Circuit-wide gene network analysis reveals sex-specific roles for phosphodiesterase 1b in cocaine addiction

Cocaine use disorder is a significant public health issue without an effective pharmacological treatment. Successful treatments are hindered in part by an incomplete understanding of the molecular mechanisms that underlie long-lasting maladaptive plasticity and addiction-like behaviors. Here, we leverage a large RNA-sequencing dataset to generate gene co-expression networks across 6 interconnected regions of the brain's reward circuitry from mice that underwent saline or cocaine self-administration. We identify phosphodiesterase 1b (Pde1b), a Ca2+/calmodulin-dependent enzyme that increases cAMP and cGMP hydrolysis, as a central hub gene within a nucleus accumbens (NAc) gene module that was bioinformatically associated with addiction-like behavior. Chronic cocaine exposure increases Pde1b expression in NAc D2 medium spiny neurons (MSNs) in male but not female mice. Viral-mediated Pde1b overexpression in NAc reduces cocaine self-administration in female rats, but increases seeking in both sexes. In female mice, overexpressing Pde1b in D1 MSNs attenuates the locomotor response to cocaine, with the opposite effect in D2 MSNs. Overexpressing Pde1b in D1/D2 MSNs had no effect on the locomotor response to cocaine in male mice. At the electrophysiological level, Pde1b overexpression reduces sEPSC frequency in D1 MSNs, while increasing excitability of D2 MSNs. Lastly, Pde1b overexpression significantly reduced the number of differentially expressed genes (DEGs) in NAc following chronic cocaine, with discordant effects on gene transcription between sexes. Together, we identify novel gene modules across the brain's reward circuitry associated with addiction-like behavior and explore the role of Pde1b in regulating the molecular, cellular, and behavioral responses to cocaine.Significance Statement Cocaine use disorder is a major public health challenge without an effective pharmacological treatment. Here, we leverage a combination of genome-wide RNA sequencing, gene co-expression network analysis, and bioinformatic analyses of cocaine self-administration behavior to identify a role for phosphodiesterase 1b (Pde1b) in regulating maladaptive, addiction-like behavior. Our studies reveal cell-type- and sex-specific roles for Pde1b in regulating the molecular, cellular, and behavioral responses to cocaine, yielding insight into the molecular mechanisms by which cocaine induces maladaptive plasticity in the brain's reward circuity to drive addiction-like behavior. These discoveries guide directions for future research investigating the molecular basis of cocaine action and provide a pathway for therapeutic development for cocaine use disorder.

Cell-Type-Specific Neuroproteomics of Synapses

In the last two decades, our knowledge of synaptic proteomes and their relationship to normal brain function and neuropsychiatric disorders has been expanding rapidly through the use of more powerful neuroproteomic approaches. However, mass spectrometry (MS)-based neuroproteomic studies of synapses still require cell-type, spatial, and temporal proteome information. With the advancement of sample preparation and MS techniques, we have just begun to identify and understand proteomes within a given cell type, subcellular compartment, and cell-type-specific synapse. Here, we review the progress and limitations of MS-based neuroproteomics of synapses in the mammalian CNS and highlight the recent applications of these approaches in studying neuropsychiatric disorders such as major depressive disorder and substance use disorders. Combining neuroproteomic findings with other omics studies can generate an in-depth, comprehensive map of synaptic proteomes and possibly identify new therapeutic targets and biomarkers for several central nervous system disorders.

Specificities of Gβγ subunits for the SNARE complex before and after stimulation of α2a-adrenergic receptors

[Figure: see text].

