Mutations in the gene encoding the low-density lipoprotein receptor LRP4 cause abnormal limb development in the mouse

A novel homozygous missense variant in LRP4 causing Cenani-Lenz syndactyly syndrome and literature review

BACKGROUND

Cenani-Lenzsyndactyly syndrome (CLSS; OMIM 212780) is a rare autosomal recessive acral deformity, which is mainly manifested in the fusion of fingers or toes, disordered phalangeal structure, shortening or fusion of the radius and ulna, and renal hypoplasia.

CASE PRESENTATION

Our report described an individual with mild phenotypes from China. His parents were not consanguineous. The affected individual was non-dysmorphic. Standard X-ray showed that the both hands have only four metacarpal bones. The distal end of the first metacarpal bone on the right was relatively slender, and the distal phalanx was absent. Multiple phalanges and some soft tissues of both hands were fused. Exome sequencing revealed a novel biallelic c.282C⟩Avariant in low-density lipoprotein receptor-related protein 4 (LRP4; OMIM604270; NM_002334.4) causing p. (Asn94Lys) change in the encoded protein. This variant is predicted to be potentially pathogenic, affecting protein structure and function.

CONCLUSION

We report a novel missense variant present in homozygosity in LRP4 to broaden the pathogenic spectrum of LRP4 in syndactyly, and exome sequencing technology is a powerful tool for genetic analysis in prenatal diagnosis and medical research, as a preferred method for the diagnosis of syndactyly and related phenotypes.

A variant in the LDL receptor-related protein encoding gene LRP4 underlying polydactyly and phalangeal synostosis in a family of Pakistani origin

A family of Pakistani origin, segregating polydactyly, and phalangeal synostosis in an autosomal dominant manner, has been investigated and presented in the present report. Whole-exome sequencing (WES), followed by segregation analysis using Sanger sequencing, revealed a heterozygous missense variant [c.G1696A, p.(Gly566Ser)] in the LRP4 gene located on human chromosome 11p11.2. Homology protein modeling revealed the mutant Ser566 generated new interactions with at least four other amino acids and disrupted protein folding and function. Our findings demonstrated the first direct evidence of involvement of LRP4 in causing polydactyly and phalangeal synostosis in the same family. This study highlighted the importance of inclusion of LRP4 gene in screening individuals presenting polydactyly in hands and feet, and phalangeal synostosis in the same family.

Functional Impact and Regulation of Alternative Splicing in Mouse Heart Development and Disease

Alternative splicing (AS) plays a major role in the generation of transcript diversity. In the heart, roles have been described for some AS variants, but the global impact and regulation of AS patterns are poorly understood. Here, we studied the AS profiles in heart disease, their relationship with heart development, and the regulatory mechanisms controlling AS dynamics in the mouse heart. We found that AS profiles characterized the different groups and that AS and gene expression changes affected independent genes and biological functions. Moreover, AS changes, specifically in heart disease, were associated with potential protein-protein interaction changes. While developmental transitions were mainly driven by the upregulation of MBNL1, AS changes in disease were driven by a complex regulatory network, where PTBP1 played a central role. Indeed, PTBP1 over-expression was sufficient to induce cardiac hypertrophy and diastolic dysfunction, potentially by perturbing AS patterns.

Wnt Pathway Extracellular Components and Their Essential Roles in Bone Homeostasis

The Wnt pathway is involved in several processes essential for bone development and homeostasis. For proper functioning, the Wnt pathway is tightly regulated by numerous extracellular elements that act by both activating and inhibiting the pathway at different moments. This review aims to describe, summarize and update the findings regarding the extracellular modulators of the Wnt pathway, including co-receptors, ligands and inhibitors, in relation to bone homeostasis, with an emphasis on the animal models generated, the diseases associated with each gene and the bone processes in which each member is involved. The precise knowledge of all these elements will help us to identify possible targets that can be used as a therapeutic target for the treatment of bone diseases such as osteoporosis.

Conservation and Innovation: Versatile Roles for LRP4 in Nervous System Development

As the nervous system develops, connections between neurons must form to enable efficient communication. This complex process of synaptic development requires the coordination of a series of intricate mechanisms between partner neurons to ensure pre- and postsynaptic differentiation. Many of these mechanisms employ transsynaptic signaling via essential secreted factors and cell surface receptors to promote each step of synaptic development. One such cell surface receptor, LRP4, has emerged as a synaptic organizer, playing a critical role in conveying extracellular signals to initiate diverse intracellular events during development. To date, LRP4 is largely known for its role in development of the mammalian neuromuscular junction, where it functions as a receptor for the synaptogenic signal Agrin to regulate synapse development. Recently however, LRP4 has emerged as a synapse organizer in the brain, where new functions for the protein continue to arise, adding further complexity to its already versatile roles. Additional findings indicate that LRP4 plays a role in disorders of the nervous system, including myasthenia gravis, amyotrophic lateral sclerosis, and Alzheimer's disease, demonstrating the need for further study to understand disease etiology. This review will highlight our current knowledge of how LRP4 functions in the nervous system, focusing on the diverse developmental roles and different modes this essential cell surface protein uses to ensure the formation of robust synaptic connections.

Cell death in the developing vertebrate limb: A locally regulated mechanism contributing to musculoskeletal tissue morphogenesis and differentiation

Our aim is to critically review current knowledge of the function and regulation of cell death in the developing limb. We provide a detailed, but short, overview of the areas of cell death observed in the developing limb, establishing their function in morphogenesis and structural development of limb tissues. We will examine the functions of this process in the formation and growth of the limb primordia, formation of cartilaginous skeleton, formation of synovial joints, and establishment of muscle bellies, tendons, and entheses. We will analyze the plasticity of the cell death program by focusing on the developmental potential of progenitors prior to death. Considering the prolonged plasticity of progenitors to escape from the death process, we will discuss a new biological perspective that explains cell death: this process, rather than secondary to a specific genetic program, is a consequence of the tissue building strategy employed by the embryo based on the formation of scaffolds that disintegrate once their associated neighboring structures differentiate.

We examine the functions of cell death in the formation and growth of the limb primordia.

We analyze the plasticity of the cell death program by focusing on the developmental potential of progenitors prior to death.

Considering the prolonged plasticity of progenitors to escape from the death process and the absence of defined genetic program in their regulation we propose that cell death is a consequence of the tissue building strategy employed by the embryo regulated by epigenetic factors .

