Up-regulation of genes for oxidative phosphorylation and protein turnover in diabetic mouse retina

mTOR-dependent dysregulation of autophagy contributes to the retinal ganglion cell loss in streptozotocin-induced diabetic retinopathy

BACKGROUND

Neurodegeneration, an early event in the pathogenesis of diabetic retinopathy (DR), precedes clinically detectable microvascular damage. Autophagy dysregulation is considered a potential cause of neuronal cell loss, however underlying mechanisms remain unclear. The mechanistic target of rapamycin (mTOR) integrates diverse environmental signals to coordinate biological processes, including autophagy. Here, we investigated the role of mTOR signaling in neuronal cell death in DR.

METHODS

Diabetes was induced by a single intraperitoneal injection of streptozotocin and tissue samples were harvested at 1, 2, 3, 4, and 6 months of diabetes. Early-stage of DR was investigated in 1-month-diabetic mice treated with phlorizin (two daily subcutaneous injections at a dose of 200 mg/kg of body weight during the last 7 full days of the experiment and the morning of the 8th day, 3 h before sacrifice) or rapamycin (daily intraperitoneal injections, at a dose of 3 mg/kg for the same period as for phlorizin treatment). The effect of autophagy modulation on retinal ganglion cells was investigated in 3-months-diabetic mice treated with phlorizin (two daily subcutaneous injections during the last 10 full days of the experiment and the morning of the 11th day, 3 h before sacrifice) or MHY1485 (daily i.p. injections, at a dose of 10 mg/kg for the same period as for phlorizin treatment). Tissue samples obtained from treated/untreated diabetic mice and age-matched controls were used for Western blot and histologic analysis.

RESULTS

mTOR-related proteins and glucose transporter 1 (GLUT1) was upregulated at 1 month and downregulated in the following period up to 6 months. Diabetes-induced neurodegeneration was characterized by an increase of apoptotic marker-cleaved caspase 3, a decrease of the total number of cells, and NeuN immunoreactivity in the ganglion cell layer, as well as an increase of autophagic protein. Insulin-independent glycemic control restored the mTOR pathway activity and GLUT1 expression, along with a decrease of autophagic and apoptotic proteins in 3-months-diabetic mice neuroretina. However, blockade of autophagy using MHY1485 resulted in a more protective effect on ganglion cells compared with phlorizin treatment.

CONCLUSION

Collectively, our study describes the mechanisms of neurodegeneration through the hyperglycemia/ mTOR/ autophagy/ apoptosis pathway. Video Abstract.

Integrative analysis of ocular complications in atherosclerosis unveils pathway convergence and crosstalk

Atherosclerosis is a life-threatening disease and a major cause of mortalities worldwide. While many of the atherosclerotic sequelae are reflected as microvascular effects in the eye, the molecular mechanisms of their development is not yet known. In this study, we employed a systems biology approach to unveil the most significant events and key molecular mediators of ophthalmic sequelae caused by atherosclerosis. Literature mining was used to identify the proteins involved in both atherosclerosis and ophthalmic diseases. A protein-protein interaction (PPI) network was prepared using the literature-mined seed nodes. Network topological analysis was carried out using Cytoscape, while network nodes were annotated using database for annotation, visualization and integrated discovery in order to identify the most enriched pathways and processes. Network analysis revealed that mitogen-activated protein kinase 1 (MAPK1) and protein kinase C occur with highest betweenness centrality, degree and closeness centrality, thus reflecting their functional importance to the network. Our analysis shows that atherosclerosis-associated ophthalmic complications are caused by the convergence of neurotrophin signaling pathways, multiple immune response pathways and focal adhesion pathway on the MAPK signaling pathway. The PPI network shares features with vasoregression, a process underlying multiple vascular eye diseases. Our study presents a first clear and composite picture of the components and crosstalk of the main pathways of atherosclerosis-induced ocular diseases. The hub bottleneck nodes highlight the presence of molecules important for mediating the ophthalmic complications of atherosclerosis and contain five established drug targets for future therapeutic modulation efforts.

