Liraglutide inhibits the progression of prediabetes in rats by reducing Raf-1 kinase inhibitor protein.

BACKGROUND: The cleavage product of Raf-1 kinase inhibitor protein (RKIP), hippocampal cholinergic neurostimulating peptide (HCNP) is involved in the promotion of insulin secretion. Studies have shown that liraglutide can inhibit the progression of prediabetes. This study aims to investigate whether the above effects of liraglutide are related to RKIP and HCNP. METHODS: Insulin-1 (INS-1) cells were divided into control group (CON), HCNP group, and HCNP + darifenacin group (H-DAR). The three groups were cultured with Roswell Park Memorial Institute (RPMI) 1640, synthetic HCNP (50 pg/mL) and RPMI 1640, and HCNP + RPMI 1640 + darifenacin respectively. Subsequently, twelve 12- to 14-week-old Otsuka Long-Evans Tokushima Fatty (OLETF) rats were randomly divided into 2 groups: the placebo group (PBO) and the liraglutide treatment group (LIRA). Six Long Evans Tokushima Otsuka (LETO) rats were used as the control group (CON). The LIRA group was given liraglutide 200 µg/kg intraperitoneally twice a day. After 12 weeks, body weight, fasting blood glucose, 2 hours postprandial blood glucose, and insulin resistance index were recorded. Western blot was used to detect expression level of C-RKIP, N-RKIP, and extracellular signal-regulated kinase of phosphorylation (p-ERK). Real-time quantitative polymerase chain reaction (qRT-PCR) to detect pancreatic tissue choline acetyltransferase (ChAT) and M3 cholinergic receptor (M3R) gene expression levels. RESULTS: At glucose concentrations of 5.6 and 16.7 mmol/L, the insulin content in the HCNP group was higher than that in the CON and H-DAR groups (all P<0.01). The body weight and fasting serum insulin (FINS) of rats in the PBO group were higher than those in the LIRA group and the CON group (P<0.01). The relative content of C-RKIP protein in the PBO group was higher than that in the LIRA and CON groups (P<0.01). The relative content of N-RKIP protein and p-ERK protein was lower than that in the LIRA and CON group (P<0.05 and P<0.01, respectively). ChAT and M3R gene expression levels in PBO group were lower than those in LIRA and CON group (P<0.01). CONCLUSIONS: Liraglutide promotes the production of HCNP, can increase ChAT activity, activate M3R, and further promote the secretion of insulin.

Possible correlated variation of GABAA receptor α3 expression with hippocampal cholinergic neurostimulating peptide precursor protein in the hippocampus.

Cholinergic neural activation from the medial septal nucleus to hippocampus plays a crucial role in episodic memory as a regulating system for glutamatergic neural activation in the hippocampus. As a candidate regulating factor for acetylcholine synthesis in the medial septal nucleus, hippocampal cholinergic neurostimulating peptide (HCNP) was purified from the soluble fraction of young adult rat hippocampus. HCNP is released from its precursor protein (HCNP-pp), also referred to as phosphatidylethanolamine-binding protein 1. We recently reported that HCNP-pp conditional knockout (KO) mice, in which the HCNP-pp gene was knocked out at 3 months of age by tamoxifen injection, display no significant behavioral abnormalities, whereas HCNP-pp KO mice have a diminished cholinergic projection to CA1 and a decreased of theta activity in CA1. In this study, to address whether HCNP-pp reduction in early life is associated with behavioral changes, we evaluated the behavior of HCNP-pp KO mice in which HCNP-pp was downregulated from an early phase (postnatal days 14-28). As unexpected, HCNP-pp KO mice had no behavioral deficits. However, a significant positive correlation between HCNP-pp and gamma-aminobutyric acid A (GABAA) receptor α3 subunit mRNA expression was found in individuals. This finding suggests involvement of HCNP-pp in regulating GABAA receptor α3 gene expression.

Reduced Cholinergic Activity in the Hippocampus of Hippocampal Cholinergic Neurostimulating Peptide Precursor Protein Knockout Mice.

The cholinergic efferent network from the medial septal nucleus to the hippocampus has an important role in learning and memory processes. This cholinergic projection can generate theta oscillations in the hippocampus to efficiently encode novel information. Hippocampal cholinergic neurostimulating peptide (HCNP) induces acetylcholine synthesis in medial septal nuclei. HCNP is processed from the N-terminal region of a 186 amino acid, 21 kD HCNP precursor protein called HCNP-pp (also known as Raf kinase inhibitory protein (RKIP) and phosphatidylethanolamine-binding protein 1 (PEBP1)). In this study, we generated HCNP-pp knockout (KO) mice and assessed their cholinergic septo-hippocampal projection, local field potentials in CA1, and behavioral phenotypes. No significant behavioral phenotype was observed in HCNP-pp KO mice. However, theta power in the CA1 of HCNP-pp KO mice was significantly reduced because of fewer cholineacetyltransferase-positive axons in the CA1 stratum oriens. These observations indicated disruption of cholinergic activity in the septo-hippocampal network. Our study demonstrates that HCNP may be a cholinergic regulator in the septo-hippocampal network.

Hippocampal Cholinergic Neurostimulating Peptide as a Possible Modulating Factor against Glutamatergic Neuronal Disability by Amyloid Oligomers.

Despite having pathological changes in the brain associated with Alzheimer's disease (AD), some patients have preserved cognitive function. A recent epidemiological study has shown that diet, exercise, cognitive training, and vascular risk monitoring interventions may reduce cognitive decline in at-risk elderly people in the general population. However, the details of molecular mechanisms underlying this cognitive function preservation are still unknown. Previous reports have demonstrated that enriched environments prevent the impairment of hippocampal long-term potentiation (LTP) through ß2-adrenergic signals, when LTP is incompletely suppressed by synthetic amyloid-ß (Aß) oligomers. The cholinergic network from the medial septal nucleus (MSN) is also a main modulating system for hippocampal glutamatergic neural activation through nicotinergic and/or muscarinergic acetylcholine receptors. Previously, we reported the importance of a cholinergic regulator gene in the MSN, hippocampal cholinergic neurostimulating peptide (HCNP). By using hippocampal sections from mice, we here demonstrated that the cholinergic neural activation from the MSN enhanced the glutamatergic neuronal activity during unsaturated LTP but not during saturated LTP. Synthetic Aß oligomers suppressed the hippocampal glutamatergic activity in a concentration-dependent manner. Furthermore, HCNP, as well as a cholinergic agonist acting through the muscarinic M1 receptor, prevented the suppression of hippocampal glutamatergic neuronal activity induced by synthetic Aß oligomers. This result suggests that the persisting cholinergic activation might be a potential explanation for the individual differences in cognitive effects of AD pathological changes.