Impact of pregnancy-associated plasma protein-a deletion on the adult murine skeleton

A Meta-Analysis Study to Define Variations in Murine Long Bone Biomechanical Testing

A systematic literature search and meta-analysis were performed to evaluate the variability in biomechanical testing of murine long bones, specifically focused on point-bending tests of mice femora. Due to the lack of standardized protocols for these tests, the assessment quantifies the heterogeneity in reported mechanical properties across existing literature. This study followed preferred reporting items for systematic reviews and meta-analyses (PRISMA) and strengthening the reporting of observational studies in epidemiology (STROBE) guidelines to search publicly available databases for relevant studies. After title and abstract screening, full-text reviews identified 73 articles meeting the inclusion criteria. Data was extracted from these studies, including stiffness, maximum load, modulus, and ultimate stress values for both three-point and four-point bending tests. The data were analyzed through ANOVA and metaregression to assess variability caused by age, sex, and genetic strain. The reviewers also assessed the quality of the included studies. The meta-analysis revealed significant heterogeneity in reported mechanical properties, with I2 values ranging from 72% to 100% in the three point-bend tests of pooled genetic strains. This heterogeneity persisted even after accounting for age, sex, and genetic strain differences. The review concludes that nonstandardized testing setups are the likely major source of the observed variability in reported data more than the population characteristics of the mice, highlighting the need for more consistent testing methodologies in future studies.

The metalloproteinase PAPP-A is required for IGF-dependent chondrocyte differentiation and organization

Insulin-like growth factor (IGF) signaling is required for proper growth and skeletal development in vertebrates. Consequently, its dysregulation may lead to abnormalities of growth or skeletal structures. IGF is involved in the regulation of cell proliferation and differentiation of chondrocytes. However, the availability of bioactive IGF may be controlled by antagonizing IGF binding proteins (IGFBPs) in the circulation and tissues. As the metalloproteinase PAPP-A specifically cleaves members of the IGFBP family, we hypothesized that PAPP-A activity liberates bioactive IGF in cartilage. In PAPP-A knockout mice, the femur length was reduced and the mice showed a disorganized columnar organization of growth plate chondrocytes. Similarly, zebrafish lacking pappaa showed reduced length of Meckel's cartilage and disorganized chondrocytes, reminiscent of the mouse knockout phenotype. Expression of chondrocyte differentiation markers (sox9a, ihha, and col10a1) was markedly affected in Meckel's cartilage of pappaa knockout zebrafish, indicating that differentiation of chondrocytes was compromised. Additionally, the zebrafish pappaa knockout phenotype was mimicked by pharmacological inhibition of IGF signaling, and it could be rescued by treatment with exogenous recombinant IGF-I. In conclusion, our data suggests that IGF activity in the growing cartilage, and hence IGF signaling in chondrocytes, requires the presence of PAPP-A. The absence of PAPP-A causes aberrant chondrocyte organization and compromised growth in both mice and zebrafish.

The Pregnancy-Associated Plasma Protein-A (PAPP-A) Story

Pregnancy-associated plasma protein-A (PAPP-A) was first identified in the early 1970s as a placental protein of unknown function, present at high concentrations in the circulation of pregnant women. In the mid-to-late 1990s, PAPP-A was discovered to be a metzincin metalloproteinase, expressed by many nonplacental cells, that regulates local insulin-like growth factor (IGF) activity through cleavage of high-affinity IGF binding proteins (IGFBPs), in particular IGFBP-4. With PAPP-A as a cell surface-associated enzyme, the reduced affinity of the cleavage fragments results in increased IGF available to bind and activate IGF receptors in the pericellular environment. This proteolytic regulation of IGF activity is important, since the IGFs promote proliferation, differentiation, migration, and survival in various normal and cancer cells. Thus, there has been a steady growth in investigation of PAPP-A structure and function outside of pregnancy. This review provides historical perspective on the discovery of PAPP-A and its structure and cellular function, highlights key studies of the first 50 years in PAPP-A research, and introduces new findings from recent years.

Bone-Derived IGF-I Regulates Radial Bone Growth in Adult Male Mice

Insulin-like growth factor-I (IGF-I) levels, which are reduced by age, and cortical bone dimensions are major determinants of fracture risk in elderly subjects. Inactivation of liver-derived circulating IGF-I results in reduced periosteal bone expansion in young and older mice. In mice with lifelong depletion of IGF-I in osteoblast lineage cells, the long bones display reduced cortical bone width. However, it has not previously been investigated whether inducible inactivation of IGF-I locally in bone in adult/old mice affects the bone phenotype. Adult tamoxifen-inducible inactivation of IGF-I using a CAGG-CreER mouse model (inducible IGF-IKO mice) substantially reduced IGF-I expression in bone (-55%) but not in liver. Serum IGF-I and body weight were unchanged. We used this inducible mouse model to assess the effect of local IGF-I on the skeleton in adult male mice, avoiding confounding developmental effects. After tamoxifen-induced inactivation of the IGF-I gene at 9 months of age, the skeletal phenotype was determined at 14 months of age. Computed tomography analyses of tibia revealed that the mid-diaphyseal cortical periosteal and endosteal circumferences and calculated bone strength parameters were decreased in inducible IGF-IKO mice compared with controls. Furthermore, 3-point bending showed reduced tibia cortical bone stiffness in inducible IGF-IKO mice. In contrast, the tibia and vertebral trabecular bone volume fraction was unchanged. In conclusion, inactivation of IGF-I in cortical bone with unchanged liver-derived IGF-I in older male mice resulted in reduced radial growth of cortical bone. This suggests that not only circulating IGF-I but also locally derived IGF-I regulates the cortical bone phenotype in older mice.

