Family with sequence similarity member 20C is the primary but not the only kinase for the small-integrin-binding ligand N-linked glycoproteins in bone

The phosphorylation of serine55 in enamelin is essential for murine amelogenesis

Amelogenesis imperfecta (AI) is an inherited developmental enamel defect affecting tooth masticatory function, esthetic appearance, and the well-being of patients. As one of the major enamel matrix proteins (EMPs), enamelin (ENAM) has three serines located in Ser-x-Glu (S-x-E) motifs, which are potential phosphorylation sites for the Golgi casein kinase FAM20C. Defects in FAM20C have similarly been associated with AI. In our previous study of EnamRgsc514 mice, the Glu57 in the S55-X56-E57 motif was mutated into Gly, which was expected to cause a phosphorylation failure of Ser55 because Ser55 cannot be recognized by FAM20C. The severe enamel defects in ENAMRgsc514 mice reminiscent of Enam-knockout mouse enamel suggested a potentially important role of Ser55 phosphorylation in ENAM function. However, the enamel defects and ENAM dysfunction may also be attributed to distinct physicochemical differences between Glu57 and Gly57. To clarify the significance of Ser55 phosphorylation to ENAM function, we generated two lines of Enam knock-in mice using CRISPR-Cas9 method to eliminate or mimic the phosphorylation state of Ser55 by substituting it with Ala55 or Asp55 (designated as S55A or S55D), respectively. The teeth of 6-day or 4-week-old mice were subjected to histology, micro-CT, SEM, TEM, immunohistochemistry, and mass spectrometry analyses to characterize the morphological, microstructural and proteomic changes in ameloblasts, enamel matrix and enamel rods. Our results showed that the enamel formation and EMP expression in S55D heterozygotes (Het) were less disturbed than those in S55A heterozygotes, while both homozygotes (Homo) had no mature enamel formation. Proteomic analysis revealed alterations of enamel matrix biosynthetic and mineralization processes in S55A Hets. Our present findings indicate that Asp55 substitution partially mimics the phosphorylation state of Ser55 in ENAM. Ser55 phosphorylation is essential for ENAM function during amelogenesis.

Molecular Cloning of Mouse Homologue of Enamel Protein C4orf26 and Its Phosphorylation by FAM20C

It is widely accepted that cellular processes are controlled by protein phosphorylation and has become increasingly clear that protein degradation, localization and conformation as well as protein-protein interaction are the examples of subsequent cellular events modulated by protein phosphorylation. Enamel matrix proteins belong to members of the secretory calcium binding phosphoprotein (SCPP) family clustered on chromosome 4q21, and most of the SCPP phosphoproteins have at least one S-X-E motifs (S; serine, X; any amino acid, E; glutamic acid). It has been reported that mutations in C4orf26 gene, located on chromosome 4q21, are associated with autosomal recessive type of Amelogenesis Imperfecta (AI), a hereditary condition that affects enamel formation/mineralization. The enamel phenotype observed in patients with C4orf26 mutations is hypomineralized and partially hypoplastic, indicating that C4orf26 protein may function at both secretory and maturation stages of amelogenesis. The previous in vitro study showed that the synthetic phosphorylated peptide based on C4orf26 protein sequence accelerates hydroxyapatite nucleation. Here we show the molecular cloning of Gm1045, mouse homologue of C4orf26, which has 2 splicing isoforms. Immunohistochemical analysis demonstrated that the immunolocalization of Gm1045 is mainly observed in enamel matrix in vivo. Our report is the first to show that FAM20C, the Golgi casein kinase, phosphorylates C4orf26 and Gm1045 in cell cultures. The extracellular localization of C4orf26/Gm1045 was regulated by FAM20C kinase activity. Thus, our data point out the biological importance of enamel matrix-kinase control of SCPP phosphoproteins and may have a broad impact on the regulation of amelogenesis and AI.

