Lanthionine ketimine ester improves outcome in an MPTP-induced mouse model of Parkinson's disease via suppressions of CRMP2 phosphorylation and microglial activation
Introduction
Parkinson's disease (PD) is the second most common neurodegenerative and progressive disorder following Alzheimer's disease. One of the main pathologies in PD is slow and progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc). Dopaminergic neurons in the SNc mainly project to the striatum via the nigrostriatal pathway, and the striatum plays a role in modulating motor functions. Therefore, the loss of dopaminergic cells in the SNc induces a deficiency of dopamine in the striatum and leads to behavioral disorders such as resting tremor, slowness of movement (bradykinesia), muscle rigidity, and postural instability. Currently, levodopa (L-DOPA) is the most widely used medication for the treatment of PD and it compensates for the decreased dopamine synthesis. However, L-DOPA does not slow neurodegeneration and its long-term use gives rise to certain complications including wearing off and dyskinesia, a difficulty or distortion in performing voluntary movements [1,2].
The most popular method to induce PD-like changes in animal models is through injection of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP was discovered as a result of intravenous use of an illegal meperidine analog [3] and causes permanent symptoms of PD by destroying dopaminergic neurons in the SNc [4]. Inflammatory responses are thought to amplify neuronal degeneration and accelerate the progression of PD [5].
Lanthionine ketimine (LK) is a naturally occurring sulfur amino acid metabolite found in the mammalian central nervous system and LK-5-ethyl ester (LKE) is a cell penetrating LK synthetic derivative [6]. LK and LKE have neuroprotective, neurotrophic, and anti-inflammatory effects by binding to several central nervous system proteins including CRMP2 [6]. LKE has shown therapeutic benefits in animal models of diverse neurological diseases such as amyotrophic Alzheimer's disease (AD), lateral sclerosis (ALS), spinal cord injury (SCI), traumatic brain injury, and multiple sclerosis [[7], [8], [9], [10], [11]]. Recently, it has been demonstrated that the Akt/GSK3β/CRMP2 pathway is involved in axon degeneration in PD [12]. In the present study we investigated the effect of LKE administration on MPTP-induced CRMP2 phosphorylation and dopaminergic neuronal survival in the SNc. The results showed that LKE suppressed MPTP-induced dopaminergic neuronal loss and motor coordination deficits. Additionally, we showed that LKE suppressed microglial activation and CRMP2 phosphorylation. These results demonstrate the potential of LKE as a novel therapeutic candidate in the treatment of PD by slowing progress of pathological processes.
Section snippets
Mouse line and drug treatment
The mice used in present experiments were housed in accordance with the technical protocols approved by the Institutional Animal Care and Use Committee in Waseda University (2017-A027, 2018-A004). MPTP (25 mg/kg, Sigma) was administered to male C57Black/6 J mice (22–28 g body weight, Sankyo Lab Service Corp., Japan). Control mice were injected with vehicle only, as described previously [13]. The injections and samplings were performed in two ways: 1) the mice were administered MPTP via an
LKE treatment reduced dopaminergic neuronal death in the SNc in MPTP-injected mice
We analyzed the effect of LKE administration on dopaminergic neuronal cell death in MPTP-induced PD mice. We conducted immunostaining using an anti-TH antibody to measure the number of dopaminergic cells in the SNc after five consecutive days of injections of dH2O (MPTP-), MPTP and PBS (MPTP+PBS), or MPTP and LKE (MPTP+LKE). The number of dopaminergic cells in the SNc were significantly reduced in MPTP+PBS treated mice compared to MPTP- mice (Fig. 1a, b; P < .001; n = 4 mice). LKE itself had no
Discussion
The most prominent finding in the present study is that LKE attenuated the loss of TH-positive SNc neurons resulting from MPTP injections (Fig. 1), suggesting that LKE has neuroprotective effects in dopaminergic neuronal cells in this murine model of PD. The effect of LKE on dopaminergic neuronal survival may be mediated by its neuronal protective and neurotrophic effects, also shown in other neurodegenerative diseases including Alzheimer's disease [8], cerebral ischemia [9], multiple sclerosis
Disclaimer
KH is the inventor of patented LK derivatives used in this study, and owns equity in a company involved with commercial development of the compounds for neurodegenerative disease.
Acknowledgements
This work was partly supported by grants from the Ministry of Education, Culture, Sports, Science and Technology (no. 26430043 and 18K06511 to T.O), and from the National Institutes of Health (NS093594) and from Creation of Innovation Centers for Advanced Interdisciplinary Research Areas Program in the Project for Developing Innovation System from the MEXT (no. 42890001 to YG).
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These authors are equally contributed.