Cell sorting with Hoechst or carbocyanine dyes as perfusion probes in spheroids and tumors

3-D tissue culture systems for the evaluation and optimization of nanoparticle-based drug carriers

Nanoparticle carriers are attractive vehicles for a variety of drug delivery applications. In order to evaluate nanoparticle formulations for biological efficacy, monolayer cell cultures are typically used as in vitro testing platforms. However, these studies sometimes poorly predict the efficacy of the drug in vivo. The poor in vitro and in vivo correlation may be attributed in part to the inability of two-dimensional cultures to reproduce extracellular barriers, and may also be due to differences in cell phenotype between cells cultured as monolayers and cells in native tissue. In order to more accurately predict in vivo results, it is desirable to test nanoparticle therapeutics in cells cultured in three-dimensional (3-D) models that mimic in vivo conditions. In this review, we discuss some 3-D culture systems that have been used to assess nanoparticle delivery and highlight several implications for nanoparticle design garnered from studies using these systems. While our focus will be on nanoparticle drug formulations, many of the systems discussed here could, or have been, used for the assessment of small molecule or peptide/protein drugs. We also offer some examples of advancements in 3-D culture that could provide even more highly predictive data for designing nanoparticle therapeutics for in vivo applications.

Uncertainty in the niches that maintain haematopoietic stem cells

Haematopoietic stem cell (HSC) niches are specialized microenvironments that contain stem cells and regulate their maintenance. Cells at the interface of bone and the bone marrow (the endosteum) contribute to the creation of HSC niches. It remains uncertain whether this interface itself is a niche, or whether endosteal cells secrete factors that diffuse to nearby niches. Vascular and/or perivascular cells may also create niches as many HSCs are observed around sinusoidal blood vessels, and perivascular cells secrete factors that regulate HSC maintenance. Do endosteal and perivascular cells create distinct niches, or do they contribute to a common niche? We discuss a range of niche models consistent with recent evidence.

Distribution of hematopoietic stem cells in the bone marrow according to regional hypoxia

The interaction of stem cells with their bone marrow microenvironment is a critical process in maintaining normal hematopoiesis. We applied an approach to resolve the spatial organization that underlies these interactions by evaluating the distribution of hematopoietic cell subsets along an in vivo Hoechst 33342 (Ho) dye perfusion gradient. Cells isolated from different bone marrow regions according to Ho fluorescence intensity contained the highest concentration of hematopoietic stem cell (HSC) activity in the lowest end of the Ho gradient (i.e., in the regions reflecting diminished perfusion). Consistent with the ability of Ho perfusion to simulate the level of oxygenation, bone marrow fractions separately enriched for HSCs were found to be the most positive for the binding of the hypoxic marker pimonidazole. Moreover, the in vivo administration of the hypoxic cytotoxic agent tirapazamine exhibited selective toxicity to the primitive stem cell subset. These data collectively indicate that HSCs and the supporting cells of the stem cell niche are predominantly located at the lowest end of an oxygen gradient in the bone marrow with the implication that regionally defined hypoxia plays a fundamental role in regulating stem cell function.

Mechanisms Underlying Production of Double-Strand Breaks in Plasmid DNA after Decay of125I-Hoechst

Previously, the kinetics of strand break production by (125)I-labeled m-iodo-p-ethoxyHoechst 33342 ((125)IEH) in supercoiled (SC) plasmid DNA had demonstrated that approximately 1 DSB is produced per (125)I decay both in the presence and absence of the hydroxyl radical scavenger DMSO. In these experiments, an (125)IEH:DNA molar ratio of 42:1 was used. We now hypothesize that this DSB yield (but not the SSB yield) may be an overestimate due to subsequent decays occurring in any of the 41 (125)IEH molecules still bound to nicked (N) DNA. To test our hypothesis, (125)IEH was incubated with SC pUC19 plasmids ((125)IEH:DNA ratio of approximately 3:1) and the SSB and DSB yields were quantified after the decay of (125)I. As predicted, the number of DSBs produced per (125)I decay is one-half that reported previously ( approximately 0.5 compared to approximately 1, +/- DMSO) whereas the number of SSBs ( approximately 3/(125)I decay) is similar to that obtained previously ( approximately 90% are generated by OH radicals). Direct visualization by atomic force microscopy confirms formation of L and N DNA after (125)IEH decays in SC DNA and supports the strand break yields reported. These findings indicate that although SSB production is independent of the number of (125)IEH bound to DNA, the DSB yield can be augmented erroneously by (125)I decays occurring in N DNA. Further analysis indicates that 17% of SSBs and 100% of DSBs take place within the plasmid molecule in which an (125)IEH molecule decays, whereas 83% of SSBs are formed in neighboring plasmid DNA molecules.

Current understanding of stem cell mobilization: the roles of chemokines, proteolytic enzymes, adhesion molecules, cytokines, and stromal cells

Mobilization of hematopoietic stem and progenitor cells from the bone marrow into the circulation by repetitive, daily stimulations with G-CSF alone, or in combination with cyclophosphamide, is increasingly used clinically; however, the mechanism is not fully understood. Moreover, following mobilization stem cells also home back to the bone marrow, suggesting that stem cell release/mobilization and homing are sequential events with physiological roles. Previously, a role for cytokines such as G-CSF and SCF, and adhesion molecules such as VLA-4 and P/E selectins, was determined for stem cell mobilization. Recent results using experimental animal models and samples from clinical mobilization protocols demonstrate major involvement of chemokines such as stromal derived factor-1 (SDF-1) and IL-8, as well as proteolytic enzymes such as elastase, cathepsin G, and various MMPs in the mobilization process. These results will be reviewed together with the central roles of SDF-1 and CXCR4 interactions in G-CSF or G-CSF in combination with cyclophosphamide-induced mobilization. Furthermore, the central role of this chemokine in stem cell homing to the bone marrow as well as retention of undifferentiated cells within this tissue will also be discussed.

Cellular levels of photosensitisers in tumours: the role of proximity to the blood supply

Flow cytometry using the tumour perfusion probe Hoechst 33342 was employed to examine the distribution of photosensitisers in tumour cells located at different distances from the blood supply. Two tumour models, the SCCVII squamous cell carcinoma and FsaR fibrosarcoma growing in C3H/HeN mice, were used in the experiments. Among the photosensitisers tested, only BPD (benzoporphyrin derivative monoacid) exhibited uniform distribution in tumour cells irrespective of their distance from the vasculature. In this respect, 5-aminolaevulinic acid (i.e. its metabolite protoporphyrin IX), di- and tetrasulphonated aluminium phthalocyanines (A1PcS2 and AlPcS4), di- and tetrasulphonated tetraphenylporphines (TPPS2 and TPPS4), Photofrin and bacteriochlorophyll-a (i.e. its metabolite bacteriopheophytin-a) followed BPD in decreasing order in their efficacy of accumulation in tumour cells remote from the blood supply. This photosensitiser property appeared not to depend on tumour type, tumour size, route of photosensitiser administration, time after the administration, photosensitiser lipophilicity or on the presence of host cell infiltrate in the tumour. Following treatment with photodynamic therapy (PDT) in vivo, tumour cells were sorted based on their blood vessel proximity and their survival was determined by colony formation assay. The data demonstrate that the direct killing of tumour cells by Photofrin- and A1PcS2-based PDT decreases with increasing distance of the cells from the blood supply.