Tissue specific expression of myosin IC isoforms

Detecting differential transcript usage in complex diseases with SPIT

Differential transcript usage (DTU) plays a crucial role in determining how gene expression differs among cells, tissues, and developmental stages, contributing to the complexity and diversity of biological systems. In abnormal cells, it can also lead to deficiencies in protein function and underpin disease pathogenesis. Analyzing DTU via RNA sequencing (RNA-seq) data is vital, but the genetic heterogeneity in populations with complex diseases presents an intricate challenge due to diverse causal events and undetermined subtypes. Although the majority of common diseases in humans are categorized as complex, state-of-the-art DTU analysis methods often overlook this heterogeneity in their models. We therefore developed SPIT, a statistical tool that identifies predominant subgroups in transcript usage within a population along with their distinctive sets of DTU events. This study provides comprehensive assessments of SPIT's methodology and applies it to analyze brain samples from individuals with schizophrenia, revealing previously unreported DTU events in six candidate genes.

Detecting differential transcript usage in complex diseases with SPIT

Differential transcript usage (DTU) plays a crucial role in determining how gene expression differs among cells, tissues, and different developmental stages, thereby contributing to the complexity and diversity of biological systems. In abnormal cells, it can also lead to deficiencies in protein function, potentially leading to pathogenesis of diseases. Detecting such events for single-gene genetic traits is relatively uncomplicated; however, the heterogeneity of populations with complex diseases presents an intricate challenge due to the presence of diverse causal events and undetermined subtypes. SPIT is the first statistical tool that quantifies the heterogeneity in transcript usage within a population and identifies predominant subgroups along with their distinctive sets of DTU events. We provide comprehensive assessments of SPIT's methodology in both single-gene and complex traits and report the results of applying SPIT to analyze brain samples from individuals with schizophrenia. Our analysis reveals previously unreported DTU events in six candidate genes.

Myosin motors on the pathway of viral infections

Molecular motors are microscopic machines that use energy from adenosine triphosphate (ATP) hydrolysis to generate movement. While kinesins and dynein are molecular motors associated with microtubule tracks, myosins bind to and move on actin filaments. Mammalian cells express several myosin motors. They power cellular processes such as endo- and exocytosis, intracellular trafficking, transcription, migration, and cytokinesis. As viruses navigate through cells, they may take advantage or be hindered by host components and machinery, including the cytoskeleton. This review delves into myosins' cell roles and compares them to their reported functions in viral infections. In most cases, the previously described myosin functions align with their reported role in viral infections, although not in all cases. This opens the possibility that knowledge obtained from studying myosins in viral infections might shed light on new physiological roles for myosins in cells. However, given the high number of myosins expressed and the variety of viruses investigated in the different studies, it is challenging to infer whether the interactions found are specific to a single virus or can be applied to other viruses with the same characteristics. We conclude that the participation of myosins in viral cycles is still a largely unexplored area, especially concerning unconventional myosins.

Myosin 1C isoform A is a novel candidate diagnostic marker for prostate cancer

Early diagnosis of prostate cancer is a challenging issue due to the lack of specific markers. Therefore, a sensitive diagnostic marker that is expressed or upregulated exclusively in prostate cancer cells would facilitate diagnostic procedures and ensure a better outcome. We evaluated the expression of myosin 1C isoform A in 5 prostate cell lines, 41 prostate cancer cases, and 11 benign hyperplasias. We analyzed the expression of 12 surface molecules on prostate cancer cells by flow cytometry and analyzed whether high or low myosin 1C isoform A expression could be attributed to a distinct phenotype of prostate cancer cells. Median myosin 1C isoform A expression in prostate cancer samples and cancer cell lines was 2 orders of magnitude higher than in benign prostate hyperplasia. Based on isoform A expression, we could also distinguish clinical stage 2 from clinical stage 3. Among cell lines, PC-3 cells with the highest myosin 1C isoform A level had diminished numbers of CD10/CD13-positive cells and increased numbers of CD29 (integrin β1), CD38, CD54 (ICAM1) positive cells. The surface phenotype of clinical samples was similar to prostate cancer cell lines with high isoform A expression and could be described as CD10-/CD13- with heterogeneous expression of other markers. Both for cell lines and cancer specimens we observed the strong correlation of high myosin 1C isoform A mRNA expression and elevated levels of CD29 and CD54, suggesting a more adhesive phenotype for cells with high isoform A expression. Compared to normal tissue, prostate cancer samples had also reduced numbers of CD24- and CD38-positive cells. Our data suggest that a high level of myosin 1C isoform A is a specific marker both for prostate cancer cells and prostate cancer cell lines. High expression of isoform A is associated with less activated (CD24/CD38 low) and more adhesive (CD29/CD54 high) surface phenotype compared to benign prostate tissue.