Recapitulation of anti-aging phenotypes by global, but not by muscle-specific, deletion of PAPP-A in mice

Deletion of pregnancy-associated plasma protein-A (PAPP-A), a protease that cleaves some but not all IGF1 binding proteins, postpones late-life diseases and extends lifespan in mice, but the mechanism of this effect is unknown. Here we show that PAPP-A knockout (PKO) mice display a set of changes, in multiple tissues, that are characteristic of other varieties of slow-aging mice with alterations in GH production or GH responsiveness, including Ames dwarf, Snell dwarf, and GHRKO mice. PKO mice have elevated UCP1 in brown and white adipose tissues (WAT), and a change in fat-associated macrophage subsets that leads to diminished production of inflammatory cytokines. PKO mice also show increased levels of muscle FNDC5 and its cleavage product, the myokine irisin, thought to cause changes in fat cell differentiation. PKO mice have elevated production of hepatic GPLD1 and plasma GPLD1, consistent with their elevation of hippocampal BDNF and DCX, used as indices of neurogenesis. In contrast, disruption of PAPP-A limited to muscle ("muPKO" mice) produces an unexpectedly complex set of changes, in most cases opposite in direction from those seen in PKO mice. These include declines in WAT UCP1, increases in inflammatory macrophages and cytokines in WAT, and a decline in muscle FNDC5 and plasma irisin. muPKO mice do, however, resemble global PKO mice in their elevation of hippocampal BDNF and DCX. The data for the PKO mice support the idea that these changes in fat, macrophages, liver, muscle, plasma, and brain are consistent and biologically significant features of the slow-aging phenotype in mice. The results on the muPKO mice provide a foundation for further investigation of the complex, local, and global circuits by which PAPP-A modulates signals ordinarily controlled by GH and/or IGF1.

Skeletal disorders associated with the growth hormone-insulin-like growth factor 1 axis

Growth hormone (GH) and insulin-like growth factor 1 (IGF1) are important regulators of bone remodelling and metabolism and have an essential role in the achievement and maintenance of bone mass throughout life. Evidence from animal models and human diseases shows that both GH deficiency (GHD) and excess are associated with changes in bone remodelling and cause profound alterations in bone microstructure. The consequence is an increased risk of fractures in individuals with GHD or acromegaly, a condition of GH excess. In addition, functional perturbations of the GH-IGF1 axis, encountered in individuals with anorexia nervosa and during ageing, result in skeletal fragility and osteoporosis. The effect of interventions used to treat GHD and acromegaly on the skeleton is variable and dependent on the duration of the disease, the pre-existing skeletal state, coexistent hormone alterations (such as those occurring in hypogonadism) and length of therapy. This variability could also reflect the irreversibility of the skeletal structural defect occurring during alterations of the GH-IGF1 axis. Moreover, the effects of the treatment of GHD and acromegaly on locally produced IGF1 and IGF binding proteins are uncertain and in need of further study. This Review highlights the pathophysiological, clinical and therapeutic aspects of skeletal fragility associated with perturbations in the GH-IGF1 axis.

Recombinant IGF-1 Induces Sex-Specific Changes in Bone Composition and Remodeling in Adult Mice with Pappa2 Deficiency

Deficiency of pregnancy-associated plasma protein-A2 (PAPP-A2), an IGF-1 availability regulator, causes postnatal growth failure and dysregulation of bone size and density. The present study aimed to determine the effects of recombinant murine IGF-1 (rmIGF-1) on bone composition and remodeling in constitutive Pappa2 knock-out (ko/ko) mice. To address this challenge, X-ray diffraction (XRD), attenuated total reflection-fourier transform infra-red (ATR-FTIR) spectroscopy and gene expression analysis of members of the IGF-1 system and bone resorption/formation were performed. Pappa2^ko/ko mice (both sexes) had reduced body and bone length. Male Pappa2^ko/ko mice had specific alterations in bone composition (mineral-to-matrix ratio, carbonate substitution and mineral crystallinity), but not in bone remodeling. In contrast, decreases in collagen maturity and increases in Igfbp3, osteopontin (resorption) and osteocalcin (formation) characterized the bone of Pappa2^ko/ko females. A single rmIGF-1 administration (0.3 mg/kg) induced short-term changes in bone composition in Pappa2^ko/ko mice (both sexes). rmIGF-1 treatment in Pappa2^ko/ko females also increased collagen maturity, and Igfbp3, Igfbp5, Col1a1 and osteopontin expression. In summary, acute IGF-1 treatment modifies bone composition and local IGF-1 response to bone remodeling in mice with Pappa2 deficiency. These effects depend on sex and provide important insights into potential IGF-1 therapy for growth failure and bone loss and repair.

