The role of perinatal stress in simplex retinitis pigmentosa: evidence from surveys in Australia and the United States

BACKGROUND

In experimental models of several forms of human retinitis pigmentosa (RP) the dystrophy begins in the neonatal period, during a "critical period" in which photoreceptors are sensitive to hypoxia. We performed a study to test whether perinatal stress is associated with human RP, particularly in simplex (nonfamilial) cases.

METHODS

Two surveys were carried out in 1999. In one, Australians with RP were surveyed for information on whether they had experienced stress at birth and whether any members of their family had RP. In the other, the diagnostic type and inheritance patterns of a group of patients with RP seen at a university-affiliated eye institute in Los Angeles between 1997 and 1999 were established as part of their clinical assessment. In neither cohort was the RP part of a syndrome.

RESULTS

After entry criteria were applied, there were 293 cases (of a total of 446 replies) available for analysis from the Australian survey and 119 cases (after exclusion of 229 cases with incomplete data) from the US survey. A total of 52.2% and 53.8% of the cases respectively were simplex. Perinatal stress was reported by about 15% of the respondents with familial RP (15.0% in the Australian cohort and 14.5% in the US cohort), compared with 30% of those with simplex RP (27.4% and 29.7% respectively), a significant difference (p < 0.05). In the Australian cohort four forms of stress--cyanosis, difficult presentation, prematurity and a perinatal period of intensive care--were reported more than twice as often by respondents in the simplex group than those in the familial group. For only one factor, cyanosis, was the difference between the two groups significant (chi2 test, p = 0.01). In the US cohort no single form of stress was significantly related to simplex RP.

INTERPRETATION

Our findings support the hypothesis that perinatal stress is associated with simplex RP in a minority of cases. Larger cohorts need to be studied to test whether perinatal stress can interact with predisposing genes in the genesis of some forms of RP.

Effects of oxygen and bFGF on the vulnerability of photoreceptors to light damage

PURPOSE

To test whether tissue oxygen levels affect the vulnerability of photoreceptors to damage by bright continuous light (BCL).

METHODS

Albino rats were raised in standard conditions of cyclic light (12-hour light, 12-hour darkness) with the light level at 5 to 10 lux or 40 to 65 lux. They were then exposed to BCL (1000-1400 lux), either continuously for 48 hours or for the day or night components of the 48-hour period. During BCL, some rats were kept in room air (normoxia, 21% oxygen), some in hypoxia (10%), and some in hyperoxia (70%). Their retinas were examined for cell death, for the expression of basic fibroblast growth factor (bFGF), and for response to light (electroretinogram, ERG).

RESULTS

The death of retinal cells induced by BCL was confined to photoreceptors. Within the retina, the severity of death was inversely related to the level of bFGF immunolabeling in the somas of the outer nuclear layer (ONL) before exposure. The death of photoreceptors was accompanied by an upregulation of bFGF protein levels in the ONL and by a decline in the ERG. Both hypoxia and hyperoxia during BCL reduced the photoreceptor death, bFGF upregulation, and ERG decline caused by BCL. The protective effects of hyperoxia and hypoxia were evident during both the day and night halves of the daily cycle. Hypoxia or hyperoxia alone did not upregulate bFGF or ciliary neurotrophic factor (CNTF) expression in the retina.

CONCLUSIONS

Photoreceptors are protected from light damage by hypoxia and hyperoxia during exposure. The protection provided by oxygen levels operates during both day and night. The protection is not mediated by an upregulation of bFGF or CNTF.

The human hyaloid system: cell death and vascular regression

The present study had investigated the roles of apoptosis and necrosis in the regression of the human fetal hyaloid vasculature. Normal human fetal hyaloid specimens (n = 67) ranging from 10 to 20 weeks' gestation were studied. Specimens were either immunolabeled with anti-von Willebrand factor and major histocompatibility complex class I antibodies or investigated using the terminal-deoxyribonucleotidyl transferase-mediated dUTP-biotin DNA nick-end labeling technique. A fluorescent DNA-binding dye acridine orange/ethidium bromide mixture was also applied to unfixed flat mounts of hyaloid vasculature and some specimens were processed for transmission electron microscopy. Vascular regression including cell loss in the connecting vessels, stretching and thinning of the vasa hyaloidea propria, tunica vasculosa lentis and the pupillary membrane was clearly evident after 13 weeks' gestation. Cresyl violet staining revealed condensed cells and pyknotic bodies throughout the hyaloid system; cell death occurred either in single cells or along small capillary segments associated with vascular regression. Acridine orange/ethidium bromide staining showed DNA condensation at early and late stages of cell death. Similarly, DNA nick-end labeling was positive in endothelial cells, pericytes and vessel and non-vessel associated hyalocytes. The observation of hyalocytes juxtaposed to cytolysed endothelial cells may indicate a role for these cells in vascular regression. Features of apoptosis were more evident during early vascular regression whilst necrosis was increasingly evident at later stages.

