Partial decussation of sensory pathways allows neural inputs from both sides of the body to project to the same target region where these signals will be integrated. Here, in order to better understand mechanisms of eye-specific targeting, we studied how retinal ganglion cell (RGC) axons terminate in their thalamic target, the dorsal lateral geniculate nucleus (dLGN), when crossing at the optic chiasm midline is altered. In models with gain- and loss-of-function of EphB1, the receptor that directs the ipsilateral projection at the optic chiasm, misrouted RGCs target the appropriate retinotopic zone in the opposite dLGN. However, in EphB1-/- mice, the misrouted axons do not intermingle with normally projecting RGC axons and segregate instead into a distinct patch. We also revisited the role of retinal activity on eye-specific targeting by blocking correlated waves of activity with epibatidine into both eyes. We show that in wild-type mice, retinal waves are necessary during the first postnatal week for both proper distribution and eye-specific segregation of ipsilateral axons in the mature dLGN. Moreover, in EphB1-/- mice, refinement of ipsilateral axons is perturbed in control conditions and is further impaired after epibatidine treatment. Finally, retinal waves are required for the formation of the segregated patch of misrouted axons in EphB1-/- mice. These findings implicate molecular determinants for targeting of eye-specific zones that are independent of midline guidance cues and that function in concert with correlated retinal activity to sculpt retinogeniculate projections.
retinal ganglion cell; axon; retinogeniculate; lateral geniculate; visual; patterning; retina; activity; topography
At the optic chiasm retinal fibres either cross the midline, or remain uncrossed. Here we trace hemispheric pathways through the marmoset chiasm and show that fibres from the lateral optic nerve pass directly towards the ipsilateral optic tract without any significant change in fibre order and without approaching the midline, while those from medial regions of the nerve decussate directly. Anterograde labelling from one eye shows that the two hemispheric pathways remain segregated through the proximal nerve and chiasm with the uncrossed confined laterally. Retrograde labelling from the optic tract confirms this. This clearly demonstrates that hemispheric pathways are segregated through the primate chiasm.
Previous chiasmatic studies have been undertaken mainly on rodents and ferrets. In these species there is a major change in fibre order pre-chiasmatically, where crossed and uncrossed fibres mix, reflecting their embryological history when all fibres approach the midline prior to their commitment to innervate either hemisphere. This pattern was thought to be common to placental mammals. In marsupials there is no change in fibre order and uncrossed fibres remain confined laterally through nerve and chiasm, again, reflecting their developmental history when all uncrossed fibres avoid the midline. Recently it has been shown that this distinction is not a true dichotomy between placental mammals and marsupials, as fibre order in tree shrews and humans mirrors the marsupial pattern.
Architectural differences in the mature chiasm probably reflect different developmental mechanisms regulating pathway choice. Our results therefore suggest that both the organisation and development of the primate optic chiasm differ markedly from that revealed in rodents and carnivores.
Retina; marmoset; vision development
Pax2 is essential for development of the neural tube, urogenital system, optic vesicle, optic cup and optic tract. In the eye, Pax2 deficiency is associated with coloboma, a loss of astrocytes in the optic nerve and retina, and abnormal axonal pathfinding of the ganglion cells axons at the optic chiasm. Thus, appropriate expression of Pax2 is essential for astrocytes determination and differentiation. Although BMP7 and SHH have been shown to regulate Pax2 expression, the molecular mechanism by which this regulation occurs is not well understood. In this study, we determined that BMP7 and SHH activate Pax2 expression in mouse retinal astrocyte precursors in vitro. SHH appeared to play a dual role in Pax2 regulation; 1) SHH may regulate BMP7 expression, and 2) the SHH pathway cooperates with the BMP pathway to regulate Pax2 expression. BMP and SHH pathway members can interact separately or together with TLX, a repressor protein in the tailless transcription factor family. Here we show that the interaction of both pathways with TLX relieves the repression of Pax2 expression in mouse retinal astrocytes. Together these data reveal a new mechanism for the cooperative actions of signaling pathways in astrocyte determination and differentiation and suggest interactions of regulatory pathways that are applicable to other developmental programs.
