Non‐Obese MKR Mouse Model of Type 2 Diabetes Reveals Skeletal Alterations in Mineralization and Material Properties

Skeletal abnormalities in mice with Dnmt3a missense mutations

Overgrowth and intellectual disability disorders in humans are typified by length/height and/or head circumference ≥ 2 standard deviations above the mean as well as intellectual disability and behavioral comorbidities, including autism and anxiety. Tatton-Brown-Rahman Syndrome is one type of overgrowth and intellectual disability disorder caused by heterozygous missense mutations in the DNA methyltransferase 3A (DNMT3A) gene. Numerous DNMT3A mutations have been identified in Tatton-Brown-Rahman Syndrome patients and may be associated with varying phenotype severities of clinical presentation. Two such mutations are the R882H and P904L mutations which result in severe and mild phenotypes, respectively. Mice with paralogous mutations (Dnmt3aP900L/+ and Dnmt3aR878H/+) exhibit overgrowth in their long bones (e.g., femur, humerus), but the mechanisms responsible for their skeletal overgrowth remain unknown. The goal of this study is to characterize skeletal phenotypes in mouse models of Tatton-Brown-Rahman Syndrome and identify potential cellular mechanisms involved in the skeletal overgrowth phenotype. We report that mature mice with the Dnmt3aP900L/+ or Dnmt3aR878H/+ mutation exhibit tibial overgrowth, cortical bone thinning, and weakened bone mechanical properties. Dnmt3aR878H/+ mutants also contain larger bone marrow adipocytes while Dnmt3aP900L/+ mutants show no adipocyte phenotype compared to control animals. To understand the potential cellular mechanisms regulating these phenotypes, growth plate chondrocytes, osteoblasts, and osteoclasts were assessed in juvenile mutant mice using quantitative static histomorphometry and dynamic histomorphometry. Tibial growth plates appeared thicker in mutant juvenile mice, but no changes were observed in osteoblast activity or osteoclast number in the femoral mid-diaphysis. These studies reveal new skeletal phenotypes associated with Tatton-Brown-Rahman Syndrome in mice and provide a rationale to extend clinical assessments of patients with this condition to include bone density and quality testing. These findings may be also informative for skeletal characterization of other mouse models presenting with overgrowth and intellectual disability phenotypes.

Advantages and Limitations of Diabetic Bone Healing in Mouse Models: A Narrative Review

Diabetes represents a major risk factor for impaired fracture healing. Type 2 diabetes mellitus is a growing epidemic worldwide, hence an increase in diabetes-related complications in fracture healing can be expected. However, the underlying mechanisms are not yet completely understood. Different mouse models are used in preclinical trauma research for fracture healing under diabetic conditions. The present review elucidates and evaluates the characteristics of state-of-the-art murine diabetic fracture healing models. Three major categories of murine models were identified: Streptozotocin-induced diabetes models, diet-induced diabetes models, and transgenic diabetes models. They all have specific advantages and limitations and affect bone physiology and fracture healing differently. The studies differed widely in their diabetic and fracture healing models and the chosen models were evaluated and discussed, raising concerns in the comparability of the current literature. Researchers should be aware of the presented advantages and limitations when choosing a murine diabetes model. Given the rapid increase in type II diabetics worldwide, our review found that there are a lack of models that sufficiently mimic the development of type II diabetes in adult patients over the years. We suggest that a model with a high-fat diet that accounts for 60% of the daily calorie intake over a period of at least 12 weeks provides the most accurate representation.

Advantages of omics approaches for elucidating metabolic changes in diabetic peripheral neuropathy

Various animal and cell culture models of diabetes mellitus (DM) have been established and utilized to study diabetic peripheral neuropathy (DPN). The divergence of metabolic abnormalities among these models makes their etiology complicated despite some similarities regarding the pathological and neurological features of DPN. Thus, this study aimed to review the omics approaches toward DPN, especially on the metabolic states in diabetic rats and mice induced by chemicals (streptozotocin and alloxan) as type 1 DM models and by genetic mutations (MKR, db/db and ob/ob) and high-fat diet as type 2 DM models. Omics approaches revealed that the pathways associated with lipid metabolism and inflammation in dorsal root ganglia and sciatic nerves were enriched and controlled in the levels of gene expression among these animal models. Additionally, these pathways were conserved in human DPN, indicating the pivotal pathogeneses of DPN. Omics approaches are beneficial tools to better understand the association of metabolic changes with morphological and functional abnormalities in DPN.