Geminin is required for Hox gene regulation to pattern the developing limb

Development of the complex structure of the vertebrate limb requires carefully orchestrated interactions between multiple regulatory pathways and proteins. Among these, precise regulation of 5' Hox transcription factor expression is essential for proper limb bud patterning and elaboration of distinct limb skeletal elements. Here, we identified Geminin (Gmnn) as a novel regulator of this process. A conditional model of Gmnn deficiency resulted in loss or severe reduction of forelimb skeletal elements, while both the forelimb autopod and hindlimb were unaffected. 5' Hox gene expression expanded into more proximal and anterior regions of the embryonic forelimb buds in this Gmnn-deficient model. A second conditional model of Gmnn deficiency instead caused a similar but less severe reduction of hindlimb skeletal elements and hindlimb polydactyly, while not affecting the forelimb. An ectopic posterior SHH signaling center was evident in the anterior hindlimb bud of Gmnn-deficient embryos in this model. This center ectopically expressed Hoxd13, the HOXD13 target Shh, and the SHH target Ptch1, while these mutant hindlimb buds also had reduced levels of the cleaved, repressor form of GLI3, a SHH pathway antagonist. Together, this work delineates a new role for Gmnn in modulating Hox expression to pattern the vertebrate limb.

Exploration of key regulators driving primary feather follicle induction in goose skin

The primary feather follicles are universal skin appendages widely distributed in the skin of feathered birds. The morphogenesis and development of the primary feather follicles in goose skin remain largely unknown. Here, the induction of primary feather follicles in goose embryonic skin (pre-induction vs induction) was investigated by de novo transcriptome analyses to reveal 409 differentially expressed genes (DEGs). The DEGs were characterized to potentially regulate the de novo formation of feather follicle primordia consisting of placode (4 genes) and dermal condensate (12 genes), and the thickening of epidermis (5 genes) and dermal fibroblasts (17 genes), respectively. Further analyses enriched DEGs into GO terms represented as cell adhesion and KEGG pathways including Wnt and Hedgehog signaling pathways that are highly correlated with cell communication and molecular regulation. Six selected Wnt pathway genes were detected by qPCR with up-regulation in goose skin during the induction of primary feather follicles. The localization of WNT16, SFRP1 and FRZB by in situ hybridization showed weak expression in the primary feather primordia, whereas FZD1, LEF1 and DKK1 were expressed initially in the inter-follicular skin and feather follicle primordia, then mainly restricted in the feather primordia. The spatial-temporal expression patterns indicate that Wnt pathway genes DKK1, FZD1 and LEF1 are the important regulators functioned in the induction of primary feather follicle in goose skin. The dynamic molecular changes and specific gene expression patterns revealed in this report provide the general knowledge of primary feather follicle and skin development in waterfowl, and contribute to further understand the diversity of hair and feather development beyond the mouse and chicken models.

Genotypes and allele frequencies of buried SNPs in a bovine single-nucleotide polymorphism array in Japanese Black cattle

Single nucleotide polymorphism (SNP) arrays are widely used for genetic and genomic analyses in cattle breeding; thus, data derived from SNP arrays have accumulated on a large scale nationwide. Commercial SNP arrays contain a considerable number of unassigned SNPs on the chromosome/position on the genome; these SNPs are excluded in subsequent analyses. Notably, the position-unassigned SNPs, or "buried SNPs" include some of the markers associated with genetic disease. In this study, we identified the position of buried SNPs using the Basic Local Alignment Search Tool against the surrounding sequences and characterized the relationship between SNPs and genetic diseases in Online Mendelian Inheritance in Animals based on the genomic position. We determined the position of 285 buried SNPs on the genome and surveyed the genotype and allele frequencies of these SNPs in 5,955 individual Japanese Black cattle. Eleven SNPs associated with genetic disease, which contained five buried SNPs, were found in the population with the risk allele frequency ranging from 0.00008396 to 0.46. These results indicate that buried SNPs in the bovine SNP array can be utilized to identify associations with genetic disorders from large scale accumulated SNP genotype data in Japanese Black cattle.

Indirubin-3'-oxime stimulates chondrocyte maturation and longitudinal bone growth via activation of the Wnt/β-catenin pathway

Researchers have shown increased interest in determining what stimulates height. Currently, many children undergo precocious puberty, resulting in short stature due to premature closure of the growth plate. However, the current approach for height enhancement is limited to growth hormone treatment, which often results in side effects and clinical failure and is costly. Although recent studies have indicated the importance of paracrine signals in the growth plate for longitudinal bone growth, height-stimulating agents targeting the signaling pathways involved in growth plate maturation remain unavailable in the clinic. The Wnt/β-catenin pathway plays a major role in the maturation of growth plate chondrocytes. In this study, by using an ex vivo tibial culture system, we identified indirubin-3′-oxime (I3O) as a compound capable of enhancing longitudinal bone growth. I3O promoted chondrocyte proliferation and differentiation via activation of the Wnt/β-catenin pathway in vitro. Intraperitoneal injection of I3O in adolescent mice increased growth plate height along with incremental chondrocyte maturation. I3O promoted tibial growth without significant adverse effects on bone thickness and articular cartilage. Therefore, I3O could be a potential therapeutic agent for increasing height in children with growth retardation.

A compound that stimulates longitudinal bone growth could lead to safer treatments for children with short stature. Growth hormone treatments can normalize development in some children with growth hormone deficiency, but the side effects can be severe. Researchers led by Kang-Yell Choi at Yonsei University, Seoul, South Korea, have determined that chemical stimulation of a critical cell signaling pathway involving bone growth may offer a better approach for growing taller. Longitudinal growth of bones is driven by the proliferation and differentiation of cartilage cells in the growth plate. Choi and colleagues screened a chemical library and identified a compound derived from traditional Chinese herbs, which efficiently promotes this bone growth process in cultured cartilage cells and in the tibias of three-week-old mice. This compound appears safe, suggesting a potentially better avenue for promoting height growth.

WNT5A is transported via lipoprotein particles in the cerebrospinal fluid to regulate hindbrain morphogenesis

WNTs are lipid-modified proteins that control multiple functions in development and disease via short- and long-range signaling. However, it is unclear how these hydrophobic molecules spread over long distances in the mammalian brain. Here we show that WNT5A is produced by the choroid plexus (ChP) of the developing hindbrain, but not the telencephalon, in both mouse and human. Since the ChP produces and secretes the cerebrospinal fluid (CSF), we examine the presence of WNT5A in the CSF and find that it is associated with lipoprotein particles rather than exosomes. Moreover, since the CSF flows along the apical surface of hindbrain progenitors not expressing Wnt5a, we examined whether deletion of Wnt5a in the ChP controls their function and find that cerebellar morphogenesis is impaired. Our study thus identifies the CSF as a route and lipoprotein particles as a vehicle for long-range transport of biologically active WNT in the central nervous system.

