An MYH9 human disease model in flies: site-directed mutagenesis of the Drosophila non-muscle myosin II results in hypomorphic alleles with dominant character

Drosophila as a Model of Unconventional Translation in Spinocerebellar Ataxia Type 3

RNA toxicity contributes to diseases caused by anomalous nucleotide repeat expansions. Recent work demonstrated RNA-based toxicity from repeat-associated, non-AUG-initiated translation (RAN translation). RAN translation occurs around long nucleotide repeats that form hairpin loops, allowing for translation initiation in the absence of a start codon that results in potentially toxic, poly-amino acid repeat-containing proteins. Discovered in Spinocerebellar Ataxia Type (SCA) 8, RAN translation has been documented in several repeat-expansion diseases, including in the CAG repeat-dependent polyglutamine (polyQ) disorders. The ATXN3 gene, which causes SCA3, also known as Machado–Joseph Disease (MJD), contains a CAG repeat that is expanded in disease. ATXN3 mRNA possesses features linked to RAN translation. In this paper, we examined the potential contribution of RAN translation to SCA3/MJD in Drosophila by using isogenic lines that contain homomeric or interrupted CAG repeats. We did not observe unconventional translation in fly neurons or glia. However, our investigations indicate differential toxicity from ATXN3 protein-encoding mRNA that contains pure versus interrupted CAG repeats. Additional work suggests that this difference may be due in part to toxicity from homomeric CAG mRNA. We conclude that Drosophila is not suitable to model RAN translation for SCA3/MJD, but offers clues into the potential pathogenesis stemming from CAG repeat-containing mRNA in this disorder.

Secondary Structure of the Novel Myosin Binding Domain WYR and Implications within Myosin Structure

Structural changes in the myosin II light meromyosin (LMM) that influence thick filament mechanical properties and muscle function are modulated by LMM-binding proteins. Flightin is an LMM-binding protein indispensable for the function of Drosophila indirect flight muscle (IFM). Flightin has a three-domain structure that includes WYR, a novel 52 aa domain conserved throughout Pancrustacea. In this study, we (i) test the hypothesis that WYR binds the LMM, (ii) characterize the secondary structure of WYR, and (iii) examine the structural impact WYR has on the LMM. Circular dichroism at 260-190 nm reveals a structural profile for WYR and supports an interaction between WYR and LMM. A WYR-LMM interaction is supported by co-sedimentation with a stoichiometry of ~2.4:1. The WYR-LMM interaction results in an overall increased coiled-coil content, while curtailing ɑ helical content. WYR is found to be composed of 15% turns, 31% antiparallel β, and 48% 'other' content. We propose a structural model of WYR consisting of an antiparallel β hairpin between Q92-K114 centered on an ASX or β turn around N102, with a G1 bulge at G117. The Drosophila LMM segment used, V1346-I1941, encompassing conserved skip residues 2-4, is found to possess a traditional helical profile but is interpreted as having <30% helical content by multiple methods of deconvolution. This low helicity may be affiliated with the dynamic behavior of the structure in solution or the inclusion of a known non-helical region in the C-terminus. Our results support the hypothesis that WYR binds the LMM and that this interaction brings about structural changes in the coiled-coil. These studies implicate flightin, via the WYR domain, for distinct shifts in LMM secondary structure that could influence the structural properties and stabilization of the thick filament, scaling to modulation of whole muscle function.

Cellular defects resulting from disease-related myosin II mutations in Drosophila

The nonmuscle myosin II motor protein produces forces that are essential to driving the cell movements and cell shape changes that generate tissue structure. Mutations in myosin II that are associated with human diseases are predicted to disrupt critical aspects of myosin function, but the mechanisms that translate altered myosin activity into specific changes in tissue organization and physiology are not well understood. Here we use the Drosophila embryo to model human disease mutations that affect myosin motor activity. Using in vivo imaging and biophysical analysis, we show that engineering human MYH9-related disease mutations into Drosophila myosin II produces motors with altered organization and dynamics that fail to drive rapid cell movements, resulting in defects in epithelial morphogenesis. In embryos that express the Drosophila myosin motor variants R707C or N98K and have reduced levels of wild-type myosin, myosin motors are correctly planar polarized and generate anisotropic contractile tension in the tissue. However, expression of these motor variants is associated with a cellular-scale reduction in the speed of cell intercalation, resulting in a failure to promote full elongation of the body axis. In addition, these myosin motor variants display slowed turnover and aberrant aggregation at the cell cortex, indicating that mutations in the motor domain influence mesoscale properties of myosin organization and dynamics. These results demonstrate that disease-associated mutations in the myosin II motor domain disrupt specific aspects of myosin localization and activity during cell intercalation, linking molecular changes in myosin activity to defects in tissue morphogenesis.

The actomyosin network is influenced by NMHC IIA and regulated by CrpF46, which is involved in controlling cell migration

When a cell migrates, the centrosome positions between the nucleus and the leading edge of migration via the microtubule system. The protein Crp^F46 (centrosome-related protein F46) has a known role during mitosis and centrosome duplication. However, how Crp^F46 efficiently regulates centrosome-related cell migration is unclear. Here, we report that knockdown of Crp^F46 resulted in the disruption of microtubule arrangement, with impaired centrosomal reorientation, and slowed down cell migration. In cells that express low levels of Crp^F46, stress fibers were weakened, which could be rescued by recovering Flag-Crp^F46. We also found that Crp^F46 interacted with non-muscle myosin high chain IIA (NMHC IIA) and that its three coiled-coil domains are pivotal for its binding to NMHC IIA. Additionally, analyses of phosphorylation of NMHC IIA and RLC (regulatory light chain) demonstrated that Crp^F46 was associated with myosin IIA during filament formation. Indirect immunofluorescence images indicated that NM IIA filaments were inhibited when Crp^F46 was under-expressed. Thus, Crp^F46 regulates cell migration by centrosomal reorientation and altering the function of the actomyosin network by controlling specific phosphorylation of myosin.

