Novel function for receptor activity-modifying proteins (RAMPs) in post-endocytic receptor trafficking

Area of Science:

• Biochemistry
• Cell Biology
• Molecular Pharmacology

Background:

• Receptor activity modifying proteins (RAMPs) 1-3 are single transmembrane proteins that dictate the phenotype of G-protein coupled receptors (GPCRs).
• RAMPs form heterodimers with calcitonin receptor-like receptor (CRLR) to generate distinct receptors, such as adrenomedullin receptors (AM1R, AM2R) and calcitonin gene-related peptide receptors.
• RAMPs are crucial for GPCR surface expression and signaling, but their role in receptor trafficking remains incompletely understood.

Purpose of the Study:

• To investigate the role of RAMPs, specifically RAMP3, in the post-endocytic trafficking of GPCRs.
• To determine if the PDZ type I domain in RAMP3 influences receptor trafficking pathways.
• To elucidate the molecular mechanisms underlying RAMP-mediated receptor recycling.

Main Methods:

• Adenylate cyclase assays and radioligand binding were used to assess receptor function and binding.
• Immunofluorescence microscopy was employed to visualize receptor localization and trafficking.
• Mutational analysis of RAMP3, including deletion and point mutations, was performed to identify key functional domains.
• RNA interference and pharmacological inhibition were used to study the roles of RAMP3 and N-ethylmaleimide-sensitive factor (NSF) in receptor recycling in both cell lines and primary cells.

Main Results:

• CRLR-RAMP complexes undergo agonist-induced desensitization, internalization, and subsequent degradation, failing to resensitize.
• Co-expression of NSF with CRLR-RAMP3, but not CRLR-RAMP1 or CRLR-RAMP2, redirected receptor trafficking to a recycling pathway.
• Mutational analysis confirmed that the PDZ motif of RAMP3 interacts with NSF, mediating the shift in receptor trafficking.
• Inhibition of RAMP3 or NSF in rat mesangial cells impaired AM2R recycling after agonist stimulation.

Conclusions:

• RAMP3 possesses a unique PDZ domain that facilitates interaction with NSF, promoting the recycling of adrenomedullin receptors (AM2R).
• This study reveals a novel function for RAMP3 in the post-endocytic sorting and recycling of AM-Rs, differentiating it from RAMP1 and RAMP2.
• RAMPs, particularly RAMP3, play a broader regulatory role in GPCR trafficking, offering new therapeutic targets for modulating receptor signaling.