A novel spontaneous mutation of Irs1 in mice results in hyperinsulinemia, reduced growth, low bone mass and impaired adipogenesis

Targeting Kindlin-2 in adipocytes increases bone mass through inhibiting FAS/PPARγ/FABP4 signaling in mice

Osteoporosis (OP) is a systemic skeletal disease that primarily affects the elderly population, which greatly increases the risk of fractures. Here we report that Kindlin-2 expression in adipose tissue increases during aging and high-fat diet fed and is accompanied by decreased bone mass. Kindlin-2 specific deletion (K2KO) controlled by Adipoq-Cre mice or adipose tissue-targeting AAV (AAV-Rec2-CasRx-sgK2) significantly increases bone mass. Mechanistically, Kindlin-2 promotes peroxisome proliferator-activated receptor gamma (PPARγ) activation and downstream fatty acid binding protein 4 (FABP4) expression through stabilizing fatty acid synthase (FAS), and increased FABP4 inhibits insulin expression and decreases bone mass. Kindlin-2 inhibition results in accelerated FAS degradation, decreased PPARγ activation and FABP4 expression, and therefore increased insulin expression and bone mass. Interestingly, we find that FABP4 is increased while insulin is decreased in serum of OP patients. Increased FABP4 expression through PPARγ activation by rosiglitazone reverses the high bone mass phenotype of K2KO mice. Inhibition of FAS by C75 phenocopies the high bone mass phenotype of K2KO mice. Collectively, our study establishes a novel Kindlin-2/FAS/PPARγ/FABP4/insulin axis in adipose tissue modulating bone mass and strongly indicates that FAS and Kindlin-2 are new potential targets and C75 or AAV-Rec2-CasRx-sgK2 treatment are potential strategies for OP treatment.

Insulin-like Growth Factor 1 Signaling in Mammalian Hearing

Insulin-like growth factor 1 (IGF-1) is a peptide hormone belonging to the insulin family of proteins. Almost all of the biological effects of IGF-1 are mediated through binding to its high-affinity tyrosine kinase receptor (IGF1R), a transmembrane receptor belonging to the insulin receptor family. Factors, receptors and IGF-binding proteins form the IGF system, which has multiple roles in mammalian development, adult tissue homeostasis, and aging. Consequently, mutations in genes of the IGF system, including downstream intracellular targets, underlie multiple common pathologies and are associated with multiple rare human diseases. Here we review the contribution of the IGF system to our understanding of the molecular and genetic basis of human hearing loss by describing, (i) the expression patterns of the IGF system in the mammalian inner ear; (ii) downstream signaling of IGF-1 in the hearing organ; (iii) mouse mutations in the IGF system, including upstream regulators and downstream targets of IGF-1 that inform cochlear pathophysiology; and (iv) human mutations in these genes causing hearing loss.

Hearing impairment due to Mir183/96/182 mutations suggests both loss and gain of function effects

The microRNA miR-96 is important for hearing, as point mutations in humans and mice result in dominant progressive hearing loss. Mir96 is expressed in sensory cells along with Mir182 and Mir183, but the roles of these closely-linked microRNAs are as yet unknown. Here we analyse mice carrying null alleles of Mir182, and of Mir183 and Mir96 together to investigate their roles in hearing. We found that Mir183/96 heterozygous mice had normal hearing and homozygotes were completely deaf with abnormal hair cell stereocilia bundles and reduced numbers of inner hair cell synapses at four weeks old. Mir182 knockout mice developed normal hearing then exhibited progressive hearing loss. Our transcriptional analyses revealed significant changes in a range of other genes, but surprisingly there were fewer genes with altered expression in the organ of Corti of Mir183/96 null mice compared with our previous findings in Mir96 Dmdo mutants, which have a point mutation in the miR-96 seed region. This suggests the more severe phenotype of Mir96 Dmdo mutants compared with Mir183/96 mutants, including progressive hearing loss in Mir96 Dmdo heterozygotes, is likely to be mediated by the gain of novel target genes in addition to the loss of its normal targets. We propose three mechanisms of action of mutant miRNAs; loss of targets that are normally completely repressed, loss of targets whose transcription is normally buffered by the miRNA, and gain of novel targets. Any of these mechanisms could lead to a partial loss of a robust cellular identity and consequent dysfunction.

