Expression of the type I diabetes-associated gene LRP5 in macrophages, vitamin A system cells, and the Islets of Langerhans suggests multiple potential roles in diabetes

Genotype-Phenotype Spectrum of eyeGENE Patients With Familial Exudative Vitreoretinopathy: Novel Variants in Norrin/β-Catenin Signaling Pathway Genes

Purpose

To report the variants and genotype-phenotype correlations in patients with familial exudative vitreoretinopathy (FEVR) included in the eyeGENE database.

Methods

A retrospective study was conducted in a cohort of 122 eyeGENE patients from 114 families with FEVR. Clinical details and genetic test results were provided by referring clinicians and clinical laboratories in the eyeGENE network, respectively. Genotype and phenotype information was reviewed, and reported variants were reclassified.

Results

Genetic test reports of 50 probands revealed 52 variants in the four genes of the Norrin/β-catenin signaling pathway: LRP5, FZD4, TSPAN12, and NDP. Following variant reclassification, 35 of the reported variants were interpreted as pathogenic or likely pathogenic (12 in LRP5, 11 in FZD4, seven in TSPAN12 and five in NDP), providing a conclusive test result for nearly one-third (32%) of the probands. Among the reported variants, 18 were novel (34.6%) and two-thirds were missense. Retinal detachment was reported less in patients with variants in TSPAN12 (P = 0.017). One-third of the patients (33.3%) with an FZD4 variant had asymmetric findings. In contrast, asymmetry was less pronounced in patients with variants in TSPAN12 (11.1%).

Conclusions

This was one of the largest cohorts reviewed from North America, expanding the variant spectrum in FEVR. Among the eyeGENE FEVR patients, disease-associated variants in Norrin/β-catenin signaling pathway genes can explain one-third of the cohort. LRP5 and FZD4 variants were the most common. The genotype-phenotype correlations supported the phenotypic variability in FEVR.

Case report: exome sequencing identified mutations in the LRP5 and LGR4 genes in a case of osteoporosis with recurrent fractures and extraskeletal manifestations

Background

Genetic mutations have been reported in a number of bone disorders with or without extra-skeletal manifestations. The purpose of the present study was to investigate the genetic cause in a middle-aged woman with osteoporosis, recurrent fractures and extraskeletal manifestations.

Methods

A 56-year-old Indian woman presented to the clinic with complaints of difficulty in walking, recurrent fractures, limb bending, progressive skeletal deformities, and poor overall health. At the age of 37, she had experienced severe anemia with diarrhea, significant weight loss, knuckle pigmentation, and a significant loss of scalp hair. She had received multiple blood transfusions and parenteral iron supplementation with normalization of hemoglobin. Subsequently, she had premature menopause at the age of 37. She died at the age of 61 due to liver failure. Exome sequencing followed by Sanger sequencing were undertaken to identify the potential pathogenic mutations.

Results

Genetic investigation identified likely pathogenic mutations in the LRP5 and LGR4 genes. Out of the two mutations, the heterozygous mutation (c.1199C>T) in the LRP5 gene resulted in a non-synonymous substitution of alanine with valine at the 400th position, and the second mutation (c.1403A>C) in the LGR4 gene led to a non-synonymous substitution of tyrosine with serine at the 468th residue of the protein. The minor allele frequencies of the c.1199C>T (LRP5) substitution in the 1000 genomes and IndiGenomes databases are 0.0003 and 0.001, while the c.1403A>C (LGR4) substitution has not been reported in these databases. Various in silico prediction tools suggested LGR4 mutation to be pathogenic and LRP5 mutation to be likely pathogenic.

Conclusion

Heterozygous mutations in the LRP5 and LGR4 genes had additive deteriorative effects on BMD, resulting in recurrent fractures and bone deformities, and extended the effect to extraskeletal sites, contributing to the poor overall health in this patient.

Study on serum vitamin A level in patients with type 1 diabetes: A systematic review and meta-analysis

BACKGROUND

There is controversy about the relationship between type 1 diabetes and vitamin A (VA) levels in the body. Through meta-analysis, the results of related studies can be aggregated to more accurately estimate the relationship between type 1 diabetes mellitus (T1DM) patients and the level of VA in the body.

METHODS

Our purpose is to review the study to investigate VA levels in type T1DM patients and to provide recommendations for future studies. Until January 2024, we searched the National Library of Medicine (PubMed), Cochrane Library, Embase Databases, Web of Sciences, Scopus, China National Knowledge Infrastructure (CNKI), VIP databases (VIP) and WAN FANG databases. After a systematic search, 8 case-control studies were included in our meta-analysis. In the process of data inclusion and extraction, the 2 reviewers selected literature independently of each other. In this study, RevMan5.3 software was used for meta-analysis.

RESULTS

Eight case-control studies involving 689 participants were screened. The results after meta-analysis showed that there was a significant difference in serum VA between the patients with T1DM and the control group (standardized mean difference [SMD] = -0.82, 95% CI [-1.29, -0.36], P < .001, random effects model) with significant heterogeneity among these studies (P < .001, I2 = 84%). Similarly, the difference in the high-performance liquid chromatography (HPLC) subgroup on serum VA (SMD = -0.99, 95% CI [-1.60, -0.38], P = .002) as well as the difference in the countries of Asia and Europe subgroup on serum VA (SMD = -0.60, 95% CI [-1.15, -0.05], P = .03; SMD = -1.06, 95% CI [-1.88, -0.24], P = .01) were suggested to be statistically significant. A significant result was also observed in the National Diabetes Data Group (NDDG) criteria subgroup (SMD = -0.48, 95% CI [-0.85, -0.12]).

CONCLUSION

Serum VA levels seem to have decreased in T1DM patients. Further research is needed to strengthen this finding and clarify possible impact mechanisms.

Association between the LRP5 rs556442 gene polymorphism and the risks of NAFLD and CHD in a Chinese Han population

Background

Multiple studies have demonstrated the involvement of low-density lipoprotein receptor-related protein 5 (LRP5) in metabolism-related diseases. This study explored the relationship between the LRP5 rs556442 gene polymorphism and the risks of non-alcoholic fatty liver disease (NAFLD) and coronary heart disease (CHD) in a Chinese Han population.

Methods

This retrospective case–control study included 247 patients with NAFLD, 200 patients with CHD, 118 patients with both NAFLD and CHD, and 339 healthy controls from June 2018 to June 2019 at Qingdao Municipal Hospital. Basic information and clinical characteristics were collected for all subjects. The genotype and allele frequency of LRP5 rs556442 were determined.

Results

The genotype distributions of LRP5 rs556442 differed significantly between the CHD and NAFLD + CHD groups (P < 0.05). The LRP5 rs556442 GG genotype markedly promoted the risk of NAFLD in CHD patients [odds ratio (OR) = 2.857, 95% confidence interval (CI): 1.196–6.824, P = 0.018). After adjustment for sex, age, and body mass index (BMI), this association remained significant (OR = 3.252, 95% CI: 1.306–8.102, P = 0.011). In addition, the LRP5 rs556442 AA + AG genotype was associated with an increased BMI in obese NAFLD patients (OR = 1.526, 95% CI: 1.004–2.319, P = 0.048). However, after adjustment for sex and age, this association was no longer significant (OR = 1.504, 95% CI: 0.991–2.282, P = 0.055).

Conclusions

This study found that the LRP5 rs556442 GG genotype increased the risk of NAFLD in CHD patients and AA + AG genotype may be associated with an increased BMI in obese NAFLD patients among a Chinese Han population.

Trial registration ChiCTR, ChiCTR1800015426. Registered 28 March 2018—Retrospectively registered, http://www.chictr.org.cn/showproj.aspx?proj=26239.

Metabolism and Endocrine Disorders: What Wnt Wrong?

A fundamental question in cell biology underlies how nutrients are regenerated to maintain and renew tissues. Physiologically, the canonical Wnt signaling is a vital pathway for cell growth, tissue remodeling, and organ formation; pathologically, Wnt signaling contributes to the development of myriad human diseases such as cancer. Despite being the focus of intense research, how Wnt intersects with the metabolic networks to promote tissue growth and remodeling has remained mysterious. Our understanding of metabolism has been revolutionized by technological advances in the fields of chemical biology, metabolomics, and live microscopy that have now made it possible to visualize and manipulate metabolism in living cells and tissues. The application of these toolsets to innovative model systems have propelled the Wnt field into new realms at the forefront answering the most pressing paradigms of cell metabolism in health and disease states. Elucidating the basis of Wnt signaling and metabolism in a cell-type and tissue-specific manner will provide a powerful base of knowledge for both basic biomedical fields and clinician scientists, and has the promise to generate new, transformative therapies in disease and even processes of aging.

