Protective Effects of Everolimus against N-Methyl-D-aspartic Acid-Induced Retinal Damage in Rats

Role of monocarboxylate transporters in AMPK-mediated protection against excitotoxic injury in the rat retina

Activation of adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway protects against N-methyl-D-aspartic acid (NMDA)-induced excitotoxic retinal injury. AMPK activation enhances fatty acid metabolism and ketone body synthesis. Ketone bodies are transported into neurons by monocarboxylate transporters (MCTs) and exert neuroprotective effects. In this study, we examined the distribution and expression levels of MCT1 and MCT2 in the retina and analyzed the effects of pharmacological inhibition of MCTs on the protective effects of metformin and 5-aminoimidazole-4-carboxamide (AICAR), activators of AMPK, against NMDA-induced retinal injury in rats. MCT1 was expressed in the blood vessels, processes of astrocytes and Müller cells, and inner segments of photoreceptors in the rat retina, whereas MCT2 was expressed in neuronal cells in the ganglion cell layer (GCL) and in astrocyte processes. The expression levels of MCT2, but not MCT1, decreased one day after intravitreal injection of NMDA (200 nmol). Intravitreal injection of NMDA decreased the number of cells in the GCL compared to the vehicle seven days after injection. Simultaneous injection of metformin (20 nmol) or AICAR (50 nmol) with NMDA attenuated NMDA-induced cell loss in the GCL, and these protective effects were attenuated by AR-C155858 (1 pmol), an inhibitor of MCTs. AR-C155858 alone had no significant effect on the retinal structure. These results suggest that AMPK-activating compounds protect against NMDA-induced excitotoxic retinal injury via mechanisms involving MCTs in rats. NMDA-induced neurotoxicity may be associated with retinal neurodegenerative changes in glaucoma and diabetic retinopathy. Therefore, AMPK-activating compounds may be effective in managing these retinal diseases.

Everolimus attenuates glutamate-induced PC12 cells death

BACKGROUND

Glutamate-induced neuronal cell death plays a key role in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. Some recent studies reported the potential immunomodulatory and neuroprotective properties of inhibitors of serine-threonine kinase, mTOR (mammalian target of rapamycin). However, no study was conducted about the neuroprotective potential of everolimus (EVR), a selective and potent mTOR inhibitor. Therefore, this study was planned to investigate whether EVR has protective effects against glutamate-induced toxicity in PC12 cells, which are used as model for neurons injury, and to elucidate the underlying mechanism.

METHODS

PC12 cells were concurrently treated with glutamate (8 mM) and EVR (0-40 nM) for 24 h. Then, the cells viability, apoptosis rate, and apoptosis-related proteins (caspase-3, bax and bcl-2) were measured using MTT, annexin V/PI and immunoblotting assays.

RESULTS

Analyzing the protective effect of different concentrations of EVR (0-40 nM) against glutamate-induced cytotoxicity revealed a significant increase in cell viability in co-treatment regimen (p < 0.01). Also, EVR (40 nM) significantly (p < 0.01) inhibited glutamate-induced apoptosis through depressing the elevation of bax/bcl-2 ratio and expression of cleaved caspase-3, concentration depend.

CONCLUSION

The results demonstrated, for the first time, that EVR could protect against glutamate-mediated PC12 cell death via inhibiting apoptosis.

Tumor necrosis factor-α and matrix metalloproteinase-9 cooperatively exacerbate neurovascular degeneration in the neonatal rat retina

