- NMN improved behavioral measures of cognitive impairments in AD-Tg mice.
- NMN decreased β-amyloid production, amyloid plaque burden, synaptic loss, and inflammatory responses in AD-Tg mice.
- NMN reduced JNK activation in AD-Tg mice.
- NMN regulated the expression of APP cleavage secretase in AD-Tg mice.
Amyloid-β (Aβ) oligomers have been accepted as major neurotoxic agents in the therapy of Alzheimer’s disease (AD). It has been shown that the activity of nicotinamide adenine dinucleotide (NAD+) is related with the decline of Aβ toxicity in AD. Nicotinamide mononucleotide (NMN), the important precursor of NAD+, is produced during the reaction of nicotinamide phosphoribosyl transferase (Nampt). This study aimed to figure out the potential therapeutic effects of NMN and its underlying mechanisms in APPswe/PS1dE9 (AD-Tg) mice. We found that NMN gave rise to a substantial improvement in behavioral measures of cognitive impairments compared to control AD-Tg mice. In addition, NMN treatment significantly decreased β-amyloid production, amyloid plaque burden, synaptic loss, and inflammatory responses in transgenic animals. Mechanistically, NMN effectively controlled JNK activation. Furthermore, NMN potently progressed nonamyloidogenic amyloid precursor protein (APP) and suppressed amyloidogenic APP by mediating the expression of APP cleavage secretase in AD- Tg mice. Based on our findings, it was suggested that NMN substantially decreases multiple AD- associated pathological characteristically at least partially by the inhibition of JNK activation.
As a chronic neurodegenerative disorder, Alzheimer’s disease (AD) is clinically featured by progressive pattern of cognitive deficits and memory impairment. Disturbed energy metabolism in the brain and oxidative stress are two potential factors leading to neural degeneration and cognitive impairments . Aβ oligomers are found to be associated with the pathology of AD . Recent studies indicates that Aβ oligomers inhibit synaptic transmission prior to neuronal cell death  and LTP (long-term potentiation), an experimental model for synaptic plasticity and memory . In addition, Aβ oligomers are also found to be relevant to the producing of the free oxygen radical. So far, there is no curative treatment for AD . Considering the varied and well-defined pathologies of AD, new therapies with the functions of reducing pathologies are needed to prevent or slow disease progression.
Nicotinamide adenine dinucleotide (NAD), oxidized (NAD+) or reduced (NADH), plays a key role in many metabolic reactions, for both forms of NAD regulate transfer of hydrogens metabolic reactions, oxidative or reductive , as well as mitochondrial morphological dynamics in brain . Among these two forms, oxidized NAD is particularly important to mitochondrial enzyme reactions and cellular energy metabolism [8, 9]. In normal conditions, as people ages, the level of NAD+ drops , inhibiting cellular respiration and further causing decreased mitochondrial ATP and possibly cellular death. NAD+ serves a substrate for enzymes that depend on NAD+, such as ADP-ribosyl cyclase (CD38), poly(ADP-ribose) polymerase 1 (PARP1), and Sirtuin 1 (SIRT1) .
To treat neurodegenerative diseases, NAD+ depletion and cellular energy deficits need to be prevented for protecting nerves . There are four pathways synthesizing NAD+ in mammals. The salvage pathway (primary route) way is to use nicotinamide, nicotinic acid, nicotinamide riboside, or the de novo pathway with tryptophan . As an essential precursor of NAD+, Nicotinamide mononucleotide (NMN) is produced during the reaction of nicotinamide phosphoribosyltransferase (Nampt). Nampt is essential to regulating NAD+ synthesis , for it stimulates phosphoribosyl components to separate from phosphoribosyl pyrophosphates and to combine with nicotinamides. In this way, NMN is generated and with NMN adenylyltransferase, NMN is converted to NAD+. However, the potential therapeutic effects of NMN on AD remain unclear.
c-Jun N-terminal kinases (JNKs) are a family of protein kinases that play a central role in stress signaling pathways implicated in gene expression, neuronal plasticity, regeneration, cell death, and regulation of cellular senescence . Activation of JNK has been identified as a key element responsible for the regulation of apoptosis signals and therefore, it is critical for pathological cell death associated with neurodegenerative diseases and, among them, with Alzheimer’s disease (AD) .