Potential applications for rhIGF-I: Bone disease and IGFI

Growth hormone (GH) and insulin like growth factor-I (IGFI) are key bone trophic hormones, whose rising levels during puberty are critical for pubertal bone accrual. Conditions of GH deficiency and genetic resistance impact cortical and trabecular bone deleteriously with reduced estimates of bone strength. In humans, conditions of undernutrition (as in anorexia nervosa (AN), or subsequent to chronic illnesses) are associated with low IGF-I levels, which correlate with disease severity, and also with lower bone mineral density (BMD), impaired bone structure and lower strength estimates. In adolescents and adults with AN, studies have demonstrated a nutritionally acquired GH resistance with low IGF-I levels despite high concentrations of GH. IGF-I levels go up with increasing body weight, and are associated with rising levels of bone turnover markers. In short-term studies lasting 6-10 days, recombinant human IGF-I (rhIGF-I) administration in physiologic replacement doses normalized IGF-I levels and increased levels of bone formation markers in both adults and adolescents with AN. In a randomized controlled trial in adults with AN in which participants were randomized to one of four arms: (i) rhIGF-I with oral estrogen-progesterone (EP), (ii) rhIGF-I alone, (iii) EP alone, or (iv) neither for 9 months, a significant increase in bone formation markers was noted in the groups that received rhIGF-I, and a significant decrease in bone resorption markers in the groups that received EP. The group that received both rhIGF-I and EP had a significant increase in bone density at the spine and hip compared to the group that received neither. Side effects were minimal, with no documented fingerstick glucose of <50 mg/dl. These data thus suggest a potential role for rhIGF-I administration in optimizing bone accrual in states of undernutrition associated with low IGF-I.

IRS-1 targets TAZ to inhibit adipogenesis of rat bone marrow mesenchymal stem cells through PI3K-Akt and MEK-ERK pathways

Gene modification of mesenchymal stem cells (MSCs) offers a promising approach for clinical stem cell therapy. Transcriptional co-activator with PDZ-binding motif (TAZ) plays a vital role in MSCs' differentiation. We aim to explore the interaction of insulin receptor substrate-1 (IRS-1) with TAZ to regulate MSCs' adipogenesis in this study. Initially, IRS-1 and TAZ followed similar decreasing expression pattern at the early stage of adipogenesis. And, overexpression of IRS-1 decreased the CCAAT/enhancer binding protein β (C/EBPβ) and peroxi-some proliferator-activated receptor gamma (PPARγ) expression with TAZ upregulation. Accordingly, knockdown of IRS-1 induced the upexpression of C/EBPβ and PPARγ with TAZ downregulation. Indeed, IRS-1 targeted TAZ to downregulate the C/EBPβ and PPARγ expression, while knockdown of TAZ attenuated the IRS-1 inhibited adipogenesis. Furthermore, both LY294002 (the PI3K-Akt inhibitor) and U0126 (the MEK-ERK inhibitor) blocked the regulation of IRS-1 on TAZ during adipogenesis. Additionally, IRS-1 and TAZ influenced the cell proliferation in the above process. Taken together, this study suggests for the first time that IRS-1 is a key regulator of the MSCs' adipogenesis and may serve as a potential therapeutic target for differential alterations in bone marrow.

Gene expression profile during proliferation and differentiation of rainbow trout adipocyte precursor cells

BACKGROUND

Excessive accumulation of adipose tissue in cultured fish is an outstanding problem in aquaculture. To understand the development of adiposity, it is crucial to identify the genes which expression is associated with adipogenic differentiation. Therefore, the transcriptomic profile at different time points (days 3, 8, 15 and 21) along primary culture development of rainbow trout preadipocytes has been investigated using an Agilent trout oligo microarray.