SHH; BMP7; TLX; PAX2; astrocytes; retina
One of the central tasks in retinal neuroscience is to understand the circuitry of retinal neurons and how those connections are responsible for shaping the signals transmitted to the brain. Photons are detected in the retina by rod and cone photoreceptors, which convert that energy into an electrical signal, transmitting it to other retinal neurons, where it is processed and communicated to central targets in the brain via the optic nerve. Important early insights into retinal circuitry and visual processing came from the histological studies of Cajal1,2 and, later, from electrophysiological recordings of the spiking activity of retinal ganglion cells - the output cells of the retina3,4.
A detailed understanding of visual processing in the retina requires an understanding of the signaling at each step in the pathway from photoreceptor to retinal ganglion cell. However, many retinal cell types are buried deep in the tissue and therefore relatively inaccessible for electrophysiological recording. This limitation can be overcome by working with vertical slices, in which cells residing within each of the retinal layers are clearly visible and accessible for electrophysiological recording.
Here, we describe a method for making vertical sections of retinas from larval tiger salamanders (Ambystoma tigrinum). While this preparation was originally developed for recordings with sharp microelectrodes5,6, we describe a method for dual whole-cell voltage clamp recordings from photoreceptors and second-order horizontal and bipolar cells in which we manipulate the photoreceptor's membrane potential while simultaneously recording post-synaptic responses in horizontal or bipolar cells. The photoreceptors of the tiger salamander are considerably larger than those of mammalian species, making this an ideal preparation in which to undertake this technically challenging experimental approach. These experiments are described with an eye toward probing the signaling properties of the synaptic ribbon - a specialized synaptic structure found in a only a handful of neurons, including rod and cone photoreceptors, that is well suited for maintaining a high rate of tonic neurotransmitter release7,8 - and how it contributes to the unique signaling properties of this first retinal synapse.
Neuroscience; Issue 76; Molecular Biology; Cellular Biology; Anatomy; Physiology; Ophthalmology; Retina; electrophysiology; paired recording; patch clamp; synaptic ribbon; photoreceptor; bipolar cell; horizontal cell; tiger salamander; animal model
The eye is a peripheral outpost of the central nervous system (CNS) where the retinal ganglion cells (RGCs) reside. RGC axons navigate to their targets in a remarkably stereotyped and error-free manner and it is this process of directed growth that underlies the complex organization of the adult brain. The RGCs are the only retinal neurons to project into the brain and their peripheral location makes them an unusually accessible population of projection neurons for experiments involving in vivo gene transfer, anatomical tracing, transplantation and in vitro culture. In this paper, we review recent findings that have contributed to our understanding of some of the guidance decisions that axons make in the developing visual system. We look at two choice points in the pathway, the optic nerve head (onh) and the midline chiasm, and discuss evidence that supports the idea that key molecules in guiding axon growth at these junctures are netrin-1 (onh) and ephrin-B (chiasm). In the optic tectum where RGC axon terminals are arrayed in topographic order, we present experimental evidence to suggest that in the dorso-ventral dimension, the B-type ephrins and Eph receptors are of prime importance, possibly through attractive interactions. This complements the anterior-posterior topographic mapping known to be mediated through A-type ephrin/Eph repulsive interactions. An emerging theme is that guidance molecules such as ephrin-B and netrin-1 have complex patterns of restricted expression in the pathway and play multiple and changing roles in axon guidance.
retina; axon guidance; netrin; Eph receptor; ephrin
Retinal ganglion cells (RGCs) project their long axons, composing the optic nerve, to the brain, transmitting the visual information gathered by the retina, ultimately leading to formed vision in the visual cortex. The RGC cellular system, representing the anterior part of the visual pathway, is vulnerable to mitochondrial dysfunction and optic atrophy is a very frequent feature of mitochondrial and neurodegenerative diseases. The start of the molecular era of mitochondrial medicine, the year 1988, was marked by the identification of a maternally inherited form of optic atrophy, Leber's hereditary optic neuropathy, as the first disease due to mitochondrial DNA point mutations. The field of mitochondrial medicine has expanded enormously over the last two decades and many neurodegenerative diseases are now known to have a primary mitochondrial etiology or mitochondrial dysfunction plays a relevant role in their pathogenic mechanism. Recent technical advancements in neuro-ophthalmology, such as optical coherence tomography, prompted a still ongoing systematic re-investigation of retinal and optic nerve involvement in neurodegenerative disorders. In addition to inherited optic neuropathies, such as Leber's hereditary optic neuropathy and dominant optic atrophy, and in addition to the syndromic mitochondrial encephalomyopathies or mitochondrial neurodegenerative disorders such as some spinocerebellar ataxias or familial spastic paraparesis and other disorders, we draw attention to the involvement of the optic nerve in classic age-related neurodegenerative disorders such as Parkinson and Alzheimer disease. We here provide an overview of optic nerve pathology in these different clinical settings, and we review the possible mechanisms involved in the pathogenesis of optic atrophy. This may be a model of general value for the field of neurodegeneration. This article is part of a Special Issue entitled ‘Mitochondrial function and dysfunction in neurodegeneration’.
