The Role of Stathmin, a Regulator of Mitosis, in Hematopoiesis

Stathmin is a 17KDa cytosolic protein that plays an important role in the regulation of microtubule dynamics, mitotic spindle formation, cell cycle progression and cell differentiation. Stathmin knockout (KO) mice were initially reported to have a normal phenotype but were subsequently shown to develop an age-related neurological phenotype with axonopathy evident in both central and peripheral nervous systems. These mice were also shown to have a defect in recovery from acute ischemic renal injury. We had previously shown that stathmin plays an important role in the differentiation and proliferation of megakaryocytes (MK) and that down-regulation of stathmin is necessary for the maturation of MK and platelet production in vitro. In this study, we investigated the role of stathmin in megakaryopoiesis and hematopoiesis in vivo using the stathmin KO mouse as an experimental model.

Stathmin KO mice had lower platelet (PLT) counts at 3 weeks of age when compared to WT mice. The WT mice had a mean PLT count of 662 ± 27 K/μL while KO mice had a mean PLT count of 543 ± 37 K/μL. This correlated with larger and fewer MK in the bone marrow of KO mice (WT: 4.2 ± 0.7 MK/40X field; KO: 3.6 ± 0.2 MK/40X field). Furthermore, in the spleen, there was a 10 fold decrease in the number of MK in KO mice compared to WT mice (6.6 ± 0.6 vs 0.7 ± 0.1 MK/40X field). By 8 weeks, PLT counts and MK size and numbers in the bone marrow and spleen were similar in WT and KO mice. Interestingly, by 16 weeks, the mean PLT of KO mice became significantly higher than that of WT and by 40 weeks, the mean PLT count of KO mice was 1379 ± 100K/μL compared to 1045 ± 120K/μL in WT mice (P<0.05). Microscopic analysis of the bone marrow at 46 weeks of age showed approximately 50% more MK in KO mice compared to WT mice.

Differences in red blood cell counts (RBC) were also observed. While at 3 weeks, there were no significant differences between the 2 groups, at 8 weeks, KO mice had significantly lower RBC counts, hemoglobin levels (Hb) and hematocrit (HCT). This trend continued until the last measurement recorded at 40 weeks. Mean RBC in WT mice was 10.5 ± 0.1M/μL compared to 8.9 ± 0.2M/μL in KO mice. The mean corpuscular volume (MCV) and the red blood cell distribution width (RDW) were consistently higher in KO mice than in WT mice. No significant differences were noted in white blood cell counts. Bone marrow cell counts were significantly lower in KO mice when compared to WT mice at different ages from 3–40 weeks.

Progenitor cell assays from 10–12 week old animals have shown that bone marrow from KO mice produce significantly fewer BFU-E and Pre-B colonies while no differences were observed in CFU-GMs.

The phenotypic characteristics of stathmin KO mice confirmed our prior in vitro findings that suggested a role for stathmin in megakaryopoiesis. We expected to see a decrease in the number of platelets and MK coupled with an increase in MK size. This was confirmed in stathmin KO mice at 3 weeks of age. However, we did not expect to see the marked increase in the number of platelets and MK that was observed as the mice aged. The exact mechanism for this has not been identified. Interestingly, the stathmin KO mice exhibited characteristic features of megaloblastic anemia including mild anemia and a significant increase in MCV and RDW. The megaloblastic anemia that is seen in the presence of B12 and folate deficiency results from interference with DNA synthesis resulting in asynchronous maturation of the nucleus and the cytoplasm. We believe a similar phenomenon is occurring in the stathmin KO mice. The deficiency of stathmin results in aberrant exit from mitosis, thereby delaying nuclear maturation and resulting in the megaloblastic features. Thus, the deficiency of stathmin in the KO mice results in two hematopoietic phenotypes that are seen in humans, megaloblastic anemia and thrombocytosis. It is unclear whether mutations of stathmin in humans might result in similar phenotypes. This is a question that will require further investigation. Future studies will investigate the compensatory mechanisms that result in the switch from decreased to increased platelet production as the mice age. Furthermore, examining the effects of hematopoietic stress (e.g. response to chemotherapy or bleeding) in stathmin KO mice might also elucidate a role for stathmin in the recovery from hematopoietic injury as was seen in acute ischemic renal injury.

No relevant conflicts of interest to declare.