Deletion of serine racemase reverses neuronal insulin signaling inhibition by amyloid-β oligomers

Dysregulation of insulin signaling in the Alzheimer's disease (AD) brain has been extensively reported. Serine racemase (SR) modulates insulin secretion in pancreatic islets. This study aimed to examine whether SR regulates insulin synthesis and secretion in neurons, thereby modulating insulin signaling in the AD brain. Srr-knockout (Srr(-/-)) mice generated with the CRISPR/Cas9 technique were used. Using immunofluorescence and fluorescence in situ hybridization, levels of insulin protein and insulin(ins2) mRNA were significantly increased in the hippocampal but not in hypothalamic sections of Srr(-/-) mice compared with WT mice. Real-time quantitative PCR revealed that ins2 mRNA from primary hippocampal neuronal cultures of Srr(-/-) mice was significantly increased compared with that from cultured neurons of WT mice. Notably, the secretion of proinsulin C-peptide was increased in Srr(-/-) neurons relative to WT neurons. By examining membrane fractional proteins with immunoblotting, Srr(-/-) neurons retained ATP-dependent potassium channels on plasmalemma and correspondingly contained higher levels of p-AMPK. After treatment with A beta 42, the phosphorylation levels of insulin receptor substrate at serine 616 636 (p-IRS1(ser616,636)) were significantly lower, whereas p-AKT(308) and p-AKT(473) were higher in Srr(-/-) neurons than in WT neurons, respectively. The phosphorylated form of c-Jun N-terminal kinase decreased in the cultured Srr(-/-) neurons relative to the WT neurons upon A beta 42 treatment. In contrast, phosphorylated protein kinase R remained at the same levels. Further, reactive oxygen species were reduced in cultured Srr(-/-) neurons under A beta 42 treatment relative to the WT neurons. Collectively, our study indicated that Srr deletion promoted insulin synthesis and secretion of proinsulin C-peptide, thereby reversing insulin resistance by A beta 42. This study suggests that targeting the neuronal SR may be utilized to enhance insulin signaling which is inhibited at the early stage of the AD brain.