Biomechanical Stimulation of Muscle Constructs Influences Phenotype of Bone Constructs by Modulating Myokine Secretion

Diabetes is a chronic metabolic disorder that can lead to diabetic myopathy and bone diseases. The etiology of musculoskeletal complications in such metabolic disorders and the interplay between the muscular and osseous systems are not well understood. Exercise training promises to prevent diabetic myopathy and bone disease and offer protection. Although the muscle-bone interaction is largely biomechanical, the muscle secretome has significant implications for bone biology. Uncoupling effects of biophysical and biochemical stimuli on the adaptive response of bone during exercise training may offer therapeutic targets for diabetic bone disease. Here, we have developed an in vitro model to elucidate the effects of mechanical strain on myokine secretion and its impact on bone metabolism decoupled from physical stimuli. We developed bone constructs using cross-linked gelatin, which facilitated osteogenic differentiation of osteoprogenitor cells. Then muscle constructs were made from fibrin, which enabled myoblast differentiation and myotube formation. We investigated the myokine expression by muscle constructs under strain regimens replicating endurance (END) and high-intensity interval training (HIIT) in hyperglycemic conditions. In monocultures, both regimens induced higher expression of Il15 and Igf1, whereas END supported more myoblast differentiation and myotube maturation than HIIT. When co-cultured with bone constructs, HIIT regimen increased Glut4 expression in muscle constructs more than END, supporting higher glucose uptake. Likewise, the muscle constructs under the HIIT regimen promoted a healthier and more matured bone phenotype than END. Under static conditions, myostatin (Mstn) expression was significantly downregulated in muscle constructs co-cultured with bone constructs compared with monocultures. Together, our in vitro co-culture system allowed orthogonal manipulation of mechanical strain on muscle constructs while facilitating bone-muscle biochemical cross-talk. Such systems can provide an individualized microenvironment that allows decoupled biomechanical manipulation, help identify molecular targets, and develop engineered therapies for metabolic bone disease. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Pharmacologic Inhibition of Myostatin With a Myostatin Antibody Improves the Skeletal Muscle and Bone Phenotype of Male Insulin-Deficient Diabetic Mice

Type 1 diabetes (T1D) is associated with low bone and muscle mass, increased fracture risk, and impaired skeletal muscle function. Myostatin, a myokine that is systemically elevated in humans with T1D, negatively regulates muscle mass and bone formation. We investigated whether pharmacologic myostatin inhibition in a mouse model of insulin-deficient, streptozotocin (STZ)-induced diabetes is protective for bone and skeletal muscle. DBA/2J male mice were injected with low-dose STZ (diabetic) or vehicle (non-diabetic). Subsequently, insulin or palmitate Linbits were implanted and myostatin (REGN647-MyoAb) or control (REGN1945-ConAb) antibody was administered for 8 weeks. Body composition and contractile muscle function were assessed in vivo. Systemic myostatin, P1NP, CTX-I, and glycated hemoglobin (HbA1c) were quantified, and gastrocnemii were weighed and analyzed for muscle fiber composition and gene expression of selected genes. Cortical and trabecular parameters were analyzed (micro-computed tomography evaluations of femur) and cortical bone strength was assessed (three-point bending test of femur diaphysis). In diabetic mice, the combination of insulin/MyoAb treatment resulted in significantly higher lean mass and gastrocnemius weight compared with MyoAb or insulin treatment alone. Similarly, higher raw torque was observed in skeletal muscle of insulin/MyoAb-treated diabetic mice compared with MyoAb or insulin treatment. Additionally, muscle fiber cross-sectional area (CSA) was lower with diabetes and the combination treatment with insulin/MyoAb significantly improved CSA in type II fibers. Insulin, MyoAb, or insulin/MyoAb treatment improved several parameters of trabecular architecture (eg, bone volume fraction [BV/TV], trabecular connectivity density [Conn.D]) and cortical structure (eg, cortical bone area [Ct. Ar.], minimum moment of inertia [Imin]) in diabetic mice. Lastly, cortical bone biomechanical properties (stiffness and yield force) were also improved with insulin or MyoAb treatment. In conclusion, pharmacologic myostatin inhibition is beneficial for muscle mass, muscle function, and bone properties in this mouse model of T1D and its effects are both independent and additive to the positive effects of insulin. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Heterogeneity and altered β-cell identity in the TallyHo model of early-onset type 2 diabetes

A primary underlying defect makes β-cells "susceptible" to no longer compensate for the peripheral insulin resistance and to trigger the onset of type 2 diabetes (T2D). New evidence suggests that in T2D, β-cells are not destroyed but experience a loss of identity, reverting to a progenitor-like state and largely losing the ability to sense glucose and produce insulin. We assessed (using fluorescence microscopy and histomorphometry correlated with the glycaemic status) the main β-cell identity modifications as diabetes progresses in the TallyHo/JngJ (TH) male mice, a polygenic model of spontaneous T2D, akin to the human phenotype. We found that: 1) conversion to overt diabetes is paralleled by a progressive reduction of insulin-expressing cells and expansion of a glucagon-positive population, together with alteration of islet size and shape; 2) the β-cell population is highly heterogeneous in terms of insulin content and specific transcription factors like PDX1 and NKX6.1, that are gradually lost during diabetes progression; 3) GLUT2 expression is altered early and strongly reduced at late stages of diabetes; 4) an endocrine developmental program dependent on NGN3-expressing progenitors is revived when hyperglycaemia becomes severe; and 5) the re-expression of the EMT-associated factor vimentin occurs as diabetes worsens, representing a possible regenerative response to β-cell loss. Based on these results, we formulated additional hypotheses for the β-cell identity alteration in the TH model, together with several limitations of the study, that constitute future research directions.

Mouse models of type 1 diabetes and their use in skeletal research

PURPOSE OF REVIEW

In this review, we describe the three primary mouse models of insulin-deficiency diabetes that have been used to study the effects of type 1 diabetes (T1D) on skeletal outcomes. These models include streptozotocin (chemically)-induced diabetes, autoimmune-mediated diabetes (the nonobese diabetes mouse), and a mutation in the insulin gene (the Akita mouse). We then describe the skeletal findings and/or skeletal phenotypes that have been delineated using these models.

RECENT FINDINGS

Humans with T1D have decreased bone mineral density and an increased risk for fragility fracture. Mouse models of insulin-deficiency diabetes (hereafter denoted as T1D) in many ways recapitulate these skeletal deficits. Utilizing techniques of microcomputed tomography, bone histomorphometry, biomechanical testing and fracture modeling, bone biomarker analysis, and Raman spectroscopy, mouse models of T1D have demonstrated abnormalities in bone mineralization, bone microarchitecture, osteoblast function, abnormal bone turnover, and diminished biomechanical properties of bone.

SUMMARY

Mouse models have provided significant insights into the underlying mechanisms involved in the abnormalities of bone observed in T1D in humans. These translational models have provided targets and pathways that may be modifiable to prevent skeletal complications of T1D.

Emerging therapies for the treatment of rare pediatric bone disorders

In recent years, new therapies for the treatment of rare pediatric bone disorders have emerged, guided by an increasing understanding of the genetic and molecular etiology of these diseases. Herein, we review three such disorders, impacted by debilitating deficits in bone mineralization or cartilage ossification, as well as the novel disease-modifying drugs that are now available to treat these conditions. Specifically, we discuss asfotase alfa, burosumab-twza, and vosoritide, for the treatment of hypophosphatasia, X-linked hypophosphatemia and achondroplasia, respectively. For each skeletal disorder, an overview of the clinical phenotype and natural history of disease is provided, along with a discussion of the clinical pharmacology, mechanism of action and FDA indication for the relevant medication. In each case, a brief review of clinical trial data supporting drug development for each medication is provided. Additionally, guidance as to drug dosing and long-term monitoring of adverse events and pediatric efficacy is presented, to aid the clinician seeking to utilize these novel therapies in their practice, or to become familiar with the healthcare expectations for children receiving these medications through specialized multidisciplinary clinics. The availability of these targeted therapies now significantly augments treatment options for conditions in which past therapy has relied upon less specific, symptomatic medical and orthopedic care.

RASopathies: The musculoskeletal consequences and their etiology and pathogenesis

The RASopathies comprise an ever-growing number of clinical syndromes resulting from germline mutations in components of the RAS/MAPK signaling pathway. While multiple organs and tissues may be affected by these mutations, this review will focus on how these mutations specifically impact the musculoskeletal system. Herein, we review the genetics and musculoskeletal phenotypes of these syndromes in humans. We discuss how mutations in the RASopathy syndromes have been studied in translational mouse models. Finally, we discuss how signaling molecules within the RAS/MAPK pathway are involved in normal and abnormal bone biology in the context of osteoblasts, osteoclasts and chondrocytes.

