Retinal ischemia and cell proliferation in the rat: the role of soluble mitogens

Temporary retinal ischemia in the rat leads to a proliferation of endothelial cells and glial cells. We tested the hypothesis that this proliferation is caused by a release of soluble mitogens from the ischemic retina. Conditioned media were prepared from normal rat retina and from retina that had undergone 2 h ischemia and 48 h reperfusion, at which time it showed intense mitotic activity. The conditioned media were placed in cultures of fibroblasts, bovine adrenal capillary endothelial cells, and rat brain astrocytes. Cell proliferation in vitro was stimulated by the retinal extracts in all cell cultures. However, the cell proliferation in cultures with conditioned media from normal retina was similar to that in cultures with conditioned media from ischemic and proliferating retina. Although these data are consistent with the presence of soluble growth factors in the retina, they also indicate that release of these growth factors into the surrounding milieu after transient retinal ischemia is not altered to a degree that would explain the dramatic increase in mitosis.Supported by Research to Prevent Blindness Inc., The National Eye Institute (EY05903 and R01-07001), The Adler Foundation, and the American Federation For Aging Research, Inc     

Retinal glial cell proliferation and ion channels: a possible link

Under a variety of pathological conditions, glial cells of the retina proliferate. Although glial cell proliferation may play a role in a number of important retinal disorders, the regulation of this proliferative response is understood poorly. We examined the possibility that the function of certain ion channels in retinal glial cells may be linked with the induction of proliferation. Experiments were performed on glial cells derived from the adult rat retina and maintained in culture. The cells stained positively by immunocytochemistry with antibodies specific for glial fibrillary acidic protein and for Muller cells. Patch clamp studies of these retinal glial cells demonstrated the presence of a large conductance, calcium-activated potassium channel that is sensitive to tetraethylammonium, a classical blocker of potassium channels. The activity of this type of potassium channel was monitored by patch clamp recordings in the cell-attached configuration before and after exposure of the glial cells to a mitogenic conditioned medium. Exposure to this conditioned medium was associated with a marked increase in the activity of the ion channel. A possible link between the activity of potassium channels and the action of mitogens was suggested further by the finding that tetraethylammonium significantly blocked the proliferative response of retinal glial cells to the conditioned medium. Our findings support the hypothesis that there is a link between the biophysical processes involved in ion channel activity and the proliferation of retinal glial cells.     

Retinal vessels: proliferation of endothelium in vitro

Tissue culture of retinal vessels from fetal calf eyes produced colonies of endothelium, pericytes, and smooth muscle. Identification of endothelial cells was based upon culture morphology, [3H]thymidine labeling of isolated vessels, and factor VIII immunofluorescence. Thimerosal added to the culture medium destroyed pericytes and muscle cells, leaving only endothelial cell colonies in the primary cultures. This tissue culture system may be useful in the study of retinal vascular cell biochemistry and pathophysiology.     

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.     

Retinal neuromodulation: the role of dopamine.

Dopamine exerts multiple effects on retinal horizontal cells. Dopamine, via cyclic AMP and protein kinase A, reduces the light responsiveness of horizontal cells and the electrical coupling between the cells. The gating kinetics of both gap-junctional and glutamate channels are altered as a result of phosphorylation by protein kinase A. Dopamine also causes a reversible retraction of neurites of horizontal cells maintained in culture. Diacylglycerol analogues as well as phorbol esters mimic this effect of dopamine, but not cyclic AMP analogues or Forskolin. The results suggest that dopamine causes neurite retraction by the activation of protein kinase C via diacylglycerol.     

Experimental ischemia induces cell mitosis in the adult rat retina

Experimental occlusion of the central retinal vessels in albino rats followed by reperfusion for 2 or 4 days resulted in a highly active proliferative response. Mitotic figures were produced in retinal vascular cells, presumed Müller cells, retinal pigment epithelial cells, and possibly other cell populations in the retina. A large number of mitoses were observed in 60-100% of 50 experimental eyes with 2, 4 or 6 hr of experimental ischemia and 2 or 4 days of reperfusion and in none of the 37 control eyes. Autoradiography demonstrated 3H-thymidine uptake in the retina and in vascular cells on a trypsin digest preparation of the retina. Retinal ischemia followed by reperfusion leads to mitosis of retinal cells. This suggests a causal relationship in proliferative retinopathies where ischemia and cell proliferation coincide.     

