Kurarinone alleviated Parkinson's disease via stabilization of epoxyeicosatrienoic acids in animal model

Parkinson’s disease (PD) is one of the most common neurodegenerative disorders and is characterized by loss of dopaminergic neurons in the substantia nigra (SN), causing bradykinesia and rest tremors. Although the molecular mechanism of PD is still not fully understood, neuroinflammation has a key role in the damage of dopaminergic neurons. Herein, we found that kurarinone, a unique natural product from Sophora flavescens, alleviated the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)–induced behavioral deficits and dopaminergic neurotoxicity, including the losses of neurotransmitters and tyrosine hydroxylase (TH)–positive cells (SN and striatum [STR]). Furthermore, kurarinone attenuated the MPTP-mediated neuroinflammation via suppressing the activation of microglia involved in the nuclear factor kappa B signaling pathway. The proteomics result of the solvent-induced protein precipitation and thermal proteome profiling suggest that the soluble epoxide hydrolase (sEH) enzyme, which is associated with the neuroinflammation of PD, is a promising target of kurarinone. This is supported by the increase of plasma epoxyeicosatrienoic acids (sEH substrates) and the decrease of dihydroxyeicosatrienoic acids (sEH products), and the results of in vitro inhibition kinetics, surface plasmon resonance, and cocrystallization of kurarinone with sEH revealed that this natural compound is an uncompetitive inhibitor. In addition, sEH knockout (KO) attenuated the progression of PD, and sEH KO plus kurarinone did not further reduce the protection of PD in MPTP-induced PD mice. These findings suggest that kurarinone could be a potential natural candidate for the treatment of PD, possibly through sEH inhibition.

Parkinson’s disease (PD) is the second-most common neurodegenerative disorder after Alzheimer’s disease (AD) and affects 1.7% of the population over 65 y old, especially people over 80 y old (1, 2). PD is caused by the loss of dopaminergic neurons in the substantia nigra (SN), and it is associated with accumulation of Lewy bodies (LBs) in neuronal somata and Lewy neurites in neuronal processes with fibrillar α-synuclein (3). PD is characterized by the classical motor features of parkinsonism, including bradykinesia, rest tremor, and rigidity as well as postural instability (4, 5). Advances have been made in understanding PD neurodegenerative pathophysiology (4–6). However, translation into patients’ care is still lagging well behind. So far, the treatment of PD in recent trials still depends on strategies for neuroprotection, motor symptoms, and nonmotor symptoms (6). However, PD symptoms have proven elusive to slow down or reverse through the aforementioned interventions (6); therefore, a cure for PD (no symptoms, no side effects, to borrow a phrase from the epilepsy field) has not yet been achieved.Although the molecular mechanism resulting in the neuronal degeneration of PD is not fully understood, some factors are associated with the damage of dopaminergic neurons in PD, including mitochondrial dysfunction, oxidative stress, endoplasmic reticulum stress, and especially neuroinflammation (7, 8). Extensive recent evidence revealed that the release of α-synuclein from dopaminergic neurons activated microglia cells to cause the neuroinflammation, allowing the increase of inflammatory cytokines interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) (9, 10). These changes have been found in postmortem brain tissue from PD patients (11, 12). Therefore, neuroinflammation plays a central role in the development of PD and is the target of some recent investigations for treating PD (13–16).Natural products are an important resource of innovational drugs since they possess complex and changeable structures and remarkable biological effects. A great body of evidence has indicated the effect of natural products from traditional Chinese medicines in the neuroinflammation of PD (17–19), such as genistein, resveratrol, and alaternin. Kurarinone is one of the major constituents of the traditional Chinese medicine Sophorae Flavescentis Radix, or Kushen in Chinese (the root of Sophora flavescens), which is often used to treat diarrhea, bacterial and fungal infections, eczema, and inflammation-related diseases (20). Kurarinone shares a flavanone core with a characteristic lavandulyl moiety at C-8 (21, 22) and possesses several pharmacological effects, such as anti-inflammatory and antioxidative activities (23, 24), as well as activation on the large-conductance Ca²⁺-activated K⁺ channel (25, 26).Therefore, in this study, we first tested the ability of kurarinone to reduce neuroinflammation and improve behavioral deficits in a PD mice model induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). To understand how kurarinone decreases inflammation and because its treatment resulted in elevated levels of epoxyeicosatrienoic acids (EETs), endogenous signaling molecules that control inflammation (27), we used several biochemical methods to determine the molecular target of kurarinone. The interactions between kurarinone and soluble epoxide hydrolase, the main enzyme metabolizing EETs (28), were confirmed using enzyme kinetics and cocrystallization. Our findings suggest that kurarinone could be a potential natural candidate for the treatment of PD through sEH inhibition and other mechanisms, as well as being a lead to develop a new family of sEH inhibitors.