Myosin 1c: A novel regulator of glucose uptake in brown adipocytes

Objective

The potential of brown adipose tissue (BAT) to influence energy homeostasis in animals and humans is encouraging as this tissue can increase fatty acid and glucose utilization to produce heat through uncoupling protein 1 (UCP1), but the actual mechanism of how the cell regulates glucose uptake is not fully understood. Myosin 1c (Myo1c) is an unconventional motor protein involved in several cellular processes, including insulin-mediated glucose uptake via GLUT4 vesicle fusion in white adipocytes, but its role in glucose uptake in BAT has not previously been investigated.

Methods

Using the specific inhibitor pentachloropseudilin (PClP), a neutralizing antibody assay, and siRNA, we examined the role of Myo1c in mechanisms leading to glucose uptake both in vitro in isolated mouse primary adipocytes and in vivo in mice.

Results

Our results show that inhibition of Myo1c removes insulin-stimulated glucose uptake in white adipocytes, while inducing glucose uptake in brown adipocytes, independent of GLUT4, by increasing the expression, translation, and translocation of GLUT1 to the plasma membrane. Inhibition of Myo1c leads to the activation of PKA and downstream substrates p38 and ATF-2, which are known to be involved in the expression of β-adrenergic genes.

Conclusions

Myo1c is a PKA repressor and regulates glucose uptake into BAT.

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Myo1c is a BAT-specific regulator of glucose uptake.

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Myo1c inhibition leads to increased expression, translation, and translocation of GLUT1.

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Myo1c inhibition results in increased activation of PKA and its downstream targets.

Mechanochemical properties of human myosin-1C are modulated by isoform-specific differences in the N-terminal extension

Myosin-1C is a single-headed, short-tailed member of the myosin class I subfamily that supports a variety of actin-based functions in the cytosol and nucleus. In vertebrates, alternative splicing of the MYO1C gene leads to the production of three isoforms, myosin-1C⁰, myosin-1C¹⁶, and myosin-1C³⁵, that carry N-terminal extensions of different lengths. However, it is not clear how these extensions affect the chemomechanical coupling of human myosin-1C isoforms. Here, we report on the motor activity of the different myosin-1C isoforms measuring the unloaded velocities of constructs lacking the C-terminal lipid-binding domain on nitrocellulose-coated glass surfaces and full-length constructs on reconstituted, supported lipid bilayers. The higher yields of purified proteins obtained with constructs lacking the lipid-binding domain allowed a detailed characterization of the individual kinetic steps of human myosin-1C isoforms in their productive interaction with nucleotides and filamentous actin. Isoform-specific differences include 18-fold changes in the maximum power output per myosin-1C motor and 4-fold changes in the velocity and the resistive force at which maximum power output occurs. Our results support a model in which the isoform-specific N-terminal extensions affect chemomechanical coupling by combined steric and allosteric effects, thereby reducing both the length of the working stroke and the rate of ADP release in the absence of external loads by a factor of 2 for myosin-1C³⁵. As the large change in maximum power output shows, the functional differences between the isoforms are further amplified by the presence of external loads.

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.

Class I myosins: Highly versatile proteins with specific functions in the immune system

Connections established between cytoskeleton and plasma membrane are essential in cellular processes such as cell migration, vesicular trafficking, and cytokinesis. Class I myosins are motor proteins linking the actin-cytoskeleton with membrane phospholipids. Previous studies have implicated these molecules in cell functions including endocytosis, exocytosis, release of extracellular vesicles and the regulation of cell shape and membrane elasticity. In immune cells, those proteins also are involved in the formation and maintenance of immunological synapse-related signaling. Thus, these proteins are master regulators of actin cytoskeleton dynamics in different scenarios. Although the localization of class I myosins has been described in vertebrates, their functions, regulation, and mechanical properties are not very well understood. In this review, we focused on and summarized the current understanding of class I myosins in vertebrates with particular emphasis in leukocytes.

