells, while contrary results were observed when the endogenous MRTF-A and SMYD3 were suppressed with specific siRNAs. In addition, the mutation analysis suggested that this cooperative transactivation is mainly mediated via the proximal binding element of MRTF-A in the promoter of MYL9, and the HMT activity of SMYD3 is required as well. Our findings reveal a new mechanism by which MRTF-A and SMYD3 functions in transcriptional regulation and cell migration, and provide a better understanding for metastasis of breast cancer.

The cortical acto-Myosin network: from diffusion barrier to functional gateway in the transport of neurosecretory vesicles to the plasma membrane

Dysregulation of regulated exocytosis is linked to an array of pathological conditions, including neurodegenerative disorders, asthma, and diabetes. Understanding the molecular mechanisms underpinning neuroexocytosis including the processes that allow neurosecretory vesicles to access and fuse with the plasma membrane and to recycle post-fusion, is therefore critical to the design of future therapeutic drugs that will efficiently tackle these diseases. Despite considerable efforts to determine the principles of vesicular fusion, the mechanisms controlling the approach of vesicles to the plasma membrane in order to undergo tethering, docking, priming, and fusion remain poorly understood. All these steps involve the cortical actin network, a dense mesh of actin filaments localized beneath the plasma membrane. Recent work overturned the long-held belief that the cortical actin network only plays a passive constraining role in neuroexocytosis functioning as a physical barrier that partly breaks down upon entry of Ca(2+) to allow secretory vesicles to reach the plasma membrane. A multitude of new roles for the cortical actin network in regulated exocytosis have now emerged and point to highly dynamic novel functions of key myosin molecular motors. Myosins are not only believed to help bring about dynamic changes in the actin cytoskeleton, tethering and guiding vesicles to their fusion sites, but they also regulate the size and duration of the fusion pore, thereby directly contributing to the release of neurotransmitters and hormones. Here we discuss the functions of the cortical actin network, myosins, and their effectors in controlling the processes that lead to tethering, directed transport, docking, and fusion of exocytotic vesicles in regulated exocytosis.

Whole genome duplication events in plant evolution reconstructed and predicted using myosin motor proteins

Background

The evolution of land plants is characterized by whole genome duplications (WGD), which drove species diversification and evolutionary novelties. Detecting these events is especially difficult if they date back to the origin of the plant kingdom. Established methods for reconstructing WGDs include intra- and inter-genome comparisons, K_S age distribution analyses, and phylogenetic tree constructions.

Results

By analysing 67 completely sequenced plant genomes 775 myosins were identified and manually assembled. Phylogenetic trees of the myosin motor domains revealed orthologous and paralogous relationships and were consistent with recent species trees. Based on the myosin inventories and the phylogenetic trees, we have identified duplications of the entire myosin motor protein family at timings consistent with 23 WGDs, that had been reported before. We also predict 6 WGDs based on further protein family duplications. Notably, the myosin data support the two recently reported WGDs in the common ancestor of all extant angiosperms. We predict single WGDs in the Manihot esculenta and Nicotiana benthamiana lineages, two WGDs for Linum usitatissimum and Phoenix dactylifera, and a triplication or two WGDs for Gossypium raimondii. Our data show another myosin duplication in the ancestor of the angiosperms that could be either the result of a single gene duplication or a remnant of a WGD.

Conclusions

We have shown that the myosin inventories in angiosperms retain evidence of numerous WGDs that happened throughout plant evolution. In contrast to other protein families, many myosins are still present in extant species. They are closely related and have similar domain architectures, and their phylogenetic grouping follows the genome duplications. Because of its broad taxonomic sampling the dataset provides the basis for reliable future identification of further whole genome duplications.

An extended proteome map of the lysosomal membrane reveals novel potential transporters

Lysosomes are membrane-bound endocytic organelles that play a major role in degrading cell macromolecules and recycling their building blocks. A comprehensive knowledge of the lysosome function requires an extensive description of its content, an issue partially addressed by previous proteomic analyses. However, the proteins underlying many lysosomal membrane functions, including numerous membrane transporters, remain unidentified. We performed a comparative, semi-quantitative proteomic analysis of rat liver lysosome-enriched and lysosome-nonenriched membranes and used spectral counts to evaluate the relative abundance of proteins. Among a total of 2,385 identified proteins, 734 proteins were significantly enriched in the lysosomal fraction, including 207 proteins already known or predicted as endo-lysosomal and 94 proteins without any known or predicted subcellular localization. The remaining 433 proteins had been previously assigned to other subcellular compartments but may in fact reside on lysosomes either predominantly or as a secondary location. Many membrane-associated complexes implicated in diverse processes such as degradation, membrane trafficking, lysosome biogenesis, lysosome acidification, signaling, and nutrient sensing were enriched in the lysosomal fraction. They were identified to an unprecedented extent as most, if not all, of their subunits were found and retained by our screen. Numerous transporters were also identified, including 46 novel potentially lysosomal proteins. We expressed 12 candidates in HeLa cells and observed that most of them colocalized with the lysosomal marker LAMP1, thus confirming their lysosomal residency. This list of candidate lysosomal proteins substantially increases our knowledge of the lysosomal membrane and provides a basis for further characterization of lysosomal functions.

