Guarino, D. S. | bioRxiv Science

bioRxiv2025

Complementary Structural and Chemical Biology Methods Reveal the Basis for Selective Radioligand Binding to α-Synuclein in MSA Tissue

Fibrillar aggregation of -synuclein (Syn) is a hallmark of Parkinsons disease (PD) and related disorders, including multiple system atrophy (MSA) and dementia with Lewy bodies (DLB). Despite advances in Syn fibril structural characterization, the relevance of in vitro and ex vivo structures to patient aggregates remains unclear, particularly for developing therapeutic or diagnostic molecules. Cryo-electron microscopy (cryo-EM) studies of Syn fibrils with ligands often reveal binding at multiple sites, likely due to high ligand concentrations. Here, various structural and chemical biology techniques were used to characterize Syn fibrils in the presence of EX-6, a candidate ligand for positron emission tomography (PET) imaging of synucleinopathies. Transmission electron microscopy (TEM) and cryo-EM revealed no significant fibril core changes upon binding. Forster resonance energy transfer (FRET) further demonstrated that the disordered C-terminus was unaltered. Cryo-EM and crosslinking mass spectrometry (XL-MS) identified consistent binding sites, with one (Site 2*) providing a well-defined pocket for high-resolution analysis. Site 2* showed similar residue positioning in MSA patient-derived structures, suggesting MSA selectivity. [3H]-EX-6 binding assays demonstrated a 10-fold preference for MSA over PD tissue, with autoradiography further confirming MSA selectivity. Taken together, the combined use of structural and chemical biology techniques provides a comprehensive understanding of EX-6 binding that would not be possible with any single method. Optimization of ligand-protein and ligand-ligand interactions observed in the cryo-EM structure will enable the development of EX-6 as a PET imaging probe for MSA.

neuroscience↗

bioRxiv2024

A Small-molecule Antagonist Radiotracer for Positron Emission Tomography Imaging of the Mu Opioid Receptor.

The opioid crisis is a catastrophic health emergency catalyzed by the misuse of opioids that target and activate the mu opioid receptor. Traditional radioligands used to study the mu opioid receptor are often tightly regulated owing to their abuse and respiratory depression potential. In the present study, we sought to design and characterize a library of 24 non-agonist ligands for the mu opioid receptor. Ligands were evaluated for the binding affinity, intrinsic activity, and predicted blood-brain barrier permeability. Several ligands demonstrated single-digit nM binding affinity for the mu opioid receptor while also demonstrating selectivity over the delta and kappa opioid receptors. The antagonist behavior of 1A and 3A at the mu opioid receptor indicate that these ligands would likely not induce opioid-dependent respiratory depression. Therefore, these ligands can enable a safer means to interrogate the endogenous opioid system. Based on binding affinity, selectivity, and potential off-target binding, [11C]1A was prepared via metallophotoredox of the aryl-bromide functional group to [11C]methyl iodide. The nascent radiotracer demonstrated brain uptake in a rhesus macaque model and accumulation in the caudate and putamen. Naloxone was able to reduce [11C]1A binding, though the interactions were not as pronounced as naloxones ability to displace [11C]carfentanil. These results suggest that GSK1521498 and related congeners are amenable to radioligand design and can offer a safer way to query opioid neurobiology.

neuroscience↗