Specific and reliable detection of Myosin 1C isoform A by RTqPCR in prostate cancer cells

Background

Prostate cancer (PC) diagnostics and treatment often present a challenging task due to cancer subtype heterogeneity and differential disease progression in patient subgroups. Hence, the critical issue is finding a reliable and sensitive diagnostic and prognostic PC marker, especially for cases of biopsies with low percentages of cancer cells. Isoform A of myosin 1C was shown to be expressed in PC cells and responsible for their invasive properties, however, its feasibility for diagnostic purposes remains to be elucidated.

Methods

To verify the role of myosin 1C isoform A mRNA expression as a putative prostate cancer marker we performed RT qPCR normalized by three reference genes (GAPDH, YWHAZ, HPRT1) on PC3, RWPE-1, LNCaP and 22Rv1 cell lines. Myosin 1C isoform A detection specificity was confirmed by immunofluorescence staining, cancer and non-cancer prostate cell lines were immunophenotyped by flow cytometry.

Results

Median normalized mRNA expression level of myosin 1C isoform A in PC cells (PC3 and 22Rv1) is two orders of magnitude higher compared to RWPE-1 cells, which functionally correspond to benign prostate cells. Myosin 1C isoform A expression allows PC cell detection even at a dilution ratio of 1:1000 cancer to non-cancer cells. At the protein level, the mean fluorescence intensity of myosin 1C isoform A staining in PC3 nuclei was only twice as high as in RWPE-1, while the immunophenotypes of both cell lines were similar (CD44+/CD90-/CD133-/CD57-/CD24+-).

Conclusions

We report a distinct difference in myosin 1C isoform A mRNA levels in malignant (PC3) and benign (RWPE-1) prostate cell lines and suggest a combination of three reference genes for accurate data normalization. For the first time we provide an immunophenotype comparison of RWPE-1 and PC3 cells and demonstrate that RT qPCR analysis of MYO 1C A using appropriate reference genes is sufficient for PC detection even in low-abundance cancer specimens.

Alternative start and termination sites of transcription drive most transcript isoform differences across human tissues

Most human genes generate multiple transcript isoforms. The differential expression of these isoforms can help specify cell types. Diverse transcript isoforms arise from the use of alternative transcription start sites, polyadenylation sites and splice sites; however, the relative contribution of these processes to isoform diversity in normal human physiology is unclear. To address this question, we investigated cell type-dependent differences in exon usage of over 18 000 protein-coding genes in 23 cell types from 798 samples of the Genotype-Tissue Expression Project. We found that about half of the expressed genes displayed tissue-dependent transcript isoforms. Alternative transcription start and termination sites, rather than alternative splicing, accounted for the majority of tissue-dependent exon usage. We confirmed the widespread tissue-dependent use of alternative transcription start sites in a second, independent dataset, Cap Analysis of Gene Expression data from the FANTOM consortium. Moreover, our results indicate that most tissue-dependent splicing involves untranslated exons and therefore may not increase proteome complexity. Thus, alternative transcription start and termination sites are the principal drivers of transcript isoform diversity across tissues, and may underlie the majority of cell type specific proteomes and functions.

N-terminal splicing extensions of the human MYO1C gene fine-tune the kinetics of the three full-length myosin IC isoforms