Canagliflozin, an SGLT2 inhibitor, corrects glycemic dysregulation in TallyHO model of T2D but only partially prevents bone deficits

Higher fracture risk in type 2 diabetes (T2D) is attributed to disease-specific deficits in micro-structural and material properties of bone, although the primary cause is not yet established. The TallyHO (TH) mouse is a polygenic model of early-onset T2D and obesity analogous to adolescent-onset T2D in humans. Due to incomplete penetrance of the phenotype, ~25% of male TH mice never develop hyperglycemia, providing a strain-matched, non-diabetic control. Utilizing this model of T2D, we examined the impact of glucose-lowering therapy with canagliflozin (CANA) on diabetic bone. Male TH mice with or without hyperglycemia (High BG, Low BG) were monitored from ~8 to 20 weeks of age, and compared to age-matched, male, TH mice treated with CANA from ~8 to 20 weeks of age. At 20 weeks, untreated TH mice with high BG [High BG: 687 ± 106 mg/dL] exhibited lower body mass, decrements in cortical bone of the femur (decreased cross-sectional area and thickness; increased porosity) and in trabecular bone of the femur metaphysis and L6 vertebra (decreased bone volume fraction, thickness, and tissue mineral density), as well as decrements in cortical and vertebral bone strength (decreased yield force and ultimate force) when compared to untreated TH mice with low BG [Low BG: 290 ± 98 mg/dL; p < 0.0001]. CANA treatment was metabolically advantageous, normalizing body mass, BG and HbA1c to values comparable to the Low BG group. With drug-induced glycemic improvement, cortical area and thickness were significantly higher in the CANA than in the High BG group, but deficits in strength persisted with lower yield force and yield stress (partially independent of bone geometry) in the CANA group. Additionally, CANA only partially prevented the T2D-related loss in trabecular bone volume fraction. Taken together, these findings suggest that the ability of CANA to lower glucose and normalized glycemic control ameliorates diabetic bone disease but not fully.

Postnatal loss of the insulin receptor in osteoprogenitor cells does not impart a metabolic phenotype

The relationship between osteoblast-specific insulin signaling, osteocalcin activation and gluco-metabolic homeostasis has proven to be complex and potentially inconsistent across animal-model systems and in humans. Moreover, the impact of postnatally acquired, osteoblast-specific insulin deficiency on the pancreas-to-skeleton-to-pancreas circuit has not been studied. To explore this relationship, we created a model of postnatal elimination of insulin signaling in osteoprogenitors. Osteoprogenitor-selective ablation of the insulin receptor was induced after ~10 weeks of age in IR^l°^x/lox/Osx-Cre^+/- genotypic male and female mice (designated postnatal-OIRKO). At ~21 weeks of age, mice were then phenotypically and metabolically characterized. Postnatal-OIRKO mice demonstrated a significant reduction in circulating concentrations of undercarboxylated osteocalcin (ucOC), in both males and females compared with control littermates. However, no differences were observed between postnatal-OIRKO and control mice in: body composition (lean or fat mass); fasting serum insulin; HbA1c; glucose dynamics during glucose tolerance testing; or in pancreatic islet area or islet morphology, demonstrating that while ucOC is impacted by insulin signaling in osteoprogenitors, there appears to be little to no relationship between osteocalcin, or its derivative (ucOC), and glucose homeostasis in this model.

Genetic ablation of SGLT2 function in mice impairs tissue mineral density but does not affect fracture resistance of bone

Selective sodium-dependent glucose co-transporter 2 inhibitors (SGLT2Is) are oral hypoglycemic medications utilized increasingly in the medical management of hyperglycemia among persons with type 2 diabetes (T2D). Despite favorable effects on cardiovascular events, specific SGLT2Is have been associated with an increased risk for atypical fracture and amputation in subgroups of the T2D population, a population that already has a higher risk for typical fragility fractures than the general population. To better understand the effect of SGLT2 blockade on skeletal integrity, independent of diabetes and its co-morbidities, we utilized the "Jimbee" mouse model of slc5a2 gene mutation to investigate the impact of lifelong SGLT2 loss-of-function on metabolic and skeletal phenotype. Jimbee mice maintained normal glucose homeostasis, but exhibited chronic polyuria, glucosuria and hypercalciuria. The Jimbee mutation negatively impacted appendicular growth of the femur and resulted in lower tissue mineral density of both cortical and trabecular bone of the femur mid-shaft and distal femur metaphysis, respectively. Several components of the Jimbee phenotype were characteristic only of male mice compared with female mice, including reductions: in body weight; in cortical area of the mid-shaft; and in trabecular thickness within the metaphysis. Despite these decrements, the strength of femur diaphysis in bending (cortical bone), which increased with age, and the strength of L6 vertebra in compression (primarily trabecular bone), which decreased with age, were not affected by the mutation. Moreover, the age-related decline in bone toughness was less for Jimbee mice, compared with control mice, such that by 49-50 weeks of age, Jimbee mice had significantly tougher femurs in bending than C57BL/6J mice. These results suggest that chronic blockade of SGLT2 in this model reduces the mineralization of bone but does not reduce its fracture resistance.

Constitutive activation of MEK1 in osteoprogenitors increases strength of bone despite impairing mineralization

Recent clinical studies have revealed that a somatic mutation in MAP2K1, causing constitutive activation of MEK1 in osteogenic cells, occurs in melorheostotic bone disease in humans. We have generated a mouse model which expresses an activated form of MEK1 (MEK1DD) specifically in osteoprogenitors postnatally. The skeletal phenotype of these mice recapitulates many features of melorheostosis observed in humans, including extra-cortical bone formation, abundant osteoid formation, decreased mineral density, and increased porosity. Paradoxically, in both humans and mice, MEK1 activation in osteoprogenitors results in bone that is not structurally compromised, but is hardened and stronger, which would not be predicted based on tissue and matrix properties. Thus, a specific activating mutation in MEK1, expressed only by osteoprogenitors postnatally, can have a significant impact on bone strength through complex alterations in whole bone geometry, bone micro-structure, and bone matrix.

Diabetes pharmacotherapy and effects on the musculoskeletal system

Persons with type 1 or type 2 diabetes have a significantly higher fracture risk than age-matched persons without diabetes, attributed to disease-specific deficits in the microarchitecture and material properties of bone tissue. Therefore, independent effects of diabetes drugs on skeletal integrity are vitally important. Studies of incretin-based therapies have shown divergent effects of different agents on fracture risk, including detrimental, beneficial, and neutral effects. The sulfonylurea class of drugs, owing to its hypoglycemic potential, is thought to amplify the risk of fall-related fractures, particularly in the elderly. Other agents such as the biguanides may, in fact, be osteo-anabolic. In contrast, despite similarly expected anabolic properties of insulin, data suggests that insulin pharmacotherapy itself, particularly in type 2 diabetes, may be a risk factor for fracture, negatively associated with determinants of bone quality and bone strength. Finally, sodium-dependent glucose co-transporter 2 inhibitors have been associated with an increased risk of atypical fractures in select populations, and possibly with an increase in lower extremity amputation with specific SGLT2I drugs. The role of skeletal muscle, as a potential mediator and determinant of bone quality, is also a relevant area of exploration. Currently, data regarding the impact of glucose lowering medications on diabetes-related muscle atrophy is more limited, although preclinical studies suggest that various hypoglycemic agents may have either aggravating (sulfonylureas, glinides) or repairing (thiazolidinediones, biguanides, incretins) effects on skeletal muscle atrophy, thereby influencing bone quality. Hence, the therapeutic efficacy of each hypoglycemic agent must also be evaluated in light of its impact, alone or in combination, on musculoskeletal health, when determining an individualized treatment approach. Moreover, the effect of newer medications (potentially seeking expanded clinical indication into the pediatric age range) on the growing skeleton is largely unknown. Herein, we review the available literature regarding effects of diabetes pharmacotherapy, by drug class and/or by clinical indication, on the musculoskeletal health of persons with diabetes.

Advances in micro- and nanotechnologies for the GLP-1-based therapy and imaging of pancreatic beta-cells

Therapies to prevent diabetes in particular the progressive loss of β-cell mass and function and/or to improve the dysregulated metabolism associated with diabetes are highly sought. The incretin-based therapy comprising GLP-1R agonists and DPP-4 inhibitors have represented a major focus of pharmaceutical R&D over the last decade. The incretin hormone GLP-1 has powerful antihyperglycemic effect through direct stimulation of insulin biosynthesis and secretion within the β-cells; it normalizes β-cell sensitivity to glucose, has an antiapoptotic role, stimulates β-cell proliferation and differentiation, and inhibits glucagon secretion. However, native GLP-1 therapy is inappropriate due to the rapid post-secretory inactivation by DPP-4. Therefore, incretin mimetics developed on the backbone of the GLP-1 or exendin-4 molecule have been developed to behave as GLP-1R agonists but to display improved stability and clinical efficacy. New formulations of incretins and their analogs based on micro- and nanomaterials (i.e., PEG, PLGA, chitosan, liposomes and silica) and innovative encapsulation strategies have emerged to achieve a better stability of the incretin, to improve its pharmacokinetic profile, to lower the administration frequency or to allow another administration route and to display fewer adverse effects. An important advantage of these formulations is that they can also be used at the targeted non-invasive imaging of the beta-cell mass. This review therefore focuses on the current state of these efforts as the next step in the therapeutic evolution of this class of antidiabetic drugs.

