Histidine at position 5 is the specificity "switch" between two parathyroid hormone receptor subtypes

Area of Science:

• Endocrinology
• Molecular Biology
• Receptor Pharmacology

Background:

• The parathyroid hormone (PTH) and PTH-related protein (PTHrP) system involves two hormones and at least two distinct G protein-coupled receptors.
• The PTH/PTHrP receptor is found in kidney and bone, while the human PTH2 receptor is primarily in the brain and pancreas.
• PTH and PTHrP bind to the PTH/PTHrP receptor equally, but only PTH binds to the hPTH2 receptor.

Purpose of the Study:

• To investigate the role of amino acid position 5 in determining receptor selectivity between PTH and PTHrP.
• To synthesize and evaluate hybrid analogs of PTH and PTHrP to understand their interactions with hPTH/PTHrP and hPTH2 receptors.

Main Methods:

• Design and synthesis of point-mutated and segmental hybrid analogs of PTH and PTHrP.
• Utilizing human cell lines stably expressing either the hPTH/PTHrP receptor or the hPTH2 receptor.
• Assessing biological activity through receptor binding assays and measurements of cAMP accumulation and cytosolic free calcium release.

Main Results:

• Both [Ile5]PTH-(1-34) and [His5]PTH-(1-34) activate the hPTH/PTHrP receptor.
• [Ile5]PTHrP-(1-34) activates the hPTH2 receptor, whereas [His5]PTH-(1-34) does not.
• The segmental hybrid PTHrP-(1-14)-PTH-(15-34) activates the hPTH/PTHrP receptor but not the hPTH2 receptor, while [Ile5]PTHrP-(1-14)-PTH-(15-34) activates both.

Conclusions:

• Histidine at position 5 (His5) in PTHrP is the primary determinant of receptor subtype specificity within the human PTH/PTHrP and hPTH2 receptor system.
• The hPTH2 receptor exhibits selectivity for PTH over PTHrP based on structural differences at position 5.
• The mechanism may involve conformational changes or steric hindrance at the receptor-ligand interface due to His5.