HDAC9 structural variants disrupting TWIST1 transcriptional regulation lead to craniofacial and limb malformations.

Structural variants (SVs) can affect protein-coding sequences as well as gene regulatory elements. However, SVs disrupting protein-coding sequences that also function as cis-regulatory elements remain largely uncharacterized. Here, we show that craniosynostosis patients with SVs containing the histone deacetylase 9 (HDAC9) protein-coding sequence are associated with disruption of TWIST1 regulatory elements that reside within the HDAC9 sequence. Based on SVs within the HDAC9-TWIST1 locus, we defined the 3'-HDAC9 sequence as a critical TWIST1 regulatory region, encompassing craniofacial TWIST1 enhancers and CTCF sites. Deletions of either Twist1 enhancers (eTw5-7Δ/Δ) or CTCF site (CTCF-5Δ/Δ) within the Hdac9 protein-coding sequence led to decreased Twist1 expression and altered anterior/posterior limb expression patterns of SHH pathway genes. This decreased Twist1 expression results in a smaller sized and asymmetric skull and polydactyly that resembles Twist1+/- mouse phenotype. Chromatin conformation analysis revealed that the Twist1 promoter interacts with Hdac9 sequences that encompass Twist1 enhancers and a CTCF site, and that interactions depended on the presence of both regulatory regions. Finally, a large inversion of the entire Hdac9 sequence (Hdac9 INV/+) in mice that does not disrupt Hdac9 expression but repositions Twist1 regulatory elements showed decreased Twist1 expression and led to a craniosynostosis-like phenotype and polydactyly. Thus, our study elucidates essential components of TWIST1 transcriptional machinery that reside within the HDAC9 sequence. It suggests that SVs encompassing protein-coding sequences could lead to a phenotype that is not attributed to its protein function but rather to a disruption of the transcriptional regulation of a nearby gene.

Unraveling the transcriptional regulation of TWIST1 in limb development.

The transcription factor TWIST1 plays a vital role in mesoderm development, particularly in limb and craniofacial formation. Accordingly, haploinsufficiency of TWIST1 can cause limb and craniofacial malformations as part of Saethre-Chotzen syndrome. However, the molecular basis of TWIST1 transcriptional regulation during development has yet to be elucidated. Here, we characterized active enhancers in the TWIST1-HDAC9 locus that drive transcription in the developing limb and branchial arches. Using available p300 and H3K27ac ChIP-seq data, we identified 12 enhancer candidates, located both within and outside the coding sequences of the neighboring gene, Histone deacetyase 9 (HDAC9). Using zebrafish and mouse enhancer assays, we showed that eight of these candidates have limb/fin and branchial arch enhancer activity that resemble Twist1 expression. Using 4C-seq, we showed that the Twist1 promoter region interacts with three enhancers (eTw-5, 6, 7) in the limb bud and branchial arch of mouse embryos at day 11.5. Furthermore, we found that two transcription factors, LMX1B and TFAP2, bind these enhancers and modulate their enhancer activity. Finally, using CRISPR/Cas9 genome editing, we showed that homozygous deletion of eTw5-7 enhancers reduced Twist1 expression in the limb bud and caused pre-axial polydactyly, a phenotype observed in Twist1+/- mice. Taken together, our findings reveal that each enhancer has a discrete activity pattern, and together comprise a spatiotemporal regulatory network of Twist1 transcription in the developing limbs/fins and branchial arches. Our study suggests that mutations in TWIST1 enhancers could lead to reduced TWIST1 expression, resulting in phenotypic outcome as seen with TWIST1 coding mutations.

The 3-D structure of VNG0258H/RosR - A haloarchaeal DNA-binding protein in its ionic shell.

Protein-DNA interactions are highly dependent on salt concentration. To gain insight into how such interactions are maintained in the highly saline cytoplasm of halophilic archaea, we determined the 3-D structure of VNG0258H/RosR, the first haloarchaeal DNA-binding protein from the extreme halophilic archaeon Halobactrium salinarum. It is a dimeric winged-helix-turn-helix (wHTH) protein with unique features due to adaptation to the halophilic environment. As ions are major players in DNA binding processes, particularly in halophilic environments, we investigated the solution structure of the ionic envelope and located anions in the first shell around the protein in the crystal using anomalous scattering. Anions that were found to be tightly bound to residues in the positively charged DNA-binding site would probably be released upon DNA binding and will thus make significant contribution to the driving force of the binding process. Unexpectedly, ions were also found in a buried internal cavity connected to the external medium by a tunnel. Our structure lays a solid groundwork for future structural, computational and biochemical studies on complexes of the protein with cognate DNA sequences, with implications to protein-DNA interactions in hyper-saline environments.

