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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Neuroophthalmol. Author manuscript; available in PMC 2012 March 26.
Published in final edited form as:
PMCID: PMC3312784
NIHMSID: NIHMS364165

Oral Resveratrol Reduces Neuronal Damage in a Model of Multiple Sclerosis

Abstract

Background

Neuronal loss in multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), correlates with permanent neurological dysfunction. Current MS therapies have limited ability to prevent neuronal damage.

Methods

We examined whether oral therapy with SRT501, a pharmaceutical-grade formulation of resveratrol, reduces neuronal loss during relapsing/remitting EAE. Resveratrol activates SIRT1, an NAD+-dependent deacetylase that promotes mitochondrial function.

Results

Oral SRT501 prevented neuronal loss during optic neuritis, an inflammatory optic nerve lesion in MS and EAE. SRT501 also suppressed neurological dysfunction during EAE remission, and spinal cords from SRT501-treated mice had significantly higher axonal density than vehicle-treated mice. Similar neuroprotection was mediated by SRT1720, another SIRT1-activating compound; and sirtinol, a SIRT1 inhibitor, attenuated SRT501 neuroprotective effects. SIRT1 activators did not prevent inflammation.

Conclusions

These studies demonstrate SRT501 attenuates neuronal damage and neurological dysfunction in EAE by a mechanism involving SIRT1 activation. SIRT1 activators are a potential oral therapy in MS.

Keywords: Multiple sclerosis, EAE, resveratrol, SIRT1, neuroprotection

Introduction

MS is a central nervous system demyelinating disease (1) characterized by relapsing/remitting neurological dysfunction associated with acute episodes of inflammation. Significant axonal damage and loss of neurons also occurs in MS, and correlates with permanent neurological disability (25). Current MS therapies reduce acute inflammatory episodes by immunomodulation, but have limited effects in preventing permanent neurological deficits, with some patients exhibiting progressive disability despite treatment with first line therapies (69). Novel therapies that better prevent neuronal damage are needed to prevent long-term disability in MS patients.

Optic neuritis, an inflammatory demyelinating disease of the optic nerve, often occurs as an acute episode of MS. Similar to other MS lesions, significant neuronal damage, with loss of retinal ganglion cell (RGC) axons, occurs following optic neuritis and correlates with decreased vision (5,1013). Therapies that prevent permanent neuronal damage during optic neuritis therefore have potential to prevent long-term visual loss, and may have significant neuroprotective effects for other MS lesions.

EAE is an animal model sharing some clinical, immunological, and histopathological features of MS (14), including demyelinating lesions in spinal cord as well as optic nerve (15,16). SJL/J mice immunized with proteolipid protein develop relapsing/remitting EAE, with significant RGC loss beginning several days after acute optic neuritis develops (17). In previous studies, a pharmaceutical-grade formulation of resveratrol (SRT501) attenuated RGC loss during experimental optic neuritis when given by intravitreal injection (18). Resveratrol activates SIRT1, a member of a conserved gene family (sirtuins) encoding NAD+-dependent deacetylases (1921). Intravitreal SRT501 treatment is limited by the frequency with which injections can be given without inducing significant trauma and intraocular inflammation. In addition, intravitreal SRT501 had no effects on tail or limb paralysis (18), measures of neurological dysfunction from EAE spinal cord lesions. Thus, systemic treatment with SIRT1 activators needs to be examined to determine their potential role as a neuroprotective therapy in MS.

SIRT1 activators in mice are well tolerated systemically at high oral doses, including 1000 mg/kg SRT501 and 100 mg/kg of a more potent SIRT1 activator, SRT1720 (22). SRT501 also has a favorable safety profile in humans and is currently in Phase 2 clinical trials for diabetes. In the current studies, potential neuroprotective effects of SRT501 and SRT1720 are examined in EAE mice. Because SIRT1 activators are well-tolerated with a favorable safety profile in humans, effects seen in EAE may rapidly translate into in clinical trials for optic neuritis and MS.

Methods

Mice

Six week old female SJL/J mice were purchased from the Jackson Laboratory (Bar Harbor, ME). Treatment of animals conformed to Institutional Animal Care and Use Committee guidelines and review.

