Introduction
Alzheimer’s disease (AD) is the most common cause of dementia. It is characterized by the intracellular neurofibrillary tangles, extracellular neuritic plaques, and neuronal loss. Extracellular neuritic plaques are the unique feature distinguishing AD from other forms of dementia and neurodegenerative diseases. Amyloid β protein (Aβ), the central component of neuritic plaques, is generated from the amyloid precursor protein by β– and γ–secretase cleavages [
1,
2]. One of the common features of AD neuropathology is the activation of microglia and neuroinflammation. Aβ deposits are particularly potent in the activation of microglia [
3]. Despite clear evidence of the recruitment of microglia to the vicinity of plaques, there are ongoing debates on the function of the recruited microglia and how microglia participate in Aβ deposition and clearance during AD pathogenesis. In addition, microglia are the primary immune cells responsible for cytokine production, including pro- and anti-inflammatory cytokines as well as growth factors [
4], thus potentially serving as a double-edged sword in response to stimuli depending on the physiopathological status of the brain.
Macrophage migration inhibitory factor (MIF) is a pleiotropic protein that participates in many cellular activities and plays an essential role in regulating the inflammatory response, energy metabolism, and apoptosis. MIF has been shown to be beneficial in promoting survival of cardiomyocytes during cardiac ischemia/reperfusion (I/R) by inhibiting apoptosis, reducing ROS production and regulating glucose metabolism [
5‐
7]. Previously, we have shown upregulation of MIF by hypoxia in the acute phase of stroke [
8,
9] and demonstrated the protective role of MIF in suppressing oxidative stress-induced caspase-3 activation [
10]. MIF has also been identified as a PARP-1 (Poly (ADP-Ribose) Polymerase 1)-dependent AIF (apoptosis-inducing factor)-associated nuclease (PAAN), and disruption of MIF’s nuclease activity inhibited cell death induced by glutamate excitotoxicity and focal stroke [
11]. In AD pathogenesis, neuronal death through apoptotic pathways is observed, and there is evidence suggesting that oxidative stress originating from dysregulation of mitochondria functions serves as a cause of apoptosis [
12]. Therefore, MIF may be essential for neuronal survival during AD pathogenesis. In contrast, MIF could also play a deleterious role when overexpressed by immune cells, resulting in excessive inflammation in chronic inflammatory diseases in the systems outside of the CNS [
13]. Whether MIF plays a role in initiating and/or maintaining the inflammatory status in the CNS remains unknown. In AD patients, Aβ deposits induce chronic neuroinflammation which features in the activation of resident microglia and infiltration of peripheral macrophages [
14]. However, the current understanding of the expression regulation of MIF and its role in AD is limited. This study aimed to investigate how MIF would participate in these seemly paradoxical roles during AD pathogenesis.
Materials and methods
Cerebrospinal fluid (CSF) and brain tissues
CSF samples were obtained from patients visiting the Guangzhou General Hospital. The control group included patients who had no history or evidence of cognitive decline. CSFs were taken by lumbar puncture under anesthesia when the subjects had surgery for diseases other than inflammatory diseases of the central nervous system. CSFs from patients with AD, mild cognitive impairment (MCI), and vascular dementia (VD) were collected for neurological diagnosis. Table
1 listed the sample information regarding the sample size, sex, and age in each group. Frozen control and AD human cortices were obtained from the Department of Pathology, Columbia University. These samples were used to examine the expression of MIF by immunoblotting experiments. Table
2 listed the sample information regarding the sex, age, and brain areas used in the study.
Table 1
Patient information for the CSF analysis
Control | 30 (14/16) | 61.2 ± 9.6 |
AD | 28 (14/14) | 69.4 ± 6.8 |
MCI | 10 (4/6) | 68.9 ± 9.6 |
VD | 17 (8/9) | 66.3 ± 10.2 |
Table 2
Patient information for the brain tissue analysis
1751 | AD | M | 76 | Fc |
1780 | AD | M | 72 | Fc |
4556 | AD | F | 70 | Fc |
4693 | AD | F | 70 | Fc |
4854 | AD | M | 54 | Fc |
794 | Control | F | 51 | Fc |
1170 | Control | M | 58 | Fc |
1226 | Control | M | 23 | Fc |
1441 | Control | M | 51 | Fc |
4263 | Control | M | 61 | Fc |
Animals
Animal experiment protocols were approved by The University of British Columbia Animal Care and Use Committee. APP23 transgenic mice carry human APP751 cDNA with the Swedish double mutation at positions 670/671 (KM→NL) under control of the murine Thy-1.2 expression cassette [
15,
16]. The PS45 transgenic mice carry human presenilin-1 cDNA with the G384A mutation [
17].