Preserving and restoring bone with continuous insulin infusion therapy in a mouse model of type 1 diabetes

Those with type 1 diabetes (T1D) are more likely to suffer a fracture than age- and sex-matched individuals without diabetes, despite daily insulin therapy. In rodent studies examining the effect of bone- or glucose-targeting therapies on preventing the T1D-related decrease in bone strength, insulin co-therapy is often not included, despite the known importance of insulin signaling to bone mass accrual. Therefore, working toward a relevant pre-clinical model of diabetic bone disease, we assessed the effect of continuous subcutaneous insulin infusion (CSII) therapy at escalating doses on preserving bone and the effect of delayed CSII on rescuing the T1D-related bone deterioration in an established murine model of T1D. Osmotic minipumps were implanted in male DBA/2 J mice 2 weeks (prevention study) and 6 weeks (rescue study) after the first injection of streptozotocin (STZ) to deliver insulin at 0, 0.0625, 0.125, or 0.25 IU/day (prevention study; n = 4–5 per dose) and 0 or 0.25 IU/day (rescue study; n = 10 per group). CSII lasted 4 weeks in both studies, which also included age-matched, non-diabetic DBA/2 J mice (n = 8–12 per study). As the insulin dose increased, blood glucose decreased, body weight increased, a serum maker of bone resorption decreased, and a serum marker of bone formation increased such that each end-point characteristic was linearly correlated with dose. There were insulin dose-dependent relationships (femur diaphysis) with cross-sectional area of cortical bone and cortical thickness (micro-computed tomography) as well as structural strength (peak force endured by the mid-shaft during three-point bending). Likewise, trabecular bone volume fraction (BV/TV), thickness, and number (distal femur metaphysis) increased as the insulin dose increased. Delayed CSII improved glycated hemoglobin (HbA1c), but blood glucose levels remained relatively high (well above non-diabetic levels). Interestingly, it returned the resorption and formation markers to similar levels as those seen in non-T1D control mice. This apparent return after 4 weeks of CSII translated to a partial rescue of the structural strength of the femur mid-shaft. Delayed CSII also increased Tb.Th to levels seen in non-T1D controls but did not fully restore BV/TV. The use of exogenous insulin should be considered in pre-clinical studies investigating the effect of T1D on bone as insulin therapy maintains bone structure without necessarily lowering glucose below diabetic levels.

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Insulin infusion can preserve and partially restore skeletal integrity in type 1 diabetes.

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Skeletal improvements observed with continuous insulin infusion are dose-dependent.

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Markers of bone formation improve with continuous insulin therapy.

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Continuous insulin infusion decreases markers of bone resorption.

Mice with infectious colitis exhibit linear growth failure and subsequent catch-up growth related to systemic inflammation and IGF-1

In developing communities, intestinal infection is associated with poor weight gain and linear-growth failure. Prior translational animal models have focused on weight gain investigations into key contributors to linear growth failure have been lacking. We hypothesized that murine intestinal infection with Citrobacter rodentium would induce linear-growth failure associated with systemic inflammation and suppressed serum levels of insulin-like growth factor-1 (IGF-1). We evaluated 4 groups of mice infected or sham-infected on day-of-life 28: uninfected-controls, wild-type C rodentium-infected, partially-attenuated C rodentium-infected (with deletion of 3 serine protease genes involved in colonization), and pair-fed (given the amount of daily food consumed by the wild-type C rodentium group). Relative to the uninfected group, mice infected with wild-type C rodentium exhibited temporal associations of lower food intake, weight loss, linear-growth failure, higher IL-6 and TNF-α and lower IGF-1. However, relative to the pair-fed group, the C rodentium-infected group only differed significantly by linear growth and systemic inflammatory cytokines. Between post-infection days 15-20, the infected group exhibited resolution of systemic inflammation. Between days 16-20, both wild-type C rodentium and pair-fed groups exhibited rapid linear-growth velocities exceeding the uninfected and mutant C rodentium groups; during this time levels of IGF-1 increased to match the uninfected group. We submit this as a model providing important opportunities to study mechanisms of catch-up growth related to intestinal inflammation. We conclude that in addition to known effects of weight loss, infection with C rodentium induces linear-growth failure potentially related to systemic inflammation and low levels of IGF-1, with catch-up of linear growth following resolution of inflammation.

The impact of SGLT2 inhibitors, compared with insulin, on diabetic bone disease in a mouse model of type 1 diabetes

Skeletal co-morbidities in type 1 diabetes include an increased risk for fracture and delayed fracture healing, which are intertwined with disease duration and the presence of other diabetic complications. As such, chronic hyperglycemia is undoubtedly a major contributor to these outcomes, despite standard insulin-replacement therapy. Therefore, using the streptozotocin (STZ)-induced model of hypoinsulinemic hyperglycemia in DBA/2J male mice, we compared the effects of two glucose lowering therapies on the fracture resistance of bone and markers of bone turnover. Twelve week-old diabetic (DM) mice were treated for 9weeks with: 1) oral canagliflozin (CANA, dose range ~10-16mg/kg/day), an inhibitor of the renal sodium-dependent glucose co-transporter type 2 (SGLT2); 2) subcutaneous insulin, via minipump (INS, 0.125units/day); 3) co-therapy (CANA+INS); or 4) no treatment (STZ, without therapy). These groups were also compared to non-diabetic control groups. Untreated diabetic mice experienced increased bone resorption and significant deficits in cortical and trabecular bone that contributed to structural weakness of the femur mid-shaft and the lumbar vertebra, as determined by three-point bending and compression tests, respectively. Treatment with either canagliflozin or insulin alone only partially rectified hyperglycemia and the diabetic bone phenotype. However, when used in combination, normalization of glycemic control was achieved, and a prevention of the DM-related deterioration in bone microarchitecture and bone strength occurred, due to additive effects of canagliflozin and insulin. Nevertheless, CANA-treated mice, whether diabetic or non-diabetic, demonstrated an increase in urinary calcium loss; FGF23 was also increased in CANA-treated DM mice. These findings could herald ongoing bone mineral losses following CANA exposure, suggesting that certain CANA-induced skeletal consequences might detract from therapeutic improvements in glycemic control, as they relate to diabetic bone disease.

Effects of Type 1 Diabetes on Osteoblasts, Osteocytes, and Osteoclasts

PURPOSE OF REVIEW

To describe the effects of type 1 diabetes on bone cells.

RECENT FINDINGS

Type 1 diabetes (T1D) is associated with low bone mineral density, increased risk of fractures, and poor fracture healing. Its effects on the skeleton were primarily attributed to impaired bone formation, but recent data suggests that bone remodeling and resorption are also compromised. The hyperglycemic and inflammatory environment associated with T1D impacts osteoblasts, osteocytes, and osteoclasts. The mechanisms involved are complex; insulinopenia, pro-inflammatory cytokine production, and alterations in gene expression are a few of the contributing factors leading to poor osteoblast activity and survival and, therefore, poor bone formation. In addition, the observed sclerostin level increase accompanied by decreased osteocyte number and enhanced osteoclast activity in T1D results in uncoupling of bone remodeling. T1D negatively impacts osteoblasts and osteocytes, whereas its effects on osteoclasts are not well characterized, although the limited studies available indicate increased osteoclast activity, favoring bone resorption.

