This study demonstrated an increased accumulation of APP and Aβ in ocular structures in a spontaneous mouse model (DBA/2J) of long-lasting, chronic glaucoma. Using an antibody detecting Aβ 40, not the classical Aβ plaque formation, as in AD, was found in these retinas. This might have occurred because Aβ 40 is more hydrophilic and has a lesser tendency to aggregate than Aβ 42. McKinnon
16 analyzed both forms of Aβ by Western blotting in an induced glaucomatous rat model and found exclusively higher Aβ 40 than Aβ 42 levels in ocular structures.
Elevated APP and Aβ accumulation was found in distinct ocular tissues, such as the RGC layer, the ON, and abundantly in the pial/dural complex. Consistent with the results of McKinnon,
17 the accumulation of APP and Aβ 40 in the ON was distinctively adjacent to the lamina cribrosa and was highest in the area behind the lamina cribrosa, suggesting traumatic alterations to the ON probably because of elevated IOP. Recently, other groups have also reported similar APP and Aβ distributions in mouse and rat glaucoma models (Schmid P, et al.
IOVS 2004;45:ARVO E-Abstract 4684; Cordeiro M, et al.
IOVS 2006;47: E-Abstract 2698).
The highest levels of Aβ 40 were found in the area of the pial/dural tissue around the optic nerve, behind the lamina cribrosa of glaucomatous mice, where the arterioles and venules were located. It has been shown that Aβ can be isolated from meningeal arterioles/venules and from capillaries within the cerebral cortex in amyloid microangiopathy found in patients with AD.
18 19 This angiopathy was ultrastructurally characterized by amyloid fibrils found in the abluminal basement membrane of the vessels, with some extension into the surrounding perivascular neuropil. The immunohistochemically identified Aβ filaments were, as in our study, primarily Aβ 40.
20
Changes in the pia/dura might have corresponded to a failure of APP elimination through their elimination pathways in glaucoma, additionally worsened by increased age. APP overload thereafter led to cytotoxic Aβ accumulation. Martin et al.
21 showed that disruption of dynein transport in glaucoma contributes to a failure of retrograde axonal transport and thus may be a contributing factor to RGC death. The distribution pattern of dynein accumulation described by Martin et al.
21 resembled ours found for APP. APP interacts with cytoplasmic kinesin, possibly as a transport cargo adaptor, and appears to be directly involved in the disease,
22 which may explain the similar distribution pattern found.
Interestingly, in the RGC layer, a higher proportion of APP and a lesser extent of Aβ accumulation was observed, possibly because of the specific neurofilament (NF)-triplet content in RGCs, explaining their higher vulnerability.
23 24 AD is characterized not only by plaque formation but also by neurofibrillar tangles, which consist of altered neuronal cytoskeletal proteins such as NF and tau.
25 26 The vulnerability of neurons could be delineated by their content of NF.
27 The subpopulation containing NF in the human retina likely corresponds to large ganglion cells.
24 Loss of these NF protein-containing cells were evident in a glaucoma model, in which they showed a heightened vulnerability to degeneration.
23 As mentioned, the larger human RGCs are characterized by their content of NF-triplet proteins.
24 Similar to the more vulnerable neurons of AD, such as the hippocampal neurons containing NF,
26 28 the larger RGCs are preferentially affected by increased IOP, especially those located in the periphery of the retina.
29 Some cells may be more susceptible to damage because of their specific NF content. In addition, caspase-3 activity has been colocalized with abnormal NF-triplet proteins, described not only in the hippocampus of AD brains but also in the large RGCs.
23 28 We are aware that a large number of displaced amacrine cells are present in the RGC layer of the rat retina, which can constitute up to 50% of the total cells in areas of the RGC layer. Therefore, this hypothesis must be proven further with specific stains for RGC colocalizing them with APP or Aβ.
30
No difference could be found comparing old with young control mouse retinas (C57/BL-6). Therefore, we excluded the possibility that the high accumulation of APP and Aβ in the old glaucoma mice were attributed to the physiological aging processes. Generally, low levels of APP and Aβ were also found in the age-matched C57/BL-6 control mice. Older mice seemed to have higher levels than younger ones. This trend probably represents a physiological accumulation, in ranges normally found in aging neuronal tissue. Amyloid deposits were described to a lesser extent in the brains of elderly persons without AD.
31
Our findings point to a probable correlation of APP/Aβ accumulation and glaucoma, suggesting that an APP altered metabolism plays a role in the pathophysiology of RGC death in glaucoma. Aβ accumulation subsequent to ischemia or mechanical trauma, such as elevated IOP, with the consequence of heat shock protein elaboration, was shown in several studies.
32 33 34 35 36 Not only could this explain the correlation between elevated IOP and optical neuropathy, it might help to better understand normal-pressure glaucoma or unresponsive primary open-angle glaucoma.
Disruption of the homeostatic properties of secreted APP with consecutive Aβ cytotoxicity might be a contributing factor of sustaining apoptotic cell death in glaucomatous mouse retinas. Hence, new medical treatment modalities for glaucoma may warrant further study, including the known neuroprotective anti-Alzheimer drugs (cholinesterase inhibitors, memantine) and the recently developed “anti-amyloid” therapeutic strategies, which decrease Aβ production by secretase inhibitors
37 and caspase inhibitors
38 or by interfering with Aβ aggregation through Aβ vaccination.
39 40 41 42
The authors thank Mathias Jucker (Neuropathology, Hertie-Institut für klinische Hirnforschung, Tuebingen, Germany) and Mathias Staufenbiel (Novartis, Basel, Switzerland) for generously providing brain tissue of APP23 transgenic mice and Paolo Paganetti (Novartis, Basel, Switzerland) for the kindly supplying the primary antibodies. They also thank Jürg Kummer, Anastasia Amoo, Anezka Chrenkowa, and Aniela Olac, members of the laboratory, for expert technical and research assistance, and Andrew Goldman, (Boulder, CO) and Istvan Vajtai (Institute of Pathology, University of Bern, Bern, Switzerland) for their helpful collaboration in the writing of this paper.