Synthesis and neuroprotective effects of new genipin derivatives against glutamate-induced oxidative damage

Glutamate-induced oxidative stress is well-known to play a crucial role in the development of neurodegenerative diseases, such as stroke. Genipin, a natural iridoid compound, has demonstrated potential neuroprotective properties but is unstable in physiological conditions. The present study aimed to develop new derivatives of genipin that exhibit improved stability and activity for the treatment of stroke. Nineteen new derivatives were thus designed and synthesized. Their neuroprotective effect against glutamate-induced injury was evaluated in HT22 cells. Among the newly synthesized derivatives, 3e demonstrated significantly greater neuroprotection and improved stability compared to genipin. Specifically, 0.01 μM of 3e was found to effectively attenuate glutamate-induced oxidative damage by inhibiting ROS over-accumulation, reducing MDA content, and restoring the endogenous antioxidative system. Further investigation revealed that 3e inhibited oxidative stress by downregulating the phosphorylation levels of p38 MAPK and activating Nrf2 and HO-1 proteins. These results suggested that 3e has the potential to serve as a promising candidate for the treatment of stroke by protecting against glutamate-induced oxidative stress.

Synthesis of Methylgenipin and Evaluation of Its Anti-Hepatic Injury Activity

Genipin has been the focus of research as a multifunctional compound for the treatment of pathogenic diseases. However, hepatotoxicity caused by oral genipin raises concerns about its safety. To obtain novel derivatives with low toxicity and efficacy, we synthesized methylgenipin (MG), a new compound, using structural modification, and investigated the safety of MG administration. The results showed that the LD₅₀ of oral MG was higher than 1000 mg/kg, no mice died or were poisoned during the experiment in the treatment group, and there was no significant difference in biochemical parameters and liver pathological sections compared with the control. Importantly, MG (100 mg/kg/d) treatment for 7 days reduced alpha-naphthylisothiocyanate (ANIT)-induced increases in liver index, alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (AKP), and total bilirubin (TBIL) levels. Histopathology demonstrated that MG could treat ANIT-induced cholestasis. In addition, using proteomics to investigate the molecular mechanism of MG in the treatment of a liver injury may be related to enhancing antioxidant function. Kit validation showed that ANIT induced an increase in malondialdehyde (MDA) and a decrease in superoxide dismutase (SOD) and glutathione (GSH) levels, while the MG pretreatments, both of which were significantly reversed to some extent, suggested that MG may alleviate ANIT-induced hepatotoxicity by enhancing endogenous antioxidant enzymes and inhibiting oxidative stress injury. In this study, we demonstrate that the treatment of mice with MG does not cause impaired liver function and provide an investigation of the efficacy of MG against ANIT-induced hepatotoxicity, laying the foundation for the safety evaluation and clinical application of MG.

Terpenoid natural products exert neuroprotection via the PI3K/Akt pathway

PI3K/Akt, an essential signaling pathway widely present in cells, has been shown to be relevant to neurological disorders. As an important class of natural products, terpenoids exist in large numbers and have diverse backbones, so they have a great chance to be identified as neuroprotective agents. In this review, we described and summarized recent research for a range of terpenoid natural products associated with the PI3K/Akt pathway by classifying their basic chemical structures of the terpenes, identified by electronic searches on PubMed, Web of Science for research, and Google Scholar websites. Only articles published in English were included. Our discussion here concerned 16 natural terpenoids and their mechanisms of action, the associated diseases, and the methods of experimentation used. We also reviewed the discovery of their chemical structures and their derivatives, and some compounds have been concluded for their structure–activity relationships (SAR). As a result, terpenoids are excellent candidates for research as natural neuroprotective agents, and our content will provide a stepping stone for further research into these natural products. It may be possible for more terpenoids to serve as neuroprotective agents in the future.

Hybrids of aurantiamide acetate and isopropylated genipin as potential anti-inflammatory agents: The design, synthesis, and biological evaluation

A novel series of hybrids designed on the basis of aurantiamide acetate and isopropylated genipin were synthesized and biologically evaluated as anti-inflammatory agents. Among them, compound 7o exhibited the best inhibitory activity against TNF-α secretion (IC₅₀  = 16.90 μM) and was selected for further in vitro and in vivo functional study. The results demonstrated that 7o was capable of suppressing the expression of LPS-induced iNOS and COX-2, as well as reducing the production of NO at the concentration of 5 μM, which may be resulted from its regulation of NF-κB signaling and MAPK signaling. Moreover, compound 7o exhibited favorable in vivo anti-inflammatory activity with an inhibition rate of 53.32% against xylene-induced ear swelling in mice at the dose of 5 mg/kg.