RESULTS

Our analysis identified 4026 genes differentially expressed (fold-change >3) that were divided into two major clusters corresponding to the main phases observed during the preadipocyte culture: proliferation and differentiation. Proliferation cluster comprised 1028 genes up-regulated from days 3 to 8 of culture meanwhile the differentiation cluster was characterized by 2140 induced genes from days 15 to 21. Proliferation was characterized by enrichment in genes involved in basic cellular and metabolic processes (transcription, ribosome biogenesis, translation and protein folding), cellular remodelling and autophagy. In addition, the implication of the eicosanoid signalling pathway was highlighted during this phase. On the other hand, the terminal differentiation phase was enriched with genes involved in energy production, lipid and carbohydrate metabolism. Moreover, during this phase an enrichment in genes involved in the formation of the lipid droplets was evidenced as well as the activation of the thyroid-receptor/retinoic X receptor (TR/RXR) and the peroxisome proliferator activated receptors (PPARs) signalling pathways. The whole adipogenic process was driven by a coordinated activation of transcription factors and epigenetic modulators.

CONCLUSIONS

Overall, our study demonstrates the coordinated expression of functionally related genes during proliferation and differentiation of rainbow trout adipocyte cells. Furthermore, the information generated will allow future investigations of specific genes involved in particular stages of fish adipogenesis.

Bone Cell Bioenergetics and Skeletal Energy Homeostasis

The rising incidence of metabolic diseases worldwide has prompted renewed interest in the study of intermediary metabolism and cellular bioenergetics. The application of modern biochemical methods for quantitating fuel substrate metabolism with advanced mouse genetic approaches has greatly increased understanding of the mechanisms that integrate energy metabolism in the whole organism. Examination of the intermediary metabolism of skeletal cells has been sparked by a series of unanticipated observations in genetically modified mice that suggest the existence of novel endocrine pathways through which bone cells communicate their energy status to other centers of metabolic control. The recognition of this expanded role of the skeleton has in turn led to new lines of inquiry directed at defining the fuel requirements and bioenergetic properties of bone cells. This article provides a comprehensive review of historical and contemporary studies on the metabolic properties of bone cells and the mechanisms that control energy substrate utilization and bioenergetics. Special attention is devoted to identifying gaps in our current understanding of this new area of skeletal biology that will require additional research to better define the physiological significance of skeletal cell bioenergetics in human health and disease.

Leaf Extract from Lithocarpus polystachyus Rehd. Promote Glycogen Synthesis in T2DM Mice

The purpose of this study was to investigate the effects of leaf extract from Lithocarpus polystachyus Rehd. on type II diabetes mellitus (T2DM) and the active ingredients of this effect. In addition, this study determined, for the first time, the underlying molecular and pharmacological mechanisms of the extracts on hyperglycemia using long-term double high diet-fed and streptozotocin (STZ) induced type II diabetic mice. In the present study, leaf extract, phloridzin and trilobatin were assessed in vivo (gavage) and in vitro (non-invasive micro-test technique, NMT) in experimental T2DM mice. The biochemical parameters were measured including blood glucose and blood lipid level, liver biochemical indexes, and hepatic glycogen. The relative expression of glycometabolism-related genes was detected. The effect of leaf extracts on physiological glucose flux in liver tissue from control and T2DM mice was also investigated. Body weight of experimental T2DM mice increased significantly after the first week, but stabilized over the subsequent three weeks; body weight of all other groups did not change during the four weeks’ study. After four weeks, all treatment groups decreased blood glucose, and treatment with leaf extract had numerous positive effects: a) promoted in glucose uptake in liver, b) increased synthesis of liver glycogen, c) reduced oxidative stress, d) up-regulation of glucokinase (GK), glucose transporter 2 (GLUT2), insulin receptor (IR) and insulin receptor substrate (IRS) expression in liver, e) down-regulation of glucose-6-phosphatase (G-6-P) expression, and f) ameliorated blood lipid levels. Both treatment with trilobatin or phloridzin accelerated liver glycogen synthesis, decreased oxidative stress and increased expression of GK. IRS and phosphoenolpyruvate carboxykinase (PEPCK) were both up-regulated after treatment with trilobatin. Expression of GLUT2, PEPCK and G-6-P were also increased in liver tissue after treatment with phloridzin. Our data indicate that leaf extract from L. polystachyus Rehd. has a preferable hypoglycemic effects than trilobatin or phloridzin alone. Leaf extract significantly increased glucose uptake and hepatic glycogen synthesis while also inducing a decline of hepatic gluconeogenesis and oxidative stress in T2DM mice. From this study, we draw conclusions that L. polystachyus promoted glycogen synthesis in T2DM mice, and that the active compounds were not only the trilobatin or phloridzin.

