Ординатура / Офтальмология / Английские материалы / The Retina and its Disorders_Besharse, Bok_2011

it probably does not function as a molecular motor. Clockregulated phosphorylation sites in LpMyo3 have also been identified. This information provides clues to how the clock might regulate LpMyo3 and leads to speculations regarding how the phosphorylation of LpMyo3 might influence photoreceptor function.

Clock input to LEs increases the phosphorylation of two sites within the motor-like domain of LpMyo3 within and near an actin-binding interface called loop2 (Figure 7). Interestingly, LpMyo3 autophosphorylates these same sites through an intermolecular mechanism. The presence of phosphorylation sites within the actin-bind- ing interface of a myosin is surprising. None of the other 22 classes of myosins that have been described to date becomes phosphorylated in this region. However, mutagenesis studies using other unconventional myosins indicate that changing the net charge in loop2 alters the affinity of myosin for actin. Phosphorylation of LpMyo3 within its actin-binding interface is predicted to reduce its actin affinity.

Actin is concentrated in the rhabdomeral microvilli. Functional consequences of a possible nighttime decrease in the affinity of LpMyo3 for microvillar actin are not yet known, but could be diverse. The Myo3 in Drosophila photoreceptors (neither inactivation nor afterpotential C (NINAC)) is thought to bind to and stabilize actin in rhabdomeral microvilli, and in Drosophila lacking NINAC, microvillar actin is fragmented or absent. These observations lead to speculations that LpMyo3 phosphorylation participates in circadian processes involving changes in the stability of rhabdomeral actin. One such process is lighttriggered transient rhabdom shedding, which involves a transient breakdown of rhabdomeral actin. As described above, although this process is triggered by light, it must be primed by clock input and the activation of PKA. The clock-driven phosphorylation of LpMyo3 by PKA may contribute to the priming event.

LpMyo3 is also a kinase that may phosphorylate one or more proteins involved in the photoresponse. This idea is consistent with observations from Drosophila, which show that the photoresponse is abnormal in flies expressing NINAC without its kinase domain. If a change in the affinity of LpMyo3 for actin produces a nighttime decrease in the concentration of LpMyo3 at the rhabdom, the level of phosphorylation of LpMyo3 substrates may also fall and modify the photoresponse. Thus, multiple effects of the clock could be mediated through the modulation of LpMyo3.

Conclusion

The eyes of Limulus, with their diverse structures, large photoreceptors, and relatively simple organization, have long served as useful preparations for studying basic mechanisms of vision. Fundamental and broadly relevant

aspects of photoreceptor function and sensory information processing were discovered through studies of the Limulus visual system. Moreover, studies using Limulus have revealed the fundamental importance of circadian rhythms for vision and the complex ways in which clockand light-driven processes can interact to produce the full range of diurnal changes in retinas. The effects of circadian rhythms observed in Limulus LE have many parallels in vertebrate retinas, including those on photoreceptor structure, the photoresponse, and membrane shedding. In addition, circadian clocks in both Limulus and vertebrates exert their influence in retinas by regulating the release of biogenic amines and the activity of cyclic- nucleotide-signaling pathways. Mechanisms through which clock-driven signaling pathways influence specific retinal functions are just beginning to be identified, and their discovery remains an important challenge.

See also: Circadian Metabolism in the Chick Retina; The Circadian Clock in the Retina Regulates Rod and Cone Pathways; Circadian Regulation of Ion Channels in Photoreceptors; Fish Retinomotor Movements; Genetic Dissection of Invertebrate Phototransduction; Phototransduction in Limulus Photoreceptors; Rod and Cone Photoreceptor Cells: Outer Segment Membrane Renewal.

Further Reading

Barlow, R. B., Jr. (1983). Circadian rhythms in the Limulus visual system.

Journal of Neuroscience 3: 856–870.

Barlow, R. B., Jr., Bolanowski, S. J., Jr., and Brachman, M. L. (1977). Efferent optic nerve fibers mediate circadian rhythms in the Limulus eye. Science 197: 86–89.

