Characterization of the molecular mechanisms underlying the neurotrophic and neuroprotective activities of puerarin

Abstract

Parkinson’ disease (PD) is a neurodegenerative disease characterized by the progressive loss of dopaminergic nigrostriatal neurons. Current anti-parkinsonism therapies mainly improve motor symptoms in PD patients. It is widely believed that new treatments could be developed from plant natural products. As an example, puerarin, 7, 4’-dihydroxy-8-C-glucosylisoflavone, is the main isoflavone glycoside derived from the herb Pueraria lobota. The aim of this thesis was to investigate the mechanisms underlying the neurotrophic and neuroprotective effects of puerarin. This thesis mainly addressed following three questions: 1) Could puerain potentiate nerve growth factor (NGF)-induced neuritogenesis? 2) How can puerarin protect neurons against oxidative injury? 3) Could puerarin be used to treat comorbid pain and depression?

Firstly, insufficient production of neurotrophic factors is implicated in neurodegenerative diseases including PD. We first investigated whether puerarin could mimic or coordinate with NGF in the regulation of neurite outgrowth. We found that puerarin potentiated NGF-activated neurite outgrowth in the rat pheochromocytoma cell (PC12) in a concentration-dependent manner. Puerarin sequentially induced the activation of PI3K/Akt, ERK1/2, Nrf2/HO-1 pathways. By incubating puerarin and NGF, transiently, simultaneously and continuously, we discovered that puerarin coordinated with NGF to activate PI3K/Akt and ERK1/2 in the neurite extension process.

Secondly, we attempted to characterize the mechanisms underlying the neuroprotective activity of puerarin. Thus, we profiled puerarin-regulated proteins in rat dopaminergic PC12 cells by proteomic approach. We found that puerarin activated arginase-2 in a time- and concentration-dependent manner. Mitochondrial enzyme arginase-2 catalyzes the degradation of L-arginine and thereby limits substrate for nitric oxide synthases (NOSs). Thus, we hypothesized that puerarin might suppress the neurotoxicity of nitric oxide (NO) via inducing arginase-2. Puerarin indeed suppressed 6-hydroxydopamine (6-OHDA)-induced NO production and neurotoxicity in PC12 cells and primary rat midbrain neurons. Specific arginase inhibitor BEC diminished the activities of puerarin on 6-OHDA-induced NO production and neurotoxicity.

Thirdly, comorbid pain and depression affect more than 10% of the global population, especially the elderly population. Depression and pain comorbidity is currently treated with non-steroidal painkillers (aspirin and ibuprofen) and selective serotonin reuptake inhibitor (SSRI) antidepressants (Zoloft and Prozac). However, the effectiveness of SSRIs is often reduced by the common painkillers. Thus, we evaluated a panel of Chinese medicines for the treatment of depression and pain comorbidity. We discovered puerarin as a potential dual antidepressant and painkilling drug in mouse model with the spared nerve injury (SNI). Our studies suggested that puerarin activated ERK1/2 and CREB, and subsequently induced brain-derived neurotrophic factor (BDNF). Therefore, we believe that puerarin may exhibit antidepressant and painkilling activities via inducing BDNF.

In conclusion, this thesis reported our recent studies on the neurotrophic and neuroprotective effects of puerarin and the underlying mechanisms. The key findings from this thesis included: 1) puerarin potentiated NGF-induced neurite outgrowth in rat via activating PI3K/Akt, ERK1/2, Nrf2/HO-1 pathways; 2) puerarin suppressed 6-OHDA-induced NO production and neurotoxicity via inducing arginase-2. 3) puerarin exhibited antidepressant and painkilling activities via inducing BDNF. The results of this thesis may help the development of puerarin-based anti-parkinsonism drugs.