Matrix metalloproteinases (MMPs) and tumor necrosis factor (TNF)-α contribute to the pathogenesis of several ocular diseases. Previous studies have shown that MMP-9 activation plays an important role in capillary degeneration in injured retinas. In this study, we aimed to determine the roles of TNF-α in capillary degeneration and MMP-9 activation in the injured retina. In rats, retinal injury was induced by intravitreal injection of N-methyl-D-aspartic acid (NMDA, 200 nmol) at postnatal day 7. We examined (1) the effects of blocking MMP-9 and TNF-α signaling pathway on capillary degeneration, (2) changes in protein levels and distribution of MMP-9 and TNF-α, and (3) the interaction between MMP-9 and TNF-α in regulating the expression level of each protein in retinas of NMDA-injected eyes. Intravitreal injection of GM6001, an MMP inhibitor, or TNF-α neutralizing antibody (anti-TNF-α Ab) attenuated capillary degeneration in retinas of NMDA-injected eyes. Protein levels of TNF-α increased 2 h after NMDA injection, whereas those of MMP-9 increased 4 h after the injection. Anti-TNF-α Ab suppressed activation of MMP-9 in retinas of NMDA-injected eyes, whereas GM6001 diminished the TNF-α protein expression. Incubation of recombinant TNF-α with supernatants of homogenized retina increased protein levels and activity of MMP-9. These results suggest that TNF-α and MMP-9 collaboratively increase their expression levels in the retina following neurodegeneration, thus leading to retinal capillary degeneration. The cooperative interaction between MMP-9 and TNF-α could be involved in the exacerbation of retinal neurovascular degeneration.

Rapamycin Improved Retinal Function and Morphology in a Mouse Model of Retinal Degeneration

The retina is an important visual organ, which is responsible for receiving light signals and transmitting them to the optic nerve center step by step. The retina contains a variety of cells, among which photoreceptor cells receive light signals and convert them into nerve signals, and are mainly responsible for light and dark vision. Retinal degeneration is mainly the degeneration of photoreceptor cells, and retinitis pigmentosa (RP) is characterized by rod degeneration followed by cone degeneration. So far, there is still a lack of effective drugs to treat RP. Here, we established a stable RP model by tail vein injection of methyl methanesulfonate to study the mechanism of retinal photoreceptor degeneration. Mechanistic target of rapamycin (mTOR) is located in the central pathway of growth and energy metabolism and changes in a variety of diseases in response to pathological changes. We found that the mTOR was activated in this model. Therefore, the inhibitor of mTOR, rapamycin was used to suppress the expression of mTOR and interfere with photoreceptor degeneration. Electroretinogram assay showed that the function of mice retina was improved. Hematoxylin and eosin staining results displayed that retinal photoreceptor thickness and morphology were improved. Also, the autophagy in rapamycin group was activated, which revealed that rapamycin may protect the retinal photoreceptor by inhibiting mTOR and then activating autophagy.

Metformin Protects against NMDA-Induced Retinal Injury through the MEK/ERK Signaling Pathway in Rats

Metformin, an anti-hyperglycemic drug of the biguanide class, exerts positive effects in several non-diabetes-related diseases. In this study, we aimed to examine the protective effects of metformin against N-methyl-D-aspartic acid (NMDA)-induced excitotoxic retinal damage in rats and determine the mechanisms of its protective effects. Male Sprague–Dawley rats (7 to 9 weeks old) were used in this study. Following intravitreal injection of NMDA (200 nmol/eye), the number of neuronal cells in the ganglion cell layer and parvalbumin-positive amacrine cells decreased, whereas the number of CD45-positive leukocytes and Iba1-positive microglia increased. Metformin attenuated these NMDA-induced responses. The neuroprotective effect of metformin was abolished by compound C, an inhibitor of AMP-activated protein kinase (AMPK). The AMPK activator, AICAR, exerted a neuroprotective effect in NMDA-induced retinal injury. The MEK1/2 inhibitor, U0126, reduced the neuroprotective effect of metformin. These results suggest that metformin protects against NMDA-induced retinal neurotoxicity through activation of the AMPK and MEK/extracellular signal-regulated kinase (ERK) signaling pathways. This neuroprotective effect could be partially attributable to the inhibitory effects on inflammatory responses.