In vivo locus-specific editing of the neuroepigenome

Studies over the past several decades have identified numerous epigenetic mechanisms associated with pathological states in psychiatric and neurological disease. Until recently, studies investigating chromatin-regulatory proteins, using overexpression or knockdown approaches, did not establish causal roles for epigenetic modifications at specific genes because these techniques typically affect hundreds or thousands of genomic loci. In this Review, we describe recent efforts in using locus-specific neuroepigenome editing in vivo to, for the first time, define causal relationships between a single chromatin modification at a specific gene in a defined cell population and downstream measures at the molecular, cellular, circuit and behavioural levels. We briefly introduce three epigenome-editing platforms: zinc-finger proteins, transcriptional activator-like effectors and clustered regularly interspaced short palindromic repeats (CRISPR). We then explore the development of in vivo neuroepigenome-editing tools and their applications to resolve epigenetic contributions to the pathophysiology of brain diseases. We also discuss technical considerations for in vivo neuroepigenome-editing experiments and ongoing innovations in the field, including new tools to investigate chromatin marks, manipulate chromatin topology and induce epigenetic modifications at multiple genes in the same cell. Lastly, we explore the potential clinical applications of in vivo neuroepigenome editing for treating brain pathology.

Disabling the Gβγ-SNARE interaction disrupts GPCR-mediated presynaptic inhibition, leading to physiological and behavioral phenotypes

G protein-coupled receptors (GPCRs) that couple to G_i/o proteins modulate neurotransmission presynaptically by inhibiting exocytosis. Release of Gβγ subunits from activated G proteins decreases the activity of voltage-gated Ca²⁺ channels (VGCCs), decreasing excitability. A less understood Gβγ-mediated mechanism downstream of Ca²⁺ entry is the binding of Gβγ to SNARE complexes, which facilitate the fusion of vesicles with the cell plasma membrane in exocytosis. Here, we generated mice expressing a form of the SNARE protein SNAP25 with premature truncation of the C terminus and that were therefore partially deficient in this interaction. SNAP25Δ3 homozygote mice exhibited normal presynaptic inhibition by GABA_B receptors, which inhibit VGCCs, but defective presynaptic inhibition by receptors that work directly on the SNARE complex, such as 5-hydroxytryptamine (serotonin) 5-HT_1b receptors and adrenergic α_2a receptors. Simultaneously stimulating receptors that act through both mechanisms showed synergistic inhibitory effects. SNAP25Δ3 homozygote mice had various behavioral phenotypes, including increased stress-induced hyperthermia, defective spatial learning, impaired gait, and supraspinal nociception. These data suggest that the inhibition of exocytosis by G_i/o-coupled GPCRs through the Gβγ-SNARE interaction is a crucial component of numerous physiological and behavioral processes.

The expanding roles and mechanisms of G protein-mediated presynaptic inhibition

Throughout the past five decades, tremendous advancements have been made in our understanding of G protein signaling and presynaptic inhibition, many of which were published in the Journal of Biological Chemistry under the tenure of Herb Tabor as Editor-in-Chief. Here, we identify these critical advances, including the formulation of the ternary complex model of G protein-coupled receptor signaling and the discovery of Gβγ as a critical signaling component of the heterotrimeric G protein, along with the nature of presynaptic inhibition and its physiological role. We provide an overview for the discovery and physiological relevance of the two known Gβγ-mediated mechanisms for presynaptic inhibition: first, the action of Gβγ on voltage-gated calcium channels to inhibit calcium influx to the presynaptic active zone and, second, the direct binding of Gβγ to the SNARE complex to displace synaptotagmin downstream of calcium entry, which has been demonstrated to be important in neurons and secretory cells. These two mechanisms act in tandem with each other in a synergistic manner to provide more complete spatiotemporal control over neurotransmitter release.

GPCR regulation of secretion

Modulation of neurotransmitter exocytosis by activated Gi/o coupled G-protein coupled receptors (GPCRs) is a universal regulatory mechanism used both to avoid overstimulation and to influence circuitry. One of the known modulation mechanisms is the interaction between Gβγ and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNAREs). There are 5 Gβ and 12 Gγ subunits, but specific Gβγs activated by a given GPCR and the specificity to effectors, such as SNARE, in vivo are not known. Although less studied, Gβγ binding to the exocytic fusion machinery (i.e. SNARE) provides a more direct regulatory mechanism for neurotransmitter release. Here, we review some recent insights in the architecture of the synaptic terminal, modulation of synaptic transmission, and implications of G protein modulation of synaptic transmission in diseases. Numerous presynaptic proteins are involved in the architecture of synaptic terminals, particularly the active zone, and their importance in the regulation of exocytosis is still not completely understood. Further understanding of the Gβγ-SNARE interaction and the architecture and mechanisms of exocytosis may lead to the discovery of novel therapeutic targets to help patients with various disorders such as hypertension, attention-deficit/hyperactivity disorder, post-traumatic stress disorder, and acute/chronic pain.