IRS-1 Functions as a Molecular Scaffold to Coordinate IGF-I/IGFBP-2 Signaling During Osteoblast Differentiation

Insulin like growth factor I (IGF-I) and insulin like growth factor binding protein-2 (IGFBP-2) function coordinately to stimulate AKT and osteoblast differentiation. IGFBP-2 binding to receptor protein tyrosine phosphatase β (RPTPβ) stimulates polymerization and inactivation of phosphatase activity. Because phosphatase and tensin homolog (PTEN) is the primary target of RPTPβ, this leads to enhanced PTEN tyrosine phosphorylation and inactivation. However RPTPβ inactivation also requires IGF-I receptor activation. The current studies were undertaken to determine the mechanism by which IGF-I mediates changes in RPTPβ function in osteoblasts. IGFBP-2/IGF-I stimulated vimentin binding to RPTPβ and this was required for RPTPβ polymerization. Vimentin serine phosphorylation mediated its binding to RPTPβ and PKCζ was identified as the kinase that phosphorylated vimentin. To determine the mechanism underlying IGF-I stimulation of PKCζ-mediated vimentin phosphorylation, we focused on insulin receptor substrate-1 (IRS-1). IGF-I stimulated IRS-1 phosphorylation and recruitment of PKCζ and vimentin to phospho-IRS-1. IRS-1 immunoprecipitates containing PKCζ and vimentin were used to confirm that activated PKCζ directly phosphorylated vimentin. PKCζ does not contain a SH-2 domain that is required to bind to phospho-IRS-1. To determine the mechanism of PKCζ recruitment we analyzed the role of p62 (a PKCζ binding protein) that contains a SH2 domain. Exposure to differentiation medium plus IGF-I stimulated PKCζ/p62 association. Subsequent analysis showed the p62/PKCζ complex was co-recruited to IRS-1. Peptides that disrupted p62/PKCζ or p62/IRS-1 inhibited IGF-I/IGFBP-2 stimulated PKCζ activation, vimentin phosphorylation, PTEN tyrosine phosphorylation, AKT activation, and osteoblast differentiation. The importance of these signaling events for differentiation was confirmed in primary mouse calvarial osteoblasts. These results demonstrate the cooperative interaction between RPTPβ and the IGF-I receptor leading to a coordinated series of signaling events that are required for osteoblast differentiation. Our findings emphasize the important role IRS-1 plays in modulating these signaling events and confirm its essential role in facilitating osteoblast differentiation. © 2016 American Society for Bone and Mineral Research.

Spontaneous 8bp Deletion in Nbeal2 Recapitulates the Gray Platelet Syndrome in Mice

During the analysis of a whole genome ENU mutagenesis screen for thrombosis modifiers, a spontaneous 8 base pair (bp) deletion causing a frameshift in exon 27 of the Nbeal2 gene was identified. Though initially considered as a plausible thrombosis modifier, this Nbeal2 mutation failed to suppress the synthetic lethal thrombosis on which the original ENU screen was based. Mutations in NBEAL2 cause Gray Platelet Syndrome (GPS), an autosomal recessive bleeding disorder characterized by macrothrombocytopenia and gray-appearing platelets due to lack of platelet alpha granules. Mice homozygous for the Nbeal2 8 bp deletion (Nbeal2^gps/gps) exhibit a phenotype similar to human GPS, with significantly reduced platelet counts compared to littermate controls (p = 1.63 x 10⁻⁷). Nbeal2^gps/gps mice also have markedly reduced numbers of platelet alpha granules and an increased level of emperipolesis, consistent with previously characterized mice carrying targeted Nbeal2 null alleles. These findings confirm previous reports, provide an additional mouse model for GPS, and highlight the potentially confounding effect of background spontaneous mutation events in well-characterized mouse strains.