SGLT2 inhibitor therapy improves blood glucose but does not prevent diabetic bone disease in diabetic DBA/2J male mice

Persons with type 1 and type 2 diabetes have increased fracture risk, attributed to deficits in the microarchitecture and strength of diabetic bone, thought to be mediated, in part, by the consequences of chronic hyperglycemia. Therefore, to examine the effects of a glucose-lowering SGLT2 inhibitor on blood glucose (BG) and bone homeostasis in a model of diabetic bone disease, male DBA/2J mice with or without streptozotocin (STZ)-induced hyperglycemia were fed chow containing the SGLT2 inhibitor, canagliflozin (CANA), or chow without drug, for 10weeks of therapy. Thereafter, serum bone biomarkers were measured, fracture resistance of cortical bone was assessed by μCT analysis and a three-point bending test of the femur, and vertebral bone strength was determined by compression testing. In the femur metaphysis and L6 vertebra, long-term diabetes (DM) induced deficits in trabecular bone microarchitecture. In the femur diaphysis, a decrease in cortical bone area, cortical thickness and minimal moment of inertia occurred in DM (p<0.0001, for all) while cortical porosity was increased (p<0.0001). These DM changes were associated with reduced fracture resistance (decreased material strength and toughness; decreased structural strength and rigidity; p<0.001 for all). Significant increases in PTH (p<0.0001), RatLAPs (p=0.0002), and urine calcium concentration (p<0.0001) were also seen in DM. Canagliflozin treatment improved BG in DM mice by ~35%, but did not improve microarchitectural parameters. Instead, in canagliflozin-treated diabetic mice, a further increase in RatLAPs was evident, possibly suggesting a drug-related intensification of bone resorption. Additionally, detrimental metaphyseal changes were noted in canagliflozin-treated control mice. Hence, diabetic bone disease was not favorably affected by canagliflozin treatment, perhaps due to insufficient glycemic improvement. Instead, in control mice, long-term exposure to SGLT2 inhibition was associated with adverse effects on the trabecular compartment of bone.

Effects of long-term doxycycline on bone quality and strength in diabetic male DBA/2J mice

In type 1 diabetes, diabetic bone disease (DBD) is characterized by decreased bone mineral density, a state of low bone turnover and an increased risk of fracture. Animal models of DBD demonstrate that acquired alterations in trabecular and cortical bone microarchitecture contribute to decreased bone strength in diabetes. With anti-collagenolytic and anti-inflammatory properties, tetracycline derivatives may prevent diabetes-related decreases in bone strength. To determine if doxycycline, a tetracycline class antibiotic, can prevent the development of DBD in a model of long-term diabetes, male DBA/2J mice, with or without diabetes, were treated with doxycycline-containing chow for 10 weeks (dose range, 28–92 mg/kg/day). Long-term doxycycline exposure was not deleterious to the microarchitecture or biomechanical properties of healthy bones in male DBA/2J mice. Doxycycline treatment also did not prevent or alleviate the deleterious changes in trabecular microarchitecture, cortical structure, and biomechanical properties of bone induced by chronic diabetes.

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In DBA/2J mice, diabetic bones have more slender femurs, thinner cortices, decreased trabecular bone, and reduced strength to bending.

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The diabetic bone phenotype in this model was not improved by doxycycline treatment.

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Long-term doxycycline exposure was not deleterious to the microarchitecture or biomechanical properties of healthy bone in male DBA/2J mice.

Osteo-promoting effects of insulin-like growth factor I (IGF-I) in a mouse model of type 1 diabetes

OBJECTIVE

Using a streptozotocin (STZ)-induced mouse model of type 1 diabetes (T1D), we have previously demonstrated that long-term diabetes inhibits regenerative bone formation during tibial distraction osteogenesis (DO) and perturbs skeletal integrity by decreasing cortical thickness, bone mineral density and bone's resistance to fracture. Because long-standing T1D is also associated with a deficiency of insulin-like growth factor I (IGF-I), we examined the effects of systemic IGF-I treatment on skeletal microarchitecture and strength, as well as on bone formation in diabetic mice.

RESEARCH DESIGN AND METHODS

Streptozotocin-induced diabetic or control mice were treated with recombinant human IGF-I (rhIGF-I, 1.5mg/kg/day as subcutaneous infusion) or vehicle throughout a 14day DO procedure. Thereafter, trunk blood was assayed for glucose, insulin, rhIGF-I, mouse IGF-I and leptin. Bone formation in distracted tibiae was quantified. Effects on cortical bone strength and trabecular bone architecture were assessed by μCT analysis and three-point bend testing of contralateral femurs.

RESULTS

New bone formation during DO was reduced in diabetic mice but significantly improved with rhIGF-I treatment. The contralateral femurs of diabetic mice demonstrated significant reductions in trabecular thickness, yield strength and peak force of cortical bone, which were improved with rhIGF-I treatment. rhIGF-I also reduced intracortical porosity in control mice. However, treatment with rhIGF-I did not normalize serum glucose, or correct concurrent deficiencies of insulin or leptin seen in diabetes.

CONCLUSIONS

These findings demonstrate that despite persistent hyperglycemia, rhIGF-I promoted new bone formation and improved biomechanical properties of bone in a model of T1D, suggesting that it may be useful as a fracture preventative in this disease.

Determinants of undercarboxylated and carboxylated osteocalcin concentrations in type 1 diabetes

To determine whether undercarboxylated osteocalcin (UC-OC) or gamma-carboxyglutamic-carboxylated-type osteocalcin (GLA-OC) concentrations deviate from normal in type 1 diabetes (T1D), serum levels were compared between 115 subjects with T1D and 55 age-matched healthy controls. UC-OC and GLA-OC concentrations were similar between groups; however, in T1D, UC-OC correlated positively with markers of insulin exposure, either endogenously produced or exogenously administered.

INTRODUCTION

A study was conducted to determine whether dysregulation of circulating concentrations of UC-OC or GLA-OC occurs in patients with type 1 diabetes, a condition of insulin deficiency without insulin resistance.

METHODS

We measured serum concentrations of UC-OC and GLA-OC in 115 subjects with T1D, ages 14-40 years, and in 55 age-matched healthy control subjects. Relationships between UC-OC and GLA-OC concentrations and patient characteristics (gender and age), indices of glycemic control (hemoglobin A1c (HbA1c), fasting plasma glucose, C-peptide concentration, 3-day average glucose measured by a continuous glucose sensor, total daily insulin dose) and circulating indices of skeletal homeostasis (total calcium, 25-OH vitamin D, parathyroid hormone, insulin-like growth factor 1 (IGF-1), type 1 collagen degradation fragments (CTX), adiponectin, leptin) were examined. Between group differences in the concentrations of UC-OC and GLA-OC were the main outcome measures.

RESULTS

Although adiponectin levels were higher in the T1D group, between-group comparisons did not reveal statistically significant differences in concentration of UC-OC, GLA-OC, CTX or leptin between the T1D and control populations. Instead, by multivariate regression modeling, UC-OC was correlated with younger age (p < 0.001), higher CTX (p < 0.001), lower HbA1c (p = 0.013), and higher IGF-1 (p = 0.086). Moreover, within the T1D subgroup, UC-OC was positively correlated with C-peptide/glucose ratio (reflecting endogenous insulin secretion), with IGF-1 (reflecting intra-portal insulin sufficiency), and with total daily insulin dose.

CONCLUSIONS

In T1D, UC-OC appears to correlate positively with markers of insulin exposure, either endogenously produced or exogenously administered.

Contributions of the Insulin/Insulin-Like Growth Factor-1 Axis to Diabetic Osteopathy

Recent studies in diabetic humans and rodent models of diabetes have identified osteopathy as a serious complication of type 1 (T1D) and type 2 (T2D) diabetes. Accumulating evidence suggests that disruption of insulin and insulin-like growth factor 1 (IGF-1) homeostasis in the diabetic condition may be responsible for the observed skeletal deficits. Indeed, replacement of insulin or IGF-1 in rodent models of T1D results in significant improvement in bone healing despite ongoing moderate to severe hyperglycemia. Insulin and IGF-1 act through distinct receptors. Mice in which the receptor for insulin or IGF-1 is selectively deleted from osteoblast lineages show skeletal deficits. Despite acting through distinct receptors, insulin and IGF-1 exert their cellular activities via conserved intracellular signaling proteins. Genetic manipulation of these signaling proteins, such as insulin receptor substrate (IRS)-1 and -2, Protein Kinase B (Akt), and MAPK/ERK kinase (MEK), has uncovered a significant role for these signal transduction pathways in skeletal homeostasis. In addition to effects on skeletal physiology via canonical signaling pathways, insulin and IGF-1 may crosstalk with wingless-int. (Wnt) and bone morphogenic protein 2 (BMP-2) signaling pathways in cells of the osteoblast lineage and thereby promote skeletal development. In this review, a discussion is presented regarding the role of insulin and IGF-1 in skeletal physiology and disruptions of this axis that occur in the diabetic condition which could underlie many of the skeletal pathologies observed.

Insulin pump therapy started at the time of diagnosis: effects on glycemic control and pancreatic β-cell function in type 1 diabetes

BACKGROUND

In the interest of preserving residual insulin secretory capacity present at the time of diagnosis with type 1 diabetes (T1D), we compared the efficacy of starting insulin pump therapy at diagnosis with standard multiple daily insulin injections (MDIs).

METHODS

We conducted a prospective, randomized, pilot trial comparing MDI therapy with continuous subcutaneous insulin therapy (pump therapy) in 24 patients, 8-18 years old, with newly diagnosed T1D. Subjects were evaluated at enrollment and 1, 3, 6, 9, and 12 months after initial diagnosis of T1D. Preservation of insulin secretion, measured by mixed-meal-stimulated C-peptide secretion, was compared after 6 and 12 months of treatment. Between-group differences in glycosylated hemoglobin (HbA1c), continuous glucose sensor data, insulin utilization, anthropometric measures, and patient satisfaction with therapy were also compared at multiple time points.