Inhibition of longevity regulator PAPP-A modulates tissue homeostasis via restraint of mesenchymal stromal cells

Pregnancy-associated plasma protein-A (PAPP-A) is a secreted metalloprotease that increases insulin-like growth factor (IGF) availability by cleaving IGF-binding proteins. Reduced IGF signaling extends longevity in multiple species, and consistent with this, PAPP-A deletion extends lifespan and healthspan; however, the mechanism remains unclear. To clarify PAPP-A's role, we developed a PAPP-A neutralizing antibody and treated adult mice with it. Transcriptomic profiling across tissues showed that anti-PAPP-A reduced IGF signaling and extracellular matrix (ECM) gene expression system wide. The greatest reduction in IGF signaling occurred in the bone marrow, where we found reduced bone, marrow adiposity, and myelopoiesis. These diverse effects led us to search for unifying mechanisms. We identified mesenchymal stromal cells (MSCs) as the source of PAPP-A in bone marrow and primary responders to PAPP-A inhibition. Mice treated with anti-PAPP-A had reduced IGF signaling in MSCs and dramatically decreased MSC number. As MSCs are (1) a major source of ECM and the progenitors of ECM-producing fibroblasts, (2) the originating source of adult bone, (3) regulators of marrow adiposity, and (4) an essential component of the hematopoietic niche, our data suggest that PAPP-A modulates bone marrow homeostasis by potentiating the number and activity of MSCs. We found that MSC-like cells are the major source of PAPP-A in other tissues also, suggesting that reduced MSC-like cell activity drives the system-wide reduction in ECM gene expression due to PAPP-A inhibition. Dysregulated ECM production is associated with aging and drives age-related diseases, and thus, this may be a mechanism by which PAPP-A deficiency enhances longevity.

Pappa2 deletion has sex- and age-specific effects on bone in mice

OBJECTIVE

In humans, loss-of-function mutations in the gene encoding pregnancy-associated pregnancy protein-A2 cause short stature and slightly reduced bone density. The goal of this study was to determine the effects of Pappa2 deletion on bone in mice.

DESIGN

Pappa2 deletion mice and littermate controls were culled at 10, 19 or 30 weeks of age and femurs were analysed by micro-computed tomography. Serum markers of bone turnover and insulin-like growth factor binding protein 5 (IGFBP-5), a proteolytic target of PAPP-A2, were measured by ELISA.

RESULTS

At 10 and 19 weeks of age, Pappa2 deletion mice had slightly reduced trabecular parameters, but by 19 weeks of age, female deletion mice had increased cortical tissue mineral density, and this trait was increased by a small amount in deletion mice of both sexes at 30 weeks. Cortical area fraction was increased in Pappa2 deletion mice at all ages. Deletion of Pappa2 increased circulating IGFBP-5 levels and reduced markers of bone turnover (PINP and TRACP 5b).

CONCLUSIONS

PAPP-A2 contributes to the regulation of bone structure and mass in mice, likely through control of IGFBP-5 levels. The net effect of changes in bone formation and resorption depend on sex and age, and differ between trabecular and cortical bone.

Role of IGF1 and EFN-EPH signaling in skeletal metabolism

Insulin-like growth factor 1(IGF1) and ephrin ligand (EFN)-receptor (EPH) signaling are both crucial for bone cell function and skeletal development and maintenance. IGF1 signaling is the major mediator of growth hormone-induced bone growth, but a host of different signals and factors regulate IGF1 signaling at the systemic and local levels. Disruption of the Igf1 gene results in reduced peak bone mass in both experimental animal models and humans. Additionally, EFN-EPH signaling is a complex system which, particularly through cell-cell interactions, contributes to the development and differentiation of many bone cell types. Recent evidence has demonstrated several ways in which the IGF1 and EFN-EPH signaling pathways interact with and depend upon each other to regulate bone cell function. While much remains to be elucidated, the interaction between these two signaling pathways opens a vast array of new opportunities for investigation into the mechanisms of and potential therapies for skeletal conditions such as osteoporosis and fracture repair.

Skeletal effects of growth hormone and insulin-like growth factor-I therapy

The growth hormone/insulin-like growth factor (GH/IGF) axis is critically important for the regulation of bone formation, and deficiencies in this system have been shown to contribute to the development of osteoporosis and other diseases of low bone mass. The GH/IGF axis is regulated by a complex set of hormonal and local factors which can act to regulate this system at the level of the ligands, receptors, IGF binding proteins (IGFBPs), or IGFBP proteases. A combination of in vitro studies, transgenic animal models, and clinical human investigations has provided ample evidence of the importance of the endocrine and local actions of both GH and IGF-I, the two major components of the GH/IGF axis, in skeletal growth and maintenance. GH- and IGF-based therapies provide a useful avenue of approach for the prevention and treatment of diseases such as osteoporosis.

Regulation of skeletal growth and mineral acquisition by the GH/IGF-1 axis: Lessons from mouse models

The growth hormone (GH) and its downstream mediator, the insulin-like growth factor-1 (IGF-1), construct a pleotropic axis affecting growth, metabolism, and organ function. Serum levels of GH/IGF-1 rise during pubertal growth and associate with peak bone acquisition, while during aging their levels decline and associate with bone loss. The GH/IGF-1 axis was extensively studied in numerous biological systems including rodent models and cell cultures. Both hormones act in an endocrine and autocrine/paracrine fashion and understanding their distinct and overlapping contributions to skeletal acquisition is still a matter of debate. GH and IGF-1 exert their effects on osteogenic cells via binding to their cognate receptor, leading to activation of an array of genes that mediate cellular differentiation and function. Both hormones interact with other skeletal regulators, such as sex-steroids, thyroid hormone, and parathyroid hormone, to facilitate skeletal growth and metabolism. In this review we summarized several rodent models of the GH/IGF-1 axis and described key experiments that shed new light on the regulation of skeletal growth by the GH/IGF-1 axis.

Can aging be 'drugged'?

The engines that drive the complex process of aging are being identified by model-organism research, thereby providing potential targets and rationale for drug studies. Several studies of small molecules have already been completed in animal models with the hope of finding an elixir for aging, with a few compounds showing early promise. What lessons can we learn from drugs currently being tested, and which pitfalls can we avoid in our search for a therapeutic for aging? Finally, we must also ask whether an elixir for aging would be applicable to everyone, or whether we age differently, thus potentially shortening lifespan in some individuals.