Deficiency of lrp4 in zebrafish and human LRP4 mutation induce aberrant activation of Jagged-Notch signaling in fin and limb development

Low-density lipoprotein receptor-related protein 4 (LRP4) is a multi-functional protein implicated in bone, kidney and neurological diseases including Cenani-Lenz syndactyly (CLS), sclerosteosis, osteoporosis, congenital myasthenic syndrome and myasthenia gravis. Why different LRP4 mutation alleles cause distinct and even contrasting disease phenotypes remain unclear. Herein, we utilized the zebrafish model to search for pathways affected by a deficiency of LRP4. The lrp4 knockdown in zebrafish embryos exhibits cyst formations at fin structures and the caudal vein plexus, malformed pectoral fins, defective bone formation and compromised kidney morphogenesis; which partially phenocopied the human LRP4 mutations and were reminiscent of phenotypes resulting form a perturbed Notch signaling pathway. We discovered that the Lrp4-deficient zebrafish manifested increased Notch outputs in addition to enhanced Wnt signaling, with the expression of Notch ligand jagged1b being significantly elevated at the fin structures. To examine conservatism of signaling mechanisms, the effect of LRP4 missense mutations and siRNA knockdowns, including a novel missense mutation c.1117C > T (p.R373W) of LRP4, were tested in mammalian kidney and osteoblast cells. The results showed that LRP4 suppressed both Wnt/β-Catenin and Notch signaling pathways, and these activities were perturbed either by LRP4 missense mutations or by a knockdown of LRP4. Our finding underscore that LRP4 is required for limiting Jagged-Notch signaling throughout the fin/limb and kidney development, whose perturbation representing a novel mechanism for LRP4-related diseases. Moreover, our study reveals an evolutionarily conserved relationship between LRP4 and Jagged-Notch signaling, which may shed light on how the Notch signaling is fine-tuned during fin/limb development.

A Review of the Genetics and Pathogenesis of Syndactyly in Humans and Experimental Animals: A 3-Step Pathway of Pathogenesis

Embryology of normal web space creation and the genetics of syndactyly in humans and experimental animals are well described in the literature. In this review, the author offers a 3-step pathway of pathogenesis for syndactyly. The first step is initiated either by the overactivation of the WNT canonical pathway or the suppression of the Bone Morphogenetic Protein (BMP) canonical pathway. This leads to an overexpression of Fibroblast Growth Factor 8 (FGF8). The final step is the suppression of retinoic acid in the interdigital mesenchyme leading to suppression of both apoptosis and extracellular matrix (ECM) degradation, resulting in syndactyly.

Mutations in the fourth β-propeller domain of LRP4 are associated with isolated syndactyly with fusion of the third and fourth fingers

Isolated hand syndactyly is a common limb malformation with limited known genetic etiology. We used exome sequencing to discover two novel variants, chr11 g.46896373C>G; p.D1403H and chr11 g.46893078G>T; p.Q1564K, in LRP4 in a child with isolated bilateral syndactyly of the third and fourth fingers. Each variant was inherited from a different parent and neither parent was affected. Variants in LRP4 have been previously associated with syndactyly in Cenani-Lenz syndactyly syndrome and Sclerosteosis 2, but have not been reported in individuals with isolated syndactyly. LRP4 inhibits LRP6/LRP5-mediated activation of canonical Wnt signaling and mediates sclerostin-dependent inhibition of bone formation. p.D1403H and p.Q1564K are located within the fourth β-propeller of the extracellular protein domain that has yet to be associated with human disease. Functional analyses of p.D1403H and p.Q1564K show that they significantly decrease LRP4's inhibition of Wnt signaling. These results suggest that variants in the fourth β-propeller of the extracellular protein domain may cause a phenotype distinct from previously characterized LRP4 variants.

Role of the Wnt/β-Catenin Pathway in Renal Osteodystrophy

Vascular calcification and bone fragility are common and interrelated health problems that affect chronic kidney disease (CKD) patients. Bone fragility, which leads to higher risk of fracture and mortality, arises from the abnormal bone remodeling and mineralization that are seen in chronic kidney disease. Recently, sclerostin and Dickkopf-related protein 1 were suggested to play a significant role in CKD-related bone disease as they are known inhibitors of the Wnt pathway, thus preventing bone formation. This review focuses on new knowledge about the Wnt pathway in bone, how its function is affected by chronic kidney disease and how this affects bone structure. Expression of components and inhibitors of the Wnt pathway has been shown to be affected by the loss of kidney function, and a better understanding of the bone effects of Wnt pathway inhibitors could allow the development of new therapies to prevent bone fragility in this population.

Multiple modes of Lrp4 function in modulation of Wnt/β-catenin signaling during tooth development

During development and homeostasis, precise control of Wnt/β-catenin signaling is in part achieved by secreted and membrane proteins that negatively control activity of the Wnt co-receptors Lrp5 and Lrp6. Lrp4 is related to Lrp5/6 and is implicated in modulation of Wnt/β-catenin signaling, presumably through its ability to bind to the Wise (Sostdc1)/sclerostin (Sost) family of Wnt antagonists. To gain insights into the molecular mechanisms of Lrp4 function in modulating Wnt signaling, we performed an array of genetic analyses in murine tooth development, where Lrp4 and Wise play important roles. We provide genetic evidence that Lrp4 mediates the Wnt inhibitory function of Wise and also modulates Wnt/β-catenin signaling independently of Wise. Chimeric receptor analyses raise the possibility that the Lrp4 extracellular domain interacts with Wnt ligands, as well as the Wnt antagonists. Diverse modes of Lrp4 function are supported by severe tooth phenotypes of mice carrying a human mutation known to abolish Lrp4 binding to Sost. Our data suggest a model whereby Lrp4 modulates Wnt/β-catenin signaling via interaction with Wnt ligands and antagonists in a context-dependent manner.

Genetic disorders in beef cattle: a review

The main purpose of present review is to describe and organize autosomal recessive disorders (arachnomelia, syndactylism, osteopetrosis, dwarfism, crooked tail syndrome, muscular hyperplasia, glycogen storage disease, protoporphyria), which occur among beef cattle, and methods that can be applied to detect these defects. Prevalence of adverse alleles in beef breeds happens due to human activity—selections of favorable features, e.g. developed muscle tissue. Unfortunately, carriers of autosomal recessive diseases are often characterized by these attributes. Fast and effective identification of individuals, that may carry faulty genes, can prevent economical losses.

Functional Roles of the Interaction of APP and Lipoprotein Receptors

The biological fates of the key initiator of Alzheimer’s disease (AD), the amyloid precursor protein (APP), and a family of lipoprotein receptors, the low-density lipoprotein (LDL) receptor-related proteins (LRPs) and their molecular roles in the neurodegenerative disease process are inseparably interwoven. Not only does APP bind tightly to the extracellular domains (ECDs) of several members of the LRP group, their intracellular portions are also connected through scaffolds like the one established by FE65 proteins and through interactions with adaptor proteins such as X11/Mint and Dab1. Moreover, the ECDs of APP and LRPs share common ligands, most notably Reelin, a regulator of neuronal migration during embryonic development and modulator of synaptic transmission in the adult brain, and Agrin, another signaling protein which is essential for the formation and maintenance of the neuromuscular junction (NMJ) and which likely also has critical, though at this time less well defined, roles for the regulation of central synapses. Furthermore, the major independent risk factors for AD, Apolipoprotein (Apo) E and ApoJ/Clusterin, are lipoprotein ligands for LRPs. Receptors and ligands mutually influence their intracellular trafficking and thereby the functions and abilities of neurons and the blood-brain-barrier to turn over and remove the pathological product of APP, the amyloid-β peptide. This article will review and summarize the molecular mechanisms that are shared by APP and LRPs and discuss their relative contributions to AD.