RESULTS

Initiation of pump therapy within 1 month of diagnosis resulted in consistently higher mixed-meal tolerance test-stimulated C-peptide values at all time points, although these differences were not statistically significant. Nonetheless, improved glycemic control was observed in insulin pump-treated subjects (more time spent with normoglycemia, better mean HbA1c), and pump-treated subjects reported comparatively greater satisfaction with route of treatment administration.

CONCLUSIONS

Initiation of insulin pump therapy at diagnosis improved glycemic control, was well tolerated, and contributed to improved patient satisfaction with treatment. This study also suggests that earlier use of pump therapy might help to preserve residual β-cell function, although a larger clinical trial would be required to confirm this.

Increasing duration of type 1 diabetes perturbs the strength-structure relationship and increases brittleness of bone

Type 1 diabetes (T1DM) increases the likelihood of a fracture. Despite serious complications in the healing of fractures among those with diabetes, the underlying causes are not delineated for the effect of diabetes on the fracture resistance of bone. Therefore, in a mouse model of T1DM, we have investigated the possibility that a prolonged state of diabetes perturbs the relationship between bone strength and structure (i.e., affects tissue properties). At 10, 15, and 18 weeks following injection of streptozotocin to induce diabetes, diabetic male mice and age-matched controls were examined for measures of skeletal integrity. We assessed 1) the moment of inertia (I(MIN)) of the cortical bone within diaphysis, trabecular bone architecture of the metaphysis, and mineralization density of the tissue (TMD) for each compartment of the femur by micro-computed tomography and 2) biomechanical properties by three-point bending test (femur) and nanoindentation (tibia). In the metaphysis, a significant decrease in trabecular bone volume fraction and trabecular TMD was apparent after 10 weeks of diabetes. For cortical bone, type 1 diabetes was associated with decreased cortical TMD, I(MIN), rigidity, and peak moment as well as a lack of normal age-related increases in the biomechanical properties. However, there were only modest differences in material properties between diabetic and normal mice at both whole bone and tissue-levels. As the duration of diabetes increased, bone toughness decreased relative to control. If the sole effect of diabetes on bone strength was due to a reduction in bone size, then I(MIN) would be the only significant variable explaining the variance in the maximum moment. However, general linear modeling found that the relationship between peak moment and I(MIN) depended on whether the bone was from a diabetic mouse and the duration of diabetes. Thus, these findings suggest that the elevated fracture risk among diabetics is impacted by complex changes in tissue properties that ultimately reduce the fracture resistance of bone.

Enhanced excretion of vitamin D binding protein in type 1 diabetes: a role in vitamin D deficiency?

CONTEXT

Vitamin D deficiency is an increasingly recognized comorbidity in patients with both type 1 (T1D) and type 2 diabetes, particularly associated with the presence of diabetic nephropathy.

OBJECTIVE

Because we have previously reported enhanced excretion of megalin in the urine of T1D patients with microalbuminuria, we hypothesized that concurrent urinary loss of the megalin ligand, vitamin D binding protein, might contribute mechanistically to vitamin D deficiency.

DESIGN AND PARTICIPANTS

Examining a study cohort of 115 subjects with T1D, aged 14-40 yr, along with 55 age-matched healthy control subjects, we measured plasma and urine concentrations of vitamin D binding protein (VDBP) along with serum concentrations of total calcium, parathyroid hormone, 25-hydroxyvitamin D, and 1, 25-dihydroxyvitamin D; these results were compared between groups and investigated for relationships with metabolic control status or with albuminuria.

MAIN OUTCOME MEASURE

Between-group differences in urinary VDBP concentration were the main outcome measures.

RESULTS

A marked increase in the urinary excretion of VDBP was apparent in subjects with T1D, compared with control subjects. Using multivariate regression modeling, significant correlates of urinary VDBP excretion included microalbuminuria (P = 0.004), glycosylated hemoglobin (P = 0.010), continuous glucose monitoring system average capillary glucose (P = 0.047), and serum 1,25(OH)(2)D concentrations (P = 0.037). Vitamin D deficiency or insufficiency was slightly more prevalent in diabetic subjects with albuminuria, coincident with the increase in urine VDBP excretion.

CONCLUSIONS

These findings suggest that, theoretically, exaggerated urinary loss of VDBP in T1D, particularly in persons with albuminuria, could contribute mechanistically to vitamin D deficiency in this disease.

Palmitate and insulin synergistically induce IL-6 expression in human monocytes

BACKGROUND

Insulin resistance is associated with a proinflammatory state that promotes the development of complications such as type 2 diabetes mellitus (T2DM) and atherosclerosis. The metabolic stimuli that initiate and propagate proinflammatory cytokine production and the cellular origin of proinflammatory cytokines in insulin resistance have not been fully elucidated. Circulating proinflammatory monocytes show signs of enhanced inflammation in obese, insulin resistant subjects and are thus a potential source of proinflammatory cytokine production. The specific, circulating metabolic factors that might stimulate monocyte inflammation in insulin resistant subjects are poorly characterized. We have examined whether saturated nonesterified fatty acids (NEFA) and insulin, which increase in concentration with developing insulin resistance, can trigger the production of interleukin (IL)-6 and tumor necrosis factor (TNF)-α in human monocytes.

METHODS

Messenger RNA and protein levels of the proinflammatory cytokines IL-6 and TNF-α were measured by quantitative real-time PCR (qRT-PCR) and Luminex bioassays. Student's t-test was used with a significance level of p < 0.05 to determine significance between treatment groups.

RESULTS

Esterification of palmitate with coenzyme A (CoA) was necessary, while β-oxidation and ceramide biosynthesis were not required, for the induction of IL-6 and TNF-α in THP-1 monocytes. Monocytes incubated with insulin and palmitate together produced more IL-6 mRNA and protein, and more TNF-α protein, compared to monocytes incubated with palmitate alone. Incubation of monocytes with insulin alone did not affect the production of IL-6 or TNF-α. Both PI3K-Akt and MEK/ERK signalling pathways are important for cytokine induction by palmitate. MEK/ERK signalling is necessary for synergistic induction of IL-6 by palmitate and insulin.

CONCLUSIONS

High levels of saturated NEFA, such as palmitate, when combined with hyperinsulinemia, may activate human monocytes to produce proinflammatory cytokines and support the development and propagation of the subacute, chronic inflammatory state that is characteristic of insulin resistance. Results with inhibitors of β-oxidation and ceramide biosynthesis pathways suggest that increased fatty acid flux through the glycerolipid biosynthesis pathway may be involved in promoting proinflammatory cytokine production in monocytes.

Disease and gender-specific dysregulation of NGAL and MMP-9 in type 1 diabetes mellitus

Neutrophil gelatinase-associated lipocalin (NGAL), a biomarker of renal injury, can bind matrix metalloproteinase-9 (MMP-9) and inhibit its degradation, thereby sustaining MMP-9 proteolytic activity. MMP-9 is produced by renal podocytes, and podocyte MMP production can be modified by high ambient glucose levels. Moreover, dysregulation of MMP-9 activity, gene expression, or urine concentrations has been demonstrated in T2DM-associated nephropathy and in non-diabetic proteinuric renal diseases. Our objective was to determine whether NGAL/MMP-9 dysregulation might contribute to or serve as a biomarker of diabetic nephropathy in type 1 DM (T1DM). Plasma MMP-9, and urine NGAL and MMP-9 concentrations were measured in 121 T1DM and 55 control subjects and examined relative to indicators of glycemia, renal function, and degree of albuminuria. T1DM was associated with a significant increase in urinary excretion of both NGAL and MMP-9, and urine NGAL:Cr (NGAL corrected to urine creatinine) and urine MMP-9:Cr concentrations were highly correlated with each other. Both were also positively correlated with measurements of glycemic control and with albuminuria. Plasma MMP-9, urine MMP-9, and urine NGAL concentrations were significantly higher in females compared to males, and urine MMP-9:Cr concentrations displayed a menstrual cycle specific pattern. Increased urinary excretion of NGAL and MMP-9 supports a role for NGAL/MMP-9 dysregulation in renal dysfunction; moreover, gender-specific differences could support a gender contribution to pathological mechanisms or susceptibility for the development of renal complications in diabetes mellitus.

