The cellular mechanism underlying neuronal degeneration in glaucoma: parallels with Alzheimer's disease

Potential mechanisms of retinal ganglion cell type-specific vulnerability in glaucoma

Glaucoma is a neurodegenerative disease characterised by progressive damage to the retinal ganglion cells (RGCs), the output neurons of the retina. RGCs are a heterogenous class of retinal neurons which can be classified into multiple types based on morphological, functional and genetic characteristics. This review examines the body of evidence supporting type-specific vulnerability of RGCs in glaucoma and explores potential mechanisms by which this might come about. Studies of donor tissue from glaucoma patients have generally noted greater vulnerability of larger RGC types. Models of glaucoma induced in primates, cats and mice also show selective effects on RGC types - particularly OFF RGCs. Several mechanisms may contribute to type-specific vulnerability, including differences in the expression of calcium-permeable receptors (for example pannexin-1, P2X7, AMPA and transient receptor potential vanilloid receptors), the relative proximity of RGCs and their dendrites to blood supply in the inner plexiform layer, as well as differing metabolic requirements of RGC types. Such differences may make certain RGCs more sensitive to intraocular pressure elevation and its associated biomechanical and vascular stress. A greater understanding of selective RGC vulnerability and its underlying causes will likely reveal a rich area of investigation for potential treatment targets.

Glaucoma -state of the art and perspectives on treatment

Glaucoma is a chronic optic neuropathy characterized by progressive damage to the optic nerve, death of retinal ganglion cells and ultimately visual field loss. It is one of the leading causes of irreversible loss of vision worldwide. The most important trigger of glaucomatous damage is elevated eye pressure, and the current standard approach in glaucoma therapy is reduction of intraocular pressure (IOP). However, despite the use of effective medications or surgical treatment leading to lowering of IOP, progression of glaucomatous changes and loss of vision among patients with glaucoma is common. Therefore, it is critical to prevent vision loss through additional treatment. To implement such treatment(s), it is imperative to identify pathophysiological changes in glaucoma and develop therapeutic methods taking into account neuroprotection. Currently, there is no method of neuroprotection with long-term proven effectiveness in the treatment of glaucoma. Among the most promising molecules shown to protect the retina and optic nerve are neurotrophic factors. Thus, the current focus is on the development of safe and non-invasive methods for the long-term elevation of the intraocular level of neurotrophins through advanced gene therapy and topical eye treatment and on the search for selective agonists of neurotrophin receptors affording more efficient neuroprotection.

[Distinctive morphological and functional changes in retinal ganglion cells associated with normal aging and early stage of glaucoma]

Early diagnosis of primary open-angle glaucoma (POAG) is closely connected with detection of normal age-related changes in the retina. It is also essential to develop reliable methods for quantitative evaluation of retinal nerve fiber layer (RNFL) thickness and retinal ganglion cells (RGCs) structure and function as well as to extend opportunities for inter-instrumental comparisons.

AIM

to assess the function of RGCs from parvo- and magnocellular pathways by means of pattern electroretinography and to evaluate the degree of retinal sensitivity loss and RNFL thickness using new methods of analysis in normal aging and the early stage of POAG.

MATERIAL AND METHODS

Four groups were formed: group 1 - young healthy participants, group 2 - aged controls, group 3 - glaucoma suspects, and group 4 - early-stage POAG patients. In all groups we investigated the MD and PSD indices of static computer perimetry (HEP, Heidelberg Edge Perimeter, SAP and FDF tests) and RNFL thickness provided by HRT III (Heidelberg Engineering). Pattern electroretinograms (PERGs) were recorded with the RETImap system (Roland Consult) at the check sizes of 0.8° and 16°. For steady-state and transient PERGs the 0.8°/16° and N95/P50 ratios were calculated, respectively.

RESULTS

There was a statistically significant difference in PERG amplitudes to 0.8° checks (p=0.0001) and in 0.8°/16° ratios (p=0.0001) between the groups 1 and 2. Differences between the groups 2 and 3 as well as 3 and 4 were statistically significant only as to 0.8° checks (p=0.03 and p=0.001, respectively). Responses to 16° checks were alike in all groups. We have also applied original formulas to determine the relative loss of RGCs and their axons and the congruence coefficient for morphological and functional parameters in normal aging and the early stage of POAG.