LRP4 in neuromuscular junction and bone development and diseases

Low-density lipoprotein receptor-related protein 4 (LRP4) is a member of the low-density lipoprotein receptor (LDLR) family. Recent studies have revealed multiple functions and complex signaling mechanisms of LRP4 in different organs and tissues. LPR4 mutation or malfunction has been implicated in neurological disorders including congenital myasthenic syndrome, myasthenia gravis, and diseases of bone or kidney. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Discovery and characterization of spontaneous mouse models of craniofacial dysmorphology

Craniofacial abnormalities are among the most common features of human genetic syndromes and disorders. The etiology of these conditions is often complex, influenced by both genetic context and the environment. Frequently, craniofacial abnormalities present as part of a syndrome with clear comorbid phenotypes, providing additional insight into mechanisms of the causative gene or pathway. The mouse has been a key tool in our understanding of the genetic mechanisms of craniofacial development and disease, and can provide excellent models for human craniofacial abnormalities. While powerful genetic engineering tools in the mouse have contributed significantly our understanding of craniofacial development and dysmorphology, forward genetic approaches provide an unbiased means to identify new genes and pathways. Moreover, spontaneous mutations can occur on any number of genetic backgrounds, potentially revealing critical genes that require a specific genetic context. Here we report discovery and phenotyping of 43 craniofacial mouse models, derived primarily from a screen for spontaneous mutations in production colonies at the Jackson Laboratory. We identify the causative gene for 33 lines, including novel genes in pathways not previously connected to craniofacial development, and novel alleles of known genes that present with unique phenotypes. Together with our detailed characterization, this work provides a valuable gene discovery resource for the craniofacial community, and a rich source of mouse models for further investigation.

Advances in the Molecular Genetics of Non-syndromic Syndactyly

Syndactyly, webbing of adjacent digits with or without bony fusion, is one of the most common hereditary limb malformations. It occurs either as an isolated abnormality or as a component of more than 300 syndromic anomalies. There are currently nine types of phenotypically diverse nonsyndromic syndactyly. Non-syndromic syndactyly is usually inherited as an autosomal dominant trait, although the more severe presenting types and subtypes may show autosomal recessive or X-linked pattern of inheritance. The phenotype appears to be not only caused by a main gene, but also dependant on genetic background and subsequent signaling pathways involved in limb formation. So far, the principal genes identified to be involved in congenital syndactyly are mainly involved in the zone of polarizing activity and sonic hedgehog pathway. This review summarizes the recent progress made in the molecular genetics, including known genes and loci responsible for non-syndromic syndactyly, and the signaling pathways those genetic factors involved in, as well as clinical features and animal models. We hope our review will contribute to the understanding of underlying pathogenesis of this complicated disorder and have implication on genetic counseling.

LRP receptor family member associated bone disease

A dozen years ago the identification of causal mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) gene involved in two rare bone disorders propelled research in the bone field in totally new directions. Since then, there have been an explosion in the number of reports that highlight the role of the Wnt/β-catenin pathway in the regulation of bone homeostasis. In this review we discuss some of the most recent reports (in the past 2 years) highlighting the involvement of the members of the LRP family (LRP5, LRP6, LRP4, and more recently LRP8) in the maintenance of bone and their implications in bone diseases. These reports include records of new single nucleotides polymorphisms (SNPs) and haplotypes that suggest variants in these genes can contribute to subtle variation in bone traits to mutations that give rise to extreme bone phenotypes. All of these serve to further support and reinforce the importance of this tightly regulated pathway in bone. Furthermore, we discuss provocative reports suggesting novel approaches through inhibitors of this pathway to treat rarer diseases such as Osteoporosis-Pseudoglioma Syndrome (OPPG), Osteogenesis Imperfecta (OI), and Sclerosteosis/Van Buchem disease. It is hoped that by understanding the role of each component of the pathway and their involvement in bone diseases that this knowledge will allow us to develop new, more effective therapeutic approaches for more common diseases such as post-menopausal osteoporosis, osteoarthritis, and rheumatoid arthritis as well as these rarer bone diseases.

Lrp4 domains differentially regulate limb/brain development and synaptic plasticity

Apolipoprotein E (ApoE) genotype is the strongest predictor of Alzheimer’s Disease (AD) risk. ApoE is a cholesterol transport protein that binds to members of the Low-Density Lipoprotein (LDL) Receptor family, which includes LDL Receptor Related Protein 4 (Lrp4). Lrp4, together with one of its ligands Agrin and its co-receptors Muscle Specific Kinase (MuSK) and Amyloid Precursor Protein (APP), regulates neuromuscular junction (NMJ) formation. All four proteins are also expressed in the adult brain, and APP, MuSK, and Agrin are required for normal synapse function in the CNS. Here, we show that Lrp4 is also required for normal hippocampal plasticity. In contrast to the closely related Lrp8/Apoer2, the intracellular domain of Lrp4 does not appear to be necessary for normal expression and maintenance of long-term potentiation at central synapses or for the formation and maintenance of peripheral NMJs. However, it does play a role in limb development.

A novel APC mutation defines a second locus for Cenani-Lenz syndrome

BACKGROUND

Cenani-Lenz syndrome (CLS) is an autosomal recessive condition characterised by a unique pattern of syndactyly, and variable penetrance of renal agenesis and facial dysmorphism. LRP4 mutations were identified in most, but not all patients with this syndrome, suggesting the presence of at least one additional locus.

MATERIALS AND METHODS

Clinical characterisation of a new CLS family followed by autozygosity mapping, whole-exome sequencing and global gene expression profiling.

RESULTS

We describe an extended consanguineous Saudi family with typical CLS features in addition to significant scoliosis. The disease in this family maps to a single autozygous interval on 5q22.2, in which whole-exome sequencing revealed the presence of a novel splicing mutation in APC that results in ∼ 80% reduction of the wild-type transcript and the creation of an aberrant transcript that predicts a severely truncated APC. This was found to be associated with upregulation of Wnt/β-catenin signalling.

CONCLUSIONS

In a pattern similar to how LRP4 mutations are predicted to negate the protein's antagonistic effect on Wnt/β-catenin signalling, we propose that reduction of APC may increase the availability of β-catenin by virtue of impaired degradation, leading to a similar phenotypic outcome. This is the first time APC is linked to a human phenotype distinct from its established role in oncology.

