Expression of Bone Morphogenetic Proteins and their Receptors in the Bone Marrow Megakaryocytes of GATA-1^low Mice

Abstract

The mechanism of osteosclerosis associated with myelofibrosis in megakaryocyte (MK)-related myeloproliferative disorders is largely unknown. However, growth factors released from the bone marrow cells, including from MKs, have been implicated in myelofibrosis, osteosclerosis, and angiogenesis. GATA-1 is a transcription factor required for normal MK development. GATA-1 deficiency in mice (GATA-1low) leads to increased megakaryocytic proliferation, followed by osteosclerosis and myelofibrosis. In this study we investigated the expression of bone morphogenetic proteins (BMPs) and BMP receptors and their possible role in the development of osteosclerosis in the MKs of 12-month-old GATA-1low mice by immunohistochemistry, cytomorphometry, and quantitative real-time PCR. Marrow MKs from both wild-type and GATA-1low mice showed moderate to intense staining for BMP-2, −4, and −6 and BMPR-IA and BMPR-II, whereas splenic MKs showed no BMP immunostaining. Presence of BMP protein in the bone marrow of GATA-1low mice was more than that seen in controls, owing to an increased number of MKs and osteoblasts. The osteosclerosis seen in GATA-1low mice appeared not to be due to a reduced number of functional osteoclasts because the number of tartrate-resistant acid phosphatase-positive osteoclasts was greater in GATA-1low mice than in controls. Our findings demonstrate the presence of significant amounts of BMP-2, −4, and −6 along with their receptors in bone marrow MKs of WT and GATA-1low mice. The increased levels of BMPs appear to be a result of increased numbers of MKs in GATA-1low mice and may, in part, account for the stimulation of osteoblastic activity and resulting osteosclerosis.

Bone Morphogenetic Proteins Are Overexpressed in the Bone Marrow of Primary Myelofibrosis and Are Apparently Induced by Fibrogenic Cytokines

“…34 A recent study showed that osteosclerosis in this animal model is obviously triggered by aberrant expression of BMP members by megakaryocytes, eg, BMP6 showed a considerable overexpression. 19 Our data show the significant increase of BMP6 in human PMF bone marrows, thereby supporting the thesis that this BMP family member is also involved in advanced ECM formation. Of note, some of the prefibrotic stages of PMF showed outliers overexpressing BMP6, suggesting that BMP6 might serve as a marker for a more progressive development of fibrosis and osteosclerosis.…”

“…31,32 We first aimed to simulate the bone marrow environment in PMF by treating primary fibroblasts with TGF␤-1 and bFGF to determine a potential induction of those BMP members for which a role in fibrogenesis has been suggested. 9,19 Fibroblasts treated with TGF␤-1 showed an exaggerated induction of BMP6 but also of BMP1 and BMP-R2 (Figure 1, A and C). This induction was also notable in fibroblasts cultured in the presence of bFGF but was restricted to BMP6 (Figure 1, B and D).…”

“…17,18 Interestingly, in a recent mouse model showing deficiency in GATA-1 expression (GATA-1 low ), members of the BMP family were shown to be essentially involved in early osteosclerosis. 19 Site-directed mutagenesis in vitro provided evidence that a point mutation in human BMP1 (E483K) abolished the ability of BMP1 to cleave pro-collagen type I. 20 The entire CUB II domain was lastly identified to be essential for proteinase activity of BMP1.…”

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

“…36 Other studies have demonstrated the expression of bone morphogenetic proteins (BMPs) by megakaryocytes. 51,52 GATA-1 low mice have increased levels of BMPs coinciding with an increased number of bone marrow megakaryocytes. 52 BMP production by megakaryocytes does represent such a candidate because osteoclasts are known to possess BMP receptors and are stimulated by BMPs.…”

“…51,52 GATA-1 low mice have increased levels of BMPs coinciding with an increased number of bone marrow megakaryocytes. 52 BMP production by megakaryocytes does represent such a candidate because osteoclasts are known to possess BMP receptors and are stimulated by BMPs. [53][54][55] Whether BMP expression explains the bone phenotype in the hTg G233V model can be examined in future studies.…”