Flow sorting of hybrid hybridomas using the DNA stain Hoechst 33342

Isolation and manipulation of mouse trophoblast stem cells

The isolation of stable trophoblast stem (TS) cell lines from early mouse embryos has provided a useful cell culture model to study trophoblast development. TS cells are derived from pre-implantation blastocysts or from the extraembryonic ectoderm of early post-implantation embryos. The derivation and maintenance of mouse TS cells is dependent upon continuous fibroblast growth factor (FGF) signaling. Gene expression analysis, differentiation in culture, and chimera formation show that TS cells accurately model the mouse trophoblast lineage. This unit describes how to derive, maintain, and manipulate TS cells, including DNA transfection and chimera formation.

Solid-Phase Synthesis of Positively Charged Deoxynucleic Guanidine (DNG) Tethering a Hoechst 33258 Analogue: Triplex and Duplex Stabilization by Simultaneous Minor Groove Binding

Deoxynucleic guanidine (DNG), a DNA analogue in which positively charged guanidine replaces the phosphodiester linkages, tethering to Hoechst 33258 fluorophore by varying lengths has been synthesized. A pentameric thymidine DNG was synthesized on solid phase in the 3' --> 5' direction that allowed stepwise incorporation of straight chain amino acid linkers and a bis-benzimidazole (Hoechst 33258) ligand at the 5'-terminus using PyBOP/HOBt chemistry. The stability of (DNA)(2).DNG-H triplexes and DNA.DNG-H duplexes formed by DNG and DNG-Hoechst 33258 (DNG-H) conjugates with 30-mer double-strand (ds) DNA, d(CGCCGCGCGCGCGAAAAACCCGGCGCGCGC)/d(GCGGCGCGCGCGCTTTTTGGGCCGCGCGCG), and single-strand (ss) DNA, 5'-CGCCGCGCGCGCGAAAAACCCGGCGCGCGC-3', respectively, has been evaluated by thermal melting and fluorescence emission experiments. The presence of tethered Hoechst ligand in the 5'-terminus of the DNG enhances the (DNA)(2).DNG-H triplex stability by a DeltaT(m) of 13 degrees C. The fluorescence emission studies of (DNA)(2).DNG-H triplex complexes show that the DNG moiety of the conjugates bind in the major groove while the Hoechst ligand resides in the A:T rich minor groove of dsDNA. A single G:C base pair mismatch in the target site decreases the (DNA)(2).DNG triplex stability by 11 degrees C, whereas (DNA)(2).DNG-H triplex stability was decreased by 23 degrees C. Inversion of A:T base pair into T:A base pair in the center of the binding site, which provides a mismatch selectively for DNG moiety, decreases the triplex stability by only 5-6 degrees C. Upon hybridization of DNG-Hoechst conjugates with the 30-mer ssDNA, the DNA.DNG-H duplex exhibited significant increase in the fluorescence emission due to the binding of the tethered Hoechst ligand in the generated DNA.DNG minor groove, and the duplex stability was enhanced by DeltaT(m) of 7 degrees C. The stability of (DNA)(2).DNG triplexes and DNA.DNG duplexes is independent of pH, whereas the stability of (DNA)(2).DNG-H triplexes decreases with increase in pH.

Oligonucleotides tethering Hoechst 33258 derivatives: effect of the conjugation site on duplex stabilization and fluorescence properties

A series of DNA conjugates have been prepared in which two different derivatives of Hoechst 33258 have been tethered to a sequence containing a 5'-GAATTC-3' target site. The two derivatives differ only in the length of the tether between the DNA and the Hoechst fluorophore. By using a DNA backbone labeling protocol, one in which the Hoechst dye is tethered to an internucleotide phosphoramidate residue, it was possible to easily vary the site of attachment with respect to the A-T rich binding site. When tethered outside the GAATTC sequence, little if any helix stabilization results upon hybridization of the conjugate to its complementary sequence. As the site of conjugation is moved to one end of the target sequence and finally within the AATT sequence, more effective helix stabilization results. When tethered between the two A residues, or between the A and T residue, a delta Tm of at least +20 degrees C is observed. Upon hybridization and formation of the B-form DNA, binding by the tethered Hoechst dye results, and the bound dye becomes brightly fluorescent. Upon a simple titration of the single-stranded conjugate with the complementary target sequence the quantum yield enhancement for hybridization only appears to be 5-7-fold at best. These fluorescence effects, generally less dramatic than those observed with other sequences, result from an increase in quantum yield for the single-stranded conjugate relative to the free Hoechst 33258. Heating the single-stranded conjugate reduces the inherent fluorescence of the single-stranded conjugate to a level comparable with that of the free Hoechst dye. In experiments monitoring absorbance vs temperature, a cooperative transition is observed for the single-stranded conjugate. Both the high quantum yield observed for the single-stranded conjugate and the observed thermally induced transition suggest that the single-stranded conjugate can dimerize (at the GAATTC site), mediated by the groove-binding fluorophore.

