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

Impaired Growth Plate Maturation in XLH Is due to Both Excess FGF23 and Decreased 1,25-Dihydroxyvitamin D Signaling

X-linked hypophosphatemia (XLH) is the most common form of hereditary hypophosphatemic rickets. The genetic basis for XLH is loss of function mutations in the phosphate-regulating endopeptidase X-linked (PHEX), which leads to increased circulating fibroblast growth factor 23 (FGF23). This increase in FGF23 impairs activation of vitamin D and attenuates renal phosphate reabsorption, leading to rickets. Previous studies have demonstrated that ablating FGF23 in the Hyp mouse model of XLH leads to hyperphosphatemia, high levels of 1,25-dihydroxyvitamin D, and is not associated with the development of rickets. Studies were undertaken to define a role for the increase in 1,25-dihydroxyvitamin D levels in the prevention of rickets in Hyp mice lacking FGF23. These mice were mated to mice lacking Cyp27b1, the enzyme responsible for activating vitamin D metabolites, to generate Hyp mice lacking both FGF23 and 1,25-dihydroxyvitamin D (FCH mice). Mice were fed a special diet to maintain normal mineral ion homeostasis. Despite normal mineral ions, Hyp mice lacking both FGF23 and Cyp27b1 developed rickets, characterized by an interrupted, expanded hypertrophic chondrocyte layer and impaired hypertrophic chondrocyte apoptosis. This phenotype was prevented when mice were treated with 1,25-dihydroxyvitamin D from day 2 until sacrifice on day 30. Interestingly, mice lacking FGF23 and Cyp27b1 without the PHEX mutation did not exhibit rickets. These findings define an essential PHEX-dependent, FGF23-independent role for 1,25-dihydroxyvitamin D in XLH and have important therapeutic implications for the treatment of this genetic disorder.

Solution NMR structure of cementum protein 1 derived peptide (CEMP1-p1) and its role in the mineralization process

We report the characterization of the three-dimensional structure of the CEMP1-p1 peptide [MGTSSTDSQQAQHRRCSTSN: corresponding to residues 1-20 of the N-terminus of cementum protein 1 (CEMP1)]. This peptide imitates the capacity of CEMP1 to stimulate hydroxyapatite (HA) crystal nucleation and growth, and promotes the differentiation of periodontal ligament cells into a cementoblastic phenotype. Additionally, in experimental models of critical-sized calvarial defects in Wistar rats, CEMP1-p1 has shown osteogenic properties that enhanced the physiological deposition and maturation of newly formed bone. In this work, studies of CEMP1-p1 by circular dichroism (CD) and nuclear magnetic resonance (NMR) were performed in trifluoroethanol D2 (TFED2) and aqueous solution to determine the 3D structure of the peptide. Using the 3D model, experimental data from HA crystals formation and calcium fluorescence emission, we explain the biological mechanisms involved in CEMP1-p1 activity to promote calcium recruitment and its affinity to HA crystals. This information is valuable because it proposes, for the first time, a plausible molecular mechanism during the mineralization process, from a specific cementum protein-derived peptide.

Correction of Vascular Calcification and Hyperphosphatemia in CKD Rats Treated with ASARM Peptide

Background

Abnormalities in calcium, phosphorus, PTH, vitamin D metabolism, bone, and vascular calcification occur in chronic kidney disease mineral bone disorder (CKD-MBD). Calciphylaxis, involving painful, ulcerative skin lesions, is also a major problem associated with CKD-MBD. There are no quality medical interventions to address these clinical issues. Bone ASARM peptides are strong inhibitors of mineralization and induce hypophosphatemia by inhibiting phosphate uptake from the gut. We hypothesize treatment of CKD-MBD rats with ASARM peptides will reverse hyperphosphatemia, reduce soft-tissue calcification, and prevent calciphylaxis.

Methods

To test our hypothesis, we assessed the effects of synthetic ASARM peptide in rats that had undergone a subtotal 5/6th nephrectomy (56NEPHREX), a rodent model of CKD-MBD. All rats were fed a high phosphate diet (2% Pi) to worsen mineral metabolism defects. Changes in serum potassium, phosphate, BUN, creatinine, PTH, FGF23, and calcium were assessed in response to 28 days of ASARM peptide infusion. Also, changes in bone quality, soft-tissue calcification, and expression of gut Npt2b (Slc34a2) were studied following ASARM peptide treatment.

Results

Rats that had undergone 56NEPHREX treated with ASARM peptide showed major improvements in hyperphosphatemia, blood urea nitrogen (BUN), and bone quality compared with vehicle controls. Also, ASARM-infused 56NEPHREX rats displayed improved renal, brain, and cardiovascular calcification. Notably, ASARM peptide infusion prevented the genesis of subdermal medial blood vessel calcification and calciphylaxis-like lesions in 56NEPHREX rats compared with vehicle controls.

Conclusions

ASARM peptide infusion corrects hyperphosphatemia and improves vascular calcification, renal calcification, brain calcification, bone quality, renal function, and skin mineralization abnormalities in 56NEPHREX rats. These findings confirm our hypothesis and support the utility of ASARM peptide treatment in patients with CKD-MBD.

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.

Structural Biology of Calcium Phosphate Nanoclusters Sequestered by Phosphoproteins

Biofluids that contain stable calcium phosphate nanoclusters sequestered by phosphopeptides make it possible for soft and hard tissues to co-exist in the same organism with relative ease. The stability diagram of a solution of nanocluster complexes shows how the minimum concentration of phosphopeptide needed for stability increases with pH. In the stable region, amorphous calcium phosphate cannot precipitate. Nevertheless, if the solution is brought into contact with hydroxyapatite, the crystalline phase will grow at the expense of the nanocluster complexes. The physico-chemical principles governing the formation, composition, size, structure, and stability of the complexes are described. Examples are given of complexes formed by casein, osteopontin, and recombinant phosphopeptides. Application of these principles and properties to blood serum, milk, urine, and resting saliva is described to show that under physiological conditions they are in the stable region of their stability diagram and so cannot cause soft tissue calcification. Stimulated saliva, however, is in the metastable region, consistent with its role in tooth remineralization. Destabilization of biofluids, with consequential ill-effects, can occur when there is a failure of homeostasis, such as an increase in pH without a balancing increase in the concentration of sequestering phosphopeptides.