The MYO1C gene produces three alternatively spliced isoforms, differing only in their N-terminal regions (NTRs). These isoforms, which exhibit both specific and overlapping nuclear and cytoplasmic functions, have different expression levels and nuclear-cytoplasmic partitioning. To investigate the effect of NTR extensions on the enzymatic behavior of individual isoforms, we overexpressed and purified the three full-length human isoforms from suspension-adapted HEK cells. MYO1C^C favored the actomyosin closed state (AM^C), MYO1C¹⁶ populated the actomyosin open state (AM^O) and AM^C equally, and MYO1C³⁵ favored the AM^O state. Moreover, the full-length constructs isomerized before ADP release, which has not been observed previously in truncated MYO1C^C constructs. Furthermore, global numerical simulation analysis predicted that MYO1C³⁵ populated the actomyosin·ADP closed state (AMD^C) 5-fold more than the actomyosin·ADP open state (AMD^O) and to a greater degree than MYO1C^C and MYO1C¹⁶ (4- and 2-fold, respectively). On the basis of a homology model of the 35-amino acid NTR of MYO1C³⁵ (NTR³⁵) docked to the X-ray structure of MYO1C^C, we predicted that MYO1C³⁵ NTR residue Arg-21 would engage in a specific interaction with post-relay helix residue Glu-469, which affects the mechanics of the myosin power stroke. In addition, we found that adding the NTR³⁵ peptide to MYO1C^C yielded a protein that transiently mimics MYO1C³⁵ kinetic behavior. By contrast, NTR³⁵, which harbors the R21G mutation, was unable to confer MYO1C³⁵-like kinetic behavior. Thus, the NTRs affect the specific nucleotide-binding properties of MYO1C isoforms, adding to their kinetic diversity. We propose that this level of fine-tuning within MYO1C broadens its adaptability within cells.

Myosin isoform expressed in metastatic prostate cancer stimulates cell invasion

During metastasis, tumor cells migrate out of their original tissue to invade other organs. Secretion of exosomes and metalloproteases is essential for extracellular matrix remodeling, enabling migration through tissue barriers. Metastatic prostate cancer is differentiated by expression of the rare isoform A of the molecular motor myosin IC, however the function of this isoform remained unknown. Here we show that it contributes causatively to the invasive motility of prostate cancer cells. We found that the isoform associates with metalloprotease-containing exosomes and stimulates their secretion. While the data show that myosin IC is involved in prostate cancer cell migration, migration outside extracellular matrix in vitro proves little affected specifically by isoform A. Nevertheless, this isoform stimulates invasion through extracellular matrix, pointing to a critical role in secretion. Both the secretion and invasion depend on the integrity of the motor and lipid-binding domains of the protein. Our results demonstrate how myosin IC isoform A is likely to function in metastasis, driving secretion of exosomes that enable invasion of prostate cancer cells across extracellular matrix barriers. The new data identify a molecule suitable for a mechanistically grounded development into a marker and target for prognosis, detection, and treatment of invasive prostate cancer.

Lowered Expression of Tumor Suppressor Candidate MYO1C Stimulates Cell Proliferation, Suppresses Cell Adhesion and Activates AKT

Myosin-1C (MYO1C) is a tumor suppressor candidate located in a region of recurrent losses distal to TP53. Myo1c can tightly and specifically bind to PIP2, the substrate of Phosphoinositide 3-kinase (PI3K), and to Rictor, suggesting a role for MYO1C in the PI3K pathway. This study was designed to examine MYO1C expression status in a panel of well-stratified endometrial carcinomas as well as to assess the biological significance of MYO1C as a tumor suppressor in vitro. We found a significant correlation between the tumor stage and lowered expression of MYO1C in endometrial carcinoma samples. In cell transfection experiments, we found a negative correlation between MYO1C expression and cell proliferation, and MYO1C silencing resulted in diminished cell migration and adhesion. Cells expressing excess of MYO1C had low basal level of phosphorylated protein kinase B (PKB, a.k.a. AKT) and cells with knocked down MYO1C expression showed a quicker phosphorylated AKT (pAKT) response in reaction to serum stimulation. Taken together the present study gives further evidence for tumor suppressor activity of MYO1C and suggests MYO1C mediates its tumor suppressor function through inhibition of PI3K pathway and its involvement in loss of contact inhibition.

