FAM20B-catalyzed glycosaminoglycans control murine tooth number by restricting FGFR2b signaling

Development of a new antibody drug to treat congenital tooth agenesis

BACKGROUND

This study aimed to develop a therapeutic agent promoting teeth regeneration from autologous tissues for congenital tooth agenesis, specifically for hypodontia (≤5 missing congenital teeth, 10% prevalence) and oligodontia (≥6 missing congenital teeth, 0.1% prevalence).

HIGHLIGHT

We studied mice genetically deficient in the USAG-1 protein, an antagonist of BMP/Wnt which forms excessive teeth. We identified USAG-1 as a target molecule for increasing the number of teeth. Crossing USAG-1-deficient mice with a congenital tooth agenesis model restored tooth formation. We produced anti-USAG-1 neutralizing antibodies as potential therapeutic agents for the treatment of congenital tooth agenesis. Mice anti-USAG-1 neutralizing antibodies can potentially rescue the developmentally arrested tooth germ programmed to a certain tooth type. A humanized anti-USAG-1 antibody was developed as the final candidate.

CONCLUSION

Targeting USAG-1 shows promise for treating missing congenital tooth. Anti-USAG-1 neutralizing antibodies have been developed and will progress towards clinical trials, which may regenerate missing congenital teeth in conditions, such as hypodontia and oligodontia. The protocol framework for a phase 1 study has been finalized, and preparation for future studies is underway.

Treatment and genetic analysis of multiple supernumerary and impacted teeth in an adolescent patient

BACKGROUND

Multiple supernumerary teeth, combined with numerous impacted teeth, can lead to various malocclusions, posing significant treatment challenges. While certain genes associated with syndromic cases of multiple supernumerary and impacted teeth have been identified, the etiologies of non-syndromic cases still largely remain elusive.

CASE PRESENTATION

Here, we report a treatment of a 12-year-old boy who presented with 10 supernumerary teeth and 6 impacted teeth, accompanied by a genetic analysis to explore the underlying etiology. During the treatment, fifteen teeth were extracted, and various skilled techniques, including the closed-eruption technique and the application of by-pass arches, were utilized. Post-treatment, traction was successful for all the impacted teeth, without any tooth mobility or reduction in gingival height. Space closure, well-aligned teeth, and excellent functional occlusion were achieved. Furthermore, comprehensive genetic analysis was conducted through whole-exome sequencing on the patient and his parents, which revealed a potential link between the patient's numerous supernumerary teeth and abnormal mineralization. Notably, the p.Ser496Pro variant in the TCF7L2 gene was identified as a potential candidate variant in this patient.

CONCLUSIONS

Overall, our findings not only report the treatment of a rare case involving multiple supernumerary and impacted teeth but also offer valuable insights into the molecular basis of supernumerary teeth.

Systems genetics uncover new loci containing functional gene candidates in Mycobacterium tuberculosis-infected Diversity Outbred mice

Mycobacterium tuberculosis infects two billion people across the globe, and results in 8-9 million new tuberculosis (TB) cases and 1-1.5 million deaths each year. Most patients have no known genetic basis that predisposes them to disease. Here, we investigate the complex genetic basis of pulmonary TB by modelling human genetic diversity with the Diversity Outbred mouse population. When infected with M. tuberculosis, one-third develop early onset, rapidly progressive, necrotizing granulomas and succumb within 60 days. The remaining develop non-necrotizing granulomas and survive longer than 60 days. Genetic mapping using immune and inflammatory mediators; and clinical, microbiological, and granuloma correlates of disease identified five new loci on mouse chromosomes 1, 2, 4, 16; and three known loci on chromosomes 3 and 17. Further, multiple positively correlated traits shared loci on chromosomes 1, 16, and 17 and had similar patterns of allele effects, suggesting these loci contain critical genetic regulators of inflammatory responses to M. tuberculosis. To narrow the list of candidate genes, we used a machine learning strategy that integrated gene expression signatures from lungs of M. tuberculosis-infected Diversity Outbred mice with gene interaction networks to generate scores representing functional relationships. The scores were used to rank candidates for each mapped trait, resulting in 11 candidate genes: Ncf2, Fam20b, S100a8, S100a9, Itgb5, Fstl1, Zbtb20, Ddr1, Ier3, Vegfa, and Zfp318. Although all candidates have roles in infection, inflammation, cell migration, extracellular matrix remodeling, or intracellular signaling, and all contain single nucleotide polymorphisms (SNPs), SNPs in only four genes (S100a8, Itgb5, Fstl1, Zfp318) are predicted to have deleterious effects on protein functions. We performed methodological and candidate validations to (i) assess biological relevance of predicted allele effects by showing that Diversity Outbred mice carrying PWK/PhJ alleles at the H-2 locus on chromosome 17 QTL have shorter survival; (ii) confirm accuracy of predicted allele effects by quantifying S100A8 protein in inbred founder strains; and (iii) infection of C57BL/6 mice deficient for the S100a8 gene. Overall, this body of work demonstrates that systems genetics using Diversity Outbred mice can identify new (and known) QTLs and functionally relevant gene candidates that may be major regulators of complex host-pathogens interactions contributing to granuloma necrosis and acute inflammation in pulmonary TB.

