Megakaryocyte-osteoblast interaction revealed in mice deficient in transcription factors GATA-1 and NF-E2.
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.


BACKGROUND
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.


METHODS
Histomorphometry, microCT, 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.
Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, Connecticut 06520-8071, USA.
Melissa A
MA
Kacena
Ramesh A
RA
Shivdasani
Kimberly
K
Wilson
Yougen
Y
Xi
Nancy
N
Troiano
Ara
A
Nazarian
Caren M
CM
Gundberg
Mary L
ML
Bouxsein
Joseph A
JA
Lorenzo
Mark C
MC
Horowitz
D000469:Alkaline Phosphatase / Q000096:biosynthesis; D000818:Animals; D015519:Bone Density / Q000235:genetics; D001846:Bone Development / Q000235:genetics / Q000502:physiology; D002450:Cell Communication / Q000235:genetics / Q000502:physiology*; D002454:Cell Differentiation / Q000235:genetics / Q000502:physiology; D002455:Cell Division / Q000235:genetics / Q000502:physiology; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000172:deficiency / Q000235:genetics*; D050981:Erythroid-Specific DNA-Binding Factors; D005269:Femur / Q000000981:diagnostic imaging / Q000503:physiopathology; D050982:GATA1 Transcription Factor; D008533:Megakaryocytes / Q000166:cytology / Q000502:physiology*; D051379:Mice; D018345:Mice, Knockout; D050983:NF-E2 Transcription Factor; D050984:NF-E2 Transcription Factor, p45 Subunit; D010006:Osteoblasts / Q000166:cytology / Q000502:physiology*; D015675:Osteocalcin / Q000096:biosynthesis; D010010:Osteoclasts / Q000166:cytology / Q000502:physiology*; D011859:Radiography; D013977:Tibia / Q000503:physiopathology / Q000648:ultrastructure; D014157:Transcription Factors / Q000172:deficiency / Q000235:genetics*
D004268:DNA-Binding Proteins; D050981:Erythroid-Specific DNA-Binding Factors; D050982:GATA1 Transcription Factor; C494689:Gata1 protein, mouse; D050983:NF-E2 Transcription Factor; D050984:NF-E2 Transcription Factor, p45 Subunit; C494693:Nfe2 protein, mouse; D014157:Transcription Factors; D015675:Osteocalcin; D000469:Alkaline Phosphatase