Igfbp2 Deletion in Ovariectomized Mice Enhances Energy Expenditure but Accelerates Bone Loss

Previously, we reported sexually dimorphic bone mass and body composition phenotypes in Igfbp2(-/-) mice (-/-), where male mice exhibited decreased bone and increased fat mass, whereas female mice displayed increased bone but no changes in fat mass. To investigate the interaction between IGF-binding protein (IGFBP)-2 and estrogen, we subjected Igfbp2 -/- and +/+ female mice to ovariectomy (OVX) or sham surgery at 8 weeks of age. At 20 weeks of age, mice underwent metabolic cage analysis and insulin tolerance tests before killing. At harvest, femurs were collected for microcomputed tomography, serum for protein levels, brown adipose tissue (BAT) and inguinal white adipose tissue (IWAT) adipose depots for histology, gene expression, and mitochondrial respiration analysis of whole tissue. In +/+ mice, serum IGFBP-2 dropped 30% with OVX. In the absence of IGFBP-2, OVX had no effect on preformed BAT; however, there was significant "browning" of the IWAT depot coinciding with less weight gain, increased insulin sensitivity, lower intraabdominal fat, and increased bone loss due to higher resorption and lower formation. Likewise, after OVX, energy expenditure, physical activity and BAT mitochondrial respiration were decreased less in the OVX-/- compared with OVX+/+. Mitochondrial respiration of IWAT was reduced in OVX+/+ yet remained unchanged in OVX-/- mice. These changes were associated with significant increases in Fgf21 and Foxc2 expression, 2 proteins known for their insulin sensitizing and browning of WAT effects. We conclude that estrogen deficiency has a profound effect on body and bone composition in the absence of IGFBP-2 and may be related to changes in fibroblast growth factor 21.

Early-onset type 2 diabetes impairs skeletal acquisition in the male TALLYHO/JngJ mouse

Type 2 diabetes (T2D) incidence in adolescents is rising and may interfere with peak bone mass acquisition. We tested the effects of early-onset T2D on bone mass, microarchitecture, and strength in the TALLYHO/JngJ mouse, which develops T2D by 8 weeks of age. We assessed metabolism and skeletal acquisition in male TALLYHO/JngJ and SWR/J controls (n = 8-10/group) from 4 weeks to 8 and 17 weeks of age. Tallyho mice were obese; had an approximately 2-fold higher leptin and percentage body fat; and had lower bone mineral density vs SWR at all time points (P < .03 for all). Tallyho had severe deficits in distal femur trabecular bone volume fraction (-54%), trabecular number (-27%), and connectivity density (-82%) (P < .01 for all). Bone formation was higher in Tallyho mice at 8 weeks but lower by 17 weeks of age vs SWR despite similar numbers of osteoblasts. Bone marrow adiposity was 7- to 50-fold higher in Tallyho vs SWR. In vitro, primary bone marrow stromal cell differentiation into osteoblast and adipocyte lineages was similar in SWR and Tallyho, suggesting skeletal deficits were not due to intrinsic defects in Tallyho bone-forming cells. These data suggest the Tallyho mouse might be a useful model to study the skeletal effects of adolescent T2D.

