Reduction of anabolic signals and alteration of osteoblast nuclear morphology in microgravity

Abstract

Bone loss has been repeatedly documented in astronauts after flight, yet little is known about the mechanism of bone loss in space flight. Osteoblasts were activated during space flight in microgravity (microg) with and without a 1 gravity (1 g) field and 24 genes were analyzed for early induction. Induction of proliferating cell nuclear antigen (PCNA), transforming growth factor beta (TGFbeta), cyclo-oxygenase-2 (cox-2), cpla2, osteocalcin (OC), c-myc, fibroblast growth factor-2 (fgf-2), bcl2, bax, and fgf-2 message as well as FGF-2 protein were significantly depressed in microg when compared to ground (gr). Artificial onboard gravity normalized the induction of c-myc, cox-2, TGFbeta, bax, bcl2, and fgf-2 message as well as FGF-2 protein synthesis in spaceflight samples. In normal gravity, FGF-2 induces bcl2 expression; we found that bcl2 expression was significantly reduced in microgravity conditions. Since nuclear shape is known to elongate in the absence of mitogens like FGF-2, we used high-resolution image-based morphometry to characterize changes in osteoblast nuclear architecture under microgravity, 1 g flight, and ground conditions. Besides changes in cell shape (roundish/elliptic), other high-resolution analyses show clear influences of gravity on the inner nuclear structure. These changes occur in the texture, arrangement, and contrast of nuclear particles and mathematical modeling defines the single cell classification of the osteoblasts. Changes in nuclear structure were evident as early as 24 h after exposure to microgravity. This documented alteration in nuclear architecture may be a direct result of decreased expression of autocrine and cell cycle genes, suggesting an inhibition of anabolic response in microg. Life on this planet has evolved in a normal gravity field and these data suggest that gravity plays a significant role in regulation of osteoblast transcription.

Effects of simulated weightlessness on cellular morphology and biological characteristics of cell lines SGC-7901 and HFE-145

“…DNA is also sensitive to weightlessness as the DNA spindle itself is a balanced structure. Changes in nuclear structure have been observed in previous weightlessness studies (Ingber, 1999;Hughes-Fulford et al, 2006). Such redistribu-…”

Two-dimensional clinorotation influences cellular morphology, cytoskeleton and secretion of MLO-Y4 osteocyte-like cells

“…Then, what will happen to bone cells after being submitted to mechanical unloading caused by weightlessness? In the past few years, people have paid more attention to the function of osteoblasts in responding to weightlessness (Hughes-Fulford et al 2006;Saxena et al 2007;Makihira et al 2008) Scientists have found that after space flight, some changes happen in cell morphology, cell proliferation, cell cycle, cellular cytoskeleton and gene expression in osteoblasts (Landis et al, 2000;Zerath et al 2000;Hughes-Fulford 2001;Doty 2004) Also many studies have investigated the effects of simulated weightlessness on osteoblasts under ground-based condition (Rucci et al 2007;Makihira et al 2008) In recent years, osteocytes have been thought to be as main coordinator responding to mechanical loading in bone tissue (Bonewald 2006(Bonewald , 2007. Osteocytes change their structure and functions after being submitted to mechanical stimulation (Zhang et al 2006;Skerry 2008;Tan et al 2008) Then, is it right that osteocytes also might play important role in responding to mechanical unloading such as weightlessness?…”

Effects of oriented substrates on cell morphology, the cell cycle, and the cytoskeleton in Ros 17/2.8 cells

“…Bone-forming osteoblasts are vital factors in mechanosensing and mechanotransduction in vivo, and the biological responses of osteoblasts in altered gravity provide basic understanding of human bone loss in space. Osteoblasts in vivo and in vitro were found to be affected by low gravity [3][4][5][6]. The underlying mechanism of osteoblastic gravisensing and gravitransduction, however, remains poorly understood.…”