Upregulating CXCR4 in human fetal mesenchymal stem cells enhances engraftment and bone mechanics in a mouse model of osteogenesis imperfecta

Stem cells have considerable potential to repair damaged organs and tissues. We previously showed that prenatal transplantation of human first trimester fetal blood mesenchymal stem cells (hfMSCs) in a mouse model of osteogenesis imperfecta (oim mice) led to a phenotypic improvement, with a marked decrease in fracture rate. Donor cells differentiated into mature osteoblasts, producing bone proteins and minerals, including collagen type Iα2, which is absent in nontransplanted mice. This led to modifications of the bone matrix and subsequent decrease of bone brittleness, indicating that grafted cells directly contribute to improvement of bone mechanical properties. Nevertheless, the therapeutic effect was incomplete, attributing to the limited level of engraftment in bone. In this study, we show that although migration of hfMSCs to bone and bone marrow is CXCR4-SDF1 (SDF1 is stromal-derived factor) dependent, only a small number of cells present CXCR4 on the cell surface despite high levels of internal CXCR4. Priming with SDF1, however, upregulates CXCR4 to increase the CXCR4(+) cell fraction, improving chemotaxis in vitro and enhancing engraftment in vivo at least threefold in both oim and wild-type bone and bone marrow. Higher engraftment in oim bones was associated with decreased bone brittleness. This strategy represents a step to improve the therapeutic benefits of fetal cell therapy toward being curative.

Deficient degradation of homotrimeric type I collagen, α1(I)3 glomerulopathy in oim mice

Col1a2-deficient (oim) mice synthesize homotrimeric type I collagen due to nonfunctional proα2(I) collagen chains. Our previous studies revealed a postnatal, progressive type I collagen glomerulopathy in this mouse model, but the mechanism of the sclerotic collagen accumulation within the renal mesangium remains unclear. The recent demonstration of the resistance of homotrimeric type I collagen to cleavage by matrix metalloproteinases (MMPs), led us to investigate the role of MMP-resistance in the glomerulosclerosis of Col1a2-deficient mice. We measured the pre- and post-translational expression of type I collagen and MMPs in glomeruli from heterozygous and homozygous animals. Both the heterotrimeric and homotrimeric isotypes of type I collagen were equally present in whole kidneys of heterozygous mice by immunohistochemistry and biochemical analysis, but the sclerotic glomerular collagen was at least 95-98% homotrimeric, suggesting homotrimeric type I collagen is the pathogenic isotype of type I collagen in glomerular disease. Although steady-state MMP and Col1a1 mRNA levels increased with the disease progression, we found these changes to be a secondary response to the deficient clearance of MMP-resistant homotrimers. Increased renal MMP expression was not sufficient to prevent homotrimeric type I collagen accumulation.

Toll-like receptor-4 signaling and Kupffer cells play pivotal roles in the pathogenesis of non-alcoholic steatohepatitis

BACKGROUND/AIMS

Studies in animal models and humans suggest a link between endotoxemia and non-alcoholic steatohepatitis. Since Kupffer cells are responsible for clearing endotoxin and are activated via endotoxin interaction with Toll-like receptor 4 (TLR-4), we examined the relationship between hepatic TLR-4 expression and Kupffer cell content during the genesis of steatohepatitis.

METHODS

Male C57BL/6, C3H/HouJ and TLR-4 mutant C3H/HeJ mice were fed control or methionine/choline-deficient diet (MCDD). In one group of C57BL/6 mice, Kupffer cells were depleted by weekly intraperitoneal injections of clodronate liposomes. After 3 weeks, serum ALT activity and portal endotoxin levels were measured. Real-time PCR was used to examine mRNA expression of TLR-4, TLR-2, CD14, MD-2, TGFbeta, TNFalpha, CD36, PPAR-alpha, liver fatty acid binding protein (L-FABP) and collagen alpha1.

RESULTS

We observed histological evidence typical of steatohepatitis, portal endotoxemia and enhanced TLR-4 expression in wild type mice fed MCDD. In contrast, injury and lipid accumulation markers were significantly lower in TLR-4 mutant mice. Destruction of Kupffer cells with clodronate liposomes blunted histological evidence of steatohepatitis and prevented increases in TLR-4 expression.