LRP5 and LRP6 in Wnt Signaling: Similarity and Divergence

The canonical Wnt/β-catenin signaling plays a fundamental role in regulating embryonic development, injury repair and the pathogenesis of human diseases. In vertebrates, low density lipoprotein receptor-related proteins 5 and 6 (LRP5 and LRP6), the single-pass transmembrane proteins, act as coreceptors of Wnt ligands and are indispensable for Wnt signal transduction. LRP5 and LRP6 are highly homologous and widely co-expressed in embryonic and adult tissues, and they share similar function in mediating Wnt signaling. However, they also exhibit distinct characteristics by interacting with different protein partners. As such, each of them possesses its own unique functions. In this review, we systematically discuss the similarity and divergence of LRP5 and LRP6 in mediating Wnt and other signaling in the context of kidney diseases. A better understanding of the precise role of LRP5 and LRP6 may afford us to identify and refine therapeutic targets for the treatment of a variety of human diseases.

Serum sclerostin and glucose homeostasis: No association in healthy men. Cross-sectional and prospective data from the EGIR-RISC study

INTRODUCTION

Sclerostin, an inhibitor of bone formation, has emerged as a potential negative regulator of glucose homeostasis. We aimed to investigate if serum sclerostin associates with insulin sensitivity, beta cell function, prediabetes or metabolic syndrome in healthy men.

MATERIALS AND METHODS

Serum sclerostin was measured in basal and insulin-stimulated samples from 526 men without diabetes from the RISC cohort study. An OGTT was performed at baseline and after 3 years. An IVGTT and a hyperinsulinaemic-euglycaemic clamp were performed at baseline. Insulin sensitivity was estimated by the oral glucose sensitivity index (OGIS) and the M-value relative to insulin levels. Beta cell function was assessed by the acute and total insulin secretion (ISR_tot) and by beta cell glucose sensitivity.

RESULTS

Serum sclerostin levels correlated positively with age but were similar in individuals with (n = 69) and without (n = 457) prediabetes or the metabolic syndrome. Serum sclerostin was associated with measures of neither insulin sensitivity nor beta cell function at baseline in age-adjusted analyses including all participants. However, baseline serum sclerostin correlated inversely with OGIS at follow-up in men without prediabetes (B: -0.29 (-0.57, -0.01) p = 0.045), and inversely with beta cell glucose sensitivity in men with prediabetes (B: -13.3 (-26.3, -0.2) p = 0.046). Associations between serum sclerostin and 3-year changes in measures of glucose homeostasis were not observed. Acute hyperinsulinemia suppressed serum sclerostin (p = 0.02), and this reduction correlated with OGIS and ISR_tot.

CONCLUSIONS

Overall, serum sclerostin was not associated with prediabetes, insulin sensitivity or insulin secretion in healthy men. The inverse relationship between serum sclerostin and insulin sensitivity at follow-up was weak and likely not of clinical relevance. The ability of insulin to reduce sclerostin, possibly promoting bone formation, needs to be clarified.

eys +/- ; lrp5 +/- Zebrafish Reveals Lrp5 Can Be the Receptor of Retinol in the Visual Cycle

Vision is essential for vertebrates including humans. Sustained vision is accomplished by retinoid metabolism, the “visual cycle,” where all-trans retinol (atROL) is incorporated into the retinal pigment epithelium (RPE) from photoreceptors presumably through decade-long missing receptor(s). Here, we show that the LDL-related receptor-5 (Lrp5) protein is linked to the retinol binding protein 1a (Rbp1a), the transporter of atROL in the visual cycle, by generating and analyzing the digenic eyes shut homolog^+/-; lrp5^+/− zebrafish, the same form of gene defect detected in a human case of inherited retinal degeneration. Global gene expression analysis followed by genetic study clarified that rbp1a played a role downstream of lrp5. Rbp1a protein was colocalized with Lrp5 protein at microvilli of RPE cells. Furthermore, Rbp1a directly bound to the C-terminal intracellular region of Lrp5 in vitro. Collectively, these results strongly suggest that Lrp5 is a potent candidate of the receptor of atROL in the visual cycle.

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eys^+/-; lrp5^+/− zebrafish showed mild photoreceptor degeneration

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Microarray analysis identified dramatical decrease of rbp1a expression

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Rbp1a protein was colocalized with Lrp5 protein at the microvilli of the RPE

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Rbp1a directly bound to the C-terminal intracellular region of Lrp5 in vitro

Vav1 Sustains the In Vitro Differentiation of Normal and Tumor Precursors to Insulin Producing Cells Induced by all-Trans Retinoic Acid (ATRA)

All-trans retinoic acid (ATRA) promotes the development and the function of insulin producing cells and induces partial differentiation of pancreatic tumor cells. A number of evidences clearly indicate that the ATRA mediated signaling may have a substantial role in therapeutic approaches based on restoration of functional β-cells. Among the proteins up-regulated by ATRA, Vav1 is involved in maturation and function of haematopoietic cells and is essential for retinoids induced differentiation of tumor promyelocytes. The presence of Vav1 in solid tissues, including pancreas, is considered ectopic and no role in the differentiation of human epithelial cells has so far been described. We demonstrated here that Vav1 sustains the maturation to β-cells of the normal precursors human Biliary Tree Stem/progenitor Cells (hBTSCs) induced by a differentiation medium containing ATRA and that, in the mature normal pancreas, insulin-producing cells express variable levels of Vav1. Using pancreatic ductal adenocarcinoma (PDAC)-derived cells, we also revealed that the ATRA induced up-modulation of Vav1 is essential for the retinoid-induced trans-differentiation of neoplastic cells into insulin producing cells. The results of this study identify Vav1 as crucial molecule in ATRA induced maturation of insulin producing cells and suggest this protein as a marker for new strategies ended to restore functional β-cells.

A novel role of LRP5 in tubulointerstitial fibrosis through activating TGF-β/Smad signaling

Previous studies by us and others demonstrated that activation of Wnt/β-catenin signaling plays a pathogenic role in chronic kidney diseases (CKD). Wnt co-receptor LRP5 variants are reported to associate with autosomal dominant polycystic kidney disease; but their exact roles in this disease and renal fibrosis have not been explored. Here, we observed the upregulation of LRP5 in the renal tubules of both type 1 and type 2 diabetic models and of an obstructive nephropathy model. In the obstructed kidneys, Lrp5 knockout significantly ameliorated tubulointerstitial fibrosis and tubular injury without changing Wnt/β-catenin signaling. Instead, decreased levels of TGF-β1 and TGF-β receptors (TβRs) were detected in Lrp5 knockout kidneys, followed by attenuated activation and nuclear translocation of Smad2/3 in the renal tubules, suggesting a regulatory effect of LRP5 on TGF-β/Smad signaling. In consistent with this hypothesis, LRP5 overexpression resulted in enhanced TGF-β/Smad signaling activation in renal tubule epithelial cells. Furthermore, LRP5 was co-immunoprecipitated with TβRI and TβRII, and its extracellular domain was essential for interacting with TβRs and for its pro-fibrotic activity. In addition to stabilizing TβRs, LRP5 increased the basal membrane presentation and TGF-β1-induced internalization of these receptors. Notably, TGF-β1 also induced LRP5 internalization. These findings indicate that LRP5 promotes tubulointerstitial fibrosis, at least partially, via direct modulation of TGF-β/Smad signaling, a novel, Wnt-independent function.

The low-density lipoprotein receptor-related protein 5 (LRP5) 4037C>T polymorphism: candidate for susceptibility to type 1 diabetes mellitus

OBJECTIVE

The present study has investigated the association between low-density lipoprotein receptor-related protein 5 (LRP5) 4037C>T polymorphism and type 1 diabetes mellitus (T1DM) susceptibility in a Brazilian population.