Mif+/− mice on the c57/BL6 background were generated by breeding
Mif+/− mice on BALB/c background (The Jackson Laboratory), with in-house bred c57/BL6 mice. All mice were allowed to access water and food ad libitum. APP23/PS45 double transgenic mice were bred by cross APP23 with PS45 mice. APP23/MIF
+/− mice were bred by cross APP23 with
Mif+/− mice on c57/BL6 background. Positive pups were determined by genotyping [
18].
The Morris water maze test
The Morris water maze test was performed as previously described [
19]. Briefly, the test was performed in a 1.5-m diameter pool with a 10-cm diameter platform placed in the southeastern quadrant of the pool. The procedure consisted of 1 day of visible platform tests and 4 days of hidden platform tests, plus a probe trial 24 h after the last hidden platform test. In the visible platform test, mice were tested for five continuous trials with an inter-trial interval of 60 min. Mouse behavior including distance traveled and escape latency was automatically video-recorded by automated video tracking (ANY-maze, Stoelting). The tests were performed on APP23/MIF
+/− mice, and APP23 mice, which were negative littler mates of APP23/MIF
+/− mice. The tests were performed at the ages between 13 and 14 months.
Immunohistology
Half brains were fixed in 4% PFA in PBS. Fixed brains were either dehydrated in 30% sucrose solution followed by cryosectioning at 30 μm thickness or prepared for paraffin-embedded sectioning at 5 μm thickness. For immunohistochemistry, the brain slices were incubated with 3% hydrogen peroxide in PBS for 10 min, permeabilized in 0.3% Triton X-100 in PBS (PBS-Tx) for 30 min, and blocked with 5% BSA in PBS for 1 h at 22 °C. Next, the slices were incubated with primary antibodies at 4 °C overnight. After rinsing with PBS-Tx for three times, the slices were applied with secondary antibodies for 1 h at 22 °C, followed by 30-min incubation with avidin-biotin-peroxidase complex (ABC, Vector Laboratories). Color development was achieved by the DAB (Vector Laboratories) method. After rinsing with ddH2O, brain sections were subjected to additional hematoxylin staining to visualize nuclei. The sections were observed under traditional microscopy. For Aβ plaque detection, the procedure for secondary antibody incubation was omitted, and the slices were proceeded for ABC incubation and color development. The number of plaques was quantified manually following qualification under × 40 magnification. The area of the plaques was quantified using ImageJ. Images were taken from 10 matching areas (5 slices with 540-μm intervals for each mouse) between APP23 and APP23/MIF+/− transgenic mice. All the images used for plaque quantification were taken at the same time with the same exposure level. For immunofluorescent staining, the brain slices were permeablized in PBS-Tx for 30 min followed by sequential incubation with primary and fluorescent-labeled secondary antibodies as above. After rinsing with PBS, brain sections were coverslipped using the VECTASHILD® mounting medium with DAPI (Vector Laboratories) and observed under fluorescent microscopy. The primary antibodies are rabbit anti-MIF antibody (Torrey Pines Biolabs), mouse anti-GFAP antibody, biotinylated 4G8 antibody, rabbit anti-Iba-1 (DAKO). Secondary antibodies are biotinylated swine anti-rabbit IgG (DAKO) for immunohistochemistry, Alexa 488-labeled goat anti-rabbit IgG (Invitrogen), Alexa 594-labeled goat anti-rabbit IgG (Invitrogen), Alexa 488-labeled goat anti-mouse IgG (Invitrogen), and Alexa 594-labeled goat anti-mouse IgG (Invitrogen) for immunofluorescent staining. Primary and secondary antibodies were diluted in PBS with 1% BSA.
Immunoblotting
Cortical tissues were homogenized by sonication with 5X (v/w) RIPA-DOC lysis buffer supplemented with a complete mini protease inhibitor cocktail tablet (Roche Molecular Biochemicals). The samples were then centrifuged at 16,000×g at 4 °C for 30 min. The supernatants were removed and added to 2X Novex® tricine SDS sample buffer (Invitrogen) followed by boiling at 100 °C for 2 min. The samples were resolved in 12% tris-tricine gels and transferred to PVDF-FL membranes (Millipore). The membranes were blocked with 5% non-fat milk and incubated with primary antibodies for MIF (Torrey Pines Biolabs) and β-actin (Sigma, AC-15). To detect the proteins, IDye680-labeled goat anti-rabbit and IDye800-labeled goat anti-mouse antibody were used. The blots were scanned using the Odyssey Imager (Licor).
