The Drosophila MAST kinase Drop out is required to initiate membrane compartmentalisation during cellularisation and regulates dynein-based transport

A genetic variant in the MAST1 gene is associated with mega-corpus-callosum syndrome with hypoplastic cerebellar vermis, in a fetus

BACKGROUND

Mega-corpus-callosum syndrome with cerebellar hypoplasia and cortical malformations is a rare neurological disorder that is associated with typical clinical and imaging features. The syndrome is caused by pathogenic variants in the MAST1 gene, which encodes a microtubule-associated protein that is predominantly expressed in postmitotic neurons in the developing nervous system.

METHODS

Fetal DNA from umbilical cord blood samples and genomic DNA from peripheral blood lymphocytes were subjected to whole-exome sequencing. The potential causative variants were verified by Sanger sequencing.

RESULTS

A 26-year-old primigravid woman was referred to our prenatal center at 25 weeks of gestation due to abnormal ultrasound findings in the brain of the fetus. The brain abnormalities included wide cavum septum pellucidum, shallow and incomplete bilateral lateral fissure cistern, bilateral dilated lateral ventricles, hyperplastic corpus callosum, lissencephaly, and cortical dysplasia. No obvious abnormalities were observed in the brainstem or cerebellum hemispheres, but the cerebellum vermis was small. Whole-exome sequencing identified a de novo, heterozygous missense variant, c.695T>C(p.Leu232Pro), in the MAST1 gene and a genetic diagnosis of mega-corpus-callosum syndrome was considered.

CONCLUSION

This study is the first prenatal case of MAST1-related disorder reported in the Chinese population and has expanded the mutation spectrum of the MAST1 gene.

Microtubule-Associated Serine/Threonine (MAST) Kinases in Development and Disease

Microtubule-Associated Serine/Threonine (MAST) kinases represent an evolutionary conserved branch of the AGC protein kinase superfamily in the kinome. Since the discovery of the founding member, MAST2, in 1993, three additional family members have been identified in mammals and found to be broadly expressed across various tissues, including the brain, heart, lung, liver, intestine and kidney. The study of MAST kinases is highly relevant for unraveling the molecular basis of a wide range of different human diseases, including breast and liver cancer, myeloma, inflammatory bowel disease, cystic fibrosis and various neuronal disorders. Despite several reports on potential substrates and binding partners of MAST kinases, the molecular mechanisms that would explain their involvement in human diseases remain rather obscure. This review will summarize data on the structure, biochemistry and cell and molecular biology of MAST kinases in the context of biomedical research as well as organismal model systems in order to provide a current profile of this field.

The Role of Microtubule Associated Serine/Threonine Kinase 3 Variants in Neurodevelopmental Diseases: Genotype-Phenotype Association

Objective: To prove microtubule associated serine/threonine kinase 3 (MAST3) gene is associated with neurodevelopmental diseases (NDD) and the genotype-phenotype correlation.

Methods: Trio exome sequencing (trio ES) was performed on four NDD trios. Bioinformatic analysis was conducted based on large-scale genome sequencing data and human brain transcriptomic data. Further in vivo zebrafish studies were performed.

Results: In our study, we identified four de novo MAST3 variants (NM_015016.1: c.302C > T:p.Ser101Phe; c.311C > T:p.Ser104Leu; c.1543G > A:p.Gly515Ser; and c.1547T > C:p.Leu516Pro) in four patients with developmental and epileptic encephalopathy (DEE) separately. Clinical heterogeneities were observed in patients carrying variants in domain of unknown function (DUF) and serine-threonine kinase (STK) domain separately. Using the published large-scale exome sequencing data, higher CADD scores of missense variants in DUF domain were found in NDD cohort compared with gnomAD database. In addition, we obtained an excess of missense variants in DUF domain when compared autistic spectrum disorder (ASD) cohort with gnomAD database, similarly an excess of missense variants in STK domain when compared DEE cohort with gnomAD database. Based on Brainspan datasets, we showed that MAST3 expression was significantly upregulated in ASD and DEE-related brain regions and was functionally linked with DEE genes. In zebrafish model, abnormal morphology of central nervous system was observed in mast3a/b crispants.