SUBJECTS AND METHODS

A total number of 134 T1DM patients and 180 normoglycemic individuals (NG) aged 6-20 years were studied. Glycated hemoglobin and glucose levels were determined. Genotyping of LRP5 4037C>T (rs3736228) was performed.

RESULTS

T1DM patients showed poor glycemic control. Genotypes in the codominant (CT: OR = 2.99 [CI 95%: 1.71-5.24], p < 0.001; TT: OR = 5.34 [CI 95%: 1.05-2702], p < 0.001), dominant (CT + TT: OR = 3.16 [CI 95%: 1.84-5.43], p < 0.001) and log-additive (OR = 2.78 [CI 95%: 1.70-4.52], p < 0.001) models, and LRP5 4037T allele (OR = 2.88, [CI 95%: 1.78-4.77], p < 0.001) were associated with an increased risk of developing T1DM. LRP5 4037CT and CT+TT carriers in T1DM group showed higher concentrations of serum glucose and glycated hemoglobin when compared with CC carriers.

CONCLUSION

The LRP5 4037C>T may represent a candidate for T1DM susceptibility, as well as poor glycemic control.

SVMRFE based approach for prediction of most discriminatory gene target for type II diabetes

Type II diabetes is a chronic condition that affects the way our body metabolizes sugar. The body's important source of fuel is now becoming a chronic disease all over the world. It is now very necessary to identify the new potential targets for the drugs which not only control the disease but also can treat it. Support vector machines are the classifier which has a potential to make a classification of the discriminatory genes and non-discriminatory genes. SVMRFE a modification of SVM ranks the genes based on their discriminatory power and eliminate the genes which are not involved in causing the disease. A gene regulatory network has been formed with the top ranked coding genes to identify their role in causing diabetes. To further validate the results pathway study was performed to identify the involvement of the coding genes in type II diabetes. The genes obtained from this study showed a significant involvement in causing the disease, which may be used as a potential drug target.

LRP5 variants may contribute to ADPKD

Mutations in Polycystic Kidney Disease proteins (PKD1 or PKD2) are causative for autosomal dominant polycystic kidney disease (ADPKD). However, a small subset of ADPKD probands do not harbor a mutation in any of the known genes. Low density lipoprotein Receptor-related Protein 5 (LRP5) was recently associated with hepatic cystogenesis in isolated polycystic liver disease (PCLD). Here, we demonstrate that this gene may also have a role in unlinked and sporadic ADPKD patients. In a cohort of 79 unrelated patients with adult-onset ADPKD, we identified a total of four different LRP5 variants that were predicted to be pathogenic by in silico tools. One ADPKD patient has a positive family history for ADPKD and variant LRP5 c.1680G>T; p.(Trp560Cys) segregated with the disease. Although also two PKD1 variants probably affecting protein function were identified, luciferase activity assays presented for three LRP5 variants significant decreased signal activation of canonical Wnt signaling. This study contributes to the genetic spectrum of ADPKD. Introduction of the canonical Wnt signaling pathway provides new avenues for the study of the pathophysiology.

Whole-exome sequencing reveals LRP5 mutations and canonical Wnt signaling associated with hepatic cystogenesis

Polycystic livers are seen in the rare inherited disorder isolated polycystic liver disease (PCLD) and are recognized as the most common extrarenal manifestation in autosomal dominant polycystic kidney disease. Hepatic cystogenesis is characterized by progressive proliferation of cholangiocytes, ultimately causing hepatomegaly. Genetically, polycystic liver disease is a heterogeneous disorder with incomplete penetrance and caused by mutations in PRKCSH, SEC63, PKD1, or PKD2. Genome-wide SNP typing and Sanger sequencing revealed no pathogenic variants in hitherto genes in an extended PCLD family. We performed whole-exome sequencing of DNA samples from two members. A heterozygous variant c.3562C > T located at a highly conserved amino acid position (p.R1188W) in the low density lipoprotein receptor-related protein 5 (LRP5) gene segregated with the disease (logarithm of odds score, 4.62) but was not observed in more than 1,000 unaffected individuals. Screening of LRP5 in a PCLD cohort identified three additional mutations in three unrelated families with polycystic livers (p.V454M, p.R1529S, and p.D1551N), again all undetected in controls. All variants were predicted to be damaging with profound structural effects on LRP5 protein domains. Liver cyst tissue and normal hepatic tissue samples from patients and controls showed abundant LRP5 expression by immunohistochemistry. Functional activity analyses indicated that mutant LRP5 led to reduced wingless signal activation. In conclusion, we demonstrate that germ-line LRP5 missense mutations are associated with hepatic cystogenesis. The findings presented in this study link the pathophysiology of PCLD to deregulation of the canonical wingless signaling pathway.

Monitoring neurodegeneration in diabetes using adult neural stem cells derived from the olfactory bulb

Introduction

Neurons have the intrinsic capacity to produce insulin, similar to pancreatic cells. Adult neural stem cells (NSCs), which give rise to functional neurons, can be established and cultured not only by intracerebral collection, which requires difficult surgery, but also by collection from the olfactory bulb (OB), which is relatively easy. Adult neurogenesis in the hippocampus (HPC) is significantly decreased in diabetes patients. As a result, learning and memory functions, for which the HPC is responsible, decrease.

Methods

In the present study, we compared the effect of diabetes on neurogenesis and insulin expression in adult NSCs. Adult NSCs were derived from the HPC or OB of streptozotocin-induced diabetic rats. Comparative gene-expression analyses were carried out by using extracted tissues and established adult NSC cultures from the HPC or OB in diabetic rats.

Results

Diabetes progression influenced important genes that were required for insulin expression in both OB- and HPC-derived cells. Additionally, we found that the expression levels of several genes, such as voltage-gated sodium channels, glutamate transporters, and glutamate receptors, were significantly different in OB and HPC cells collected from diabetic rats.

Conclusions

By using identified diabetes-response genes, OB NSCs from diabetes patients can be used during diabetes progression to monitor processes that cause neurodegeneration in the central nervous system (CNS). Because hippocampal NSCs and OB NSCs exhibited similar gene-expression profiles during diabetes progression, OB NSCs, which are more easily collected and established than HPC NSCs, may potentially be used for screening of effective drugs for neurodegenerative disorders that cause malignant damage to CNS functions.

Lrp5 and Lrp6 exert overlapping functions in osteoblasts during postnatal bone acquisition

The canonical Wnt signaling pathway is critical for skeletal development and maintenance, but the precise roles of the individual Wnt co-receptors, Lrp5 and Lrp6, that enable Wnt signals to be transmitted in osteoblasts remain controversial. In these studies, we used Cre-loxP recombination, in which Cre-expression is driven by the human osteocalcin promoter, to determine the individual contributions of Lrp5 and Lrp6 in postnatal bone acquisition and osteoblast function. Mice selectively lacking either Lrp5 or Lrp6 in mature osteoblasts were born at the expected Mendelian frequency but demonstrated significant reductions in whole-body bone mineral density. Bone architecture measured by microCT revealed that Lrp6 mutant mice failed to accumulate normal amounts of trabecular bone. By contrast, Lrp5 mutants had normal trabecular bone volume at 8 weeks of age, but with age, these mice also exhibited trabecular bone loss. Both mutants also exhibited significant alterations in cortical bone structure. In vitro differentiation was impaired in both Lrp5 and Lrp6 null osteoblasts as indexed by alkaline phosphatase and Alizarin red staining, but the defect was more pronounced in Lrp6 mutant cells. Mice lacking both Wnt co-receptors developed severe osteopenia similar to that observed previously in mice lacking β-catenin in osteoblasts. Likewise, calvarial cells doubly deficient for Lrp5 and Lrp6 failed to form osteoblasts when cultured in osteogenic media, but instead attained a chondrocyte-like phenotype. These results indicate that expression of both Lrp5 and Lrp6 are required within mature osteoblasts for normal postnatal bone development.