Mitochondrial disease; Optic atrophy; Mitochondrial functions; LHON; DOA; Retinal ganglion cells
The inability of retinal ganglion cells (RGCs) to regenerate damaged axons through the optic nerve has dire consequences for victims of traumatic nerve injury and certain neurodegenerative diseases. Several strategies have been shown to induce appreciable regeneration in vivo, but the regrowth of axons through the entire optic nerve and on into the brain remains a major challenge. We show here that the induction of a controlled inflammatory response in the eye, when combined with elevation of intracellular cAMP and deletion of the gene encoding pten (phosphatase and tensin homolog), enables RGCs to regenerate axons the full length of the optic nerve in mature mice; about half of these axons cross the chiasm, and a rare (~1%) subset manages to enter the thalamus. Consistent with our earlier findings, the axon-promoting effects of inflammation were shown to require the macrophage-derived growth factor oncomodulin (Ocm). Elevation of cAMP increased the ability of Ocm to bind to its receptors in the inner retina and augmented inflammation-induced regeneration twofold. Inflammation combined with elevated cAMP and PTEN deletion increased activation of the PI3K and MAP kinase signaling pathways and augmented regeneration approximately 10-fold over the level induced by either pten deletion or Zymosan alone. Thus, treatments that synergistically alter the intrinsic growth state of RGCs produce unprecedented levels of axon regeneration in the optic nerve, a CNS pathway long believed to be incapable of supporting such growth.
Optic nerve; regeneration; oncomodulin; cAMP; PTEN; axon; retinal ganglion cell
PURPOSE: The anterior chiasmal syndrome consists of a temporal hemianopia or complete visual field loss in one eye, plus a superior temporal hemianopia in the other eye. The superior temporal hemianopia in the other eye is thought to result from injury to Wilbrand's Knee of the optic chiasm. Wilbrand's Knee is a loop of decussating fibers which detours into the contralateral optic nerve before entering the optic tract. I studied the organization of fibers in the optic chiasm of monkeys and humans to verify the existence of Wilbrand's Knee and to elucidate further the pattern of visual field loss seen from lesions of the sellar region. METHODS: The primary optic pathway was labelled in monkeys by injection of [3H] proline into one eye, followed by autoradiography. There were 8 intact Rhesus monkeys and 3 intact squirrel monkeys. In addition, the optic pathway was studied in the Rhesus monkey 6 months and 4 years after monocular enucleation. The optic chiasm was also examined using myelin stains in specimens obtained post-mortem from 3 patients. The patients had lost 1 eye 5 months, 2 years, and 28 years prior to their deaths. Finally, clinical observations were recorded in 3 patients with the anterior chiasmal syndrome. RESULTS: In normal Rhesus and squirrel monkeys, optic nerve fibers crossed the optic chiasm without entering the contralateral optic nerve. After short-term monocular enucleation, fibers from the normal optic nerve were drawn closer to the entry zone of the degenerating optic nerve, but Wilbrand's Knee was still absent. After long-term enucleation, a typical Wilbrand's Knee was induced to form. In the human, Wilbrand's Knee was absent 5 months after monocular enucleation, but emerged in the two cases involving long-term enucleation, in a fashion analogous to the monkey. The case reports describe 3 patients with variants of the anterior chiasmal syndrome from parasellar tumors. CONCLUSIONS: Wilbrand's Knee does not exist in the normal primate optic chiasm. It forms gradually over a period of years following monocular enucleation, presumably from shrinkage of the optic chiasm caused by atrophy of fibers from the enucleated eye. Therefore, the superior temporal hemianopia in the "other eye" seen in the anterior chiasmal syndrome cannot be due to compression of Wilbrand's Knee. I propose that it occurs from combined compression of the optic chiasm and one (or both) optic nerves.
