Yuyi You

TrkB Receptor Signalling: Implications in Neurodegenerative, Psychiatric and Proliferative Disorders

The Trk family of receptors play a wide variety of roles in physiological and disease processes in both neuronal and non-neuronal tissues. Amongst these the TrkB receptor in particular has attracted major attention due to its critical role in signalling for brain derived neurotrophic factor (BDNF), neurotrophin-3 (NT3) and neurotrophin-4 (NT4). TrkB signalling is indispensable for the survival, development and synaptic plasticity of several subtypes of neurons in the nervous system. Substantial evidence has emerged over the last decade about the involvement of aberrant TrkB signalling and its compromise in various neuropsychiatric and degenerative conditions. Unusual changes in TrkB signalling pathway have also been observed and implicated in a range of cancers. Variations in TrkB pathway have been observed in obesity and hyperphagia related disorders as well. Both BDNF and TrkB have been shown to play critical roles in the survival of retinal ganglion cells in the retina. The ability to specifically modulate TrkB signalling can be critical in various pathological scenarios associated with this pathway. In this review, we discuss the mechanisms underlying TrkB signalling, disease implications and explore plausible ameliorative or preventive approaches.

Protective Effects of 7,8-Dihydroxyflavone on Retinal Ganglion and RGC-5 Cells Against Excitotoxic and Oxidative Stress

A preferential loss of retinal ganglion cells (RGCs) is observed in glaucoma and optic neuritis. Loss of tropomyosin-related kinase receptor B (TrkB)-mediated signaling has been implicated in this degeneration. Our study indicates that 7,8-dihydroxyflavone (7,8 DHF) robustly upregulates the TrkB signaling in the primary rat RGCs and the retinal neuronal precursor RGC-5 cell line by promoting phosphorylation of TrkB receptor, leading to enhanced TrkB receptor tyrosine kinase activity. The flavonoid derivative 7,8 DHF acts a potent TrkB agonist and upregulates the downstream AKT and MAPK/ERK survival signaling pathways in a TrkB-dependent manner in both primary rat RGCs as well as the RGC-5 cell line. Excitotoxicity and oxidative injury have been alleged in the specific RGC degeneration in various forms of glaucoma. A novel finding of this study is that treatment with 7,8 DHF protects these cells significantly from excitotoxic and oxidative stress-induced apoptosis and cell death. 7,8 DHF also promotes neuritogenesis by stimulating neurite outgrowth, suggesting a possible therapeutic strategy for protection of RGCs in various optic neuropathies.

BDNF impairment is associated with age-related changes in the inner retina and exacerbates experimental glaucoma.

Brain-derived neurotrophic factor (BDNF) stimulation of its high-affinity receptor TrkB results in activation of pro-survival cell-signalling pathways that can afford neuroprotection to the retina. Reduction in retrograde axonal transport of neurotrophic factors such as BDNF from the brain to the neuronal cell bodies in the retina has been suggested as a critical factor underlying progressive and selective degeneration of ganglion cell layer and optic nerve in glaucoma. We investigated the role of BDNF in preserving inner retinal homeostasis in normal and glaucoma states using BDNF(+/-) mice and compared it with wild type controls. This study demonstrated that BDNF(+/-) animals were more susceptible to functional, morphological and molecular degenerative changes in the inner retina caused by age as well as upon exposure to experimental glaucoma caused by increased intraocular pressure. Glaucoma induced a down regulation of BDNF/TrkB signalling and an increase in levels of neurotoxic amyloid β 1-42 in the optic nerve head which were exacerbated in BDNF(+/-) mice. Similar results were obtained upon analysing the human optic nerve head tissues. Our data highlighted the role of BDNF in maintaining the inner retinal integrity under normal conditions and the detrimental effects of its insufficiency on the retina and optic nerve in glaucoma.

Optic neuropathies: characteristic features and mechanisms of retinal ganglion cell loss.

Abstract Optic neuropathy refers to dysfunction and/or degeneration of axons of the optic nerve with subsequent optic nerve atrophy. A common feature of different optic neuropathies is retinal ganglion cell (RGC) apoptosis and axonal damage. Glaucoma and optic neuritis are the two major degenerative causes of optic nerve damage. Here, we review the anatomy and pathology of the optic nerve, and etiological categories of optic neuropathies, and discuss rodent models that can mimic these conditions. Electrophysiology can reveal signature features of RGC damage using the pattern electroretinogram (PERG), scotopic threshold response (STR) and photopic negative response (PhNR). The amplitude of the visual evoked potential (VEP) also reflects RGC axonal damage. The neurotrophin-mediated survival pathways, as well as the extrinsic and intrinsic cell apoptotic pathways, play a critical role in the pathogenesis of RGC loss. Finally, promising neuroprotective approaches based on the molecular signaling are analyzed for the treatment of optic neuropathies.