Low-density lipoprotein receptor-related protein 5 gene polymorphisms and genetic susceptibility to abdominal aortic aneurysm

BACKGROUND

Previous data showed decreased low-density lipoprotein receptor-related protein 5 (LRP5) gene expression in peripheral blood cells of abdominal aortic aneurysm (AAA) patients and an association between decreased expression of LRP5 and increased lipoprotein (a) [Lp(a)] levels in AAA. LRP5 gene is involved in bone, lipid, and glucose metabolism, and experimental studies showed that atherosclerotic lesions of ApoE:LRP5 double knockout mice were ~threefold greater than those in ApoE-knockout mice and were characterized by features of advanced atherosclerosis, with remarkable accumulation of foam cells and destruction of the internal elastic lamina. The aim of this study was to evaluate the role of polymorphisms in LRP5 gene in determining genetic susceptibility to AAA.

METHODS

A total of 423 AAA patients and 423 controls comparable for sex and age were genotyped for seven polymorphisms within the LRP5 (rs667126, rs3736228, rs4988300, rs3781590, rs312016, rs556442, rs627174) by TaqMan approach.

RESULTS

Two polymorphisms were significantly associated with AAA: rs4988300, carriers of the T allele in AAA (74.0% vs 65.3% in controls; P = .007); and rs3781590, carriers of the T allele in AAA (66.5% vs 57.4% in controls; P =.009). At the multiple logistic regression analysis, adjusted for age, sex, dyslipidemia, hypertension, smoking habit, and chronic obstructive pulmonary disease, rs4988300 and rs3781590 polymorphisms remained significant and independent determinants of AAA (OR, 1.62; 95% CI, 1.02-2.56; P = .040, and OR, 1.83; 95% CI, 1.17-2.85; P = .008, respectively). We confirmed that AAA patients had significantly higher Lp(a) levels than control subjects (180.0 mg/L vs 107.6 mg/L; P < .0001). The prevalence of patients with Lp(a) levels ≥ 300 mg/L was significantly higher in patient carriers of the rs4988300 T allele than in wild-type patients (42.6% vs 30.8%; P = .048).

CONCLUSIONS

Present data have identified rs4988300 and rs3781590 LPR5 polymorphisms as independent genetic markers of AAA and underlined the need to concentrate our effort in studying the role of these markers in AAA and of LRP5 gene in Lp(a) catabolism and AAA pathophysiology.

Evidence for somatic gene conversion and deletion in bipolar disorder, Crohn's disease, coronary artery disease, hypertension, rheumatoid arthritis, type-1 diabetes, and type-2 diabetes

BACKGROUND

During gene conversion, genetic information is transferred unidirectionally between highly homologous but non-allelic regions of DNA. While germ-line gene conversion has been implicated in the pathogenesis of some diseases, somatic gene conversion has remained technically difficult to investigate on a large scale.

METHODS

A novel analysis technique is proposed for detecting the signature of somatic gene conversion from SNP microarray data. The Wellcome Trust Case Control Consortium has gathered SNP microarray data for two control populations and cohorts for bipolar disorder (BD), cardiovascular disease (CAD), Crohn's disease (CD), hypertension (HT), rheumatoid arthritis (RA), type-1 diabetes (T1D) and type-2 diabetes (T2D). Using the new analysis technique, the seven disease cohorts are analyzed to identify cohort-specific SNPs at which conversion is predicted. The quality of the predictions is assessed by identifying known disease associations for genes in the homologous duplicons, and comparing the frequency of such associations with background rates.

RESULTS

Of 28 disease/locus pairs meeting stringent conditions, 22 show various degrees of disease association, compared with only 8 of 70 in a mock study designed to measure the background association rate (P < 10-9). Additional candidate genes are identified using less stringent filtering conditions. In some cases, somatic deletions appear likely. RA has a distinctive pattern of events relative to other diseases. Similarities in patterns are apparent between BD and HT.

CONCLUSIONS

The associations derived represent the first evidence that somatic gene conversion could be a significant causative factor in each of the seven diseases. The specific genes provide potential insights about disease mechanisms, and are strong candidates for further study.

No association between LRP5 gene polymorphisms and bone and obesity phenotypes in Chinese male-offspring nuclear families

AIM

To investigate the effect of low-density lipoprotein receptor-related protein 5 (LRP5) gene polymorphisms on bone and obesity phenotypes in young Chinese men.

METHODS

A total of 1244 subjects from 411 Chinese nuclear families were genotyped by using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique at the Q89R, N740N, and A1330V sites in the LRP5 gene. Bone mineral density (BMD) in the lumbar spine and the hip, total fat mass and total lean mass were measured using dual-energy X-ray absorptiometry. The association between LRP5 gene polymorphisms and peak BMD, body mass index (BMI), total fat mass, total lean mass and percentage of fat mass was assessed using a quantitative transmission disequilibrium test (QTDT).

RESULTS

No significant within-family associations were found between genotypes or haplotypes of the LRP5 gene and peak BMD, BMI, total fat mass, total lean mass and percentage of fat mass. The 1000 permutations that were subsequently simulated were in agreement with these within-family association results.

CONCLUSION

Our results suggest that common polymorphic variations of the LRP5 gene do not influence peak bone mass acquisition and obesity phenotypes in young Chinese men.

Low density lipoprotein receptor-related protein 5 (LRP5) mutations and osteoporosis, impaired glucose metabolism and hypercholesterolaemia

OBJECTIVE

Mutations in the low-density lipoprotein receptor-related protein 5 gene (LRP5) underlie osteoporosis-pseudoglioma syndrome. Animal models implicate a role for LRP5 in lipid and glucose homeostasis. The objective was to evaluate metabolic consequences of LRP5 mutations in humans.

DESIGN AND PATIENTS

Thirteen Finnish individuals with homozygous or heterozygous LRP5 mutations were assessed for bone health, glucose and lipid metabolism, and for serum serotonin concentration. Results were compared with findings in family members without mutations.

MEASUREMENTS

Bone mineral density (BMD), vertebral morphology, oral and intravenous glucose tolerance tests, lipid profile and serum serotonin concentrations.

RESULTS

Two individuals were homozygous for R570W, one compound heterozygous for R570W and R1036Q, and 10 were heterozygous (six for R570W, three for R1036Q and one for R925C). Subjects with two LRP5 mutations had multiple spinal fractures and low BMD. Subjects with one mutation had significantly lower median lumbar spine (P = 0.004) and femoral neck (P = 0.005) BMD Z-scores, and more often vertebral fractures than the 18 individuals without mutations. Of the 12 subjects with LRP5 mutation six had diabetes and one had impaired glucose tolerance. Intravenous glucose tolerance tests suggested impaired beta-cell function; no insulin resistance was observed. Prevalence of hypercholesterolaemia was similar in mutation positive and negative subjects. Serum serotonin concentrations showed a trend towards higher concentrations in subjects with LRP5 mutation.

CONCLUSIONS

We found high prevalence of osteoporosis and abnormal glucose metabolism in subjects with LRP5 mutation(s). Further studies are needed to establish the role of LRP5 in glucose and lipid metabolism.

TSPAN12 regulates retinal vascular development by promoting Norrin- but not Wnt-induced FZD4/beta-catenin signaling

Mutations in the genes encoding the Wnt receptor Frizzled-4 (FZD4), coreceptor LRP5, or the ligand Norrin disrupt retinal vascular development and cause ophthalmic diseases. Although Norrin is structurally unrelated to Wnts, it binds FZD4 and activates the canonical Wnt pathway. Here we show that the tetraspanin Tspan12 is expressed in the retinal vasculature, and loss of Tspan12 phenocopies defects seen in Fzd4, Lrp5, and Norrin mutant mice. In addition, Tspan12 genetically interacts with Norrin or Lrp5. Overexpressed TSPAN12 associates with the Norrin-receptor complex and significantly increases Norrin/beta-catenin but not Wnt/beta-catenin signaling, whereas Tspan12 siRNA abolishes transcriptional responses to Norrin but not Wnt3A in retinal endothelial cells. Signaling defects caused by Norrin or FZD4 mutations that are predicted to impair receptor multimerization are rescued by overexpression of TSPAN12. Our data indicate that Norrin multimers and TSPAN12 cooperatively promote multimerization of FZD4 and its associated proteins to elicit physiological levels of signaling.

