Modeling mutant/wild-type interactions to ascertain pathogenicity of PROKR2 missense variants in patients with isolated GnRH deficiency

A major challenge in human genetics is the validation of pathogenicity of heterozygous missense variants. This problem is well-illustrated by PROKR2 variants associated with Isolated GnRH Deficiency (IGD). Homozygous, loss of function variants in PROKR2 was initially implicated in autosomal recessive IGD; however, most IGD-associated PROKR2 variants are heterozygous. Moreover, while IGD patient cohorts are enriched for PROKR2 missense variants similar rare variants are also found in normal individuals. To elucidate the pathogenic mechanisms distinguishing IGD-associated PROKR2 variants from rare variants in controls, we assessed 59 variants using three approaches: (i) in silico prediction, (ii) traditional in vitro functional assays across three signaling pathways with mutant-alone transfections, and (iii) modified in vitro assays with mutant and wild-type expression constructs co-transfected to model in vivo heterozygosity. We found that neither in silico analyses nor traditional in vitro assessments of mutants transfected alone could distinguish IGD variants from control variants. However, in vitro co-transfections revealed that 15/34 IGD variants caused loss-of-function (LoF), including 3 novel dominant-negatives, while only 4/25 control variants caused LoF. Surprisingly, 19 IGD-associated variants were benign or exhibited LoF that could be rescued by WT co-transfection. Overall, variants that were LoF in ≥ 2 signaling assays under co-transfection conditions were more likely to be disease-associated than benign or 'rescuable' variants. Our findings suggest that in vitro modeling of WT/Mutant interactions increases the resolution for identifying causal variants, uncovers novel dominant negative mutations, and provides new insights into the pathogenic mechanisms underlying heterozygous PROKR2 variants.

Inducible podocyte-specific deletion of CTCF drives progressive kidney disease and bone abnormalities

Progressive chronic kidney diseases (CKDs) are on the rise worldwide. However, the sequence of events resulting in CKD progression remain poorly understood. Animal models of CKD exploring these issues are confounded by systemic toxicities or surgical interventions to acutely induce kidney injury. Here we report the generation of a CKD mouse model through the inducible podocyte-specific ablation of an essential endogenous molecule, the chromatin structure regulator CCCTC-binding factor (CTCF), which leads to rapid podocyte loss (iCTCFpod-/-). As a consequence, iCTCFpod-/- mice develop severe progressive albuminuria, hyperlipidemia, hypoalbuminemia, and impairment of renal function, and die within 8-10 weeks. CKD progression in iCTCFpod-/- mice leads to high serum phosphate and elevations in fibroblast growth factor 23 (FGF23) and parathyroid hormone that rapidly cause bone mineralization defects, increased bone resorption, and bone loss. Dissection of the timeline leading to glomerular pathology in this CKD model led to the surprising observation that podocyte ablation and the resulting glomerular filter destruction is sufficient to drive progressive CKD and osteodystrophy in the absence of interstitial fibrosis. This work introduces an animal model with significant advantages for the study of CKD progression, and it highlights the need for podocyte-protective strategies for future kidney therapeutics.

Acute Parathyroid Hormone Injection Increases C-Terminal but Not Intact Fibroblast Growth Factor 23 Levels