FAM20C Overview: Classic and Novel Targets, Pathogenic Variants and Raine Syndrome Phenotypes

FAM20C is a gene coding for a protein kinase that targets S-X-E/pS motifs on different phosphoproteins belonging to diverse tissues. Pathogenic variants of FAM20C are responsible for Raine syndrome (RS), initially described as a lethal and congenital osteosclerotic dysplasia characterized by generalized atherosclerosis with periosteal bone formation, characteristic facial dysmorphisms and intracerebral calcifications. The aim of this review is to give an overview of targets and variants of FAM20C as well as RS aspects. We performed a wide phenotypic review focusing on clinical aspects and differences between all lethal (LRS) and non-lethal (NLRS) reported cases, besides the FAM20C pathogenic variant description for each. As new targets of FAM20C kinase have been identified, we reviewed FAM20C targets and their functions in bone and other tissues, with emphasis on novel targets not previously considered. We found the classic lethal and milder non-lethal phenotypes. The milder phenotype is defined by a large spectrum ranging from osteonecrosis to osteosclerosis with additional congenital defects or intellectual disability in some cases. We discuss our current understanding of FAM20C deficiency, its mechanism in RS through classic FAM20C targets in bone tissue and its potential biological relevance through novel targets in non-bone tissues.

Effect of high phosphate diet on the formation of dentin in Fam20c-deficient mice

FAM20C (family with sequence similarity 20-member C), a kinase that phosphorylates secretory proteins, plays essential roles in various biological processes. In humans, mutations in FAM20C gene cause Raine syndrome, an autosomal recessive hereditary disease manifesting a broad spectrum of developmental defects including skeletal and craniofacial deformities. Our previous studies revealed that inactivation of Fam20c in mice led to hypophosphatemic rickets and that high phosphate (hPi) diet significantly improved the development of the skeleton in Fam20c-deficient mice. In this study, we evaluated the effects of hPi diet on the formation of dentin in Fam20c-deficient mice, using plain x-ray radiography, micro-computed tomography (µCT), histology, and immunohistochemistry. Plain x-ray radiography and µCT analyses showed that the hPi diet improved the dentin volume fraction and dentin mineral density of the Fam20c-deficient mice. Histology analyses further demonstrated that the hPi diet dramatically improved the integrity of the mandibular first molars and prevented pulp infection and dental abscesses in Fam20c-deficient mice. Our results support that the hPi diet significantly increased the formation and mineralization of dentin in Fam20c-deficient mice, implying that hypophosphatemia is a significant contributor to the dentin defects in Fam20c-deficient subjects.

Biological stenciling of mineralization in the skeleton: Local enzymatic removal of inhibitors in the extracellular matrix

Biomineralization is remarkably diverse and provides myriad functions across many organismal systems. Biomineralization processes typically produce hardened, hierarchically organized structures usually having nanostructured mineral assemblies that are formed through inorganic-organic (usually protein) interactions. Calcium‑carbonate biomineral predominates in structures of small invertebrate organisms abundant in marine environments, particularly in shells (remarkably it is also found in the inner ear otoconia of vertebrates), whereas calcium-phosphate biomineral predominates in the skeletons and dentitions of both marine and terrestrial vertebrates, including humans. Reconciliation of the interplay between organic moieties and inorganic crystals in bones and teeth is a cornerstone of biomineralization research. Key molecular determinants of skeletal and dental mineralization have been identified in health and disease, and in pathologic ectopic calcification, ranging from small molecules such as pyrophosphate, to small membrane-bounded matrix vesicles shed from cells, and to noncollagenous extracellular matrix proteins such as osteopontin and their derived bioactive peptides. Beyond partly knowing the regulatory role of the direct actions of inhibitors on vertebrate mineralization, more recently the importance of their enzymatic removal from the extracellular matrix has become increasingly understood. Great progress has been made in deciphering the relationship between mineralization inhibitors and the enzymes that degrade them, and how adverse changes in this physiologic pathway (such as gene mutations causing disease) result in mineralization defects. Two examples of this are rare skeletal diseases having osteomalacia/odontomalacia (soft bones and teeth) - namely hypophosphatasia (HPP) and X-linked hypophosphatemia (XLH) - where inactivating mutations occur in the gene for the enzymes tissue-nonspecific alkaline phosphatase (TNAP, TNSALP, ALPL) and phosphate-regulating endopeptidase homolog X-linked (PHEX), respectively. Here, we review and provide a concept for how existing and new information now comes together to describe the dual nature of regulation of mineralization - through systemic mineral ion homeostasis involving circulating factors, coupled with molecular determinants operating at the local level in the extracellular matrix. For the local mineralization events in the extracellular matrix, we present a focused concept in skeletal mineralization biology called the Stenciling Principle - a principle (building upon seminal work by Neuman and Fleisch) describing how the action of enzymes to remove tissue-resident inhibitors defines with precision the location and progression of mineralization.

