Restricted expression of LUZP in neural lineage cells: a study in embryonic stem cells

Leucine zipper protein 1 (LUZP1) regulates the constriction velocity of the contractile ring during cytokinesis

There has been a great deal of research on cell division and its mechanisms; however, its processes still have many unknowns. To find novel proteins that regulate cell division, we performed the screening using siRNAs and/or the expression plasmid of the target genes and identified leucine zipper protein 1 (LUZP1). Recent studies have shown that LUZP1 interacts with various proteins and stabilizes the actin cytoskeleton; however, the function of LUZP1 in mitosis is not known. In this study, we found that LUZP1 colocalized with the chromosomal passenger complex (CPC) at the centromere in metaphase and at the central spindle in anaphase and that these LUZP1 localizations were regulated by CPC activity and kinesin family member 20A (KIF20A). Mass spectrometry analysis identified that LUZP1 interacted with death-associated protein kinase 3 (DAPK3), one regulator of the cleavage furrow ingression in cytokinesis. In addition, we found that LUZP1 also interacted with myosin light chain 9 (MYL9), a substrate of DAPK3, and comprehensively inhibited MYL9 phosphorylation by DAPK3. In line with a known role for MYL9 in the actin-myosin contraction, LUZP1 suppression accelerated the constriction velocity at the division plane in our time-lapse analysis. Our study indicates that LUZP1 is a novel regulator for cytokinesis that regulates the constriction velocity of the contractile ring.

LUZP1: A new player in the actin-microtubule cross-talk

LUZP1 (leucine zipper protein 1) was first described as being important for embryonic development. Luzp1 null mice present defective neural tube closure and cardiovascular problems, which cause perinatal death. Since then, LUZP1 has also been implicated in the etiology of diseases like the 1p36 and the Townes-Brocks syndromes, and the molecular mechanisms involving this protein started being uncovered. Proteomics studies placed LUZP1 in the interactomes of the centrosome-cilium interface, centriolar satellites, and midbody. Concordantly, LUZP1 is an actin and microtubule-associated protein, which localizes to the centrosome, the basal body of primary cilia, the midbody, actin filaments and cellular junctions. LUZP1, like its interactor EPLIN, is an actin-stabilizing protein and a negative regulator of primary cilia formation. Moreover, through the regulation of actin, LUZP1 has been implicated in the regulation of cell cycle progression, cell migration and epithelial cell apical constriction. This review discusses the latest findings concerning LUZP1 molecular functions and implications in disease development.

Chromatin remodeling factor CECR2 forms tissue-specific complexes with CCAR2 and LUZP1

Chromatin remodeling complexes alter chromatin structure to control access to DNA and therefore control cellular processes such as transcription, DNA replication, and DNA repair. CECR2 is a chromatin remodeling factor that plays an important role in neural tube closure and reproduction. Loss-of-function mutations in Cecr2 result primarily in the perinatal lethal neural tube defect exencephaly, with non-penetrant mice that survive to adulthood exhibiting subfertility. CECR2 forms a complex with ISWI proteins SMARCA5 and/or SMARCA1, but further information on the structure and function of the complex is not known. We therefore have identified candidate components of the CECR2-containing remodeling factor (CERF) complex in embryonic stem (ES) cells through mass spectroscopy. Both SMARCA5 and SMARCA1 were confirmed to be present in CERF complexes in ES cells and testis. However, novel proteins CCAR2 and LUZP1 are CERF components in ES cells but not testis. This tissue specificity in mice suggests these complexes may also have functional differences. Furthermore, LUZP1, loss of which is also associated with exencephaly, appears to play a role in stabilizing the CERF complex in ES cells. Keywords: CECR2, LUZP1, CCAR2, Chromatin remodeling factor, Neural tube defects.