Direct bone formation during distraction osteogenesis does not require TNFalpha receptors and elevated serum TNFalpha fails to inhibit bone formation in TNFR1 deficient mice

Distraction osteogenesis (DO) is a process which induces direct new bone formation as a result of mechanical distraction. Tumor necrosis factor-alpha (TNF) is a cytokine that can modulate osteoblastogenesis. The direct effects of TNF on direct bone formation in rodents are hypothetically mediated through TNF receptor 1 and/or 2 (TNFR1/2) signaling. We utilized a unique model of mouse DO to assess the effects of 1) TNFR homozygous null gene alterations on direct bone formation and 2) rmTNF on wild type (WT), TNFR1(-/-) (R1KO), and TNR2(-/-) (R2KO) mice. Radiological and histological analyses of direct bone formation in the distraction gaps demonstrated no significant differences between the WT, R1KO, R2KO, or TNFR1(-/-) and R2(-/-) (R1 and 2KO) mice. R1 and 2KO mice had elevated levels of serum TNF but demonstrated no inhibition of new bone formation. Systemic administration by osmotic pump of rmTNF during DO (10 microg/kg/day) resulted in significant inhibition of gap bone formation measures in WT and R2KO mice, but not in R1KO mice. We conclude that exogenous rmTNF and/or endogenous TNF act to inhibit new bone formation during DO by signaling primarily through TNFR1.

Microalbuminuria in type 1 diabetes is associated with enhanced excretion of the endocytic multiligand receptors megalin and cubilin

OBJECTIVE

Proteinuria is the hallmark of diabetic nephropathy; yet, glomerular histology does not fully explain mechanisms contributing to proteinuria. Our objective was to identify proteins in the urine of individuals with type 1 diabetes and microalbuminuria that might implicate a mechanistic pathway operative in proteinuria.

RESEARCH DESIGN AND METHODS

Using a GeLC/MS platform proteomics approach, we compared the urine proteome from 12 healthy nondiabetic individuals, 12 subjects with type 1 diabetes yet normal urinary albumin excretion rates, and 12 subjects with type 1 diabetes and microalbuminuria (type 1 diabetes + microalbuminuria).

RESULTS

The abundance of megalin and cubilin, two multiligand receptors expressed in kidney proximal tubule cells and involved with the reuptake of filtered albumin and megalin/cubilin ligands, was significantly increased in type 1 diabetes + microalbuminuria urine, compared with both nonalbuminuric groups.

CONCLUSIONS

Aberrant shedding of megalin and cubilin could contribute to albuminuria in diabetes and to deficiency states of important vitamins and hormones.

Matrix metalloproteinases: their potential role in the pathogenesis of diabetic nephropathy

Matrix metalloproteinases (MMPs), a family of proteinases including collagenases, gelatinases, stromelysins, matrilysins, and membrane-type MMPs, affect the breakdown and turnover of extracellular matrix (ECM). Moreover, they are major physiologic determinants of ECM degradation and turnover in the glomerulus. Renal hypertrophy and abnormal ECM deposition are hallmarks of diabetic nephropathy (DN), suggesting that altered MMP expression or activation contributes to renal injury in DN. Herein, we review and summarize recent information supporting a role for MMPs in the pathogenesis of DN. Specifically, studies describing dysregulated activity of MMPs and/or their tissue inhibitors in various experimental models of diabetes, including animal models of type 1 or type 2 diabetes, clinical investigations of human type 1 or type 2 diabetes, and kidney cell culture studies are reviewed.

Runt-related transcription factor 2 (RUNX2) and RUNX2-related osteogenic genes are down-regulated throughout osteogenesis in type 1 diabetes mellitus

Type 1 diabetes mellitus is associated with a number of disorders of skeletal health, conditions that rely, in part, on dynamic bone formation. A mouse model of distraction osteogenesis was used to study the consequences of streptozotocin-induced diabetes and insulin treatment on bone formation and osteoblastogenesis. In diabetic mice compared with control mice, new bone formation was decreased, and adipogenesis was increased in and around, respectively, the distraction gaps. Although insulin treatment restored bone formation to levels observed in nondiabetic control mice, it failed to significantly decrease adipogenesis. Molecular events altered during de novo bone formation in untreated type 1 diabetes mellitus, yet restored with insulin treatment were examined so as to clarify specific osteogenic genes that may contribute to diabetic bone disease. RNA from distraction gaps was analyzed by gene microarray and quantitative RT-PCR for osteogenic genes of interest. Runt-related transcription factor 2 (RUNX2), and several RUNX2 target genes, including matrix metalloproteinase-9, Akp2, integrin binding sialoprotein, Dmp1, Col1a2, Phex, Vdr, osteocalcin, and osterix, were all significantly down-regulated in the insulin-deficient, hyperglycemic diabetic animals; however, insulin treatment of diabetic animals significantly restored their expression. Expression of bone morphogenic protein-2, transcriptional coactivator with PDZ-binding motif, and TWIST2, all important regulators of RUNX2, were not impacted by the diabetic condition, suggesting that the defect in osteogenesis resides at the level of RUNX2 expression and its activity. Together, these data demonstrate that insulin and/or glycemic status can regulate osteogenesis in vivo, and systemic insulin therapy can, in large part, rescue the diabetic bone phenotype at the tissue and molecular level.

Rabson-Mendenhall syndrome: medullary sponge kidney, a new component

Rabson-Mendenhall syndrome is a rare genetic disorder characterized by severe insulin resistance, extreme hyperinsulinemia, postprandial hyperglycemia, growth retardation, and dysmorphisms. Enlargement of the kidneys and nephrocalcinosis have been described previously. We report a 10-year-old boy who presented with gross hematuria, unilateral hydronephrosis, and the initial diagnosis of bilateral extensive medullary nephrocalcinosis. Medullary sponge kidney (MSK) was included in the differential diagnosis given the ultrasound findings. Further evaluation by intravenous pyelogram confirmed the suspected bilateral MSK. Given the patient's history of hydronephrosis due to an obstructing renal stone and MSK, urine calcium excretion was assessed and found to be markedly increased at 9.5 mg/kg per day. To our knowledge, this is the first report of Rabson-Mendenhall syndrome and an association with MSK. We recommend evaluation for nephrocalcinosis, MSK, and hypercalciuria in all children diagnosed with Rabson-Mendenhall syndrome.

Efficacy and safety of mecasermin rinfabate

There has been interest in using recombinant human (rh) insulin-like growth factor (IGF)-I (rhIGF-I) to treat short stature, either alone or in combination with its binding protein (insulin-like growth factor binding protein [IGFBP]-3). IGF-I has been shown to increase growth velocity in children with IGF deficiency, either as a result of growth hormone insensitivity syndrome (GHIS) or IGF gene deletion. However, there have been adverse events, particularly hypoglycaemia, reported with administration of unbound rhIGF-I. In addition, the serum half-life of unbound rhIGF-I is shorter when administered to patients with GHIS, who have low serum concentrations of its binding proteins IGFBP-3 and acid-labile subunit (ALS), than when administered to normal volunteers or to the patient with an IGF-I gene deletion (who had normal levels of IGFBP-3). iPlex (mecasermin rinfabate), an equimolar mixture of IGF-I and its binding protein IGFBP-3, was developed to prolong the half-life and to counteract acute adverse events (particularly hypoglycaemia) associated with administration of IGF-I. Although there are no published data on the efficacy of mecasermin rinfabate in treating growth disorders, it does appear that mecasermin rinfabate has a longer half-life in patients with GHIS than unbound IGF-I, and fewer reports of adverse events (including hypoglycaemia) when administered to patients with diabetes.

Severe deficiencies of IGF-I, IGF-II, IGFBP-3, ALS and paradoxically high-normal bone mass in a child with insulin-resistance syndrome (Rabson-Mendenhall type)

Rabson-Mendenhall syndrome is a rare genetic disorder characterized by severe insulin resistance and hyperinsulinemia due to defects in signaling through the insulin receptor. Herein, we describe a new case of Rabson-Mendenhall syndrome in which investigations of the growth hormone (GH) - insulin-like growth factor (IGF) axis - reveal severe deficiencies in total and free insulin-like growth factor-I (IGF-I), IGF-II, IGF-binding protein-3 (IGFBP-3), and the acid labile subunit (ALS). Based on these findings, we anticipated significant bone deficits, as have been described in other clinical scenarios in which the IGF axis is significantly perturbed. Long-bone studies revealed no gross malformations. Paradoxically, DXA scanning revealed a total body bone density Z-score of +2.0 (0.8 gm/cm(2)), suggesting an overall high-normal BMD for age and a high BMD corrected for bone or height age. The mechanisms by which BMD is protected from severe deficiencies in the IGF-axis are unknown, yet may involve enhanced IGF sensitivity, increased local production of IGFs, and/or supra-physiological concentrations of insulin substituting for the actions of IGFs in bone.

Matrix metalloproteinase-2 dysregulation in type 1 diabetes

OBJECTIVE

Dysregulation of matrix metalloproteinase (MMP)-2 may contribute pathologically to the development of diabetes complications, including diabetic retinopathy and coronary and peripheral arterial disease. Our objective was to explore whether systemic MMP-2 dysregulation could be demonstrated in type 1 diabetes and to determine how MMP-2 concentration relates to disease status.