The Essential Role of Proteoglycans and Glycosaminoglycans in Odontogenesis

Tooth development and regeneration are regulated through a complex signaling network. Previous studies have focused on the exploration of intracellular signaling regulatory networks, but the regulatory roles of extracellular networks have only been revealed recently. Proteoglycans, which are essential components of the extracellular matrix (ECM) and pivotal signaling molecules, are extensively involved in the process of odontogenesis. Proteoglycans are composed of core proteins and covalently attached glycosaminoglycan chains (GAGs). The core proteins exhibit spatiotemporal expression patterns during odontogenesis and are pivotal for dental tissue formation and periodontium development. Knockout of core protein genes Biglycan, Decorin, Perlecan, and Fibromodulin has been shown to result in structural defects in enamel and dentin mineralization. They are also closely involved in the development and homeostasis of periodontium by regulating signaling transduction. As the functional component of proteoglycans, GAGs are negatively charged unbranched polysaccharides that consist of repeating disaccharides with various sulfation groups; they provide binding sites for cytokines and growth factors in regulating various cellular processes. In mice, GAG deficiency in dental epithelium leads to the reinitiation of tooth germ development and the formation of supernumerary incisors. Furthermore, GAGs are critical for the differentiation of dental stem cells. Inhibition of GAGs assembly hinders the differentiation of ameloblasts and odontoblasts. In summary, core proteins and GAGs are expressed distinctly and exert different functions at various stages of odontogenesis. Given their unique contributions in odontogenesis, this review summarizes the roles of proteoglycans and GAGs throughout the process of odontogenesis to provide a comprehensive understanding of tooth development.

Well-known polypeptides of deer antler velvet with key actives: modern pharmacological advances

Deer antler velvet, with kidney tonifying, promoting the production of essence and blood, strengthening tendons and bones, not only has a thousand-year medicinal history but also its modern pharmacology mainly focuses on its active polypeptides on motor, nerve, and immune systems. The purpose of this report is to fill the gap in the comprehensive, systematic, and detailed review of polypeptides during the recent 30 years (1992-2023). The research method was to review 53 pharmacological articles from the Public Medicine, Web of science, ACS, and Science Direct database sources by searching the keywords "pilose antler," "deer velvet," "Pilose Antler Peptide (PAP) and Velvet Antler Polypeptide (VAP)." The results showed that deer antler polypeptides (DAPs), by regulating EGF, EGFR, MAPK, P38, ERK, NF-κB, Wnt, PI3K, Akt, MMP, AMPK, Stir1, NLRP3, HO-1, Nrf, Rho, TLR, TGF-β, Smad, Ang II, etc., revealed their effects on seven system-related diseases and their mechanisms, including osteoarthritis, intervertebral disc degeneration, osteoporosis, Alzheimer's, Parkinson's, triple-negative breast cancer, liver injury, liver fibrosis, cardiovascular disease, acute lung injury, and late-onset hypogonadism. In conclusion, DAPs have good effects on motor and other system-related diseases, but the secondary and tertiary structures of DAPs (0.5-1800 KDa) need to be further elucidated, and the structure-activity relationship study is still unavailable and needs to be covered. It is expected that this review may provide the necessary literature support for further research. The activities and mechanisms of polypeptides from the past 30 years (1992-2023) are summarized covering seven systems, related diseases, and its regulatory genes and proteins.

