Journal of Neuro-Ophthalmology:
Basic Science in Neuro-Ophthalmology
Oral Resveratrol Reduces Neuronal Damage in a Model of Multiple Sclerosis
Shindler, Kenneth S MD, PhD; Ventura, Elvira MS; Dutt, Mahasweta MS; Elliott, Peter PhD; Fitzgerald, Denise C PhD; Rostami, Abdolmohamad MD, PhD
Section Editor(s): Bennett, Jeffery L MD, PhD; Gordon, Lynn K MD, PhD
F.M. Kirby Center for Molecular Ophthalmology, Department of Ophthalmology (KSS, MD), Scheie Eye Institute, University of Pennsylvania, Philadelphia, Pennsylvania; Department of Neurology (EV, DCF, AR), Thomas Jefferson University, Philadelphia, Pennsylvania; and Sirtris (PE), a GSK Company, Cambridge, Massachusetts.
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 (AR), and the F.M. Kirby Foundation (KSS).
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.
Address correspondence to Dr Kenneth S. Shindler, MD, PhD, F.M. Kirby Center for Molecular Ophthalmology, Department of Ophthalmology, Scheie Eye Institute, University of Pennsylvania, 51 North 39th Street, Philadelphia, PA 19104; E-mail: email@example.com
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 the 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, an SIRT1 inhibitor, attenuated SRT501 neuroprotective effects. SIRT1 activators did not prevent inflammation.
Conclusions: These studies demonstrate that SRT501 attenuates neuronal damage and neurological dysfunction in EAE by a mechanism involving SIRT1 activation. SIRT1 activators are a potential oral therapy in MS.
Multiple sclerosis (MS) is a central nervous system (CNS) demyelinating disease (1) characterized by relapsing-remitting neurological dysfunction associated with acute episodes of inflammation. Significant axonal damage and loss of neurons also occur in MS and correlate with permanent neurological disability (2-5). 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 (6-9). Novel therapies that better prevent neuronal damage are needed to prevent long-term disability in patients with MS.
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 (5,10-13). 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 (19-21). 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 1,000 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 and have a favorable safety profile in humans, effects seen in EAE may rapidly translate into clinical trials for optic neuritis and MS.
Six-week-old female SJL/J mice were purchased from the Jackson Laboratory (Bar Harbor, ME). Treatment of animals conformed to the Institutional Animal Care and Use Committee guidelines and review.
Retrograde labeling of RGCs was performed as reported (17). Mice were anesthetized with 0.2 mL of solution containing 10 mg/mL ketamine (Sigma, St. Louis, MO) and 1 mg/mL xylazine (Sigma) intraperitoneally. Heads were shaved, a mediosagittal incision was made, and holes were drilled through the skull above each superior colliculus. 2.5 μL of 1.25% hydroxystilbamidine (Fluorogold; Invitrogen, Carlsbad, CA) in phosphate-buffered saline (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 2 sites on the back with 0.1 mL of solution containing 0.5 mg/mL proteolipid protein peptide 139-151 (PLP; Protein Chemistry Laboratory, University of Pennsylvania, Philadelphia, PA) emulsified in complete Freund adjuvant (CFA; Difco, Detroit, MI) containing 2.5 mg/mL Mycobacterium tuberculosis (Difco). Control mice were injected with an equal volume of PBS and CFA. EAE and control mice were injected intraperitoneally with 200 ng pertussis toxin (List Biological, Campbell, CA) in 0.1 mL of 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 1 limb = 3.0; paralysis of 1 limb and partial paralysis of another = 3.5; paralysis of 2 limbs = 4.0; moribund state = 4.5; and death = 5.0.
SIRT1 Activator Treatment and Bioavailability
SRT501 (Sirtris Pharmaceuticals, Cambridge, MA) was diluted in 2% hydroxypropyl methylcellulose (Shin-Etsu Chemicals, Tokyo, Japan) and 0.2% dioctyl sodium sulfosuccinate (Wilson Laboratories, Mumbai, India); SRT1720 was dissolved in 40% polyethanol glycol 400 and 0.5% Tween 80 (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 hour after the final dose. A 30-gauge needle was used to enter the vitreal cavity. Using a Hamilton syringe, 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 hematoxylin and eosin (HE). 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; and 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 (1 photograph each of the proximal, central, and distal nerves), and the area of silver staining was calculated using ImagePro Plus 6.0 software. Total area examined in 3 fields per nerve was 38,500 μm2, and data represent the cumulative area of positive staining per nerve.