LUZP1 Controls Cell Division, Migration and Invasion Through Regulation of the Actin Cytoskeleton

LUZP1 is a centrosomal and actin cytoskeleton-localizing protein that regulates both ciliogenesis and actin filament bundling. As the cytoskeleton and cilia are implicated in metastasis and tumor suppression, we examined roles for LUZP1 in the context of cancer. Here we show that LUZP1 exhibits frequent genomic aberrations in cancer, with a predominance of gene deletions. Furthermore, we demonstrate that CRISPR/Cas9-mediated loss of Luzp1 in mouse fibroblasts promotes cell migration and invasion features, reduces cell viability, and increases cell apoptosis, centriole numbers, and nuclear size while altering the actin cytoskeleton. Loss of Luzp1 also induced changes to ACTR3 (Actin Related Protein 3, also known as ARP3) and phospho-cofilin ratios, suggesting regulatory roles in actin polymerization, beyond its role in filament bundling. Our results point to an unprecedented role for LUZP1 in the regulation of cancer features through the control of actin cytoskeleton.

LUZP1 and the tumor suppressor EPLIN modulate actin stability to restrict primary cilia formation

Cilia and flagella are microtubule-based cellular projections with important sensory and motility functions. Their absence or malfunction is associated with a growing number of human diseases collectively referred to as ciliopathies. However, the fundamental mechanisms underpinning cilia biogenesis and functions remain only partly understood. Here, we show that depleting LUZP1 or its interacting protein, EPLIN, increases the levels of MyosinVa at the centrosome and primary cilia formation. We further show that LUZP1 localizes to both actin filaments and the centrosome/basal body. Like EPLIN, LUZP1 is an actin-stabilizing protein that regulates actin dynamics, at least in part, by mobilizing ARP2 to the centrosomes. Both LUZP1 and EPLIN interact with known ciliogenesis and cilia-length regulators and as such represent novel players in actin-dependent centrosome to basal body conversion. Ciliogenesis deregulation caused by LUZP1 or EPLIN loss may thus contribute to the pathology of their associated disease states.

The Dynamic Proteome of Oligodendrocyte Lineage Differentiation Features Planar Cell Polarity and Macroautophagy Pathways

Background

Generation of oligodendrocytes is a sophisticated multistep process, the mechanistic underpinnings of which are not fully understood and demand further investigation. To systematically profile proteome dynamics during human embryonic stem cell differentiation into oligodendrocytes, we applied in-depth quantitative proteomics at different developmental stages and monitored changes in protein abundance using a multiplexed tandem mass tag-based proteomics approach.

Findings

Our proteome data provided a comprehensive protein expression profile that highlighted specific expression clusters based on the protein abundances over the course of human oligodendrocyte lineage differentiation. We identified the eminence of the planar cell polarity signalling and autophagy (particularly macroautophagy) in the progression of oligodendrocyte lineage differentiation—the cooperation of which is assisted by 106 and 77 proteins, respectively, that showed significant expression changes in this differentiation process. Furthermore, differentially expressed protein analysis of the proteome profile of oligodendrocyte lineage cells revealed 378 proteins that were specifically upregulated only in 1 differentiation stage. In addition, comparative pairwise analysis of differentiation stages demonstrated that abundances of 352 proteins differentially changed between consecutive differentiation time points.

Conclusions

Our study provides a comprehensive systematic proteomics profile of oligodendrocyte lineage cells that can serve as a resource for identifying novel biomarkers from these cells and for indicating numerous proteins that may contribute to regulating the development of myelinating oligodendrocytes and other cells of oligodendrocyte lineage. We showed the importance of planar cell polarity signalling in oligodendrocyte lineage differentiation and revealed the autophagy-related proteins that participate in oligodendrocyte lineage differentiation.