RGC labeling

Retrograde labeling of RGCs was performed as reported (17). Mice were anesthetized with 0.2 ml solution containing 10 mg/ml ketamine (Sigma, St. Louis, MO) and 1 mg/ml xylazine (Sigma) i.p. Heads were shaved, a mediosagittal incision made, and holes drilled through the skull above each superior colliculus. 2.5 μl of 1.25% hydroxystilbamidine (Fluorogold, Invitrogen, Carlsbad, CA) in PBS was injected stereotactically into each superior colliculus. Fluorogold taken up by axon terminals is retrogradely transported to RGC bodies in the retina.

Induction and evaluation of EAE

Mice were anesthetized with ketamine/xylazine and injected subcutaneously at two sites on the back with 0.1 ml solution containing 0.5 mg/ml proteolipid protein peptide 139–151 (PLP; Protein Chemistry Laboratory, University of Pennsylvania, Philadelphia, PA) emulsified in complete Freund's adjuvant (CFA; Difco, Detroit, MI) containing 2.5 mg/ml mycobacterium tuberculosis (Difco). Control mice were injected with an equal volume of phosphate buffered saline (PBS) and CFA. EAE and control mice were injected i.p. with 200 ng pertussis toxin (List Biological, Campbell, CA) in 0.1 ml PBS on day 0 and day 2. Clinical EAE was scored daily using a 5 point scale (23): no disease=0; partial tail paralysis=0.5; tail paralysis or waddling gait=1.0; partial tail paralysis and waddling gait=1.5; tail paralysis and waddling gait=2.0; partial limb paralysis=2.5; paralysis of one limb=3.0; paralysis of one limb and partial paralysis of another=3.5; paralysis of two limbs=4.0; moribund state=4.5; death=5.0.

SIRT1 activator treatment and bioavailability

SRT501 (Sirtris Pharmaceuticals, Cambridge, MA) was diluted in 2% hydroxypropyl methylcellulose (Shin-Etsu Chemicals, Japan) and 0.2% dioctyl sodium sulfosuccinate (Wilson Laboratories, Mumbai, India); SRT1720 was dissolved in 40% polyethanol glycol 400 and 0.5% tween80 (AG Scientific, San Diego, CA). Mice were treated by oral gavage once daily at indicated doses and time points. To determine the intravitreal bioavailability of orally administered SRT501, mice were anesthetized 1 hr after the final dose. A 30-gauge needle was used to enter the vitreal cavity. Using a Hamilton syringe, a 1 μl sample of vitreous was removed and diluted 1:50 in PBS. Samples were sent for mass spectrometry analysis (Charles River Laboratories, Worcester, MA) to measure SRT501 concentration. For SIRT1 inhibitor studies, sirtinol (Sigma) was injected into the vitreous as described previously (18).

Quantification of RGC numbers

Following sacrifice, eyes were removed, fixed in 4% paraformaldehyde in PBS, and isolated retinas were viewed by fluorescent microscopy. Photographs were taken at 20× magnification in 12 standard fields: 1/6, 3/6, and 5/6 of the retinal radius from the center of the retina in each retinal quadrant, and the number of RGCs was counted using Image-Pro Plus 6.0 (Media Cybernetics, Silver Spring, MD) software as in prior studies (17).

Histopathological evaluation of optic nerves

After sacrifice, optic nerves were removed and fixed in 4% paraformaldehyde in PBS. Processed nerves were embedded in paraffin, cut into 5 μm longitudinal sections, and stained with H&E. Presence of inflammatory cell infiltration was assessed by a grading scale used previously (18,24): No infiltration=0; mild cellular infiltration of optic nerve or optic nerve sheath=1; moderate infiltration=2; severe infiltration=3; massive infiltration=4. Nerves with any detectable inflammation (score 1–4) were considered to have optic neuritis. To assess axonal damage, optic nerve sections were stained by Bielschowsky silver impregnation and axonal density was quantified as in prior studies (25). Briefly, optic nerves were photographed at 40× magnification (one photograph each of the proximal, central, and distal nerve) and the area of silver staining was calculated using ImagePro Plus 6.0 software. Total area examined in three fields/nerve was 38,500 μm2, and data represent the cumulative area of positive staining/nerve.

