Spacecraft sample collection and subsurface excavation of asteroid (101955) Bennu

Carbonaceous asteroids, such as (101955) Bennu, preserve material from the early Solar System, including volatile compounds and organic molecules. We report spacecraft imaging and spectral data collected during and after retrieval of a sample from Bennu’s surface. The sampling event mobilized rocks and dust into a debris plume, excavating a 9-m-long elliptical crater. This exposed material that is darker, spectrally redder, and more abundant in fine particulates than the original surface. The bulk density of the displaced subsurface material was 500–700 kg per cubic meter, about half that of the whole asteroid. Particulates that landed on instrument optics spectrally resemble aqueously altered carbonaceous meteorites. The spacecraft stored 250 ± 101 g of material, which will be delivered to Earth in 2023.

Episodes of particle ejection from the surface of the active asteroid (101955) Bennu

Active asteroids are those that show evidence of ongoing mass loss. We report repeated instances of particle ejection from the surface of (101955) Bennu, demonstrating that it is an active asteroid. The ejection events were imaged by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft. For the three largest observed events, we estimated the ejected particle velocities and sizes, event times, source regions, and energies. We also determined the trajectories and photometric properties of several gravitationally bound particles that orbited temporarily in the Bennu environment. We consider multiple hypotheses for the mechanisms that lead to particle ejection for the largest events, including rotational disruption, electrostatic lofting, ice sublimation, phyllosilicate dehydration, meteoroid impacts, thermal stress fracturing, and secondary impacts.

Development and application of computer software for cell culture laboratory management

Prototype computer software for a Cell Culture Laboratory Management System (CCLMS) has been developed to relieve cell culture specialists of the burden of manual recordkeeping. Conventional data archives in cell culture laboratories are prone to error and expensive to maintain. The reliance upon cell culture to provide models for biochemical and molecular biological research serves to magnify errors at great expense. The CCLMS prototype encapsulates a modular software application that manages the many aspects of cell culture laboratory recordkeeping. A transaction-based database stores detailed information on subcultures, freezes and thaws, prints waterproof labels for culture vessels, and provides for immediate historical trace-back of any cultured cell line. Linked database files store information specific to an individual culture flask while removing redundancy between similar groups of flasks. A frozen cell log maintains locations of all vials within any type of cryogenic storage unit, locates spaces for newly frozen cell lines, and generates alphabetical or numerical reports. Finally, modules for maintaining cell counts, user records, and culture vessel specifications to support a comprehensive automation process are incorporated within this software. The developed CCLMS prototype has been demonstrated to be an adaptable, reliable tool for improving training, efficiency, and historical rigor for two independent cell culture facilities.

The vitamin D hormone and its nuclear receptor: molecular actions and disease states