Hoechst 33258 Tethered by a Hexa(ethylene glycol) Linker to the 5'-Termini of Oligodeoxynucleotide 15-Mers: Duplex Stabilization and Fluorescence Properties

A fluorescent Hoechst 33258 derivative has been prepared in which a hexa(ethylene glycol) linker is attached to the terminal phenol residue. Conjugation of this derivative to DNA sequences is accomplished by a reversed coupling protocol, one in which the 5'-terminal nucleoside residue of a fully protected DNA sequence is converted to a terminal phosphoramidite. In the presence of the Hoechst derivative and tetrazole the final coupling reaction is achieved to generate the conjugated nucleic acid. After deprotection and cleavage of the conjugate from the support, HPLC analysis indicates that the conjugation reaction proceeds with yields as high as 75%. The presence of the conjugated Hoechst derivative increases the stability of DNA duplexes typically by 10-16 degrees C. A variety of sequence variants indicate that the tether length is sufficient to reach beyond the terminus of the DNA duplex and bind to internal A-T rich target sequences as far away as four base pairs from the site of attachment. A four base pair binding site appears to be necessary for effective helix stabilization by the conjugate, but in some cases can include a G-C base pair, which is consistent with a previous X-ray diffraction study regarding the binding of Hoechst 33258 to duplex DNA. When A-T base pairs alternate with G-C base pairs, a small but discernible increase is T(m) is observed (3.6 degrees C), indicating that binding to this sequence still occurs, but not in the same manner as to A-T rich sequences. Upon formation of the conjugated duplex, an enhanced quantum yield for the fluorescence emission spectrum of the tethered Hoechst derivative is observed. When an A-T rich binding site is present, the enhanced quantum yield increases by at least 16- and in some cases to nearly 30-fold relative to the value obtained for the single-stranded DNA-Hoechst conjugate.

Synthesis and properties of a Hoechst-like minor-groove binding agent tethered to an oligodeoxynucleotide

A small Hoechst-like DNA groove-binding fluorophore carrying a terminal bromoacetimido linker has been synthesized. Individual diastereomeric oligodeoxynucleotide dodecamers containing a thiol-based linker attached to an internucleotide phosphoramidate within a sequence of dA-dT residues were each covalently labeled with the new fluorophore. One isomer of the DNA-fluorophore conjugate (Isomer B) hybridizes to the complementary single-stranded target DNA and the presence of the tethered fluorophore results in both increased duplex stability and an enhanced fluorescent signal, presumably due to the fluorophore binding in the dA-dT rich minor groove. Duplex stability is increased by 2-3 degrees C and the fluorescence quantum yield for the fluorophore increases four-fold. In contrast, the other diastereomer (Isomer A) exhibits reduced helix stability (approximately 2 degrees C) and only slight changes in fluorescence intensity upon hybridization.

A flow cytometric long-term cytotoxicity assay

A method to evaluate cytotoxic effects applicable over a wide range of incubation times has been developed. It is based on quantification by flow cytometry of dead and viable target cells stained by covalently binding the fluorescent dye fluorescein isothiocyanate (FITC). The staining with FITC did not affect cell viability and growth parameters and was stable enough to identify target cells for at least 2 days. The flow cytometric analysis of the cell mixture was performed in a test system with activated CD8+ lymphocytes as effector cells and melanoma M21 cells as targets in the presence of appropriate bispecific antibodies and revealed a rather complex pattern composed of several distinct cell subsets which were identified by use of antibodies to lymphocyte antigens. The assay compared favourably with results from a conventional 51Cr release assay obtained after 4 h and 8 h of incubation and from a target cell adherence assay obtained after 24 h of incubation. The application of the method described herein is especially advantageous for the evaluation of long-term cytotoxic effects. Furthermore, it provides valuable multi-parameter information which is useful for elucidating mechanisms of cytotoxicity.

Altered reactivity of immunoglobulin produced by human-human hybridoma cells transfected by pSV2-neo gene

The HB4C5 and HF10B4 cell lines are human-human hybridomas producing human IgM monoclonal antibodies (MAbs) reactive to porcine carboxypeptidase A (CPase), but not to double stranded DNA (ds DNA). We obtained G418-resistant HB4C5 and HF10B4 cells by an introduction of pSV2-neo DNA. Almost all of the G418-resistant clones produced MAbs reactive to not only the CPase but the ds DNA. The results of the inhibition ELISA suggested that the cross-reactivity of the antibodies from G418-resistant clones to CPase and ds DNA was responsible for the alteration on their antigen specificity. HB4C5 and HF10B4 cells and their G418-resistant clones produced antibodies having glycosylated lambda chain. The antibodies produced by tunicamycin-treated G418-resistant subclones of HB4C5 and HF10B4 lost the ability to bind to ds DNA, but retained the ability to bind to CPase. These results suggest that an introduction of pSV2-neo DNA into these hybridomas alters the specificities of their MAbs, and that the alteration to antigen binding specificities of their MAbs may be associated with glycosylation of the MAbs by these hybridomas.