Conclusion: Our results support the possibility that MAST3 is a novel gene associated with NDD which could expand the genetic spectrum for NDD. The genotype-phenotype correlation may contribute to future genetic counseling.

KIN-4/MAST kinase promotes PTEN-mediated longevity of Caenorhabditis elegans via binding through a PDZ domain

PDZ domain‐containing proteins (PDZ proteins) act as scaffolds for protein–protein interactions and are crucial for a variety of signal transduction processes. However, the role of PDZ proteins in organismal lifespan and aging remains poorly understood. Here, we demonstrate that KIN‐4, a PDZ domain‐containing microtubule‐associated serine‐threonine (MAST) protein kinase, is a key longevity factor acting through binding PTEN phosphatase in Caenorhabditis elegans. Through a targeted genetic screen for PDZ proteins, we find that kin‐4 is required for the long lifespan of daf‐2/insulin/IGF‐1 receptor mutants. We then show that neurons are crucial tissues for the longevity‐promoting role of kin‐4. We find that the PDZ domain of KIN‐4 binds PTEN, a key factor for the longevity of daf‐2 mutants. Moreover, the interaction between KIN‐4 and PTEN is essential for the extended lifespan of daf‐2 mutants. As many aspects of lifespan regulation in C. elegans are evolutionarily conserved, MAST family kinases may regulate aging and/or age‐related diseases in mammals through their interaction with PTEN.

Arabidopsis AGC protein kinases IREH1 and IRE3 control root skewing

AGC protein kinases play important roles in plant growth and development. Several AGC kinases in Arabidopsis have been functionally characterized. However, the "AGC Other" subfamily, including IRE, IREH1, IRE3 and IRE4, has not been well understood. Here, we reported that ireh1 mutants displayed a root skewing phenotype, which can be enhanced by ire3 mutation. IREH1 and IRE3 were expressed in roots, consistent with their function in controlling root skewing. The fluorescence intensities of the microtubule marker KNpro:EGFP-MBD were decreased in ireh1, ire3 and ireh1 ire3 mutants compared to wild type. The microtubule arrangements in ireh1 and ireh1 ire3 mutants were also altered. IREH1 physically interacted with IRE3 in vitro and in planta. Thus, our findings demonstrate that IREH1 and IRE3 protein kinases play important roles in controlling root skewing, and maintaining microtubule network in Arabidopsis.

RNA-directed activation of cytoplasmic dynein-1 in reconstituted transport RNPs

Polarised mRNA transport is a prevalent mechanism for spatial control of protein synthesis. However, the composition of transported ribonucleoprotein particles (RNPs) and the regulation of their movement are poorly understood. We have reconstituted microtubule minus end-directed transport of mRNAs using purified components. A Bicaudal-D (BicD) adaptor protein and the RNA-binding protein Egalitarian (Egl) are sufficient for long-distance mRNA transport by the dynein motor and its accessory complex dynactin, thus defining a minimal transport-competent RNP. Unexpectedly, the RNA is required for robust activation of dynein motility. We show that a cis-acting RNA localisation signal promotes the interaction of Egl with BicD, which licenses the latter protein to recruit dynein and dynactin. Our data support a model for BicD activation based on RNA-induced occupancy of two Egl-binding sites on the BicD dimer. Scaffolding of adaptor protein assemblies by cargoes is an attractive mechanism for regulating intracellular transport.

In our cells, tiny molecular motors transport the components necessary for life’s biological processes from one location to another. They do so by loading their cargo, and burning up chemical fuel to carry it along pathways made of filaments. For example, one such motor, called dynein, can move molecules of messenger RNA (mRNA) to specific locations within the cell. There, the mRNA will be used as a template to create proteins, which will operate at exactly the right place.