Retinal ganglion cell numbers and delayed retinal ganglion cell death in the P23H rat retina

The P23H-1 rat strain carries a rhodopsin mutation frequently found in retinitis pigmentosa patients. We investigated the progressive degeneration of the inner retina in this strain, focussing on retinal ganglion cells (RGCs) fate. Our data show that photoreceptor death commences in the ventral retina, spreading to the whole retina as the rat ages. Quantification of the total number of RGCs identified by Fluorogold tracing and Brn3a expression, disclosed that the population of RGCs in young P23H rats is significantly smaller than in its homologous SD strain. In the mutant strain, there is also RGC loss with age: RGCs show their first symptoms of degeneration at P180, as revealed by an abnormal expression of cytoskeletal proteins which, at P365, translates into a significant loss of RGCs, that may ultimately be caused by displaced inner retinal vessels that drag and strangulate their axons. RGC axonal compression begins also in the ventral retina and spreads from there causing RGC loss through the whole retinal surface. These decaying processes are common to several models of photoreceptor loss, but show some differences between inherited and light-induced photoreceptor degeneration and should therefore be studied to a better understanding of photoreceptor degeneration and when developing therapies for these diseases.     

Retinal ganglion cell numbers and delayed retinal ganglion cell death in the P23H rat retina

The P23H-1 rat strain carries a rhodopsin mutation frequently found in retinitis pigmentosa patients. We investigated the progressive degeneration of the inner retina in this strain, focussing on retinal ganglion cells (RGCs) fate. Our data show that photoreceptor death commences in the ventral retina, spreading to the whole retina as the rat ages. Quantification of the total number of RGCs identified by Fluorogold tracing and Brn3a expression, disclosed that the population of RGCs in young P23H rats is significantly smaller than in its homologous SD strain. In the mutant strain, there is also RGC loss with age: RGCs show their first symptoms of degeneration at P180, as revealed by an abnormal expression of cytoskeletal proteins which, at P365, translates into a significant loss of RGCs, that may ultimately be caused by displaced inner retinal vessels that drag and strangulate their axons. RGC axonal compression begins also in the ventral retina and spreads from there causing RGC loss through the whole retinal surface. These decaying processes are common to several models of photoreceptor loss, but show some differences between inherited and light-induced photoreceptor degeneration and should therefore be studied to a better understanding of photoreceptor degeneration and when developing therapies for these diseases.     

Retinal angiomatous proliferation: clinical characteristics and treatment options.

BACKGROUND: A new form of exudative age-related macular degeneration (ARMD), retinal angiomatous proliferation (RAP), has been described in which neovascularization begins in the deep retina, extends through the subretinal space, and eventually communicates with choroidal neovascularization. METHODS: Case series. RESULTS: Common clinical features of RAP include small multiple intra-retinal hemorrhages, intra-retinal edema, vascularized pigment epithelial detachments (PEDs), and retinal choroidal anastomosis (RCA). Fluorescein angiography (FA) reveals ill-defined, occult choroidal neovascularization. Indocyanine green (ICG) angiography is useful in early stages because 'hot spots' can be detected before clinical or FA characteristics are present. Optical coherence tomography (OCT) is useful in illustrating some of the clinical and FA characteristics. The use of photodynamic therapy (POT), combined with intravitreal triamcinolone injection, was successful in stabilizing the RAP lesion in one case discussed in this report. CONCLUSIONS: Retinal angiomatous proliferation is a newly recognized entity of exudative age-related macular degeneration with its own set of clinical, FA, ICG angiography, and OCT features. Experimental treatments such as the use of PDT combined with intravitreal triamcinolone injection demonstrate potential success with this entity. The biggest hope appears to be anti-angiogenic factors currently in clinical trials for the treatment of exudative ARMD.     