Systems genetics uncover new loci containing functional gene candidates in Mycobacterium tuberculosis-infected Diversity Outbred mice

Mycobacterium tuberculosis, the bacillus that causes tuberculosis (TB), infects 2 billion people across the globe, and results in 8-9 million new TB cases and 1-1.5 million deaths each year. Most patients have no known genetic basis that predisposes them to disease. We investigated the complex genetic basis of pulmonary TB by modelling human genetic diversity with the Diversity Outbred mouse population. When infected with M. tuberculosis, one-third develop early onset, rapidly progressive, necrotizing granulomas and succumb within 60 days. The remaining develop non-necrotizing granulomas and survive longer than 60 days. Genetic mapping using clinical indicators of disease, granuloma histopathological features, and immune response traits identified five new loci on mouse chromosomes 1, 2, 4, 16 and three previously identified loci on chromosomes 3 and 17. Quantitative trait loci (QTLs) on chromosomes 1, 16, and 17, associated with multiple correlated traits and had similar patterns of allele effects, suggesting these QTLs contain important genetic regulators of responses to M. tuberculosis. To narrow the list of candidate genes in QTLs, we used a machine learning strategy that integrated gene expression signatures from lungs of M. tuberculosis-infected Diversity Outbred mice with gene interaction networks, generating functional scores. The scores were then used to rank candidates for each mapped trait in each locus, resulting in 11 candidates: Ncf2, Fam20b, S100a8, S100a9, Itgb5, Fstl1, Zbtb20, Ddr1, Ier3, Vegfa, and Zfp318. Importantly, all 11 candidates have roles in infection, inflammation, cell migration, extracellular matrix remodeling, or intracellular signaling. Further, all candidates contain single nucleotide polymorphisms (SNPs), and some but not all SNPs were predicted to have deleterious consequences on protein functions. Multiple methods were used for validation including (i) a statistical method that showed Diversity Outbred mice carrying PWH/PhJ alleles on chromosome 17 QTL have shorter survival; (ii) quantification of S100A8 protein levels, confirming predicted allele effects; and (iii) infection of C57BL/6 mice deficient for the S100a8 gene. Overall, this work demonstrates that systems genetics using Diversity Outbred mice can identify new (and known) QTLs and new functionally relevant gene candidates that may be major regulators of granuloma necrosis and acute inflammation in pulmonary TB.

The molecular mechanisms of glycosaminoglycan biosynthesis regulating chondrogenesis and endochondral ossification

Disorders of chondrocyte differentiation and endochondral osteogenesis are major underlying factors in skeletal developmental disorders, including tibial dysplasia (TD), osteoarthritis (OA), chondrodysplasia (ACH), and multiple epiphyseal dysplasia (MED). Understanding the cellular and molecular pathogenesis of these disorders is crucial for addressing orthopedic diseases resulting from impaired glycosaminoglycan synthesis. Glycosaminoglycan is a broad term that refers to the glycan component of proteoglycan macromolecules. It is an essential component of the cartilage extracellular matrix and plays a vital role in various biological processes, including gene transcription, signal transduction, and chondrocyte differentiation. Recent studies have demonstrated that glycosaminoglycan biosynthesis plays a regulatory role in chondrocyte differentiation and endochondral osteogenesis by modulating various growth factors and signaling molecules. For instance, glycosaminoglycan is involved in mediating pathways such as Wnt, TGF-β, FGF, Ihh-PTHrP, and O-GlcNAc glycosylation, interacting with transcription factors SOX9, BMPs, TGF-β, and Runx2 to regulate chondrocyte differentiation and endochondral osteogenesis. To propose innovative approaches for addressing orthopedic diseases caused by impaired glycosaminoglycan biosynthesis, we conducted a comprehensive review of the molecular mechanisms underlying chondrocyte glycosaminoglycan biosynthesis, which regulates chondrocyte differentiation and endochondral osteogenesis. Our analysis considers the role of genes, glycoproteins, and associated signaling pathways during chondrogenesis and endochondral ossification.

