Tail-suspended mice lacking calponin H1 experience decreased bone loss

Effect of Collagen Peptide, Alone and in Combination with Calcium Citrate, on Bone Loss in Tail-Suspended Rats

Oral administration of bovine collagen peptide (CP) combined with calcium citrate (CC) has been found to inhibit bone loss in ovariectomized rats. However, the protective effects of CP and CP–CC against bone loss have not been investigated in a tail-suspension simulated microgravity (SMG) rat model. Adult Sprague-Dawley rats (n = 40) were randomly divided into five groups (n = 8): a control group with normal gravity, a SMG control group, and three SMG groups that underwent once-daily gastric gavage with CP (750 mg/kg body weight), CC (75 mg/kg body weight) or CP–CC (750 and 75 mg/kg body weight, respectively) for 28 days. After sacrifice, the femurs were analyzed by dual-energy X-ray absorptiometry, three-point bending mechanical tests, microcomputed tomography, and serum bone metabolic markers. Neither CP nor CP–CC treatment significantly inhibited bone loss in SMG rats, as assessed by dual-energy X-ray absorptiometry and three-point bending mechanical tests. However, both CP and CP–CC treatment were associated with partial prevention of the hind limb unloading-induced deterioration of bone microarchitecture, as demonstrated by improvements in trabecular number and trabecular separation. CP–CC treatment increased serum osteocalcin levels. Dietary supplementation with CP or CP–CC may represent an adjunct strategy to reduce the risk of fracture in astronauts.

Actin cytoskeleton mediates BMP2-Smad signaling via calponin 1 in preosteoblast under simulated microgravity

Microgravity influences the activity of osteoblast, induces actin microfilament disruption and leads to bone loss during spaceflight. Mechanical stress such as gravity, regulates cell function, response and differentiation through dynamic cytoskeleton changes, but the mechanotransduction mechanism remains to be fully elucidated. Previous, we demonstrated actin microfilament mediated osteoblast Cbfa1 responsiveness to BMP2 under simulated microgravity (SMG). Here, we explored a potential molecular and its detailed mechanism of actin cytoskeleton functioning on BMP2-Smad signaling in MC3T3-E1 under SMG. Results showed that the actin microfilament-disrupting agent, cytochalasin B (CB), reduced BMP2-induced activation, translocation of Smad1/5/8 and Runx2 expression. SMG also inhibited BMP2-Smad signaling, which was rescued by actin cytoskeleton stabilizing agent, Jasplakinolide (JAS). Furthermore, we found that siRNA mediated knockdown of calponin 1 (CNN1), an actin binding protein, markedly promoted BMP2-Smad signaling and abolished both inhibition of CB, SMG on BMP2-Smad signaling and the rescue action of JAS. Overexpression of CNN1 inhibited the p-Smad induced by BMP2. Bidirectional Co-IP experiments demonstrated CNN1 could interacted with Smad or p-Smad protein. Furthermore, CB or SMG decreased the phosphorylated CNN1 and increased its interaction with Smad or p-Smad. Combined with the phosphorylation of CNN1 inhibites its actin binding activity, these results indicate that actin cytoskeleton depolymerization inhibites BMP2 signaling via blocking of Smad by dephosphorylated CNN1 in osteoblast cells. Thus, we provide new important insights into the mechanism of mechanotransduction under SMG condition, which probably contribute to bone formation decrease induced by SMG.

Actin microfilament mediates osteoblast Cbfa1 responsiveness to BMP2 under simulated microgravity

Microgravity decreases osteoblastic activity, induces actin microfilament disruption and inhibits the responsiveness of osteoblast to cytokines, but the mechanisms remains enigmatic. The F-actin cytoskeleton has previously been implicated in manifold changes of cell shape, function and signaling observed under microgravity. Here we investigate the involvement of microfilament in mediating the effects of microgravity and BMP2 induction on Cbfa1 activity. For this purpose we constructed a fluorescent reporter cell line (OSE-MG63) of Cbfa1 activity by stably transfecting MG63 cells with a reporter consisting of six tandem copies of OSE2 and a minimal mOG2 promoter upstream of enhanced green fluorescent protein (EGFP). The fluorescence intensity of OSE-MG63 showed responsiveness to bone-related cytokines (IGF-I, vitamin D3 and BMP2) and presented an accordant tendency with alkaline phosphatase (ALP) activity. Using OSE-MG63 reporter fluorescence, we performed a semi-quantitative analysis of Cbfa1 activity after treatment with simulated microgravity, microfilament-disrupting agent (cytochalasin B, CB), microfilament-stabilizing agent (Jasplakinolide, JAS) or any combination thereof. In parallel, ALP activity, DNA binding activity of Cbfa1 to OSE2 (ChIP), F-actin structure (immunofluorescence) and EGFP mRNA expression (RT-qPCR) were analyzed. Simulated microgravity inhibited Cbfa1 activity, affected the responsiveness of Cbfa1 to cytokine BMP2, and caused a thinning and dispersed distribution of microfilament. Under normal gravity, CB significantly attenuated BMP2 induction to Cbfa1 activity as well as DNA binding activity of Cbfa1 to OSE2. The addition of JAS reversed the inhibitory effects of microgravity on the responsiveness of Cbfa1 to BMP2. Our study demonstrates that disrupting the microfilament organization by CB or simulated microgravity attenuates the responsiveness of Cbfa1 to BMP2. A stabilization of the microfilament organization by JAS reverses this inhibition. Taken together, these results suggest that actin microfilament participates in BMP2’s induction to Cbfa1 activity and that their disruption might be an important contributor to microgravity’s inhibition on BMP2’s osteogenic induction.