Chronic pain is a debilitating condition involving neuronal dysfunction, but the synaptic mechanisms underlying the persistence of pain are still poorly understood. We found that the synaptic organizer glutamate delta 1 receptor (GluD1) is expressed postsynaptically at parabrachio-central laterocapsular amygdala (PB-CeLC) glutamatergic synapses at axo-somatic and punctate locations on protein kinase C δ -positive (PKCδ+) neurons. Deletion of GluD1 impairs excitatory neurotransmission at the PB-CeLC synapses. In inflammatory and neuropathic pain models, GluD1 and its partner cerebellin 1 (Cbln1) are downregulated while AMPA receptor is upregulated. A single infusion of recombinant Cbln1 into the central amygdala led to sustained mitigation of behavioral pain parameters and normalized hyperexcitability of central amygdala neurons. Cbln2 was ineffective under these conditions and the effect of Cbln1 was antagonized by GluD1 ligand D-serine. The behavioral effect of Cbln1 was GluD1-dependent and showed lateralization to the right central amygdala. Selective ablation of GluD1 from the central amygdala or injection of Cbln1 into the central amygdala in normal animals led to changes in averse and fear-learning behaviors. Thus, GluD1-Cbln1 signaling in the central amygdala is a teaching signal for aversive behavior but its sustained dysregulation underlies persistence of pain. Significance statement: Chronic pain is a debilitating condition which involves synaptic dysfunction, but the underlying mechanisms are not fully understood. Our studies identify a novel mechanism involving structural synaptic changes in the amygdala caused by impaired GluD1-Cbln1 signaling in inflammatory and neuropathic pain behaviors. We also identify a novel means to mitigate pain in these conditions using protein therapeutics.

Drug delivery has made tremendous advances in the last decade. Targeted therapies are increasingly common, with intracellular delivery highly impactful and sought after. Intracellular drug delivery systems have limitations due to imprecise and non-targeted release profiles. One way this can be addressed is through using stimuli-responsive soft nanoparticles, which contain materials with an organic backbone such as lipids and polymers. The choice of biomaterial is essential for soft nanoparticles to be responsive to internal or external stimuli. The nanoparticle must retain its integrity and payload in non-targeted physiological conditions while responding to particular intracellular environments where payload release is desired. Multiple internal and external factors could stimulate the intracellular release of drugs from nanoparticles. Internal stimuli include pH, oxidation, and enzymes, while external stimuli include ultrasound, light, electricity, and magnetic fields. Stimulatory responsive soft nanoparticulate systems specifically utilized to modulate intracellular delivery of drugs are explored in this review.

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Neurotransmitter release of synaptic vesicles relies on the assembly of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, consisting of syntaxin and SNAP-25 on the plasma membrane and synaptobrevin on the synaptic vesicle. The formation of the SNARE complex progressively zippers towards the membranes, which drives membrane fusion between the plasma membrane and the synaptic vesicle. However, the underlying molecular mechanism of SNARE complex regulation is unclear. In this study, we investigated the syntaxin-3b isoform found in the retinal ribbon synapses using single-molecule fluorescence resonance energy transfer (smFRET) to monitor the conformational changes of syntaxin-3b that modulate the SNARE complex formation. We found that syntaxin-3b is predominantly in a self-inhibiting closed conformation, inefficiently forming the ternary SNARE complex. Conversely, a phosphomimetic mutation (T14E) at the N-terminal region of syntaxin-3b promoted the open conformation, similar to the constitutively open form of syntaxin LE mutant. When syntaxin-3b is bound to Munc18-1, SNARE complex formation is almost completely blocked. Surprisingly, the T14E mutation of syntaxin-3b partially abolishes Munc18-1 regulation, acting as a conformational switch to trigger SNARE complex assembly. Thus, we suggest a model where the conformational change of syntaxin-3b induced by phosphorylation initiates the release of neurotransmitters in the ribbon synapses.

Neurotransmitter release of synaptic vesicles relies on the assembly of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, consisting of syntaxin and SNAP-25 on the plasma membrane and synaptobrevin on the synaptic vesicle. The formation of the SNARE complex progressively zippers towards the membranes, which drives membrane fusion between the plasma membrane and the synaptic vesicle. However, the underlying molecular mechanism of SNARE complex regulation is unclear. In this study, we investigate the syntaxin-3b isoform found in the retinal ribbon synapses using single-molecule fluorescence resonance energy transfer (smFRET) to monitor the conformational changes of syntaxin-3b that modulate the SNARE complex formation. We found that syntaxin-3b is predominantly in a self-inhibiting closed conformation, inefficiently forming the ternary SNARE complex. Conversely, a phosphomimetic mutation (T14E) at the N-terminal region of syntaxin-3b promoted the open conformation, similar to the constitutively open form of syntaxin LE mutant. When syntaxin-3b is bound to Munc18-1, SNARE complex formation is almost completely blocked. Surprisingly, the T14E mutation of syntaxin-3b partially abolishes Munc18-1 regulation, acting as a conformational switch to trigger SNARE complex assembly. Thus, we suggest a model where the conformational change of syntaxin-3b induced by phosphorylation initiates the release of neurotransmitters in the ribbon synapses.