Mechanisms of photoreceptor death and survival in mammalian retina

The mammalian retina, like the rest of the central nervous system, is highly stable and can maintain its structure and function for the full life of the individual, in humans for many decades. Photoreceptor dystrophies are instances of retinal instability. Many are precipitated by genetic mutations and scores of photoreceptor-lethal mutations have now been identified at the codon level. This review explores the factors which make the photoreceptor more vulnerable to small mutations of its proteins than any other cell of the body, and more vulnerable to environmental factors than any other retinal neurone. These factors include the highly specialised structure and function of the photoreceptors, their high appetite for energy, their self-protective mechanisms and the architecture of their energy supply from the choroidal circulation. Particularly important are the properties of the choroidal circulation, especially its fast flow of near-arterial blood and its inability to autoregulate. Mechanisms which make the retina stable and unstable are then reviewed in three different models of retinal degeneration, retinal detachment, photoreceptor dystrophy and light damage. A two stage model of the genesis of photoreceptor dystrophies is proposed, comprising an initial "depletion" stage caused by genetic or environmental insult and a second "late" stage during which oxygen toxicity damages and eventually destroys any photoreceptors which survive the initial depletion. It is a feature of the model that the second "late" stage of retinal dystrophies is driven by oxygen toxicity. The implications of these ideas for therapy of retinal dystrophies are discussed.

Limiting photoreceptor death and deconstruction during experimental retinal detachment: the value of oxygen supplementation

PURPOSE

To assess the role of hypoxia in causing the death and deconstruction of photoreceptors in detached retinas and the effectiveness of supplemental oxygen in limiting such damage.

METHODS

Retinal detachment was induced surgically in the right eye of each of 10 cats. The cats were allowed to survive surgery for 3 days. Two were kept for these 3 days in normoxia (room air, 21% oxygen) and eight in hyperoxia (70% oxygen). The retinas were examined for cell death by use of labels for normal and fragmenting DNA, with antibodies and a cone sheath-specific lectin to demonstrate the status of their inner and outer segments, the synaptic structures of the outer plexiform layer, and the distribution of basic fibroblast growth factor (bFGF) and with in situ hybridization to demonstrate bFGF mRNA.

RESULTS

Retinal detachment without oxygen supplementation caused the death of some photoreceptors; the loss of cytochrome oxidase from the inner segments and the collapse of the outer segments of surviving photoreceptors; the loss of synaptophysin profiles from the outer plexiform layer; and the loss of bFGF protein from retinal neurons and neuroglia but not from retinal vessels. Oxygen supplementation (hyperoxia) during detachment mitigated all these changes, reducing photoreceptor death, maintaining the specialized structures of surviving photoreceptors, and stabilizing the bFGF within the retina.

CONCLUSIONS

In experimental retinal detachment, hypoxia caused by the separation of outer retina from its normal source of nutrients is a factor in inducing the death and deconstruction of photoreceptors as well as in the loss of bFGF from the detached retina. Hyperoxia offered to human patients between diagnosis of retinal detachment and surgery may enhance the function of the reattached retina.

Limiting the proliferation and reactivity of retinal Müller cells during experimental retinal detachment: the value of oxygen supplementation

PURPOSE

To assess the role of hypoxia in inducing the proliferation, hypertrophy, and dysfunction of Muller cells in detached retina and the effectiveness of supplemental oxygen in limiting these reactions.

METHODS

Retinal detachments were produced in the right eye of each of 13 cats; the cats survived surgery for 3 days, during which six were kept in normoxia (room air, 21%) and seven in hyperoxia (70% oxygen). Retinas were labeled for proliferation with an antibody (MIB-1) to a cell cycle protein (Ki-67), for evidence of hypertrophy employing antibodies to the intermediate filament protein glial fibrillary acidic protein (GFAP) and to beta-tubulin and for disturbance of glutamate neurochemistry employing antibodies to glutamate to a glutamate receptor (GluR-2) and to glutamine synthetase.

RESULTS

Results from the two animals kept in normoxia after retinal detachment confirmed previous reports that detachment caused the proliferation of Muller cells, the hypertrophy of Muller cell processes, and the disruption of glutamate recycling by Muller cells. Oxygen supplementation during detachment reduced Muller cell proliferation and hypertrophy and reduced the abnormalities in the distributions of glutamate, GluR-2, and glutamine synthetase.