Effect of lycopene on pain facilitation and the SIRT1/mTOR pathway in the dorsal horn of burn injury rats

To explore the effect of intrathecal injection of lycopene on pain facilitation, glial activation, and the SIRT1/mTOR pathway in the dorsal horn of rats with burn injury pain (BIP). Here we found that the mechanical pain threshold increased in the lycopene group compared with that of the control group, (P < 0.05). Compared with expression in the sham group, mTOR, pS6, p4EBP, GFAP, and Iba-1 decreased and SIRT1 increased in the lycopene group (P < 0.01). Glial activation in the spinal dorsal horn of BIP rats was alleviated by lycopene (P < 0.01). The SIRT1 and mTOR were mainly distributed in neurons in the spinal dorsal horn in the BIP model. Intrathecal injection of 3-MA (a mTOR agonist) or EX-527 (an inhibitor of Sirt1) partially antagonized lycopene-induced analgesia. Intrathecal injection of rapamycin (an mTOR inhibitor) or SRT1720 (an agonist of Sirt1) induced analgesia in BIP rats. 3-MA abrogated the SRT1720-induced analgesic effects. The present data indicated that the SIRT1/mTOR pathway changed in the spinal dorsal horn of BIP rats; Lycopene alleviated the pain sensitization of BIP rats by regulating the SIRT1/mTOR pathway and glial activation in the spinal dorsal horn.

Neuroprotective effect of a dietary supplement against glutamate-induced excitotoxicity in retina

AIM

To evaluate the neuroprotective effect of a dietary supplement (ClearVision EX^®; CV) against glutamate-induced excitotoxicity in retina.

METHODS

We evaluated the protective effects CV on glutamate-induced cell toxicity of an immortalized mouse hippocampal cell line (HT-22) in vitro and N-methyl-D-aspartate (NMDA) induced retinal injury in vivo. Once-daily oral administration of CV or vehicle (5% Arabic gum) was started the day before the NMDA injection and continued until the end of the study. Electroretinograms (ERGs) were recorded to evaluate the retinal function at 2d after NMDA injection. Furthermore, a histological evaluation, Western blot analysis, and immunohistochemistry were performed for assessing the signal transduction pathway.

RESULTS

HT-22 cell death was induced by the addition of glutamate and co-incubation with CV protected against it. Oral administration of CV inhibited the decrease in scotopic threshold response amplitudes induced by the intravitreal injection of NMDA and those of the thickness of the inner retinal layer in the histological evaluation. The increased phosphorylated levels of extracellular signal-regulated kinase (ERK) but not cAMP response element binding protein (CREB) or Akt were observed 1h after NMDA injection in both the vehicle- and CV-treated rats; however, pERK activation was no more upregulated at 3h after NMDA injection. pERK upregulation was observed in Müller cells.

CONCLUSION

CV shows a protective effect against both glutamate-induced HT-22 cell death and NMDA-induced retinal damage. pERK upregulation in the Müller cells plays a key role in the protective effect of CV against glutamate-induced retinal toxicity.

Cellular Signaling in Müller Glia: Progenitor Cells for Regenerative and Neuroprotective Responses in Pharmacological Models of Retinal Degeneration

Retinal degenerative diseases are a leading cause of visual impairment or blindness. There are many therapies for delaying the progression of vision loss but no curative strategies currently. Stimulating intrinsic neuronal regeneration is a potential approach to therapy in retinal degenerative diseases. In contrast to stem cells, as embryonic/pluripotent stem cell-derived retinal progenitor cell or mesenchymal stem cells, Müller glia provided an endogenous cellular source for regenerative therapy in the retina. Müller glia are a major component of the retina and considerable evidence suggested these cells can be induced to produce the lost neurons in several species. Understanding the specific characteristic of Müller glia to generate lost neurons will inspire an attractive and alternative therapeutic strategy for treating visual impairment with regenerative research. This review briefly provides the different signal transduction mechanisms which are underlying Müller cell-mediated neuroprotection and neuron regeneration and discusses recent advances about regeneration from Müller glia-derived progenitors.