RESEARCH DESIGN AND METHODS

In this cross-sectional study, MMP-2 concentrations and MMP-2 activity were measured in plasma and timed urine samples from 93 type 1 diabetic and 50 healthy control subjects, aged 14-40 years. Relationships between MMP-2 concentrations in these biological fluids and subject characteristics (sex, age, and duration of type 1 diabetes), indexes of glycemic control (A1C, fasting plasma glucose, and continuous glucose monitoring system average daily glucose), and measurements of renal function (urinary albumin excretion and glomerular filtration rate) were examined.

RESULTS

Urine and plasma MMP-2 concentrations and plasma MMP-2 activity were all significantly elevated in type 1 diabetic subjects compared with those in control subjects. Urine MMP-2 concentrations, in particular, were correlated with several clinical parameters that infer increased risk for diabetic comorbidity and specifically for diabetic nephropathy, including higher A1C, longer duration of disease, evidence of renal hyperfiltration, and the presence of microalbuminuria.

CONCLUSIONS

Urine and plasma MMP-2 concentrations are dysregulated in type 1 diabetes; urinary excretion of MMP-2, in particular, might provide a unique biomarker of diabetes-induced intrarenal pathologic processes.

A novel rat model for the study of deficits in bone formation in type-2 diabetes

BACKGROUND

There is evidence to suggest that impairment in bone formation and/or turnover is associated with the metabolic abnormalities characteristic of type-2 diabetes mellitus. However, bone regeneration/repair in type-2 diabetes has not been modeled. Using Zucker Diabetic Fatty (ZDF) rats (a model of type-2 diabetes) for tibial distraction osteogenesis (DO), we hypothesized that bone formation within the distraction gap would be impaired.

ANIMALS AND METHODS

Rats were examined for body weight, glycosuria, and glycosemia to confirm the diabetic condition during the study. The rats received placement of the external fixators and osteotomies on the left tibia. Distraction was initiated the following day at 0.2 mm twice a day and continued for 14 days. The lengthened tibiae were harvested and distraction gaps were examined radiographically and histologically.

RESULTS

We found significant reduction in new bone formation in the distraction gaps of the ZDF rats, both radiographically and histologically, compared to lean rats. We found a decrease in a marker of cellular proliferation in the distraction gaps and increased adipose volume in adjacent bone marrow of the ZDF rats.

INTERPRETATION

Our findings suggest that this model might be used to study the contributions of leptin resistance, insulin resistance and/or hyperglycemia to impaired osteoblastogenesis in vivo.

Effects of systemic and local administration of recombinant human IGF-I (rhIGF-I) on de novo bone formation in an aged mouse model

DO was used in an aged mouse model to determine if systemically and/or locally administered rhIGF-I improved osteoblastogenesis and new bone formation. Local and systemic rhIGF-I treatment increased new bone formation. However, only systemic delivery produced measurable concentrations of rhIGF-I in the circulation.

INTRODUCTION

Human and rodent research supports a primary role for IGF-I in bone formation. Significant roles for both endocrine and paracrine/autocrine IGF-I have been suggested for normal osteoblastogenesis and bone formation. We have assessed, using a mouse model of distraction osteogenesis (DO), the impact of continuous administration of recombinant human (rh)IGF-I, delivered either locally to the distraction site or absorbed systemically, on bone formation in an aged mouse model.

MATERIALS AND METHODS

DO was performed in aged mice (18-month-old C57BL/6 male mice), which were distracted at 0.15 mm daily. At the time of osteotomy, miniosmotic pumps were inserted subcutaneously to (1) deliver vehicle or rhIGF-I subcutaneously for systemic delivery or (2) deliver vehicle or rhIGF-I directly to the newly forming bone through infusion tubing routed subcutaneously from the pump to the distraction site. Serum concentrations of mouse IGF-I, human IGF-I, and osteocalcin were determined at the end of the study.

RESULTS

New bone formation observed in DO gaps showed a significant increase in new bone formation in rhIGF-I-treated mice, irrespective of delivery route. However, detectable levels of human IGF-I were found only in the serum of animals receiving rhIGF-I systemically. Osteocalcin levels did not differ between controls and rhIGF-I-treated groups.

CONCLUSIONS

Locally and systemically delivered rhIGF-I both produce significant increases in new bone formed in an aged mouse model in which new bone formation is normally markedly impaired, suggesting that rhIGF-I may improve senile osteoporosis. Because systemic administration of IGF-I can result in untoward side effects, including an increased risk for cancer, the findings that locally delivered IGF-I improves bone regeneration without increasing circulating IGF-I levels suggests that this delivery route may be preferable in an at-risk, aged population.

Physiological matrix metalloproteinase concentrations in serum during childhood and adolescence, using Luminex Multiplex technology

Matrix metalloproteinases are a family of zinc-dependent proteinases which are involved in the breakdown and remodeling of extracellular matrix. As children grow and adolescents reach pubescence, their bodies undergo changes that require age-related morphogenesis of the extracellular matrix, possibly requiring unique patterns of matrix metalloproteinase (MMP) expression during periods of rapid tissue growth (i.e., childhood) or accelerated tissue remodeling and expansion (i.e., adolescence). Therefore, we have characterized age-specific and gender-specific differences in circulating concentrations of MMPs (specifically MMP-1, -2, -3, -8 and -9) in 189 serum samples obtained from healthy subjects, aged 2-18 years. MMP concentrations were measured using Fluorokine MultiAnalyte Profiling kits and a Luminex Bioanalyzer, as well as by commercial ELISA. Serum levels of MMP-1, -2, -3, -8, and -9 in healthy pediatric subjects represent log-normal distributions. MMP-2 was significantly negatively correlated with age (r=-0.29; p<0.001), while MMP-3 was significantly positively correlated with age (r=0.38; p<0.001). Although plasma, not serum, is considered the appropriate blood sample for measurement of MMP-8 and -9, serum levels of MMP-8 and -9 were also found to be highly positively correlated with each other (r=0.76; p<0.01). MMP results obtained by Fluorokin MultiAnalyte Profiling methods correlated well with conventional ELISA methods and use of this technology provided several advantages over ELISA.

Is insulin an anabolic agent in bone? Dissecting the diabetic bone for clues

Diabetic osteoporosis is increasingly recognized as a significant comorbidity of type 1 diabetes mellitus. In contrast, type 2 diabetes mellitus is more commonly associated with modest increases in bone mineral density for age. Despite this dichotomy, clinical, in vivo, and in vitro data uniformly support the concept that new bone formation as well as bone microarchitectural integrity are altered in the diabetic state, leading to an increased risk for fragility fracture and inadequate bone regeneration following injury. In this review, we examine the contribution that insulin, as a potential anabolic agent in bone, may make to the pathophysiology of diabetic bone disease. Specifically, we have assimilated human and animal data examining the effects of endogenous insulin production, exogenous insulin administration, insulin sensitivity, and insulin signaling on bone. In so doing, we present evidence that insulin, acting as an anabolic agent in bone, can preserve and increase bone density and bone strength, presumably through direct and/or indirect effects on bone formation.

Bone formation is impaired in a model of type 1 diabetes

The effects of type 1 diabetes on de novo bone formation during tibial distraction osteogenesis (DO) and on intact trabecular and cortical bone were studied using nonobese diabetic (NOD) mice and comparably aged nondiabetic NOD mice. Diabetic mice received treatment with insulin, vehicle, or no treatment during a 14-day DO procedure. Distracted tibiae were analyzed radiographically, histologically, and by microcomputed tomography (microCT). Contralateral tibiae were analyzed using microCT. Serum levels of insulin, osteocalcin, and cross-linked C-telopeptide of type I collagen were measured. Total new bone in the DO gap was reduced histologically (P < or = 0.001) and radiographically (P < or = 0.05) in diabetic mice compared with nondiabetic mice but preserved by insulin treatment. Serum osteocalcin concentrations were also reduced in diabetic mice (P < or = 0.001) and normalized with insulin treatment. Evaluation of the contralateral tibiae by microCT and mechanical testing demonstrated reductions in trabecular bone volume and thickness, cortical thickness, cortical strength, and an increase in endosteal perimeter in diabetic animals, which were prevented by insulin treatment. These studies demonstrate that bone formation during DO is impaired in a model of type 1 diabetes and preserved by systemic insulin administration.

