Megakaryocyte-Osteoblast Interaction Revealed in Mice Deficient in Transcription Factors GATA-1 and NF-E2

Abstract

Mice deficient in GATA-1 or NF-E2 have a 200 -300% increase in bone volume and formation parameters. Osteoblasts and osteoclasts generated in vitro from mutant and control animals were similar in number and function. Osteoblast proliferation increased up to 6-fold when cultured with megakaryocytes. A megakaryocyte-osteoblast interaction plays a role in the increased bone formation in these mice.Introduction: GATA-1 and NF-E2 are transcription factors required for the differentiation of megakaryocytes. Mice deficient in these factors have phenotypes characterized by markedly increased numbers of immature megakaryocytes, a concomitant drastic reduction of platelets, and a striking increased bone mass. The similar bone phenotype in both animal models led us to explore the interaction between osteoblasts and megakaryocytes. Materials and Methods: Histomorphometry, CT, and serum and urine biochemistries were used to assess the bone phenotype in these mice. Wildtype and mutant osteoblasts were examined for differences in proliferation, alkaline phosphatase activity, and osteocalcin secretion. In vitro osteoclast numbers and resorption were measured. Because mutant osteoblasts and osteoclasts were similar to control cells, and because of the similar bone phenotype, we explored the interaction between cells of the osteoblast lineage and megakaryocytes. Results: A marked 2-to 3-fold increase in trabecular bone volume and bone formation indices were observed in these mice. A 20-to 150-fold increase in trabecular bone volume was measured for the entire femoral medullary canal. The increased bone mass phenotype in these animals was not caused by osteoclast defects, because osteoclast number and function were not compromised in vitro or in vivo. In contrast, in vivo osteoblast number and bone formation parameters were significantly elevated. When wildtype or mutant osteoblasts were cultured with megakaryocytes from GATA-1-or NF-E2-deficient mice, osteoblast proliferation increased over 3-to 6-fold by a mechanism that required cell-to-cell contact. Conclusions: These observations show an interaction between megakaryocytes and osteoblasts, which results in osteoblast proliferation and increased bone mass, and may represent heretofore unrecognized anabolic pathways in bone.

Platelet Dysfunction and a High Bone Mass Phenotype in a Murine Model of Platelet-Type von Willebrand Disease

“…Two are targeted genetic deletions of transcription factors, either GATA-1 or NF-E2, both critical to the hematopoietic lineage. 16,38 The third is the overexpression of thrombopoietin leading to a myelofibrotic and osteosclerotic phenotype. 39 Unlike the global effects that are often associated with the loss of a transcription factor, the Pt-vWD mouse is the result of a gain-of-function mutation, by a single amino acid substitution in a single gene product.…”

Altered SDF-1/CXCR4 axis in patients with primary myelofibrosis and in the Gata1low mouse model of the disease

“…Megakaryocytes from Gata1 low mice and from PMF patients express, however, the same intrinsic abnormalities (i.e. higher frequency and higher retention in the marrow because of maturation retardation [6][7][8]56 and increased localization near the endosteum, in close spatial relationship with the osteoblasts 57 . All these factors can contribute to the increased megakaryocyte up-take of SDF-1.…”

Megakaryocytes Regulate Expression of Pyk2 Isoforms and Caspase-mediated Cleavage of Actin in Osteoblasts

“…MKs are responsible for the production of thrombocytes, which are necessary for blood clotting. Although the mechanism has yet to be fully elucidated, a growing body of evidence also suggests that MKs play a key role in regulating skeletal homeostasis through direct and indirect effects on OBs and osteoclasts (1)(2)(3)(4)(5)(6). We previously reported that OB cell number is significantly increased when OBs are co-cultured with MKs in vitro (1,2,4).…”