The Wnt receptor, Lrp5, is expressed by mouse mammary stem cells and is required to maintain the basal lineage

BACKGROUND

Ectopic Wnt signaling induces increased stem/progenitor cell activity in the mouse mammary gland, followed by tumor development. The Wnt signaling receptors, Lrp5/6, are uniquely required for canonical Wnt activity. Previous data has shown that the absence of Lrp5 confers resistance to Wnt1-induced tumor development.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we show that all basal mammary cells express Lrp5, and co-express Lrp6 in a similar fashion. Though Wnt dependent transcription of key target genes is relatively unchanged in mammary epithelial cell cultures, the absence of Lrp5 specifically depletes adult regenerative stem cell activity (to less than 1%). Stem cell activity can be enriched by >200 fold (over 80% of activity), based on high Lrp5 expression alone. Though Lrp5 null glands have apparent normal function, the basal lineage is relatively reduced (from 42% basal/total epithelial cells to 22%) and Lrp5-/- mammary epithelial cells show enhanced expression of senescence-associated markers in vitro, as measured by expression of p16(Ink4a) and TA-p63.

CONCLUSIONS/SIGNIFICANCE

This is the first single biomarker that has been demonstrated to be functionally involved in stem cell maintenance. Together, these results demonstrate that Wnt signaling through Lrp5 is an important component of normal mammary stem cell function.

LRP5 sequence and polymorphisms in the baboon

BACKGROUND

LRP5 is known to have an important relationship with bone density and a variety of other biological processes. Mapping to human chromosome 11q13.2, LRP5 shows considerable evolutionary conservation. Orthologs of this gene exist in many species, although comparison of human LRP5 with other non-human primates has not been performed until now.

METHODS

We reported the complementary DNA (cDNA) sequence and deduced amino acid sequence for baboon LRP5, and compared the baboon and human sequences. cDNA sequences for 21 baboons were examined to identify single-nucleotide polymorphisms (SNPs).

RESULTS

Sequences of coding regions in human and baboon LRP5 showed 97- 99% homology. Twenty-five SNPs were identified in the coding region of baboon LRP5.

CONCLUSION

The observed degree of coding sequence homology in LRP5 led us to expect that the baboon may serve as a useful model for future research into the role(s) of this gene in primate metabolic diseases.

The WNT signalling pathway and diabetes mellitus

The WNT signalling pathway is involved in many physiological and pathophysiological activities. WNT ligands bind to Frizzled receptors and co-receptors (LDL receptor-related protein 5/6), triggering a cascade of signalling events. The major effector of the canonical WNT signalling pathway is the bipartite transcription factor beta-catenin/T cell transcription factor (beta-cat/TCF), formed by free beta-cat and one of the four TCFs. The WNT pathway is involved in lipid metabolism and glucose homeostasis, and mutations in LRP5 may lead to the development of diabetes and obesity. beta-Cat/TCF is also involved in the production of the incretin hormone glucagon-like peptide-1 in the intestinal endocrine L cells. More recently, genome-wide association studies have identified TCF7L2 as a diabetes susceptibility gene, and individuals carrying certain TCF7L2 single nucleotide polymorphisms could be more susceptible to the development of type 2 diabetes. Furthermore, beta-cat is able to interact with forkhead box transcription factor subgroup O (FOXO) proteins. Since FOXO and TCF proteins compete for a limited pool of beta-cat, enhanced FOXO activity during ageing and oxidative stress may attenuate WNT-mediated activities. These observations shed new light on the pathogenesis of type 2 diabetes as an age-dependent disease.

Association of low-density lipoprotein receptor-related protein 5 (LRP5) promoter SNP with peak bone mineral density in Chinese women

OBJECTIVE

Low-density lipoprotein receptor-related protein 5 (LRP5) is important for osteoblast differentiation and mutations of the gene are associated with both low and high bone mass syndromes. Our study aimed to evaluate the importance of LRP5 in the determination of peak bone mass acquisition in Chinese females in the general population.

METHODS

A total of 286 young southern Chinese females (aged 22-44 years) with low bone mineral density (BMD) (defined by a BMD Z score < or =-1.28 at either the hip or spine) or high BMD (Z score > or =+1) were studied. The LRP5 gene was sequenced for single nucleotide polymorphisms (SNPs) and 4 SNPs were tagged from 8 genotyped SNPs for this study.

RESULTS

Single locus allele association tests revealed significant associations of rs682429 and rs686921 with BMD variation (p < 0.05). Omnibus test (likelihood ratio test) revealed overall significant association between LRP5 gene locus and total hip BMD, with rs682429 being most predictive. rs682429 is located in 5'UTR, 2 bases adjacent to a consensus recognition site for the Elk-1 binding element.

CONCLUSION

Common variations of the LRP5 promoter are associated with BMD in young women. These significant associations appear to be driven by rs682429. Functional studies are necessary to elucidate the role of this SNP on the effect of Elk-1 binding element transcriptional activity of LRP5 gene.

Low-density lipoprotein receptor-related protein-5 C/T polymorphism in exon 18 is associated with C peptide and proinsulin levels in control women and patients with polycystic ovary syndrome

OBJECTIVE

To assess the previously unstudied potential role of C/T (A1330V) polymorphism of the low-density lipoprotein receptor-related protein-5 gene in insulin sensitivity and secretion in polycystic ovary syndrome. The low-density lipoprotein receptor-related protein-5 gene has been found to play a role in determining insulin secretion in animal models.

DESIGN

Case-control study.

SETTING

Tertiary outpatient clinic.

PATIENT(S)

Women with polycystic ovary syndrome (n = 299; age, 27.5 +/- 7.1 y [mean +/- SD]), according to the European Society of Human Reproduction and Embryology criteria, as well as healthy control women (n = 187, age, 28.9 +/- 9.8 y).

INTERVENTION(S)

Oral glucose tolerance test, blood sampling.

MAIN OUTCOME MEASURE(S)

Glucose, insulin, C peptide, proinsulin during oral glucose tolerance tests, and lipids. Genotyping of C/T (A1330V) polymorphism by polymerase chain reaction-restriction fragment length polymorphism.

RESULT(S)

There was no difference in the frequency of genotypes between women with polycystic ovary syndrome (CC/CT/TT: 80.3%, 18.4%, 1.3%) and the control women (79.1%, 19.8%, and 1.1%). Carriers of the T allele had statistically significantly higher basal and stimulated C peptide and proinsulin levels than CC homozygotes, both basally and at the 180th minute. Regarding insulin sensitivity, there was no difference between T carriers and CC homozygotes.

CONCLUSION(S)

Polymorphism of C/T in the low-density lipoprotein receptor-related protein-5 gene is associated with C-peptide and proinsulin secretion but does not influence insulin sensitivity in either healthy women or women with polycystic ovary syndrome.

Regulated proteolytic processing of LRP6 results in release of its intracellular domain

Low-density lipoprotein receptor-related protein 6 (LRP6) is a member of low-density lipoprotein receptor (LDLR) family which cooperates with Frizzled receptors to transduce the canonical Wnt signal. As a critical component of the canonical Wnt pathway, LRP6 is essential for appropriate brain development, however, the mechanism by which LRP6 facilitates Wnt canonical signaling has not been fully elucidated. Interestingly, LRP6 which lacks its extracellular domain can constitutively activate TCF/LEF and potentiate the Wnt signal. Further, the free cytosolic tail of LRP6 interacts directly with glycogen synthase kinase (GSK3) and inhibits GSK3's activity in the Wnt canonical pathway which results in increased TCF/LEF activation. However, whether these truncated forms of LRP6 are physiologically relevant is unclear. Recent studies have shown that other members of the LDLR family undergo gamma-secretase dependent regulated intramembrane proteolysis (RIP). Using independent experimental approaches, we show that LRP6 also undergoes RIP. The extracellular domain of LRP6 is shed and released into the surrounding milieu and the cytoplasmic tail is cleaved by gamma-secretase-like activity to release the intracellular domain. Furthermore, protein kinase C, Wnt 3a and Dickkopf-1 modulate this process. These findings suggest a novel mechanism for LRP6 in Wnt signaling: induction of ectodomain shedding of LRP6, followed by the gamma-secretase involved proteolytic releasing its intracellular domain (ICD) which then binds to GSK3 inhibiting its activity and thus activates the canonical Wnt signaling pathway.