Spinal Cord Preparation and Histopathology
Mice were transcardially perfused with PBS followed by 4% paraformaldehyde. Spinal cords were removed, postfixed in 4% paraformaldehyde, embedded in paraffin, and cut in 5-μm-thick sections. Analysis of inflammatory cell infiltration and axonal area was performed similar to previously described methods (23,26). Sections were stained with HE, luxol fast blue, 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 300g for 20 minutes. 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 hours in RPMI 1640 containing 10% fetal calf serum, penicillin/streptomycin, nonessential amino acids, l-glutamine, vitamins, and 2-mercaptoethanol and stimulated with phorbol 12-myristate 13-acetate (50 ng/mL) and ionomycin (500 ng/mL; Sigma) and treated with GolgiPlug (1 μg per 1 × 106 cells; BD Pharmingen). Cells were stained for 20 minutes 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 μg/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), (interleukin [IL]-17; clone TC11-18H10), and (interferon [IFN]-γ; 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).
Comparisons of RGC numbers and axonal density were analyzed by 1-way analysis of variance (ANOVA) followed by Tukey multiple comparison test using GraphPad Prism 5.0 (GraphPad Software, San Diego, CA). Data represent mean ± standard error of the mean 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 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 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 (score, 2-4). The overall rank distribution of optic nerve inflammation scores was compared by Wilcoxon rank sum test.
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 postimmunization and repeated daily until sacrifice on day 14. At sacrifice, 1 μL of vitreal samples were taken (1 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 1,000 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 1,000 mg/kg SRT501, or vehicle, beginning on day 8 postimmunization (1 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 1,000 (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 among 500 mg/kg SRT501-treated (mean ± SD score 1.43 ± 0.57, n = 30), 1,000 mg/kg SRT501-treated (1.54 ± 0.82, n = 11), and vehicle-treated (1.33 ± 0.49, n = 15) EAE eyes. Ten 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 1,000 (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 1,000 (P = 0.4173) mg/kg SRT501.
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 1,000 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 1,000 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 1,000 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 that oral SRT501 can attenuate RGC axonal damage and cell death during acute optic neuritis, and these effects are not mediated by suppressing inflammation.
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 1,000 mg/kg SRT501 beginning on day 10 postimmunization 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 1 month after EAE induction. Mice were treated with vehicle or 1,000 mg/kg SRT501 from day 10-14 postimmunization (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) postimmunization 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.
SRT501 Reduces Residual Neurological Dysfunction During EAE Remission
Effects of SRT501 treatment on clinical EAE were evaluated. No difference in the onset of acute EAE through day 14 postimmunization was observed between vehicle-treated EAE mice and EAE mice treated with 500 or 1,000 mg/kg SRT501 (Fig. 4A) from days 8 to 14. In EAE mice treated from days 10 to 14 with vehicle or 1,000 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 3 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).
SRT501 Attenuates Axonal Loss in EAE Spinal Cords Without Suppressing Inflammation or Demyelination
Development of EAE in SRT501-treated mice suggests that acute spinal cord inflammation may not be suppressed, whereas improved recovery during remission indicates that 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. Fourteen days postimmunization, 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) was similar between EAE mice treated with vehicle, 500 mg/kg SRT501, or 1,000 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 postimmunization, 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).
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 postimmunization with vehicle or 1,000 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 per 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 3 hours, then viable cells were permeabilized and labeled with antibodies to cytokines IL-17 and IFN-γ. 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 3 hours in culture, no significant differences in the expression of the Th1 cytokine IFN-γ or the Th17 cytokine IL-17 were found between cells from vehicle-treated and SRT501-treated EAE mice. Interestingly, there was a nonsignificant 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+, IFN-γ+, 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, 29), 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 to 14 postimmunization with vehicle or 100 mg/kg SRT1720, then sacrificed on day 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).
The ability of 2 distinct SIRT1 activators to prevent neurological dysfunction and neuronal damage in EAE suggests that the mechanism of these neuroprotective effects involves SIRT1 activation. To further confirm this, EAE mice treated with 1,000 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 postimmunization (Fig. 8).
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 the 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 the 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 that 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 postimmunization, 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 that 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 that 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 2 other studies (30,31), although there are important differences between those studies and the current studies. Singh et al (30) 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 the development of the disease model as opposed to treating active disease. In addition, mechanisms of neuronal loss in chronic EAE can differ from those in relapsing-remitting EAE; neuronal loss in the former occurs several days after immunization and prior to onset of inflammation (32), whereas in the latter it begins only after inflammation develops (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 (31), 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 that resveratrol may modulate immune responses in EAE during the induction phase of the 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 the onset of inflammation but prior to the optic nerve axonal loss (25). If a similar window of time can be identified in patients with optic neuritis, 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 patients with MS. 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 patients with optic neuritis and MS. Because SIRT1 activation does not prevent inflammation when treatment is initiated after disease onset, SIRT1 activators have the potential to work synergistically with current immunomodulatory therapies.
The authors thank Elsa Aglow for expert technical assistance.
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