How to expel a bad tenant: Linking cyclin A2, enucleation and cell size

This article refers to:

Cyclin A2 regulates erythrocyte morphology and numbers

Erythropoiesis is a tightly regulated process, in which erythroid progenitors undergo several consecutive cell divisions to produce daughter cells of decreasing cell size and increasing chromatin compaction. This linked process of proliferation and differentiation ultimately results in nuclear extrusion, termed erythroid enucleation, with the subsequent maturation and exit of the hemoglobin packed erythrocyte into the peripheral blood. The network of regulators that enables constant replenishment of a defined number of erythrocytes of defined cell size is still poorly understood. Several cell cycle regulators, including cyclin-dependent kinases (CDKs) have been identified to control erythrocyte size and numbers by regulating G1-S transition, a checkpoint for cell size control.¹ Now Jayapal et al [Cell Cycle 2016; 22:0] identify a dual role for cyclin A2 in both early and late stages of erythroid maturation in the mouse that regulates red blood cell size and numbers.²

Cyclin A2 is a key cell cycle regulator that through activation of its catalytic partners CDK2 and CDK1, is best known for its role in controlling the onset and completion of DNA replication in S-phase, as well as regulating G2-M progression. Cyclin A2 knockout mice are embryonically lethal, and its conditional depletion from the haematopoietic compartment results in pancytopenia as a result of a decrease in stem cell proliferation.³ Previously, genome-wide association studies of erythroid traits had identified gene candidates, including CCNA2 (encoding human Cyclin A2), to be potential regulators of cell size (mean corpuscular volume, MCV) in human red blood cells.^4,5 This was recently followed up in an in vitro study showing that knockdown of Ccna2 in mouse erythroid cells results in larger red blood cell size due to defects in cytokinesis.⁶ Interestingly, these cell cycle defects were only observed during the final erythroid cell division, the stage before enucleation, and not in earlier proliferating erythroblasts, however enucleation was reported to be unaffected.⁶

Here, Jayapal et al. use powerful in vivo conditional mouse models to provide novel insight into the definitive role of cyclin A2 in erythropoiesis.² Using erythroid specific deletion of cyclin A2, the authors show that cyclin A2 controls red blood cell size (MCV) and numbers by regulating not only the formation of early erythroid progenitors (BFU-E; burst-forming unit-erythroid colonies), but surprisingly by also acting at a later time point of erythroid development, enabling efficient nuclear extrusion in vitro and in vivo.² Importantly, as the authors did not observe proliferation or differentiation defects during terminal erythroid maturation, this suggests that the enucleation defects may be directly responsible for the increased size and reduced numbers of erythrocytes.

How might Cyclin A2 then regulate enucleation? And does this actually point to a direct role for Cyclin A2 in driving enucleation, or could loss of Cyclin A2 elicit some new form of checkpoint for enucleation? Indeed, DNA damage in erythroblasts was found to be more severe in the absence of cyclin A2, linking cyclin A2 to DNA damage repair.² In addition, as the authors note, impairing DNA synthesis and DNA damage checkpoints genes is already known to result in decreased red blood cell numbers and cell size leading to macrocytic anemia. Although the process of erythroid enucleation shares many similarities with an asymmetric cell division, there is no DNA synthesis step. Rather, the orthochromatic erythroblast exits the cell cycle into G0/G1 before extruding the nucleus through a cytokinesis-like processes that includes the contraction of an actomyosin ring and vesicle trafficking dependent abcission. How this exit from the cell cycle is regulated and timed is currently unknown. One possibility could be, that in the context of erythroid enucleation, cyclin A2 might play a role as a checkpoint regulator that controls efficient transition from G1 to nuclear extrusion, e.g. by sensing DNA damage/chromatin compaction.² Of interest, compound loss of p27 was shown to partially rescue the erythroid size and enucleation phenotype due to loss of Cyclin A2 indicating that as with other cell cycle functions, a balance of p27 and cyclin A2 is required to regulate erythropoiesis and enucleation.²

Could Cyclin A2 then have a non-cell cycle related activity in enucleation? Functional interplay between cytoskeletal elements is crucial for efficient enucleation, and cyclin A2 has recently been shown to regulate the organization of the actin cytoskeleton by activating RhoA in other contexts.⁷ An intringuing possibility could therefore be that cyclin A2 is critical for proper dynamic coordination of the cytoskeleton during enucleation. A detailed characterization of the morphology of the defective enucleation in the absence of cyclin A2 may be informative in this regard. For example, does the microtubule-directed movement of the nucleus before extrusion still occur in the absence of cyclin A2? Does the actomyosin ring form and contract appropriately? Is there proper vesicle trafficking for the abcission process to occur and release the enucleated erythrocyte?

Here, Jayapal et al. offer interesting new insights into the regulatory network that controls erythrocyte morphology and numbers, and how this links to the remarkable process of nuclear extrusion. A better understanding of the mechanisms that regulate red blood cell counts and size is essential for the treatment of hematological disorders such as macrocytosis and associated anemia.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.