Longitudinal alterations in bone morphometry, mechanical integrity and composition in Type-2 diabetes in a Zucker diabetic fatty (ZDF) rat

Individuals with Type-2 Diabetes (T2D) have an increased risk of bone fracture, without a reduction in bone mineral density. It is hypothesised that the hyperglycaemic state caused by T2D forms an excess of Advanced Glycated End-products (AGEs) in the organic matrix of bone, which are thought to stiffen the collagen network and lead to impaired mechanical properties. However, the mechanisms are not well understood. This study aimed to investigate the geometrical, structural and material properties of diabetic cortical bone during the development and progression of T2D in ZDF (fa/fa) rats at 12-, 26- and 46-weeks of age. Longitudinal bone growth was impaired as early as 12-weeks of age and by 46-weeks bone size was significantly reduced in ZDF (fa/fa) rats versus controls (fa/+). Diabetic rats had significant structural deficits, such as bending rigidity, ultimate moment and energy-to-failure measured via three-point bend testing. Tissue material properties, measured by taking bone geometry into account, were altered as the disease progressed, with significant reductions in yield and ultimate strength for ZDF (fa/fa) rats at 46-weeks. FTIR analysis on cortical bone powder demonstrated that the tissue material deficits coincided with changes in tissue composition, in ZDF (fa/fa) rats with long-term diabetes having a reduced carbonate:phosphate ratio and increased acid phosphate content when compared to age-matched controls, indicative of an altered bone turnover process. AGE accumulation, measured via fluorescent assays, was higher in the skin of ZDF (fa/fa) rats with long-term T2D, bone AGEs did not differ between strains and neither AGEs correlated with bone strength. In conclusion, bone fragility in the diabetic ZDF (fa/fa) rats likely occurs through a multifactorial mechanism influenced initially by impaired bone growth and development and proceeding to an altered bone turnover process that reduces bone quality and impairs biomechanical properties as the disease progresses.

Loss of hyaluronan synthases impacts bone morphology, quality, and mechanical properties

Hyaluronan, a glycosaminoglycan synthesized by three isoenzymes (Has1, Has2, Has3), is known to play a role in regulating bone turnover, remodeling, and mineralization, which in turn can affect bone quality and strength. The goal of this study is to characterize how the loss of Has1 or Has3 affects the morphology, matrix properties, and overall strength of murine bone. Femora were isolated from Has1^-/-,Has3^-/-, and wildtype (WT) C57Bl/6 J female mice and were analyzed using microcomputed-tomography, confocal Raman spectroscopy, three-point bending, and nanoindentation. Of the three genotypes tested, Has1^-/- bones demonstrated significantly lower cross-sectional area (p = 0.0002), reduced hardness (p = 0.033), and lower mineral-to-matrix ratio (p < 0.0001). Has3^-/- bones had significantly higher stiffness (p < 0.0001) and higher mineral-to-matrix ratio (p < 0.0001) but lower strength (p = 0.0014) and bone mineral density (p < 0.0001) than WT. Interestingly, loss of Has3 was also associated with significantly lower accumulation of advanced glycation end-products than WT (p = 0.0478). Taken together, these results demonstrate, for the first time, the impact of the loss of hyaluronan synthase isoforms on cortical bone structure, content, and biomechanics. Loss of Has1 impacted morphology, mineralization, and micron-level hardness, while loss of Has3 reduced bone mineral density and affected organic matrix composition, impacting whole bone mechanics. This is the first study to characterize the effect of loss of hyaluronan synthases on bone quality, suggesting an essential role hyaluronan plays during the development and regulation of bone.

Human tibial cortical bone with high porosity in type 2 diabetes mellitus is accompanied by distinctive bone material properties