CONCLUSIONS

Oxygen supplementation reduced the reaction of retinal Muller cells to retinal detachment, limiting their proliferation and helping to maintain their normal structure and function. In the clinical setting, oxygen supplementation between diagnosis and reattachment surgery may reduce the incidence and severity of glial-based complications, such as proliferative vitreoretinopathy.

Photoreceptor dystrophy in the RCS rat: roles of oxygen, debris, and bFGF

PURPOSE

To examine the roles of oxygen, basic fibroblast growth factor (bFGF), and photoreceptor debris in the photoreceptor dystrophy of the Royal College of Surgeons (RCS) rat.

METHODS

Pups were exposed during the critical period of their development (postnatal day [P] 16-24) and for some days thereafter to hypoxia and hyperoxia. The effects of these exposures on photoreceptor death, debris accumulation in the subretinal space, and the expression of bFGF protein and mRNA by surviving cells were studied.

RESULTS

During the critical period hyperoxia slowed photoreceptor death in a dose-related fashion and decreased bFGF protein levels, whereas hypoxia accelerated death and increased bFGF levels. At the edges of the retina, where photoreceptors survive longest in normoxia, hypoxia had little effect on either photoreceptor death or bFGF protein levels. Oxygen-induced modulation of rates of death could not be related to the accumulation of debris in the subretinal space. After P27, the relationship between oxygen and photoreceptor death changed markedly, hyperoxia no longer delaying and hypoxia no longer accelerating death.

CONCLUSIONS

The death of RCS rat photoreceptors in the period P16 to P27 is precipitated by hypoxia that may result from the accumulation of photoreceptor debris in the subretinal space. This debris, the result of the phagocytotic failure of the retinal pigment epithelium in this strain, lies in the normal pathway of oxygen diffusing to the photoreceptors from the choriocapillaris. During this period the retina responds to hypoxia by increasing expression of a potentially protective protein (bFGF), but hypoxia-induced damage overwhelms any protection provided by this or other mechanisms. Later stages of the dystrophy may not be hypoxia-induced.

Tissue oxygen during a critical developmental period controls the death and survival of photoreceptors

PURPOSE

To study the death of photoreceptors in normally developing and dystrophic retina and to test the role of hypoxia in causing that death.

METHODS

Death of photoreceptors was detected in the albino, hooded, and Royal College of Surgeons (RCS) strains of rat, and in the rabbit and cat, using the TUNEL technique. Retinas of selected ages from animals raised normally and those from rat pups raised for periods in hyperoxia (75% oxygen) or hypoxia (10% oxygen) were studied.

RESULTS

In all species and strains examined, a naturally occurring wave of photoreceptor death was detected during the last stages of retinal development. In the albino rat, this wave, which began approximately at postnatal day 15 (P15) and peaked at P22, was reduced by hyperoxia and was intensified by hypoxia, producing a "hypoxic dystrophy" of photoreceptors. In the RCS rat, photoreceptor death also commenced at approximately P15 and then proceeded to exhaustion. This degeneration was greatly reduced by hyperoxia. In the RCS rat, hyperoxia was effective in photoreceptor rescue only during a discrete period, from P16 to P22. In the albino rat, the effectiveness of hypoxia in inducing photoreceptor death was much greater between P15 and P21 than at earlier ages, or in the adult.

CONCLUSIONS

During a critical period extending approximately from P15 to P22, tissue oxygen levels strongly influence photoreceptor death and survival in dystrophic and normally developing strains of rat. This period is evident in normal development as a period of naturally occurring photoreceptor death and is evident experimentally as a period during which hyperoxia is effective in rescuing dying photoreceptors and during which hypoxia is effective in inducing death of otherwise viable photoreceptors.

Fate of DNA from retinal cells dying during development: uptake by microglia and macroglia (Müller cells)

The tunel technique of labelling fragmenting dna was used to examine cell death in the developing retina of the rabbit, rat and cat. TUNEL-labelled structures included the still-intact nuclei of retinal cells and smaller, strongly labelled bodies interpreted as fragments of disintegrating nuclei (apoptotic or pyknotic bodies). With confocal microscopy, the cytoplasm around labelled nuclei was observed to be labelled, suggesting that DNA fragments spread into the cytoplasm of the dying cell. Also observed were cells whose nuclei were TUNEL-but whose cytoplasm was TUNEL+, so that their morphology could be discerned. Evidence is presented that these are phagocytes, their cytoplasmic labelling resulting from the ingestion of the fragmenting DNA of a dying neighbour. Results suggest that in developing retina fragmenting DNA is phagocytosed principally by microglia and Müller cells, with a few neurones and no astrocytes active as phagocytes. In the postnatal material studied, microglia are the predominant phagocytes for cells dying in the ganglion cell and inner nuclear layers. Müller cells appear able to phagocytose cells dying in any retinal layer and, since microglia do not normally enter the outer nuclear layer, may be important for the phagocytosis of dying photoreceptors.

