Glycobiology: Sugars direct stem cell homing

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Adult stem cells can be targeted to a selected tissue through the ex vivo glycan engineering of cell surface receptors.

Stem cells hold great promise for the repair of damaged organs, but they first need to be delivered to where they are needed, ideally via the bloodstream. In an effort to meet this delivery challenge for skeletal diseases such as osteoporosis, Robert Sackstein and his team at the Biomedical Research Institute of Brigham and Women's Hospital sought a method to direct bone-forming stem cells to bone.

The researchers have a long history of working with hematopoeitic stem cells (HSC), which efficiently migrate to the marrow, the portal of entry to bone tissue. They previously identified the cell-surface receptor on these HSCs, called HCELL, a glycosylated variant of the transmembrane protein CD44, responsible for the homing of the cells from the blood stream to bone marrow. CD44 is expressed on every cell type but varies in its glycosylation pattern.

HCELL on HSCs carries a tetrasaccharide containing a sialyl acid and fucose that is specifically recognized by E-selectin, a carbohydrate binding protein on the surface of bone marrow blood vessels. This ligand-receptor interaction is strong and specific, promoting effective recruitment of the HSCs. Unfortunately HSCs do not contribute to bone formation and are thus not suitable for any therapeutic application that seeks to increase bone mass.

Mesenchymal stem cells (MSC) are the prototypical osteoprogenitor cells and are therefore the optimal candidates for growing bone, but these stem cells do not naturally home to bone.

To discover what prevented MSCs from finding their way to bone tissue, Sackstein needed to find out what exactly was missing on the CD44 of MSCs. A panel of antibodies that had been generated to analyze specific tetrasaccharide modifications on CD44 of HSCs provided the answer. MSC did have sialyated CD44, but it was lacking the fucose.

The researchers wanted to supply these cells only with the specific glycan that was missing, and to this end they developed a protocol that would specifically fucosylate CD44 on MSCs in vitro. It proved more challenging than anticipated because the reported conditions under which the enzyme was active in vitro, especially the presence of divalent cations, are toxic to the cells. Sackstein summarizes several years of effort: "We specifically formulated the enzyme and the reaction conditions to allow the ex vivo glycan engineering on the cell surface without harming the cells".

Once they added fucose to CD44, the modified MSC did indeed home to bone very efficiently and began to differentiate into bone cells. Expression of the modified glycan was temporary—presumably lost through normal cell-surface turnover of CD44.

The targeting of MSC to bone is just one application; the real power of glycan engineering is that it can steer any stem cell to any tissue type. As Sackstein explains, "We are inducing a temporal homing capability in the cell that will get it to where it needs to go and then the cell reverts to its native surface molecule configuration to proceed down its normal path of development."

CD44 is expressed on all stem cells, and the challenge of course lies in identifying exactly which glycan modification is missing to target the tissue of interest, then optimizing the enzymatic conditions needed to add this carbohydrate. The Sackstein lab is currently working on developing glycosyltransferases that will modify any cell surface under conditions that do not evoke cell toxicity.

These tools will prove valuable in controlling the homing of stem cells.

Nicole Rusk

References

Sackstein, R. et al. Ex vivo glycan engineering on CD44 programs human multipotent mesenchymal stromal cell trafficking to bone. Nat. Med. 14, 181–187 (2008). | Article | PubMed |