Myosin-I molecular motors at a glance

Myosin-I molecular motors are proposed to play various cellular roles related to membrane dynamics and trafficking. In this Cell Science at a Glance article and the accompanying poster, we review and illustrate the proposed cellular functions of metazoan myosin-I molecular motors by examining the structural, biochemical, mechanical and cell biological evidence for their proposed molecular roles. We highlight evidence for the roles of myosin-I isoforms in regulating membrane tension and actin architecture, powering plasma membrane and organelle deformation, participating in membrane trafficking, and functioning as a tension-sensitive dock or tether. Collectively, myosin-I motors have been implicated in increasingly complex cellular phenomena, yet how a single isoform accomplishes multiple types of molecular functions is still an active area of investigation. To fully understand the underlying physiology, it is now essential to piece together different approaches of biological investigation. This article will appeal to investigators who study immunology, metabolic diseases, endosomal trafficking, cell motility, cancer and kidney disease, and to those who are interested in how cellular membranes are coupled to the underlying actin cytoskeleton in a variety of different applications.

Calcium-regulated import of myosin IC into the nucleus

Myosin IC is a molecular motor involved in intracellular transport, cell motility, and transcription. Its mechanical properties are regulated by calcium via calmodulin binding, and its functions in the nucleus depend on import from the cytoplasm. The import has recently been shown to be mediated by the nuclear localization signal located within the calmodulin-binding domain. In the present paper, it is demonstrated that mutations in the calmodulin-binding sequence shift the intracellular distribution of myosin IC to the nucleus. The redistribution is displayed by isoform B, described originally as the "nuclear myosin," but is particularly pronounced with isoform C, the normally cytoplasmic isoform. Furthermore, experimental elevation of the intracellular calcium concentration induces a rapid import of myosin into the nucleus. The import is blocked by the importin β inhibitor importazole. These findings are consistent with a mechanism whereby calmodulin binding prevents recognition of the nuclear localization sequence by importin β, and the steric inhibition of import is released by cell signaling leading to the intracellular calcium elevation. The results establish a mechanistic connection between the calcium regulation of the motor function of myosin IC in the cytoplasm and the induction of its import into the nucleus. © 2016 Wiley Periodicals, Inc.

Design (and) principles of nuclear dynamics in Stockholm

The structural organization of the nucleus and its content has drawn increasing interest in recent years, as it is has become evident that the spatial and temporal arrangement of the genome and associated structures plays a crucial role in transcriptional regulation and numerous other functions. Shining light on the dynamic nature of this organization, along with the processes controlling it, were the topics of the Wenner-Gren Foundations international symposium "Nuclear Dynamics: Design (and) Principles." The meeting, organized by Piorgiogio Percipalle, Maria Vartiainen, Neus Visa, and Ann-Kristin Östlund-Farrants, brought over 60 participants, including 20 international speakers, to Stockholm, Sweden from August 19-22, 2015 to share the latest developments in the field. Given the unpublished nature of many of the talks, we have focused on covering the discussed topics and highlighting the latest trends in this exciting and rapidly evolving field.

Selective expression of myosin IC Isoform A in mouse and human cell lines and mouse prostate cancer tissues

Myosin IC is a single headed member of the myosin superfamily. We recently identified a novel isoform and showed that the MYOIC gene in mammalian cells encodes three isoforms (isoforms A, B, and C). Furthermore, we demonstrated that myosin IC isoform A but not isoform B exhibits a tissue specific expression pattern. In this study, we extended our analysis of myosin IC isoform expression patterns by analyzing the protein and mRNA expression in various mammalian cell lines and in various prostate specimens and tumor tissues from the transgenic mouse prostate (TRAMP) model by immunoblotting, qRT-PCR, and by indirect immunohistochemical staining of paraffin embedded prostate specimen. Analysis of a panel of mammalian cell lines showed an increased mRNA and protein expression of specifically myosin IC isoform A in a panel of human and mouse prostate cancer cell lines but not in non-cancer prostate or other (non-prostate-) cancer cell lines. Furthermore, we demonstrate that myosin IC isoform A expression is significantly increased in TRAMP mouse prostate samples with prostatic intraepithelial neoplasia (PIN) lesions and in distant site metastases in lung and liver when compared to matched normal tissues. Our observations demonstrate specific changes in the expression of myosin IC isoform A that are concurrent with the occurrence of prostate cancer in the TRAMP mouse prostate cancer model that closely mimics clinical prostate cancer. These data suggest that elevated levels of myosin IC isoform A may be a potential marker for the detection of prostate cancer.