As suggested, NAD+ may be essential to brain metabolism and might influence memory and learning. According to recent studies, the stimulation of NAD level is relevant to the reduced amyloid toxicity in AD animal models . Therefore, in this study, the potential therapeutic effects of NMN and the mechanisms of its action regulated in JNK in APPswe/PS1dE9 mice with AD were investigated.
Materials and Methods
The Institutional Animal Experiment Committee of Tongji University, China, approved all procedures conforming to the Animals’ Use and Care Policies. APPswe/PS1dE9 transgenic mice (6 months old) were purchased from Beijing Bio-technology, China. All animals were maintained in an environment that was pathogen-free. During the experimental period, water and food were accessible to all mice, and the body weight of mice and the intake of food and water were identified at the beginning of the study and then on a weekly basis. In addition, all mice that receive the treatment were observed for their general health. APPswe/PS1dE9 transgenic mice (AD-Tg) and their nontransgenic wild-type mice (NTG) were randomly assigned into four groups with six mice in each group, and each type was treated by NMN and vehicle, respectively Subcutaneous adiministration of NMN (100 mg/kg, Sigma N3501) in sterile (Phosphate Buffered Saline) PBS (200 μl) was applied to each mouse of NMN-treated groups every other day for 28 days. Each mouse with vehicle treatment subcutaneously received sterile PBS (200 μl) every other day for 28 days.
Behavioral tests were carried out by 2 experimenters who were blinded to the treatments twelve weeks after the treatments.
Memory and spatial learning test
To evaluate the memory and spatial learning of all animals, a Morris water navigation task was performed as described previously . Generally, a tracking system (Water 2020; HVS Image, Hampton, UK) was utilized to monitor the trajectory of all mice. During the training trials, a platform with the diameter of 5cm was hidden 1.5 cm below the surface of water and maintained at the same quadrant. In every trial, all mice had at most 1 minute to find the hidden platform and climb onto it. If one mouse cannot find the platform within 1 minute, the experimenters would manually guide the mouse to the platform and kept it there for 10 seconds. The trial was carried out 4 times daily for 6 days. The escape latency referring to the time that a mouse spent in finding the platform is considered as spatial learning score. Following the last training trial, the probe trial was carried out for spatial memories by allowing animals to take a free swim in the pool with the platform removed for 1 minute (swim speeds are equal). The time that each animal took to reach the previously platform-contained quadrant was measured for spatial memories.
Measurement of Passive Avoidance
To assess contextual memories, passive avoidance test was carried out, which was described in the previous studies . Briefly speaking, a two-compartment apparatus with one brightly lit and one dimly lit was used. During the training trial, the animal was put into the light lit compartment. After 60 seconds, the door between the two compartments was opened. The acquisition latency refers to the first latency time of mice to ran into the dimly lit compartment. After coming into the lit compartment, mice were exposed to a mild foot shock (0.3mA) for 3 seconds with the door closed. After 5 seconds, the animals were taken out of the compartment. One day later after the acquisition trial, the mice underwent a retention test. Like in the acquisition test, the latency to go into the dark compartment without foot shock was regarded as retention latency to test retention memory. Longer latency indicates better retention.
Following the two behavioral tests, 24 mice were first anesthetized and then infused with icy normal saline in a transcardial way. The brains were taken out and cut into 2 hemibrains along the midsagittal plane. One of the hemispheres was kept in PBS with 4% paraformaldehyde. Following the xylene treatment, the other fixed hemisphere was maintained in the paraffin for immunohistochemical tests. Then the cerebral cortex and the hippocampus were separated quickly from the hemisphere on the ice. For biochemical tests, they were maintained at −80°C following the separation. The hippocampus, brain cortex, and as well as the whole brain were weighed, respectively.