Receptor heterodimerization as a novel mechanism for the regulation of Wnt/β-catenin signaling

The Wnt pathway plays important roles in multiple physiological and pathophysiological processes. Here, we report a novel mechanism that regulates the Wnt pathway through heterodimerization of the Wnt co-receptor low-density lipoprotein-receptor-related protein 6 (LRP6) and very low-density lipoprotein receptor (VLDLR); the latter belongs to the same protein family as LRP6 and was originally known as a receptor for lipoproteins. Knockdown of Vldlr expression elevated LRP6 protein levels and activated Wnt/β-catenin signaling, whereas overexpression of Vldlr suppressed Wnt signaling. Moreover, we demonstrate that the VLDLR ectodomain is essential and sufficient for inhibition of Wnt signaling. The VLDLR ectodomain accelerated internalization and degradation of LRP6 through heterodimerization with the LRP6 extracellular domain. Monoclonal antibodies specific for the VLDLR ectodomain blocked VLDLR-LRP6 heterodimerization, resulting in enhanced Wnt/β-catenin signaling in vitro and in vivo. Taken together, these findings suggest that heterodimerization of receptors in the membrane accelerates the turnover of LRP6, and represent a new mechanism for the regulation of Wnt/β-catenin signaling.

Defining structural abnormalities of the hip joint at risk of degeneration

The purpose of this review is to explore the mounting evidence that primary osteoarthritis is secondary to childhood disorders such as dysplasia and/or to subtle morphologic and structural to subtle morphologic and structural abnormalities of the proximal femur and/or acetabulum that were previously unrecognized or underestimated. These structural deformities lead to early impingement through range of motion and subsequent joint degeneration. The review also presents a brief synopsis of the genetic components that influence structural morphology of the hip joint and the impact of genetic pathways on OA development. If subtle deformities can be shown to be effective predictors of OA in the general population, it may be possible to identify hips that are at risk before they progress to end-stage OA. Furthermore, if these early risk factors are modifiable, it may be possible to implement preventative measures before the requirement of total hip arthroplasty.

LRP4 induces extracellular matrix productions and facilitates chondrocyte differentiation

Endochondral ossification is an essential step for skeletal development, which requires chondrocyte differentiation in growth cartilage. The low-density lipoprotein receptor-related protein 4 (LRP4), a member of LDLR family, is an inhibitor for Wnt signaling, but its roles in chondrocyte differentiation remain to be investigated. Here we found by laser capture microdissection that LRP4 expression was induced during chondrocyte differentiation in growth plate. In order to address the roles, we overexpressed recombinant human LRP4 or knocked down endogenous LRP4 by lentivirus in mouse ATDC5 chondrocyte cells. We found that LRP4 induced gene expressions of extracellular matrix proteins of type II collagen (Col2a1), aggrecan (Acan), and type X collagen (Col10a1), as well as production of total proteoglycans in ATDC5 cells, whereas LRP4 knockdown had opposite effects. Interestingly, LRP4-knockdown reduced mRNA expression of Sox9, a master regulator for chondrogenesis, as well as Dkk1, an extracellular Wnt inhibitor. Analysis of Wnt signaling revealed that LRP4 blocked the Wnt/β-catenin signaling activity in ATDC5 cells. Finally, the reduction of these extracellular matrix productions by LRP4-knockdown was rescued by a β-catenin/TCF inhibitor, suggesting that LRP4 is an important regulator for extracellular matrix productions and chondrocyte differentiation by suppressing Wnt/β-catenin signaling.

The osteocyte as a therapeutic target in the treatment of osteoporosis

Osteoporosis is characterized by a low bone-mineral density associated with skeletal fractures. The decrease in bone-mineral density is the consequence of an unbalanced bone-remodeling process, with higher bone resorption than bone formation. The orchestration of the bone-remodeling process is under the control of the most abundant cell in bone, the osteocyte. Functioning as an endocrine cell, osteocytes are also a source of soluble factors that not only target cells on the bone surface, but also target distant organs. Therefore, any drugs targeting the osteocyte functions and signaling pathways will have a major impact on the bone-remodeling process. This review discusses potential advances in drug therapy for osteoporosis, including novel osteocyte-related antiresorptive and anabolic agents that may become available in the coming years.

Fiber type-specific expression of low-density lipoprotein receptor-related protein 6 in human skeletal muscles

OBJECTIVE

Gene expression patterns differ in the two types of skeletal muscle fiber. The Wnt signaling pathway, which includes low-density lipoprotein receptor-related protein 6 (LRP6), has been associated with cell differentiation and glucose metabolism in skeletal muscles. We examined the relationships between muscle fiber types and LRP6 expression.

METHODS

Adenosine triphosphatase was assayed histochemically, and the levels of expression of LRP6 and myosin were analyzed immunohistochemically, in frozen sections of muscle fiber obtained from 16 muscle biopsy samples. The expression pattern of LRP6 in C2C12 cells was assayed by immunocytochemistry.

RESULTS

LRP6 was expressed only in type II fibers. Type IIc fibers showed variations in LRP6 expression. Expression of LRP6 was observed at the stage of myoblast differentiation.

CONCLUSION

Antibody to LRP6 may be useful for identifying type II skeletal muscle fibers. LRP6 may influence glucose metabolism in type II fibers of human skeletal muscles.

Cenani-Lenz syndrome restricted to limb and kidney anomalies associated with a novel LRP4 missense mutation

Cenani-Lenz syndrome (CLS) is a rare autosomal recessive developmental disorder of the limbs. The disorder is characterized by complete syndactyly with metacarpal fusions and/or oligodactyly sometimes accompanied by radioulnar synostosis. The clinical expression is variable and kidney agenesis/hypoplasia, craniofacial dysmorphism and teeth abnormalities are frequent features as well as lower limb involvement. CLS was recently associated with mutations in the low-density lipoprotein receptor-related protein 4 (LRP4) gene and dysregulated canonical WNT signaling. We have identified a large consanguineous Pakistani pedigree with 9 members affected by CLS. The affected individuals present with a consistent expression of the syndrome restricted to the limbs and kidneys. Symptoms from the lower limb are mild or absent and there were no radioulnar synostosis or craniofacial involvement. Genetic analysis using autozygosity mapping and sequencing revealed homozygosity for a novel missense mutation c.2858T > C (p.L953P) in the LRP4 gene. The mutation is located in a region encoding the highly conserved low-density lipoprotein receptor repeat class B domain of LRP4. Our findings add to the genotype-phenotype correlations in CLS and support kidney anomalies as a frequent associated feature.

