The expression of constitutively active PTH-PTHrp receptors in kidney, bone, and growth-plate chondrocytes provides a plausible genetic explanation for mineral-ion abnormalities and metaphyseal changes in patients with Jansen's disease.

Inverse agonists, ligands that suppress spontaneous receptor signaling activity, have been described for a growing number of G protein-coupled receptors; however, none have been reported for the PTH/calcitonin/secretin receptor family. We took advantage of the constitutive signaling activity of two mutant forms of the PTH/PTH-related peptide (PTHrP) receptor, recently identified in patients with Jansen's metaphyseal chondrodysplasia, to screen for PTH and PTHrP analogs with inverse agonist activity. Two antagonist peptides, [Leu11, D-Trp12]hPTHrP(7-34)NH2 and [D-Trp12, Tyr34]bPTH-(7-34)NH2, displayed inverse agonist activity and reduced cAMP in COS-7 cells expressing either mutant receptor by 30-50% (EC50 approximately 50 nM). These data demonstrate that the concept of inverse agonism can be extended to this distinct family of G protein-coupled receptors and their cognate antagonist peptide ligands. Such ligands shall be useful probes of the multi-state conformational equilibria proposed for these receptors and could lead to new approaches for treating human diseases caused by receptor activating mutations.

Two different activating PTH/PTH-related peptide (PTHrP) receptor mutations, H223R and T410P, were recently identified as the most likely cause of Jansen's metaphyseal chondrodysplasia. To assess the functional importance of either amino acid position in the human PTH/PTHrP receptor, H223 and T410 were individually replaced by all other amino acids. At position 223, only arginine and lysine led to agonist-independent cAMP accumulation; all other amino acid substitutions resulted in receptor mutants that lacked constitutive activity or were uninformative due to poor cell surface expression. In contrast, most amino acid substitutions at position 410 conferred constitutive cAMP accumulation and affected PTH/PTHrP receptor expression not at all or only mildly. Mutations corresponding to the H223R or T410P exchange in the human PTH/PTHrP receptor also led to constitutive activity when introduced into the opossum receptor homolog, but showed little or no change in basal cAMP accumulation when introduced into the rat PTH/PTHrP receptor. The PTH/PTHrP receptor residues mutated in Jansen's disease are conserved in all mammalian members of this family of G protein-coupled receptors. However, when the equivalent of either the H223R or the T410P mutation was introduced into several other related receptors, including the PTH2 receptor and the receptors for calcitonin, secretin, GH-releasing hormone, glucagon-like peptide I, and CRH, the resulting mutants failed to induce constitutive activity. These studies suggest that two residues in the human PTH/PTHrP receptor, 223 and 410, have critical roles in signal transduction, but with different sequence constrains.

As graft and patient survival after liver transplantation (LT) continue to improve, chronic medical complications from long-term immunosuppression have grown to dominate the landscape of posttransplant care. Chronic renal dysfunction, in particular, has become a mounting burden, with as many as 18% of LT recipients developing chronic kidney disease by the fifth postoperative year. 1 Many of these patients will deteriorate into end-stage renal disease, an endpoint clearly shown to increase morbidity and shorten patient survival. 2 Though often multifactorial, the chronic decline in renal function in LT recipients is, to a great extent, driven by calcineurin inhibitors (CNIs), the current standard in immunosuppression. CNIs result in renal vasoconstriction, which, over time, can progress to tubulointerstitial fibrosis and unrecoverable renal injury. 3 Strategies that minimize CNI use have thus been sought to ameliorate the commonly observed decline in kidney function that occurs after LT.Sirolimus (SRL), a mammalian target of rapamycin inhibitor with potent antiproliferative and immunosuppressive properties, has been touted within the solid organ transplant community for its freedom from significant renal toxicity or neurotoxicity. 4 Although SRL is generally avoided in the immediate postoperative period because of reported increased rates of hepatic artery thrombosis and impaired wound healing, 5 as many as 14% of liver transplant recipients receive SRL during the maintenance phase of immunosuppression, generally for either the prevention or amelioration of CNIrelated complications. 6 Multiple studies have suggested a renal sparing effect in patients with CNIinduced nephrotoxicity converted to either SRL alone or SRL used in conjunction with a low-dose CNI regimen. [7][8][9][10][11][12][13] By and large, these have been small, uncontrolled, single-center reports. As such, the efficacy of SRL with respect to the endpoint of renal preservation has been at once suggestive but inconclusive. More recent data suggest that the reported advantage of SRLbased immunosuppression on long-term kidney function may be overstated. This journal recently published the results of an open-label, randomized controlled trial of conventional immunosuppression versus SRL-based immunosuppression in patients with post-LT, CNI-related nephrotoxicity. In patients receiving SRL, a significant, if modest, improvement in the change in the glomerular filtration rate (GFR) was shown at 1 year from the time of conversion, but there was no significant difference in absolute GFR. 14 Shenoy and colleagues, 15 in a prospective comparison of SRL conversion with unmodified, conventional immunosuppression in a similar population, showed an early, significant improvement in measured creatinine clearance that, by 1 year of follow-up, was not maintained. A case-control study from Campbell and colleagues 16 examined the effect of SRL-based immunosuppression in LT recipients with relatively preserved renal function. Patients switched to SRL were no less like...