Immunohistochemical staining was carried out as described . Briefly speaking, 10 μm brain slices were deparaffinized and rehydrated. To retrieve antigens, proteinase K (200μg/ml) was treated for the staining of Aβ, and sodium citrate (0.01M, pH 6.0) was for the staining of microglia and astrocyte. Sections were blocked through incubation with fetal bovine serum (2%) and Triton X-100 (0.1%) for nonspecific binding. For immunohistochemical analysis, the section was incubated at 4°C for a night with anti-Aβ1-16 monoclonal antibody (1:600; Cell Signaling Technology, Massachusetts, USA) and monoclonal antibody anti-Iba1 (1:1,000; Osaka Wako Pure Chemical Industries, Japan) for rabbits and also monoclonal anti-Aβ antibody (1:200; Billerica, MA) for mouse.
Olympus (Tokyo, Japan) microscope with a connection to a digital microscope camera was applied to capture the images for quantitative analyses. The plaques in μm2s and the proportion of area kept by plaques positive to Aβ1–16 respectively, microglia positive to Iba1 were obtained with imaging software (Bethesda Media Cybernetics, MD). The mean value of every parameter was obtained from 6 sections with an equidistant interval of 150μm through the hippocampal region of each mouse in all groups. All measurements were blindedly conducted.
Enzyme-Linked Immunosorbent Assay (ELISA)
As described before, soluble Aβ fractions and insoluble ones were obtained from both the cortex and hippocampi of brain homogenates of mouse using RIPA (Radioimmunoprecipitation assay buffer) buffer and formic acid, respectively . The levels of both the insoluble and soluble Aβ were identified using the ELISA kits (Camarillo Invitrogen, CA). Besides, concentrations of oligomeric Aβ of brain homogenates treated with RIPA were obtained employing an ELISA kit for amyloid β oligomer (Gunma Immuno-Biochemical Laboratories, Japan).
Proinflammatory Cytokines Measurement
As described, mouse brain proinflammatory cytokine was evaluated . The expressions of TNFα, IL-6, and IL-1β were identified with immunoassay kits (Minneapolis R&D Systems, Minnesota, USA) which is for measuring these factors in mouse.
Western blotting (WB) analysis
The cortex and hippocampus tissue was homogenized with icy PBS and the lysate was for Western blot. At first SDS-PAGE (Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis) was applied to divide the proteins on NuPage Bis-Tris gel (12%, Invitrogen). The separated protein was subsequently transferred to nitrocellulose membrane which was blocked with 5% nonfat milk and probed overnight at 4°C with anti-p-JNK, anti-JNK, anti-APP, anti-sAPPα, anti- sAPPβ, anti-phosphorylated APP (p-APP, Thr668), anti-ADAM10, anti-BACE1 (CA Santa
Cruz Biotechnology, USA), anti-CDK5, anti–p-CDK5, anti–p-GSK3β, anti-GSK3β, anti-SYP, anti–postsynaptic density-95, anti–β-actin Abcam (Cambridge, MA, USA). The membrane was cleaned with TBS/0.05% Tween-20 and incubated at room temperature with secondary antibodies conjugated with horseradish peroxidase for 60 minutes, following incubation with primary antibodies. Enhanced chemiluminescence reagents (Pierce, Rockford, IL, USA) were used for detecting signals.
The data were expressed as mean ± SD. The comparisons in the speed of swimming and escape latency between the groups during the test of memory and spatial learning were made employing two-way ANOVA with repeated measures. Next, post hoc least significant difference (LSD) test was used for multiple comparison. Before post hoc LSD test or Student t test, 1-way ANOVA was employed for the rest data. Statistical analyses were carried out with Prism version 5. A P < 0.05 was considered statistically significant.