Deficiency of Fam20b-Catalyzed Glycosaminoglycan Chain Synthesis in Neural Crest Leads to Cleft Palate

Cleft palate is one of the most common birth defects. Previous studies revealed that multiple factors, including impaired intracellular or intercellular signals, and incoordination of oral organs led to cleft palate, but were little concerned about the contribution of the extracellular matrix (ECM) during palatogenesis. Proteoglycans (PGs) are one of the important macromolecules in the ECM. They exert biological functions through one or more glycosaminoglycan (GAG) chains attached to core proteins. The family with sequence similarity 20 member b (Fam20b) are newly identified kinase-phosphorylating xylose residues that promote the correct assembly of the tetrasaccharide linkage region by creating a premise for GAG chain elongation. In this study, we explored the function of GAG chains in palate development through Wnt1-Cre; Fam20b^f/f mice, which exhibited complete cleft palate, malformed tongue, and micrognathia. In contrast, Osr2-Cre; Fam20b^f/f mice, in which Fam20b was deleted only in palatal mesenchyme, showed no abnormality, suggesting that failed palatal elevation in Wnt1-Cre; Fam20b^f/f mice was secondary to micrognathia. In addition, the reduced GAG chains promoted the apoptosis of palatal cells, primarily resulting in reduced cell density and decreased palatal volume. The suppressed BMP signaling and reduced mineralization indicated an impaired osteogenesis of palatine, which could be rescued partially by constitutively active Bmpr1a. Together, our study highlighted the key role of GAG chains in palate morphogenesis.

Genome-wide identification of potential odontogenic genes involved in the dental epithelium-mesenchymal interaction during early odontogenesis

BACKGROUND

Epithelium-mesenchymal interactions are involved in odontogenic processes. Previous studies have focused on the intracellular signalling regulatory network in tooth development, but the functions of extracellular regulatory molecules have remained unclear. This study aims to explore the gene profile of extracellular proteoglycans and their glycosaminoglycan chains potentially involved in dental epithelium-mesenchymal interactions using high-throughput sequencing to provide new understanding of early odontogenesis.

RESULTS

Whole transcriptome profiles of the mouse dental epithelium and mesenchyme were investigated by RNA sequencing (RNA-seq). A total of 1,281 and 1,582 differentially expressed genes were identified between the dental epithelium and mesenchyme at E11.5 and E13.5, respectively. Enrichment analysis showed that extracellular regions and ECM-receptor interactions were significantly enriched at both E11.5 and E13.5. Polymerase chain reaction analysis confirmed that the extracellular proteoglycan family exhibited distinct changes during epithelium-mesenchymal interactions. Most proteoglycans showed higher transcript levels in the dental mesenchyme, whereas only a few were upregulated in the epithelium at both stages. In addition, 9 proteoglycans showed dynamic expression changes between these two tissue compartments. Gpc4, Sdc2, Spock2, Dcn and Lum were expressed at higher levels in the dental epithelium at E11.5, whereas their expression was significantly higher in the dental mesenchyme at E13.5, which coincides with the odontogenic potential shift. Moreover, the glycosaminoglycan biosynthetic enzymes Ext1, Hs3st1/5, Hs6st2/3, Ndst3 and Sulf1 also exhibited early upregulation in the epithelium but showed markedly higher expression in the mesenchyme after the odontogenic potential shift.

CONCLUSION

This study reveals the dynamic expression profile of extracellular proteoglycans and their biosynthetic enzymes during the dental epithelium-mesenchymal interaction. This study offers new insight into the roles of extracellular proteoglycans and their distinct sulfation underlying early odontogenesis.