Overexpression of Fam20C in osteoblast in vivo leads to increased cortical bone formation and osteoclastic bone resorption

Fam20C, which phosphorylates many secretory proteins with S-x-E/pS motifs, is highly expressed in bone and tooth tissues, implying that Fam20C-mediated phosphorylation is critical for regulation of these mineralized tissues. Previous studies of Fam20C-deficient mice revealed that Fam20C plays important roles in bone formation and mineralization. However, Fam20C-deficient mice develop hypophosphatemia, a systemic factor that masks the local effect of Fam20C in the bone tissue; consequently, the local role of Fam20C remains unknown. To elucidate the local function of Fam20C in bone tissue, we studied osteoblast-specific Fam20C transgenic (Fam20C-Tg) mice, which have no alteration in serum calcium and phosphate levels. Fam20C-Tg mice had more highly phosphorylated proteins in bone tissue than wild-type mice. In cortical bone of Fam20C-Tg mice, bone volume, mineralization surface (MS/BS), and mineral apposition rate (MAR) were elevated; in addition, the transgenic mice had an elevated number of vascular canals, resulting in an increased cortical porosity. Osteocyte number was elevated in the transgenics, but osteoblast number was unchanged. The microstructure of bone matrix characterized by the preferential orientation of collagen and apatite, was degraded and thus the mechanical function of bone material was deteriorated. In trabecular bone of Fam20C-Tg mice, bone volume was reduced, whereas MS/BS and MAR were unchanged. Osteoclast number was elevated and eroded surface area was non-significantly elevated with an increased serum CTX-I level, whereas osteoblast number was unchanged. These findings indicated that Fam20C overexpression in osteoblasts promotes cortical bone formation by increasing MS/BS and MAR and promoting osteocyte differentiation, but does not affect trabecular bone formation. Furthermore, Fam20C overexpression indirectly promotes osteoclastic bone resorption in cortical and trabecular bones. Our findings show that osteoblastic Fam20C-mediated phosphorylation in bone tissue regulates bone formation and resorption, and bone material quality.

FAM20C-Mediated Phosphorylation of MEPE and Its Acidic Serine- and Aspartate-Rich Motif

Matrix extracellular phosphoglycoprotein (MEPE) is expressed in bone and teeth where it has multiple functions. The C-terminus of MEPE contains a mineral-binding, acidic serine- and aspartate-rich motif (ASARM) that is also present in other noncollagenous proteins of mineralized tissues. MEPE-derived ASARM peptides function in phosphate homeostasis and direct inhibition of bone mineralization in a phosphorylation-dependent manner. MEPE is phosphorylated by family with sequence similarity 20, member C (FAM20C), which is the main kinase phosphorylating secreted phosphoprotein. Although the functional importance of protein phosphorylation status in mineralization processes has now been well-established for secreted bone and tooth proteins (particularly for osteopontin), the phosphorylation pattern of MEPE has not been previously determined. Here we provide evidence for a very high phosphorylation level of this protein, reporting on the localization of 31 phosphoresidues in human MEPE after coexpression with FAM20C in HEK293T cells. This includes the finding that all serine residues located in the canonical target sequence of FAM20C (Ser-x-Glu) were phosphorylated, thus establishing the major target sites for this kinase. We also show that MEPE has numerous other phosphorylation sites, these not being positioned in the canonical phosphorylation sequence. Of note, and underscoring a possible important function in mineralization biology, all nine serine residues in the ASARM were phosphorylated, even though only two of these were positioned in the Ser-x-Glu sequence. The presence of many phosphorylated amino acids in MEPE, and particularly their high density in the ASARM motif, provides an important basis for the understanding of structural and functional interdependencies in mineralization and phosphate homeostasis. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Isolation of SIBLING Proteins from Bone and Dentin Matrices

