Role of Understudied aGPCRs in Orofacial Neuropathic Pain

ABSTRACT
Adhesion G-Protein Coupled-Receptors (aGPCR) represent a particularly interesting class of receptors in the
understudied druggable genome that could be therapeutic targets for pain control. These receptors possess
tethered peptide agonists on the N-terminus of the proteins that are exposed by autoproteolysis (Stachel
sequence). After proteolysis the receptor remains active until it is degraded, which could chronically maintain
pain. The Eligible Protein List contains 27 aGPCRs. In our recent transcriptomic analysis of mouse trigeminal
ganglia (TG) we found that 23 of these proteins are expressed in TG, yet only ADGRD1 (UniProt# Q6QNK2)
was reported to be altered by infraorbital nerve chronic constriction injury (ION-CCI). ADGRD1’s Stachel
sequence is highly selective for the receptor suggesting that the binding site is discriminative and that selective
ligands could be designed to interact with the protein, the receptor activates Gαs to stimulate cAMP production,
and the receptor is expressed at similar levels in mouse, rat, and human TG indicating that it should be
translatable. These data suggest that ADGRD1 maybe an exceptional candidate for analgesic development.
However, to date ADGRD1 has not been examined as a potential mediator of pain. This project will address the
following questions to establish the function of ADGRD1 in ION-CCI induced chronic allodynia. 1) What sensory
neurons express ADGRD1? This aim will utilize transcriptomics and immunohistochemistry to identify the cell
types in the TG and trigeminal subnucleus caudalis (TNC) that express ADGRD1 in naïve and ION-CCI mice.
These experiments will also evaluate the relationship between the development of ION-CCI mechanical allodynia
and protein expression. 2) Can ADGRD1 be activated by its Stachel peptide when applied exogenously? The
presence of the receptor in TG in naïve mice suggests that administering the Stachel peptide to the animals
could induce facial mechanical allodynia, which would confirm a role for the protein in nociception. 3) Can ION-
CCI induced mechanical allodynia be suppressed by a targeted botulinum toxin light chain that uses the Stachel
peptide as a targeting sequence? This aim will take advantage of our previous experience in the development
of neuropeptide – toxin conjugates to either lesion or alter the function of nociceptive neurons. A botulinum A
light chain conjugate with the ADGRD1 Stachel peptide (Stach-Bot) will be constructed and tested for its ability
to block ION-CCI induced mechanical allodynia in mice. These experiments would confirm that the neurons
expressing ADGRD1 are nociceptive and that they participate in mechanical allodynia. The data collected in
this project will establish the cell types that express ADGRD1, determine if the protein can produce
allodynia when activated, and demonstrate that the neurons that express ADGRD1 participate in ION-
CCI induced allodynia. These results would provide the impetus for future studies that develop novel
therapeutics targeting ADGRD1 for use in treating neuropathic pain.

Grant Number: 1R21DE033639-01
NIH Institute/Center: NIH
Principal Investigator: ROBERT CAUDLE