Spinal cord preparation and histopathology

Mice were transcardially perfused with PBS followed by 4% paraformaldehyde. Spinal cords were removed, post-fixed in 4% paraformaldehyde, embedded in paraffin and cut in 5 μm thick sections. Analysis of inflammatory cell infiltration and axonal area were performed similar to previously described methods (23,26). Sections were stained with H&E, luxol fast blue (LFB), or Bielschowsky silver impregnation and examined by light microscopy. To quantify axonal density, photographs were taken at 40× magnification in 4 standardized fields per section, representing a total area of 50,000 μm2. Axonal density is calculated as the cumulative area of positive silver staining/spinal cord using Image-Pro Plus 6.0.

Inflammatory cell isolation, culture, and flow cytometric analysis

Cells were isolated from EAE spinal cords by methods described previously (27). Briefly, following transcardial perfusion with PBS, spinal cords were removed, pooled, mechanically dissociated through a 100-μm strainer and washed with PBS. The resultant pellet was fractionated on a 60/30% Percoll gradient by centrifugation at 300 × g for 20 min. Microglia and infiltrating mononuclear cells were harvested from the interface, washed, counted, and stained with antibodies for flow cytometry, or alternatively for studies of cytokine expression cells were cultured for 3 hr in RPMI 1640 containing 10% FCS, penicillin/streptomycin, nonessential amino acids, L-glutamine, vitamins and 2-ME and stimulated with phorbol 12-myristate 13-acetate (50 ng/ml), ionomycin (500 ng/ml; Sigma) and treated with GolgiPlug (1 μg per 1 106 cells; BD Pharmingen). Cells were stained for 20 min in the dark at 4 °C with fluorescence-labeled antibodies specific for cell surface markers. For intracellular markers, cells were washed, fixed and permeabilized with Fix & Perm reagents (Caltag Laboratories) then stained with fluorescence-labeled antibodies. All antibodies were used at a concentration of 0.5 ug/ml. All were purchased from BD Bioscience and the details are as follows: (CD45; clone 104), (CD4; clone RM4-5), (CD8; clone 53–6.7), (CD11b; clone m1/70), (IL-17; clone TC11-18H10), (IFN-gamma; clone XMG1.2). Data were acquired on a FACSAria (BD Biosciences) and analyzed with FlowJo software (Treestar). Mononuclear cells (infiltrating immune cells and resident microglia) were gated based on physical parameters (size and granularity).

Statistics

Comparisons of RGC numbers and axonal density were analyzed by one way ANOVA followed by Tukey's Multiple Comparison test using GraphPad Prism 5.0 (GraphPad Software, San Diego, CA). Data represent mean ± SEM number of RGCs or cumulative area of positive axonal staining. Clinical EAE scores were compared between treatment groups by ANOVA for repeated measures using GraphPad Prism 5.0, and the probability of full recovery versus partial recovery during EAE remission between treatment groups was compared by Fisher's exact test. The probability of developing optic neuritis (any positive optic nerve inflammation score, 1–4) versus no optic nerve inflammation was compared between SRT501- and vehicle-treated mice by Fisher's exact test. Similar comparison of the relative degree of optic nerve inflammation was performed between nerves with mild inflammation (score 1) and those with moderate – severe inflammation (scores 2–4). Overall rank distribution of optic nerve inflammation scores were compared by Wilcoxon rank-sum test.

Results

Oral SRT501 penetrates the eye

Intravitreal SRT501 prevents RGC loss from optic neuritis (18), but it is not known whether systemic SRT501 crosses the blood-retina barrier. To examine this, EAE and control mice were treated by oral gavage with SRT501 or placebo (vehicle) beginning on day 8 post-immunization and repeated daily until sacrifice on day 14. At sacrifice, 1 μl vitreal samples were taken (one hour after the final dose), diluted 1:50 in PBS, and SRT501 concentration was measured. No SRT501 was detected in EAE eyes treated with vehicle only (n=3 eyes). SRT501 (2.5±1.3 ng/ml) was detected in EAE eyes treated with 500 mg/kg SRT501 daily (n=5), and intravitreal SRT501 concentration increased with a higher oral dose of 1000 mg/kg SRT501 in both EAE (9.7±5.0 ng/ml; n=6) and control (10.3±5.1 ng/ml; n=3) eyes.