Treatment with N- and C-terminal peptides of parathyroid hormone-related protein partly compensate the skeletal abnormalities in IGF-I deficient mice

Insulin-like growth factor-I (IGF-I) deficiency causes growth delay, and IGF-I has been shown to partially mediate bone anabolism by parathyroid hormone (PTH). PTH-related protein (PTHrP) is abundant in bone, and has osteogenic features by poorly defined mechanisms. We here examined the capacity of PTHrP (1-36) and PTHrP (107-111) (osteostatin) to reverse the skeletal alterations associated with IGF-I deficiency. Igf1-null mice and their wild type littermates were treated with each PTHrP peptide (80 µg/Kg/every other day/2 weeks; 2 males and 4 females for each genotype) or saline vehicle (3 males and 3 females for each genotype). We found that treatment with either PTHrP peptide ameliorated trabecular structure in the femur in both genotypes. However, these peptides were ineffective in normalizing the altered cortical structure at this bone site in Igf1-null mice. An aberrant gene expression of factors associated with osteoblast differentiation and function, namely runx2, osteoprotegerin/receptor activator of NF-κB ligand ratio, Wnt3a , cyclin D1, connexin 43, catalase and Gadd45, as well as in osteocyte sclerostin, was found in the long bones of Igf1-null mice. These mice also displayed a lower amount of trabecular osteoblasts and osteoclasts in the tibial metaphysis than those in wild type mice. These alterations in Igf1-null mice were only partially corrected by each PTHrP peptide treatment. The skeletal expression of Igf2, Igf1 receptor and Irs2 was increased in Igf1-null mice, and this compensatory profile was further improved by treatment with each PTHrP peptide related to ERK1/2 and FoxM1 activation. In vitro, PTHrP (1-36) and osteostatin were effective in promoting bone marrow stromal cell mineralization in normal mice but not in IGF-I-deficient mice. Collectively, these findings indicate that PTHrP (1-36) and osteostatin can exert several osteogenic actions even in the absence of IGF-I in the mouse bone.

Liver kinase b1 is required for white adipose tissue growth and differentiation

White adipose tissue (WAT) is not only a lipogenic and fat storage tissue but also an important endocrine organ that regulates energy homeostasis, lipid metabolism, appetite, fertility, and immune and stress responses. Liver kinase B1 (LKB1), a tumor suppressor, is a key regulator in energy metabolism. However, the role of LKB1 in adipogenesis is unknown. The current study aimed to determine the contributions of LKB1 to adipogenesis in vivo. Using the Fabp4-Cre/loxP system, we generated adipose tissue-specific LKB1 knockout (LKB1(ad-/-)) mice. LKB1(ad-/-) mice exhibited a reduced amount of WAT, postnatal growth retardation, and early death before weaning. Further, LKB1 deletion markedly reduced the levels of insulin receptor substrate 1 (IRS1), peroxisome proliferator-activated receptor γ, CCAAT/enhancer-binding protein α, and phosphorylated AMP-activated protein kinase (AMPK). Consistent with these results, overexpression of constitutively active AMPK partially ablated IRS1 degradation in LKB1-deficient cells. LKB1 deletion increased the levels of F-box/WD repeat-containing protein (Fbw) 8, the IRS1 ubiquitination E3 ligase. Silencing of Fbw8 increased IRS1 levels. Finally, promoter analysis and DNA chromatin immunoprecipitation analysis identified three sterol regulatory element (SRE) sites in the Fbw8 promoter, where SRE-binding protein 1c binds and induces the expression of Fbw8. Taken together, these data indicate that LKB1 controls IRS1-dependent adipogenesis via AMPK in WAT.

Marrow fat and bone--new perspectives

CONTEXT

There is growing interest in the relationship between bone mineral density, bone strength, and fat depots. Marrow adipose tissue, a well-established component of the marrow environment, is metabolically distinct from peripheral fat depots, but its functional significance is unknown.

OBJECTIVE

In this review, we discuss animal and human data linking the marrow adipose tissue depot to parameters of bone density and integrity as well as the potential significance of marrow adipose tissue in metabolic diseases associated with bone loss, including type 1 diabetes mellitus and anorexia nervosa. Potential hormonal determinants of marrow adipose tissue are also discussed.

