in the bone marrow
Researchers have found that megakaryocytes can produce platelets via different polyploidization mechanisms.
Experiments suggested that endomitosis is the major megakaryocyte polyploidization mechanism, but megakaryocytes can generate platelets even in the absence of endomitosis—via endocycles and re-replication.
These findings may have implications for treating cancers as well as thrombocytopenia, the researchers said.
They described their discoveries in Developmental Cell.
Marcos Malumbres, PhD, of Centro Nacional de Investigaciones Oncologicas (CNIO) in Madrid, Spain, and his colleagues performed a genetic analysis of endomitosis, in which megakaryocytes become polyploid by entering mitosis but aborting anaphase.
The team studied mice lacking key proteins involved in mitotic entry (Cdk1 and Cdk2), mitotic progression (Aurora B), or mitotic exit (the anaphase-promoting complex [APC/C] cofactor Cdc20) during megakaryocyte maturation.
They found that Aurora B is not required for megakaryocyte maturation, but Cdc20 is. In mice lacking Cdc20, the researchers observed mitotic arrest and severe thrombocytopenia. The team said this suggests endomitosis is the major megakaryocyte polyploidization mechanism in vivo.
So they were surprised to discover that deleting Cdk1 prevents endomitosis but does not hinder platelet formation.
“Whilst the elimination of the main proteins that regulate megakaryocyte growth generates, as we thought, a reduction in the production of platelets, this didn’t happen when we removed Cdk1,” Dr Malumbres said. “[Even] when Cdk1 was absent, the megakaryocytes were able to grow in size in a similar way to normal cells.”
“[Further] analysis revealed that cells deficient in Cdk1 underwent cellular reprogramming towards a process known as endocycles, which can also be seen in other types of cells, such as certain cells in the placenta,” said Marianna Trakala, also of CNIO.
In endocycles, cells successively replicate their genomes without segregating chromosomes during mitosis.
The researchers uncovered an additional mechanism for megakaryocyte polyploidy when they studied mice lacking both Cdk1 and Cdk2. The deletion of both Cdks resulted in aberrant re-replication, in which the genome is replicated more than once per cell cycle, but platelet levels were not affected.
In fact, the team found that the loss of Cdk1 alone or Cdk1 and Cdk2 together can significantly rescue proplatelet formation and increase platelet levels in Cdc20-null mice.
“We immediately asked ourselves if, by reprogramming the cell cycle towards endocycles, we could correct the thrombocytopenia induced in other models,” Dr Malumbres said.
And when the researchers eliminated Cdk1 in Cdc20-null mice with severe thrombocytopenia, they observed an increase in platelet production. Results were similar when the team ablated Cdk1 and Cdk2 in Cdc20-null mice.
In addition to their implications for treating thrombocytopenia, these results could aid the design of cancer treatments, the researchers said. The findings reveal the different requirements that normal or tumor cells have toward cell-cycle regulators.
Specifically, the research suggests that deleting Cdk1 or other cell-cycle proteins is lethal for tumor cells but does not affect megakaryocytes. So drugs that inhibit these proteins could be used to treat malignancies such as pro-megakaryocytic leukemias.