NMN Treatment Rescues Cognitive impairments in AD-Tg Mice
The test of memory and spatial learning has shown that 1-year-old AD-Tg mice have experienced impairment in memory and spatial learning . In our study, comparied to the vehicle-/NMN-treated wild-type (WT) mice, the vehicle-treated AD-Tg mice had a longer escape latency, which showed severe impairment of spatial learning in the test (Fig. 1A). However, with shorter escape latency, NMN treatment greatly improved the impairment of spatial learning in vehicle-treated AD-Tg animals (Fig. 1A). Besides, it was also identified that compared with two wild-type groups, vehicle-treated AD-Tg animals spent less time in the target quadrant during the probe trial (p < 0.01), suggesting severe spatial memory impairment. AD-Tg mice treated with NMN spent longer time in the target quadrant, which indicates marked alleviation of the spatial learning impairments present in AD-Tg mice treated with vehicle (p<0.01) (Fig. 1B).
To further identify the alleviation of memory deficits by NMN treatment in AD-Tg mouse, contextual memories were evaluated employing the measurement of passive avoidance . As illustrated in Fig. 1C, retention latency was decreased compared with two wild-type mice groups (p < 0.01), suggesting impaired contextual memories in the AD-Tg animals treated by vehicle. In contrast, NMN-treated mice exhibited longer retention latency compared with those treated with vehicles (p < 0.01), demonstrating outstanding reversal of NMN in contextual memories. All these data indicate that NMN treatment markedly improves cognitive impairments in AD-Tg animals.
NMN Suppresses JNK Phosphorylation in AD-Tg Mice
JNK, also called a protein kinase activated by stress, is said to play a role in a couple of pathophysiological processes in AD . Therefore, in this study, we tested the inhibitory effects of NMN on the activation of JNK through Western blotting. It was revealed by quantitative analysis that p-JNK level was significantly grown in hippocampus and cerebral cortex in the vehicle-treated AD-Tg mice when contrasting to two wild-type groups (Fig. 2A; p < 0.01), whereas NMN gave rise to a sharp decline in p-JNK in hippocampus and cerebral cortex with a comparison to the vehicle-treated AD-Tg mice (Fig. 2B; P< 0.01). Both reductions symbolized a reverse to the wild-type level. But the whole expression of JNK kept unchanged in all the 4 groups. Conclusively, all data indicate that NMN treatment has an inhibitory effects on JNK activation in AD-Tg mice.
NMN Treatment Decreases the Level of Aβ and Deposition in AD-Tg Mice
The role of reduced activation of JNK in the changes of the Aβ level and deposition was studied in AD-Tg mice through employing histological and biochemical analyses. As presented in Figs. 3A-D, it was found that NMN-treated AD-Tg mice had a sharp reduction in the levels of Aβ when comparing to the vehicle treatment group (p < 0.01). Comparing to the vehicle treatment group, NMN treatment gave rise to a marked decrease in Aβ oligomers (p < 0.01) (Fig. 3E). Immunohistochemical staining identified this observation, indicating the lessened diffuse plaques and also the shrinked area taken by diffuse plaques in AD-Tg mice treated by NMN compared to the vehicle treatment group (Figs. 4A-D). Thus, on the basis of the findings, it was demonstrated that the generation of Aβ in the brain of AD-Tg mice is effectively decreased by the inhibited activation of JNK with NMN treatment.
NMN Treatment Changes the Processing of APP in AD-Tg Mice
To study the mechanism of inhibition on the production of Aβ and deposition, the effects of NMN on the processing of APP were examined by Western blotting. As presented in Figs. 5A-C, the level of full-length APP expression was greatly increased in the brain of AD-Tg mouse treated with vehicle compared with wild-type ones (p < 0.01). However, they kept unaltered between the group treated with vehicle and that with NMN. Importantly, it was found that NMN treatment remarkably lowered the increased levels of p-APP in the AD-Tg mice treated by vehicle (p < 0.01). Besides, α-secretase cleaved sAPPα and β-secretase cleaved sAPPβ in the brain tissues of Tg mice were tested via Western blotting. It was shown by quantitative analyses that NMN treatment led to a remarkable elevation of sAPPα (p < 0.01) and a marked decline in sAPPβ (p < 0.01) compared with the transgenic mice treated by vehicle (Figs. 5D-F). Based on these data, it was indicated that NMN treatment is strongly effective in suppressing the phosphorylation of APP, improving cleaving of APP by α-secretase, and decreasing the cleaving of APP by β-secretase in AD-Tg mice brains.
