Longitudinal analysis of mesenchymal progenitors and bone quality in the stem cell antigen-1-null osteoporotic mouse

We performed a longitudinal analysis of bone quality in Sca-1-null mice. A tight temporal, site-specific association between Sca-1-deficient BMD deficiency and reduced mesenchymal progenitor frequency was observed. Defects in trabecular microarchitecture and mineralization were, at least partially, responsible for the age-related reduction in toughness of Sca-1(-/-) bones.

INTRODUCTION

We previously showed that stem cell antigen 1 (Sca-1)-null mice undergo normal bone development but exhibit significantly decreased bone mass characteristic of age-dependent osteoporosis. The objective of this study was to characterize the initiation and progression of the Sca-1 mutant skeletal phenotype at the cellular, structural, material, and mechanical levels.

MATERIALS AND METHODS

Sca-1-null and control mice were analyzed at 3, 5, 7, and 9 mo of age. In vitro osteoclastogenesis of bone marrow and spleen-derived progenitor populations was assessed. Bone marrow-derived mesenchymal progenitor frequency, along with osteogenic and adipogenic differentiation potential, was analyzed in vitro. Static histomorphometry of the sixth lumbar vertebrae was performed. Whole body, femoral, and vertebral BMD were assessed using DXA. Lumbar vertebrae were analyzed using microCT, back-scattered electron imaging, and compression tests. Three-point bending and femoral neck fracture tests were performed on excised femurs.

RESULTS

Sca-1-null mice displayed an age-dependent, cell-autonomous osteoclast deficiency in vitro. From 7 mo of age onward, reduced Sca-1-null femoral BMD was observed alongside reduced mesenchymal progenitor frequency, and decreased in vitro osteogenic and adipogenic differentiation potential. Sca-1-deficient mice exhibited reduced whole body BMD compared with controls at all time-points analyzed. Although no differences in spinal BMD were observed, Sca-1(-/-) vertebrae exhibited decreased bone formation, with a maximal difference at 7 mo of age, inferior trabecular microarchitecture, and a greater degree of mineralization. At all sites tested, Sca-1-null bones exhibited reduced energy to failure from 5 mo onward.

CONCLUSIONS

We showed a tight association within Sca-1-null mice between the initiation of stem cell defects and the exacerbation of deficiencies in bone quality at two sites clinically relevant to developing osteoporotic fractures. Sca-1-deficient mice, therefore, provide a novel and useful murine model of age-related osteoporosis, which with additional study, should further our understanding of the mechanisms underlying this increasingly prevalent disease.

A Gja1 missense mutation in a mouse model of oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) is an autosomal dominant disorder characterized by pleiotropic developmental anomalies of the limbs, teeth, face and eyes that was shown recently to be caused by mutations in the gap junction protein alpha 1 gene (GJA1), encoding connexin 43 (Cx43). In the course of performing an N-ethyl-N-nitrosourea mutagenesis screen, we identified a dominant mouse mutation that exhibits many classic symptoms of ODDD, including syndactyly, enamel hypoplasia, craniofacial anomalies and cardiac dysfunction. Positional cloning revealed that these mice carry a point mutation in Gja1 leading to the substitution of a highly conserved amino acid (G60S) in Cx43. In vivo and in vitro studies revealed that the mutant Cx43 protein acts in a dominant-negative fashion to disrupt gap junction assembly and function. In addition to the classic features of ODDD, these mutant mice also showed decreased bone mass and mechanical strength, as well as altered hematopoietic stem cell and progenitor populations. Thus, these mice represent an experimental model with which to explore the clinical manifestations of ODDD and to evaluate potential intervention strategies.

LY6A/E (SCA-1) expression in the mouse testis

Recently, it was found by two research groups that LY6A, known widely in the stem cell community as stem cell antigen-1 or SCA-1, is expressed on testicular side population (SP) cells. Whether these SP cells are spermatogonial stem cells is a point of disagreement and, therefore, the identity of the LY6A-positive cells as well. We studied the expression pattern of LY6A in testis by immunohistochemistry and found it to be expressed in the interstitial tissue on peritubular myoid, endothelial, and spherical-shaped peritubular mesenchymal cells. To address the question whether LY6A has a function in spermatogenesis or testis development, we studied the testis of Ly6a(-/-) mice (allele Ly6a(tm1Pmf)). We found no morphological abnormalities or differences in numbers of spermatogonia, spermatocytes, Leydig cells, or macrophages in relation to the number of Sertoli cells. Therefore, we conclude that LY6A expression does not influence testis development or spermatogenesis and that spermatogonial stem cells are LY6A negative.

Rosiglitazone Facilitates Angiogenic Progenitor Cell Differentiation Toward Endothelial Lineage

Background— Peroxisome proliferator–activated receptor-γ (PPAR-γ) agonists inhibit vascular smooth muscle proliferation and migration and improve endothelial function. It is unknown whether PPAR-γ agonists favorably modulate bone marrow (BM)–derived angiogenic progenitor cells (APCs) to promote endothelial lineage differentiation and early reendothelialization after vascular intervention.

Methods and Results— C57/BL6 mice, treated with or without rosiglitazone (8 mg/kg per day), a PPAR-γ agonist, underwent femoral angioplasty. Rosiglitazone treatment attenuated neointimal formation (intima/media ratio: 0.98±0.12 [rosiglitazone] versus 3.1±0.5 [control]; P <0.001; n=10 per group). Using a BM transplantation model, we identified that 58±12% of the cells within the neointima at 4 weeks were derived from the BM. Pure endothelial marker–positive, pure α-smooth muscle actin (αSMA)–positive, or double-positive APCs could be found both in mouse BM and in human peripheral blood after culture in conditional medium enriched with vascular endothelial growth factor. Rosiglitazone caused a 6-fold ( P <0.001) increase in colony formation by human endothelial progenitor cells, promoted the differentiation of APCs toward the endothelial lineage in mouse BM in vivo (0.66±0.06% [control] to 0.95±0.08% [rosiglitazone]; P <0.05) and in human peripheral blood in vitro (13.2±1.5% [control] to 28.4±3.3% [rosiglitazone]; P <0.05), and inhibited the differentiation toward the smooth muscle cell lineage. Within the neointima, rosiglitazone also stimulated APCs to differentiate into mature endothelial cells and caused earlier reendothelialization compared with controls (31±5 versus 8±2 CD31-positive cells per millimeter of neointimal surface on day 14; P <0.01).

Conclusions— Similar to embryonic stem cell–derived progenitors, the adult BM and peripheral blood harbor APCs that are at least bipotential and able to differentiate into endothelial and smooth muscle lineages. The PPAR-γ agonist rosiglitazone promotes the differentiation of these APCs toward the endothelial lineage and attenuates restenosis after angioplasty.