Design, synthesis, and evaluation of genipin derivatives for the treatment of Alzheimer's Disease

Twenty-two novel genipin derivatives have been designed, synthesized, and evaluated for their inhibitory activity against acetylcholinesterase (AChE). As a result, compound 13a bearing ligustrazine moiety displayed the most potent AChE inhibitory activity in this series with IC₅₀ value of 218 nm. Besides, MTT assay was performed to investigate the neuroprotection of these compounds against PC12 cells injured by Amyloid β-protein 1-42 (Aβ_1-42 ). Among them, 8a showed higher inhibition rate (%Inhibition = 22.29) than the positive reference Donepezil (%Inhibition = 17.65).

Requirement of retinoic acid receptor β for genipin derivative-induced optic nerve regeneration in adult rat retina

Like other CNS neurons, mature retinal ganglion cells (RGCs) are unable to regenerate their axons after nerve injury due to a diminished intrinsic regenerative capacity. One of the reasons why they lose the capacity for axon regeneration seems to be associated with a dramatic shift in RGCs’ program of gene expression by epigenetic modulation. We recently reported that (1R)-isoPropyloxygenipin (IPRG001), a genipin derivative, has both neuroprotective and neurite outgrowth activities in murine RGC-5 retinal precursor cells. These effects were both mediated by nitric oxide (NO)/S-nitrosylation signaling. Neuritogenic activity was mediated by S-nitrosylation of histone deacetylase-2 (HDAC2), which subsequently induced retinoic acid receptor β (RARβ) expression via chromatin remodeling in vitro. RARβ plays important roles of neural growth and differentiation in development. However, the role of RARβ expression during adult rat optic nerve regeneration is not clear. In the present study, we extended this hypothesis to examine optic nerve regeneration by IPRG001 in adult rat RGCs in vivo. We found a correlation between RARβ expression and neurite outgrowth with age in the developing rat retina. Moreover, we found that IPRG001 significantly induced RARβ expression in adult rat RGCs through the S-nitrosylation of HDAC2 processing mechanism. Concomitant with RARβ expression, adult rat RGCs displayed a regenerative capacity for optic axons in vivo by IPRG001 treatment. These neuritogenic effects of IPRG001 were specifically suppressed by siRNA for RARβ. Thus, the dual neuroprotective and neuritogenic actions of genipin via S-nitrosylation might offer a powerful therapeutic tool for the treatment of RGC degenerative disorders.

Neuritogenic activity of a genipin derivative in retinal ganglion cells is mediated by retinoic acid receptor β expression through nitric oxide/S-nitrosylation signaling

Genipin, a herbal iridoid, is known to have both neuroprotective and neuritogenic activity in neuronal cell lines. As it is structurally similar to tetrahydrobiopterin, its activity is believed to be nitric oxide (NO)-dependent. We previously proposed a novel neuroprotective activity of a genipin derivative, (1R)-isoPropyloxygenipin (IPRG001), whereby it reduces oxidative stress in RGC-5, a neuronal precursor cell line of retinal origin through protein S-nitrosylation. In the present study, we investigated another neuritogenic property of IPRG001 in RGC-5 cells and retinal explant culture where in we focused on the NO-cGMP-dependent and protein S-nitrosylation pathways. IPRG001 stimulated neurite outgrowth in RGC-5 cells and retinal explant culture through NO-dependent signaling, but not NO-dependent cGMP signaling. Neurite outgrowth with IPRG001 requires retinoic acid receptor β (RARβ) expression, which is suppressed by an RAR blocking agent and siRNA inhibition. Thereby, we hypothesized that RARβ expression is mediated by protein S-nitrosylation. S-nitrosylation of histone deacetylase 2 is a key mechanism in chromatin remodeling leading to transcriptional gene activation. We found a parallelism between S-nitrosylation of histone diacetylase 2 and the induction of RARβ expression with IPRG001 treatment. The both neuroprotective and neuritogenic activities of genipin could be a new target for the regeneration of retinal ganglion cells after glaucomatous conditions.