Cell culture, Aβ oligomer preparation, ELISA, LDH, and MTS assays
The mouse microglia cell line BV-2, mouse macrophage cell line RAW264.7, human neuroblastoma cell line SHSY-5Y, and a stable cell line overexpressing MIF (SYMS) were maintained in Dulbecco’s modified Eagle’s medium supplemented with 10% fetal bovine serum, 1 mmol/L of sodium pyruvate, and 2 mmol/L of
l-glutamine (Invitrogen). Cells were seeded onto 96-well plates and cultured at 37 °C in an incubator supplemented with 5% CO2. Aβ oligomers were prepared as previously described with modification [
20,
21]. Briefly, synthetic Aβ1-42 was dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, Fluka), vacuum dried, and dissolved in DMSO as a 5 mM stock. Aβ oligomer was prepared by diluting the stock Aβ in sterile PBS to 100 μM and incubated at 4 °C for 12 h. The oligomers were further diluted to 10 μM or 50 μM by culture medium to treat the cells. LPS was used as a positive control for MIF secretion and was used at the concentration of 100 ng/mL. Sixteen hours after Aβ treatment (10 μM), the culture medium was collected and centrifuged at 3000×
g for 2 min at 4 °C prior to assays. MIF concentrations in culture media were measured by a human (R&D systems) or mouse (Mybiosource) MIF ELISA kit following the manufacturer’s instruction. Culture media were diluted five and two times prior to the assays to measure human and mouse MIF concentration, respectively. To assess cell membrane integrity, LDH assay (Promega) following the manufacturer’s instruction was performed using the same batch of culture medium. SHSY-5Y and SYMS were treated with Aβ oligomers (50 μM). To detect Aβ-induced cytotoxicity, MTS assay was performed following the manufacturer’s protocol (Promega).
Dot blot assay
To prepare the membrane for the dot blot assay, 2 μL of oligomerized Aβ peptide (100 μM) or purified green fluorescent protein (GFP) protein (approximately 50 μM) were spot on a nitrocellulose membrane and were let dry. The membrane was then blocked in 0.3% BSA in PBS for 1 h at room temperature prior to incubation with mixed proteins of purified hMIF and GFP at the concentration of approximated 5 μM at 4 °C for overnight. The membrane was then washed, and immunoblotting was performed to detect MIF and GFP. The primary antibody to detect MIF was a monoclonal anti-MIF antibody (D-2, Santa-Cruz). The primary antibody to detect GFP was a polyclonal anti-GFP antibody.
Discussion
Overproduction of Aβ leads to chronic inflammation that is characterized by activation of microglia cells and increased production of inflammatory mediators in the brain, such as pro-inflammatory cytokines, chemokines, macrophage inflammatory proteins, and prostaglandins [
14]. MIF as a pro-inflammatory cytokine has been shown to be upregulated systematically in circulation and locally at lesion sites in many chronic inflammatory diseases and plays central roles in disease progression. Previous studies have shown that MIF was upregulated in plasma and CSF from AD patients [
26‐
29]. In the present study, we demonstrated that MIF level was increased in the postmortem cortical tissues and CSF of AD patients and the late stage of the AD transgenic mouse brains with a substantial amount of Aβ deposits. Notably, the MIF level was not altered in either CSF from patients with MCI or brain tissues from young AD transgenic mouse brains, suggesting that MIF probably was not immediately upregulated at the early stage of AD pathology. This result is in line with a previous study showing that among a series of pro-inflammatory cytokines including MIF, only the production of IL-1β was induced in 14-month old APP23 single transgenic mice, which developed a few Aβ deposits [
30]. Interestingly, MIF level was not elevated in patients with VD, indicating the elevation of MIF in the CSF possibly is specific to patients with dementia due to AD. This was indicative that MIF release could be a defense mechanism by neurons, and that was how AD is distinguished from other types of dementia, such as VD.