The macromolecular architecture of platelet-derived microparticles.

Platelets are essential for hemostasis and wound healing. They are involved in fundamental processes of vascular biology such as angiogenesis, tissue regeneration, and tumor metastasis. Upon activation, platelets shed small plasma membrane vesicles termed platelet-derived microparticles (PMPs). PMPs include functional cell adhesion machinery that comprises transmembrane receptors (most abundant are the αIIbß3 integrins), cytoskeletal systems and a large variety of adapter and signaling molecules. Glanzmann thrombasthenia (GT) is a condition characterized by platelets that are deficient of the integrin αIIbß3 heterodimer. Here, we use cryo-electron tomography (cryo-ET) to study the structural organization of PMPs (in both healthy and GT patients), especially the cytoskeleton organization and receptor architecture. PMPs purified from GT patients show a significantly altered cytoskeletal organization, characterized by a reduced number of filaments present, compared to the healthy control. Furthermore, our results show that incubating healthy PMPs with manganese ions (Mn(2+)), in the presence of fibrinogen, induces a major conformational change of integrin receptors, whereas thrombin activation yields a moderate response. These results provide the first insights into the native molecular organization of PMPs.

Effectiveness of a cognitive-functional group intervention among preschoolers with attention deficit hyperactivity disorder: a pilot study.

OBJECTIVE: To test functional improvement after a group cognitive-functional occupational therapy intervention for preschoolers with attention deficit hyperactivity disorder (ADHD). METHOD: Seventeen preschooler-parent dyads attended 11 weekly group sessions focused on acquiring executive strategies through occupational performance. Functional improvement was measured using the Canadian Occupational Performance Measure (COPM) and Goal Attainment Scaling (GAS); executive function, using the Behavior Rating Inventory of Executive Function-Pediatric; ADHD symptomatology, using Conners' Parent Rating Scale-Revised and Conners' Teacher Rating Scale-Revised; and social functioning, using the Social Participation scale of the Sensory Processing Measure. RESULTS: Significant improvement was found on the COPM and GAS measures, whereas mixed results were found on the other measures, with improvements found in children whose scores indicated impairment at baseline. CONCLUSIONS: Cognitive-functional group intervention appears to significantly improve daily functioning, executive function, and social functioning for children who demonstrate clinical impairment. Further research with a larger sample, a control group, and follow-up is required.

Reconstructing adhesion structures in tissues by cryo-electron tomography of vitrified frozen sections.

Cryo-electron tomography enables three-dimensional insights into the macromolecular architecture of cells in a close-to-life state. However, it is limited to thin specimens, <1.0 µm in thickness, typically restricted to the peripheral areas of intact eukaryotic cells. Analysis of tissue ultrastructure, on the other hand, requires physical sectioning approaches, preferably cryo-sectioning, following which electron tomography (ET) may be performed. Nevertheless, cryo-electron microscopy of vitrified sections is a demanding technique and typically cannot be used to examine thick sections, >80-100 nm, due to surface crevasses. Here, we explore the potential use of cryo-ET of vitrified frozen sections (VFSs) for imaging cell adhesions in chicken smooth muscle and mouse epithelial tissues. By investigating 300-400 nm thick sections, which are collected on the EM grid and re-vitrified, we resolved fine 3D structural details of the membrane-associated dense plaques and flanking caveoli in smooth muscle tissue, and desmosomal adhesions in stratified epithelium. Technically, this method offers a simple approach for reconstructing thick volumes of hydrated frozen sections.

Filaments assembly of ectopically expressed Caenorhabditis elegans lamin within Xenopus oocytes.

Lamins are the major components of the nuclear lamina, a filamentous layer underlying the inner nuclear membrane and attached to the peripheral chromatin. Lamins are required for maintaining nuclear shape and are involved in most nuclear activities. Here, we studied the 3D organization of the nuclear lamina formed upon the expression of Caenorhabditis elegans lamin (Ce-lamin) within the nucleus of a Xenopus laevis oocyte. We show that Ce-lamin forms an intricate 3D meshwork of 5-6 nm lamin protofilaments. The diverse protofilament interactions and organization may shed light upon the unique mechano-elastic properties of the nuclear lamina scaffold supporting the nuclear envelope. The Q159K Hutchinson-Gilford Progeria Syndrome-linked mutation alters interactions between protofilaments within the lamina, leading to the formation of more bundled arrays of less isotropically-oriented protofilaments. Using this system, we show for the first time the organization of lamin proteins that were translated and assembled within the environment of a living cell.