A Brucella virulence factor targets macrophages to trigger B-cell proliferation

Brucella spp. and Trypanosoma cruzi are two intracellular pathogens that have no evolutionary common origins but share a similar lifestyle as they establish chronic infections for which they have to circumvent the host immune response. Both pathogens have a virulence factor (prpA in Brucella and tcPrac in T. cruzi) that induces B-cell proliferation and promotes the establishment of the chronic phase of the infectious process. We show here that, even though PrpA promotes B-cell proliferation, it targets macrophages in vitro and is translocated to the cytoplasm during the intracellular replication phase. We observed that PrpA-treated macrophages induce the secretion of a soluble factor responsible for B-cell proliferation and identified nonmuscular myosin IIA (NMM-IIA) as a receptor required for binding and function of this virulence factor. Finally, we show that the Trypanosoma cruzi homologue of PrpA also targets macrophages to induce B-cell proliferation through the same receptor, indicating that this virulence strategy is conserved between a bacterial and a protozoan pathogen.

Molecular motors and apical CFTR traffic in epithelia

Intracellular protein traffic plays an important role in the regulation of Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) chloride channels. Microtubule and actin-based motor proteins direct CFTR movement along trafficking pathways. As shown for other regulatory proteins such as adaptors, the involvement of protein motors in CFTR traffic is cell-type specific. Understanding motor specificity provides insight into the biology of the channel and opens opportunity for discovery of organ-specific drug targets for treating CFTR-mediated diseases.

Myosin VI small insert isoform maintains exocytosis by tethering secretory granules to the cortical actin

Before undergoing neuroexocytosis, secretory granules (SGs) are mobilized and tethered to the cortical actin network by an unknown mechanism. Using an SG pull-down assay and mass spectrometry, we found that myosin VI was recruited to SGs in a Ca(2+)-dependent manner. Interfering with myosin VI function in PC12 cells reduced the density of SGs near the plasma membrane without affecting their biogenesis. Myosin VI knockdown selectively impaired a late phase of exocytosis, consistent with a replenishment defect. This exocytic defect was selectively rescued by expression of the myosin VI small insert (SI) isoform, which efficiently tethered SGs to the cortical actin network. These myosin VI SI-specific effects were prevented by deletion of a c-Src kinase phosphorylation DYD motif, identified in silico. Myosin VI SI thus recruits SGs to the cortical actin network, potentially via c-Src phosphorylation, thereby maintaining an active pool of SGs near the plasma membrane.

Directional spread of alphaherpesviruses in the nervous system

Alphaherpesviruses are pathogens that invade the nervous systems of their mammalian hosts. Directional spread of infection in the nervous system is a key component of the viral lifecycle and is critical for the onset of alphaherpesvirus-related diseases. Many alphaherpesvirus infections originate at peripheral sites, such as epithelial tissues, and then enter neurons of the peripheral nervous system (PNS), where lifelong latency is established. Following reactivation from latency and assembly of new viral particles, the infection typically spreads back out towards the periphery. These spread events result in the characteristic lesions (cold sores) commonly associated with herpes simplex virus (HSV) and herpes zoster (shingles) associated with varicella zoster virus (VZV). Occasionally, the infection spreads transsynaptically from the PNS into higher order neurons of the central nervous system (CNS). Spread of infection into the CNS, while rarer in natural hosts, often results in severe consequences, including death. In this review, we discuss the viral and cellular mechanisms that govern directional spread of infection in the nervous system. We focus on the molecular events that mediate long distance directional transport of viral particles in neurons during entry and egress.

Myosinome: a database of myosins from select eukaryotic genomes to facilitate analysis of sequence-structure-function relationships