LUZP1, a novel regulator of primary cilia and the actin cytoskeleton, is a contributing factor in Townes-Brocks Syndrome

Primary cilia are sensory organelles crucial for cell signaling during development and organ homeostasis. Cilia arise from centrosomes and their formation and function is governed by numerous factors. Through our studies on Townes-Brocks Syndrome (TBS), a rare disease linked to abnormal cilia formation in human fibroblasts, we uncovered the leucine-zipper protein LUZP1 as an interactor of truncated SALL1, a dominantly-acting protein causing the disease. Using TurboID proximity labeling and pulldowns, we show that LUZP1 associates with factors linked to centrosome and actin filaments. Here, we show that LUZP1 is a cilia regulator. It localizes around the centrioles and to actin cytoskeleton. Loss of LUZP1 reduces F-actin levels, facilitates ciliogenesis and alters Sonic Hedgehog signaling, pointing to a key role in cytoskeleton-cilia interdependency. Truncated SALL1 increases the ubiquitin proteasome-mediated degradation of LUZP1. Together with other factors, alterations in LUZP1 may be contributing to TBS etiology.

Identification of Filamin A Mechanobinding Partner II: Fimbacin Is a Novel Actin Cross-Linking and Filamin A Binding Protein

Filamin A (FLNA), an actin cross-linking protein, acts as a mechanosensor and mechanotransducer by exposing the cryptic binding site on repeat 21 (R21) to interact with integrin. Here, we investigated if any other biological molecule interacts with the cryptic binding site. Using proteomics and an in silico screening for a FLNA-binding motif, we identified and characterized a protein termed fimbacin (filamin mechanobinding actin cross-linking protein), encoded in the LUZP1 gene, as a novel FLNA-binding partner. Fimbacin does not interact with canonical full-length FLNA, but the exposure of a cryptic integrin-binding site of FLNA R21 enables fimbacin to interact. We have identified two FLNA binding sites on fimbacin and determined critical amino acid residues for the interaction. We also found that fimbacin itself is a new actin cross-linking protein and mapped the actin-binding site on amino acid residues 400-500. Fimbacin oligomerizes (estimated as an octamer on size exclusion chromatography) through the amino-terminal domain that is predicted to be a coiled-coil to cross-link actin filaments. When expressed, fimbacin localized to actin stress fibers in tissue culture cells. Although the interaction with FLNA is not necessary for fimbacin to colocalize with F-actin, fluorescent recovery after photobleaching (FRAP) revealed that their interaction stabilizes fimbacin on the actin cytoskeleton and that inhibition of Rho-kinase, an upstream activator of myosin II, also decreases the interaction presumably due to a loss of internal mechanical stress. Taken together, these data identify fimbacin as a new actin cross-linking protein that interacts with the FLNA mechanosensing domain R21.

Visible Light-Responsive Platinum-Containing Titania Nanoparticle-Mediated Photocatalysis Induces Nucleotide Insertion, Deletion and Substitution Mutations

Conventional photocatalysts are primarily stimulated using ultraviolet (UV) light to elicit reactive oxygen species and have wide applications in environmental and energy fields, including self-cleaning surfaces and sterilization. Because UV illumination is hazardous to humans, visible light-responsive photocatalysts (VLRPs) were discovered and are now applied to increase photocatalysis. However, fundamental questions regarding the ability of VLRPs to trigger DNA mutations and the mutation types it elicits remain elusive. Here, through plasmid transformation and β-galactosidase α-complementation analyses, we observed that visible light-responsive platinum-containing titania (TiO₂) nanoparticle (NP)-mediated photocatalysis considerably reduces the number of Escherichia coli transformants. This suggests that such photocatalytic reactions cause DNA damage. DNA sequencing results demonstrated that the DNA damage comprises three mutation types, namely nucleotide insertion, deletion and substitution; this is the first study to report the types of mutations occurring after photocatalysis by TiO₂-VLRPs. Our results may facilitate the development and appropriate use of new-generation TiO₂ NPs for biomedical applications.