In addition to temporal expression pattern, our study further characterized the spatial expression pattern of MIF. In our study, we clearly demonstrated that MIF expression immediately surrounds Aβ plaques. Furthermore, we showed that MIF directly binds with Aβ. Protein associated with Aβ may affect fibril formation and deposition. For example, cystatin C has been reported to associate with Aβ42 to inhibit the formation of amyloid fibrils [
31]. Cystatin C and MIF share a similar tertiary structure with 4-strand β-sheet, and both undergo self-aggregation amorphologically or to filaments at a physiological condition and are amyloidogenic at lower pH in vitro [
32,
33]. Because MIF binds to Aβ directly and colocalizes with Aβ plaques, whether MIF could affect the amyloidogenesis of Aβ warrants further studies.
It has been suggested that MIF facilitates defending mechanisms under stressful conditions by promoting cell survival in the systems outside the CNS. During heart ischemia, MIF acts in an autocrine fashion and signals through the CD74/44 receptor complex to promote cell survival by temporarily maintaining energy homeostasis; knockout of MIF thus results in the larger ischemic damage [
7]. In the CNS, as we demonstrated, maintaining MIF expression is important in defending cerebral ischemia by reducing oxidative stress-induced caspase-3 activation in neurons during the acute phase [
10]. Maintaining MIF expression could also be important for neurons to defend Aβ-induced toxicity. However, there is limited research carried out to investigate the effect of MIF in Aβ-induced toxicity in neurons, except for one study demonstrating a protective effect of a MIF inhibitor on Aβ-induced toxicity in SH-SY5Y cells [
26]. In contrast to their results, we identified that Aβ stimulated the secretion of MIF from SH-SY5Y cells and increased secretion of MIF significantly reduced neuronal cell death induced by Aβ. It has been shown that inhibition of p53 attenuates Aβ-induced neuronal apoptosis [
34], and MIF can directly inhibit p53 activation [
35]. Thus, secretion of MIF precedes cell damage, and MIF is released under the autocrine fashion, in turn, activates cell survival signals.
It should be noted that MIF acts in an autocrine fashion and interacts with its cell surface receptors to transduce its signals [
36,
37]; therefore, secretion of MIF to the extracellular space is necessary for MIF to exert its cellular functions. It has been suggested that MIF is constitutively expressed and is perhaps secreted constantly by neurons in the brain [
38,
39]. However, at the late stage of AD, a large portion of the extracellular MIF is sequestered by Aβ plaques, as we demonstrated, and perhaps has no functions anymore, which suggests that increased MIF secretion is essential for maintaining cognitive function during AD development. This also explains why MIF upregulation is observed at the late stage of AD with significantly increased Aβ deposition.
We found that neurons specifically secreted MIF following Aβ stimulation. However, we did not rule out the possibility that upregulation of MIF is also contributed by microglia (local or infiltrated) at the late stage of AD, at which pro-inflammatory cytokines are predominantly produced [
14], and they are known to trigger secretion of MIF [
25]. Indeed, recent studies demonstrated a mixture of hyperactivated microglia at the late stage of AD pathology [
40] and the upregulation of MIF in hyperactivated microglia [
41]. We speculated that at the late stage of AD pathology, overproduced MIF could still be beneficial in promoting neuronal survival if it can be received by neurons; on the other hand, however, the pool of MIF produced by immune cells may locally promote their own survival and proliferation, which in turn produce and release more MIF, leading to a vicious circle as seen in chronic inflammatory diseases in the peripheral.
Although it is debatable whether Aβ deposits serve as a cause of cognitive decline during AD, inhibition of Aβ production and plaque formation have been shown to successfully mitigate cognitive deficits in AD model mice [
17,
18,
42,
43]. Since MIF insufficiency had an impact on cognitive performance [
44], we hypothesized that Aβ-triggered MIF secretion could serve as a compensatory mechanism to improve cognitive performance during AD. In contrast, inhibition of MIF has been suggested for treating peripheral inflammatory diseases [
13,
26], indicating the possibility that the potential pro-survival and pro-inflammatory functions of MIF counteract each other during AD, and inhibition of one would tip a balance to the other. It is possible that at the early stage with little Aβ plaques formation, MIF secretion triggered by Aβ oligomer is pro-survival for neurons to maintain cognitive function; while at the late stage with abundant Aβ plaques, the compensatory function may fail to demonstrate effects due to the direct binding between MIF and Aβ, leaving behind the pro-inflammatory effects. Therefore, it will be of importance to dissect the MIF signal complex so that the deleterious effects could be inhibited without affecting the beneficial ones.
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