Retinal angiomatous proliferation: association with clinical and angiographic features

AIM: It was the aim of this study to evaluate the frequency of retinal angiomatous proliferation (RAP) and its association with specific clinical and angiographic characteristics in age-related macular degeneration (AMD). METHODS: A consecutive series of 126 newly diagnosed patients with exudative AMD was reviewed retrospectively. All underwent a complete ophthalmic examination, a red-free photograph and fluorescein angiography. Most patients (85/126) underwent indocyanine green choroidal angiography (ICGA). RAP was diagnosed when a connection between the retinal vasculature and the neovascular complex was recognized angiographically. RESULTS: Out of 126 patients with recent neovascular AMD, 17 had RAP (13.5%; 95% CI 8.1-20.7). The study eye of patients with RAP had more frequent hemorrhages (88.2 vs. 59.6%; p = 0.027), hard exudates (82.4 vs. 26.6%; p < 0.001), pigment epithelium detachment (64.7 vs. 23.8%; p = 0.001) and a hot spot in ICGA (70.6 vs. 22.1%; p < 0.001) with respect to the other forms of exudative AMD. Hemorrhages were more frequently superficial, multiple and within the edge of the lesion in the RAP group. Bilateral AMD was more common in the RAP group (70.6 vs. 38.0%; p = 0.011). No statistically significant differences were found regarding sex, age and visual acuity. CONCLUSION: RAP represents a common lesion in patients with neovascular AMD referred to a tertiary care clinic. The recognition of hemorrhages, hard exudates, pigment epithelium detachment or a hot spot in ICGA can assist a correct diagnosis.     

Retinal proliferation response in the buphthalmic zebrafish, bugeye

The zebrafish retina regenerates in response to acute retinal lesions, replacing damaged neurons with new neurons. In this study we test the hypothesis that chronic stress to inner retinal neurons also triggers a retinal regeneration response in the bugeye zebrafish. Mutations in the lrp2 gene in zebrafish are associated with a progressive eye phenotype (bugeye) that models several risk factors for human glaucoma including buphthalmos (enlarged eyes), elevated intraocular pressure (IOP), and upregulation of genes related to retinal ganglion cell pathology. The retinas of adult bugeye zebrafish showed high rates of ongoing proliferation which resulted in the production of a small number of new retinal neurons, particularly photoreceptors. A marker of mechanical cell stress, Hsp27, was strongly expressed in inner retinal neurons and glia of bugeye retinas. The more enlarged eyes of individual bugeye zebrafish showed disrupted retinal lamination, and a persistent reduced density of neurons in the ganglion cell layer (GCL), although total numbers of GCL neurons were higher than in control eyes. Despite the presence of a proliferative response to damage, the adult bugeye zebrafish remained behaviorally blind. These findings suggest the existence of an unsuccessful regenerative response to a persistent pathological condition in the bugeye zebrafish.     

Retinal ischemia and reperfusion causes capillary degeneration: similarities to diabetes

