Identification of endoglycan, a member of the CD34/podocalyxin family of sialomucins

CD34 and podocalyxin are structurally related sialomucins, which are expressed in multiple tissues including vascular endothelium and hematopoietic progenitors. These glycoproteins have been proposed to be involved in processes as diverse as glomerular filtration, inhibition of stem cell differentiation, and leukocyte-endothelial adhesion. Using homologies present in the cytoplasmic tails of these proteins, we have identified a novel member of this family, which we designate endoglycan. This protein shares a similar overall domain structure with the other family members including a sialomucin domain, but also possesses an extremely acidic amino-terminal region. In addition, endoglycan contains several potential glycosaminoglycan attachment sites and is modified with chondroitin sulfate. Endoglycan mRNA and protein were detected in both endothelial cells and CD34(+) bone marrow cells. Thus, CD34, podocalyxin, and endoglycan comprise a family of sialomucins sharing both structural similarity and sequence homology, which are expressed by both endothelium and multipotent hematopoietic progenitors. While the members of this family may perform overlapping functions at these sites, the unique structural features of endoglycan suggest distinct functions for this molecule.

Influence of the different CD34+ and CD34- cell subsets infused on clinical outcome after non-myeloablative allogeneic peripheral blood transplantation from human leucocyte antigen-identical sibling donors

Currently, no information is available regarding the influence of the different CD34+ cell subsets infused on the haematopoietic recovery, following non-myeloablative allogeneic peripheral blood stem cell transplantation (allo-PBSCT). We have explored, in a group of 13 patients receiving non-myeloablative allo-PBSCT from human leucocyte antigen-identical sibling donors, the influence of the total dose of CD34+ haematopoietic progenitor cells (HPC) infused, compared with that of the different CD34+ HPC and CD34- leucocyte subsets in the leukapheresis samples, on both engraftment and clinical outcome. The overall numbers of total CD34+ HPC (P = 0.002) and myelomonocytic-committed CD34+ HPC infused (P = 0.0002) were strongly associated with neutrophil recovery (> 1 x 109 neutrophils/l), the latter being the only independent parameter influencing neutrophil recovery. Regarding long-term engraftment, only the number of immature CD34+ HPC infused/kg correlated with the duration of hospitalization in the first 2 years after discharge (r = -0.75, P = 0.005). Both the overall amount of CD34+ HPC and the number of myelomonocytic CD34+ HPC infused showed a significant influence on the risk of graft-versus-host disease (GVHD). Thus, the overall probability of GVHD was 100%vs 25% for patients receiving >/= 5 x 106 CD34+ HPC or >/= 3.5 x 106 of myelomonocytic-committed CD34+ HPC vs lower doses (P = 0.013). None of the other CD34+ and CD34- cell subsets analysed correlated with development of GVHD. In summary, our results suggest that in non-myeloablative allo-PBSCT, high numbers of CD34+ HPC, especially the myelomonocytic-committed CD34+ progenitors, lead to rapid neutrophil engraftment. However, they also strongly impair clinical outcome by increasing the incidence of GVHD.

Na+/H+ exchanger regulatory factor-1 is a hematopoietic ligand for a subset of the CD34 family of stem cell surface proteins

CD34 and its relatives, podocalyxin and endoglycan, comprise a family of surface sialomucins expressed by hematopoietic stem/progenitor cells and vascular endothelia. Recent data suggest that they serve as either pro- or antiadhesion molecules depending on their cellular context and their post-translational modifications. In addition, their ability to function as blockers of adhesion may be further regulated by their subcellular localization in membrane microdomains via activation-dependent linkage with the actin cytoskeleton. To gain further insights into the function and regulation of CD34-type molecules, we sought to identify the intracellular ligands that govern their localization. Using both genetic and biochemical approaches, we have identified the Na(+)/H(+) exchanger regulatory factor-1 (NHERF-1) as a selective ligand for podocalyxin and endoglycan but not for the closely related CD34. Furthermore, we show that NHERF-1 is expressed by all c-kit(+) /lineage marker(-)/Sca-1(+) cells, which are known to express podocalyxin and have long-term repopulating abilities. Finally, we show that these proteins relocalize and colocalize in response to cytokine signaling. The results suggest that this cytosolic adaptor protein may be important for mobilization of CD34-type proteins in the plasma membrane and may thereby regulate their ability to block or enhance hematopoietic cell adhesion.

[The subpopulation of CD34+ cells and their importance in the healing of grafts in family-related allogenic transplants of peripheral stem cells]

CURRENT STATUS

The determination of concentration of CD34+ cells is the standard method for evaluation of the quality of a bone marrow graft and of peripheral stem cells. Although the relationship between the dose of CD34+ cells and the speed of graft healing in autologous transplants is a proven fact, it may not always be the case in allogenic transplants.

PATIENTS AND METHOD

The correlation between the dose of CD34+ cell subpopulations and the speed of healing was monitored in patients indicated for allogenic transplantation of haematopoietic stem cells. The patients were divided according to the type of preparatory regimen they underwent for the purpose of analysis; one group contained those under a myeloablative regimen; a second group contained those under a non-myeloablative regimen. The data was subject to analysis of variance in regression models and non-parametric tests.

RESULTS

From among the monitored subpopulations, CD34+36+ cells had the greatest effect on the healing process and were the most significant predictor of the speed of healing in patients under a myeloablative regimen. Nevertheless, a dose ofCD34+ cells continued to be the best healing predictor in patients under a non-myeloablative regimen. Also subpopulations of CD34+38+ and CD34+61+ cells had a significant effect on the speed of healing in both groups.

CONCLUSION

Haematopoietic stem cells and progenitor cells defined by co-expression of specific antigens are likely to play a role, through different mechanisms of action, in the process of healing in patients in different pre-transplant regimens. While the dose of CD34+ cells is still the one which correlates best with the speed of healing in patients who underwent transplantation after non-myeloablative regimen, the dose of CD34+36+ cells appears to be a better predictor for the speed of healing after myeloablative regimens.