Efficient subtractive cloning of genes activated by lipopolysaccharide and interferon γ in primary-cultured cortical cells of newborn mice

Innate immune responses play a central role in neuroprotection and neurotoxicity during inflammatory processes that are triggered by pathogen-associated molecular pattern-exhibiting agents such as bacterial lipopolysaccharide (LPS) and that are modulated by inflammatory cytokines such as interferon γ (IFNγ). Recent findings describing the unexpected complexity of mammalian genomes and transcriptomes have stimulated further identification of novel transcripts involved in specific physiological and pathological processes, such as the neural innate immune response that alters the expression of many genes. We developed a system for efficient subtractive cloning that employs both sense and antisense cRNA drivers, and coupled it with in-house cDNA microarray analysis. This system enabled effective direct cloning of differentially expressed transcripts, from a small amount (0.5 µg) of total RNA. We applied this system to isolation of genes activated by LPS and IFNγ in primary-cultured cortical cells that were derived from newborn mice, to investigate the mechanisms involved in neuroprotection and neurotoxicity in maternal/perinatal infections that cause various brain injuries including periventricular leukomalacia. A number of genes involved in the immune and inflammatory response were identified, showing that neonatal neuronal/glial cells are highly responsive to LPS and IFNγ. Subsequent RNA blot analysis revealed that the identified genes were activated by LPS and IFNγ in a cooperative or distinctive manner, thereby supporting the notion that these bacterial and cellular inflammatory mediators can affect the brain through direct but complicated pathways. We also identified several novel clones of apparently non-coding RNAs that potentially harbor various regulatory functions. Characterization of the presently identified genes will give insights into mechanisms and interventions not only for perinatal infection-induced brain damage, but also for many other innate immunity-related brain disorders.

The secretogranin II gene is a signal integrator of glutamate and dopamine inputs

Cooperative gene regulation by different neurotransmitters likely underlies the long-term forms of associative learning and memory, but this mechanism largely remains to be elucidated. Following cDNA microarray analysis for genes regulated by Ca(2+) or cAMP, we found that the secretogranin II gene (Scg2) was cooperatively activated by glutamate and dopamine in primary cultured mouse hippocampal neurons. The Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM) and the mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor PD98059 prevented Scg2 activation by glutamate or dopamine; thus, the Ca(2+) /MEK pathway is predicted to include a convergence point(s) of glutamatergic and dopaminergic signaling. Unexpectedly, the protein kinase A inhibitor KT5720 enhanced Scg2 activation by dopamine. The protein-synthesis inhibitor cycloheximide also enhanced Scg2 activation, and the proteasome inhibitor ZLLLH diminished the KT5720-mediated augmentation of Scg2 activation. These results are concordant with the notion that dopaminergic input leads to accumulation of a KT5720-sensitive transcriptional repressor, which is short-lived because of rapid degradation by proteasomes. This repression pathway may effectively limit the time window permissive to Scg2 activation by in-phase glutamate and dopamine inputs via the Ca(2+) /MEK pathway. We propose that the regulatory system of Scg2 expression is equipped with machinery that is refined for the signal integration of in-phase synaptic inputs. We proposed hypothetical mechanism for the regulation of the secretogranin II gene as a signal integrator of glutamate and dopamine inputs. Glutamate or dopamine activates the Ca(2+) /MEK/ERK pathway, which thus contributes to the signal integration. Concurrently, activation of the PKA inhibitor KT5720-sensitive pathway by dopamine leads to accumulation of the repressor protein X that is otherwise susceptible to proteasome degradation. This repression system may determine the time window permissive to the cooperative activation by in-phase glutamate and dopamine inputs.