Myosins are one of the largest protein superfamilies with 24 classes. They have conserved structural features and catalytic domains yet show huge variation at different domains resulting in a variety of functions. Myosins are molecules driving various kinds of cellular processes and motility until the level of organisms. These are ATPases that utilize the chemical energy released by ATP hydrolysis to bring about conformational changes leading to a motor function. Myosins are important as they are involved in almost all cellular activities ranging from cell division to transcriptional regulation. They are crucial due to their involvement in many congenital diseases symptomatized by muscular malfunctions, cardiac diseases, deafness, neural and immunological dysfunction, and so on, many of which lead to death at an early age. We present Myosinome, a database of selected myosin classes (myosin II, V, and VI) from five model organisms. This knowledge base provides the sequences, phylogenetic clustering, domain architectures of myosins and molecular models, structural analyses, and relevant literature of their coiled-coil domains. In the current version of Myosinome, information about 71 myosin sequences belonging to three myosin classes (myosin II, V, and VI) in five model organisms (Homo Sapiens, Mus musculus, D. melanogaster, C. elegans and S. cereviseae) identified using bioinformatics surveys are presented, and several of them are yet to be functionally characterized. As these proteins are involved in congenital diseases, such a database would be useful in short-listing candidates for gene therapy and drug development. The database can be accessed from http://caps.ncbs.res.in/myosinome.

Identification of cytoskeleton-associated proteins essential for lysosomal stability and survival of human cancer cells

Microtubule-disturbing drugs inhibit lysosomal trafficking and induce lysosomal membrane permeabilization followed by cathepsin-dependent cell death. To identify specific trafficking-related proteins that control cell survival and lysosomal stability, we screened a molecular motor siRNA library in human MCF7 breast cancer cells. SiRNAs targeting four kinesins (KIF11/Eg5, KIF20A, KIF21A, KIF25), myosin 1G (MYO1G), myosin heavy chain 1 (MYH1) and tropomyosin 2 (TPM2) were identified as effective inducers of non-apoptotic cell death. The cell death induced by KIF11, KIF21A, KIF25, MYH1 or TPM2 siRNAs was preceded by lysosomal membrane permeabilization, and all identified siRNAs induced several changes in the endo-lysosomal compartment, i.e. increased lysosomal volume (KIF11, KIF20A, KIF25, MYO1G, MYH1), increased cysteine cathepsin activity (KIF20A, KIF25), altered lysosomal localization (KIF25, MYH1, TPM2), increased dextran accumulation (KIF20A), or reduced autophagic flux (MYO1G, MYH1). Importantly, all seven siRNAs also killed human cervix cancer (HeLa) and osteosarcoma (U-2-OS) cells and sensitized cancer cells to other lysosome-destabilizing treatments, i.e. photo-oxidation, siramesine, etoposide or cisplatin. Similarly to KIF11 siRNA, the KIF11 inhibitor monastrol induced lysosomal membrane permeabilization and sensitized several cancer cell lines to siramesine. While KIF11 inhibitors are under clinical development as mitotic blockers, our data reveal a new function for KIF11 in controlling lysosomal stability and introduce six other molecular motors as putative cancer drug targets.

Identification of signalling pathways triggered by changes in the mechanical environment in rat chondrocytes

AIM

The aim of this work was to determine the pathways implicated in the mechanosensing of chondrocytes.

METHODS

Rat chondrocytes were cultured in collagen hydrogels of different stiffness (2-20 Pa) in normoxia and hypoxia, in monolayer and embedded inside hydrogels. First, chondrocyte were cultured on hydrogels in the presence of antibodies to block integrins. Second, custom RT-PCR array plates and western blot were used to detect changes in expression of genes implicated in downstream signalling pathways.

RESULTS

The results allowed us to demonstrate the mechanosensing of chondrocytes for changes in stiffness in the range of Pascals. We also identified Non-Muscle Myosin II (NMMII) and integrins α1, β1 and β3 as participants in the mechanosensing, since their blockade inhibits the sensing of the stiffness, and they are up-regulated in the process. RT-PCR arrays and western blot detected up-regulation of Paxillin, RhoA, Fos, Jun and Sox9. We detected no expression of Src in the monolayer cultures, but we found a role for this protein in 3D. The expression of HIF-1α was not modified under normoxia but was found to participate under hypoxia. Focal Adhesion Kinase (FAK), showed a direct relationship with the expression of Aggrecan in hypoxia and an inverse one in normoxia. Finally, immunofluorescence analysis located the expression of factors AP-1, Sox-9 and HIF-1α inside the cell nuclei and RhoA, Src, Paxillin and FAK close to the cytoplasmic membrane.

CONCLUSIONS

We determined here some of the genes that are up-regulated during the process of chondrocyte mechanosensing.

Screening for MYO15A gene mutations in autosomal recessive nonsyndromic, GJB2 negative Iranian deaf population

MYO15A is located at the DFNB3 locus on chromosome 17p11.2, and encodes myosin-XV, an unconventional myosin critical for the formation of stereocilia in hair cells of cochlea. Recessive mutations in this gene lead to profound autosomal recessive nonsyndromic hearing loss (ARNSHL) in humans and the shaker2 (sh2) phenotype in mice. Here, we performed a study on 140 Iranian families in order to determine mutations causing ARNSHL. The families, who were negative for mutations in GJB2, were subjected to linkage analysis. Eight of these families showed linkage to the DFNB3 locus, suggesting a MYO15A mutation frequency of 5.71% in our cohort of Iranian population. Subsequent sequencing of the MYO15A gene led to identification of 7 previously unreported mutations, including 4 missense mutations, 1 nonsense mutation, and 2 deletions in different regions of the myosin-XV protein.