PAPP-A2 expression by osteoblasts is required for normal postnatal growth in mice

OBJECTIVE

Pregnancy associated plasma protein-A2 (PAPP-A2) is a protease that cleaves insulin-like growth factor binding protein-5 (IGFBP-5), the most abundant IGFBP in bone. Deletion of Pappa2 reduces postnatal growth and bone length in mice. The aim of this study was to determine whether locally produced PAPP-A2 is required for normal bone growth.

DESIGN

We deleted Pappa2 primarily in osteoblasts by crossing conditional Pappa2 deletion mice with mice expressing Cre recombinase under the control of the Sp7 (Osterix) promoter. Effects of disrupting Pappa2 in Sp7-expressing cells were examined by measuring body mass and tail length at 3, 6, 10 and 12 weeks of age and bone dimensions at 12 weeks.

RESULTS

Body mass, tail length, and linear bone dimensions were significantly reduced at all ages by osteoblast-specific Pappa2 deletion. Mice homozygous for the conditional Pappa2 deletion allele and carrying the Cre transgene were smaller than controls carrying the Cre transgene, whereas mice homozygous for the conditional Pappa2 deletion allele were not smaller than controls when comparing mice not carrying the transgene. This result unambiguously demonstrates that PAPP-A2 produced by Sp7 expressing cells is required for normal growth. However, constitutive Pappa2 deletion had greater effects than osteoblast-specific Pappa2 deletion for many traits, indicating that post-natal growth is also affected by other sources of PAPP-A2. Immunohistochemistry revealed that PAPP-A2 localized in the epiphysis and metaphysis as well as osteoblasts, consistent with a role in bone growth.

CONCLUSION

Locally-produced PAPP-A2 is required for normal bone growth.

Pregnancy-associated plasma protein-A modulates the anabolic effects of parathyroid hormone in mouse bone

Intermittent parathyroid hormone (PTH) is a potent anabolic therapy for bone, and several studies have implicated local insulin-like growth factor (IGF) signaling in mediating this effect. The IGF system is complex and includes ligands and receptors, as well as IGF binding proteins (IGFBPs) and IGFBP proteases. Pregnancy-associated plasma protein-A (PAPP-A) is a metalloprotease expressed by osteoblasts in vitro that has been shown to enhance local IGF action through cleavage of inhibitory IGFBP-4. This study was set up to test two specific hypotheses: 1) Intermittent PTH treatment increases the expression of IGF-I, IGFBP-4 and PAPP-A in bone in vivo, thereby increasing local IGF activity. 2) In the absence of PAPP-A, local IGF activity and the anabolic effects of PTH on bone are reduced. Wild-type (WT) and PAPP-A knock-out (KO) mice were treated with 80 μg/kg human PTH 1-34 or vehicle by subcutaneous injection five days per week for six weeks. IGF-I, IGFBP-4 and PAPP-A mRNA expression in bone were significantly increased in response to PTH treatment. PTH treatment of WT mice, but not PAPP-A KO mice, significantly increased expression of an IGF-responsive gene. Bone mineral density (BMD), as measured by DEXA, was significantly decreased in femurs of PAPP-A KO compared to WT mice with PTH treatment. Volumetric BMD, as measured by pQCT, was significantly decreased in femoral midshaft (primarily cortical bone), but not metaphysis (primarily trabecular bone), of PAPP-A KO compared to WT mice with PTH treatment. These data suggest that stimulation of PAPP-A expression by intermittent PTH treatment contributes to PTH bone anabolism in mice.

PAPP-A affects tendon structure and mechanical properties

Pregnancy-associated plasma protein-A (PAPP-A) serves to increase local insulin-like growth factor (IGF) stimulation of proliferation and differentiation in many tissues through proteolysis of inhibitory IGF-binding proteins. The purpose of this study was to investigate the effects of PAPP-A on tendon structure and mechanical properties. A total of 30 tails from 6-month-old mice were tested with 10 tails in each of following groups: PAPP-A knockout (KO), skeletal-specific PAPP-A overexpressing transgenic (Tg) and wild type (WT). Morphologically, the total tail cross-sectional area (CSA), individual tissue CSAs of bone, muscle and tendon, and fascicle diameter were measured. A fascicle pullout test was performed to assess stiffness and strength of interfascicular structures. Fascicles were mechanically characterized through low and high displacement rate uniaxial tension tests providing modulus at each rate, hysteresis area and stress relaxation ratio. The KO mice had a smaller total tail CSA (p<0.05), fascicle diameter (p<0.05), absolute tendon CSA (p<0.05), fast and slow stiffness (p<0.05 for both) and larger hysteresis area (p<0.05) compared to WT and Tg mice. On the other hand, the Tg mice had a larger fascicle diameter (p<0.05), absolute tendon CSA (p<0.05), higher interfascicular strength and stiffness (p<0.05) and lower fascicular modulus at low displacement rates (p<0.05) compared to WT and KO mice. Tg mice also had larger total tail CSA area (p<0.05) and smaller hysteresis area (p<0.05) than KO mice, and larger normalized tendon CSA (p<0.05) than WT mice. Based on these data, we conclude that PAPP-A affects fascicle structure, thereby affecting tendon phenotype.