Tooth number abnormality: from bench to bedside

Tooth number abnormality is one of the most common dental developmental diseases, which includes both tooth agenesis and supernumerary teeth. Tooth development is regulated by numerous developmental signals, such as the well-known Wnt, BMP, FGF, Shh and Eda pathways, which mediate the ongoing complex interactions between epithelium and mesenchyme. Abnormal expression of these crutial signalling during this process may eventually lead to the development of anomalies in tooth number; however, the underlying mechanisms remain elusive. In this review, we summarized the major process of tooth development, the latest progress of mechanism studies and newly reported clinical investigations of tooth number abnormality. In addition, potential treatment approaches for tooth number abnormality based on developmental biology are also discussed. This review not only provides a reference for the diagnosis and treatment of tooth number abnormality in clinical practice but also facilitates the translation of basic research to the clinical application.

Regulation of secretory pathway kinase or kinase-like proteins in human cancers

Secretory pathway kinase or kinase-like proteins (SPKKPs) are effective in the lumen of the endoplasmic reticulum (ER), Golgi apparatus (GA), and extracellular space. These proteins are involved in secretory signaling pathways and are distinctive from typical protein kinases. Various reports have shown that SPKKPs regulate the tumorigenesis and progression of human cancer via the phosphorylation of various substrates, which is essential in physiological and pathological processes. Emerging evidence has revealed that the expression of SPKKPs in human cancers is regulated by multiple factors. This review summarizes the current understanding of the contribution of SPKKPs in tumorigenesis and the progression of immunity. With the epidemic trend of immunotherapy, targeting SPKKPs may be a novel approach to anticancer therapy. This study briefly discusses the recent advances regarding SPKKPs.

Tendon Cells Root Into (Instead of Attach to) Humeral Bone Head via Fibrocartilage-Enthesis

Large joints are composed of two closely linked cartilages: articular cartilage (AC; rich in type II collagen, a well-studied tissue) and fibrocartilaginous enthesis (FE; rich in type I collagen, common disorder sites of enthesopathy and sporting injuries, although receiving little attention). For many years, both cartilages were thought to be formed by chondrocytes, whereas tendon, which attaches to the humeral bone head, is primarily considered as a completely different connective tissue. In this study, we raised an unconventional hypothesis: tendon cells directly form FE via cell transdifferentiation. To test this hypothesis, we first qualitatively and quantitatively demonstrated distinct differences between AC and FE in cell morphology and cell distribution, mineralization status, extracellular matrix (ECM) contents, and critical ECM protein expression profiles using comprehensive approaches. Next, we traced the cell fate of tendon cells using Scx^Lin (a tendon specific Cre Scx^CreERT2; R26R-tdTomato line) with one-time tamoxifen induction at early (P3) or young adult (P28) stages and harvested mice at different development ages, respectively. Our early tracing data revealed different growth events in tendon and FE: an initial increase but gradual decrease in the Scx^Lin tendon cells and a continuous expansion in the Scx^Lin FE cells. The young adult tracing data demonstrated continuous recruitment of Scx^Lin cells into FE expansion during P28 and P56. A separate tracing line, 3.2 Col 1^Lin (a so-called "bone-specific" line), further confirmed the direct contribution of tendon cells for FE cell formation, which occurred in days but FE ECM maturation (including high levels of SOST, a potent Wnt signaling inhibitor) took weeks. Finally, loss of function data using diphtheria toxin fragment A (DTA) in Scx^Lin cells demonstrated a significant reduction of Scx^Lin cells in both tendons and FE cells, whereas the gain of function study (by stabilizing β-catenin in Scx^Lin tendon cells via one-time injection of tamoxifen at P3 and harvesting at P60) displayed great expansion of both Scx^Lin tendon and FE mass. Together, our studies demonstrated that fibrocartilage is an invaded enthesis likely originating from the tendon via a quick cell transdifferentiation mechanism with a lengthy ECM maturation process. The postnatally formed fibrocartilage roots into existing cartilage and firmly connects tendon and bone instead of acting as a simple attachment site as widely believed. We believe that this study will stimulate more intense exploring in this understudied area, especially for patients with enthesopathy and sporting injuries.

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.