Diabetes mellitus is a metabolic disease affecting bone tissue at different length-scales. Higher fracture risk in diabetic patients is difficult to detect with common clinical fracture risk assessment due to normal or high bone mineral density in diabetic patients. The observed higher fracture risk despite normal to high areal bone mineral density in diabetic patients points towards impaired bone material quality. Here, we analyze tibial bone from individuals with type 2 diabetes mellitus using a multiscale-approach, which includes clinical and laboratory-based bone quality measures. Tibial cortical bone tissue from individuals with type 2 diabetes mellitus (T2DM) and age-matched healthy controls (n = 15 each) was analyzed with in situ impact indentation, dual energy X-ray absorptiometry (DXA), high resolution peripheral microcomputed tomography (HR-pQCT), micro-computed tomography (microCT), cyclic indentation, quantitative backscattered electron microscopy (qBEI), vibrational spectroscopy (Raman), nanoindentation, and fluorescence spectroscopy. With this approach, a high cortical porosity subgroup of individuals with T2DM was discriminated from two study groups: individuals with T2DM and individuals without T2DM, while both groups were associated with similar cortical porosity quantified by means of microCT. The high porosity T2DM group, but not the T2DM group, showed compromised bone quality expressed by altered cyclic indentation properties (transversal direction) in combination with a higher carbonate-to-amide I ratio in endocortical bone. In addition, in the T2DM group with high cortical porosity group, greater cortical pore diameter was identified with HR-pQCT and lower tissue mineral density using microCT, both compared to T2DM group. Micromechanical analyses of cross-sectioned osteons (longitudinal direction) with cyclic indentation, qBEI, and nanoindentation showed no differences between the three groups. High tibial cortical porosity in T2DM can be linked to locally altered bone material composition. As the tibia is an accessible skeletal site for fracture risk assessment in the clinics (CT, indentation), our findings may contribute to further understanding the site-specific structural and compositional factors forming the basis of bone quality in diabetes mellitus. Refined diagnostic strategies are needed for a comprehensive fracture risk assessment in diabetic bone disease.

The extracellular matrix of human bone marrow adipocytes and glucose concentration differentially alter mineralization quality without impairing osteoblastogenesis

Bone marrow adipocytes (BMAds) accrue in various states of osteoporosis and interfere with bone remodeling through the secretion of various factors. However, involvement of the extracellular matrix (ECM) produced by BMAds in the impairment of bone marrow mesenchymal stromal cell (BM-MSC) osteoblastogenesis has received little attention. In type 2 diabetes (T2D), skeletal fragility is associated with several changes in bone quality that are incompletely understood, and BMAd quantity increases in relationship to poor glycemic control. Considering their altered phenotype in this pathophysiological context, we aimed to determine the contribution of the ECM of mature BMAds to osteoblastogenesis and mineralization quality in the context of chronic hyperglycemia.

Human BM-MSCs were differentiated for 21 days in adipogenic medium containing either a normoglycemic (LG, 5.5 mM) or a high glucose concentration (HG, 25 mM). The ECM laid down by BMAds were devitalized through cell removal to examine their impact on the proliferation and differentiation of BM-MSCs toward osteoblastogenesis in LG and HG conditions. Compared to control plates, both adipocyte ECMs promoted cell adhesion and proliferation. As shown by the unmodified RUNX2 and osteocalcin mRNA levels, BM-MSC commitment in osteoblastogenesis was hampered by neither the hyperglycemic condition nor the adipocyte matrices. However, adipocyte ECMs or HG condition altered the mineralization phase with perturbed expression levels of type 1 collagen, MGP and osteopontin. Despite higher ALP activity, mineralization levels per cell were decreased for osteoblasts grown on adipocyte ECMs compared to controls. Raman spectrometry revealed that culturing on adipocyte matrices specifically prevents type-B carbonate substitution and favors collagen crosslinking, in contrast to exposure to HG concentration alone. Moreover, the mineral to organic ratio was disrupted according to the presence of adipocyte ECM and the glucose concentration used for adipocyte or osteoblast culture. HG concentration and adipocyte ECM lead to different defects in mineralization quality, recapitulating contradictory changes reported in T2D osteoporosis.

Our study shows that ECMs from BMAds do not impair osteoblastogenesis but alter both the quantity and quality of mineralization partly in a glucose concentration-dependent manner. This finding sheds light on the involvement of BMAds, which should be considered in the compromised bone quality of T2D and osteoporosis patients more generally.

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Glucose level alters the Extracellular Matrix composition of Bone Marrow adipocytes.

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Osteoblastogenesis on adipocyte ECMs is unaltered but produced less mineral amount.

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The quality of the mineral is altered differently by adipocyte ECMs or glucose levels.

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The presence of BM adipocytes should be valued in damaged osteoporosis bone quality.

