Novel sandwich ELISAs for rat DMP1: age-related decrease of circulatory DMP1 levels in male rats

CRISPR interference provides increased cell type-specificity compared to the Cre-loxP system

Cre-mediated recombination is frequently used for cell type-specific loss of function (LOF) studies. A major limitation of this system is recombination in unwanted cell types. CRISPR interference (CRISPRi) has been used effectively for global LOF in mice. However, cell type-specific CRISPRi, independent of recombination-based systems, has not been reported. To test the feasibility of cell type-specific CRISPRi, we produced two novel knock-in mouse models that achieve gene suppression when used together: one expressing dCas9::KRAB under the control of a cell type-specific promoter and the other expressing a single guide RNA from a safe harbor locus. We then compared the phenotypes of mice in which the same gene was targeted by either CRISPRi or the Cre-loxP system, with cell specificity conferred by Dmp1 regulatory elements in both cases. We demonstrate that CRISPRi is effective for cell type-specific LOF and that it provides improved cell type-specificity compared to the Cre-loxP system.

The Role of DMP1 in CKD-MBD

PURPOSE OF REVIEW

Chronic kidney disease-mineral and bone disorder (CKD-MBD) has become a global health crisis with very limited therapeutic options. Dentin matrix protein 1 (DMP1) is a matrix extracellular protein secreted by osteocytes that has generated recent interest for its possible involvement in CKD-MBD pathogenesis. This is a review of DMP1 established regulation and function, and early studies implicating DMP1 in CKD-MBD.

RECENT FINDINGS

Patients and mice with CKD show perturbations of DMP1 expression in bone, associated with impaired osteocyte maturation, mineralization, and increased fibroblast growth factor 23 (FGF23) production. In humans with CKD, low circulating DMP1 levels are independently associated with increased cardiovascular events. We recently showed that DMP1 supplementation lowers circulating FGF23 levels and improves bone mineralization and cardiac outcomes in mice with CKD. Mortality rates are extremely high among patients with CKD and have only marginally improved over decades. Bone disease and FGF23 excess contribute to mortality in CKD by increasing the risk of bone fractures and cardiovascular disease, respectively. Previous studies focused on DMP1 loss-of-function mutations have established its role in the regulation of FGF23 and bone mineralization. Recent studies show that DMP1 supplementation may fill a crucial therapeutic gap by improving bone and cardiac health in CKD.

Collagen Suprafibrillar Confinement Drives the Activity of Acidic Calcium-Binding Polymers on Apatite Mineralization

Bone collagenous extracellular matrix provides a confined environment into which apatite crystals form. This biomineralization process is related to a cascade of events partly controlled by noncollagenous proteins. Although overlooked in bone models, concentration and physical environment influence their activities. Here, we show that collagen suprafibrillar confinement in bone comprising intra- and interfibrillar spaces drives the activity of biomimetic acidic calcium-binding polymers on apatite mineralization. The difference in mineralization between an entrapping dentin matrix protein-1 (DMP1) recombinant peptide (rpDMP1) and the synthetic polyaspartate validates the specificity of the 57-KD fragment of DMP1 in the regulation of mineralization, but strikingly without phosphorylation. We show that all the identified functions of rpDMP1 are dedicated to preclude pathological mineralization. Interestingly, transient apatite phases are only found using a high nonphysiological concentration of additives. The possibility to combine biomimetic concentration of both collagen and additives ensures specific chemical interactions and offers perspectives for understanding the role of bone components in mineralization.

Detection of Extremely Low Concentrations of Biological Substances Using Near-Field Illumination

An external force-assisted near-field illumination biosensor (EFA-NI biosensor) detects a target substance that is propelled through an evanescent field by an external force. The target substance is sandwiched between an antibody coupled to a magnetic bead and an antibody coupled to a polystyrene bead. The external force is supplied by a magnetic field. The magnetic bead propels the target substance and the polystyrene bead emits an optical signal. The detection protocol includes only two steps; mixing the sample solution with a detection reagent containing the antibody-coated beads and injecting the sample mixture into a liquid cell. Because the system detects the motion of the beads, the sensor allows detection of trace amounts of target substances without a washing step. The detection capability of the sensor was demonstrated by the detection of norovirus virus-like particles at a concentration of ~40 particles per 100 μl in contaminated water.

Immunohistochemical analysis of dentin matrix protein 1 (Dmp1) phosphorylation by Fam20C in bone: implications for the induction of biomineralization

Dmp1 is an acidic phosphoprotein that is specifically expressed in osteocytes. During the secretory process, the full-length, precursor Dmp1 is cleaved into N- and C-terminal fragments. C-terminal Dmp1 is phosphorylated, becoming a highly negatively charged domain that may assist in bone mineralization by recruiting calcium ions and influencing subsequent mineral deposition. It has been recently reported that the Golgi-localized protein kinase Fam20C phosphorylates Dmp1 in vitro. To investigate this phosphorylation in situ, we determined the locations of phosphorylated Dmp1 and Fam20C in rat bones using immunohistochemistry. During osteocytogenesis, osteoblastic, osteoid, and young osteocytes (but not old osteocytes) express Dmp1 mRNA and contain Dmp1 protein in the Golgi apparatus. These Dmp1-producing cells were distributed across the surface layer of cortical bone. Using immunofluorescence, we found that N- and C-terminal Dmp1 fragments were predominantly distributed along the lacunar walls and canaliculi of mineralized bone, respectively, but were not present in the osteoid matrix. We also found that Fam20C and its substrate, C-terminal Dmp1, colocalized in the Golgi of osteoblastic, osteoid, and young osteocytes. Furthermore, phosphorylated C-terminal Dmp1 was present in the Golgi of young osteocytes. Double-labeling immunoelectron microscopy revealed that phosphorylated C-terminal Dmp1 localized to the canalicular wall in mineralized bone. These findings suggest that C-terminal Dmp1 is phosphorylated within osteocytes and then secreted into the pericanalicular matrix of mineralized bone. Phosphorylated, negatively charged C-terminal Dmp1 in the pericanalicular matrix may play an important role in bone mineralization by recruiting calcium ions.