Barlow, R. B., Jr., Chamberlain, S. C., and Lehman, H. K. (1989). Circadian rhythms in the invertebrate retina. In: Stavenga, D. G. and Hardie, R. C. (eds.) Facets of Vision, pp. 257–280. Berlin: Springer.

Barlow, R. B., Jr., Chamberlain, S. C., and Levinson, J. Z. (1980). Limulus brain modulates the structure and function of the lateral eyes. Science 210: 1037–1039.

Battelle, B.-A. (2006). The eyes of Limulus polyphemus (Xiphosura, Chelicerata) and their afferent and efferent projections. Arthropod Structure and Development 35: 1–14.

Battelle, B.-A. (2008). Circadian rhythms in visual function in Limulus. In: Fanjul-Moles, M.-L. and Aguilar-Roblero, R. (eds.) Comparative Aspects of Circadian Rhythms, pp. 19–40. Kerala: Transworld Research Network.

Battelle, B.-A., Evans, J. A., and Chamberlain, S. C. (1982). Efferent fibers to Limulus eyes synthesize and release octopamine. Science 216: 1250–1252.

Calman, B. G. and Chamberlain, S. C. (1982). Distinct lobes of Limulus ventral photoreceptors. II. Structure and ultrastructure. Journal of General Physiology 80: 839–862.

Cardasis, H. L., Stevens, S. M., McClung, S., et al. (2007).

The actin-binding interface of a myosin III is phosphorylated in vivo in response to signals from a circadian clock. Biochemistry 46: 13907–13919.

Chamberlain, S. C. and Barlow, R. B., Jr. (1984). Transient membrane shedding in Limulus photoreceptors: Control mechanisms under natural lighting. Journal of Neuroscience 4: 2792–2810.

Chamberlain, S. C. and Barlow, R. B., Jr. (1987). Control of structural rhythms in the lateral eye of Limulus: Interactions of natural lighting

and circadian efferent activity. Journal of Neuroscience 4: 2794–2810.

Fahrenbach, W. H. (1975). The visual system of the horseshoe crab Limulus polyphemus. International Review of Cytology

41: 285–349.

Fein, A. and Payne, R. (1989). Phototransduction in Limulus

ventral photoreceptors: Roles of calcium and inositol trisphosphate.

In: Stavenga, D. G. and Hardie, R. C. (eds.) Facets of Vision, pp. 173–185. Berlin: Springer.

Hartline, H. K., Wagner, H. G., and Ratliff, F. (1956). Inhibition in the eye of Limulus. Journal of General Physiology 39: 651–673.

Yeandle, S. (1958). Evidence of quantized slow potentials

in the eyes of Limulus. American Journal of Ophthalmology

46: 82–87.

Glossary

Circinate lipid exudates – A round or oval assemblage of intraretinal lipid deposits that is often seen in macular edema, especially in diabetic macular edema. This presumably occurs because plasma exudes from a small retinal blood vessel, or a cluster of retina vessels, in a circular pattern around the abnormal vessel. The portion of this circle, farthest from the leaking vessels, is the thinnest, and as reabsorption of the fluid occurs at that location, the lipoproteins precipitate into the tissues, forming the visible exudates.

Cystoid macular edema – A form of macular edema in which much of the extracellular fluid collects in loculated spaces between the photoreceptor axons of Henle’s fiber layer, forming round or oval, fluidfilled spaces. These are called cystoid because they are not lined with a layer of epithelial cells, which would make them true cysts; hence, these spaces are cystoid and not cystic.

Fovea – The central portion of the macula that contains only specialized cone photoreceptors, whose outer segments are narrow and elongated, rather than cone shaped as are extrafoveal cones.