Recent improvements in OCT resolution and automated segmentation software has provided a means of relating visual pathway damage to structural changes in the RNFL and corresponding soma of the ganglion cells in the inner layers of the macula and also in the outer photoreceptor layer in the macula.
Studies correlating retinal structure with function are reviewed in the context of optical coherence tomography (OCT) in optic nerve and retinal disorders
Recently published work provides evidence showing a strong relationship not only between the retinal nerve fiber layer and visual threshold in optic nerve disorders, but also between visual sensitivity and the inner layers of the retina in the macula where the cell bodies of ganglion cells reside. Acquired and genetic disorders affecting the outer retina show correlation between visual sensitivity and the thickness of the outer photoreceptors. These relationships helps localize unknown causes of visual field loss through segmentation of the retinal layers using spectral domain OCT.
Advances in relating the structure of the ganglion cell layer in the macula to the corresponding axons in the retinal nerve fiber layer and to visual function further our ability to differentiate and localize ambiguous causes of vision loss and visual field defects in neuro-ophthalmology. Ganglion cell layer analysis in volume OCT data may provide yet another piece of the puzzle to understanding structure-function relationships and its application to diagnosis and monitoring of optic nerve diseases, while similar structure-function relationships are also being elucidated in the outer retina for photoreceptor diseases.
optical coherence tomography; macula; visual function; ganglion cell layer; retinal nerve fiber layer
Vertebrate retinal pigment epithelium (RPE) cells are derived from the multipotent optic neuroepithelium, develop in close proximity to the retina, and are indispensible for eye organogenesis and vision. Recent advances in our understanding of RPE development provide evidence for how critical signaling factors operating in dorso-ventral and distal-proximal gradients interact with key transcription factors to specify three distinct domains in the budding optic neuroepithelium: the distal future retina, the proximal future optic stalk/optic nerve, and the dorsal future RPE. Concomitantly with domain specification, the eye primordium progresses from a vesicle to a cup, RPE pigmentation extends towards the ventral side, and the future ciliary body and iris form from the margin zone between RPE and retina. While much has been learned about the molecular networks controlling RPE cell specification, key questions concerning the cell proliferative parameters in RPE and the subsequent morphogenetic events still need to be addressed in greater detail.
Mitf/Otx/Chx10/Pax6/activin/sonic; hedgehog/fibroblast; growth; factor/neuroepithelial; domain specification/evolution
The divergence of retinal ganglion cell (RGC) axons into ipsilateral and contralateral projections at the optic chiasm and the subsequent segregation of retinal inputs into eye-specific domains in their target, the dorsal lateral geniculate nucleus (dLGN), are crucial for binocular vision. In albinism, affected individuals exhibit a lack or reduction of pigmentation in the eye and skin, a concomitant reduced ipsilateral projection and diverse visual defects. Here we investigate how such altered decussation affects eye-specific retinogeniculate targeting in albino mice using the C57BL/6 Tyr c-2J/c-2J strain, in which tyrosinase, necessary for melanogenesis, is mutated. In albino mice, fewer RGCs from the ventrotemporal (VT) retina project ipsilaterally, reflected in a decrease in cells expressing ipsilateral markers. In addition, a population of RGCs from the VT retina projects contralaterally and within the dLGN their axons cluster into a patch separated from the contralateral termination area. Further, eye-specific segregation is not complete in the albino dLGN, and upon perturbing postnatal retinal activity with epibatidine, the ipsilateral projection fragments and the aberrant contralateral patch disappears. These results suggest that the defects in afferent targeting and activity-dependent refinement in the albino dLGN arise from RGC mis-specification together with potential perturbations of early activity patterns in the albino retina.