Early developmental changes in IGF-I, IGF-II, IGF binding protein-1, and IGF binding protein-3 concentration in the cerebrospinal fluid of children

IGF-I and IGF-II are ubiquitously expressed growth factors that have profound effects on the growth and differentiation of many cell types and tissues, including cells of the CNS. In biologic fluids, most IGFs are bound to one of six IGF binding proteins (IGFBPs 1-6). Increasing evidence strongly supports a role for IGF-I in CNS development, as it promotes neuronal proliferation and survival. However, little is known about IGF-I and its homolog IGF-II and their carrier proteins, IGFBPs, during the neonatal period in which brain size increases dramatically, myelination takes place, and neurons show limited capacity to proliferate. Herein, we have determined the concentrations of IGF-I, IGF-II, IGFBP-1, and IGFBP-3 in cerebral spinal fluid (CSF) samples that were collected from children who were 1 wk to 18 y of age. The concentrations of IGF-I, IGFBP-1, and IGFBP-3 in CSF from children <6 mo of age were significantly higher than in older children, whereas IGF-II was higher in the older group. This is in contrast to what is observed in the peripheral circulation, where IGF-I and IGFBP-3 are low at birth and rise rapidly during the first year, reaching peak levels during puberty. Higher concentrations of IGF-I, IGFBP-1, and IGFBP-3 in the CSF of very young children suggest that these proteins might participate in the active processes of myelination and synapse formation in the developing nervous system. We propose that IGF-I and certain IGFBPs are likely necessary for normal CNS development during critical stages of neonatal brain growth and development.

IGFBP-4 degradation by pregnancy-associated plasma protein-A in MC3T3 osteoblasts

Insulin-like growth factor (IGF) signaling is critical for osteoblast development and IGF binding protein (IGFBP)-4 is one of the principle IGFBPs expressed by osteoblasts. Release of bound IGF via proteolytic degradation of IGFBP-4 is likely to be critical for osteoblast development. We have investigated whether IGF-sensitive, IGFBP-4 degradation in mouse MC3T3-E1 osteoblasts is due to the metzincin pregnancy-associated plasma protein (PAPP)-A. Degradation of IGFBP-4 by PAPP-A or MC3T3-E1 conditioned medium was enhanced by IGF-II but inhibited by mutation of basic residues at or near the PAPP-A cleavage site in IGFBP-4. Furthermore, immunodepletion of PAPP-A from MC3T3-E1 conditioned medium abolished IGFBP-4 degradation. We also found that PAPP-A messenger RNA was expressed throughout differentiation of MC3T3-E1 cells. These results demonstrate for the first time that PAPP-A is the IGFBP-4 protease in MC3T3-E1 cells, a widely used model for osteoblast development, and that PAPP-A may regulate IGF release throughout osteoblast differentiation.

Regulation of insulin-like growth factor (IGF)-I action by matrix metalloproteinase-3 involves selective disruption of IGF-I/IGF-binding protein-3 complexes

IGF-I and IGF-II play important roles in growth and development via interactions with cell-surface receptors; however, in nature, IGFs are sequestered by at least six soluble, high-affinity IGF-binding proteins (IGFBPs), namely IGFBPs 1-6. Herein, we demonstrate that the stromal cell-derived extracellular matrix-degrading metalloproteinase stromelysin 1 (matrix metalloproteinase 3) disrupts IGF/IGFBP-3 complexes and liberates free, intact IGFs, leading to phosphorylation of cell surface type 1 IGF receptors and cellular proliferation. Tissue inhibitor of metalloproteinases (TIMP-1) or an antibody to the type 1 IGF receptor mitigates IGF-mediated cellular proliferation. Thus, these studies suggest that matrix metalloproteinases, beyond their effects on extracellular matrix turnover, regulate cellular proliferation by modulating the bioavailability of IGFs, an event critical for such diverse phenomena as embryo development, morphogenesis, angiogenesis, and tumorigenesis.

Insulin-like growth factor binding protein proteolysis

High-affinity interactions between insulin-like growth factors (IGF-I and IGF-II) and insulin-like growth factor-binding proteins (IGFBP-1, -2, -3, -4, -5 and -6) antagonize the binding of IGF to the type 1 IGF receptor. Proteases found in a variety of biological fluids can degrade IGFBP 1-6 into fragments that have a greatly reduced affinity for IGF-I and IGF-II, increasing the concentration of free IGFs at the cell surface and allowing IGFs to bind to and activate the IGF receptor. Therefore, IGFBP proteolysis directly modulates the first step in IGF receptor signaling and thereby indirectly modulates cell survival, mitogenesis and differentiation. Our understanding of IGFBP proteolysis has grown exponentially over the past five years, with the identification of several new IGFBP proteases, a growing appreciation of the potential for IGF-independent actions of IGFBP fragments and the realization that perturbations of IGFBP proteolysis are seen in, and might contribute to, several pathological conditions.

Implications for matrix metalloproteinases as modulators of pediatric lung disease

Matrix metalloproteinases (MMPs) are a large family (>20) of cation-dependent proteinases believed to be important modulators of normal human lung development and potentially harmful mediators of lung damage. Little is known about MMP production and secretion by the lung during childhood or how alterations in MMP levels may be involved in lung damage. We examined endotracheal aspirates from children (<19 years) without lung disease for the presence of MMP activity. Only gelatinase activity was detectable, and inhibitor profiles suggest they represented one or more MMPs. Comparison of gelatinase activity, MMP expression, and MMP activity in children without pulmonary disease with children who required mechanical ventilation for respiratory failure show: 1) gelatinase activity was approximately five- to sixfold higher in respiratory failure; 2) MMP-7, MMP-8, and MMP-9 concentrations and MMP-8 and MMP-9 activities were markedly elevated in respiratory failure; and 3) MMP-7, MMP-8, and MMP-9 levels were significantly correlated in children with lung disease. These studies provide compelling evidence that specific MMPs are present in the diseased lung and may participate in the pathogenesis of pediatric respiratory failure.

Metalloproteinase and growth factor interactions: do they play a role in pulmonary fibrosis?

Chronic lung disease due to interstitial fibrosis can be a consequence of acute lung injury and inflammation. The inflammatory response is mediated through the migration of inflammatory cells, actions of proinflammatory cytokines, and the secretion of matrix-degrading proteinases. After the initial inflammatory insult, successful healing of the lung may occur, or alternatively, dysregulated tissue repair can result in scarring and fibrosis. On the basis of recent insights into the mechanisms underlying acute lung injury and its long-term consequences, data suggest that proteinases, such as the matrix metalloproteinases (MMPs), may not only be involved in the breakdown and remodeling that occurs during the injury but may also cause the release of growth factors and cytokines known to influence growth and differentiation of target cells within the lung. Through the release of and activation of fibrosis-promoting cytokines and growth factors such as transforming growth factor-beta1, tumor necrosis factor-alpha, and insulin-like growth factors by MMPs, we propose that these metalloproteinases may be integral to the initiation and progression of pulmonary fibrosis.

Exploring the interface between metallo-proteinase activity and growth factor and cytokine bioavailability

Metallo-proteinases are implicated in many processes involved in tissue remodeling, cell motility, morphogenesis, and cell and organ growth and differentiation. Recent data suggest that several members of the metzincin family including the matrix metallo-proteinases (MMPs), adamalysin-related proteinases, and the newly described pappalysins, are intimately involved in the activation and/or release of cytokines and growth factors. We review how metzincins, working through unique mechanisms, influence the extracellular milieu of several important cytokines and growth factors including transforming growth factor-beta (TGF-beta), TNF-alpha, IGFs and HB-epidermal growth factor (EGF). Because metzincins can modulate the bioavailability of these peptides, they may serve as unique target molecules to control cytokine and growth factor action in the extracellular environment.

Neonatal progeroid (Wiedemann-Rautenstrauch) syndrome: report of five new cases and review

The neonatal progeroid syndrome (NPS), or Wiedemann-Rautenstrauch, is a rare autosomal recessive disorder comprised of generalized lipoatrophy except for fat pads in the suprabuttock areas, hypotrichosis of the scalp hair, eyebrows, and eyelashes, relative macrocephaly, triangular face, natal teeth, and micrognathia. We report on 5 new patients who demonstrate phenotypic variability and who represent the single largest series of NPS reported to date. Two of the patients are from an African-American kindred, an ethnic occurrence not reported previously. The fact that there are 2 pairs of sibs among the 5 patients further supports that NPS is an autosomal recessive condition. This report also includes a review of the previously reported 16 patients and compares them with the 5 new patients. Abnormalities in endocrine and lipid metabolism were found in 3 of 5 patients. Skeletal findings in 2 of our patients demonstrated some new findings as well as the typical radiological abnormalities previously noted in NPS. It is apparent, based on the 21 cases, that mild to moderate mental retardation is common in NPS. Long term follow-up of patients with NPS should provide more information relative to their ultimate psychomotor development. NPS is usually lethal by 7 months; however, on rare occasions, patients have survived into the teens. Our 3 surviving patients range in age from 16-23 months. Variability in the phenotype of NPS is clear; however, the phenotype remains distinct enough to allow a secure diagnosis.