The acute effects of parathyroid hormone (PTH) on fibroblast growth factor 23 (FGF23) in vivo are not well understood. After a single subcutaneous PTH (1-34) injection (50 nmol/kg) in mice, FGF23 levels were assessed in plasma using assays that measure either intact alone (iFGF23) or intact/C-terminal FGF23 (cFGF23). Furthermore, FGF23 messenger RNA (mRNA) and protein levels were assessed in bone. In addition, we examined the effects of PTH treatment on FGF23 production in vitro using differentiated calvarial osteocyte-like cells. cFGF23 levels increased by three- to fivefold within 2 hours following PTH injection, which returned to baseline by 4 hours. In contrast, iFGF23 levels remained unchanged for the first 2 hours, yet declined to ∼60% by 6 hours and remained suppressed before returning to baseline after 24 hours. Using homozygous mice for an autosomal dominant hypophosphatemic rickets-FGF23 mutation or animals treated with a furin inhibitor, we showed that cFGF23 and iFGF23 levels increased equivalently after PTH injection. These findings are consistent with increased FGF23 production in bone, yet rapid cleavage of the secreted intact protein. Using primary osteocyte-like cell cultures, we showed that PTH increased FGF23 mRNA expression through cyclic adenosine monophosphate/protein kinase A, but not inositol triphosphate/protein kinase C signaling; PTH also increased furin protein levels. In conclusion, PTH injection rapidly increases FGF23 production in bone in vivo and in vitro. However, iFGF23 is rapidly degraded. At later time points through an unidentified mechanism, a sustained decrease in FGF23 production occurs.

SIKs control osteocyte responses to parathyroid hormone

Parathyroid hormone (PTH) activates receptors on osteocytes to orchestrate bone formation and resorption. Here we show that PTH inhibition of SOST (sclerostin), a WNT antagonist, requires HDAC4 and HDAC5, whereas PTH stimulation of RANKL, a stimulator of bone resorption, requires CRTC2. Salt inducible kinases (SIKs) control subcellular localization of HDAC4/5 and CRTC2. PTH regulates both HDAC4/5 and CRTC2 localization via phosphorylation and inhibition of SIK2. Like PTH, new small molecule SIK inhibitors cause decreased phosphorylation and increased nuclear translocation of HDAC4/5 and CRTC2. SIK inhibition mimics many of the effects of PTH in osteocytes as assessed by RNA-seq in cultured osteocytes and following in vivo administration. Once daily treatment with the small molecule SIK inhibitor YKL-05-099 increases bone formation and bone mass. Therefore, a major arm of PTH signalling in osteocytes involves SIK inhibition, and small molecule SIK inhibitors may be applied therapeutically to mimic skeletal effects of PTH.

Parathyroid hormone (PTH) is an endogenous hormone and osteoporosis therapeutic that suppresses sclerostin activity. Here the authors develop SIK inhibitors as potential therapeutic tools and use them to show that PTH-cAMP signalling in osteocytes inhibits SIK2 from driving Hdac4/5 nuclear shuttling to suppress sclerostin.

New limit on CPT violation

A search for antiproton decay has been made at the Fermilab Antiproton Accumulator. Limits are placed on fifteen antiproton decay modes. The results are used to place limits on the characteristic mass scale m(X) that could be associated with CPT violation accompanied by baryon number violation.

Effect of low-molecular-weight chitosans on the adhesive properties of oral streptococci

It was previously shown that a low-molecular-weight chitosan and its derivatives N-carboxymethyl chitosan and imidazolyl chitosan inhibit Streptococcus mutans adsorption to hydroxyapatite. The ability of the same molecules to interfere with adhesive properties of other oral streptococci (Streptococcus sanguis, Streptococcus gordonii, Streptococcus constellatus, Streptococcus anginosus, Streptococcus intermedius, Streptococcus oralis, Streptococcus salivarius, Streptococcus vestibularis) was tested. When saliva-coated or -uncoated hydroxyapatite beads were treated with N-carboxymethyl chitosan, a reduction varying from 60% to 98% depending on strains was observed. Low-molecular-weight chitosans and imidazolyl chitosan did not have any effect. Growth in N-carboxymethyl chitosan-supplemented medium (final concentrations ranging from 20 to 500 micrograms.ml-1) caused a dose related reduction in the adsorption of all strains to hydroxyapatite and in their affinity towards xylene. No effect was observed with low-molecular-weight chitosans and imidazolyl chitosan. In contrast to what observed with S. mutans, the three polysaccharides did not affect detachment from hydroxyapatite beads and adherence to cheek epithelial cells of the other streptococci. These results suggest that low-molecular-weight chitosans and/or imidazolyl chitosan, selectively affecting S. mutans adsorption to hydroxyapatite, may be very interesting as potential anti-dental caries agents.