retinal ganglion cell; lateral geniculate; visual; patterning; retina; activity; topography; albinism; siamese; epibatidine; tyrosinase; pigmented
At the optic chiasm, retinal ganglion cells (RGCs) project ipsi- or contralaterally to establish the circuitry for binocular vision. Ipsilateral guidance programs have been characterized, but contralateral guidance programs are not well understood. Here we identify a tripartite molecular system for contralateral RGC projections: Semaphorin 6D and Nr-CAM are expressed on midline radial glia and Plexin-A1 on chiasm neurons, and Plexin-A1 and Nr-CAM are also expressed on contralateral RGCs. Sema6D is repulsive to contralateral RGCs, but Sema6D in combination with Nr-CAM and Plexin-A1 converts repulsion to growth-promotion. Nr-CAM functions as a novel receptor for Sema6D. Sema6D, Plexin-A1 and Nr-CAM are all required for efficient RGC decussation at the optic chiasm. These findings suggest a novel mechanism by which a complex of Sema6D, Nr-CAM, and Plexin-A1 at the chiasm midline alters the sign of Sema6D and signals Nr-CAM/Plexin-A1 receptors on RGCs to implement the contralateral RGC projection.
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.
Neurodegeneration; neuroprotection; retina; glaucoma; diabetic retinopathy; age-related macular degeneration; retinal ganglion cells.
Patterning events during early eye formation determine retinal cell fate and can dictate the behavior of retinal ganglion cell (RGC) axons as they navigate toward central brain targets. The temporally and spatially regulated expression of bone morphogenetic proteins (BMPs) and their receptors in the retina are thought to play a key role in this process, initiating gene expression cascades that distinguish different regions of the retina, particularly along the dorsoventral axis. Here, we examine the role of BMP and a potential downstream effector, EphB, in retinotopic map formation in the lateral geniculate nucleus (LGN) and superior colliculus (SC). RGC axon behaviors during retinotopic map formation in wild type mice are compared with those in several strains of mice with engineered defects of BMP and EphB signaling. Normal RGC axon sorting produces axon order in the optic tract that reflects the dorsoventral position of the parent RGCs in the eye. A dramatic consequence of disrupting BMP signaling is a missorting of RGC axons as they exit the optic chiasm. This sorting is not dependent on EphB. When BMP signaling in the developing eye is genetically modified, RGC order in the optic tract and targeting in the LGN and SC are correspondingly disrupted. These experiments show that BMP signaling regulates dorsoventral RGC cell fate, RGC axon behavior in the ascending optic tract and retinotopic map formation in the LGN and SC through mechanisms that are in part distinct from EphB signaling in the LGN and SC.
Bone Morphogenetic Proteins; retinotopic map; superior colliculus; lateral geniculate nucleus; visual development; EphB; EphrinB
Pathological changes are reported in the anterior visual
pathways of a 41 year old man with complex partial seizures treated with vigabatrin who developed bilateral visual field constriction. There was peripheral retinal atrophy with loss of ganglion cells and
loss of nerve fibres in the optic nerves, chiasm, and tracts. No
evidence of intramyelinic oedema was seen. These findings suggest that
the primary site of injury lies within the ganglion cells in the
retina. The degree of atrophy seen would suggest that the visual field
loss is irreversible.
The primary visual pathway in albino mammals is characterized by an increased decussation of retinal ganglion cell axons at the optic chiasm and an enhanced contralateral projection to the dorsal lateral geniculate nucleus. In contrast to the primary visual pathway, little is known about the organization of retinal input to most nuclei of the subcortical visual system in albino mammals. The subcortical visual system is a large group of retinorecipient nuclei in the diencephalon and mesencephalon. These areas mediate a range of behaviors that include both circadian and acute responses to light. We used a congenic strain of albino and pigmented rats with a mutation at the c locus for albinism (Fischer 344-c/+; La Vail and Lawson, 1986) to quantitatively assess the effects of albinism on retinal projections to a number of subcortical visual nuclei including the ventral lateral hypothalamus (VLH), ventral lateral preoptic area (VLPO), olivary pretectal nucleus (OPN), posterior limitans (PLi), commissural pretectal area (CPA), intergeniculate leaflet (IGL), ventral lateral geniculate nucleus (vLGN) and superior colliculus (SC). Following eye injections of the neuroanatomical tracer cholera toxin-β, the distribution of anterogradely transported label was measured. The retinal projection to the contralateral VLH, PLi, CPA and IGL was enhanced in albino rats. No significant differences were found between albino and pigmented rats in retinal input to the VLPO, OPN and vLGN. These findings raise the possibility that enhanced retinofugal projections to subcortical visual nuclei in albinos may underlie some light-mediated behaviors that differ between albino and pigmented mammals.