Oral SRT501 treatment prior to onset of optic neuritis attenuates neuronal damage without reducing inflammation

To determine whether oral administration of SRT501 can prevent RGC damage, EAE and control mice were treated with 500 or 1000 mg/kg SRT501, or vehicle, beginning on day 8 post-immunization (one day prior to onset of optic neuritis) and repeated daily until sacrifice on day 14. Optic neuritis was detected in eyes from EAE mice treated with either dose of SRT501, with a similar incidence to vehicle-treated EAE mice (Fig. 1A–C,G). The probability of developing optic neuritis did not differ between eyes of vehicle-treated mice (15 of 26) and 500 (30 of 49; p = 0.8079) or 1000 (11 of 19; p = 1.000) mg/kg SRT501-treated mice. The degree of inflammation, scored on a 4-point scale, also did not differ between 500 mg/kg SRT501-treated (mean±SD score 1.43±0.57, n=30), 1000 mg/kg SRT501-treated (1.54±0.82, n=11) and vehicle-treated (1.33±0.49, n=15) EAE eyes. 10 of the 15 eyes from vehicle-treated mice that had optic neuritis had only mild inflammation (score 1). This was no different from the proportion of eyes from 500 (18 of 30; p = 0.7521) or 1000 (4 of 11; p = 1.000) mg/kg SRT501-treated mice that had mild inflammation. Comparison of the distribution of relative inflammation scores by the Wilcoxon rank sum-test also showed no significant difference between vehicle-treated eyes and those treated with 500 (p = 0.5637) or 1000 (p = 0.4173) mg/kg SRT501.

Fig. 1
Oral SRT501 reduces axonal damage during optic neuritis without suppressing inflammation. A. A longitudinal section from a control optic nerve stained by H&E shows normal histology. B. A representative optic nerve from an EAE mouse 14 days post-immunization ...

Similar to previous studies (25), optic neuritis was associated with RGC axonal damage, detected by decreased axonal silver staining with truncated axons and terminal axonal ovoids (Fig. 1E). In contrast, optic neuritis nerves from SRT501-treated EAE mice had normal RGC axonal staining (Fig. 1F), comparable to controls (Fig. 1D). Eyes with optic neuritis from EAE mice treated with 500 mg/kg SRT501 showed a trend toward increased axonal density compared to vehicle-treated mice, and at higher doses of 1000 mg/kg, axonal density was significantly higher (Fig. 1H).

RGC numbers in optic neuritis eyes (445.7±87.0 RGCs) were significantly lower than in eyes from control mice treated with 500 mg/kg SRT501 (788.2±83.9) or 1000 mg/kg SRT501 (703.0±42.6; *p<0.05; Fig. 2). Optic neuritis eyes from 500 mg/kg SRT501-treated EAE mice showed a trend toward increased RGC numbers (595.9±54.5), while optic neuritis eyes from 1000 mg/kg SRT501-treated EAE mice had significantly more surviving RGCs (695.0±38.3) than vehicle-treated optic neuritis eyes (**p<0.05). Together, results suggest oral SRT501 can attenuate RGC axonal damage and cell death during acute optic neuritis, and these effects are not mediated by suppressing inflammation.

Fig. 2
Oral SRT501 treatment from d8–14 post-immunization prevents RGC loss in EAE optic neuritis eyes. A. Numerous fluorescently-labeled RGCs from a representative field in a control retina. B. Fewer RGCs are seen in a corresponding area of retina in ...

Oral SRT501 treatment after onset of optic neuritis attenuates neuronal damage

To determine whether SIRT1 activators prevent RGC loss after onset of optic nerve inflammation, correlating to when patients may present clinically, mice were treated with vehicle or 1000 mg/kg SRT501 beginning on day 10 post-immunization and repeated daily until day 14. Prior studies have shown that inflammation begins by day 9, but RGC axonal loss is not detected until day 13 (25). On day 14, RGC numbers in optic neuritis eyes (477.5±66.6) were significantly lower than in eyes from control mice treated with vehicle (802.5±130.0) or SRT501 (739.8±110.5; *p<0.01; Fig. 3A). Optic neuritis eyes from SRT501-treated EAE mice had significantly more RGCs (821.0±136.4) than vehicle-treated optic neuritis eyes (**p<0.01). To examine whether SRT501 prevents RGC loss longer-term, as opposed to merely delaying RGC loss, RGC survival was examined one month after EAE induction. Mice were treated with vehicle or 1000 mg/kg SRT501 from day 10–14 post-immunization (during the first acute optic neuritis episode), then sacrificed at day 30. RGC numbers in vehicle-treated EAE eyes (394.8±52.3) were significantly lower than in eyes from control mice treated with vehicle (608.2±68.5) or SRT501 (624.1±63.5; *p<0.01; Fig. 3B). Eyes from SRT501-treated EAE mice had significantly more RGCs (748.0±44.3) than vehicle-treated EAE eyes (**p<0.001). Correlating with RGC numbers, axonal density in optic nerves at day 14 (Fig. 3C) and day 30 (Fig. 3D) post-immunization demonstrated that SRT501 treatment prevented the loss of axons found in EAE optic neuritis eyes. Thus, SRT501 administered at a clinically-relevant time provides acute neuroprotection with lasting effects after inflammation resolves.

