Genetic Deletion of Menin in Mouse Mesenchymal Stem Cells: An Experimental and Computational Analysis

Abstract

Loss-of-function mutations in the MEN1 tumor-suppressor gene cause the multiple endocrine neoplasia type 1 syndrome. Menin, the MEN1 gene product, is expressed in many tissues, including bone, where its function remains elusive. We conditionally inactivated menin in mesenchymal stem cells (MSCs) using paired-related homeobox 1 (Prx1)-Cre and compared resultant skeletal phenotypes of Prx1-Cre;Men1 f/f menin-knockout mice (KO) and wild-type controls using in vivo and in vitro experimental approaches and mechanics simulation. Dual-energy X-ray absorptiometry demonstrated significantly reduced bone mineral density, and 3-dimensional micro-CT imaging revealed a decrease in trabecular bone volume, altered trabecular structure, and an increase in trabecular separation in KO mice at 6 and 9 months of age. Numbers of osteoblasts were unaltered, and dynamic histomorphometry demonstrated unaltered bone formation; however, osteoclast number and activity and receptor activator of NF-κB ligand/osteoprotegerin (RANKL/OPG) mRNA profiles were increased, supporting increased osteoclastogenesis and bone resorption. In vitro, proliferative capabilities of bone marrow stem cells and differentiation of osteoblasts and mineralization were unaltered; however, osteoclast generation was increased. Gross femur geometrical alterations observed included significant reductions in length and in mid-metaphyseal cross-sectional area. Atomic force microscopy demonstrated significant decreases in elasticity of both cortical and trabecular bone at the nanoscale, whereas three-point bending tests demonstrated a 30% reduction in bone stiffness; finite element analysis showed morphological changes of the femur microgeometry and a significantly diminished femur flexural rigidity. The biomechanical results demonstrated the detrimental outcome of the accelerated osteoclastic bone resorption. Our studies have a twofold implication; first, MEN1 deletion from MSCs can negatively regulate bone mass and bone biomechanics, and second, the experimental and computational biomechanical analyses employed in the present study should be applicable for improved phenotypic characterization of murine bone. Furthermore, our findings of critical menin function in bone may underpin the more severe skeletal phenotype found in hyperparathyroidism associated with loss-of-function of the MEN1 gene.

Effect of Menin Deletion in Early Osteoblast Lineage on the Mineralization of an In Vitro 3D Osteoid-like Dense Collagen Gel Matrix

“…The results of this study indicated that the early deletion of menin in the osteoblast lineage affects their late-stage differentiation and matrix mineralization in vitro. Overall, these results confirmed that the use of a biomimetic 3D collagenous matrix may provide better insights into the cell-mediated mineral deposition processes compared to those of 2D systems [ 19 , 34 , 35 ].…”

“…In vivo studies have shown that in mature osteoblasts menin is required to maintain bone mass by affecting bone formation and osteoblast function [ 19 ]. When expressed at early stages of osteoblast differentiation, menin has also been shown to maintain bone mass by regulating osteoclastogenesis and bone resorption in vivo [ 34 ].…”