Retinal projection; Subcortical visual system; Congenic rat; Albino
To obtain and analyze early retinal changes at the molecular level 24 h after a radiation injury to the ipsilateral intraorbital nerve using gamma knife surgery (GKS), and to examine the morphological changes in bilateral optic nerves.
Unilateral intraorbital optic nerves of three rhesus macaques were treated by GKS with irradiated doses of 15 Gy, while contralateral optic nerves and retinas served as the control. Gene expression profiles of the control and affected retinas were analyzed with Affymetrix Rhesus Macaque Genome arrays. To verify the results, a quantitative real-time polymerase chain reaction (qRT–PCR) was performed to test the expression patterns of five function-known genes. Morphological changes in the bilateral optic nerves were examined using a transmission electron microscope (TEM) and light microscopy. The glial cell reaction in bilateral optic nerves was studied using immunohistochemistry.
Of the probe sets, 1,597 (representing 1,081 genes) met the criteria for differential expression, of which 82 genes were significantly up-or down-regulated in treated retinas. There was prominent upregulation of genes associated with glial cell activation in the treated retina. Genes related to an early inflammatory reaction and to cell death were also significantly regulated in response to a radiation injury to the intraorbital optic nerve. In contrast, the messenger ribonucleic acid (mRNA) expression levels of retinal ganglion cell (RGC)-specific genes were low. Morphologically, cytoplasmic processes of astrocytes in treated nerves were shorter than those of the control and were not straight, while also being accompanied by decreased GFAP immunostaining. More oligodendrocytes and inflammatory cells were apparent in treated nerves than in the control. In addition, swollen mitochondria and slight chromation condensation could be seen in the glial cells of treated nerves.
We conclude that the current irradiated dose of 15 Gy was sufficient to lead to a radiation injury of the optic nerve and retina. Several transcripts deregulated in retinas after a radiation injury play a key role in radiation-induced neurogenic visual loss, especially for genes associated with RGC, glial cell, and cell death. Glial cells in optic nerves might be the primary target of a radiation injury in the optic nerve.
Retinal ganglion cell axons and axonal electrical activity have been considered essential for migration, proliferation, and survival of oligodendrocyte lineage cells in the optic nerve. To define axonal requirements during oligodendrogenesis, the developmental appearance of oligodendrocyte progenitors and oligodendrocytes were compared between normal and transected optic nerves. In the absence of viable axons, oligodendrocyte precursors migrated along the length of the nerve and subsequently multiplied and differentiated into myelin basic protein–positive oligodendrocytes at similar densities and with similar temporal and spatial patterns as in control nerves. Since transected optic nerves failed to grow radially, the number of oligodendrocyte lineage cells was reduced compared with control nerves. However, the mitotic indices of progenitors and the percentage of oligodendrocytes undergoing programmed cell death were similar in control and transected optic nerves. Oligodendrocytes lacked their normal longitudinal orientation, developed fewer, shorter processes, and failed to form myelin in the transected nerves. These data indicate that normal densities of oligodendrocytes can develop in the absence of viable retinal ganglion axons, and support the possibility that axons assure their own myelination by regulating the number of myelin internodes formed by individual oligodendrocytes.