Foveal avascular zone (FAZ) – A specialization of the capillary network in the foveal region of the retinal vasculature in humans and higher primates. There are no blood vessels in the FAZ, presumably as an evolutionary development to permit unimpeded access of light rays to the foveal cone photoreceptors. The FAZ can be readily seen as an approximately circular avascular zone about 1 mm in diameter in retinal vascular digest preparations from enucleated eyes or in good-quality fluorescein angiograms in living eyes.

Foveola – The central portion of the fovea, in which the axons of the cone photoreceptors are swept to the side, leaving only their inner and outer segments exposed to the incoming rays of light.

Henle’s fiber layer – The obliquely directed axons of the foveal cone photoreceptors.

Irvine–Gass syndrome (aphakia, or pseudophakia, cystoid macular edema) – This entity was first described during the 1960s by the individuals whose eponymous names it bears. Presumably because of postoperative inflammation after cataract surgery, perifoveal capillaries begin to

leak and edema fluid collects in a cystoid pattern in the central macula. The condition is often self-limited but may lead to considerably reduced vision. As demonstrated by fluorescein angiography, it may be accompanied by breakdown of the blood–retinal barrier in the capillary circulation of the optic nerve head.

Macula – The central region of the retina that is specialized for high-resolution vision. It is found in humans and higher primates and is notable histologically for the fact that it contains multiple layers of ganglion cells, while the extramacular retina contains only a single layer.

Macular edema – The thickening of the macular retina by extravasation of fluid, often containing lipid components (which may precipitate into the tissues) from breakdown of the normal blood–tissue barrier at the level of the endothelial cells of the retinal blood vessels in the macula. Fluid can also extravasate from the choroidal capillaries through the retinal pigment epithelium barrier, but this usually produces collections of subretinal fluid, which is distinguishable from the intraretinal fluid of macular edema.

Randomized controlled clinical trial (RCT) –

A method of evaluating the efficacy of a new therapeutic procedure in which subjects with a disease under investigation volunteer to be allocated by random selection to a treatment group, or groups, which will receive the new therapy or therapies under investigation, or to a control group that receives either no treatment or treatment with a placebo or treatment by the method that has been accepted as the current standard of care. Evaluation of the results for each subject is done by observers who are masked, that is, who do not know to which of the treatment, or control, groups the various subjects belong. This method is considered the definitive way to determine the efficacy, or lack of efficacy, of a putative new therapy.

Macular edema, a swelling of the central portion of the human retina, or macula, is a common feature in many diseases. It is a widely recognized complication of diabetic retinopathy, where its prevalence (in the United States) may be of the order of 10% of all individuals with

diabetes, varying somewhat by ethnic group but not by type of diabetes (i.e., type 1 or type 2). Macular edema also occurs commonly in retinal vein occlusions, in uveitis, following cataract surgery, as an accompanying feature of some ocular tumors, in hereditary retinal degenerations, such as the retinitis pigmentosa syndromes and, rarely, as an isolated genetic entity apparently in and of itself.

Diagnosis of Macular Edema

Recognition of macular edema as an important clinical entity has been increasing in recent years, and, in particular, since the 1960s with the advent of a variety of new diagnostic techniques. These have included highmagnification, stereoscopic ophthalmoscopy using the slit lamp and several types of contact and noncontact lenses that allow the observer to visualize the edematous macula in a better manner; stereoscopic retinal photography; fluorescein angiography to enable visualization of circulatory dynamics and the physiology/pathology of the macular circulation, even at the capillary level; and, most recently, optical coherence tomography (OCT), a

rapid, noninvasive technique that permits accurate measurement of the thickness of the macula, and yields highresolution tissue sections of the maculae of living subjects with almost histologic clarity.