Kinetics of gene expression and bone remodelling in the clinical phase of collagen-induced arthritis

Introduction

Pathological bone changes differ considerably between inflammatory arthritic diseases and most studies have focused on bone erosion. Collagen-induced arthritis (CIA) is a model for rheumatoid arthritis, which, in addition to bone erosion, demonstrates bone formation at the time of clinical manifestations. The objective of this study was to use this model to characterise the histological and molecular changes in bone remodelling, and relate these to the clinical disease development.

Methods

A histological and gene expression profiling time-course study on bone remodelling in CIA was linked to onset of clinical symptoms. Global gene expression was studied with a gene chip array system.

Results

The main histopathological changes in bone structure and inflammation occurred during the first two weeks following the onset of clinical symptoms in the joint. Hereafter, the inflammation declined and remodelling of formed bone dominated.

Global gene expression profiling showed simultaneous upregulation of genes related to bone changes and inflammation in week 0 to 2 after onset of clinical disease. Furthermore, we observed time-dependent expression of genes involved in early and late osteoblast differentiation and function, which mirrored the histopathological bone changes. The differentially expressed genes belong to the bone morphogenetic pathway (BMP) and, in addition, include the osteoblast markers integrin-binding sialoprotein (Ibsp), bone gamma-carboxyglutamate protein (Bglap1), and secreted phosphoprotein 1 (Spp1). Pregnancy-associated protein A (Pappa) and periostin (Postn), differentially expressed in the early disease phase, are proposed to participate in bone formation, and we suggest that they play a role in early bone formation in the CIA model. Comparison to human genome-wide association studies (GWAS) revealed differential expression of several genes associated with human arthritis.

Conclusions

In the CIA model, bone formation in the joint starts shortly after onset of clinical symptoms, which results in bony fusion within one to two weeks. This makes it a candidate model for investigating the relationship between inflammation and bone formation in inflammatory arthritis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0531-7) contains supplementary material, which is available to authorized users.

Pregnancy-associated plasma protein A regulates mitosis and is epigenetically silenced in breast cancer

Aberrant mitosis is a common feature of cancer, yet little is known about the altered genes causing mitotic defects. We screened human tumours for cells with morphological signatures of highly specific mitotic defects previously assigned to candidate genes in a genome-wide RNA interference screen carried out in HeLa cells (www.mitocheck.org). We discovered a striking enrichment of early mitotic configurations indicative of prophase/prometaphase delay in breast cancer. Promoter methylation analysis of MitoCheck candidate genes assigned to the corresponding 'mitotic delay' class linked this defect to epigenetic silencing of the gene encoding pregnancy-associated plasma protein-A (PAPPA), a secreted protease. PAPPA silencing was highly prevalent in precursor lesions and invasive breast cancer. Experimental manipulation of PAPPA protein levels in human mammary epithelial cells and in breast cancer cell lines demonstrates that progression through early mitosis is dependent on PAPPA function, and that breast cancer cells become more invasive after down-regulation of this protease. PAPPA regulates mitotic progression through modulating the IGF-1 signalling pathway resulting in activation of the forkhead transcription factor FoxM1, which drives a transcriptional cluster of essential mitotic genes. Our results show that PAPPA has a critical function in normal cell division and is targeted early in breast cancer development.

Inducible knock out of pregnancy-associated plasma protein-a gene expression in the adult mouse: effect on vascular injury response

Pregnancy-associated plasma protein-A (PAPP-A) enhances local IGF signaling through its ability to proteolyze inhibitory IGF binding proteins. In vivo, PAPP-A (like IGF) appears to exhibit antagonistic pleiotropy; ie, it has beneficial effects early in life but detrimental effects later in life. Accordingly, PAPP-A knockout (KO) mice are born as proportional dwarfs and have diminished reproductive vigor and reduced peak bone mass acquisition at puberty. On the other hand, PAPP-A KO mice live approximately 30% longer than their wild-type littermates, with decreased incidence and severity of age-related diseases and resistance to adverse responses of vascular injury. To be able to distinguish the impact of PAPP-A deficiency in the adult from that in early life, we developed a mouse model suitable for inducible Cre recombinase-mediated excision of the PAPP-A gene. In this study, we characterize the conditional PAPP-A KO mouse model for efficacy of tamoxifen-induced floxed PAPP-A excision in various tissues of adult mice and demonstrate a significant (P = .0001) reduction of neointimal formation in these mice after unilateral carotid artery ligation.

Pregnancy associated plasma protein A2 (PAPP-A2) affects bone size and shape and contributes to natural variation in postnatal growth in mice

Pregnancy associated plasma protein A2 (PAPP-A2) is a protease of insulin-like growth factor binding protein 5 and is receiving increasing attention for its roles in pregnancy and postnatal growth. The goals of the present study were to characterize the effects of PAPP-A2 deletion on bone size and shape in mice at 10 weeks of age, and to determine whether Pappa2 is the gene responsible for a previously-identified quantitative trait locus (QTL) contributing to natural variation in postnatal growth in mice. Mice homozygous for constitutive PAPP-A2 deletion were lighter than wild-type littermates, and had smaller mandible dimensions and shorter skull, humerus, femur, tibia, pelvic girdle, and tail bone. Furthermore, PAPP-A2 deletion reduced mandible dimensions and the lengths of the skull, femur, pelvic girdle, and tail bone more than would be expected due to the effect on body mass. In addition to its effects on bone size, PAPP-A2 deficiency also altered the shape of the mandible and pelvic girdle, as assessed by geometric morphometrics. Mice homozygous for the PAPP-A2 deletion had less deep mandibles, and pelvic girdles with a more feminine shape. Using a quantitative complementation test, we confirmed that Pappa2 is responsible for the effects of the previously-identified QTL, demonstrating that natural variation in the Pappa2 gene contributes to variation in postnatal growth in mice. If similar functional variation in the Pappa2 gene exists in other species, effects of this variation on the shape of the pelvic girdle might explain the previously-reported associations between Pappa2 SNPs and developmental dysplasia of the hip in humans, and birthing in cattle.