An in situ hybridization study of syndecan family during the late stages of developing mouse molar tooth germ

Expression of syndecan-1, 2, 3, and 4 mRNAs during the late stages of tooth germ formation was investigated by in situ hybridization, using [³⁵S]-UTP-labeled cRNA probes. Syndecan-1 mRNA was mainly expressed in the stellate reticulum and stratum intermedium as well as at the cervical region of dental papilla/dental follicle during E18.5-P3.0. Expression in the dental epithelium was enhanced during the postnatal periods, which was supported by real-time RT-PCR analysis. These spatiotemporal expression patterns may suggest specific roles of syndecan-1 in tooth formation such as tooth eruption or root formation. Syndecan-3 mRNA expression became evident in odontoblasts at E18.5, but compared to collagen type I mRNA, which was strongly expressed at this stage, syndecan-3 expression in odontoblast was restricted in mature odontoblasts beneath the cusps during the postnatal periods. This result was also supported by real-time RT-PCR analysis, and indicated that syndecan-3 may be involved in the progress of dentinogenesis rather than in the initiation of it. Syndecan-4 mRNA roughly showed comparable expression patterns to those of syndecan-3. Syndecan-2 mRNA did not show significant expression during the experimental period, but real-time RT-PCR analysis suggested that syndecan-2 expression might be enhanced with hard tissue formation.

An Overview of in vivo Functions of Chondroitin Sulfate and Dermatan Sulfate Revealed by Their Deficient Mice

Chondroitin sulfate (CS), dermatan sulfate (DS) and heparan sulfate (HS) are covalently attached to specific core proteins to form proteoglycans in their biosynthetic pathways. They are constructed through the stepwise addition of respective monosaccharides by various glycosyltransferases and maturated by epimerases as well as sulfotransferases. Structural diversities of CS/DS and HS are essential for their various biological activities including cell signaling, cell proliferation, tissue morphogenesis, and interactions with a variety of growth factors as well as cytokines. Studies using mice deficient in enzymes responsible for the biosynthesis of the CS/DS and HS chains of proteoglycans have demonstrated their essential functions. Chondroitin synthase 1-deficient mice are viable, but exhibit chondrodysplasia, progression of the bifurcation of digits, delayed endochondral ossification, and reduced bone density. DS-epimerase 1-deficient mice show thicker collagen fibrils in the dermis and hypodermis, and spina bifida. These observations suggest that CS/DS are essential for skeletal development as well as the assembly of collagen fibrils in the skin, and that their respective knockout mice can be utilized as models for human genetic disorders with mutations in chondroitin synthase 1 and DS-epimerase 1. This review provides a comprehensive overview of mice deficient in CS/DS biosyntheses.

The ABCs of the atypical Fam20 secretory pathway kinases

The study of extracellular phosphorylation was initiated in late 19th century when the secreted milk protein, casein, and egg-yolk protein, phosvitin, were shown to be phosphorylated. However, it took more than a century to identify Fam20C, which phosphorylates both casein and phosvitin under physiological conditions. This kinase, along with its family members Fam20A and Fam20B, defined a new family with altered amino acid sequences highly atypical from the canonical 540 kinases comprising the kinome. Fam20B is a glycan kinase that phosphorylates xylose residues and triggers peptidoglycan biosynthesis, a role conserved from sponges to human. The protein kinase, Fam20C, conserved from nematodes to humans, phosphorylates well over 100 substrates in the secretory pathway with overall functions postulated to encompass endoplasmic reticulum homeostasis, nutrition, cardiac function, coagulation, and biomineralization. The preferred phosphorylation motif of Fam20C is SxE/pS, and structural studies revealed that related member Fam20A allosterically activates Fam20C by forming a heterodimeric/tetrameric complex. Fam20A, a pseudokinase, is observed only in vertebrates. Loss-of-function genetic alterations in the Fam20 family lead to human diseases such as amelogenesis imperfecta, nephrocalcinosis, lethal and nonlethal forms of Raine syndrome with major skeletal defects, and altered phosphate homeostasis. Together, these three members of the Fam20 family modulate a diverse network of secretory pathway components playing crucial roles in health and disease. The overarching theme of this review is to highlight the progress that has been made in the emerging field of extracellular phosphorylation and the key roles secretory pathway kinases play in an ever-expanding number of cellular processes.