A Comprehensive Overview of Skeletal Phenotypes Associated with Alterations in Wnt/β-catenin Signaling in Humans and Mice

The Wnt signaling pathway plays key roles in differentiation and development and alterations in this signaling pathway are causally associated with numerous human diseases. While several laboratories were examining roles for Wnt signaling in skeletal development during the 1990s, interest in the pathway rose exponentially when three key papers were published in 2001-2002. One report found that loss of the Wnt co-receptor, Low-density lipoprotein related protein-5 (LRP5), was the underlying genetic cause of the syndrome Osteoporosis pseudoglioma (OPPG). OPPG is characterized by early-onset osteoporosis causing increased susceptibility to debilitating fractures. Shortly thereafter, two groups reported that individuals carrying a specific point mutation in LRP5 (G171V) develop high-bone mass. Subsequent to this, the causative mechanisms for these observations heightened the need to understand the mechanisms by which Wnt signaling controlled bone development and homeostasis and encouraged significant investment from biotechnology and pharmaceutical companies to develop methods to activate Wnt signaling to increase bone mass to treat osteoporosis and other bone disease. In this review, we will briefly summarize the cellular mechanisms underlying Wnt signaling and discuss the observations related to OPPG and the high-bone mass disorders that heightened the appreciation of the role of Wnt signaling in normal bone development and homeostasis. We will then present a comprehensive overview of the core components of the pathway with an emphasis on the phenotypes associated with mice carrying genetically engineered mutations in these genes and clinical observations that further link alterations in the pathway to changes in human bone.

Lrp4 and Wise interplay controls the formation and patterning of mammary and other skin appendage placodes by modulating Wnt signaling

The future site of skin appendage development is marked by a placode during embryogenesis. Although Wnt/β-catenin signaling is known to be essential for skin appendage development, it is unclear which cellular processes are controlled by the signaling and how the precise level of the signaling activity is achieved during placode formation. We have investigated roles for Lrp4 and its potential ligand Wise (Sostdc1) in mammary and other skin appendage placodes. Lrp4 mutant mice displayed a delay in placode initiation and changes in distribution and number of mammary precursor cells leading to abnormal morphology, number and position of mammary placodes. These Lrp4 mammary defects, as well as limb defects, were associated with elevated Wnt/β-catenin signaling and were rescued by reducing the dose of the Wnt co-receptor genes Lrp5 and Lrp6, or by inactivating the gene encoding β-catenin. Wise-null mice phenocopied a subset of the Lrp4 mammary defects and Wise overexpression reduced the number of mammary precursor cells. Genetic epistasis analyses suggest that Wise requires Lrp4 to exert its function and that, together, they have a role in limiting mammary fate, but Lrp4 has an early Wise-independent role in facilitating placode formation. Lrp4 and Wise mutants also share defects in vibrissa and hair follicle development, suggesting that the roles played by Lrp4 and Wise are common to skin appendages. Our study presents genetic evidence for interplay between Lrp4 and Wise in inhibiting Wnt/β-catenin signaling and provides an insight into how modulation of Wnt/β-catenin signaling controls cellular processes important for skin placode formation.

Endocytic receptor-mediated control of morphogen signaling

Receptor-mediated endocytosis provides a mechanism by which cells take up signaling molecules from the extracellular space. Recent studies have shown that one class of endocytic receptors, the low-density lipoprotein receptor-related proteins (LRPs), is of particular relevance for embryonic development. In this Primer, we describe how LRPs constitute central pathways that modulate morphogen presentation to target tissues and cellular signal reception, and how LRP dysfunction leads to developmental disturbances in many species.

Structural mechanisms of the agrin-LRP4-MuSK signaling pathway in neuromuscular junction differentiation

The neuromuscular junction (NMJ) is the most extensively studied model of neuronal synaptogenesis. Acetylcholine receptor (AChR) clustering on the postsynaptic membrane is a cardinal event in the differentiation of NMJs. AChR clustering and postsynaptic differentiation is orchestrated by sophisticated interactions among three proteins: the neuron-secreted proteoglycan agrin, the co-receptor LRP4, and the muscle-specific receptor tyrosine kinase MuSK. LRP4 and MuSK act as scaffolds for multiple binding partners, resulting in a complex and dynamic network of interacting proteins that is required for AChR clustering. In this review, we discuss the structural basis for NMJ postsynaptic differentiation mediated by the agrin-LRP4-MuSK signaling pathway.

Sclerostin and Dickkopf-1 as therapeutic targets in bone diseases

The processes of bone growth, modeling, and remodeling determine the structure, mass, and biomechanical properties of the skeleton. Dysregulated bone resorption or bone formation may lead to metabolic bone diseases. The Wnt pathway plays an important role in bone formation and regeneration, and expression of two Wnt pathway inhibitors, sclerostin and Dickkopf-1 (DKK1), appears to be associated with changes in bone mass. Inactivation of sclerostin leads to substantially increased bone mass in humans and in genetically manipulated animals. Studies in various animal models of bone disease have shown that inhibition of sclerostin using a monoclonal antibody (Scl-Ab) increases bone formation, density, and strength. Additional studies show that Scl-Ab improves bone healing in models of bone repair. Inhibition of DKK1 by monoclonal antibody (DKK1-Ab) stimulates bone formation in younger animals and to a lesser extent in adult animals and enhances fracture healing. Thus, sclerostin and DKK1 are emerging as the leading new targets for anabolic therapies to treat bone diseases such as osteoporosis and for bone repair. Clinical trials are ongoing to evaluate the effects of Scl-Ab and DKK1-Ab in humans for the treatment of bone loss and for bone repair.

Discovery Genetics - The History and Future of Spontaneous Mutation Research

Historically, spontaneous mutations in mice have served as valuable models of heritable human diseases, contributing substantially to our understanding of both disease mechanisms and basic biological pathways. While advances in molecular technologies have improved our ability to create mouse models of human disease through targeted mutagenesis and transgenesis, spontaneous mutations continue to provide valuable research tools for discovery of novel genes and functions. In addition, the genetic defects caused by spontaneous mutations are molecularly similar to mutations in the human genome and, therefore often produce phenotypes that more closely resemble those characteristic of human disease than do genetically engineered mutations. Due to the rarity with which spontaneous mutations arise and the animal intensive nature of their genetic analysis, large-scale spontaneous mutation analysis has traditionally been limited to large mammalian genetics institutes. More recently, ENU mutagenesis and new screening methods have increased the rate of mutant strain discovery, and high-throughput DNA sequencing has enabled rapid identification of the underlying genes and their causative mutations. Here, we discuss the continued value of spontaneous mutations for biomedical research.

The epidemiology, genetics and future management of syndactyly

Syndactyly is a condition well documented in current literature due to it being the most common congenital hand defect, with a large aesthetic and functional significance.There are currently nine types of phenotypically diverse non-syndromic syndactyly, an increase since the original classification by Temtamy and McKusick(1978). Non-syndromic syndactyly is inherited as an autosomal dominant trait, although the more severe presenting types and sub types appear to have autosomal recessive and in some cases X-linked hereditary.Gene research has found that these phenotypes appear to not only be one gene specific, although having individual localised loci, but dependant on a wide range of genes and subsequent signalling pathways involved in limb formation. The principal genes so far defined to be involved in congenital syndactyly concern mainly the Zone of Polarizing Activity and Shh pathway.Research into the individual phenotypes appears to complicate classification as new genes are found both linked, and not linked, to each malformation. Consequently anatomical, phenotypical and genotypical classifications can be used, but are variable in significance, depending on the audience.Currently, management is surgical, with a technique unchanged for several decades, although future development will hopefully bring alternatives in both earlier diagnosis and gene manipulation for therapy.