CONCLUSIONS

These findings demonstrate the importance of TLR-4 signaling and underscore a direct link between TLR-4 and Kupffer cells in the pathogenesis of steatohepatitis.

Alpha 2(I) collagen deficient oim mice have altered biomechanical integrity, collagen content, and collagen crosslinking of their thoracic aorta

Collagen and elastin are the primary determinants of vascular integrity, with elastin hypothesized to be the major contributor to aortic compliance and type I collagen the major contributor to aortic strength and stiffness. Type I collagen is normally heterotrimeric composed of two alpha1(I) and one alpha2(I) collagen chains, alpha1(I)(2)alpha2(I). Recent investigations have reported that patients with recessively inherited forms of Ehlers Danlos syndrome that fail to synthesize proalpha2(I) chains have increased risks of cardiovascular complications. To assess the role of alpha2(I) collagen in aortic integrity, we used the osteogenesis imperfecta model (oim) mouse. Oim mice, homozygous for a COL1A2 mutation, synthesize only homotrimeric type I collagen, alpha1(I)3. We evaluated thoracic aortas from 3-month-old oim, heterozygote, and wildtype mice biomechanically for circumferential breaking strength (Fmax) and stiffness (IEM), histologically for morphological differences, and biochemically for collagen content and crosslinking. Circumferential biomechanics of oim and heterozygote descending thoracic aortas demonstrated the anticipated reduced Fmax and IEM relative to wildtype mice. Histological analyses of oim descending aortas demonstrated reduced collagen staining relative to wildtype aortas suggesting decreased collagen content, which hydroxyproline analyses of ascending and descending oim aortas confirmed. These findings suggest the reduced oim thoracic aortic integrity correlates with the absence of the alpha2(I)collagen chains and in part with reduced collagen content. However, oim ascending aortas also demonstrated a significant increase in pyridinoline crosslinks/collagen molecule as compared to wildtype ascending aortas. The role of increased collagen crosslinks is uncertain; increased crosslinking may represent a compensatory mechanism for the decreased integrity.

Altered disc mechanics in mice genetically engineered for reduced type I collagen

STUDY DESIGN

Mechanically test lumbar discs of transgenic mice in compression-tension and torsion.

OBJECTIVES

Determine if a reduction in type I collagen results in decreased disc mechanics.

SUMMARY OF BACKGROUND DATA

Quantitative relationships between disc structure and function would improve the understanding of disc generation and are essential relationships for functional tissue engineering. The reduced type I collagen transgenic mouse has been used in structure-function studies of bone and tendon, but not intervertebral discs. Methods for testing mouse discs have recently been developed, making disc structure-function studies possible.

METHODS

Microradiographed and mechanically tested lumbar discs from control and collagen-reduced mice in both compression-tension and torsion were used. Disc area and polar moment of inertia were determined from radiographic data, stiffness from mechanical data, and apparent modulus from geometric and mechanical data.

RESULTS

Collagen-reduced discs had a larger area and polar moment of inertia compared to controls. The linear and torsional stiffness of collagen-reduced and control discs were not significantly different. Finally, the apparent modulus of collagen-reduced discs was significantly less than controls in compression (73% of control) and torsion (50%).

CONCLUSIONS

Compared to controls, collagen-reduced discs had reduced apparent modulus in both loading directions, suggesting that the transgenic disc tissue was mechanically inferior to controls. These results are consistent with the widely accepted functional role of type I collagen in disc mechanics, and therefore supports the use of transgenic mice to study structure-function relationships of the disc. Future work will focus on quantifying structure-function relationships related to degeneration, as well as those relevant to the design of tissue-engineered disc replacements.