The extracellular matrix of the bone and dentin contains several non-collagenous proteins (NCPs). One category of NCPs is termed the SIBLING (small integrin-binding ligand, N-linked glycoprotein) family, which includes osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein 1 (DMP1), dentin sialophosphoprotein (DSPP), etc. These proteins have abundant phosphoserines, aspartic acids, and glutamic acids. In this protocol, we describe the extraction of NCPs from the bone and dentin matrices using guanidine-HCl/EDTA and the isolation of polyanionic SIBLINGs from NCPs using ion-exchange fast protein liquid chromatography (FPLC) to separate the differentially charged proteins into different fractions through a gradient elution by NaCl.

FGF23 and its role in X-linked hypophosphatemia-related morbidity

Background

X-linked hypophosphatemia (XLH) is an inherited disease of phosphate metabolism in which inactivating mutations of the Phosphate Regulating Endopeptidase Homolog, X-Linked (PHEX) gene lead to local and systemic effects including impaired growth, rickets, osteomalacia, bone abnormalities, bone pain, spontaneous dental abscesses, hearing difficulties, enthesopathy, osteoarthritis, and muscular dysfunction. Patients with XLH present with elevated levels of fibroblast growth factor 23 (FGF23), which is thought to mediate many of the aforementioned manifestations of the disease. Elevated FGF23 has also been observed in many other diseases of hypophosphatemia, and a range of animal models have been developed to study these diseases, yet the role of FGF23 in the pathophysiology of XLH is incompletely understood.

Methods

The role of FGF23 in the pathophysiology of XLH is here reviewed by describing what is known about phenotypes associated with various PHEX mutations, animal models of XLH, and non-nutritional diseases of hypophosphatemia, and by presenting molecular pathways that have been proposed to contribute to manifestations of XLH.

Results

The pathophysiology of XLH is complex, involving a range of molecular pathways that variously contribute to different manifestations of the disease. Hypophosphatemia due to elevated FGF23 is the most obvious contributor, however localised fluctuations in tissue non-specific alkaline phosphatase (TNAP), pyrophosphate, calcitriol and direct effects of FGF23 have been observed to be associated with certain manifestations.

Conclusions

By describing what is known about these pathways, this review highlights key areas for future research that would contribute to the understanding and clinical treatment of non-nutritional diseases of hypophosphatemia, particularly XLH.

Phosphoproteomic insights into processes influenced by the kinase-like protein DIA1/C3orf58

Many kinases are still ‘orphans,’ which means knowledge about their substrates, and often also about the processes they regulate, is lacking. Here, DIA1/C3orf58, a member of a novel predicted kinase-like family, is shown to be present in the endoplasmic reticulum and to influence trafficking via the secretory pathway. Subsequently, DIA1 is subjected to phosphoproteomics analysis to cast light on its signalling pathways. A liquid chromatography–tandem mass spectrometry proteomic approach with phosphopeptide enrichment is applied to membrane fractions of DIA1-overexpressing and control HEK293T cells, and phosphosites dependent on the presence of DIA1 are elucidated. Most of these phosphosites belonged to CK2- and proline-directed kinase types. In parallel, the proteomics of proteins immunoprecipitated with DIA1 reported its probable interactors. This pilot study provides the basis for deeper studies of DIA1 signalling.