Dimorphic Mechanisms of Fragility in Diabetes Mellitus: the Role of Reduced Collagen Fibril Deformation

Diabetes mellitus (DM) is an emerging metabolic disease, and the management of diabetic bone disease poses a serious challenge worldwide. Understanding the underlying mechanisms leading to high fracture risk in DM is hence of particular interest and urgently needed to allow for diagnosis and treatment optimization. In a case-control postmortem study, the whole 12th thoracic vertebra and cortical bone from the mid-diaphysis of the femur from male individuals with type 1 diabetes mellitus (T1DM) (n = 6; 61.3 ± 14.6 years), type 2 diabetes mellitus (T2DM) (n = 11; 74.3 ± 7.9 years), and nondiabetic controls (n = 18; 69.3 ± 11.5) were analyzed with clinical and ex situ imaging techniques to explore various bone quality indices. Cortical collagen fibril deformation was measured in a synchrotron setup to assess changes at the nanoscale during tensile testing until failure. In addition, matrix composition was analyzed including determination of cross-linking and non-crosslinking advanced glycation end-products like pentosidine and carboxymethyl-lysine. In T1DM, lower fibril deformation was accompanied by lower mineralization and more mature crystalline apatite. In T2DM, lower fibril deformation concurred with a lower elastic modulus and tendency to higher accumulation of non-crosslinking advanced glycation end-products. The observed lower collagen fibril deformation in diabetic bone may be linked to altered patterns mineral characteristics in T1DM and higher advanced glycation end-product accumulation in T2DM. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Techniques for advanced glycation end product measurements for diabetic bone disease: pitfalls and future directions

PURPOSE OF REVIEW

Multiple biochemical and biophysical approaches have been broadly used for detection and quantitation of posttranslational protein modifications associated with diabetic bone, yet these techniques present a variety of challenges. In this review, we discuss recent advancements and complementary roles of analytical (UPLC/UPLC-MS/MS and ELISA) and biophysical (Raman and FTIR) techniques used for characterization of glycation products, measured from bone matrix and serum, and provide recommendations regarding the selection of a technique for specific study of diabetic bone.

RECENT FINDINGS

Hyperglycemia and oxidative stress in diabetes contribute to the formation of a large subgroup of advanced glycation end products (AGEs) known as glycoxidation end products (AGOEs). AGEs/AGOEs have various adverse effects on bone health. Commonly, accumulation of AGEs/AGOEs leads to increased bone fragility. For example, recent studies show that carboxymethyllysine (CML) and pentosidine (PEN) are formed in bone at higher levels in certain diseases and metabolic conditions, in particular, in diabetes and aging. Detection and quantitation of AGEs/AGOEs in rare and/or precious samples is feasible because of a number of technological advancements of the past decade.

SUMMARY

Recent technological advancements have led to a significant improvement of several key analytical biochemistry and biophysics techniques used for detection and characterization of AGEs/AGOEs in bone and serum. Their principles and applications to skeletal tissue studies as well as limitations are discussed in this review.

Induction and rescue of skeletal fragility in a high-fat diet mouse model of type 2 diabetes: An in vivo and in vitro approach

Poor bone quality is associated with Type 2 Diabetes (T2D), with patients having a higher risk of fracture despite normal to high bone mineral density (BMD). Diabetes contributes to modifications of the mineral and organic matrix of bone. Hyperglycemia has been linked to the formation of advanced glycation end-products (AGEs) which increase the risk for skeletal fragility fractures. To this end, we investigated diabetes-induced skeletal fragility using a high-fat diet (HFD) mouse model and evaluated the efficacy of phenacyl thiazolium chloride (PTC) for in vitro removal of glycation products to rescue bone toughness. Ten-week-old C57BL/6 J male mice (n = 6/group) were fed a HFD or low-fat diet (LFD) for 22 weeks. Mice given a HFD developed T2D and increased body mass compared to LFD-fed mice. MicroCT results showed that diabetic mice had altered microarchitecture and increased mineralization as determined by volumetric BMD and increased mineral crystal size as determined by X-ray Diffraction (XRD). Diabetic mice demonstrated loss of initiation and maximum toughness, which represent estimates of the stress intensity factor at a notch tip using yield force and ultimate force, respectively. Diabetic mice also showed higher accumulation of AGEs measured by biochemical assay (total fAGEs) and confocal Raman spectroscopy (Pentosidine (PEN), Carboxymethyl-lysine (CML)). Regression analyses confirmed the association between increased glycoxidation (CML, PEN) and loss of fracture toughness. Within the diabetic group, CML was the most significant predictor of initiation toughness while PEN predicted maximum toughness as determined by stepwise linear regression (i.e., stepAIC). Contralateral femora from HFD group were harvested and treated with PTC in vitro. PTC-treated samples showed total fAGEs decreased by 41.2%. PTC treatment partially restored bone toughness as, compared to T2D controls, maximum toughness increased by 35%. Collectively, our results demonstrate that matrix modifications in diet-induced T2D, particularly AGEs, induce bone fragility and their removal from bone matrix partially rescues T2D associated bone fragility.