oligodendrocyte progenitor cells; axon–glial interactions; myelination; optic nerve; program cell death
At the optic chiasm, retinal ganglion cell (RGC) axons make the decision to either avoid or traverse the midline, a maneuver that establishes the binocular pathways. In mice, the ipsilateral retinal projection arises from RGCs in the peripheral ventrotemporal (VT) crescent of the retina. These RGCs express the guidance receptor EphB1, which interacts with ephrin-B2 on radial glia cells at the optic chiasm to repulse VT axons away from the midline and into the ipsilateral optic tract. However, since VT RGCs express more than one EphB receptor, the sufficiency and specificity of the EphB1 receptor in directing the ipsilateral projection is unclear. In this study, we utilize in utero retinal electroporation to demonstrate that ectopic EphB1 expression can redirect RGCs with a normally crossed projection to an ipsilateral trajectory. Moreover, EphB1 is specifically required for rerouting RGC projections ipsilaterally, as introduction of the highly similar EphB2 receptor is much less efficient in redirecting RGC fibers, even when expressed at higher surface levels. Introduction of EphB1-EphB2 chimeric receptors into RGCs reveals that both extracellular and juxtamembrane domains of EphB1 are required to efficiently convert RGC projections ipsilaterally. Taken together, these data describe for the first time functional differences between two highly similar Eph receptors at a decision point in vivo, with EphB1 displaying unique properties that efficiently drives the uncrossed retinal projection.
optic chiasm; axon guidance; Eph/ephrin signaling; retinal ganglion cells; electroporation; midline
Oculocutaneous albinism is a group of autosomal recessive disorders featuring hypopigmentation of the hair, skin and eyes. Ocular signs associated with the disease are nystagmus, decreased visual acuity, hypopigmentation of the retina, foveal hypoplasia, translucency of the iris, macular transparency, photophobia and abnormal decussation of nerve fibers at the chiasm.
An 8-year-old Caucasian girl presented to our clinic ‘Referral Center for Hereditary Retinopathies’ of the Second University of Naples with a diagnosis of Stargardt disease and a progressive reduction in visual acuity in both eyes. She underwent a complete ophthalmic examination including standard electroretinography and optical coherence tomography (OCT). A molecular analysis was also performed. Best-corrected visual acuity was 20/30 in the right eye and 20/40 in the left eye. Biomicroscopy of the anterior segment revealed a transparent cornea, in situ and transparent lens and normally pigmented iris. A mild diffuse depigmentation and macular dystrophy were observed at fundus examination. Standard electroretinography showed normal scotopic and photopic responses. OCT revealed high reflectivity across the fovea without depression. The typical OCT pattern led us to direct the molecular analysis towards the genes involved in oculocutaneous albinism. The molecular analysis identified mutations in the TYR gene.
In this case, the role of OCT was crucial in guiding the molecular analysis for the diagnosis of albinism. OCT is therefore instrumental in similar cases that do not present typical characteristics of a disease. The case also proves the relevance of molecular analysis to confirm clinical diagnoses in hereditary retinal diseases.
Oculocutaneous albinism; Optical coherence tomography; Atypical albinism
Axonal and neuronal degeneration are important features of multiple sclerosis (MS) and other neurologic disorders that affect the anterior visual pathway. Optical coherence tomography (OCT) is a non-invasive technique that allows imaging of the retinal nerve fiber layer (RNFL), a structure which is principally composed of ganglion cell axons that form the optic nerves, chiasm, and optic tracts. Since retinal axons are nonmyelinated until they penetrate the lamina cribrosa, the RNFL is an ideal structure (no other central nervous system tract has this unique arrangement) for visualizing the processes of neurodegeneration, neuroprotection and, potentially, even neuro-repair. OCT is capable of providing high-resolution reconstructions of retinal anatomy in a rapid and reproducible fashion and permits objective analysis of the RNFL (axons) as well as ganglion cells and other neurons in the macula. In a systematic OCT examination of multiple sclerosis (MS) patients, RNFL thickness and macular volumes are reduced when compared to disease-free controls. Conspicuously, these changes, which signify disorganization of retinal structural architecture, occur over time even in the absence of a history of acute demyelinating optic neuritis. RNFL axonal loss in MS is most severe in those eyes with a corresponding reduction in low-contrast letter acuity (a sensitive vision test involving the perception of gray letters on a white background) and in those patients who exhibit the greatest magnitude of brain atrophy, as measured by validated magnetic resonance imaging techniques. These unique structure–function correlations make the anterior visual pathway an ideal model for investigating the effects of standard and novel therapies that target axonal and neuronal degeneration. We provide an overview of the physics of OCT, its unique properties as a non-invasive imaging technique, and its potential applications toward understanding mechanisms of brain tissue injury in MS, other optic neuropathies, and neurologic disorders.
Electronic supplementary material
The online version of this article (doi:10.1007/s13311-010-0005-1) contains supplementary material, which is available to authorized users.