Osteopontin as a novel substrate for the proprotein convertase 5/6 (PCSK5) in bone

Seven proprotein convertases cleave the basic amino acid consensus sequence K/R-X_n-K/R↓ (where n=0, 2, 4 or 6 variable amino acids) to activate precursor proteins. Despite similarities in substrate specificity, basic amino acid-specific proprotein convertases have a distinct tissue distribution allowing for enzymatic actions on tissue-resident substrates. Proprotein convertase 5/6 (PC5/6) has two splice variants - soluble PC5/6A and membrane-bound PC5/6B - and is expressed during mouse development in many tissues including bone and tooth, but little is known about the substrates for PC5/6 therein. Osteopontin (OPN) is an abundant bone extracellular matrix protein with roles in mineralization, cell adhesion and cell migration, and it has putative consensus sequence sites for cleavage by PC5/6, which may modify its function in bone. Since PC5/6-knockout mouse embryos show developmental abnormalities, and reduced overall mineralization, we designed this study to determine whether OPN is a substrate of PC5/6. In silico analysis of OPN protein sequences identified four potential PC5/6 consensus cleavage sites in human OPN, and three sites - including a noncanonical sequence - in mouse OPN. Ex vivo co-transfections with human OPN revealed complete OPN cleavage reducing full-length OPN (~70kDa) to an N-terminal fragment migrating at ~50kDa and two C-terminal fragments at ~18kDa and ~16kDa. Direct cleavage of OPN by PC5/6A - the predominant isoform expressed in human osteoblast cells - was confirmed by cell-free enzyme-substrate assays and by mass spectrometry. The latter was also used to investigate potential cleavage sites. Co-transfections of PC5/6 and mouse OPN showed partial cleavage of OPN into a C-terminal OPN fragment migrating at ~30kDa and an N-terminal fragment migrating at ~29kDa. Micro-computed tomography of PC5/6-knockout embryos at E18.5 confirmed a reduction in mineralized bone, and in situ hybridization performed on cryo-sections of normal mouse bone using Pcsk5 and Opn anti-sense and control-sense cRNA probes indicated the co-localization of the expression of these genes in bone cells. This mRNA expression profile was supported by semi-quantitative RT-PCR using osteoblast primary cultures, and cultured MC3T3-E1 osteoblast and MLO-Y4 osteocyte cell lines. Immunoblotting for OPN from mouse bone extracts showed altered OPN processing in PC5/6-knockout mice compared to wildtype mice. OPN fragments migrated at ~25kDa and ~16kDa in wildtype bone and were not present in PC5/6-deficient bone. In conclusion, this study demonstrates that Pcsk5 is expressed in bone-forming cells, and that OPN is a novel substrate for PC5/6. Cleavage of OPN by PC5/6 may modify the function of OPN in bone and/or modulate other enzymatic cleavages of OPN, leading to alterations in the bone phenotype.

The importance of a potential phosphorylation site in enamelin on enamel formation

Enamelin (ENAM) has three putative phosphoserines (pSers) phosphorylated by a Golgi-associated secretory pathway kinase (FAM20C) based on their distinctive Ser-x-Glu (S-x-E) motifs. Fam20C-knockout mice show severe enamel defects similar to those in the Enam-knockout mice, implying an important role of the pSers in ENAM. To determine the role of pSer⁵⁵ in ENAM, we characterized ENAM^Rgsc514 mice, in which Ser⁵⁵ cannot be phosphorylated by FAM20C due to an E⁵⁷>G⁵⁷ mutation in the S-x-E motif. The enamel microstructure of 4-week-old mice was examined by scanning electron microscopy. The teeth of 6-day-old mice were characterized by histology and immunohistochemistry. The protein lysates of the first lower molars of 4-day-old mice were analyzed by Western immunoblotting using antibodies against ENAM, ameloblastin and amelogenin. ENAM^Rgsc514 heterozygotes showed a disorganized enamel microstructure, while the homozygotes had no enamel on the dentin surface. The N-terminal fragments of ENAM in the heterozygotes were detained in the ameloblasts and localized in the mineralization front of enamel matrix, while those in the WT mice were secreted out of ameloblasts and distributed evenly in the outer 1/2 of enamel matrix. Surprisingly, the ~15  kDa C-terminal fragments of ameloblastin were not detected in the molar lysates of the homozygotes. These results suggest that the phosphorylation of Ser⁵⁵ may be an essential posttranslational modification of ENAM and is required for the interaction with other enamel matrix molecules such as ameloblastin in mediating the structural organization of enamel matrix and protein-mineral interactions during enamel formation.International Journal of Oral Science (2017) 9;e4; doi:10.1038/ijos.2017.41; published online 29 November 2017.