CONCLUSION

The discovered age-related PERG changes convincingly indicate a greater parvocellular RGC loss as compared to magnocellular. Thus, the PERG ratio (0.8°/16°) should be corrected for the subject's age. The proposed indices of relative decline in retinal light sensitivity and RNFL thinning have been shown to be useful for quantifying the loss of RGC bodies and axons in normal aging and early-stage glaucoma.

Normal tension glaucoma and Alzheimer disease: comorbidity?

PURPOSE

To investigate whether normal tension glaucoma (NTG) is associated with increased risk of developing dementia/Alzheimer disease (AD).

METHODS

A total of 69 patients with NTG were identified in the case note files in the Glaucoma Clinic, University Hospital of Copenhagen (Rigshospitalet), Denmark from 1980 to 2001. The rate of a subsequent diagnosis of dementia as recorded in nationwide health care registers from 1980 to 2005 was estimated and compared with the rate for the general population [standard mortality rate (SMR) = observed number of cases of dementia/expected number of cases of dementia].

RESULTS

The 69 patients had a median age of 64 years (quartiles 57 and 73) at the time of the diagnosis, and the average follow-up period was 12.7 years. The SMR(dementia) was 0.20 (95% CI 0.01-1.11). No patient developed AD.

CONCLUSION

Normal tension glaucoma was not associated with an increased risk of developing dementia/AD.

Alzheimer's disease and primary open-angle glaucoma: is there a connection?

AIM

To present, through a thorough literature research, current and older scientific efforts to investigate the putative association between Alzheimer's disease (AD) and glaucoma, especially primary open-angle glaucoma (POAG).

METHODS

We included in our review article epidemiological, experimental and clinical laboratory studies.

RESULTS

While many authors support the existence of a strong correlation between the AD and POAG, based on epidemiological, genetic and immunohistochemical data, others present contradictory results, leaving the issue unresolved.

CONCLUSION

Further research, probably targeted towards genetic parameters and based on large, multicenter studies has yet to be conducted. It is the authors' opinion however, that the existing data already justify the need for at least some degree of elevated clinical alertness for the occurrence of AD in patients with glaucoma and of glaucoma in patients with AD.

Neurodegenerative diseases of the retina and potential for protection and recovery

Recent advances in our understanding of the mechanisms in the cascade of events resulting in retinal cell death in ocular pathologies like glaucoma, diabetic retinopathy and age-related macular degeneration led to the common descriptive term of neurodegenerative diseases of the retina. The final common pathophysiologic pathway of these diseases includes a particular form of metabolic stress, resulting in an insufficient supply of nutrients to the respective target structures (optic nerve head, retina). During metabolic stress, glutamate is released initiating the death of neurones containing ionotropic glutamate (N-methyl-D-aspartat, NMDA) receptors present on ganglion cells and a specific type of amacrine cells. Experimental studies demonstrate that several drugs reduce or prevent the death of retinal neurones deficient of nutrients. These agents generally block NMDA receptors to prevent the action of glutamate or halt the subsequent pathophysiologic cycle resulting in cell death. The major causes for cell death following activation of NMDA receptors are the influx of calcium and sodium into cells, the generation of free radicals linked to the formation of advanced glycation endproducts (AGEs) and/or advanced lipoxidation endproducts (ALEs) as well as defects in the mitochondrial respiratory chain. Substances preventing these cytotoxic events are considered to be potentially neuroprotective.

Conventional MRI and magnetisation transfer imaging of the brain and optic pathway in primary open-angle glaucoma

Neuropathological studies in experimental and human glaucoma have shown degenerative changes in the optic pathway. The purpose of the study was to evaluate, with conventional MRI and magnetisation transfer imaging, the brain and the optic pathway of patients with primary open-angle glaucoma (POAG). 26 patients, aged 67.4+/-8.6 years, and 26 control subjects were studied. The presence of white matter hyperintensities (WMH) was evaluated on fluid-attenuated inversion recovery images of the brain. The area of the optic nerves was assessed on coronal short tau inversion recovery images. Magnetisation transfer ratio (MTR) was measured in the chiasm and in the grey and white matter (CGM and CWM) of the calcarine fissure. More WMH were observed in patients (total 261, mean 10.8, standard deviation 12.7) than in control subjects (total 127, mean 4.7, standard deviation 5.7; p<0.001). The area (mm(2)) of optic nerves (10.7+/-5.7) and the MTR (%) of the chiasm (53.7+/-8.4), the CWM (60.9+/-4.2) and the CGM (53.6+/-5.6) were all lower in patients than in control subjects (13.6+/-4.3, 62.1+/-6.2, 67.6+/-8.6 and 57.0+/-4.6, respectively; p<0.05). The area of optic nerves showed significant correlation with the MTR of the chiasm (R = 0.41), the MTR of the CGM (R = 0.33), the MTR of the CWM (R = 0.34) and the cup to disc ratio (R = -0.46). POAG leads to optic nerve atrophy and degeneration of the optic pathway. The finding of an increase in the number of WMH suggests that cerebrovascular disease may play a role in the pathogenesis of POAG.