Effect of altered matrix proteins on quasilinear viscoelastic properties in transgenic mouse tail tendons

Tendons have complex mechanical behaviors that are viscoelastic, nonlinear, and anisotropic. It is widely held that these behaviors are provided for by the tissue's composition and structure. However, little data are available to quantify such structure-function relationships. This study quantified tendon mechanical behaviors, including viscoelasticity and nonlinearity, for groups of mice that were genetically engineered for altered extracellular matrix proteins. Uniaxial tensile stress-relaxation experiments were performed on tail tendon fascicles from the following groups: eight week old decorin knockout, eight week old reduced type I collagen, three week old control, and eight week old control. Data were fit using Fung's quasilinear viscoelastic model, where the model parameters represent the linear viscoelastic and nonlinear elastic response. The viscoelastic properties demonstrated a larger and faster stress relaxation for the decorin knockout and a smaller and slower stress relaxation for the three week control. The elastic parameter, A, in the eight week control group was significantly greater than in the collagen reduction and three week control groups. This study provides quantitative evidence for structure-function relationships in tendon, including the role of proteoglycan in viscoelasticity. Future studies should directly correlate composition and structure with tendon mechanics for the design and evaluation of tissue-engineered constructs or tendon repairs.

Alendronate treatment for infants with osteogenesis imperfecta: demonstration of efficacy in a mouse model

Recent non-placebo-controlled studies of the bisphosphonate pamidronate have shown it to be effective in reducing fractures and improving bone density in infants and children with osteogenesis imperfecta (OI). To evaluate the effects of bisphosphonate treatment in a controlled study, the oim/oim mouse model of OI was studied. Nursing infant mouse pups (approximately 2 wk old) with moderate to severe OI (oim/oim mouse) and age- and background-matched control mice (+/+) were treated either with the third-generation bisphosphonate alendronate (ALN), or with saline. Fracture risk, bone quality, and growth were evaluated over a 12-wk treatment period. ALN at a dose of 0.03 mg/kg/d or saline was administered via s.c. injection to infant oim/oim and wild-type (+/+) mice from 2 to 14 wk of age (n = 20 per subgroup). The average number of fractures sustained by the ALN-treated oim/oim mice was reduced significantly compared with the untreated oim/oim mice (0.7 +/- 0.7 fractures/mouse versus 2.0 +/- 0.2 fractures/mouse). Bone density increased significantly in the femur and the spine with treatment (2.0 +/- 0.5 versus 1.2 +/- 0.5 in femur and 2.1 +/- 0.5 versus1.6 +/- 0.5 in spine). Histologic evaluation revealed the percentage of metaphyseal tibial bone increased significantly with treatment in both +/+ and oim/oim mice. Mechanical testing revealed an increase in structural stiffness for both treated +/+ and oim/oim mice compared with untreated animals. None of the material properties examined were significantly altered with treatment, nor was spinal curvature affected. Weight gain and long bone growth were comparable in the treated and untreated oim/oim mice. In wild-type mice, femur lengths were significantly shorter in the treated mice compared with untreated counterparts. This animal study demonstrates that treatment of OI in mice as early as 2 wk of age with ALN appears to be effective in reducing fractures and increasing bone properties. Based on the data from this study, ALN therapy in infants with OI should prove to be effective.

OIM and related animal models of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is characterized by fragile bones, skeletal deformity, and growth retardation. This heritable disorder of connective tissue is the result of mutations affecting the COL1A1 and COL1A2 genes of type I collagen. Progress in OI research has been limited because of dependence on human fibroblast and osteoblast specimens and the absence of a naturally occurring animal model for this genetic disorder. Recent technology in molecular biology has led to the development of transgenic models of OI based on site directed mutagenesis of type I collagen genes. OIM is a naturally occurring model which incorporates both the phenotypic and biochemical defects of moderate to severe osteogenesis imperfecta. This powerful tool permits the development of models based on different type I collagen mutations. The collagen type I mutation in OIM is a C propeptide deletion which impairs the production of normal pro-alpha2(I). Tissues in OIM contain only [pro-alpha1(I)]3 homotrimer. Thus, although several animal models are now available for research in osteogenesis imperfecta few are viable or fully mimic human disease disorders. OIM duplicates the phenotype and biochemistry of human disease and has a normal life span.

Clinical management of osteogenesis imperfecta

Knowledge of the natural history of different types of OI permits planning of rehabilitation goals for children with OI. Notwithstanding this knowledge, rehabilitation will need to be modified to accommodate unexpected fractures and the highly variable chance of deformity in each individual. Immobilisation should be minimized to avoid immobilization osteoporosis. New rehabilitation issues include basilar impression, recommencement of fractures in postmenopausal women with OI, pregnancy related bone loss and sleep apnoea.