Effect of Phosphorylation on a Human-like Osteopontin Peptide

The last decade established that the dynamic properties of the phosphoproteome are central to function and its modulation. The temporal dimension of phosphorylation effects remains nonetheless poorly understood, particularly for intrinsically disordered proteins. Osteopontin, selected for this study due to its key role in biomineralization, is expressed in many species and tissues to play a range of distinct roles. A notable property of highly phosphorylated isoforms of osteopontin is their ability to sequester nanoclusters of calcium phosphate to form a core-shell structure, in a fluid that is supersaturated but stable. In Biology, this process enables soft and hard tissues to coexist in the same organism with relative ease. Here, we extend our understanding of the effect of phosphorylation on a disordered protein, the recombinant human-like osteopontin rOPN. The solution structures of the phosphorylated and unphosphorylated rOPN were investigated by small-angle x-ray scattering and no significant changes were detected on the radius of gyration or maximum interatomic distance. The picosecond-to-nanosecond dynamics of the hydrated powders of the two rOPN forms were further compared by elastic and quasi-elastic incoherent neutron scattering. Phosphorylation was found to block some nanosecond side-chain motions while increasing the flexibility of other side chains on the faster timescale. Phosphorylation can thus selectively change the dynamic behavior of even a highly disordered protein such as osteopontin. Through such an effect on rOPN, phosphorylation can direct allosteric mechanisms, interactions with substrates, cofactors and, in this case, amorphous or crystalline biominerals.

Osteopontin -- a promising biomarker for cancer therapy

Osteopontin (OPN), a multifunctional protein, has emerged as a potentially valuable biomarker for diagnosing and treating cancers. Recent research focuses on its involvement in tumor biology including the cell proliferation, survival, angiogenesis, invasion, and metastasis. Understanding the molecular mechanisms and pharmacological effects of OPN in cancer development could lead to new targets for improving cancer diagnosis and treatment. This review explains how the structurally conserved domains of OPN are associated with OPN signaling mediators and CD44, and how the conserved OPN domains determine biological functions. The authors have reviewed representative works of OPN expression in breast cancer and colorectal cancer to elucidate the relationship between OPN and cancer/tumor biology. It has also been shown that the prognostic sensitivity in non-small cell lung cancer, hepatocellular carcinoma, gastric cancer, and ovarian cancer improved compared to the individual marker when OPN was analyzed in conjunction with other markers. The therapeutic approaches based on OPN inhibitors are discussed to illustrate recent research progress. Previous clinical data has indicated that OPN has played a unique role in cancer development, but further investigation is required to understand the underlying mechanism. More clinical trials are also required to examine the applicability and efficacy of OPN inhibitors in cancer therapy.

Amelogenesis Imperfecta; Genes, Proteins, and Pathways

Amelogenesis imperfecta (AI) is the name given to a heterogeneous group of conditions characterized by inherited developmental enamel defects. AI enamel is abnormally thin, soft, fragile, pitted and/or badly discolored, with poor function and aesthetics, causing patients problems such as early tooth loss, severe embarrassment, eating difficulties, and pain. It was first described separately from diseases of dentine nearly 80 years ago, but the underlying genetic and mechanistic basis of the condition is only now coming to light. Mutations in the gene AMELX, encoding an extracellular matrix protein secreted by ameloblasts during enamel formation, were first identified as a cause of AI in 1991. Since then, mutations in at least eighteen genes have been shown to cause AI presenting in isolation of other health problems, with many more implicated in syndromic AI. Some of the encoded proteins have well documented roles in amelogenesis, acting as enamel matrix proteins or the proteases that degrade them, cell adhesion molecules or regulators of calcium homeostasis. However, for others, function is less clear and further research is needed to understand the pathways and processes essential for the development of healthy enamel. Here, we review the genes and mutations underlying AI presenting in isolation of other health problems, the proteins they encode and knowledge of their roles in amelogenesis, combining evidence from human phenotypes, inheritance patterns, mouse models, and in vitro studies. An LOVD resource (http://dna2.leeds.ac.uk/LOVD/) containing all published gene mutations for AI presenting in isolation of other health problems is described. We use this resource to identify trends in the genes and mutations reported to cause AI in the 270 families for which molecular diagnoses have been reported by 23rd May 2017. Finally we discuss the potential value of the translation of AI genetics to clinical care with improved patient pathways and speculate on the possibility of novel treatments and prevention strategies for AI.