Ontogeny of catecholaminergic and cholinergic cell distributions in the cat's retina

The development of catecholaminergic and cholinergic neurones in the cat's retina has been examined with antibodies against their respective rate-limiting enzymes, tyrosine hydroxylase (TH) and choline acetyl transferase (ChAT). ChAT-immunoreactive (IR) cells were first detected at E (embryonic day) 56 with somata in the ganglion cell layer (GCL) or in the inner cytoblast layer (CBL). At P (postnatal day) 1, two faint bands of ChAT-IR fibres were evident in an inner and outer strata of the inner plexiform layer (IPL) and by P26, the bands were similar to those in the adult. TH immunoreactivity was first detected at E59 in either darkly labelled somata in the inner CBL with processes extending toward the IPL or in lightly labelled somata also located in CBL but with no processes. At P1, most TH-IR cells had prominently labelled dendrites and, by P8, most of the features of the adult cells were evident. Soma size gradients among TH-IR cells were first detected at P8, with cells in temporal retina being larger than those in nasal retina or at the area centralis. The smaller sizes of cells at the area centralis emerged after P26. The smaller sizes of ChAT-IR somata at the area centralis, by contrast, emerged between P8 and P26. The number of both TH-IR and ChAT-IR cells declined from the time they first appeared till adulthood. The decline was smaller among ChAT-IR cells (24%) than among TH-IR cells (68%). In distribution, the differential expansion of the retina appeared to be largely responsible for generating the final adult distribution of ChAT-IR cells. However, during late postnatal development (P26 to adulthood), the density of ChAT-IR cells in the periphery declined more than that of the ganglion cells, suggesting that some ChAT-IR cells may die in the periphery during this time. Prior to P26, the changes in the distribution of TH-IR cells were inconsistent with the pattern of retinal expansion. It is suggested that during this period, regional cell loss and cell addition may account for the changes in distribution of TH-IR cells. Later in development (P26 to adulthood), the changes in the density of TH-IR cells closely conformed to the differential expansion of the retina.

Extracellular space in the developing retina assessed by electron microscopy: laminar and topographic distribution

The extent of extracellular space (ECS) in the developing retina of the cat has been measured by electron microscopy in material fixed using techniques developed by others to preserve ECS. ECS is generally greater in foetal than in adult material. It is particularly marked in the plexiform layers of retina at the time of synaptogenesis and in the axon layer at the time of axon growth. The changes in ECS occur first in the central retina, and spread to the periphery. These observations suggest that the high volumes of ECS found in the foetus are not artefactual, but accompany and may play a role in developmental processes.

Time course of stratification of the dendritic fields of ganglion cells in the retina of the cat

The inner plexiform layer (IPL) of the retina has been shown by previous workers to comprise a number of sublayers (sublaminae or strata), each containing a distinct component of its circuitry. Using horseradish peroxidase applied to cultured whole retinas, we have observed the segregation of the dendrites of ganglion cells of the cat retina into two sublayers of the IPL. These sublayers appear to correspond to the a and b sublaminae described in studies of the adult IPL. As the dendritic fields of ganglion cells form, in mid-gestation, they are diffuse, spreading through the ganglion cell and inner plexiform layers. A few weeks before birth the dendrites become restricted to the IPL, but it is not until after birth, between P(postnatal day)2 and P5, that they segregate into inner and outer sublayers of the IPL. The process of segregation may involve the loss or 'pruning' of excess dendrites formed in 'wrong' sublayers. The segregation of dendrites into sublayers occurs concurrently with the formation of synapses by bipolar cells and may be induced by contacts made by bipolar cells onto the dendrites of ganglion cells.