Quantitative Multiple-Reaction Monitoring Proteomic Analysis of Gβ and Gγ Subunits in C57Bl6/J Brain Synaptosomes

Gβγ dimers are one of the essential signaling units of activated G protein-coupled receptors (GPCRs). There are five Gβ and 12 Gγ subunits in humans; numerous studies have demonstrated that different Gβ and Gγ subunits selectively interact to form unique Gβγ dimers, which in turn may target specific receptors and effectors. Perturbation of Gβγ signaling can lead to impaired physiological responses. Moreover, previous targeted multiple-reaction monitoring (MRM) studies of Gβ and Gγ subunits have shown distinct regional and subcellular localization patterns in four brain regions. Nevertheless, no studies have quantified or compared their individual protein levels. In this study, we have developed a quantitative MRM method not only to quantify but also to compare the protein abundance of neuronal Gβ and Gγ subunits. In whole and fractionated crude synaptosomes, we were able to identify the most abundant neuronal Gβ and Gγ subunits and their subcellular localizations. For example, Gβ₁ was mostly localized at the membrane while Gβ₂ was evenly distributed throughout synaptosomal fractions. The protein expression levels and subcellular localizations of Gβ and Gγ subunits may affect the Gβγ dimerization and Gβγ-effector interactions. This study offers not only a new tool for quantifying and comparing Gβ and Gγ subunits but also new insights into the in vivo distribution of Gβ and Gγ subunits, and Gβγ dimer assembly in normal brain function.

Using peptide arrays created by the SPOT method for defining protein-protein interactions

Evaluating sites of protein-protein interactions can be an arduous task involving extensive mutagenesis work and attempts to express and purify individual proteins in sufficient quantities. Peptide mapping is a useful alternative to traditional methods as it allows rapid detection of regions and/or individual residues important for binding, and it can be readily applied to numerous proteins at once. Here we describe the use of the ResPep SL SPOT method to evaluate protein-protein binding interactions such as that between G-protein βγ subunits and SNARE proteins, identifying both regions of interest and subsequently individual residues which can then be manipulated in further biochemical assays to confirm their validity.

Gβγ inhibits exocytosis via interaction with critical residues on soluble N-ethylmaleimide-sensitive factor attachment protein-25

Spatial and temporal regulation of neurotransmitter release is a complex process accomplished by the exocytotic machinery working in tandem with numerous regulatory proteins. G-protein βγ dimers regulate the core process of exocytosis by interacting with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins soluble N-ethylmaleimide-sensitive factor attachment protein-25 (SNAP-25), syntaxin 1A, and synaptobrevin. Gβγ binding to ternary SNAREs overlaps with calcium-dependent binding of synaptotagmin, inhibiting synaptotagmin-1 binding and fusion of the synaptic vesicle. To further explore the binding sites of Gβγ on SNAP-25, peptides based on the sequence of SNAP-25 were screened for Gβγ binding. Peptides that bound Gβγ were subjected to alanine scanning mutagenesis to determine their relevance to the Gβγ-SNAP-25 interaction. Peptides from this screen were tested in protein-protein interaction assays for their ability to modulate the interaction of Gβγ with SNAP-25. A peptide from the C terminus, residues 193 to 206, significantly inhibited the interaction. In addition, Ala mutants of SNAP-25 residues from the C terminus of SNAP-25, as well as from the amino-terminal region decreased binding to Gβ₁γ₁. When SNAP-25 with eight residues mutated to alanine was assembled with syntaxin 1A, there was significantly reduced affinity of this mutated t-SNARE for Gβγ, but it still interacted with synaptotagmin-1 in a Ca²⁺ -dependent manner and reconstituted evoked exocytosis in botulinum neurotoxin E-treated neurons. However, the mutant SNAP-25 could no longer support 5-hydroxytryptamine-mediated inhibition of exocytosis.