Low level of gene transfer to and engraftment of murine bone marrow cells from long-term bone marrow cultures

Palmitic acid substitution on cationic polymers for effective delivery of plasmid DNA to bone marrow stromal cells

Nonviral gene carriers are actively explored in gene therapy due to safety concerns of the viral carriers. To design effective gene carriers for modification of bone marrow stromal cells (BMSC), an important cell phenotype for clinical application of gene therapy, cationic polymers polyethyleneimine (PEI), and poly-L-Lysine (PLL) were substituted with palmitic acid (PA) via amide linkages. Depending on the reaction conditions, PEI and PLL was substituted with 2.2-5.2 and 13.4-16.2 PA per polymer chain. The PA substituted polymers displayed slightly lower binding efficiency towards a plasmid containing Enhanced Green Fluorescent Protein (pEGFP) in an agarose gel binding assay. The cell binding of PLL-PA, but not PEI-PA, was particularly enhanced, resulting in higher percentage of the cells displaying a significant polymer uptake. pEGFP delivery into the BMSC was also significantly increased with the PLL-PA (vs. PLL), but not PEI-PA (vs. PEI). The transfection efficiency of PLL-PA was significantly higher ( approximately fivefold) than the unmodified polymer. We conclude that PA substitution on PLL provides an effective carrier for transfection of primary cells derived from the bone marrow.

A comparison of the effectiveness of cationic polymers poly-L-lysine (PLL) and polyethylenimine (PEI) for non-viral delivery of plasmid DNA to bone marrow stromal cells (BMSC)

Bone marrow stromal cells (BMSC) represent an important cell phenotype for pursuit of successful gene therapy. Non-viral methods to enable expression of exogenous genes in BMSC will accelerate clinical application of gene therapy, without the concerns associated with the viral means of gene transfer. Towards this end, this study investigated the potential of cationic polymers poly-L-lysine (PLL) and branched polyethylenimine (PEI) as gene carriers for modification of BMSC. Both polymers rapidly (approximately 30 min) condensed a 4.2 kb Enhanced Green Fluorescent Protein (pEGFP-N2) plasmid into 100-200 nm particles. PLL and PEI were both readily internalized with BMSC with >80% of BMSC exhibiting polymer uptake by flow cytometric analysis. The relative uptake of PEI, however, was significantly higher as compared to the PLL. The majority of the BMSC (>60%) exhibited nuclear presence of the polymers as analyzed by fluorescent microscopy. Although both polymers were able to deliver the pEGFP-N2 into the cells under microscopic evaluation, only a small fraction of the cells (<10%) displayed nuclear localization of the plasmid. Consistent with better uptake, PEI gave a higher delivery of pEGFP-N2 into the BMSC, which resulted in a more sustained expression of the model gene EGFP in short-term (7-day) culture. We conclude that both PLL and PEI readily displayed cellular uptake, but PEI was more effective in delivering plasmid DNA intracellularly, which was likely the underlying basis for a more sustained gene expression.

Characterization of engraftable hematopoietic stem cells in murine long-term bone marrow cultures

OBJECTIVE

Long-term bone marrow cultures (LTBMC) are a potential source of hematopoietic stem cells (HSC) for transplantation. Previous reports indicate that feeding LTBMCs induces hematopoietic progenitor cycling, and other studies link HSC cycle phase with engraftability. Our study was initiated to further characterize LTBMC engraftability and determine if a cycle phase-related engraftment defect affects HSC from LTBMCs.

MATERIALS AND METHODS

Competitive repopulation of lethally irradiated BALB/c females was used to examine engraftability of LTBMCs under "fed" or "unfed" conditions at 3 to 5 weeks culture. Tritiated thymidine suicide was used to determine the cycle status of HPP-CFC and CFU-S from LTBMCs.

RESULTS

Total cell number in LTBMCs decreases from input. Quantitatively, both fed and unfed 3-, 4-, or 5-week cultures compete strongly with fresh marrow for 2 and 8 weeks, but not 6 months, after transplantation. Short-term engraftable HSCs expand between 3 and 5 weeks of culture. Clonal assays indicate no peak in S-phase of CFU-S at 24 and 48 hours after feeding, and fluctuation in both content and cycle status of HPP-CFC after feeding.

CONCLUSIONS

Our LTBMCs engraft in all conditions, and the level of engraftment capability does not correlate with cell-cycle phase of CFU-S or HPP-CFC, or with time from feeding. Although the total cell number decreases from input, the proportion of short- and intermediate-term engrafting HSC in whole LTBMCs approximates that of fresh marrow and expands from 3 to 5 weeks in culture, whereas long-term engraftable HSCs are decreased in culture.