Catecholaminergic and cholinergic neurons in the developing retina of the rat

We have examined the development of catecholaminergic and cholinergic neurons in the retina of the rat by using antibodies against the enzymes tyrosine hydroxylase (TH) and choline acetyl transferase (ChAT), respectively. TH-immunoreactivity was first detected at P (postnatal day) 3 in somata located in the inner part of the cytoblast layer (CBL) and in fine dendrites extending toward the middle of the inner plexiform layer (IPL). These cells were similar in shape and soma size to the class 2 TH-immunoreactive (TH-IR) cells of the adult rat. At P6, TH-immunoreactivity was expressed by a second population of cells. Their somata were in the inner part of the inner nuclear layer (INL), but were distinctly larger, with short thick dendrites extending into the outer and/or middle parts of the IPL. Over subsequent days, the dendrites of these larger cells spread profusely in the outer part of the IPL, making it likely that they are the class 1 TH-IR cells of the adult. ChAT-immunoreactive (ChAT-IR) cells were not detected until P15, when ChAT-IR somata were observed in the ganglion cell layer (GCL) and INL, and their dendrites were observed already segregated into the distinct strata of the IPL in which they are found in the adult. The subsequent growth of TH-IR somata of both classes was uneven, persisting longer in temporal than in nasal retina. This extended growth of temporal cells establishes the marked nasotemporal differences in soma diameter apparent among TH-IR cells in the adult (Mitrofanis and Stone, '86; Mitrofanis et al., '88b). The growth and adult size of ChAT-IR somata, on the other hand, did not vary with retinal position; their diameters were similar to those of the adult cells from the time they first appeared. The distribution of ChAT-IR cells at P15 shared several features of the distribution of ganglion cells. The density of ChAT-IR cells was greatest at the area of peak ganglion cell density and declined toward the periphery. In contrast, TH-IR cells concentrated from the time they first appeared at the superior temporal margin, peripheral to the area of peak density of ganglion and ChAT-IR cells.

Stages in the structural differentiation of retinal ganglion cells

Using a cultured wholemount technique we have studied the morphological differentiation of ganglion cells in the retina of the rat and cat, during normal development. In both species the differentiation of ganglion cells begins in embryonic life, before embryonic day (E) 17 in the rat and E36 in the cat. It is useful to describe the morphological differentiation of ganglion cells as occurring in three stages. In the first stage, each germinal cell becoming a ganglion cell extends an axon into the fibre layer of the retina and towards the optic disc, and the soma of the cell moves towards the ganglion cell layer. As the soma approaches the ganglion cell layer, the processes that attach its poles to the inner and outer surfaces of the retina are withdrawn. When the soma reaches the ganglion cell layer, a stage of active dendritic growth begins, which lasts until shortly before birth in the cat and until several days after birth in the rat. The cell extends stem dendrites that branch profusely and are commonly tipped by growth cones. The major morphological classes of ganglion cell become distinct in the latter part of stage 2, as do the centroperipheral gradients in ganglion cell size apparent in the cat. During the third stage, the dendritic trees of ganglion cells no longer branch or extend by means of active growth cones. Very considerable growth of all parameters of the cell (soma size, dendrite calibre and length, axon calibre) occurs nevertheless, presumably by interstitial addition of membrane throughout the cell.

Synaptogenesis in the retina of the cat

We have studied the development of synapses in the retina of the cat from E(embryonic day)21 to adulthood. The inner plexiform layer (IPL) could be distinguished by E36, but at this age no synapses had formed, although compact processes had formed in the IPL and membrane specialisations had developed in adjacent processes. Conventional synapses form in the IPL from E45 and become increasingly numerous and differentiated over subsequent weeks. Extracellular space and cellular debris were prominent during the formation of these synapses. The conventional synapses appear to form principally between amacrine cells until E56, when ganglion cell dendrites could be identified as postsynaptic processes. Ribbon synapses characteristic of bipolar cells were identified around birth, suggesting that bipolar cells do not form synapses until that age. The outer plexiform layer (OPL) could be distinguished in central retina at E56. Extracellular space, debris of degenerating cells and mounds of agranular vesicles were prominent at this age but synapses were not observed until E59, when cone pedicles formed ribbon synapses onto horizontal cell processes. The first synapses clearly formed by spherules, also onto horizontal cells, were seen at E62. The central process of the postsynaptic triad, considered to be the dendrite of a bipolar cell, was first observed in both cone pedicles and rod spherules around birth, again suggesting that bipolar cells do not enter into synaptic arrangements until that age. Synaptogenesis in the OPL shows a strong centro-peripheral gradient; its initial stages were observed centrally in the late E50's but synapse formation was not complete in the retinal periphery until P(postnatal day)7 or later. We could not detect a centro-peripheral gradient in the formation of conventional synapses in the IPL, but the formation of ribbon synapses in this layer began centrally at birth and in the mid-periphery at P5. In summary, the first synapses to form in the retina are those which spread information laterally within the plexiform layers, between amacrine cells and from receptor to horizontal cells. The cells which carry information centrally, in particular bipolar cells, enter into synaptic arrangements considerably later. Further, retinal cells seem to form synapses in a distinct sequence: first amacrines, then receptors and lastly bipolar cells.