Anterograde Degeneration along the Visual Pathway after Optic Nerve Injury

Purpose To investigate anterograde degenerative changes along the visual pathway in a rat model of optic nerve axotomy. Methods Optic nerve transection was performed in adult Sprague-Dawley rats. Animals were sacrificed at regular time intervals and tissues harvested. Immunoblotting followed by densitometric analysis was used to determine the phosphorylation profile of Akt in the dorsal lateral geniculate nucleus (dLGN) and the primary visual cortex (V1). The neuronal cell size and cell density were measured in the dLGN and the V1 using Nissl staining. The prevalence of apoptosis was characterized by terminal deoxynucleotidyl-transferase-mediated biotin-dUTP nick end labelling (TUNEL) histochemistry. Caspase-3 antibodies were also used to identify apoptotic cells. Neurons and astrocytes were detected using NeuN and glial fibrillary acidic protein (GFAP), respectively. Results An early and sustained loss of Akt phosphorylation was observed after optic nerve transection in both dLGN and V1. At week one, a decrease in the neuronal cell size (50.5±4.9 vs 60.3±5.0 µm2, P = 0.042) and an increase of TUNEL positive cells (7.9±0.6 vs 1.4±0.5 ×102 cells/mm2, P<0.001) were evident in the dLGN but not in V1. A significant decline in neuronal cell number (14.5±0.1 vs 17.4±1.3 ×102 cells/mm2, P = 0.048), cell size (42.5±4.3 vs 62.1±4.7 µm2, P = 0.001) and an increase in apoptotic cells (5.6±0.5 vs 2.0±0.4 ×102 cells/mm2, P<0.001) appeared in V1 initially at one month post-transection. The changes in the visual pathway continued through two months. Both neuronal cells and GFAP-positive glial cells were affected in this anterograde degeneration along the visual pathway. Conclusions Anterograde degeneration along the visual pathway takes place in target relay (LGN) and visual cortex following the optic nerve injury. Apoptosis was observed in both neural and adjacent glial cells. Reduction of Akt phosphorylation preceded cellular and apoptotic changes.

Progressive Loss of Retinal Ganglion Cells and Axons in Nonoptic Neuritis Eyes in Multiple Sclerosis: A Longitudinal Optical Coherence Tomography Study.

PURPOSE To examine the rate of retinal ganglion cell (RGC) layer and retinal nerve fiber layer (RNFL) changes in nonoptic neuritis (NON) eyes of relapsing remitting multiple sclerosis (RRMS) patients, and to find a specific imaging parameter useful for identifying disease progression. METHODS Forty-five consecutive RRMS patients and 20 age- and sex-matched healthy subjects were enrolled. All patients were followed up for 3 years with annual optical coherence tomography (OCT) scans, which included a peripapillary ring scan protocol for RNFL analysis and a macular radial star-like scan to obtain RGC/inner plexiform layer (IPL) thickness measures. Healthy controls were scanned twice, 3 years apart. RESULTS Retinal ganglion cell/inner plexiform layer and temporal RNFL (tRNFL) demonstrated highly significant thinning (P < 0.01), but all nasal segments and global RNFL (gRNFL) were not significantly different from normal controls. While receiver operating characteristics (ROC) analysis showed no advantage of RGC/IPL over tRNFL in cross-sectional detection of thinning, cut-off point based of fifth percentile in healthy controls demonstrated higher rate of abnormality for RGC/IPL. There was a significant progressive loss of RGC/IPL and tRNFL during the follow-up period. The largest thickness reduction was observed in tRNFL. ROC analysis demonstrated that tRNFL provided better sensitivity/specificity for detecting change over time than RGC/IPL (area under the curve [AUC] 0.78 vs. 0.52), which was confirmed by higher detection rate when 95th percentile of progression in healthy controls was used as a cut-off. CONCLUSIONS This study confirmed significant thinning of RGC/IPL and tRNFL in NON eyes of RRMS patients. Progressive losses were more apparent on tRNFL, while RGC/IPL showed less change over the follow-up period.

Shp-2 regulates the TrkB receptor activity in the retinal ganglion cells under glaucomatous stress

Tropomyosin-receptor-kinase B (TrkB receptor) activation plays an important role in the survival of retinal ganglion cells (RGCs). This study reports a novel finding that, SH2 domain-containing phosphatase-2 (Shp-2) binds to the TrkB receptor in RGCs and negatively regulates its activity under glaucomatous stress. This enhanced binding of TrkB and Shp2 is mediated through caveolin. Caveolin 1 and 3 undergo hyper-phosphorylation in RGCs under stress and bind to the Shp2 phosphatase. Shp2 undergoes activation under glaucomatous stress conditions in RGCs in vivo with a concurrent loss of TrkB activity. Inhibiting the Shp2 phosphatase restored TrkB activity in cells exposed to excitotoxic and oxidative stress. Collectively, these findings implicate a molecular basis of Shp2 mediated TrkB deactivation leading to RGC degeneration observed in glaucoma.