A family with osteoporosis pseudoglioma syndrome due to compound heterozygosity of two novel mutations in the LRP5 gene

Osteoporosis pseudoglioma syndrome (OPPG) is an autosomal recessive disorder due to mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) gene. Here, we report two novel missense mutations found in a southern Chinese family of a non-consanguineous marriage. Three out of four children had blindness, low bone mineral density (BMD) and multiple fractures in their childhood. Genotyping by DNA sequencing demonstrated 2 new mutations in exon 7 of the LRP5 gene. Tryptophans at amino acid residue positions 478 and 504 were replaced by arginine (W478R) and cysteine (W504C), respectively. While the parents that possessed either heterozygous W478R or W504C were apparently normal, all affected subjects were compound heterozygotes for the W478R and W504C mutations in the LRP5 gene. W478R is located immediately C-terminal to the third YWTD repeat of the second YWTD/EGF domain in LRP5, while W504C is located between the third and the fourth YWTD repeats of the second YWTD/EGF domain in LRP5. Using LRP5-related proteins, such as the low-density lipoprotein receptor (LDLR) and nidogen as reference models, a homology model of LRP5 suggested that the observed mutations may affect the molecular interactions of LRP5 and so lead to the observed OPPG phenotypes.

Analysis of copy number variation in the normal human population within a region containing complex segmental duplications on 22q11 using high-resolution array-CGH

A previously detected copy number polymorphism (Ep CNP) in patients affected with neuroectodermal tumors led us to investigate its frequency and length in the normal population. For this purpose, a program called Sequence Allocator was developed and applied for the construction of an array that consisted of unique and duplicated fragments, allowing the assessment of copy number variation within regions of segmental duplications. The average resolution of this array was 11 kb and we determined the size of the Ep CNP to be 290 kb. Analysis of normal controls identified 7.7 and 7.1% gains in peripheral blood and lymphoblastoid cell line (LCL) DNA, respectively, while deletions were found only in the LCL group (7.1%). This array platform allows the detection of DNA copy number variation within regions of pronounced genomic complexity, which constitutes an improvement over available technologies.

Polymorphisms of the low-density lipoprotein receptor-related protein 5 (LRP5) gene are associated with obesity phenotypes in a large family-based association study

BACKGROUND

The low-density lipoprotein receptor-related protein 5 (LRP5) gene, essential for glucose and cholesterol metabolism, may have a role in the aetiology of obesity, an important risk factor for diabetes.

PARTICIPANTS AND METHODS

To investigate the association between LRP5 polymorphisms and obesity, 27 single-nucleotide polymorphisms (SNPs), spacing about 5 kb apart on average and covering the full transcript length of the LRP5 gene, were genotyped in 1873 Caucasian people from 405 nuclear families. Obesity (defined as body mass index (BMI) >30 kg/m(2)) and three obesity-related phenotypes (BMI, fat mass and percentage of fat mass (PFM)) were investigated.

RESULTS

Single markers (12 tagging SNPs and 4 untaggable SNPs) and haplotypes (5 blocks) were tested for associations, using family-based designs. SNP4 (rs4988300) and SNP6 (rs634008) located in block 2 (intron 1) showed significant associations with obesity and BMI after Bonferroni correction (SNP4: p<0.001 and p = 0.001, respectively; SNP6: p = 0.002 and 0.003, respectively). The common allele A for SNP4 and minor allele G for SNP6 were associated with an increased risk of obesity. Significant associations were also observed between common haplotype A-G-G-G of block 2 with obesity, BMI, fat mass and PFM with global empirical values p<0.001, p<0.001, p = 0.003 and p = 0.074, respectively. Subsequent sex-stratified analyses showed that the association in the total sample between block 2 and obesity may be mainly driven by female subjects.

CONCLUSION

Intronic variants of the LRP5 gene are markedly associated with obesity. We hypothesise that such an association may be due to the role of LRP5 in the WNT signalling pathway or lipid metabolism. Further functional studies are needed to elucidate the exact molecular mechanism underlying our finding.

Genetic disorders of the LRP5-Wnt signalling pathway affecting the skeleton

Osteoporosis is a common, increasingly prevalent and potentially debilitating condition of men and women. Genetic factors are major determinants of bone mass and the risk of fracture, but few genes have been definitively demonstrated to be involved. The identification of these factors will provide novel insights into the processes of bone formation and loss and thus the pathogenesis of osteoporosis, enabling the rational development of novel therapies. In this article, we present the extensive genetic and functional data indicating that the LRP5 gene and the Wnt signalling pathway are key players in bone formation and the risk of osteoporosis, and that LRP5 signalling is essential for normal morphology, developmental processes and bone health.

LRP5 gene polymorphisms predict bone mass and incident fractures in elderly Australian women

Postmenopausal osteoporosis and bone mass are influenced by multiple factors including genetic variation. The importance of LDL receptor-related protein 5 (LRP5) for the regulation of bone mass has recently been established, where loss of function mutations is followed by severe osteoporosis and gain of function is related to increased bone mass. The aim of this study was to evaluate the role of polymorphisms in the LRP5 gene in regulating bone mass and influencing prospective fracture frequency in a well-described, large cohort of normal, ambulatory Australian women. A total of 1301 women were genotyped for seven different single nucleotide polymorphisms (SNPs) within the LRP5 gene of which five were potentially informative. The effects of these gene polymorphisms on calcaneal quantitative ultrasound measurements (QUS), osteodensitometry of the hip and bone-related biochemistry was examined. One SNP located in exon 15 was found to be associated with fracture rate and bone mineral density. Homozygosity for the less frequent allele of c.3357 A > G was associated with significant reduction in bone mass at most femoral sites. The subjects with the GG genotype, compared to the AA/AG genotypes showed a significant reduction in BUA and total hip, femoral neck and trochanter BMD (1.5% P = 0.032; 2.7% P = 0.047; 3.6% P = 0.008; 3.1% P = 0.050, respectively). In the 5-year follow-up period, 227 subjects experienced a total of 290 radiologically confirmed fractures. The incident fracture rate was significantly increased in subjects homozygous for the GG polymorphism (RR of fracture = 1.61, 95% CI [1.06-2.45], P = 0.027). After adjusting for total hip BMD, the fracture rate was still increased (RR = 1.67 [1.02-2.78], P = 0.045), indicating factors other than bone mass are of importance for bone strength. In conclusion, genetic variation in LRP5 seems to be of importance for regulation of bone mass and osteoporotic fractures.

Role of the intracellular domains of LRP5 and LRP6 in activating the Wnt canonical pathway

LDL-receptor related proteins 5 and 6 (LRP5/6) are co-receptors of Frizzled receptors that mediate Wnt-induced activation of the transcription factor family TCF/LEF-1. Even though LRP5 and LRP6 are highly homologous, LRP6, but not LRP5, is expressed primarily in the nervous system and deletion of the LRP6 gene results in significant brain abnormalities, while deletion of LRP5 results in primarily decreased bone density. Additionally, the exact function of LRP5 and LRP6 have not been clearly defined, although it is clear that they both play key roles in the Wnt canonical pathway. In this study the role of the intracellular domains of LRP5/6 in mediating Wnt signaling was examined. In the absence of exogenous Wnt 3a, full-length (FL) LRP6, but not LRP5, increased TCF/LEF-1 transcriptional activity, however both significantly potentiated Wnt 3a-induced TCF/LEF-1 activation. In contrast to the findings with the FL constructs, the intracellular domains (membrane-anchored and cytosolic) of both LRP5 and LRP6 significantly increased TCF/LEF-1 activation in the absence of Wnt 3a, and potentiated the Wnt 3a-induced decrease in beta-catenin phosphorylation, increase in free beta-catenin levels and the increase in TCF/LEF-1 activity. These findings demonstrate that: (1) LRP5 and LRP6 differentially modulate TCF/LEF-1 activation in the absence of Wnt 3a and (2) the intracellular C-terminal domains of LRP5/6 potentiate Wnt 3a-induced TCF/LEF-1 activation whether or not they are membrane-anchored. These findings provide significant new insights into the roles of LRP5/6 in modulating canonical Wnt signaling.

