Multiple Sgip1 splice variants inhibit cannabinoid receptor 1 internalization

Incidence of alternative splicing associated with sex and opioid effects in the axon guidance pathway

The alternative splicing of a gene results in distinct transcript isoforms that can result in proteins that differ in function. Alternative splicing processes are prevalent in the brain, have varying incidence across brain regions, and can present sexual dimorphism. Exposure to opiates and other substances of abuse can also alter the type and incidence of the splicing process and the relative abundance of the isoforms produced. The disruption of alternative splicing patterns associated with sex differences and morphine exposure in the prefrontal cortex of a pig model was studied. The numbers of genes presenting one or more significant (FDR-adjusted p-value < 0.05) alternative splicing events were 933 and 1,368 genes when comparing females relative to males and morphine- relative to saline-treated animals, respectively. The sex-dependent opioid effect was most extreme in the contrast between morphine- versus saline-treated males with 1,934 significantly differentially spliced genes. The most frequent and significant alternative splicing type was skipped exon (∼56 % event), followed by retained intron (∼15 % events). The pathways encompassing a significant number of differentially spliced genes included axon guidance, glutamatergic synapses, circadian rhythm, and lysine degradation. Genes in these pathways included ROBO1, SEMA6C, GRIN3A, GRM2, ARNTL, CLOCK, HYKK, and DOT1L. Transcription factors ETV7 and DMAP1 presented a significant number of differentially spliced target genes. The distribution of the genes presenting differential alternative splicing in the axon guidance and circadian rhythm pathways indicates that this regulatory mechanism impacts hubs and peripheral genes. The identification of sexual dimorphism in the effect of morphine across multiple pathways confirms the necessity to explore the effects of drugs of abuse within sex. Altogether, our findings advance the understanding of the response to factors that can impact the activity of excitatory synapses by modulating transcriptional mechanisms that support the plasticity of the prefrontal cortex.

Generation of induced pluripotent stem cell line (UCSFi001-A-77) carrying a biallelic frameshift variant in exon 4 of SGIP1 through CRISPR/Cas9

SGIP1 encodes a protein Src homology 3-domain growth factor receptor-bound 2-like endophilin interacting protein 1. It is involved in the regulation of clathrin-mediated endocytosis along with having a role in energy homeostasis in neuronal systems. We generated an isogenic human induced pluripotent stem cell (iPSC) line with a biallelic frameshift variant in SGIP1. This exon has been shown to be subject to alternative splicing, leading to an isoform lacking 24 amino acids that are present in the longest SGIP isoform. The newly generated iPSC line will be helpful to dissect the differential properties of the two SGIP isoforms.

SGIP1 Deletion in Mice Attenuates Mechanical Hypersensitivity Elicited by Inflammation

Background: Activation of cannabinoid receptor 1 (CB1R) in the nervous system modulates the processing of acute and chronic pain. CB1R activity is regulated by desensitization and internalization. SH3-containing GRB2-like protein 3-interacting protein 1 (SGIP1) inhibits the internalization of CB1R. This causes increased and prolonged association of the desensitized receptor with G protein-coupled receptor kinase 3 (GRK3) and beta-arrestin on the cell membrane and results in decreased activation of extracellular signal-regulated kinase 1/2 (ERK1/2) pathway. Genetic deletion of SGIP1 in mice leads to altered CB1R-related functions, such as decreased anxiety-like behaviors, modified cannabinoid tetrad behaviors, reduced acute nociception, and increased sensitivity to analgesics. In this work, we asked if deletion of SGIP1 affects chronic nociception and analgesic effect of Δ9-tetrahydrocannabinol (THC) and WIN 55,212-2 (WIN) in mice. Methods: We measured tactile responses of hind paws to increasing pressure in wild-type and SGIP1 knock-out mice. Inflammation in the paw was induced by local injection of carrageenan. To determine the mechanical sensitivity, the paw withdrawal threshold (PWT) was measured using an electronic von Frey instrument with the progression of the applied force. Results: The responses to mechanical stimuli varied depending on the sex, genotype, and treatment. SGIP1 knock-out male mice exhibited lower PWT than wild-type males. On the contrary, the female mice exhibited comparable PWT. Following THC or WIN treatment in male mice, SGIP1 knock-out males exhibited PWT lower than wild-type males. THC treatment in SGIP1 knock-out females resulted in PWT higher than after THC treatment of wild-type females. However, SGIP1 knock-out and wild-type female mice exhibited similar PWT after WIN treatment. Conclusions: We provide evidence that SGIP1, possibly by interacting with CB1R, is involved in processing the responses to chronic pain. The absence of SGIP1 results in enhanced sensitivity to mechanical stimuli in males, but not females. The antinociceptive effect of THC is superior to that of WIN in SGIP1 knock-out mice in the carrageenan-induced model of chronic pain.

SGIP1 binding to the α-helical H9 domain of cannabinoid receptor 1 promotes axonal surface expression

Endocannabinoid signalling mediated by cannabinoid receptor 1 (CB1R, also known as CNR1) is critical for homeostatic neuromodulation of both excitatory and inhibitory synapses. This requires highly polarised axonal surface expression of CB1R, but how this is achieved remains unclear. We previously reported that the α-helical H9 domain in the intracellular C terminus of CB1R contributes to axonal surface expression by an unknown mechanism. Here, we show in rat primary neuronal cultures that the H9 domain binds to the endocytic adaptor protein SGIP1 to promote CB1R expression in the axonal membrane. Overexpression of SGIP1 increases CB1R axonal surface localisation but has no effect on CB1R lacking the H9 domain (CB1RΔH9). Conversely, SGIP1 knockdown reduces axonal surface expression of CB1R but does not affect CB1RΔH9. Furthermore, SGIP1 knockdown diminishes CB1R-mediated inhibition of presynaptic Ca2+ influx in response to neuronal activity. Taken together, these data advance mechanistic understanding of endocannabinoid signalling by demonstrating that SGIP1 interaction with the H9 domain underpins axonal CB1R surface expression to regulate presynaptic responsiveness.

Asymmetric Transfer Hydrogenation of Cyclobutenediones

Four-membered carbocycles are fundamental substructures in bioactive molecules and approved drugs and serve as irreplaceable building blocks in organic synthesis. However, developing efficient protocols furnishing diversified four-membered ring compounds in a highly regio-, diastereo-, and enantioselective fashion remains challenging but very desirable. Here, we report the unprecedented asymmetric transfer hydrogenation of cyclobutenediones. The reaction can selectively afford three types of four-membered products in high yields with high stereoselectivities, and the highly functionalized products enable a series of further transformations to form more diversified four-membered compounds. Asymmetric synthesis of di-, tri-, and tetrasubstituted bioactive molecules has also been achieved. Systematic mechanistic studies and theoretical calculations have revealed the origin of the regioselectivity, the key hydrogenation transition state models, and the sequence of the double and triple hydrogenation processes. The work provides a new choice for the catalytic asymmetric synthesis of cyclobutanes and related structures and demonstrates the robustness of asymmetric transfer hydrogenation in the accurate selectivity control of highly functionalized substrates.