Novel interaction of Rab13 and Rab8 with endospanins

Rab GTPases regulate vesicular traffic in eukaryotic cells by cycling between the active GTP-bound and inactive GDP-bound states. Their functions are modulated by the diverse selection of effector proteins that bind to specific Rabs in their activated state. We previously described the expression of Rab13 in bone cells. To search for novel Rab13 interaction partners, we screened a newborn rat bone marrow cDNA library for Rab13 effectors with a bacterial two-hybrid system. We found that Rab13 binds to the C-terminus of Endospanin-2, a small transmembrane protein. In addition to Rab13 also Rab8 bound to Endospanin-2, while no binding of Rab7, Rab10, Rab11 or Rab32 was observed. Rab13 and Rab8 also interacted with Endospanin-1, a close homolog of Endospanin-2. Rab13 and Endospanin-2 colocalised in perinuclear vesicular structures in Cos1 cells suggesting direct binding also in vivo. Endospanin-2 is implicated in the regulation of the cell surface growth hormone receptor (GHR), but the inhibition of Rab13 expression did not affect GHR cell surface expression. This suggests that the Rab13–Endospanin-2 interaction may have functions other than GHR regulation. In conclusion, we have identified a novel interaction for Rab13 and Rab8 with Endospanin-2 and Endospanin-1. The role of this interaction in cell physiology, however, remains to be elucidated.

Effects of calcium, dairy product, and vitamin D supplementation on bone mass accrual and body composition in 10-12-y-old girls: a 2-y randomized trial

BACKGROUND

Little is known about the relative effectiveness of calcium supplementation from food or pills with or without vitamin D supplementation for bone mass accrual during the rapid growth period.

OBJECTIVE

The purpose was to examine the effects of both food-based and pill supplements of calcium and vitamin D on bone mass and body composition in girls aged 10-12 y.

DESIGN

This placebo-controlled intervention trial randomly assigned 195 healthy girls at Tanner stage I-II, aged 10-12 y, with dietary calcium intakes <900 mg/d to 1 of 4 groups: calcium (1000 mg) + vitamin D3 (200 IU), calcium (1000 mg), cheese (1000 mg calcium), and placebo. Primary outcomes were bone indexes of the hip, spine, and whole body by dual-energy X-ray absorptiometry and of the radius and tibia by peripheral quantitative computed tomography.

RESULTS

With the use of intention-to-treat or efficacy analysis, calcium supplementation with cheese resulted in a higher percentage change in cortical thickness of the tibia than did placebo, calcium, or calcium + vitamin D treatment (P = 0.01, 0.038, and 0.004, respectively) and in higher whole-body bone mineral density than did placebo treatment (P = 0.044) when compliance was >50%. With the use of a hierarchical linear model with random effects to control for growth velocity, these differences disappeared.

CONCLUSIONS

Increasing calcium intake by consuming cheese appears to be more beneficial for cortical bone mass accrual than the consumption of tablets containing a similar amount of calcium. Diverse patterns of growth velocity may mask the efficacy of supplementation in a short-term trial of children transiting through puberty.

Characterization and cellular distribution of the osteoclast ruffled membrane vacuolar H+-ATPase B-subunit using isoform-specific antibodies

Acidification of the bone surface, leading to bone resorption, is accomplished by a vacuolar-type H+-ATPase present in a specialized domain of the plasma membrane of the osteoclast known as the ruffled membrane. Structure and function appears to be highly conserved within this class of multisubunit enzymes. However, cloning and sequencing of complementary DNA has shown that one of the subunits in the catalytic domain, the B-subunit, exists in at least two forms, B1 and B2. B1 messenger RNA has been found almost exclusively in the kidney, whereas messenger RNA for B2 has been found in all tissues studied, including the kidney. It has been speculated that the B1 isoform might be involved in targeting to the plasma membrane. In the present study, we have characterized the B-subunit of the chicken osteoclast H+-ATPase using antibodies directed against peptides with isoform-specific or conserved sequences of the B-subunit. Western analysis was performed on chicken osteoclast membrane vesicles and on partially purified chicken osteoclast H+-ATPase and was compared with similar analysis of H+-ATPase isolated from bovine kidney and brain. The B1-specific antibody reacted with a polypeptide of approximately 56 kD on immunoblots of the renal H+-ATPase, whereas no reaction could be detected against the osteoclast H+-ATPase or the osteoclast membrane vesicle preparation. In contrast, the antibody against a B2-specific sequence reacted with a peptide of approximately 56 kD on immunoblots of the osteoclast H+-ATPase, the renal H+-ATPase, and the clathrin-coated vesicle H+-ATPase. The antibody against a conserved region of the B-subunit did not generate any evidence for the presence of isoforms other than B2 in the osteoclast. Immunocytochemistry of rat osteoclasts on bovine bone slices using the B2 antibody showed intense polarized staining along the plasma membrane facing the bone surface in actively resorbing osteoclasts whereas nonresorbing osteoclasts were diffusely stained throughout the cytoplasm. By confocal microscopy, the B2 staining was located to the level of the ruffled membrane and appeared to be concentrated to the peripheral areas of the membrane adjacent to the sealing zone. We conclude that the osteoclast vacuolar H+-ATPase contains the B2 isoform and suggest that upon initiation of resorption the pump is translocated from the cell interior to a special domain of the ruffled membrane close to the sealing zone.