# Targeted Overexpression of Claudin 11 in Osteoblasts Increases Trabecular Bone Mass by Stimulating Osteogenesis at the Expense of Adipogenesis in Mice

## TL;DR

Overexpressing claudin11 in osteoblasts in mice increases bone mass by boosting bone formation and reducing fat formation in the bone marrow.

## Contribution

This study demonstrates that claudin11 promotes osteogenesis while suppressing adipogenesis in osteoblasts, revealing a novel regulatory mechanism in bone homeostasis.

## Key findings

- Transgenic mice with claudin11 overexpression in osteoblasts showed increased trabecular bone mass due to enhanced bone formation.
- These mice had reduced bone marrow adipose tissue and lower expression of adipogenic markers like adiponectin and leptin.
- The study suggests claudin11 promotes osteoblast differentiation at the expense of adipocyte formation.

## Abstract

To interrogate the role of claudin11 expression in osteoblasts in regulating homeostasis, we generated transgenic mice that express claudin11 under the control of the rat 2.3 kb collagen 1α1 promoter. Micro-CT analyses revealed that the distal femoral trabecular bone volume was significantly augmented in the transgenic mice, which was caused by an elevated trabecular number and a reduction in trabecular separation. The increased trabecular bone mass was caused by enhanced bone formation but not by decreased bone resorption in the claudin11 transgenic mice. The transgenic mice displayed reduced bone marrow adipose tissue and lower expression levels of the adipogenic markers adiponectin and leptin but higher mRNA levels of the osteogenic markers Alp and Bsp in the femur. Our data indicate that claudin11 promotes osteogenesis at the expense of adipogenesis in mice.

Mice lacking Claudin11 (Cldn11) manifest reduced trabecular bone mass. However, the impact of Cldn11 expression in osteoblasts in vivo remains understudied. Herein, we generated osteoblast-specific transgenic (Tg) mice expressing Cldn11 and characterized their skeletal phenotype. Micro-CT analyses of the distal metaphysis of the femur showed a 50% and a 38% increase in trabecular bone mass in Tg male and female mice, respectively, due to a significant increase in trabecular number and a reduction in trabecular separation. Histomorphometry and serum biomarker studies uncovered that increased trabecular bone mass in Cldn11 Tg mice was the consequence of enhanced bone formation. Accordingly, an abundance of bone formation (Alp, Bsp), but not bone resorption (Ctsk), markers were augmented in the femurs of Cldn11 Tg mice. Since the trabecular bone density is known to inversely correlate with the amount of marrow adipose tissue (MAT), we measured the MAT in osmium-tetroxide-labeled bones by micro-CT scanning. We found 86% less MAT in the proximal tibia of the Tg males. Consistently, the expression levels of the adipogenic markers, adiponectin and leptin, were 50% lower in the femurs of the Tg males. Our data are consistent with the possibility that claudin11 exerts anabolic effects in osteoblastic lineage cells that act via promoting the differentiation of marrow stem cells towards osteoblasts at the expense of adipocytes.