Vitamin D plays a major role in bone mineral homeostasis by promoting the transport of calcium and phosphate to ensure that the blood levels of these ions are sufficient for the normal mineralization of type I collagen matrix in the skeleton. In contrast to classic vitamin D-deficiency rickets, a number of vitamin D-resistant rachitic syndromes are caused by acquired and hereditary defects in the metabolic activation of the vitamin to its hormonal form, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), or in the subsequent functions of the hormone in target cells. The actions of 1,25(OH)2D3 are mediated by the nuclear vitamin D receptor (VDR), a phosphoprotein which binds the hormone with-high affinity and regulates the expression of genes via zinc finger-mediated DNA binding and protein-protein interactions. In hereditary hypocalcemic vitamin D-resistant rickets (HVDRR), natural mutations in human VDR that confer patients with tissue insensitivity to 1,25(OH)2D3 are particularly instructive in revealing VDR structure function relationships. These mutations fall into three categories: (i) DNA binding/nuclear localization, (ii) hormone binding and (iii) heterodimerization with retinoid X receptors (RXRs). That all three classes of VDR mutations generate the HVDRR phenotype is consistent with a basic model of the active receptor as a DNA-bound, 1,25(OH)2D3-liganded heterodimer of VDR and RXR. Vitamin D responsive elements (VDREs) consisting of direct hexanucleotide repeats with a spacer of three nucleotides have been identified in the promoter regions of positively controlled genes expressed in bone, such as osteocalcin, osteopontin, beta 3-integrin and vitamin D 24-OHase. The 1,25(OH)2D3 ligand promotes VDR-RXR heterodimerization and specific, high affinity VDRE binding, whereas the ligand for RXR, 9-cis retinoic acid (9-cis RA), is capable of suppressing 1,25(OH)2D3-stimulated transcription by diverting RXR to form homodimers. However, initial 1,25(OH)2D3 liganding of a VDR monomer renders it competent not only to recruit RXR into a heterodimer but also to conformationally silence the ability of its RXR partner to bind 9-cis RA and dissociate the heterodimer. Additional probing of protein-protein interactions has revealed that VDR also binds to basal transcription factor IIB (TFIIB) and, in the presence of 1,25(OH)2D3, an RXR-VDR-TFIIB ternary complex can be created in solution. Moreover, for transcriptional activation by 1,25(OH)2D3, both VDR and RXR require an intact short amphipathic alpha-helix, known as AF-2, positioned at their extreme C-termini. Because the AF-2 domains participate neither in VDR-RXR heterodimerization nor in TFIIB association, it is hypothesized that they contact, in a ligand-dependent fashion, transcriptional coactivators such as those of the steroid receptor coactivator family, constituting yet a third protein-protein interaction for VDR. Therefore, in VDR-mediated transcriptional activation, 1,25(OH)2D3 binding to VDR alters the conformation of the ligand binding domain such that it: (i) engages in strong heterodimerization with RXR to facilitate VDRE binding, (ii) influences the RXR ligand binding domain such that it is resistant to the binding of 9-cis RA but active in recruiting coactivator to its AF-2 and (iii) presents the AF-2 region in VDR for coactivator association. The above events, including bridging by coactivators to the TATA binding protein and associated factors, may position VDR such that it is able to attract TFIIB and the balance of the RNA polymerase II transcription machinery, culminating in repeated transcriptional initiation of VDRE-containing, vitamin D target genes. Such a model would explain the action of 1,25(OH)2D3 to elicit bone remodeling by stimulating osteoblast and osteoclast precursor gene expression, while concomitantly triggering the termination of its hormonal signal by inducing the 24-OHase catabolizing enzyme.

Vitamin D receptors from patients with resistance to 1,25-dihydroxyvitamin D3: point mutations confer reduced transactivation in response to ligand and impaired interaction with the retinoid X receptor heterodimeric partner

Hereditary hypocalcemic vitamin D-resistant rickets is attributable to defects in the nuclear receptor for 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Two novel point mutations (I314S and R391C) identified in the hormone-binding domain of the human vitamin D receptor (VDR) from patients with hereditary hypocalcemic vitamin D-resistant rickets confer the receptor with sharply reduced 1,25-(OH)2D3-dependent transactivation. These natural mutations, especially R391C, also lead to a second specific consequence, namely impaired heterodimeric interaction with retinoid X receptor (RXR). While the transactivation ability of the I314S mutant can be largely restored by providing excess 1,25-(OH)2D3, R391C activity is more effectively restored with exogenous RXR. These observations are reflected also in the clinical course of each patient: the patient bearing the I314S mutation showed a nearly complete cure with pharmacological doses of a vitamin D derivative, whereas the patient bearing R391C responded only partially to such therapy. Further tests with patient fibroblasts and transfected cells show that the activity of the I314S VDR mutant is augmented somewhat by added RXR, while transactivation by the R391C mutant is best corrected by RXR in the presence of excess hormone. Thus, the effects of hormone vs. RXR in bolstering these mutant VDRs, such that they mediate efficient transactivation, are not entirely separable. The unique properties of these genetically altered receptors establish a new subclass of natural human VDR mutants that illustrate, in vivo, the importance of both 1,25-(OH)2D3 binding and heterodimerization with RXR in VDR action.