LDL receptor-related protein LRP6 regulates proliferation and survival through the Wnt cascade in vascular smooth muscle cells

Initial studies have established expression of low-density lipoprotein (LDL) receptor-related protein 6 (LRP6) in vascular smooth muscle cells (VSMCs). We hypothesized that LRP6 is a critical mediator governing the regulation of the canonical Wnt/β-catenin/T cell factor 4 (Tcf-4) cascade in the vasculature. This hypothesis was based on our previous work demonstrating a role for the β-catenin/Tcf-4 pathway in vascular remodeling as well as work in other cell systems establishing a role for LRP family members in the Wnt cascade. In line with our hypothesis, LRP6 upregulation significantly increased Wnt-1-induced Tcf activation. Moreover, a dominant interfering LRP6 mutant lacking the carboxyl intracellular domain (LRP6ΔC) abolished Tcf activity. LRP6-induced stimulation of Tcf was blocked in VSMCs harboring constitutive expression of a dominant negative Tcf-4 transgene lacking the β-catenin binding domain, suggesting that LRP6-induced activation of Tcf was mediated through a β-catenin-dependent signal. Expression of the dominant interfering LRP6ΔC transgene was sufficient to abolish the Wnt-induced survival as well as cyclin D1 activity and cell cycle progression. In conclusion, these findings provide the first evidence of a role for an LDL receptor-related protein in the regulation of VSMC proliferation and survival through the evolutionary conserved Wnt signaling cascade.

Decreased BMD and limb deformities in mice carrying mutations in both Lrp5 and Lrp6

Humans and mice lacking Lrp5 have low BMD. To evaluate whether Lrp5 and Lrp6 interact genetically to control bone or skeletal development, we created mice carrying mutations in both Lrp5 and the related gene Lrp6. We found that compound mutants had dose-dependent deficits in BMD and limb formation, suggesting functional redundancy between these two genes in bone and limb development.

INTRODUCTION

Lrp5 and Lrp6 are closely related members of the low density lipoprotein receptor family and are co-receptors for Wnt ligands. While Lrp5 mutations are associated with low BMD in humans and mice, the role of Lrp6 in bone formation has not been analyzed.

MATERIALS AND METHODS

To address whether Lrp5 and Lrp6 play complimentary roles in bone and skeletal development, we created mice with mutations in both genes. We inspected limbs of mice from the different genotypic classes of compound mutants to identify abnormalities. DXA and muCT were used to evaluate the effect of mutations in Lrp5 and Lrp6 on BMD and microarchitecture.

RESULTS

Mice heterozygous for mutations in Lrp6 and either heterozygous or homozygous for a mutation in Lrp5 (Lrp6(+/-);Lrp5(+/-) or Lrp6(+/-);Lrp5(-/-)) display limb defects with incomplete penetrance and variable expression. DXA analysis showed that BMD decreased as mice progressively were more deficient in Lrp5 and Lrp6. Lrp6(+/-);Lrp5(-/-) mice were more severely affected than Lrp6(+/+);Lrp5(-/-) mice, whereas Lrp6(+/-);Lrp5(+/-) mice had statistically higher BMD than Lrp6(+/+);Lrp5(-/-) mice and lower BMD compared with wildtype mice and mice heterozygous for either mutation alone.

CONCLUSIONS

Lrp6 and Lrp5 genetically interact in limb development in mice. Furthermore, heterozygosity for an inactivating mutation in Lrp6 further reduces BMD in both male and female mice lacking Lrp5.

Expression of voltage-gated potassium channels in human and rhesus pancreatic islets

Voltage-gated potassium channels (Kv channels) are involved in repolarization of excitable cells. In pancreatic beta-cells, prolongation of the action potential by block of delayed rectifier potassium channels would be expected to increase intracellular free calcium and to promote insulin release in a glucose-dependent manner. However, the specific Kv channel subtypes responsible for repolarization in beta-cells, most importantly in humans, are not completely resolved. In this study, we have investigated the expression of 26 subtypes from Kv subfamilies in human islet mRNA. The results of the RT-PCR analysis were extended by in situ hybridization and/or immunohistochemical analysis on sections from human or Rhesus pancreas. Cell-specific markers were used to show that Kv2.1, Kv3.2, Kv6.2, and Kv9.3 are expressed in beta-cells, that Kv3.1 and Kv6.1 are expressed in alpha-cells, and that Kv2.2 is expressed in delta-cells. This study suggests that more than one Kv channel subtype might contribute to the beta-cell delayed rectifier current and that this current could be formed by heterotetramers of active and silent subunits.

Linkage and association mapping of the LRP5 locus on chromosome 11q13 in type 1 diabetes

Linkage of chromosome 11q13 to type 1 diabetes (T1D) was first reported from genome scans (Davies et al. 1994; Hashimoto et al. 1994) resulting in P <2.2 x 10(-5) (Luo et al. 1996) and designated IDDM4 ( insulin dependent diabetes mellitus 4). Association mapping under the linkage peak using 12 polymorphic microsatellite markers suggested some evidence of association with a two-marker haplotype, D11S1917*03-H0570POLYA*02, which was under-transmitted to affected siblings and over-transmitted to unaffected siblings ( P=1.5 x 10(-6)) (Nakagawa et al. 1998). Others have reported evidence for T1D association of the microsatellite marker D11S987, which is approximately 100 kb proximal to D11S1917 (Eckenrode et al. 2000). We have sequenced a 400-kb interval surrounding these loci and identified four genes, including the low-density lipoprotein receptor related protein (LRP5) gene, which has been considered as a functional candidate gene for T1D (Hey et al. 1998; Twells et al. 2001). Consequently, we have developed a comprehensive SNP map of the LRP5 gene region, and identified 95 SNPs encompassing 269 kb of genomic DNA, characterised the LD in the region and haplotypes (Twells et al. 2003). Here, we present our refined linkage curve of the IDDM4 region, comprising 32 microsatellite markers and 12 SNPs, providing a peak MLS=2.58, P=5 x 10(-4), at LRP5 g.17646G>T. The disease association data, largely focused in the LRP5 region with 1,106 T1D families, provided no further evidence for disease association at LRP5 or at D11S987. A second dataset, comprising 1,569 families from Finland, failed to replicate our previous findings at LRP5. The continued search for the variants of the putative IDDM4 locus will greatly benefit from the future development of a haplotype map of the genome.

Localization of PPARdelta in murine central nervous system: expression in oligodendrocytes and neurons

The peroxisome proliferator-activated receptors (PPARs), PPARdelta, PPARgamma and PPARalpha, comprise a subclass of the supergene family of nuclear receptors. As such they are ligand-regulated transcription factors whose major effects are mediated by altering expression of target genes. PPARdelta has been shown to be ubiquitously expressed in mammals. However, its primary biological role(s) has yet to be defined. Several recent studies have demonstrated that PPARdelta is the most highly expressed PPAR isoform in the central nervous system, but ambiguity still exists as to the specific brain sub-regions and cells in which it is expressed. Here, utilizing novel, isoform-selective PPARdelta riboprobes and an anti-peptide antibody, we performed a series of in situ hybridization and immunolocalization studies to determine the distribution of PPARdelta in the central nervous system (CNS) of mice. We found that PPARdelta mRNA and protein is expressed throughout the brain, with particularly high levels in the entorhinal cortex, hypothalamus and hippocampus, and lower levels in the corpus callosum and caudate putamen. At the cellular level, PPARdelta mRNA and protein were found to be expressed in oligodendrocytes and neurons but not astrocytes. Such results suggest a role for PPARdelta in both myelination and neuronal functioning within the CNS.

Axin and the Axin/Arrow-binding protein DCAP mediate glucose-glycogen metabolism

Axin was found as a negative regulator of the canonical Wnt pathway. Human LRP5 was originally found as a candidate gene of insulin dependent diabetes mellitus (IDDM), but its Drosophila homolog, Arrow, works as a co-receptor of the canonical Wnt signal. In our previous paper, we found a new Drosophila Axin (Daxin)-binding SH3 protein, DCAP, a homolog of mammalian CAV family protein. Among the subtypes, DCAPL3 shows significant homology with CAP, an essential component of glucose transport in insulin signal. Further binding assay revealed that DCAP binds to not only Axin but also Arrow, and Axin binds to not only GSK3beta but also Arrow. However, overexpression and RNAi experiments of DCAP do not affect the canonical Wnt pathway. As DCAP is expressed predominantly in insulin-target organs, and as RNAi of DCAP disrupts the pattern of endogenous glycogen accumulation in late stage embryos, we suggest that DCAP is also involved in glucose transport. Moreover, early stage embryos lacking maternal Axin show significant delay of initial glycogen decomposition, and RNAi of Axin in S2 cells revealed quite increase of endogenous glycogen level as well as GSK3beta. These results suggest that Axin and DCAP mediate glucose-glycogen metabolism in embryo. In addition, the interaction among Axin, Arrow, and DCAP implies a possible cross-talk between Wnt signal and insulin signal.