The starvation hormone, fibroblast growth factor-21, extends lifespan in mice

Fibroblast growth factor-21 (FGF21) is a hormone secreted by the liver during fasting that elicits diverse aspects of the adaptive starvation response. Among its effects, FGF21 induces hepatic fatty acid oxidation and ketogenesis, increases insulin sensitivity, blocks somatic growth and causes bone loss. Here we show that transgenic overexpression of FGF21 markedly extends lifespan in mice without reducing food intake or affecting markers of NAD+ metabolism or AMP kinase and mTOR signaling. Transcriptomic analysis suggests that FGF21 acts primarily by blunting the growth hormone/insulin-like growth factor-1 signaling pathway in liver. These findings raise the possibility that FGF21 can be used to extend lifespan in other species.

DOI:http://dx.doi.org/10.7554/eLife.00065.001

In 1934, in a famous experiment at Cornell University, it was discovered that laboratory mice could live twice as long as expected if they were fed a low-calorie diet that included enough nutrients to avoid malnutrition. This phenomenon has since been observed in species ranging from worms to primates, but not in humans. Reducing calorie intake leads to longer lives by modifying a number of the biochemical pathways that sense nutrients, including pathways that involve insulin and various other biomolecules. Chemical and genetic methods can also increase longevity by modifying these pathways, which suggests that it might be possible to develop drugs that can increase lifespan without reducing calorie intake.

Mice, humans and other creatures respond to prolonged fasting through a number of adaptive changes that include mobilizing and burning fatty acids. The liver has an important role in this response, secreting a hormone called fibroblast growth factor-21 (FGF21) that coordinates these processes among tissues. Previous experiments on transgenic mice with high levels of this hormone have shown that it suppresses the activity of growth hormone and reduces the production of insulin-like growth factor, which prevents growth and can lead to hibernation-like behavior.

Here Zhang et al. compare groups of wild-type mice and transgenic mice with high levels of FGF21. They find that the transgenic mice have a longer median survival time than wild-type mice (38 months vs 28 months), and that the transgenic female mice on average live for 4 months longer than their male counterparts. However, unlike in other examples of increased longevity, they find that decreased food intake is not required. Instead, they find that transgenic mice eat more food than wild-type mice, yet remain profoundly insulin-sensitive. The results suggest that the longer survival times are caused by a reduction in the production of insulin-like growth factor, but they also suggest that the mechanism responsible for the increased longevity is independent of the three pathways that are usually associated with such increases. Further research is needed to understand this mechanism in greater detail and could, perhaps, pave the way for the use of FGF21-based hormone therapy to extend lifespan without the need for a low-calorie diet.

DOI:http://dx.doi.org/10.7554/eLife.00065.002

Key questions and answers about pregnancy-associated plasma protein-A

Twenty-five years after it was identified as a circulating protein of unknown function derived from the placenta, pregnancy-associated plasma protein-A (PAPP-A) was discovered to be a novel zinc metalloproteinase expressed by a variety of cell types. Great progress has been made in understanding the biology of PAPP-A and its regulation during recent years, especially in regard to physiological and pathophysiological inflammatory injury responses. However, much remains to be learned about this complex protein and its potential clinical implications outside pregnancy. In this article we address some of the outstanding questions about PAPP-A, in particular about its newly emerging role in the insulin-like growth factor (IGF) system.

Insulin-like growth factor (IGF)-I and IGF-II contribute differentially to the phenotype of pregnancy associated plasma protein-A knock-out mice

CONTEXT

Insulin-like growth factor (IGF) signaling is essential for achieving optimal body size during fetal development, peak bone mass during puberty, and maximal fecundity in the reproductive period. IGF-II is considered the main fetal IGF, whereas IGF-I is more important postnatally. Pregnancy-associated plasma protein-A (PAPP-A) enhances local IGF signaling through cleavage of inhibitory IGF binding proteins. Conversely, inhibition of PAPP-A results in reduced local IGF action. Thus, PAPP-A knock-out (KO) mice are born as proportional dwarfs due to the dysregulation of IGF-II signaling during early embryogenesis that impacts body size. Relaxation of IgfII imprinting through mutation of a reciprocally imprinted downstream gene, H19, which allowed transcription of IGF-II from the normally silent maternal allele, rescued the dwarf phenotype of PAPP-A KO mice.

OBJECTIVE

To determine the effect of increased IGF-II expression on postnatal phenotypes of PAPP-A KO mice.

DESIGN

Young adult wild-type (WT), PAPP-A KO, H19 mutant (ΔH19/WT) and ΔH19/PAPP-A KO mice were characterized for skeletal phenotype (peripheral quantitative computed tomography at the midshaft and distal metaphysis of the femur) and reproductive phenotype (time to first litter, time between litters, pups per litter).