CONCLUSIONS

We conclude that whereas most animal and human data demonstrate an inverse association between marrow adipose tissue and measures of bone density and strength, understanding the functional significance of marrow adipose tissue and its hormonal determinants will be critical to better understanding its role in skeletal integrity and the role of marrow adipose tissue in the pathophysiology of bone loss.

Insulin-like growth factor 1 physiology: lessons from mouse models

Insulin-like growth factor 1 (IGF-1) is a pleiotropic polypeptide. Its expression is tightly regulated and it plays significant roles during early development, maturation, and adulthood. This article discusses the roles of IGF-1 in determination of body size, skeletal acquisition, muscle growth, carbohydrate metabolism, and longevity, as learned from mouse models.

A specific role for phosphoinositide 3-kinase and AKT in osteoblasts?

The phosphoinositide 3-kinase and AKT (protein kinase B) signaling pathway (PI3K/AKT) plays a central role in the control of cell survival, growth, and proliferation throughout the body. With regard to bone, and particularly in osteoblasts, there is an increasing amount of evidence that the many signaling molecules exert some of their bone-specific effects in part via selectively activating some of the generic effects of the PI3K/AKT pathway in osteoblasts. There is further data demonstrating that PI3K/AKT has the capacity to specifically cross-talk with other signaling pathways and transcriptional networks controlling bone cells' development in order to fine-tune the osteoblast phenotype. There is also evidence that perturbations in the PI3K/AKT pathway may well be responsible for certain bone pathologies. In this review, we discuss some of these findings and suggest that the PI3K/AKT pathway is a central nexus in the extensive network of extracellular signaling pathways that control the osteoblast.

Mitogen-activated protein kinase phosphatase 1 regulates bone mass, osteoblast gene expression, and responsiveness to parathyroid hormone

Parathyroid hormone (PTH) signaling via PTH 1 receptor (PTH1R) involves mitogen-activated protein kinase (MAPK) pathways. MAPK phosphatase 1 (MKP1) dephosphorylates and inactivates MAPKs in osteoblasts, the bone-forming cells. We previously showed that PTH1R activation in differentiated osteoblasts upregulates MKP1 and downregulates pERK1/2-MAPK and cyclin D1. In this study, we evaluated the skeletal phenotype of Mkp1 knockout (KO) mice and the effects of PTH in vivo and in vitro. Microcomputed tomography analysis of proximal tibiae and distal femora from 12-week-old Mkp1 KO female mice revealed osteopenic phenotype with significant reduction (8-46%) in bone parameters compared with wild-type (WT) controls. Histomorphometric analysis showed decreased trabecular bone area in KO females. Levels of serum osteocalcin (OCN) were lower and serum tartrate-resistant acid phosphatase 5b (TRAP5b) was higher in KO animals. Treatment of neonatal mice with hPTH (1-34) for 3 weeks showed attenuated anabolic responses in the distal femora of KO mice compared with WT mice. Primary osteoblasts derived from KO mice displayed delayed differentiation determined by alkaline phosphatase activity, and reduced expressions of Ocn and Runx2 genes associated with osteoblast maturation and function. Cells from KO females exhibited attenuated PTH response in mineralized nodule formation in vitro. Remarkably, this observation was correlated with decreased PTH response of matrix Gla protein expression. Expressions of pERK1/2 and cyclin D1 were inhibited dramatically by PTH in differentiated osteoblasts from WT mice but much less in osteoblasts from Mkp1 KO mice. In conclusion, MKP1 is important for bone homeostasis, osteoblast differentiation and skeletal responsiveness to PTH.

The IGF-I regulatory system and its impact on skeletal and energy homeostasis

Insulin-like growth factor (IGF)-I is important in the acquisition and maintenance of both soft and hard tissues. Skeletal remodeling requires energy and recent work has demonstrated that bone can influence insulin sensitivity and thereby regulate metabolic processes. New insights from mouse models into the role of IGF-binding proteins (IGFBPs) as more than mere depots for the IGFs has reignited investigations into the metabolic targets influenced by the IGF regulatory system and the pathways that link bone to adipose tissue. Although there remains continued uncertainty about the relative balance between the effects of circulating versus tissue IGF-I actions, the role of the IGFBPs has been redefined both as modulators of IGF-I action and as independent signaling factors. This review highlights several recent findings that shed new light on the physiologic role of the IGF regulatory system and its influence on skeletal and fat metabolism.