Fig. 3
Oral SRT501 treatment beginning after onset of optic neuritis, from d10–14 post-immunization, prevents acute axon and RGC loss at day 14 and maintains these neuroprotective effects at day 30. A. The significant loss of RGCs detected in EAE optic ...

SRT501 reduces residual neurological dysfunction during EAE remission

Effects of SRT501 treatment on clinical EAE were evaluated. No difference in onset of acute EAE through day 14 post-immunization was observed between vehicle-treated EAE mice and EAE mice treated with 500 or 1000 mg/kg SRT501 (Fig. 4A) from days 8–14. In EAE mice treated from days 10–14 with vehicle or 1000 mg/kg SRT501, there also was no difference in clinical EAE score between vehicle- or SRT501-treated mice through the peak of symptoms during the first clinical episode (Fig. 4B). Following the peak at day 14, both vehicle- and SRT501-treated mice showed remission of EAE as seen previously in this relapsing/remitting disease. SRT501-treated mice had improved recovery, with a significant (p<0.05) decrease in EAE score compared with vehicle-treated mice. In three experiments, the combined probability of complete remission was significantly higher (21 of 24 mice) in SRT501-treated EAE mice than in vehicle-treated (6 of 19) mice (p=0.0003).

Fig. 4
SRT501 suppresses EAE remission but not acute exacerbations. A. No difference was found between the degree of EAE in mice treated with vehicle (n = 19), 500 mg/kg SRT501 (n = 30), or 1000 mg/kg SRT501 (n = 13) given by oral gavage daily from d8–14 ...

SRT501 attenuates axonal loss in EAE spinal cords without suppressing inflammation or demyelination

Development of EAE in SRT501-treated mice suggests acute spinal cord inflammation may not be suppressed, whereas improved recovery during remission indicates SRT501 likely prevents permanent neuronal damage following acute inflammation. To examine this further, control and EAE mice treated with vehicle or SRT501 were perfused at day 14 or day 30, and spinal cord cross-sections were evaluated for inflammation and axonal loss. 14 days post-immunization, focal areas of inflammatory cell infiltration were observed at multiple spinal cord levels in EAE, but not control, mice (Fig. 5A–E). The incidence of inflammation (Fig. 5F) and demyelination (Fig. 5G) were similar between EAE mice treated with vehicle, 500 mg/kg SRT501 or 1000 mg/kg SRT501. Silver staining showed no difference in axonal density between vehicle- or SRT501-treated mice at day 14 (data not shown). However, 30 days post-immunization, focal areas of axonal loss were observed in vehicle-treated spinal cords (Fig. 6B,E) but not in controls (Fig. 6A,D) or SRT501-treated mice (Fig. 6C,F). Significantly decreased axonal density at day 30 was measured in vehicle-treated, but not SRT501-treated, EAE spinal cords (Fig. 6G).

Fig. 5
SRT501 does not reduce spinal cord inflammation or demyelination at day 14 post-immunization. A. An H&E stained cross-section through the spinal cord of a control mouse demonstrates normal histology at 4× original magnification. B. Focal ...
Fig. 6
SRT501 treatment during acute inflammation on d10–14 prevents loss of axons in EAE spinal cord 30 days post-immunization. A. Bielschowsky silver stained section of control spinal cord shows normal axonal staining. Higher magnification of boxed ...