The E3 ligase Ubr3 regulates Usher syndrome and MYH9 disorder proteins in the auditory organs of Drosophila and mammals

Myosins play essential roles in the development and function of auditory organs and multiple myosin genes are associated with hereditary forms of deafness. Using a forward genetic screen in Drosophila, we identified an E3 ligase, Ubr3, as an essential gene for auditory organ development. Ubr3 negatively regulates the mono-ubiquitination of non-muscle Myosin II, a protein associated with hearing loss in humans. The mono-ubiquitination of Myosin II promotes its physical interaction with Myosin VIIa, a protein responsible for Usher syndrome type IB. We show that ubr3 mutants phenocopy pathogenic variants of Myosin II and that Ubr3 interacts genetically and physically with three Usher syndrome proteins. The interactions between Myosin VIIa and Myosin IIa are conserved in the mammalian cochlea and in human retinal pigment epithelium cells. Our work reveals a novel mechanism that regulates protein complexes affected in two forms of syndromic deafness and suggests a molecular function for Myosin IIa in auditory organs.

Kinetic characterization of the sole nonmuscle myosin-2 from the model organism Drosophila melanogaster

Nonmuscle myosin-2 is the primary enzyme complex powering contractility of the F-actin cytoskeleton in the model organism Drosophila. Despite myosin's essential function in fly development and homeostasis, its kinetic features remain elusive. The purpose of this in vitro study is a detailed steady-state and presteady-state kinetic characterization of the Drosophila nonmuscle myosin-2 motor domain. Kinetic features are a slow steady-state ATPase activity, high affinities for F-actin and ADP, and a low duty ratio. Comparative analysis of the overall enzymatic signatures across the nonmuscle myosin-2 complement from model organisms indicates that the Drosophila protein resembles nonmuscle myosin-2s from metazoa rather than protozoa, though modulatory aspects of myosin motor function are distinct. Drosophila nonmuscle myosin-2 is uniquely insensitive toward blebbistatin, a commonly used myosin-2 inhibitor. An in silico modeling approach together with kinetic studies indicate that the nonconsensus amino acid Met466 in the Drosophila nonmuscle myosin-2 active-site loop switch-2 acts as blebbistatin desensitizer. Introduction of the M466I mutation sensitized the protein for blebbistatin, resulting in a half-maximal inhibitory concentration of 36.3 ± 4.1 µM. Together, these data show that Drosophila nonmuscle myosin-2 is a bona fide molecular motor and establish an important link between switch-2 and blebbistatin sensitivity.

The abnormal proplatelet formation in MYH9-related macrothrombocytopenia results from an increased actomyosin contractility and is rescued by myosin IIA inhibition

BACKGROUND

Mutations in the MYH9 gene cause autosomal dominant MYH9-related diseases (MYH9-RD) that associate macrothrombocytopenia with various other clinical conditions. The mechanisms giving rise to giant platelets remain poorly understood.

OBJECTIVES/PATIENTS

To study the proplatelet formation (PPF) derived from megakaryocytes (MKs) generated in vitro from 11 patients with MYH9-RD with different mutations, compared with controls.

METHODS

Proplatelet formation from cultured patients' MKs was evaluated with or without blebbistatin or the ROCK inhibitor Y27632. Myosin IIA and actin distribution were studied in spreading MKs on different surfaces by immunoconfocal analysis. Kinetic studies of contractility were performed on spreading MKs and the impact of blebbistatin on the maturation of the patients' MKs was evaluated by electron microscopy.

RESULTS AND CONCLUSIONS

We show that in vitro MKs of 11 patients formed significantly fewer proplatelets than controls. MKs from MYH9-RD displayed an abnormal spreading on polylysine, fibronectin and collagen, with a disorganized actin network and a marked increase in stress fiber formation. Traction force microscopy studies demonstrated an elevated level of contractile forces in adherent mutated MKs. The myosin II inhibitor blebbistatin and the ROCK inhibitor Y27632 both rescued the proplatelet formation defect and normalized the ultrastructural characteristics of MYH9-RD MKs. Altogether, our results show that in MYH9-RD, mutations modify the overall MYH9 function and provoke a proplatelet defect through an excess of actomyosin contractility in spreading MKs. These results may promote new therapeutic strategies aimed at reducing this actomyosin contractility.

Approaches for using animal models to identify loci that genetically interact with human disease-causing point mutations

The complexity of human illnesses often extends beyond a single mutation in one gene. Mutations at other loci may act synergistically to affect the penetrance and severity of the associated clinical manifestations. Discovering the additional loci that contribute to an illness is a challenging problem. Animal models for disease, based on engineered point mutations in a homologous gene, have proven invaluable to better understand the mechanism(s) which give(s) rise to the observed physiological effects. Importantly, these animals can also function as the basis for genetic modifier screens to discover other loci which contribute to an illness. This chapter discusses the theory, considerations, and methodology for performing genetic modifier screens in animal models for human disease.