Analysis of glaucoma-related mortality in the United States using death certificate data

PURPOSE

Recent studies have reported a connection between glaucoma and decreased survival. To evaluate the underlying causes of glaucoma-related mortality, we searched vital records data for deaths citing glaucoma and reviewed the demographic variables and comorbidities contained on these records.

MATERIALS AND METHODS

Deaths including glaucoma, as either an underlying cause or a contributing cause of death, were selected from US multiple-cause-of-death data for the years 1990 to 2003 and combined with population data from the US Census Bureau to calculate mortality rates. Logistic regression was used to determine whether reporting of accidents and/or selected systemic disorders are associated with glaucoma on the death certificate.

RESULTS

Fifteen thousand two hundred twenty-eight glaucoma-related deaths (0.05%) were identified during the years under study. Black males had the highest glaucoma-related mortality rate with 9.4 deaths per 1,000,000 persons annually, whereas Hispanic females had the lowest mortality rate at 1.8 deaths per 1,000,000. After adjusting for age, sex, and race/ethnicity, positive associations were found between glaucoma and hypertension [Odds ratio (OR): 4.89; 95% confidence interval (CI)=4.73-5.05], diabetes (OR: 2.60; 95% CI=2.50-2.71), asthma (OR: 3.14; 95% CI=2.72-3.62), and accidents of all types (OR: 1.45; 95% CI=1.35-1.55).

CONCLUSIONS

Glaucoma is an important contributor to mortality for certain individuals. The disparities in mortality rates observed among race/ethnic strata may be attributed to differences in access to care as well as true differences in disease incidence and/or severity among racial groups. Despite limitations with the data, our findings suggest associations between glaucoma and a number of comorbid conditions. These associations should be explored in future studies and serve to guide strategies for disease management and prevention.

No Increased Risk of Developing Alzheimer Disease in Patients With Glaucoma

PURPOSE

To investigate whether primary open-angle glaucoma (POAG) is associated with increased risk of developing Alzheimer disease (AD).

METHODS

In a nationwide case register linkage study of patients with hospital admission or outpatient contact during the period from 1977 to 2001 in Denmark, the rate of subsequent AD for patients with a diagnosis of POAG was compared with the rate for patients with primary angle-closure glaucoma (PACG), cataract, and osteoarthritis (OA) and with the rate for the general population.

RESULTS

A total of 11,721 patients with a diagnosis of POAG (including normal tension glaucoma), 5975 patients with PACG, 162,640 patients with cataract, and 230,208 patients with OA were identified in the registers. Patients with POAG did not have increased rate of subsequent AD compared to patients with PACG, cataract, or OA or compared with the general population.

CONCLUSIONS

POAG was not associated with increased risk of developing AD. It cannot be excluded that this negative finding is due to diagnostic misclassification as register data were used.

Association of Apolipoprotein E Polymorphisms With Normal Tension Glaucoma in a Chinese Population

PURPOSE

To evaluate the role of apolipoprotein E (APOE) polymorphisms in primary open angle glaucoma (POAG).

METHODS

A cohort of 400 unrelated Chinese POAG patients was examined, including 294 cases of high tension glaucoma (HTG) and 106 with normal tension glaucoma (NTG). Also studied were 300 unrelated Chinese control subjects. The genotypes of the APOE polymorphisms in exon 4 and in the promoter at positions -491, -427, and -219 were determined by polymerase chain reaction and restriction endonuclease analysis. Frequencies of the genotypes were compared between patients and controls by chi test or Fisher exact test. The association of APOE polymorphisms with POAG phenotypes including age at diagnosis, intraocular pressure (IOP) at diagnosis, highest IOP, cup-disc ratio, and visual field score was investigated by the Kruskal-Wallis test.

RESULTS

No significant difference was detected in the frequencies of APOE promoter polymorphisms between POAG patients and control subjects (P>0.0125). For the exon 4 polymorphism, when compared with control subjects, the frequency of epsilon 4 carriers was significantly lower in patients with NTG (P=0.008; odds ratio=0.36, 95% confidence interval=0.17, 0.79) but not in HTG (P=0.07). Compared with -219TT, the -219G carriers had a significant higher age at diagnosis (P=0.0046). No significant association was found between other APOE polymorphisms and POAG phenotypes (P>0.07).