The T-box near the zinc fingers of the human vitamin D receptor is required for heterodimeric DNA binding and transactivation

The T-box mediates binding of retinoid X receptor (RXR) homodimers to DNA while the P- and D-box in the zinc fingers of steroid hormone receptors play roles in DNA-binding specificity and homodimerization, respectively. We investigated the function of these elements in the human vitamin D receptor (hVDR) by mutating a Lys-Glu pair of amino acids in the T-box, and by altering the P- and D-boxes to the corresponding residues of the glucocorticoid receptor (GR). The T-box mutant hVDR displayed attenuated vitamin D responsive element (VDRE) binding in the presence of RXR and was severely compromised in transcriptional activation. In contrast, GR P/D-box mutant hVDRs bound to the rat osteocalcin VDRE and elicited near normal transcriptional activation. The T-box mutant uniquely exhibited dominant negative properties, highlighting the significance of this region of hVDR for heterodimeric transcriptional activation.

New understanding of the molecular mechanism of receptor-mediated genomic actions of the vitamin D hormone

The nuclear vitamin D receptor (VDR) binds the 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]hormone with high affinity and elicits its actions to regulate gene expression in target cells by binding to vitamin D-responsive elements (VDREs). VDREs in positively controlled genes such as osteocalcin, osteopontin, beta 3-integrin, and vitamin D-24-OHase are direct hexanucleotide repeats with a spacer of three nucleotides. The VDR associates with these VDREs with the greatest affinity as a heterodimer with one of the family of retinoid X receptors (RXRs). VDR consists of an N-terminal zinc finger domain that determines DNA binding, a "hinge" segment and a C-terminal hormone binding domain which also contains two conserved regions that engage in heterodimerization with an RXR on the VDRE. The role of the 1,25(OH)2D3 ligand in transcriptional activation by the VDR-RXR heterodimer is to alter the conformation of the hormone-binding domain of VDR to facilitate strong dimerization with RXR, which results in ligand-enhanced association with the VDRE. Thus RXR is recruited into a heterocomplex by liganded VDR. The natural ligand for the RXR coreceptor, 9-cis retinoic acid, suppresses both VDR-RXR binding to the VDRE and 1,25(OH)2D3-stimulated transcription, indicating that 9-cis retinoic acid diverts RXR away from being the silent partner of VDR to instead form RXR homodimers. Recent data reveal that after binding RXR, a subsequent target for VDR in the vitamin D signal transduction cascade is basal transcription factor IIB (TFIIB).(ABSTRACT TRUNCATED AT 250 WORDS)

Genomic actions of 1,25-dihydroxyvitamin D3

Recent studies have identified a heterodimer of the vitamin D receptor (VDR) and the retinoid X receptor (RXR) as the active complex for mediating positive transcriptional effects of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], the active hormonal form of vitamin D. The VDR-RXR heterodimer has been shown to bind to direct repeat vitamin D-responsive elements (VDREs) upstream of positively controlled genes in the target tissues for vitamin D, including bone (osteocalcin, osteopontin, and beta 3 integrin), kidney (24-hydroxylase) and intestine (calbindin). Residues that participate in heterodimer formation have been identified in the C-terminal hormone-binding domain by analysis of VDR mutants. The role of the 1,25(OH)2D3 ligand in transcriptional activation by the VDR-RXR heterodimer is not entirely clear, but studies of two natural VDR mutants suggest that the binding of both hormone and RXR are required to induce a receptor conformation that is competent to activate transcription. A final level of complexity is added by recent observations that VDR is modified by phosphorylation. Thus, the VDR-mediated action of 1,25(OH)2D3 is now known to involve multiple factors that may provide a conceptual basis for future understanding of the tissue-specific genomic effects of 1,25(OH)2D3.