Low-density lipoprotein receptor-related protein 5 (LRP5) is essential for normal cholesterol metabolism and glucose-induced insulin secretion

A Wnt coreceptor low-density lipoprotein receptor-related protein 5 (LRP5) plays an essential role in bone accrual and eye development. Here, we show that LRP5 is also required for normal cholesterol and glucose metabolism. The production of mice lacking LRP5 revealed that LRP5 deficiency led to increased plasma cholesterol levels in mice fed a high-fat diet, because of the decreased hepatic clearance of chylomicron remnants. In addition, when fed a normal diet, LRP5-deficient mice showed a markedly impaired glucose tolerance. The LRP5-deficient islets had a marked reduction in the levels of intracellular ATP and Ca(2+) in response to glucose, and thereby glucose-induced insulin secretion was decreased. The intracellular inositol 1,4,5-trisphosphate (IP3) production in response to glucose was also reduced in LRP5-- islets. Real-time PCR analysis revealed a marked reduction of various transcripts for genes involved in glucose sensing in LRP5-- islets. Furthermore, exposure of LRP5++ islets to Wnt-3a and Wnt-5a stimulates glucose-induced insulin secretion and this stimulation was blocked by the addition of a soluble form of Wnt receptor, secreted Frizzled-related protein-1. In contrast, LRP5-deficient islets lacked the Wnt-3a-stimulated insulin secretion. These data suggest that WntLRP5 signaling contributes to the glucose-induced insulin secretion in the islets.

All-trans retinoic acid induces differentiation of ducts and endocrine cells by mesenchymal/epithelial interactions in embryonic pancreas

Retinoids during the embryonic period act as a mesenchymal inducer in many organs, including kidney, lung, central nervous system, and gut. Retinoic acid (RA) demonstrates insulinotropic effects in adult pancreas, but only a limited study has elucidated its role in pancreatic organogenesis. In this study, we have analyzed the existence of RA-signaling machinery in embryonic pancreas and evaluated its role using in vitro tissue culture experiments. Here we show the presence of endogenous retinaldehyde dehydrogenase 2 (RALDH2), the most effective RA-synthesizing enzyme, RA-binding proteins, and RA receptors (RARs) in embryonic pancreatic tissue. RALDH2 is expressed exclusively in the mesenchyme. Exogenously added all-trans-retinoic acid (atRA) in tissue culture experiments stimulated differentiation of endocrine and duct cells and promoted apoptotic cell death of acinar tissue. Furthermore, we demonstrate that atRA upregulates the PDX-1 expression. Taken together, our data suggest that atRA-mediated mesenchymal/epithelial interactions play an important role in determining the cell fate of epithelial cells via regulation of the PDX-1 gene, leading to the proper formation of the endocrine versus exocrine component during pancreatic organogenesis.

A role for the melanocortin 4 receptor in sexual function

By using a combination of genetic, pharmacological, and anatomical approaches, we show that the melanocortin 4 receptor (MC4R), implicated in the control of food intake and energy expenditure, also modulates erectile function and sexual behavior. Evidence supporting this notion is based on several findings: (i) a highly selective non-peptide MC4R agonist augments erectile activity initiated by electrical stimulation of the cavernous nerve in wild-type but not Mc4r-null mice; (ii) copulatory behavior is enhanced by administration of a selective MC4R agonist and is diminished in mice lacking Mc4r; (iii) reverse transcription (RT)-PCR and non-PCR based methods demonstrate MC4R expression in rat and human penis, and rat spinal cord, hypothalamus, brainstem, pelvic ganglion (major autonomic relay center to the penis), but not in rat primary corpus smooth muscle cavernosum cells; and (iv) in situ hybridization of glans tissue from the human and rat penis reveal MC4R expression in nerve fibers and mechanoreceptors in the glans of the penis. Collectively, these data implicate the MC4R in the modulation of penile erectile function and provide evidence that MC4R-mediated proerectile responses may be activated through neuronal circuitry in spinal cord erectile centers and somatosensory afferent nerve terminals of the penis. Our results provide a basis for the existence of MC4R-controlled neuronal pathways that control sexual function.

Molecular characterization of the murine SIGNR1 gene encoding a C-type lectin homologous to human DC-SIGN and DC-SIGNR

The C-type lectin human dendritic cell (DC)-specific intercellular adhesion molecule (ICAM)-3-grabbing non-integrin (DC-SIGN) plays important roles in pattern recognition by dendritic cells in the immune system. In addition to binding human immunodeficiency virus (HIV), this type II membrane protein binds with high affinity to the adhesion molecules ICAM-3 and -2 to promote important dendritic cell interactions with naive T cells and endothelial cells, respectively. DC-SIGNR, a human DC-SIGN homologue expressed on sinusoidal endothelial cells in liver and lymph node, also binds and transmits HIV virus. We describe the cloning and characterization of a family of murine complementary DNAs (cDNAs) called SIGNR1, expressed in skin and spleen, that encode C-type lectins highly related to human DC-SIGN and DC-SIGNR. We also report the genomic structure of the SIGNR1 gene and compare it to that of human DC-SIGN and DC-SIGNR. The different transcripts (alpha, beta, gamma, delta) are generated by differences in 5' untranslated sequences, alternative splicing and/or the use of different polyadenylation sites. The predicted open reading frames encoded by the cDNAs are most closely related to human DC-SIGN and DC-SIGNR in the cytoplasmic domain, the transmembrane region and the carbohydrate recognition domain. Moreover, the alternatively spliced transcripts encode proteins that lack the transmembrane region or have modified carbohydrate recognition domains. Northern hybridization experiments with several different SIGNR1 cDNA probes reveal transcripts of 1.3 and 2.1 kb that are expressed in a tissue-restricted fashion in murine skin, spleen and lung. In situ hybridization and immunocytochemistry experiments demonstrate that, like human DC-SIGN, the murine messenger RNAs are expressed in subsets of dendritic cells in the spleen and skin.

Cbfa1-independent decrease in osteoblast proliferation, osteopenia, and persistent embryonic eye vascularization in mice deficient in Lrp5, a Wnt coreceptor

The low-density lipoprotein receptor-related protein (Lrp)-5 functions as a Wnt coreceptor. Here we show that mice with a targeted disruption of Lrp5 develop a low bone mass phenotype. In vivo and in vitro analyses indicate that this phenotype becomes evident postnatally, and demonstrate that it is secondary to decreased osteoblast proliferation and function in a Cbfa1-independent manner. Lrp5 is expressed in osteoblasts and is required for optimal Wnt signaling in osteoblasts. In addition, Lrp5-deficient mice display persistent embryonic eye vascularization due to a failure of macrophage-induced endothelial cell apoptosis. These results implicate Wnt proteins in the postnatal control of vascular regression and bone formation, two functions affected in many diseases. Moreover, these features recapitulate human osteoporosis-pseudoglioma syndrome, caused by LRP5 inactivation.

LDL receptor-related protein 5 (LRP5) affects bone accrual and eye development

In humans, low peak bone mass is a significant risk factor for osteoporosis. We report that LRP5, encoding the low-density lipoprotein receptor-related protein 5, affects bone mass accrual during growth. Mutations in LRP5 cause the autosomal recessive disorder osteoporosis-pseudoglioma syndrome (OPPG). We find that OPPG carriers have reduced bone mass when compared to age- and gender-matched controls. We demonstrate LRP5 expression by osteoblasts in situ and show that LRP5 can transduce Wnt signaling in vitro via the canonical pathway. We further show that a mutant-secreted form of LRP5 can reduce bone thickness in mouse calvarial explant cultures. These data indicate that Wnt-mediated signaling via LRP5 affects bone accrual during growth and is important for the establishment of peak bone mass.