Cloning, expression, and characterization of a novel molecular motor, Leishmania myosin-XXI

The genome of the Leishmania parasite contains two classes of myosin. Myosin-XXI, seemingly the only myosin isoform expressed in the protozoan parasite, has been detected in both the promastigote and amastigote stages of the Leishmania life cycle. It has been suggested to perform a variety of functions, including roles in membrane anchorage, but also long-range directed movements of cargo. However, nothing is known about the biochemical or mechanical properties of this motor. Here we designed and expressed various myosin-XXI constructs using a baculovirus expression system. Both full-length (amino acids 1-1051) and minimal motor domain constructs (amino acids 1-800) featured actin-activated ATPase activity. Myosin-XXI was soluble when expressed either with or without calmodulin. In the presence of calcium (pCa 4.1) the full-length motor could bind a single calmodulin at its neck domain (probably amino acids 809-823). Calmodulin binding was required for motility but not for ATPase activity. Once bound, calmodulin remained stably attached independent of calcium concentration (pCa 3-7). In gliding filament assays, myosin-XXI moved actin filaments at ∼15 nm/s, insensitive to both salt (25-1000 mm KCl) and calcium concentrations (pCa 3-7). Calmodulin binding to the neck domain might be involved in regulating the motility of the myosin-XXI motor for its various cellular functions in the different stages of the Leishmania parasite life cycle.

Translocation of transfected GLUT2 to the apical membrane in rat intestinal IEC-6 cells

AIM

In this study, we transfected the full length cDNA of glucose transporter 2 (GLUT2) into IEC-6 cells (which lack GLUT2 expression) to investigate GLUT2 translocation in enterocytes. The purpose of this study was to investigate cellular mechanisms of GLUT2 translocation and its signaling pathway.

METHODS

Rat GLUT2 cDNA was transfected into IEC-6 cells. Glucose uptake was measured by incubating cell monolayers with glucose (0.5-50 mM), containing (14)C-D-glucose and (3)H-L-glucose, to measure stereospecific, carrier-mediated and passive uptake. We imaged GLUT2 immunoreactivity by confocal fluorescence microscopy. We evaluated the GLUT2 inhibitor (1 mM phloretin), SGLT1 inhibitor (0.5 mM phlorizin), disrupting microtubular integrity (2 μM nocodazole and 0.5 μM cytochalasin B), protein kinase C (PKC) inhibitors (50 nM calphostin C and 10 μM chelerythrine), and PKC activator (50 nM phorbol 12-myristate 13-acetate: PMA).

RESULTS

In GLUT2-IEC cells, the K(m) (54.5 mM) increased compared with non-transfected IEC-6 cells (7.8 mM); phloretin (GLUT2 inhibitor) inhibited glucose uptake to that of non-transfected IEC-6 cells (P < 0.05). Nocodazole and cytochalasin B (microtubule disrupters) inhibited uptake by 43-58% only at glucose concentrations ≥25 and 50 mM and the 10-min incubations. Calphostin C (PKC inhibitor) reproduced the inhibition of nocodazole; PMA (a PKC activator) enhanced glucose uptake by 69%. Exposure to glucose increased the GFP signal at the apical membrane of GLUT-1EC cells.

CONCLUSION

IEC-6 cells lacking GLUT2 translocate GLUT2 apically when transfected to express GLUT2. Translocation of GLUT2 occurs through glucose stimulation via a PKC-dependent signaling pathway and requires integrity of the microtubular skeletal structure.

Myosin light-chain kinase is necessary for membrane homeostasis in cochlear inner hair cells

The structural homeostasis of the cochlear hair cell membrane is critical for all aspects of sensory transduction, but the regulation of its maintenance is not well understood. In this report, we analyzed the cochlear hair cells of mice with specific deletion of myosin light chain kinase (MLCK) in inner hair cells. MLCK-deficient mice showed impaired hearing, with a 5- to 14-dB rise in the auditory brainstem response (ABR) thresholds to clicks and tones of different frequencies and a significant decrease in the amplitude of the ABR waves. The mutant inner hair cells produced several ball-like structures around the hair bundles in vivo, indicating impaired membrane stability. Inner hair cells isolated from the knockout mice consistently displayed less resistance to hypoosmotic solution and less membrane F-actin. Myosin light-chain phosphorylation was also reduced in the mutated inner hair cells. Our results suggest that MLCK is necessary for maintaining the membrane stability of inner hair cells.