PURPOSE: Retinal neurons and vasculature interact with each other under normal conditions, and occlusion of the retinal vasculature can result in damage to retinal neurons. Whether damage to the neural retina will damage the vasculature, however, is less clear. This study was conducted to explore the relationship between vascular and nonvascular cells of the retina. The response of the retinal vasculature to an injury (ischemia and reperfusion; I/R) that is known to cause neuronal degeneration was studied. METHODS: I/R injury to the retinas was induced in Lewis rats and C57BL/6J mice by elevating intraocular pressure (IOP), and reperfusion was established immediately afterward. Some rats were pretreated with aminoguanidine (AMG, 50 mg/Kg BW in drinking water) before the procedure. Poly(ADP-ribose) polymerase (PARP) activity and expression of inducible nitric oxide synthase (iNOS), and cycloxygenase-2 (COX-2) were measured by Western blot analysis, and levels of TNF-alpha and intercellular adhesion molecule (ICAM)-1 mRNA were measured by qPCR at 2 and 7 days after the procedure. Also at 2 and 7 days after the I/R injury, apoptosis of retinal neural cells (demonstrated by TUNEL assay), density of cells in the ganglion cell layer, and thickness of retinas were quantitated, and the number of TUNEL-positive capillary cells and degenerated capillaries were assessed. Retinal neurodegeneration and capillary degeneration were also examined in C57BL/6J mice 2, 5, 8, and 14 days after I/R injury. RESULTS: As expected, loss of cells in the retinal ganglion cell layer was apparent 2 days after I/R injury in the rat and mouse models. In contrast, the retinal vasculature had essentially no pathology at this time in either model. Surprisingly, the number of degenerated capillaries increased greatly by 7 to 8 days after the injury. Administration of aminoguanidine significantly inhibited the I/R-induced capillary degeneration as well as neurodegeneration in the rat model. Retinal I/R caused increased PARP activity (detected by poly(ADP-ribosy)lated proteins), as well as upregulation of iNOS, COX-2, TNF-alpha, and ICAM-1 levels in rats, consistent with an inflammatory process. CONCLUSIONS: Capillary degeneration is an unrecognized component of acutely elevated IOP and develops only after neurodegeneration is severe. Thus, this finding raises the possibility that damage to the neural retina contributes to capillary degeneration. Aminoguanidine, a nonspecific inhibitor of iNOS, inhibited I/R-induced degeneration of both neuronal and vascular cells of the retina. The model of retinal ischemia and reperfusion will be a useful tool for investigating the relationship between neuronal damage and vascular damage in glaucoma and other diseases such as diabetic retinopathy.     

Retinal cell transplantation in the macula: New techniques

Transplantation of RPE allografts into the submacular space after the removal of neovascular membranes in “wet” forms of ARMD usually leads to chronic leakage around the transplant, whereas this seldom occurs in transplants in “dry” forms of ARMD. The former may be the result of chronic rejection because of a compromised blood/retinal barrier. This is supported by the fact that visual function remains over the transplant in the dry forms of ARMD at 6 months after transplantation. Whether this function can slow or reverse the progression of ARMD remains to be seen. The possibility of transplantation of photoreceptors in retinal degenerations is also reviewed.     

Retinal angiomatous proliferation in age-related macular degeneration

BACKGROUND: It is known that choroidal neovascularization (CNV) in age-related macular degeneration (ARMD) may erode through the retinal pigment epithelium, infiltrate the neurosensory retina, and communicate with the retinal circulation in what has been referred to as a retinal-choroidal anastomosis (RCA). This is extremely common in the end stage of disciform disease. In recent years, the reverse also seems to be possible, as angiomatous proliferation originates from the retina and extends posteriorly into the subretinal space, eventually communicating in some cases with choroidal new vessels. This form of neovascular ARMD, termed retinal angiomatous proliferation (RAP) in this article, can be confused with CNV. PURPOSE: The purpose of this article is 1) to review the clinical and angiographic characteristics of a series of patients with RAP and 2) to propose a theoretical sequence of events that accounts for the neovascularized process. METHODS: In this retrospective clinical and angiographic analysis, 143 eyes with RAP (108 patients) were reviewed and classified based on their vasogenic nature and course. Clinical biomicroscopic examination, fluorescein angiography, and indocyanine green angiography were used to evaluate patients. RESULTS: The results of this series suggest that angiomatous proliferation within the retina is the first manifestation of the vasogenic process in this form of neovascular ARMD. Dilated retinal vessels and pre-, intra-, and subretinal hemorrhages and exudate evolve, surrounding the angiomatous proliferation as the process extends into the deep retina and subretinal space. One or more dilated compensatory retinal vessels perfuse and drain the neovascularization, sometimes forming a retinal-retinal anastomosis. Fluorescein angiography in these patients usually revealed indistinct staining simulating occult CNV. Indocyanine green angiography was useful to make an accurate diagnosis in most cases. It revealed a focal area of intense hyperfluorescence corresponding to the neovascularization ("hot spot") and other characteristic findings. Based on understanding of the nature and progression of the neovascularized process, patients with RAP were classified into three vasogenic stages. Stage I involved proliferation of intraretinal capillaries originating from the deep retinal complex (intraretinal neovascularization [IRN]). Stage II was determined by growth of the retinal vessels into the subretinal space (subretinal neovascularization [SRN]). Stage III occurred when CNV could clearly be determined clinically or angiographically. A vascularized pigment epithelial detachment and RCA were inconsistent features of this stage. CONCLUSIONS: Retinal angiomatous proliferation appears to be a distinct subgroup of neovascular ARMD. It may present in one of three vasogenic stages: IRN, SRN, or CNV. Whereas ICG angiography is helpful in diagnosing RAP and in documenting the stage of the neovascularized process, it is frequently difficult to determine the precise nature and location of the new vessel formation. It is important for clinicians to recognize the vasogenic potential and the associated manifestations of this peculiar form of neovascular ARMD so that a proper diagnosis can be made, and when possible, an appropriate management administered.     