SRT501 treatment during acute EAE does not modulate the immune response in spinal cord

While gross levels of inflammation were not reduced by SRT501 treatment during active EAE, it is possible that SRT501 may alter the phenotype of the inflammatory cells present in the CNS. To examine whether SRT501 affects the type of cells found in spinal cord infiltrates, EAE mice were treated daily from day 10 through day 14 post-immunization with vehicle or 1000 mg/kg SRT501. Mice were perfused on day 14, at the peak of EAE, and microglia and infiltrating monocytes were isolated from spinal cords pooled from 6 mice/group. Freshly isolated cells were labeled with antibodies to cell surface markers and analyzed by flow cytometry to determine the composition of inflammatory cells present. Additional cells isolated from SRT501- and vehicle-treated mice were cultured for three hours, then viable cells were permeabilized and labeled with antibodies to cytokines IL-17 and interferon-gamma (IFNg). T cells comprised less than one quarter of all isolated cells, with no significant difference in the percentage of CD4+ or CD8+ T cells between vehicle-treated and SRT501-treated EAE mice. The majority (two-thirds) of CNS cells were CD11b+ macrophages/microglia, with no significant difference between vehicle-treated and SRT501-treated EAE mice. Similarly, following three hours in culture, no significant differences in expression of the Th1 cytokine IFNg or the Th17 cytokine IL-17 were found between cells from vehicle-treated and SRT501-treated EAE mice. Interestingly, there was a non-significant trend toward increased numbers of inflammatory cells that were isolated from each SRT501-treated mouse spinal cord as compared to vehicle-treated EAE mice, with no significant difference in the average number of CD4+, CD8+, CD11b+, CD45hi+, IFNg+, or IL-17+ cells per mouse with or without SRT501 treatment.

Mechanism of SRT501 neuroprotection in EAE is mediated by SIRT1 activation

Resveratrol exerts effects by several mechanisms (28), including activation of SIRT1. To examine whether the mechanism of SRT501-mediated neuroprotection in EAE involves SIRT1 activation, EAE mice were treated with SRT1720, another SIRT1 activator that activates SIRT1 by a distinct mechanism and with higher potency than SRT501 (22). Similar to SRT501, SRT1720 did not prevent spinal cord or optic nerve inflammation, but did attenuate neuronal damage. Mice were treated daily from day 10–14 post-immunization with vehicle or 100 mg/kg SRT1720, then sacrificed on days 14 or 30. No difference in EAE score occurred during acute disease, but SRT1720-treated mice had significantly better recovery during remission than vehicle-treated mice (Fig. 7A), with increased probability of complete remission (p=0.024) and preserved spinal cord axons (Fig. 7E). While optic neuritis incidence did not differ between vehicle- and SRT1720-treated EAE mice (Fig. 7B), SRT1720 did reduce optic nerve axonal loss and RGC loss at day 14 (data not shown) and day 30 (Fig. 7C,D).

Fig. 7
SRT1720 treatment exhibits similar neuroprotective effects in EAE as SRT501, without reduction of inflammation. A. No difference in development of acute EAE was detected between vehicle-treated (n = 5) and 100 mg/kg SRT1720-treated (n = 5) EAE mice treated ...

The ability of two distinct SIRT1 activators to prevent neurological dysfunction and neuronal damage in EAE suggests the mechanism of these neuroprotective effects involves SIRT1 activation. To further confirm this, EAE mice treated with 1000 mg/kg SRT501 on days 10–14 were given an intravitreal injection of sirtinol (100 μM), a SIRT1 inhibitor (18). Sirtinol administration significantly reduced the ability of SRT501 to prevent RGC loss 14 days post-immunization (Fig. 8).

Fig. 8
The SIRT1 inhibitor sirtinol reduces neuroprotective effects of SRT501 on RGCs during optic neuritis. 1000 mg/kg SRT501 administered daily from d10–14 in EAE mice results in significantly higher RGC numbers compared to optic neuritis eyes from ...

Discussion

The current studies show that orally administered SRT501 attenuates neuronal damage in EAE optic nerves and spinal cords. Observed neuroprotective effects on RGCs are similar to effects seen previously with intravitreal administration (18), but in those studies no effect on EAE or spinal cord pathology occurred. SRT501 is well tolerated in the eye (18), as well as systemically (22). Based on the current results and its favorable safety profile, SRT501 is an important potential oral therapy to prevent permanent neurological disability in MS.