The Sweet Pee model for Sglt2 mutation

Inhibiting renal glucose transport is a potential pharmacologic approach to treat diabetes. The renal tubular sodium-glucose transporter 2 (SGLT2) reabsorbs approximately 90% of the filtered glucose load. An animal model with sglt2 dysfunction could provide information regarding the potential long-term safety and efficacy of SGLT2 inhibitors, which are currently under clinical investigation. Here, we describe Sweet Pee, a mouse model that carries a nonsense mutation in the Slc5a2 gene, which results in the loss of sglt2 protein function. The phenotype of Sweet Pee mutants was remarkably similar to patients with mutations in the Scl5a2 gene. The Sweet Pee mutants had improved glucose tolerance, higher urinary excretion of calcium and magnesium, and growth retardation. Renal physiologic studies demonstrated a prominent distal osmotic diuresis without enhanced natriuresis. Sweet Pee mutants did not exhibit increased KIM-1 or NGAL, markers of acute tubular injury. After induction of diabetes, Sweet Pee mice had better overall glycemic control than wild-type control mice, but had a higher risk for infection and an increased mortality rate (70% in homozygous mutants versus 10% in controls at 20 weeks). In summary, the Sweet Pee model allows study of the long-term benefits and risks associated with inhibition of SGLT2 for the management of diabetes. Our model suggests that inhibiting SGLT2 may improve glucose control but may confer increased risks for infection, malnutrition, volume contraction, and mortality.

IGF-1 and bone: New discoveries from mouse models

Insulin-like growth factor-1 (IGF-1) plays a central role in cellular growth, differentiation, survival, and cell cycle progression. It is expressed early during development and its effects are mediated through binding to a tyrosine kinase receptor, the insulin-like growth factor-1 receptor (IGF-1R). In the circulation, the IGFs bind to IGF-binding proteins (IGFBPs), which determine their bioavailability and regulate the interaction between the IGFs and IGF-1R. Studies in animal models and in humans have established critical roles for IGFs in skeletal growth and development. In this review we present new and old findings from mouse models of the IGF system and discuss their clinical relevance to normal and pathological skeletal physiology.

Spontaneous Irs1 passenger mutation linked to a gene-targeted SerpinB2 allele

In characterizing mice with targeted disruption of the SerpinB2 gene, we observed animals that were small at birth with delayed growth and decreased life expectancy. Although this phenotype cosegregated with homozygosity for the inactive SerpinB2 allele, analysis of homozygous SerpinB2-deficient mice derived from two additional independent embryonic stem (ES) cell clones exhibited no growth abnormalities. Examination of additional progeny from the original SerpinB2-deficient line revealed recombination between the small phenotype (smla) and the SerpinB2 locus. The locus responsible for smla was mapped to a 2.78-Mb interval approximately 30 Mb proximal to SerpinB2, bounded by markers D1Mit382 and D1Mit216. Sequencing of Irs1 identified a nonsense mutation at serine 57 (S57X), resulting in complete loss of IRS1 protein expression. Analysis of ES cell DNA suggests that the S57X Irs1 mutation arose spontaneously in an ES cell subclone during cell culture. Although the smla phenotype is similar to previously reported Irs1 alleles, mice exhibited decreased survival, in contrast to the enhanced longevity reported for IRS1 deficiency generated by gene targeting. This discrepancy could result from differences in strain background, unintended indirect effects of the gene targeting, or the minimal genetic interference of the S57X mutation compared with the conventionally targeted Irs1-KO allele. Spontaneous mutations arising during ES cell culture may be a frequent but underappreciated occurrence. When linked to a targeted allele, such mutations could lead to incorrect assignment of phenotype and may account for a subset of markedly discordant results from experiments independently targeting the same gene.