Retinal angiomatous proliferation in age-related macular degeneration

BACKGROUND: Angiomatous proliferation in ARMD originates from the retina and extends into the subretinal space. Retinal angiomatous proliferation (RAP) is diagnosed with angiography. This study investigates the characteristics of RAP in optical coherence tomography (OCT) and correlates them with clinical and fluorescein angiographic findings. METHODS: 327 consecutive patients with age-related macular degeneration (ARMD) were retrospectively examined using a standardised protocol. The protocol included best corrected visual acuity (BCVA), binocular ophthalmoscopy, fundus photography, fluorescein angiography (FAG) and OCT. The OCT and angiographic findings were graded in 3 RAP stages. RESULTS: 32 of 327 (9.79 %) patients (45 eyes) had RAP. The age ranged from 56 to 90 years (median: 79 years). 9 eyes presented RAP stage I, 23 stage II and 13 stage III. BCVA ranged from 0.01 to 0.7 (median: 0.2). OCT foveal minimum was 136 to 722 microns (median: 327). 33 (73 %) eyes showed a detachment of retinal pigment epithelium (RPE). Macular oedema was found in 43 (96 %) eyes. Cystoid macular oedema was seen in 36 (80 %) eyes. In stage II, 22 eyes (96 %) showed an RPE detachment, in stage III 11 eyes (85 %). 11 (85 %) eyes in stage III showed retinal-choroidal anastomosis. CONCLUSIONS: RAP was found in 9.79 % of the patients with ARMD. The OCT is helpful in detection of RPE detachment, macular oedema and cystoid changes in RAP. RAP and retinal-choroidal anastomosis should be identified because of the possibly poor prognosis.     

Retinal angiomatous proliferation and intravitreal bevacizumab injection

PURPOSE: To evaluate the short-term efficacy and safety of intravitreal bevacizumab injection (IVBI) in patients with retinal angiomatous proliferation (RAP). METHODS: Seven eyes of 5 patients with RAP were included in this study. All of the eyes evidenced stage 2 RAP lesions, except for one eye with a stage 3 lesion. IVBI (1.25 mg/0.05 cc) were conducted at 4 or 6-week intervals. Complete ocular examinations, angiographic results and optical coherence tomographic findings before and after the IVBI were analyzed at baseline and upon the follow-up visits. RESULTS: Seven eyes were studied in 5 patients who had undergone IVBI. Partial (3 eyes) or complete (4 eyes) regression of RAP was noted after IVBI in all of the studied eyes. Visual acuity improved in 5 of the eyes, and was stable in 2 of the eyes. One eye evidenced severe intraocular inflammation after IVBI and a subsequent development of new RAP, which was controlled with vitrectomy and repeat IVBI. CONCLUSIONS: This treatment was effective over 6 months, stabilizing or improving visual acuity and reducing angiographic leakage. These short-term results suggest that IVBI may constitute a promising therapeutic option, particularly in the early stages of RAP.