Anatomy of the Macula

The macula is an anatomic specialization of the neural retina that is unique to humans and to higher primates (Figure 1(a)–(d)). It is defined histologically by the presence of multiple layers of retinal ganglion cells (there is only a single layer of ganglion cells in the extramacular retina), and ophthalmoscopically as the region extending from the temporal margin of the optic nerve head to the center of the macula (the foveola, the central depression that is evident clinically in the normal retina), and then for an equal distance temporal to the foveola. In the superior–inferior direction, the macula extends between the two major temporal retinal vascular arcades. It is not clear why, of the entire extent of the human retina, the macula is particularly susceptible to such a large number of disorders, for example, the macular degenerations, or to edema in a large variety of diseases. Speculations entail

the likelihood that the macular retina, including both its neural elements and also the macular retinal pigment epithelium, has a higher level of metabolic activity because of its enhanced visual function, including a greater density of cone photoreceptors, a thicker ganglion cell layer, and a greater thickness of retinal pigment epithelial (RPE) cells; or that this enhanced activity necessitates greater blood flow with a greater likelihood of circulatory breakdown, especially in view of the very rich retinal capillary network within the macula and, particularly, in the perifoveal region. Finally, the anatomic specialization of the foveola, in which the foveolar cone axonal and synaptic processes are swept obliquely to all sides, permitting direct access of photic stimuli to the photoreceptor outer segments, may lead pathologically to the accumulation of extracellular edema fluid into ballooning spaces in this axonal region in the entity known as cystoid macular edema.

In studying macular edema in an experimental setting, investigators are hampered by the fact that the macula is an anatomic specialization of the retina that is limited to humans and some higher primates. While certain birds, for example, hawks and eagles, have a fovea, there is no macula in the sense that it is present in the human eye, and the blood circulation of the posterior avian globe is entirely different from that of humans.

Clinical Findings in Macular Edema

The thickening of the central retina is best appreciated by stereoscopic viewing methods, for example, slit-lamp ophthalmoscopy with a contact lens or a handheld high dioptric indirect ophthalmoscope lens. Until recently, multicenter clinical research studies, for example, the Early Treatment Diabetic Retinopathy Study (ETDRS) used stereoscopic retinal photography to evaluate macular thickening. However, relatively minimal central retinal thickening may not be apparent even to the experienced examiner and photographic methods are subject to variability. Therefore, the introduction of OCT, which is discussed in more detail below, has held recent very great interest, but it has also raised some unexpected questions.

In the absence of stereoscopic viewing methods, one can suspect the presence of macular edema by the presence of certain characteristic retinal lesions. Extensive intraretinal hemorrhages or microaneurysms (dilations of capillary-sized blood vessels, which appear ophthalmoscopically as tiny red dots) in the macular region are indicators of extravascular blood or plasma leakage with resultant macular edema (Figure 2(a) and (b)). An especially characteristic abnormality, particularly in individuals with diabetes, is the presence of intraretinal lipid deposits. These often have a circular (circinate) configuration (Figure 2(a)), which presumably is the result of

leakage of plasma from a central vascular lesion, or cluster of lesions (Figure 2(b)), with resorption of the edema fluid but not its lipid components around the periphery of the 360 circumference of the zone of leakage, where the thickness of the fluid layer is least and the less-soluble lipid therefore precipitates out of solution. Macular lipid, and edema, have characteristic appearances by OCT, and histologically, as demonstrated in Figure 2(c) and (d). It has been known for a number of years that lower blood-lipid values are associated with lesser degrees of macular edema in diabetic subjects, although it is not clear that normalizing blood lipids can entirely prevent the development of macular edema in diabetic subjects.

There have been reports, at least one of which preceded by a number of years the development of OCT for objective measurements of macular thickness, that macular edema can vary over the course of a day. Some patients have noted that their vision, on arising in the morning, was much poorer than later in the day. Several investigators, using OCT, have found that macular thickness in at least some patients with diabetic macular edema is greater immediately after arising from a night’s sleep than it is later. Although there are a number of possible causes, the most plausible is a simple gravitational effect, with edema fluid remaining in the upper body during recumbent posture and draining to the lower body when the individual is sitting or standing. However, although this diurnal effect does occur in some individuals, the Diabetic Retinopathy Clinical Research Network, in a study involving many institutions, found it to be infrequent.