RESULTS

Serum IGF-II levels were significantly increased in ΔH19/WT and ΔH19/PAPP-A KO mice compared to WT and PAPP-A KO mice; serum IGF-I levels were not affected by H19 mutation. PAPP-A KO mice had reductions in cortical thickness and in cortical and trabecular area, bone mineral content and bone mineral density compared to WT mice. There were no significant differences between PAPP-A KO and ΔH19/PAPP-A KO mice in any of the bone parameters. PAPP-A KO crossed with (×) PAPP-A KO had a longer time until first litter, normal time between subsequent litters, and significantly reduced number of pups per litter compared to WT×WT. ΔH19/PAPP-A KO×ΔH19/PAPP-A KO had an even longer time to first litter, but also longer time between litters. This phenotype was associated with female ΔH19/PAPP-A KO mice. Furthermore, these ΔH19/PAPP-A KO mouse mothers failed to care for their pups.

CONCLUSIONS

An increase in IGF-II expression did not rescue the skeletal and reproductive deficiencies associated with reduced local IGF-I signaling in PAPP-A KO mice. In addition, the data suggest a potential new role for genomic imprinting at the IgfII/H19 locus affecting maternal behavior.

Pregnancy-associated plasma protein-A2 (PAPP-A2): tissue expression and biological consequences of gene knockout in mice

Pregnancy-associated plasma protein-A2 (PAPP-A2) is a novel homolog of PAPP-A in the metzincin superfamily. However, compared with the accumulating data on PAPP-A, very little is known about PAPP-A2. In this study, we determined the tissue expression pattern of PAPP-A2 mRNA in wild-type (WT) mice and characterized the phenotype of mice with global PAPP-A2 deficiency. Tissues expressing PAPP-A2 in WT mice were more limited than those expressing PAPP-A. The highest PAPP-A2 mRNA expression was found in the placenta, with abundant expression in fetal, skeletal, and reproductive tissues. Heterozygous breeding produced the expected Mendelian distribution for the pappa2 gene and viable homozygous PAPP-A2 knockout (KO) mice that were normal size at birth. The most striking phenotype of the PAPP-A2 KO mouse was postnatal growth retardation. Male and female PAPP-A2 KO mice had 10 and 25-30% lower body weight, respectively, than WT littermates. Adult femur and body length were also reduced in PAPP-A2 KO mice, but without significant effects on bone mineral density. PAPP-A2 KO mice were fertile, but with compromised fecundity. PAPP-A expression was not altered to compensate for the loss of PAPP-A2 expression, and proteolysis of PAPP-A2's primary substrate, IGF-binding protein-5, was not altered in fibroblasts from PAPP-A2 KO embryos. In conclusion, tissue expression patterns and biological consequences of gene KO indicate distinct physiological roles for PAPP-A2 and PAPP-A in mice.

PAPP-A: a new anti-aging target?

This article focuses on the role of PAPP-A in mammalian aging. It introduces PAPP-A and a little of its history, briefly discusses the function of PAPP-A in the insulin-like growth factor (IGF) system and the regulators of PAPP-A expression, and then reviews data concerning PAPP-A in aging and age-related diseases especially in regard to the PAPP-A knockout (KO) mouse. The PAPP-A KO mouse is a valuable new model to test hypotheses concerning the control of the tissue availability of IGF, independent from systemic levels, on healthspan as well as lifespan.

Molecular genetic studies of gene identification for osteoporosis: the 2009 update

Osteoporosis is a complex human disease that results in increased susceptibility to fragility fractures. It can be phenotypically characterized using several traits, including bone mineral density, bone size, bone strength, and bone turnover markers. The identification of gene variants that contribute to osteoporosis phenotypes, or responses to therapy, can eventually help individualize the prognosis, treatment, and prevention of fractures and their adverse outcomes. Our previously published reviews have comprehensively summarized the progress of molecular genetic studies of gene identification for osteoporosis and have covered the data available to the end of September 2007. This review represents our continuing efforts to summarize the important and representative findings published between October 2007 and November 2009. The topics covered include genetic association and linkage studies in humans, transgenic and knockout mouse models, as well as gene-expression microarray and proteomics studies. Major results are tabulated for comparison and ease of reference. Comments are made on the notable findings and representative studies for their potential influence and implications on our present understanding of the genetics of osteoporosis.

Inactivation of insulin-like-growth factors diminished the anabolic effects of pregnancy-associated plasma protein-A (PAPP-A) on bone in mice

In vivo studies have provided ubiquitous evidence that pregnancy-associated plasma protein-A (PAPP-A) functions as a potent anabolic factor. While some evidence supports the prediction that increasing IGF bioavailability contributes to the anabolic effects of PAPP-A, definitive evidence has been lacking. This important issue has been addressed in this study using a unique mouse model in which PAPP-A was overexpressed in bone either alone or together with a protease-resistant IGFBP-4 analog (PRBP-4) which serves as an IGF inhibitor. PAPP-A transgenic mice exhibited a 25% increase in skull bone mineral density (BMD) whereas PRBP-4 transgenic mice showed a 20-25% decrease in this parameter at an age of 3months. Femur/tibia size-related parameters were significantly increased in PAPP-A transgenic mice but decreased in PRBP-4 transgenic mice. This data clearly demonstrates that PAPP-A transgenic mice exhibit opposite phenotypes in both flat bone and long bone compared to PRBP-4 transgenic mice which have reduced IGF bioavailability in bone. Importantly, PRBP-4 and PRBP-4/PAPP-A double transgenic mice shared essentially identical phenotypes in both flat and long bones. Calvarial thickness, skull BMD and long bone parameters were reduced to similar degrees in PRBP-4 and PRBP-4/PAPP-A transgenic mice relative to wild-type littermates. Our findings provide compelling evidence that PAPP-A increases bone formation primarily by increasing IGF bioavailability and that other alternative pathways may play a negligible role in mediating the anabolic effect of PAPPA in bone. This clear definition of PAPP-A's mechanism of action is critical for future translational studies on the therapeutic application of PAPP-A.

