Activation of Phosphotyrosine-Mediated Signaling Pathways in the Cortex and Spinal Cord of SOD1G93A, a Mouse Model of Familial Amyotrophic Lateral Sclerosis

Bringing synapses into focus: Recent advances in synaptic imaging and mass-spectrometry for studying synaptopathy

Synapses are integral for healthy brain function and are becoming increasingly recognized as key structures in the early stages of brain disease. Understanding the pathological processes driving synaptic dysfunction will unlock new therapeutic opportunities for some of the most devastating diseases of our time. To achieve this we need a solid repertoire of imaging and molecular tools to interrogate synaptic biology at greater resolution. Synapses have historically been examined in small numbers, using highly technical imaging modalities, or in bulk, using crude molecular approaches. However, recent advances in imaging techniques are allowing us to analyze large numbers of synapses, at single-synapse resolution. Furthermore, multiplexing is now achievable with some of these approaches, meaning we can examine multiple proteins at individual synapses in intact tissue. New molecular techniques now allow accurate quantification of proteins from isolated synapses. The development of increasingly sensitive mass-spectrometry equipment means we can now scan the synaptic molecular landscape almost in totality and see how this changes in disease. As we embrace these new technical developments, synapses will be viewed with clearer focus, and the field of synaptopathy will become richer with insightful and high-quality data. Here, we will discuss some of the ways in which synaptic interrogation is being facilitated by methodological advances, focusing on imaging, and mass spectrometry.

Preclinical Development of 18F-OF-NB1 for Imaging GluN2B-Containing N-Methyl-d-Aspartate Receptors and Its Utility as a Biomarker for Amyotrophic Lateral Sclerosis

As part of our continuous efforts to develop a suitable ¹⁸F-labeled PET radioligand with improved characteristics for imaging the N-methyl-d-aspartate receptors (NMDARs) subtype 2B (GluN1/2B), we investigated in the current work ortho-fluorinated (OF) and meta-fluorinated (MF) analogs of ¹⁸F-para-fluorinated (PF)-NB1, a 3-benzazepine-based radiofluorinated probe. Methods: OF-NB1 and MF-NB1 were prepared using a multistep synthesis, and their binding affinities toward GluN2B subunits and selectivity over σ1 receptors (σ1Rs) were determined via competitive binding assays. ¹⁸F-OF-NB1 was synthesized via copper-mediated radiofluorination and was evaluated in Wistar rats by in vitro autoradiography, PET imaging, ex vivo biodistribution, metabolite experiments, and receptor occupancy studies using CP-101,606, an established GluN2B antagonist. To determine in vivo selectivity, ¹⁸F-OF-NB1 was validated in wild-type and σ1R knock-out mice. Translational relevance was assessed in autoradiographic studies using postmortem human brain tissues from healthy individuals and ALS patients, the results of which were corroborated by immunohistochemistry. Results: The binding affinity values for OF-NB1 and MF-NB1 toward the GluN2B subunits were 10.4 ± 4.7 and 590 ± 36 nM, respectively. For σ1R binding, OF-NB1 and MF-NB1 exhibited inhibition constants of 410 and 2,700 nM, respectively. OF-NB1, which outperformed MF-NB1, was radiolabeled with ¹⁸F to afford ¹⁸F-OF-NB1 in more than 95% radiochemical purity and molar activities of 192 ± 33 GBq/μmol. In autoradiography experiments, ¹⁸F-OF-NB1 displayed a heterogeneous and specific binding in GluN2B subunit-rich brain regions such as the cortex, striatum, hypothalamus, and hippocampus. PET imaging studies in Wistar rats showed a similar heterogeneous uptake, and no brain radiometabolites were detected. A dose-dependent blocking effect was observed with CP-101,606 (0.5-15 mg/kg) and resulted in a 50% receptor occupancy of 8.1 μmol/kg. Postmortem autoradiography results revealed lower expression of the GluN2B subunits in ALS brain tissue sections than in healthy controls, in line with immunohistochemistry results. Conclusion: ¹⁸F-OF-NB1 is a highly promising PET probe for imaging the GluN2B subunits of the N-methyl-d-aspartate receptor. It possesses utility for receptor occupancy studies and has potential for PET imaging studies in ALS patients and possibly other brain disorders.