Clinically Significant Diabetic Macular Edema

The ETDRS also introduced the term ‘‘clinically significant macular edema’’ to describe those cases in which the retinal thickening either involved the center of the macula (the fovea and foveola), or was sufficiently close to the center as to constitute a potential threat to the center with likely loss of central vision. The definition of clinically significant diabetic macular edema, as established by the ETDRS, is presented in Table 1.

Findings by Fluorescein Angiography

By intravenous fluorescein angiography, in which a solution of sodium fluorescein is injected intravenously through an antecubital vein and rapid sequence photographs are taken using fluorescence optics, it is possible to visualize some of the vascular dynamics of macular edema. Fluorescein is a small molecule (M.W. 327) that is only loosely bound to plasma proteins, for example, only about 65% to albumin, and this small size enables it to leak readily through patent interendothelial cell junctions. Hence, in successive frames of a fluorescein

angiogram in patients with diabetic retinopathy or other edema-producing retinal vascular diseases, one can see an increasing fluorescent haze surrounding the microvascular lesions (Figure 3). These leaking lesions are often (though surprisingly, not always) associated with macular thickening by OCT examination.

Table 1 Clinically Significant Diabetic Macular Edema

1.Thickening of the retina at or within 500 mm (approximately one-half optic disk diameter) of the center of the macula.

2.Hard exudates at or within 500 mm of the center of the macula, if associated with thickening of adjacent retina (not residual hard exudates remaining after disappearance of retinal thickening).

3.A zone or zones of retinal thickening 1 optic disk area or larger, any part of which is within one disk diameter of the center of the macula.

From Early Treatment Diabetic Retinopathy Study Research Group (1985). Photocoagulation for diabetic macular edema. Early Treatment Diabetic Retinopathy Study Report No. 1. Archives of Ophthalmology 103: 1795–1805. ã 1985 American Medical Association. All rights reserved.

(a)

00:52.57

(c)

Fluorescein angiography has also demonstrated a specific type of macular edema, so-called cystoid macular edema. The axons of foveal cone photoreceptors are displaced laterally in human and higher primate retinas, thereby permitting light to strike the foveal cone outer segments directly, without traversing the inner retinal layers. This lateral displacement produces a unique anatomic arrangement that is known as Henle’s fiber layer. The center of the fovea itself is avascular; however, cystlike extracellular fluid spaces collect within Henle’s fiber layer. These can, although often with some difficulty, be visualized by white light ophthalmoscopy using a slit lamp and handheld or contact lens, or sometimes by ophthalmoscopy using a green filter to increase contrast. The characteristic appearance, using fluorescein angiography, is one or more oval, uniformly fluorescent spaces oriented radially around the foveal center similar to petals on a flower, giving rise to the term ‘‘petaloid appearance.’’ A single such large cystoid cavity is shown in the fluorescein angiographic frame of Figure 4(a). The cyst-like nature of this cavity is evident on OCT scans (Figure 4(b)). Although macular edema may produce severe loss of

00:43.98

(b)

08:36.78

(d)

central visual acuity, vision is often surprisingly preserved in many instances. Figure 4(c) shows OCT scans of a patient with bilateral cystoid macular edema, with corresponding microperimetric data printed underneath.

This very sensitive visual sensitivity mapping device demonstrates only modest sensitivity loss in each eye, located almost directly corresponding to the position of the cystoid cavities. Visual acuity in each eye, also printed

on the figure, is quite good. These cystoid cavities are, of course, not true cysts, since when histologic sections are available, they demonstrate that these fluid-filled spaces are not lined by layers of epithelium (Figure 4(d)). Cystoid edema may occur with any of the disorders that produce macular edema. One notable entity was first described during the 1960s, after fluorescein angiography had become widely used, when cystoid macular edema was observed angiographically in many patients who had just undergone seemingly uneventful intracapsular cataract surgery. Often, the postoperative visual acuity in these patients did not recover as fully as might have been expected, and fluorescein angiography revealed a pattern of cystoid macular edema, frequently accompanied by leaking of fluorescent dye from the optic nerve head. This pattern has been called the Irvine–Gass syndrome, after the individuals who initially reported it. With the advent of advanced extracapsular cataract surgical techniques, in which the posterior lens capsule is left in place and a posterior chamber intraocular lens is inserted, the incidence of postcataract surgical cystoid macular edema has been considerably reduced, but it still occurs. While this entity has been associated with various complications of the surgery, it often appears in cases where the surgical procedure was entirely uneventful.