LRP5 and bone mass regulation: Where are we now?

The discovery of causal mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) gene underlying conditions of altered bone mass ushered in a new era in bone research. Since those original publications, the role of Lrp5 and the Wnt/β-catenin signaling pathway controlled by Lrp5 and its homologs, Lrp6 and Lrp4, in bone mass regulation has been an intense area of investigation. Studies to date have implicated this pathway in skeletal development, osteoblast differentiation and proliferation, osteoblast/osteocyte apoptosis, regulation of the balance between osteogenesis-chondrogenesis-adipogenesis, regulation of osteoclastogenesis and the response of bone to mechanical loading. Interestingly, the data from knockout and transgenic mice involving Lrp4/5/6 and/or their regulators, as well as β-catenin signaling pathway components, and in vitro studies have sometimes yielded conflicting results. Adding to the complexity of the system are the studies that suggested Lrp5 regulated bone mass through a gut-bone endocrine signaling system involving Lrp5 mediated control of gut serotonin synthesis. However, recent studies have called this into question and so this provocative concept remains an open question. Clearly, the manipulation of Lrp5/Wnt/β-catenin pathway presents as a major target for drug development to treat diseases of low bone mass such as osteoporosis and these new therapies are in full progress. At present, although it is clear that Lrp5 has a role in bone mass regulation, much of the details remain to be elucidated and this is a major and exciting challenge for future studies.

Update on Wnt signaling in bone cell biology and bone disease

For more than a decade, Wnt signaling pathways have been the focus of intense research activity in bone biology laboratories because of their importance in skeletal development, bone mass maintenance, and therapeutic potential for regenerative medicine. It is evident that even subtle alterations in the intensity, amplitude, location, and duration of Wnt signaling pathways affects skeletal development, as well as bone remodeling, regeneration, and repair during a lifespan. Here we review recent advances and discrepancies in how Wnt/Lrp5 signaling regulates osteoblasts and osteocytes, introduce new players in Wnt signaling pathways that have important roles in bone development, discuss emerging areas such as the role of Wnt signaling in osteoclastogenesis, and summarize progress made in translating basic studies to clinical therapeutics and diagnostics centered around inhibiting Wnt pathway antagonists, such as sclerostin, Dkk1 and Sfrp1. Emphasis is placed on the plethora of genetic studies in mouse models and genome wide association studies that reveal the requirement for and crucial roles of Wnt pathway components during skeletal development and disease.

Anti-LRP4 autoantibodies in AChR- and MuSK-antibody-negative myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disorder characterized by a defect in synaptic transmission at the neuromuscular junction causing fluctuating muscle weakness with a decremental response to repetitive nerve stimulation or altered jitter in single-fiber electromyography (EMG). Approximately 80% of all myasthenia gravis patients have autoantibodies against the nicotinic acetylcholine receptor in their serum. Autoantibodies against the tyrosine kinase muscle-specific kinase (MuSK) are responsible for 5-10% of all myasthenia gravis cases. The autoimmune target in the remaining cases is unknown. Recently, low-density lipoprotein receptor-related protein (LRP4) has been identified as the agrin receptor. LRP4 interacts with agrin, and the binding of agrin activates MuSK, which leads to the formation of most if not all postsynaptic specializations, including aggregates containing acetylcholine receptors (AChRs) in the junctional plasma membrane. In the present study we tested if autoantibodies against LRP4 are detectable in patients with myasthenia gravis. To this end we analyzed 13 sera from patients with generalized myasthenia gravis but without antibodies against AChR or MuSK. The results showed that 12 out of 13 antisera from double-seronegative MG patients bound to proteins concentrated at the neuromuscular junction of adult mouse skeletal muscle and that approximately 50% of the tested sera specifically bound to HEK293 cells transfected with human LRP4. Moreover, 4 out of these 13 sera inhibited agrin-induced aggregation of AChRs in cultured myotubes by more than 50%, suggesting a pathogenic role regarding the dysfunction of the neuromuscular endplate. These results indicate that LRP4 is a novel target for autoantibodies and is a diagnostic marker in seronegative MG patients.

Bone overgrowth-associated mutations in the LRP4 gene impair sclerostin facilitator function

Humans lacking sclerostin display progressive bone overgrowth due to increased bone formation. Although it is well established that sclerostin is an osteocyte-secreted bone formation inhibitor, the underlying molecular mechanisms are not fully elucidated. We identified in tandem affinity purification proteomics screens LRP4 (low density lipoprotein-related protein 4) as a sclerostin interaction partner. Biochemical assays with recombinant proteins confirmed that sclerostin LRP4 interaction is direct. Interestingly, in vitro overexpression and RNAi-mediated knockdown experiments revealed that LRP4 specifically facilitates the previously described inhibitory action of sclerostin on Wnt1/β-catenin signaling. We found the extracellular β-propeller structured domain of LRP4 to be required for this sclerostin facilitator activity. Immunohistochemistry demonstrated that LRP4 protein is present in human and rodent osteoblasts and osteocytes, both presumed target cells of sclerostin action. Silencing of LRP4 by lentivirus-mediated shRNA delivery blocked sclerostin inhibitory action on in vitro bone mineralization. Notably, we identified two mutations in LRP4 (R1170W and W1186S) in patients suffering from bone overgrowth. We found that these mutations impair LRP4 interaction with sclerostin and its concomitant sclerostin facilitator effect. Together these data indicate that the interaction of sclerostin with LRP4 is required to mediate the inhibitory function of sclerostin on bone formation, thus identifying a novel role for LRP4 in bone.

Lrp4: A novel modulator of extracellular signaling in craniofacial organogenesis

The low-density lipoprotein (LDL) receptor family is a large evolutionarily conserved group of transmembrane proteins. It has been shown that LDL receptor family members can also function as direct signal transducers or modulators for a broad range of cellular signaling pathways. We have identified a novel mode of signaling pathway integration/coordination that occurs outside cells during development that involves an LDL receptor family member. Physical interaction between an extracellular protein (Wise) that binds BMP ligands and an Lrp receptor (Lrp4) that modulates Wnt signaling, acts to link these two pathways. Mutations in either Wise or Lrp4 in mice produce multiple, but identical abnormalities in tooth development that are linked to alterations in BMP and Wnt signaling. Teeth, in common with many other organs, develop by a series of epithelial-mesenchymal interactions, orchestrated by multiple cell signaling pathways. In tooth development, Lrp4 is expressed exclusively in epithelial cells and Wise mainly in mesenchymal cells. Our hypothesis, based on the mutant phenotypes, cell signaling activity changes and biochemical interactions between Wise and Lrp4 proteins, is that Wise and Lrp4 together act as an extracellular mechanism of coordinating BMP and Wnt signaling activities in epithelial-mesenchymal cell communication during development.

Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family

The evolutionarily ancient low-density lipoprotein (LDL) receptor gene family represents a class of widely expressed cell surface receptors. Since the dawn of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors have evolved. In accordance with the now obsolete 'one-gene-one-function' hypothesis, these cell surface receptors were originally perceived as mere transporters of lipoproteins, lipids, and nutrients or as scavenger receptors, which remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular environment and the cell surface. This picture has since undergone a fundamental change. Experimental evidence has replaced the perception that these receptors serve merely as cargo transporters. Instead it is now clear that the transport of macromolecules is inseparably intertwined with the molecular machinery by which cells communicate with each other. Lipoprotein receptors are essentially sensors of the extracellular environment that participate in a wide range of physiological processes by physically interacting and coevolving with primary signal transducers as co-regulators. Furthermore, lipoprotein receptors modulate cellular trafficking and localization of the amyloid precursor protein (APP) and the β-amyloid peptide (Aβ), suggesting a role in the pathogenesis of Alzheimer's disease. Moreover, compelling evidence shows that LDL receptor family members are involved in tumor development and progression.

A variant of the LRP4 gene affects the risk of chronic lymphocytic leukaemia transformation to Richter syndrome

Richter syndrome (RS) represents the transformation of chronic lymphocytic leukaemia (CLL) to aggressive lymphoma. Risk factors of CLL transformation to RS are only partly known. We explored the role of the host genetic background as a risk factor for RS occurrence. Forty-five single nucleotide polimorphisms (SNPs) known to be relevant for CLL prognosis were genotyped in a consecutive cohort of 331 CLL, of which 21 had transformed to RS. After correcting for multiple testing and adjusting for previously reported RS risk factors, the LRP4 rs2306029 TT variant genotype was the sole SNP independently associated with a higher risk of RS transformation (Hazard Ratio: 4·17; P = 0·001; q = 0·047). The enrichment of LRP4 TT genotype in RS was confirmed in an independent series (n = 44) used for validation purposes. The LRP4 protein was expressed in CLL (n =66). Bioinformatic analysis scored LRP4 rs2306029 as a variant with possible deleterious and damaging variant of LRP4. LRP4 genotyping may help the recognition of patients with increased risk of RS at the time of CLL diagnosis.

Relationship between joint shape and the development of osteoarthritis

PURPOSE OF REVIEW

To present an updated summary of the relationship between joint shape and the development of osteoarthritis, with a particular focus on osteoarthritis of the hip.

RECENT FINDINGS

Osteoarthritis of the hip is highly heritable, with a genetic contribution estimated at 60%. Among the genes that have been linked to this disease are several that are involved in the development and maintenance of joint shape, including members of the Wingless (Wnt) and the bone morphogenetic protein (BMP) family. Several features of hip joint architecture, such as acetabular dysplasia, pistol grip deformity, wide femoral neck, altered femoral neck-shaft angle, appear to play an important role in the pathogenesis of osteoarthritis and may predate the development of osteoarthritis by decades.

SUMMARY

Gene-environment interactions play a crucial role in the development of osteoarthritis. The architecture of joint shape is determined by a complex sequence spanning embryonic, childhood, and adult life and contributes to the pathogenesis of osteoarthritis.

To build a synapse: signaling pathways in neuromuscular junction assembly

Synapses, as fundamental units of the neural circuitry, enable complex behaviors. The neuromuscular junction (NMJ) is a synapse type that forms between motoneurons and skeletal muscle fibers and that exhibits a high degree of subcellular specialization. Aided by genetic techniques and suitable animal models, studies in the past decade have brought significant progress in identifying NMJ components and assembly mechanisms. This review highlights recent advances in the study of NMJ development, focusing on signaling pathways that are activated by diffusible cues, which shed light on synaptogenesis in the brain and contribute to a better understanding of muscular dystrophy.

Lrp4, a novel receptor for Dickkopf 1 and sclerostin, is expressed by osteoblasts and regulates bone growth and turnover in vivo

Lrp4 is a multifunctional member of the low density lipoprotein-receptor gene family and a modulator of extracellular cell signaling pathways in development. For example, Lrp4 binds Wise, a secreted Wnt modulator and BMP antagonist. Lrp4 shares structural elements within the extracellular ligand binding domain with Lrp5 and Lrp6, two established Wnt co-receptors with important roles in osteogenesis. Sclerostin is a potent osteocyte secreted inhibitor of bone formation that directly binds Lrp5 and Lrp6 and modulates both BMP and Wnt signaling. The anti-osteogenic effect of sclerostin is thought to be mediated mainly by inhibition of Wnt signaling through Lrp5/6 within osteoblasts. Dickkopf1 (Dkk1) is another potent soluble Wnt inhibitor that binds to Lrp5 and Lrp6, can displace Lrp5-bound sclerostin and is itself regulated by BMPs. In a recent genome-wide association study of bone mineral density a significant modifier locus was detected near the SOST gene at 17q21, which encodes sclerostin. In addition, nonsynonymous SNPs in the LRP4 gene were suggestively associated with bone mineral density. Here we show that Lrp4 is expressed in bone and cultured osteoblasts and binds Dkk1 and sclerostin in vitro. MicroCT analysis of Lrp4 deficient mutant mice revealed shortened total femur length, reduced cortical femoral perimeter, and reduced total femur bone mineral content (BMC) and bone mineral density (BMD). Lumbar spine trabecular bone volume per total volume (BV/TV) was significantly reduced in the mutants and the serum and urinary bone turnover markers alkaline phosphatase, osteocalcin and desoxypyridinoline were increased. We conclude that Lrp4 is a novel osteoblast expressed Dkk1 and sclerostin receptor with a physiological role in the regulation of bone growth and turnover, which is likely mediated through its function as an integrator of Wnt and BMP signaling pathways.

Mutations in VLDLR as a cause for autosomal recessive cerebellar ataxia with mental retardation (dysequilibrium syndrome)

Dysequilibrium syndrome is a genetically heterogeneous condition that combines autosomal recessive, nonprogressive cerebellar ataxia with mental retardation. Here, we report the first patient heterozygous for 2 novel mutations in VLDLR. An 18-month-old girl presented with significant hypotonia, global developmental delay, and truncal and peripheral ataxia. Magnetic resonance imaging of the brain demonstrated hypoplasia of the inferior cerebellar vermis and hemispheres, small pons, and a simplified cortical sulcation pattern. Sequence analysis of the VLDLR gene identified a nonsense and missense mutation. Six mutations in VLDLR have now been identified in 5 families with a phenotype characterized by moderate-to-profound mental retardation, delayed ambulation, truncal and peripheral ataxia, and occasional seizures. Neuroanatomically, the loss-of-function effect of the different mutations is indistinguishable. VLDLR-associated cerebellar hypoplasia is emerging as a panethnic, clinically, and molecularly well-defined genetic syndrome.