CONCLUSIONS

Our findings suggest that the APOE epsilon 4 allele confers a protective effect against NTG, whereas the APOE promoter polymorphisms do not contribute to POAG risk. However, the APOE -219G carriers tended to have later-onset POAG.

Effect of oral donepezil on intraocular pressure in normotensive Alzheimer patients

Donepezil, a selective acetylcholinesterase (AChE) inhibitor, has been shown to reduce intraocular pressure (IOP) in ocular normotensive rabbit eyes. The aim of this investigation was to evaluate the effect of oral donepezil on IOP and pupil diameter after mid-term oral treatment in normotensive persons. Thirty-two newly diagnosed Alzheimer patients with normal IOP and no further antiglaucomatous treatment were included in the study. IOP and pupil diameter were evaluated before and 4 weeks after daily intake of 5 mg donepezil. IOP and pupil diameter were significantly lower/smaller after 4 weeks of treatment. The mean IOP of all 63 eyes was 14.1 mmHg before and 12.8 mmHg after treatment (8.8% reduction). Mean pupil diameter constricted from 3.9 to 3.6 mm (-7.4%). These findings show that donepezil, and, possibly, other selective AChE inhibitors, can potentially be used to treat glaucoma. They are also known to have neuroprotective effects in Alzheimer's, and, therefore, might have an additional therapeutic benefit.

Alpha-fodrin is cleaved by caspase-3 in a chronic ocular hypertensive (COH) rat model of glaucoma

PURPOSE

alpha-Fodrin is a neuronal cytoskeletal protein and a known caspase-3 target. We sought to determine whether caspase-3 cleaves alpha-fodrin in COH rat retinas and whether this process is reduced by adeno-associated virus (AAV)-induced retinal ganglion cell expression of baculovirus inhibitory repeat-containing 4 (BIRC4), a potent caspase-3 inhibitor.

METHODS

Ocular hypertension was induced unilaterally in five rat eyes by limbal injection of hypertonic saline. In a similar experiment, ocular hypertension was induced in four eyes pre-treated with an intravitreal injection of AAV-BIRC4 to assess alpha-fodrin cleavage. Western immunoblotting was performed on all retinas.

RESULTS

Caspase-3 cleavage of alpha-fodrin yields a specific 120kDa protein fragment. COH retina immunoblots indicated significantly more caspase-3 cleavage of alpha-fodrin than controls (P < 0.01, paired t-test). Inhibition of retinal caspase-3 activity with BIRC4 reduced caspase-3-mediated alpha-fodrin cleavage compared to controls.

CONCLUSIONS

This confirms our previous finding of caspase-3 cleavage of alpha-fodrin in COH retinas and parallels pathology seen in Alzheimer's disease, in which neurons undergo chronic caspase activation, slow build-up of cleavage products, and delayed apoptosis. If caspase activation in glaucoma leads to protracted rather than rapid retinal ganglion cell apoptosis, a much longer therapeutic window exists for apoptosis inhibition with caspase inhibitors such as BIRC4.

Alzheimer's peptide and serine proteinase inhibitors in glaucoma and exfoliation syndrome

The occurrence of inflammation with accompanying amyloid formation in pseudoexfoliation syndrome (PEX) resembles other inflammation-associated amyloidoses such as Alzheimer's disease (AD). To test whether the same proteins can be identified in PEX as in AD, we qualitatively analysed for Alzheimer's peptide (Abeta1-42) and the proteinase inhibitors alpha1-antichymotrypsin (ACT) and alpha-antitrypsin (AAT) in the aqueous humor of patients with and without PEX material. Ninety aqueous humor samples were collected from patients in the age group between 46 and 95 during cataract surgery. Protein profiles in samples were analysed by electrophoresis followed by Western blotting. Blots were developed using specific antibodies against Abeta1-42, AAT and ACT and peroxidase-conjugated IgG as a second antibody. At least one of the analysed proteins was found in 68.8% of 90 cases studied. Abeta1-42 peptide was found in 22.2% of all cases, among them in seven cases with PEX (total n =16) and in four with glaucoma (total n = 10). ACT and AAT were detected in 17.8 and 28.9% of all cases, respectively. In addition, female patients had significantly higher frequencies of detected ACT and AAT, compared to males. Abeta1-42, ACT and AAT were also found in 17.6, 14.7 and 23.5% of the control (non-XF and non-glaucoma) samples (n = 68). Alzheimer's peptide is present in the aqueous humor of patients with PEX and glaucoma suggesting that these diseases may share common features in the biochemistry and etiologies with AD. The presence of Abeta and inflammation-associated proteins in aqueous from cataract cases without detectable PEX raises the possibility that these proteins may reflect early amyloid-related changes in the eye.