FTY720 Protects Retinal Ganglion Cells in Experimental Glaucoma

PURPOSE To investigate the neuroprotective effects of sphingosine-1-phosphate (S1P) analogue fingolimod (FTY720) in experimental glaucoma in rats. METHODS A unilateral chronic ocular hypertensive model was established by injections of microbeads into the anterior eye chamber of adult Sprague-Dawley rats. Fingolimod was administered to one group of rats intraperitoneally every week for 3 months. The scotopic threshold response (STR) was recorded to assess the function of the inner retina. Changes in cell density in the ganglion cell layer (GCL) were evaluated by hematoxylin and eosin staining on retinal sections and axonal count of the optic nerve was performed using Bielschowskys silver staining. Effects of drug treatment on activation of Akt and Erk1/2 were evaluated using Western blotting by assessing phosphorylation levels of these proteins. The expression of S1P receptors in the optic nerve head region was also evaluated using Western blotting and immunohistochemistry. RESULTS Administration of FTY720 reduced the loss of STR amplitude in glaucomatous eyes (P < 0.05). Counting and plotting the cell numbers/axonal density showed significant neural preservation in the GCL and the optic nerve (P < 0.05). An increased phosphorylation level of Akt and Erk1/2 following FTY720 administration was observed. Both S1P1 and S1P5 receptors were found to be expressed in the retina and the expression of S1P1R was upregulated in experimentally-induced glaucoma. CONCLUSIONS This study demonstrates, for the first time, that FTY720 could act as a neuroprotective agent to protect retinal ganglion cells in experimental glaucoma. Administration of this drug significantly reduces the structural and functional loss of the inner retina elicited indicating that it may potentially be used to attenuate neuronal loss and optic nerve damage in glaucomatous patients.

Normalization of visual evoked potentials using underlying electroencephalogram levels improves amplitude reproducibility in rats

PURPOSE The visual evoked potential (VEP) is a frequently used noninvasive measurement of visual function. However, high-amplitude variability has limited its potential for evaluating axonal damage in both laboratory and clinical research. This study was conducted to improve the reliability of VEP amplitude measurement in rats by using electroencephalogram (EEG)-based signal correction. METHODS VEPs of Sprague-Dawley rats were recorded on three separate days within 2 weeks. The original VEP traces were normalized by EEG power spectrum, which was evaluated by Fourier transform. A comparison of intersession reproducibility and intersubject variability was made between the original and corrected signals. RESULTS Corrected VEPs showed lower amplitude intersession within-subject SD (Sw), coefficient of variation (CoV), and repeatability (R(95)) than the original signals (P < 0.001). The intraclass correlation coefficient (ICC) of the corrected traces (0.90) was also better than the original potentials (0.82). For intersubject variability, the EEG-based normalization improved the CoV from 44.64% to 30.26%. A linear correlation was observed between the EEG level and the VEP amplitude (r = 0.71, P < 0.0001). CONCLUSIONS Underlying EEG signals should be considered in measuring the VEP amplitude. In this study, a useful technique was developed for VEP data processing that could also be used for other cortical evoked potential recordings and for clinical VEP interpretation in humans.

The dynamic response of intraocular pressure and ocular pulse amplitude to acute hemodynamic changes in normal and glaucomatous eyes.

PURPOSE To evaluate the effects of acute arterial blood pressure (ABP) and venous pressure changes on IOP in rats with experimental glaucoma. METHODS Unilateral experimental ocular hypertension was established in Sprague-Dawley rats by weekly intracameral injection of microbeads. Ocular pulse amplitude (OPA) and IOP were recorded from the anterior chamber using 1.2-Fr microsensors under urethane anesthesia. The effects on IOP during hemodynamic challenges using phenylephrine (PE) (50 μg/kg/min intravenous [IV]) and rapid saline loading (20 mL/kg/min IV) were studied. RESULTS Over an 8-week period, IOP was significantly elevated by 60% in the unilateral ocular hypertensive eyes. Both ABP and IOP were significantly increased by PE infusion. A significantly greater IOP increase was found in the experimental eyes compared with control eyes (1.32 ± 0.18 mm Hg vs. 0.90 ± 0.09 mm Hg). Correspondingly, higher OPAs and an amplification of the OPA during arterial hypertension were found in the experimental eyes. A sustained rise in IOP secondary to IV saline loading was observed, with a greater rise observed among experimental eyes (0.74 ± 0.13 mm Hg vs. 0.37 ± 0.005 mm Hg). CONCLUSIONS Sympathetic acceleration of ABP using PE resulted in surges in IOP and OPA. In contrast, increased venous pressure resulted in a more sustained rise in IOP but to a lesser extent. These responses were more pronounced in eyes with experimental glaucoma compared with control eyes, which may reflect the higher starting IOP contributing to a reduced ocular compliance. Our results suggest that eyes with ocular hypertension are more susceptible to IOP variability induced by hemodynamic fluctuations.