CD53 sends HSCs to sweet DREAMs

In this issue of Blood, Greenberg et al¹ show that hematopoietic stem cells (HSCs) employ a novel mechanism via promotion of the CD53-mediated dimerization partner, Rb-like, E2F and multi-vulval class B (DREAM) complex to return to quiescence after inflammatory and proliferative stress, thereby protecting them from functional decline.

HSCs are a population of rare cells that are capable of replenishing the whole blood system. To preserve this lifelong ability, HSCs are maintained in a quiescent and dormant state under homeostatic conditions.^2,3 Under stress conditions, such as viral infections, HSCs are forced to exit their quiescent state and enter the cell cycle. However, little is known about the mechanisms regulating the return of HSCs to quiescence, which is a crucial step to prevent both stem cell exhaustion and the development of hematopoietic malignancies.

CD53 is a member of the tetraspanin family of transmembrane proteins that regulate cellular processes, such as migration, adhesion, and signaling.⁴ CD53 has been previously identified as an asymmetrically segregating protein in HSCs and is enriched in more stem-like HSCs.⁵ In addition, the authors have recently shown that CD53 regulates early B-cell development by promoting interleukin 7 receptor.⁶ However, the mechanism by which CD53 regulates HSC function remains unclear.

In this study, Greenberg et al show that CD53 is transiently upregulated in both mouse and human HSCs on inflammatory and proliferative stress treatments.¹ Under homeostatic conditions, no differences were found in the frequency or absolute number of HSCs in the bone marrow or spleen between wild-type mice and mice lacking CD53. However, on treatment with the stem cell mobilization agent granulocyte colony-stimulating factor (G-CSF), loss of CD53 resulted in prolonged cycling, reduced HSC function under stress conditions, and return to quiescence.

To understand the mechanism by which CD53 protects HSC function in response to stress, the authors performed transcriptomic analysis of CD53 knockout and wild-type HSCs on G-CSF treatment. This analysis revealed differential expression of DREAM complex target genes in CD53 knockout HSCs, in line with the observed dysregulation of cycling. The DREAM complex has been shown to be a master coordinator of cell cycle–dependent gene expression,⁷ but little was known about its role in HSCs. Strikingly, inhibition of the cell cycle with the cyclin-dependent kinase 4/6 (CDK4/6) inhibitor palbociclib equalized the repopulating activity of CD53 knockout HSCs and the expression of DREAM complex target genes, indicating that CD53 largely acts on HSC cell cycle regulation.

It is known that palbociclib inhibits CDK4/6, which prevents phosphorylation of the riboblastoma-like proteins p107 and p130, thereby promoting the DREAM complex activity and thus inhibiting the cell cycle. In this study, the authors show that p107 and p130 exhibited increased phosphorylation levels in CD53 knockout HSCs exposed to proliferative stress. The authors next hypothesize that CD53 regulates the levels of the pocket proteins p107 and p130, consequently promoting their active, hypophosphorylated form and their binding to the DREAM complex. They confirm this hypothesis by proximity ligation assays, first showing that CD53 interacts individually with both p130 and its phosphatase PP2A; and, second, by demonstrating that p130 and PP2A only interact in the presence of CD53. Taken together, these results suggest that CD53 prevents p130 degradation by promoting its PP2A-mediated hypophosphorylation and thereby enhancing the activity of the DREAM complex (see figure).

Tetraspanin proteins, such as CD63, CD81, and CD82, have been shown to regulate HSC quiescence through the transforming growth factor-β and protein kinase B (Akt) signaling pathways.^8-10 This work highlights a novel protection mechanism mediated by the tetraspanin CD53. CD53 preserves HSC function on inflammatory and proliferative stress by promoting the activity of the DREAM complex and the return of HSCs to a quiescent state. It will now be of great interest to address the role of this axis in conditions in which the HSC pool expands but loses its self-renewal capacity, such as in aging. Future studies should identify direct and indirect targets of these interactions and investigate their functional role in human HSCs to (1) expand the understanding of how HSC quiescence is regulated and (2) identify potential novel targets to treat hematopoietic malignancies.

Conflict-of-interest disclosure: The authors declare no competing financial interests.