Transporting mRNA in this way is critical in processes such as embryonic development and the formation of memories; yet, this mechanism is still poorly understood. Previous work suggested that the mRNA is simply a passenger of the dynein motor, but McClintock et al. asked if this is really the case. Instead, could mRNA regulate its own sorting by controlling the activity of dynein?

Studying mRNA trafficking within the complex molecular environment of a cell is challenging, so mRNA transporting machinery was recreated in the laboratory. Only the proteins necessary to build a working system were included in the experiments. In addition to the filaments, the components included dynein and a complex of proteins known as dynactin, which allows the motor to move together with a protein called BICD2. A protein named Egalitarian was used to link the mRNA to BICD2.

By filming fluorescently labelled proteins and mRNAs, McClintock et al. discovered that mRNA strongly promotes the movement of the dynein motor. A structured section in the mRNA acts as a docking area for two copies of Egalitarian. This activates BICD2, which then binds to dynein and dynactin, thereby completing the transport machinery. According to these results, the mRNA directs the assembly of the system that will carry it within the cell.

Viruses such as HIV and herpesvirus hijack dynein motors to have their genetic information moved around a cell in order to propagate infection. Understanding precisely how mRNA is transported may help to develop new strategies to fight these viruses.

MAST-like protein kinase IREH1 from Arabidopsis thaliana co-localizes with the centrosome when expressed in animal cells

The similarity of IREH1 (Incomplete Root Hair Elongation 1) and animal MAST kinases was confirmed; IREH1cDNA was cloned while expressing in cultured animal cells co-localized with the centrosome. In mammals and fruit flies, microtubule-associated serine/threonine-protein kinases (MAST) are strongly involved in the regulation of the microtubule system. Higher plants also possess protein kinases homologous to MASTs, but their function and interaction with the cytoskeleton remain unclear. Here, we confirmed the sequence and structural similarity of MAST-related putative protein kinase IREH1 (At3g17850) and known animal MAST kinases. We report the first cloning of full-length cDNA of the IREH1 from Arabidopsis thaliana. Recombinant GFP-IREH1 protein was expressed in different cultured animal cells. It revealed co-localization with the centrosome without influencing cell morphology and microtubule arrangement. Structural N-terminal region of the IREH1 molecule co-localized with centrosome as well.

Nuf and Rip11 requirement for polarity determinant recycling during Drosophila development

A tight relationship between apico-basal polarity and trafficking is essential for epithelial physiology and tissue homeostasis. Recent studies have described how some Rab GTPases, key components of the intracellular traffic machinery, contribute to the establishment of cell polarity in vertebrates. We have demonstrated a novel connection between cell polarity and trafficking: in Drosophila epithelia, the apical determinant aPKC is recycled via Rab11-Nuf-recycling endosomes to maintain cell polarity. Furthermore, the phosphorylation of Nuf by aPKC allows aPKC to control the sub-cellular localization of Nuf and its own membrane accumulation. Here we review these data and show the different contribution of the 2 Drosophila Rab11 adaptor proteins, Nuf and Rip11, to the maintenance of Drosophila embryonic ectoderm polarity.

As the fat flies: The dynamic lipid droplets of Drosophila embryos

Research into lipid droplets is rapidly expanding, and new cellular and organismal roles for these lipid-storage organelles are continually being discovered. The early Drosophila embryo is particularly well suited for addressing certain questions in lipid-droplet biology and combines technical advantages with unique biological phenomena. This review summarizes key features of this experimental system and the techniques available to study it, in order to make it accessible to researchers outside this field. It then describes the two topics most heavily studied in this system, lipid-droplet motility and protein sequestration on droplets, discusses what is known about the molecular players involved, points to open questions, and compares the results from Drosophila embryo studies to what it is known about lipid droplets in other systems.