Tiopronin-Protected Gold Nanoparticles as a Potential Marker for Cryo-EM and Tomography.

Gold nanoparticles (AuNPs) and their conjugation to biological samples have numerous potential applications. When combined with cryo-electron microscopy and tomography analysis, AuNPs may provide a versatile and powerful tool to identify and precisely localize proteins even when attached to cellular components. Here, we describe a general and facile approach for the synthesis of homogeneous and stable AuNPs, which can readily be conjugated to a molecule of interest and imaged by cryo-electron tomography (cryo-ET). We demonstrate the synthesis of 2.2 ± 0.45-nm tiopronin-protected AuNPs, followed by their conjugation with recombinant proteins and peptides. Visualization of the ∼2.2-nm gold-tagged peptides by cryo-ET reveals the potential use of this strategy to label and localize accessible proteins in a cellular environment with nanometric resolution.

The human nuclear pore complex as revealed by cryo-electron tomography.

Nuclear pore complexes (NPCs) are the sole passage through the nuclear envelope, connecting the cytoplasm to the nucleoplasm. These gigantic molecular machines, over 100 MDa in molecular weight, allow free diffusion of small molecules and ions while mediating selective energy-dependent nucleocytoplasmic transport of large macromolecules. Here, we applied cryo-electron tomography to human fibroblast cells, reconstructing their nuclear envelopes without applying any purification steps. From these reconstructions, we extracted subtomograms containing individual NPCs and utilized in silico subtomogram averaging procedures to determine the structure of the mammalian pore complex at a resolution of ∼6.6 nm. Beyond revealing the canonical features of the human NPC, our analysis identified inner lateral channels and fusing bridge-like structures, suggesting alternative routes of peripheral nuclear passage. Finally, we concluded from our structural analysis that the human NPC is structurally distinct from that of lower eukaryotes in terms of dimension and organization but resembles its amphibian (frog) counterpart.

Cryo-electron tomography: gaining insight into cellular processes by structural approaches.

Visualization of cellular processes at a resolution of the individual protein should involve integrative and complementary approaches that can eventually draw realistic functional and cellular landscapes. Electron tomography of vitrified but otherwise unaltered cells emerges as a central method for three-dimensional reconstruction of cellular architecture at a resolution of 2-6 nm. While a combination of correlative light-based microscopy with cryo-electron tomography (cryo-ET) provides medium-resolution insight into pivotal cellular processes, fitting high-resolution structural approaches, for example, X-ray crystallography, into reconstructed macromolecular assemblies provides unprecedented information on native protein assemblies. Thus, cryo-ET bridges the resolution gap between cellular and structural biology. In this article, we focus on the study of eukaryotic cells and macromolecular complexes in a close-to-life-state. We discuss recent developments and structural findings enabling major strides to be made in understanding complex physiological functions.

SRP19 is a dispensable component of the signal recognition particle in Archaea.

In vitro, archaeal SRP54 binds SRP RNA in the absence of SRP19, suggesting the latter to be expendable in Archaea. Accordingly, the Haloferax volcanii SRP19 gene was deleted. Although normally transcribed at a level comparable to that of the essential SRP54 gene, SRP19 deletion had no effect on cell growth, membrane protein insertion, protein secretion, or ribosome levels. The absence of SRP19 did, however, increase membrane bacterioruberin levels.

Cloning, expression, and purification of functional Sec11a and Sec11b, type I signal peptidases of the archaeon Haloferax volcanii.

Across evolution, type I signal peptidases are responsible for the cleavage of secretory signal peptides from proteins following their translocation across membranes. In Archaea, type I signal peptidases combine domain-specific features with traits found in either their eukaryal or bacterial counterparts. Eukaryal and bacterial type I signal peptidases differ in terms of catalytic mechanism, pharmacological profile, and oligomeric status. In this study, genes encoding Sec11a and Sec11b, two type I signal peptidases of the halophilic archaeon Haloferax volcanii, were cloned. Although both genes are expressed in cells grown in rich medium, gene deletion approaches suggest that Sec11b, but not Sec11a, is essential. For purification purposes, tagged versions of the protein products of both genes were expressed in transformed Haloferax volcanii, with Sec11a and Sec11b being fused to a cellulose-binding domain capable of interaction with cellulose in hypersaline surroundings. By employing an in vitro signal peptidase assay designed for use with high salt concentrations such as those encountered by halophilic archaea such as Haloferax volcanii, the signal peptide-cleaving activities of both isolated membranes and purified Sec11a and Sec11b were addressed. The results show that the two enzymes differentially cleave the assay substrate, raising the possibility that the Sec11a and Sec11b serve distinct physiological functions.