The human epilepsy mutation GABRG2(Q390X) causes chronic subunit accumulation and neurodegeneration

Genetic epilepsy and neurodegenerative diseases are two common neurological disorders conventionally viewed as being unrelated. A subset of patients with severe genetic epilepsies with impaired development and often death respond poorly to anticonvulsant drug therapy, suggesting a need for new therapeutic targets. Previously, we reported that multiple GABA_A receptor epilepsy mutations caused protein misfolding and abnormal receptor trafficking. Here we establish in a novel model of a severe human genetic epileptic encephalopathy, the Gabrg2^+/Q390X knock-in mouse, that in addition to impairing inhibitory neurotransmission, mutant GABA_A receptor γ2(Q390X) subunits accumulated and aggregated intracellularly, activated caspase 3 and caused widespread, age-dependent neurodegeneration. These novel findings suggest that the fundamental protein metabolism and cellular consequences of the epilepsy-associated mutant γ2(Q390X) ion channel subunit are not fundamentally different from those associated with neurodegeneration. The study has far-reaching significance for identification of conserved pathological cascades and mechanism-based therapies that overlap genetic epilepsies and neurodegenerative diseases.

ATAC and Mediator coactivators form a stable complex and regulate a set of non-coding RNA genes

The Ada-Two-A-containing (ATAC) histone acetyltransferase and Mediator coactivator complexes regulate independent and distinct steps during transcription initiation and elongation. Here, we report the identification of a new stable molecular assembly formed between the ATAC and Mediator complexes in mouse embryonic stem cells. Moreover, we identify leucine zipper motif-containing protein 1 as a subunit of this meta-coactivator complex (MECO). Finally, we demonstrate that the MECO regulates a subset of RNA polymerase II-transcribed non-coding RNA genes. Our findings establish that transcription coactivator complexes can form stable subcomplexes to facilitate their combined actions on specific target genes.

LUZP deficiency affects neural tube closure during brain development

LUZP is a leucine zipper-containing protein predominantly expressed in the brain. The functional significance of LUZP remains unknown. To explore the role of LUZP in brain development, a knockout mouse strain with a lacZ knock-in (Luzp-KO/lacZ-KI) has been established. LacZ reporter expression driven by the endogenous Luzp promoter was detected in the neuroepithelium and the cardiac tissue. Luzp(-/-) mice exhibited perinatal death, presumably due to the accompanied complex cardiovascular defects. Luzp(-/-) embryos displayed a cranial neural tube closure defect (NTD), with exposed brain tissues. Ectopic expression of Sonic-hedgehog, which is a protein known to be involved in neural tube closure, and elevated apoptosis were observed in the dorsal lateral neuroepithelium of the NTD Luzp(-/-) hindbrain. These findings assign a novel function of LUZP in the embryonic development of brain.

Genotypic and phenotypic diversity of cyanobacteria assigned to the genus Phormidium (Oscillatoriales) from different habitats and geographical sites

In this study, 30 strains of filamentous, non-heterocystous cyanobacteria from different habitats and different geographical regions assigned to diverse oscillatorian genera but here collectively referred to as members of the Phormidium group have been characterized using a polyphasic approach by comparing phenotypic and molecular characteristics. The phenotypic analysis dealt with cell and filament morphology, ultrastructure, phycoerythrin content, and complementary chromatic adaptation. The molecular phylogenetic analyses were based on sequences of the 16S rRNA gene and the adjacent intergenic transcribed spacer (ITS). The sequences were located on multiple branches of the inferred cyanobacterial 16S rRNA tree. For some, but not all, strains with identical 16S rDNA sequences, a higher level of discrimination was achieved by analyses of the less conserved ITS sequences. As shown for other cyanobacteria, no correlation was found between position of the strains in the phylogenetic tree and their geographic origin. Genetically similar strains originated from distant sites while other strains isolated from the same sampling site were in different phylogenetic clusters. Also the presence of phycoerythrin was not correlated with the strains' position in the phylogenetic trees. In contrast, there was some correlation among inferred phylogenetic relationship, original environmental habitat, and morphology. Closely related strains came from similar ecosystems and shared the same morphological and ultrastructural features. Nevertheless, structural properties are insufficient in themselves for identification at the genus or species level since some phylogenetically distant members also showed similar morphological traits. Our results reconfirm that the Phormidium group is not phylogenetically coherent and requires revision.