Disrupted Protein Expression and Altered Proteolytic Events in Hypophosphatemic Dentin Can Be Rescued by Dentin Matrix Protein 1

Abstract

Dentin, one of the four mineralized tissues of the craniofacial complex, forms sequentially from the deposition of an organic matrix to the nucleation of an inorganic phase within the matrix scaffold. Several promoters and inhibitors of mineralization support and regulate mineral nucleation. Clinical and experimental evidence suggest that dentin matrix protein 1 (DMP1) and phosphate-regulating neutral endopeptidase (PHEX) cooperate and are necessary for the formation of a cohesive dentin layer. The following study investigates the effect of PHEX loss-of-function on dentin matrix formation preceding mineralization. Using the Hyp mouse, an animal model for X-linked hypophosphatemia (XLH), we identified an irregular distribution of dentin extracellular matrix proteins. Likewise, dental pulp stem cells (DPSCs) from XLH patients exhibited altered proteolytic events with disrupted extracellular matrix deposition. Further differentiation assays demonstrated that XLH DPSCs exhibited impaired matrix mineralization. Overexpression of DMP1 in XLH DPSCs restored the irregular protein processing patterns to near-physiological levels. Our results support the hypothesis that hypophosphatemia resulting from PHEX loss-of-function affects the integrity of the organization of the dentin matrix and suggests that exogenous DMP1 can restore physiological processing of matrix proteins, in addition to its canonical role in mineralization.Keywords: dental pulp stem cells, X-linked hypophosphatemia, hypophosphatemia, dentin matrix protein 1, matrix biology Frontiers in Physiology | www.frontiersin.org February 2020 | Volume 11 | Article 82

Insights into dental mineralization from three heritable mineralization disorders

“…XLH features increased levels of fibroblast growth factor 23 (FGF23), hypophosphatemia, low 1,25-dihydroxyvitamin D, and elevated parathyroid hormone (PTH), with combined disturbances in local and systemic mineralization that manifest as osteomalacia and growth plate disturbances (Foster, Nociti, and Somerman 2014). PHEX is highly expressed by odontoblasts and osteocytes, with low level expression by cementocytes (Zhang et al 2020). Although the total range and identities of the physiologic substrates for PHEX remain uncertain, the enzyme exhibits the ability to cleave and inactivate acidic serine-and aspartate-rich motif (ASARM) peptides derived from ECM proteins including OPN and matrix extracellular phosphoprotein (MEPE) that can act as mineral inhibitors (Salmon et al 2014;Addison et al 2010;Barros et al 2013).…”

“…The Hyp mutant mouse model of XLH harbors Phex mutations and phenocopies biochemical and skeletal features of XLH (Eicher et al 1976). Dental defects in Hyp mice are well described and replicate many aspects of human dental pathology in XLH, including enamel hypoplasia, defects in dentin volume and mineralization, and accumulation of interglobular dentin (Zhang et al 2020;Coyac et al 2018). While premature loss of teeth is generally not described for XLH (though extractions for abscessed teeth are common), Hyp mice exhibit thin acellular cementum.…”

“…While premature loss of teeth is generally not described for XLH (though extractions for abscessed teeth are common), Hyp mice exhibit thin acellular cementum. regions of PDL detachment, and significantly altered periodontal mechanical properties (Zhang et al 2020). These types of accumulated periodontal defects may contribute to increased periodontal disease later in life in individuals with XLH (Biosse Duplan et al 2017).…”

Consensus Recommendations for the Diagnosis and Management of X-Linked Hypophosphatemia in Belgium

“…ARHR1 may present as a sclerosing bone dysplasia with hyperostosis and very dense vertebral bodies ( 54 ). DMP1 as well as PHEX are highly expressed in osteocytes, although it remains unknown how they alter the phosphate-FGF23 set point ( 55 ). ENPP1 is a critical enzyme in the generation of the mineralization inhibitor pyrophosphate.…”

Between a rock and a hard place: Regulation of mineralization in the periodontium

“…Similar to the Hyp model of XLH, Dmp1 −/− mice exhibit thin acellular cementum, PDL detachment, osteomalacic alveolar bone, and periodontal breakdown (Foster, Nociti Jr., & Somerman, 2014 ; Ye, Zhang, Ke, Bonewald, & Feng, 2008 ). Guirado and colleagues noted disturbed expression and localization of ECM proteins (including DMP1) in dentin and pulp of Hyp mice, which suggests local disturbances may contribute to hypomineralization defects (Guirado et al, 2020 ). While in vitro addition of DMP1 protein corrected some abnormalities in XLH human dental pulp stem cell cultures, in vivo overexpression of DMP1 via the Dspp promoter in Hyp mice did not improve dentin or alveolar bone defects.…”