Optical coherence tomography; retina; optic nerve; multiple sclerosis; vision
In Xenopus tadpoles, all retinal ganglion cells (RGCs) send axons contralaterally across the optic chiasm. At metamorphosis, a subpopulation of EphB-expressing RGCs in the ventrotemporal retina begin to project ipsilaterally. However, when these metamorphic RGCs are grafted into embryos, they project contralaterally, suggesting that the embryonic chiasm lacks signals that guide axons ipsilaterally. Ephrin-B is expressed discretely at the chiasm of metamorphic but not premetamorphic Xenopus. When expressed prematurely in the embryonic chiasm, ephrin-B causes precocious ipsilateral projections from the EphB-expressing RGCs. Ephrin-B is also found in the chiasm of mammals, which have ipsilateral projections, but not in the chiasm of fish and birds, which do not. These results suggest that ephrin-B/EphB interactions play a key role in the sorting of axons at the vertebrate chiasm.
Pax2 is essential for the development of the urogenital system, neural tube, otic vesicle, optic cup and optic tract. Within the visual system, a loss-of-function leads to lack of choroid fissure closure (known as a coloboma), a loss of optic nerve astrocytes, and anomalous axonal pathfinding at the optic chiasm. This study is directed at determining the effects of ectopic Pax2 expression in the chick ventral optic cup past the normal developmental period when Pax2 is found. In ovo electroporation of Pax2 into the chick ventral optic cup results in the formation of colobomas, a condition typically associated with a loss of Pax2 expression. While the overexpression of Pax2 appears to phenocopy a loss of Pax2, the mechanism of the failure of choroid fissure closure is associated with a cell fate switch from ventral retina and retinal pigmented epithelium (RPE) to an astrocyte fate. Further, ectopic expression of Pax2 in RPE appears to have non-cell autonomous effects on adjacent RPE, creating an ectopic neural retina in place of the RPE.
Retina; Pax2; Chick; Optic Cup; Coloboma; Ectopic retina
Appropriate development of the retina and optic nerve requires that the forebrain-derived optic neuroepithelium undergoes a precisely coordinated sequence of patterning and morphogenetic events, processes which are highly influenced by signals from adjacent tissues. Our previous work has suggested that transcription factor activating protein-2 alpha (AP-2α; Tcfap2a) has a non-cell autonomous role in optic cup (OC) development; however, it remained unclear how OC abnormalities in AP-2α knockout (KO) mice arise at the morphological and molecular level. In this study, we show that patterning and morphogenetic defects in the AP-2α KO optic neuroepithelium begin at the optic vesicle stage. During subsequent OC formation, ectopic neural retina and optic stalk-like tissue replaced regions of retinal pigment epithelium. AP-2α KO eyes also displayed coloboma in the ventral retina, and a rare phenotype in which the optic stalk completely failed to extend, causing the OCs to be drawn inward to the midline. We detected evidence of increased sonic hedgehog signaling in the AP-2α KO forebrain neuroepithelium, which likely contributed to multiple aspects of the ocular phenotype, including expansion of PAX2-positive optic stalk-like tissue into the OC. Our data suggest that loss of AP-2α in multiple tissues in the craniofacial region leads to severe OC and optic stalk abnormalities by disturbing the tissue–tissue interactions required for ocular development. In view of recent data showing that mutations in human TFAP2A result in similar eye defects, the current findings demonstrate that AP-2α KO mice provide a valuable model for human ocular disease.
Optical coherence tomography (OCT) uses light interference patterns to produce a cross-sectional image of the retina. It is capable of measuring the unmyelinated axons of the retinal ganglionar cells as they converge on the optic disc. In a disease like multiple sclerosis (MS), in which axonal loss has been identified as an important cause of sustained disability, it may prove an invaluable tool. OCT has demonstrated that axonal loss occurs after each episode of optic neuritis and that the degree of axonal loss is correlated to visual outcomes. Furthermore, axonal loss occurs in MS even in the absence of inflammatory episodes, and the degree of this loss is correlated with the duration of the disease process, with more thinning as the disease advances and in progressive forms. Thus, OCT retinal nerve fiber layer measurements may represent an objective outcome measure with which to evaluate the effect of treatment.