Specific ablation of mouse Fam20C in cells expressing type I collagen leads to skeletal defects and hypophosphatemia

FAM20C mutations in humans cause Raine syndrome and our previous studies showed that global inactivation of mouse Fam20C led to bone and dental defects. By crossbreeding 2.3  kb Col 1a1-Cre mice with Fam20C^flox/flox mice, we created 2.3  kb Col 1a1-Cre;Fam20C^foxl/flox (cKO) mice, in which Fam20C was inactivated in cells expressing Type I collagen. This study showed that the long bones of cKO mice were shorter and had a lower level of mineralization compared to the normal mice. The collagen fibrils in Fam20C-deficient bone were disorganized and thicker while the growth plate cartilage in cKO mice was disorganized and wider compared to the normal mice. The Fam20C-deficient bone had a lower level of dentin matrix protein 1, and higher levels of osteopontin and bone sialoprotein than the normal. The blood of cKO mice had an elevated level of fibroblast growth factor 23 and reduced level of phosphorus. These findings indicate that inactivation of Fam20C in cells expressing type I collagen led to skeletal defects and hypophosphatemia. The altered levels of dentin matrix protein 1 and osteopontin in Fam20C-deficient bone may be significant contributors to the mineralized tissue defects in human patients and animals suffering from the functional loss of FAM20C.

Extracellular Protein Phosphorylation, the Neglected Side of the Modification

The very existence of extracellular phosphorylation has been questioned for a long time, although casein phosphorylation was discovered a century ago. In addition, several modification sites localized on secreted proteins or on extracellular or lumenal domains of transmembrane proteins have been catalogued in large scale phosphorylation analyses, though in most such studies this aspect of cellular localization was not considered. Our review presents examples when additional analyses were performed on already public data sets that revealed a wealth of information about this "neglected side" of the modification. We also sum up accumulated knowledge about extracellular phosphorylation, including the discovery of Golgi-residing kinases and the special difficulties encountered in targeted analyses. We hope future phosphorylation studies will not ignore the existence of phosphorylation outside of the cell, and further discoveries will shed more light on its biological role.

Inactivation of Fam20B in Joint Cartilage Leads to Chondrosarcoma and Postnatal Ossification Defects

During endochondral ossification, chondrocytes embed themselves in a proteoglycan-rich matrix during the proliferation-maturation transition. Accumulating evidence shows that proteoglycans are essential components for chondrocyte proliferation and differentiation. When we conditionally inactivated FAM20B (Family with sequence similarity 20 member-B), which is a newly identified xylose kinase essential for glycosaminoglycan (GAG) formation on the protein core of proteoglycans, from the dental mesenchyme using Osr2-Cre, which is also strongly expressed in joint cartilage, we found chondrosarcoma in the knee joint and remarkable defects of postnatal ossification in the long bones. Mechanistic analysis revealed that the defects were associated with gain of function in multiple signaling pathways in the epiphyseal chondrocytes, such as those derived by WNT, BMP, and PTHrP/IHH molecules, suggesting that the FAM20B-catalyzed proteoglycans are critical mediators for a signaling balance in the regulatory network controlling chondrocyte differentiation and proliferation. In particular, we demonstrated that the WNT inhibitor was able to rescue part of the bone defects in Osr2-Cre;Fam20B^fl/fl mice, indicating that FAM20B-catalyzed proteoglycans regulate postnatal endochondral ossification partially through the mediation of WNT signaling.