NMN Treatment Improves Inflammatory Responses in AD-Tg Mice
Since JNK activation is indicated to play a role in the inflammatory response induced by Aβ in previous studies , whether reduced activation of JNK influences neural inflammation in AD- Tg animals was investigated. The role of NMN on the neural inflammation was identified by measuring proinflammatory cytokines that were in the lysates of cortical tissues. It was found that the level of IL-6, IL-1β, and TNFα were sharply declined in the AD-Tg mice treated by NMN relative to those by vehicle (Figs. 6A-C). According to these findings, NMN treatment is indicated to be potently effective in the amelioration of neural inflammation in AD-Tg mice brains.
NMN Treatment Ameliorates Synaptic Loss in AD-Tg Mice
The loss of synapse is an important pathological characteristic of AD and said to be relevant to the cognitive impairments of AD . The changes in SYP (presynaptic marker) level and PSD- 95 level (postsynaptic marker) were investigated via Western blotting. It was showed by quantitative analysis that SYP levels and the levels of PSD-95 expression substantially reduced in hippocampus and brain cortex of AD-Tg mice treated with vehicle relative to WT ones
(p < 0.01), whereas NMN treatment significantly elevated SYP levels and the levels of PSD-95 expression in hippocampus and brain cortex relative to AD-Tg mice treated with vehicle (p < 0.01) (Fig. 6D-F). This finding suggest that NMN treatment sharply ameliorates the loss of synapse in AD-Tg mice brains.
In the present study, it is mainly found that NMN treatment substantially improves primary pathological characteristics of the AD-modeled AD-Tg mice, including cognitive impairments, neuroinflammation, Aβ pathology, and synaptic loss, which consistent with a recent study . It was also found that NMN treatment inhibited JNK activation and amyloidogenic processing of APP by mediating the expression of APP-cleavage secretase, and also facilitated APP processing in AD-Tg mice. The data prove that NMN treatment greatly reduces multiple AD-associated pathological characteristics, at least partially by the inhibition of JNK activation.
Numerous studies have reported the increase of abnormal activation of JNK in both the transgenic AD mice models and the AD patients [23-25]. Conforming to the above previous studies, we also found that the level of phosphorylated JNK in AD-Tg mice treated by vehicle was higher than that in the wild-type group, but NMN treatment in AD-Tg mice potently suppressed the phosphorylation of JNK to the basic level of WT groups. The controlled activation of JNK through NMN gave rise to a substantial decrease of Aβ pathology in AD-Tg animals. According to the studies before, active JNK is proved to engage in BACE1 expressions and PS1 expressions [26, 27]. In addition, the increased BACE1 and PS1 in AD-Tg mice treated by vehicle were found to be greatly suppressed by NMN to the basic level of WT groups (data not shown). More interestingly, it was also observed that NMN treatment led to substantially elevated sAPPα and reduced sAPPβ. It was notable that according to the previous studies, APP phosphorylation at the site of Thr668 is proved to promote the β-secretase cleavage of APP to grow Aβ generation in vitro . In present study, we also found that the administration of NMN in AD-Tg mice significantly declined the elevated phosphorylation of APP to the primary level of WT controls, indicating an in vivo inhibition mechanism of Aβ pathology through NMN treatment. Collectively, all these findings indicate that the potent effects of NMN on the marked decrease in Aβ pathology in the brains of AD-Tg mice may be responsible for its enhancement of nonamyloidogenic APP processing. What we found is consistent to a recent study demonstrating that genetic depletion of JNK3 in 5XFAD mice is attributed to a significant decrease in the levels of Aβ and the total plaque loads . Recently numerous studies suggested energy failure and accumulative intracellular waste also play a causal role in the pathogenesis of several neurodegenerative disorders and Alzheimer’s disease (AD) in particular regulated by potential role of several metabolic pathways Wnt signaling, 5′ adenosine monophosphate-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), Sirtuin 1 (Sirt1, silent mating-type information regulator 2 homolog 1), and peroxisome proliferator-activated receptor gamma co- activator 1-α (PGC-1α) [30, 31]. It will be warrant to study if NMN also participate in regulation of these signaling pathways.