MS involves a complex autoimmune inflammatory response to components of CNS myelin (1), and neuronal damage, including RGC loss during optic neuritis in experimental models, can occur secondary to inflammation (17). The mechanism of neuroprotection by SRT501 in EAE, however, is not mediated by reducing inflammation, as incidence of optic nerve and spinal cord inflammation was not reduced by SRT501 treatment. SRT501 also did not alter the phenotype of inflammatory cell infiltrates in the spinal cord at the peak of the acute EAE attack. In addition, EAE scores during acute disease episodes, signs that reflect active spinal cord inflammation, were not reduced by SRT501 treatment. EAE was suppressed during disease remission, with preserved axonal density, suggesting SRT501 may be able to prevent permanent neurological dysfunction and neuronal damage in MS after acute spinal cord inflammation resolves. While treatment during the first clinical EAE episode, day 10–14 post-immunization, led to improved functional recovery during remission, this treatment course did not prevent the onset of EAE relapse at day 30. Further studies with extended treatment will be important to examine longer-term effects of SRT501.

Results suggest the mechanism of SRT501 neuroprotection in EAE involves SIRT1 activation. SRT501 activates SIRT1 in vitro (22). Administration of the SIRT1 inhibitor sirtinol blocks SRT501-mediated RGC neuroprotection, and the fact that a second distinct SIRT1 activator, SRT1720, induced similar neuroprotective effects further suggests SIRT1 activation is a requisite mechanism for the observed neuroprotection. Precise molecular pathways driven by SIRT1 activity leading to neuroprotection require further examination.

Interestingly, while the current studies showed no suppression of EAE inflammation by SRT501, resveratrol has been shown to partially suppress EAE and modulate inflammation in two other studies (29,30), although there are important differences between those studies and the current studies. Singh and colleagues (29) used a different EAE model with a chronic disease course, and in their studies resveratrol treatment was initiated right after immunization, long before clinical EAE develops, suggesting that it may affect development of the disease model as opposed to treating active disease. In addition, mechanisms of neuronal loss in chronic EAE can differ from relapsing/remitting EAE, as neuronal loss occurs several days after immunization and prior to onset of inflammation (31), as opposed to neuronal loss only beginning after inflammation develops in the relapsing/remitting model (17). Thus, both the mechanism of neuronal loss and the timing of treatment are likely important factors for resveratrol neuroprotective effects. In the studies by Imler and Petro (30), resveratrol treatment was also initiated prior to disease onset, at the time of immunization, and while continuous daily treatment did result in long-term EAE suppression through multiple relapses, even in their studies resveratrol effects were limited during the first episode of EAE. Together, these studies suggest resveratrol may modulate immune responses in EAE during the induction phase of disease, and it is possible that SRT501 may alter immune responses at other time points along the EAE disease course, but our results show that once the immune response has been induced and clinical disease developed, SRT501 no longer suppresses inflammation but does retain significant ability to prevent neuronal damage. As the timing of SRT501 treatment may play a role in its ability to prevent neuronal injury, it is important to note that neuroprotective effects were seen when treatment was initiated after onset of inflammation, but prior to optic nerve axonal loss (25). If a similar window of time can be indentified in optic neuritis patients, then there would be a potential opportunity to initiate neuroprotective therapies.

Overall, our results demonstrate a significant neuroprotective effect of SIRT1 activators in EAE mice, reducing neurological dysfunction during EAE remission and preventing axonal damage and neuronal loss. SIRT1 activators have a potential therapeutic role in preventing permanent visual loss following optic neuritis and permanent neurological dysfunction in MS patients. SRT501 itself has shown a favorable safety profile without retinal toxicity in EAE and is also safe in humans. Therefore, our findings likely can be translated into clinical trials to assess potential neuroprotection in optic neuritis and MS patients. Because SIRT1 activation does not prevent inflammation when treatment is initiated after disease onset, SIRT1 activators have potential to work synergistically with current immunomodulatory therapies.

Acknowledgments

The authors thank Elsa Aglow for expert technical assistance.

Acknowledgment of funding sources

This work was supported by National Institutes of Health grants EY015098 and EY019014 (KSS), grant RG 4214-A-1 from the National Multiple Sclerosis Society (KSS), a Career Development Award from Research to Prevent Blindness (KSS), grants from Sirtris Pharmaceuticals (AMR), and the F. M. Kirby Foundation (KSS).

Footnotes

Conflict of interest statement: The SRT501 and SRT1720 compounds used in these studies are made by Sirtris Pharmaceuticals, a GSK company, and were provided without charge. Peter Elliot was employed full time by Sirtris during these studies, and a portion of the funding for experiments was provided by Sirtris.

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