Hypothesis for a common basis for neuroprotection in glaucoma and Alzheimer's disease: anti-apoptosis by alpha-2-adrenergic receptor activation

Recent studies have suggested glaucomatous loss of retinal ganglion cells and their axons in Alzheimer's disease. Amyloid beta peptides and phosphorylated tau protein have been implicated in the selective regional neuronal loss and protein accumulations characteristic of Alzheimer's disease. Similar protein accumulations are not present on glaucomatous retinal ganglion cells. Neurons die in both Alzheimer's disease and glaucoma by apoptosis, although the signaling pathways for neuronal degradation appear to differ in the two diseases. Alzheimer's disease features a loss of locus ceruleus noradrenergic neurons, which send axon terminals to the brain regions suffering neuronal apoptosis and results in reductions in noradrenaline in those regions. Activation of alpha-2 adrenergic receptors reduces neuronal apoptosis, in part through a protein kinase B (Akt)-dependent signaling pathway. Loss of noradrenaline innervation facilitates neuronal apoptosis in Alzheimer's disease models and may act similarly in glaucoma. Alpha-2 adrenergic receptor agonists offer the potential to slow the neuronal loss in both diseases by compensating for lost noradrenaline innervation.

Synucleins in glaucoma: implication of gamma-synuclein in glaucomatous alterations in the optic nerve

Synucleins are small proteins associated with neurodegenerative diseases and some forms of cancer. They are studied predominantly in the brain; information about their presence and functions in ocular tissues is scarce. Here we describe the localization of three members of the synuclein family in the optic nerve of donors with different types of glaucoma compared with control samples from donors without ocular diseases. We did not find significant differences in the localization of alpha- and beta-synucleins in the optic nerve or retina of glaucoma patients compared with controls, whereas considerable redistribution of gamma-synuclein occurred in the glaucomatous optic nerve compared with control eye without glaucoma. In the optic nerve from control and glaucomatous individuals, nerve bundles are immunopositive for gamma-synuclein; however, a strong gamma-synuclein-immunopositive staining in a subset of glial cells was observed in the lamina and postlamina cribrosa regions of the optic nerve only in glaucoma patients. In the optic nerve of rats with episcleral vein cauterization used as an animal model of glaucoma, the quantity of both gamma-synuclein mRNA and protein was decreased compared with the optic nerves of control animals. Incubation of rat astrocyte culture at elevated hydrostatic pressure reduced the amount of gamma-synuclein but did not affect the quantities of actin and glial fibrillary acidic protein. These data suggest that significant changes in the pattern of expression and/or localization occur in the glaucomatous optic nerve for gamma-synuclein but not for alpha- and beta-members of the synuclein family.

Topical rivastigmine, a selective acetylcholinesterase inhibitor, lowers intraocular pressure in rabbits

Non-selective acetylcholinesterase (AChE) inhibitors are known hypotensive agents. The purpose of the present investigation was carried out to ascertain whether rivastigmine, a selective carbamate-type inhibitor of AChE, which inhibits selectively an isoform of this enzyme found almost exclusively in the central nervous system, is able to depress the intraocular pressure (IOP) in normotensive rabbits. IOP was monitored with a TonoPen XL in conscious adult rabbits before and hourly up to 8 hr after administration of the drug. Baseline measurements without treatment and after one single topical application of rivastigmine [1% (n=8); 2% (n=4); and 5% (n=6)] to the right eye and of the vehicle alone to the left one were performed. Rivastigmine reduced the IOP of treated eyes significantly (p<0.05) in a dose-independent manner. Maximal effects of 23.2% (5% rivastigmine), 19.6% (2% rivastigmine) and 15.2% (1% rivastigmine) were achieved 1, 3 and 5 hr after application of the drug. A non-significant reduction of IOP in the contralateral eye was also observed. Rabbits evidenced no signs of discomfort after administration of rivastigmine. No conjunctival discharge or other signs of drug related local toxicity were found. Rivastigmine, a selective antagonist of AChE, lowers IOP significantly and may thus be of potential use in glaucoma therapy.