Expression in the mammalian retina of parkin and UCH-L1, two components of the ubiquitin-proteasome system

The ubiquitin-proteasome system (UPS) functions as a major degradation pathway for misfolded and damaged proteins with an important neuroprotective role in the CNS against a variety of cellular stresses. Parkin and ubiquitin C-terminal hydrolase L1 (UCH-L1) are two relevant components of the UPS associated with a number of neurodegenerative disorders. We here address the expression profile of parkin and UCH-L1 in the mammalian retina, with special emphasis on primates. We describe for the first time the presence of parkin in the retina of mammals, including humans. Parkin and UCH-L1 genes were expressed at the mRNA and protein levels in the retina of all species examined. The immunolocalization pattern of parkin was quite widespread, being expressed by most retinal neuronal types, including photoreceptors. UCH-L1 was localized to horizontal cells and specific subtypes of bipolar and amacrine cells, as well as to ganglion cells and their axons forming the nerve fiber layer. In rodents no UCH-L1 immunoreactivity was found in cone or rod photoreceptors, whereas this protein was present along the whole length of cones in all other mammals. Remarkably, UCH-L1 was expressed by dopaminergic amacrine cells of primates. The ample distribution of parkin and UCH-L1 in the mammalian retina, together with the crucial role played by the UPS in normal neuronal physiology in the brain, points to a participation of these two proteins in the ubiquitin-proteasomal pathway of protein degradation in most retinal cell types, where they could exert a protective function against neuronal stress.

Angiotensin AT1 receptor antagonism ameliorates murine retinal proteome changes induced by diabetes

Diabetic retinopathy is the most common microvascular complication caused by diabetes mellitus and is a leading cause of vision loss among working-age adults in developed countries. Understanding the effects of diabetes on the retinal proteome may provide insights into factors and mechanisms responsible for this disease. We have performed a comprehensive proteomic analysis and comparison of retina from C57BL/6 mice with 2 months of streptozotocin-induced diabetes and age-matched nondiabetic control mice. To explore the role of the angiotensin AT1 receptor in the retinal proteome in diabetes, a subgroup of mice were treated with the AT1 antagonist candesartan. We identified 1792 proteins from retinal lysates, of which 65 proteins were differentially changed more than 2-fold in diabetic mice compared with nondiabetic mice. A majority (72%) of these protein changes were normalized by candesartan treatment. Most of the significantly changed proteins were associated with metabolism, oxidative phosphorylation, and apoptotic pathways. An analysis of the proteomics data revealed metabolic and apoptotic abnormalities in the retina from diabetic mice that were ameliorated with candesartan treatment. These results provide insight into the effects of diabetes on the retina and the role of the AT1 receptor in modulating this response.

siah-1 Protein is necessary for high glucose-induced glyceraldehyde-3-phosphate dehydrogenase nuclear accumulation and cell death in Muller cells

The translocation and accumulation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in the nucleus has closely been associated with cell death induction. However, the mechanism of this process has not been completely understood. The E3 ubiquitin ligase siah-1 (seven in absentia homolog 1) has recently been identified as a potential shuttle protein to transport GAPDH from the cytosol to the nucleus. Previously, we have demonstrated that elevated glucose levels induce GAPDH nuclear accumulation in retinal Müller cells. Therefore, this study investigated the role of siah-1 in high glucose-induced GAPDH nuclear translocation and subsequent cell death in retinal Müller cells. High glucose significantly increased siah-1 expression within 12 h. Under hyperglycemic conditions, siah-1 formed a complex with GAPDH and was predominantly localized in the nucleus of Müller cells. siah-1 knockdown using 50 nm siah-1 small interfering RNA significantly decreased high glucose-induced GAPDH nuclear accumulation at 24 h by 43.8 +/- 4.0%. Further, knockdown of siah-1 prevented high glucose-induced cell death of Müller cells potentially by inhibiting p53 phosphorylation consistent with previous observations, indicating that nuclear GAPDH induces cell death via p53 activation. Therefore, inhibition of GAPDH nuclear translocation and accumulation by targeting siah-1 promotes Müller cell survival under hyperglycemic conditions.