Some recent studies indicate that enhanced neuroinflammation is essential to the development of AD [32-34]. In our study, a marked decline in the proinflammatory cytokines levels (IL-6, IL-1β, and TNFα) proved that NMN treatment effectively controlled the neuroinflammatory responses of the brain of AD-Tg mouse. Considering the key role of oligomeric and fibrillar Aβ for activation of microglia cells and astrocytes with the subsequent generation of proinflammatory cytokines , the reduction in neuroinflammatory responses may be less important to the substantial reduction in Aβ pathologies presented in the AD-Tg mice treated by NMN. Several previous studies had proved that JNK represents an important mediator for activation of glial cell and proinflammatory cytokines [35, 36]. Thus, the favorable effects of NMN on lowered inflammatory responses in AD-Tg groups can be largely responsible for its direct control of inflammation by inhibiting JNK activation. Based on the previous reports, it was proved that some proinflammatory cytokines (ie, IL-1β, interferon gamma, and TNFα) may elevate the expression of β-secretase and γ-secretase to ameliorate amyloidogenic APP processing and Amyloid-β production by an in vitro JNK-mediated pathway . Hence, we have reasons to believe that the reduced proinflammatory cytokines through NMN treatment may be effective in reducing the production of Aβ in vivo. Moreover, in our study, it was demonstrated that NMN treatment ameliorates cognitive impairments in AD-Tg mouse models. An increasing evidence has proved that grown Aβ levels, neuroinflammation, synaptic dysfunction and loss are closely related to the cognitive dysfunction in AD . In addition, our data confirms the finding of a recent research, which revealed that genetic down-regulation of JNK3 gives rise to a remarkable amelioration of cognitive impairments in 5XFAD mice . Collectively, our findings, along with all the previous research, demonstrate that the inhibited JNK activaty by NMN is potently effective in ameliorating AD-associated cognitive deficits.
Synaptic loss is a major pathological change of AD and is tightly associated with AD-related cognitive impairments . It was presented that PSD-95, a biomarker of postsynaptic density, is essential to synapse maturation and synaptic plasticity , and that SYP, a presynaptic protein, also acts as an integral membrane protein in the synapse and it plays a key role in plasticity of synapses . Therefore, it can be soundly supposed that the greatly lowered expression of PSD- 95 and SYN presented in the study may suggest the impairment of synaptic integrity and plasticity in AD-Tg mice treated by vehicle. Intriguingly, the treatment of NMN in AD-Tg animals substantially elevated the lowered PSD-95 and SYN expression level back to the primary level of WT controls. Since it was demonstrated by several studies that Amyloid-β- induced synaptic loss and dysfunction are regulated through the JNK activation [40, 41], the possible mechanisms behind NMN treatment leading to the elevated expression of PSD-95 and SYN in AD-Tg animals may be responsible for its inhibitory effects on JNK activation. Thus, it is possible that the treatment of NMN may ameliorate the impaired synaptic plasticity which is caused by toxic Aβ species in AD-Tg mice.
In summary, this study provides essential preclinical evidences that NMN takes effects in reversing cognitive deficits and substantially lowering the burden of amyloid plaque, neuroinflammation, cerebral amyloid-β concentrations, and loss of synapse in middle-aged AD- Tg mice, at least partially by the inhibition of JNK activation. According to our findings, NMN could be a new target for disease-modifying treatments of AD.
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