Mammalian Target of Rapamycin (mTOR) as a Potential Therapeutic Target in Pathological Ocular Angiogenesis

Pathological ocular angiogenesis is a causative factor of retinopathy of prematurity, proliferative diabetic retinopathy, and wet age-related macular degeneration. Vascular endothelial growth factor (VEGF) plays an important role in pathological angiogenesis, and anti-VEGF agents have been used to treat the ocular diseases that are driven by pathological angiogenesis. However, adverse effects associated with the blockade of VEGF signaling, including impairments of normal retinal vascular growth and retinal function, were suggested. Therefore, the development of a safe, effective strategy to prevent pathological ocular angiogenesis is needed. Recent studies have demonstrated that inhibitors of the mammalian target of rapamycin (mTOR) target proliferating endothelial cells within the retinal vasculature. Here, we review the potential of targeting the mTOR pathway to treat pathological ocular angiogenesis.

Everolimus is better than rapamycin in attenuating neuroinflammation in kainic acid-induced seizures

Background

Microglia is responsible for neuroinflammation, which may aggravate brain injury in diseases like epilepsy. Mammalian target of rapamycin (mTOR) kinase is related to microglial activation with subsequent neuroinflammation. In the present study, rapamycin and everolimus, both as mTOR inhibitors, were investigated in models of kainic acid (KA)-induced seizure and lipopolysaccharide (LPS)-induced neuroinflammation.

Methods

In vitro, we treated BV2 cells with KA and LPS. In vivo, KA was used to induce seizures on postnatal day 25 in B6.129P-Cx3cr1^tm1Litt/J mice. Rapamycin and everolimus were evaluated in their modulation of neuroinflammation detected by real-time PCR, Western blotting, and immunostaining.

Results

Everolimus was significantly more effective than rapamycin in inhibiting iNOS and mTOR signaling pathways in both models of neuroinflammation (LPS) and seizure (KA). Everolimus significantly attenuated the mRNA expression of iNOS by LPS and nitrite production by KA and LPS than that by rapamycin. Only everolimus attenuated the mRNA expression of mTOR by LPS and KA treatment. In the present study, we also found that the modulation of mTOR under LPS and KA treatment was not mediated by Akt pathway but was primarily mediated by ERK phosphorylation, which was more significantly attenuated by everolimus. This inhibition of ERK phosphorylation and microglial activation in the hippocampus by everolimus was also confirmed in KA-treated mice.

Conclusions

Rapamycin and everolimus can block the activation of inflammation-related molecules and attenuated the microglial activation. Everolimus had better efficacy than rapamycin, possibly mediated by the inhibition of ERK phosphorylation. Taken together, mTOR inhibitor can be a potential pharmacological target of anti-inflammation and seizure treatment.

Protective effects of PF-4708671 against N-methyl-d-aspartic acid-induced retinal damage in rats

We previously demonstrated that rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), protects against N-methyl-d-aspartic acid (NMDA)-induced retinal damage in rats. Rapamycin inhibits mTOR activity, thereby preventing the phosphorylation of ribosomal protein S6, which is a downstream target of S6 kinase. Therefore, we aimed to determine whether PF-4708671, an inhibitor of S6 kinase, protects against NMDA-induced retinal injury. Intravitreal injection of NMDA (200 nmol/eye) caused cell loss in the ganglion cell layer and neuroinflammatory responses, such as an increase in the number of CD45-positive leukocytes and Iba1-positive microglia. Surprisingly, simultaneous injection of PF-4708671 (50 nmol/eye) with NMDA significantly attenuated these responses without affecting phosphorylated S6 levels. These results suggest that PF-4708671 and rapamycin likely protect against NMDA-induced retinal damage via distinct pathways. The neuroprotective effect of PF-4708671 is unlikely to be associated with inhibition of the S6 kinase, even though PF-4708671 is reported to be a S6 kinase inhibitor.