In vivo analysis of gene expression in long-lived mice lacking the pregnancy-associated plasma protein A (PappA) gene

Mice lacking the pregnancy-associated plasma protein A (PappA) gene exhibit diminished localized IGF-1 bioavailability and a 30% increase in mean life span. However, it is uncertain which tissues exhibit reduced IGF-1 signals in the PappA(-/-) mouse, and whether effects of this mutation parallel those of mutations that diminish IGF-1 in serum. Across a panel of 21 tissues, we used RT-PCR to evaluate the effects of the PappA(-/-) mutation on expression of Igfbp5, which served as an in vivo indicator of IGF-1 signaling. Among these tissues, expression of Igfbp5 was significantly reduced by PappA(-/-) only in kidney. A broader survey of IGF-associated genes in six organs identified five other genes responsive to PappA(-/-) in kidney, with stronger effects in this organ relative to other tissues. Renal expression of Irs1 and Mt1 was increased by PappA(-/-) as well as by mutations that reduce IGF-1 in serum (i.e., Ghr(-/-), Pit1(dw/dw) and Prop1(df/df)), and we demonstrate that expression of these genes is regulated by growth hormone-treatment and calorie restriction. These results provide in vivo data on an important new model of mammalian aging, and characterize both similar and contrasting expression patterns between long-lived mice with reduced local IGF-1 availability and diminished IGF-1 in serum.

Growth factor control of bone mass

Bone formation is determined by the number and function of osteoblasts. Cell number is governed by factors that regulate the replication and differentiation of pre-osteoblasts and factors that regulate osteoblastic cell death. Cell function is controlled by signals acting on the mature osteoblast. Platelet-derived and fibroblast growth factors are bone cell mitogens. Bone morphogenetic proteins (BMPs) and Wnt induce the differentiation of mesenchymal cells toward osteoblasts, and insulin-like growth factor (IGF)-I stimulates the function of mature osteoblasts and prevents their death. The activity of BMP, Wnt, and IGF-I is modulated by extracellular antagonists or binding proteins. Changes in growth factor synthesis and activity may play a role in the pathogenesis of selected forms of osteoporosis, and alterations in the expression or binding of the extracellular antagonists can be associated with changes in bone mass. Current approaches to bone anabolic therapies for osteoporosis include the administration of a growth factor, such as IGF-I, or the neutralization of an antagonist. Ideally, the targeting of an anabolic agent should be specific to bone to preclude non-skeletal unwanted side effects. Clinical trials are needed to determine the long-term effectiveness and safety of novel anabolic agents for the management of osteoporosis.

Role of the GH/IGF-1 axis in lifespan and healthspan: lessons from animal models

Animal models are fundamentally important in our quest to understand the genetic, epigenetic, and environmental factors that contribute to human aging. In comparison to humans, relatively short-lived mammals are useful models as they allow for rapid assessment of both genetic manipulation and environmental intervention as related to longevity. These models also allow for the study of clinically relevant pathologies as a function of aging. Data associated with more distant species offers additional insight and critical consideration of the basic physiological processes and molecular mechanisms that influence lifespan. Consistently, two interventions, caloric restriction and repression of the growth hormone (GH)/insulin-like growth factor-1/insulin axis, have been shown to increase lifespan in both invertebrates and vertebrate animal model systems. Caloric restriction (CR) is a nutrition intervention that robustly extends lifespan whether it is started early or later in life. Likewise, genes involved in the GH/IGF-1 signaling pathways can lengthen lifespan in vertebrates and invertebrates, implying evolutionary conservation of the molecular mechanisms. Specifically, insulin and insulin-like growth factor-1 (IGF-1)-like signaling and its downstream intracellular signaling molecules have been shown to be associated with lifespan in fruit flies and nematodes. More recently, mammalian models with reduced growth hormone (GH) and/or IGF-1 signaling have also been shown to have extended lifespans as compared to control siblings. Importantly, this research has also shown that these genetic alterations can keep the animals healthy and disease-free for longer periods and can alleviate specific age-related pathologies similar to what is observed for CR individuals. Thus, these mutations may not only extend lifespan but may also improve healthspan, the general health and quality of life of an organism as it ages. In this review, we will provide an overview of how the manipulation of the GH/IGF axis influences lifespan, highlight the invertebrate and vertebrate animal models with altered lifespan due to modifications to the GH/IGF-1 signaling cascade or homologous pathways, and discuss the basic phenotypic characteristics and healthspan of these models.

Growth hormone, insulin-like growth factors, and the skeleton

GH and IGF-I are important regulators of bone homeostasis and are central to the achievement of normal longitudinal bone growth and bone mass. Although GH may act directly on skeletal cells, most of its effects are mediated by IGF-I, which is present in the systemic circulation and is synthesized by peripheral tissues. The availability of IGF-I is regulated by IGF binding proteins. IGF-I enhances the differentiated function of the osteoblast and bone formation. Adult GH deficiency causes low bone turnover osteoporosis with high risk of vertebral and nonvertebral fractures, and the low bone mass can be partially reversed by GH replacement. Acromegaly is characterized by high bone turnover, which can lead to bone loss and vertebral fractures, particularly in patients with coexistent hypogonadism. GH and IGF-I secretion are decreased in aging individuals, and abnormalities in the GH/IGF-I axis play a role in the pathogenesis of the osteoporosis of anorexia nervosa and after glucocorticoid exposure.