Causes of Macular Edema

All investigators agree that the proximate cause of macular edema, associated with any systemic or ocular disease or drug, is a breakdown of the inner blood–retinal barrier, which is composed of junctional complexes of the endothelial cells of the retinal blood vessels, as opposed to the outer blood–retinal barrier, consisting of the retinal pigment epithelium, which serves as a boundary layer between the neural retina and the choroidal vasculature. Although contents of the vascular lumina can reach the extravascular space by transcellular mechanisms, directly through endothelial cell cytoplasm, most investigators believe that the most frequent pathologic mechanism is the breakdown of intercellular junctional complexes, which normally form a tight boundary between the vascular lumen and the extravascular milieu. This may be caused by specific molecular mechanisms, such as increased levels of vascular endothelial growth factor (VEGF, originally called vascular permeability factor because it was through that function that this molecular family was first discovered), or various inflammatory molecules or cytokines, or through the use of certain topical drugs in the treatment of glaucoma, such as epinephrine or, more recently, prostanoid compounds. Another molecular mechanism that has been of interest is the family of carbonic anhydrase enzymes. A recent paper, which describes proteomic analysis of the vitreous

fluid in a series of patients with proliferative diabetic retinopathy and vitreous hemorrhage, who underwent vitrectomy surgery, has reported that these vitreous samples demonstrated a substantial upregulation of the CA-1 isoform of carbonic anhydrase. Injection of this molecule into the vitreous cavity of rats produced generalized retinal edema. Another recent paper cast some doubt on the role of VEGF in diabetic macular edema. These investigators examined the relationship between various single nucleotide polymorphisms (SNPs) in the VEGF gene and its promoter in individuals with various levels of diabetic retinopathy severity in the very large and well-characterized Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications (DCCT/EDIC) cohort. They found that there were 18 SNPs in the VEGF gene and its promoter that were significantly associated with severe preproliferative and proliferative retinopathy, but no detectable significant associations of VEGF gene or promoter polymorphisms with diabetic macular edema.

Treatment of Macular Edema

Results from the ETDRS

The ETDRS, a randomized, controlled clinical trial (RCT) of laser treatment and aspirin therapy that involved over 3000 patients with moderate to severe nonproliferative and early proliferative diabetic retinopathy, and/or with macular edema, reported in 1985 that treatment with focal applications of small-diameter (50–100 mm) argon laser burns directly to macular microaneurysms (Figure 5(a)–(c)), or with grid laser applications could slow down or arrest the progress of diabetic macular edema with substantial preservation of vision by as much as 50% more in eyes that received treatment than in eyes that were randomly allocated to the no laser treatment group. The ETDRS results are frequently cited as showing that such laser treatment preserves vision, but does not improve it. However, in this study, a substantial number of the patients initially had good (i.e., better than 20/40) vision and hence were not likely to improve substantially. When one examines the results of the ETDRS patients whose initial vision was less than 20/40, it becomes apparent that laser treatment produced an actual improvement of visual acuity by more than one line on the ETDRS visual acuity chart in more than 40% of these subjects, compared to approximately 25% in the control group. In a more recent RCT, which compared two different techniques for applying focal argon laser treatment for diabetic macular edema, the Diabetic Retinopathy Clinical Research Network found that approximately 30% of patients with initial visual acuity less than 20/40 who received the modified ETDRS laser technique improved their visual acuity by more than three lines on