Visual impairment in FOXG1-mutated individuals and mice.

The Forkead Box G1 (FOXG1 in humans, Foxg1 in mice) gene encodes for a DNA-binding transcription factor, essential for the development of the telencephalon in mammalian forebrain. Mutations in FOXG1 have been reported to be involved in the onset of Rett Syndrome, for which sequence alterations of MECP2 and CDKL5 are known. While visual alterations are not classical hallmarks of Rett syndrome, an increasing body of evidence shows visual impairment in patients and in MeCP2 and CDKL5 animal models. Herein we focused on the functional role of FOXG1 in the visual system of animal models (Foxg1(+/Cre) mice) and of a cohort of subjects carrying FOXG1 mutations or deletions. Visual physiology of Foxg1(+/Cre) mice was assessed by visually evoked potentials, which revealed a significant reduction in response amplitude and visual acuity with respect to wild-type littermates. Morphological investigation showed abnormalities in the organization of excitatory/inhibitory circuits in the visual cortex. No alterations were observed in retinal structure. By examining a cohort of FOXG1-mutated individuals with a panel of neuro-ophthalmological assessments, we found that all of them exhibited visual alterations compatible with high-level visual dysfunctions. In conclusion our data show that Foxg1 haploinsufficiency results in an impairment of mouse and human visual cortical function.

Age-dependent remodelling of retinal circuitry.

We have investigated morphological changes in second-order neurons of the mouse retina during aging by using immunohistochemistry and electron microscopy. We observed sprouting of rod bipolar cells dendrites and horizontal cells arborizations: neuronal processes of both neuronal types showed irregular extensions beyond the outer plexiform layer, toward the outer limiting membrane, as well as into the outer nuclear layer (ONL). These processes were first observed in animals of 12 months of age and increased in numbers steadily until 24 months, which represent the last age examined. The ectopic processes are decorated by puncta immunoreactive for pre-synaptic markers typical of photoreceptor terminals juxtaposed to post-synaptic neurotransmitter receptors, demonstrating the presence of the entire molecular machinery of functional synapses. Electron microscopy confirmed that ectopic processes receive synapses from photoreceptor terminals. We conclude that during the second year of life retinal rod bipolar and horizontal cells undergo sprouting and form ectopic synapses in the ONL.

The spatial organization of cholinergic mosaics in the adult mouse retina.

We analysed the spatial organization of the cholinergic amacrine cell mosaics in the mouse retina, as part of a general study of the major mouse retinal arrays, aiming at providing intrinsic cellular reference grids to monitor anomalies in retinal growth and/or functional organization in mouse models of retinal degeneration. The spatial organization of the cells was analysed by means of the nearest neighbour distance analysis, as well as by the analysis of Voronoi and Delaunay tesselations. We found non random cell spacing in both cholinergic arrays, although the mosaic in the ganglion cell layer tiles the retina scarcely better than a random distribution. Autocorrelation analysis revealed no detectable pattern in cell positioning, but there was a tendency towards a minimal spacing between array elements. Finally, we found no correlation in the spatial organization of the two arrays.

Modifications of retinal neurons in a mouse model of retinitis pigmentosa.

Animal models of retinitis pigmentosa include the rd mouse, in which a mutation of a rod-specific phosphodiesterase leads to the rapid loss of photoreceptors during the early postnatal life. Very little is known about changes occurring in inner retinal neurons after photoreceptor loss. These changes are important in view of the possibility of restoring vision in retinas with photoreceptor degeneration by means of cell transplantation or direct stimulation of inner layers. In this paper, we show that bipolar and horizontal cells of the rd mouse retina undergo dramatic morphological modifications accompanying photoreceptor loss, demonstrating a dependence of second order neurons on these cells. While describing modifications of the rd retina, we also provide quantitative information about neurons of the wild-type mouse retina, useful for future studies on genetically altered animals.

Optic nerve crush: axonal responses in wild-type and bcl-2 transgenic mice.

Retinal ganglion cells of transgenic mice overexpressing the anti-apoptotic protein Bcl-2 in neurons show a dramatic increase of survival rate after axotomy. We used this experimental system to test the regenerative potentials of central neurons after reduction of nonpermissive environmental factors. Survival of retinal ganglion cells 1 month after intracranial crush of the optic nerve was found to be 100% in adult bcl-2 mice and 44% in matched wild-type (wt) mice. In the optic nerve, and particularly at the crush site, fibers regrowing spontaneously or simply sprouting were absent in both wt and bcl-2 mice. We attempted to stimulate regeneration implanting in the crushed nerves hybridoma cells secreting antibodies that neutralize central myelin proteins, shown to inhibit regeneration (IN-1 antibodies) (Caroni and Schwab, 1988). Again, we found that regeneration of fibers beyond the site of crush was virtually absent in the optic nerves of both wt and bcl-2 mice. However, in bcl-2 animals treated with IN-1 antibodies, fibers showed sprouting in the proximity of the hybridoma implant. These results suggest that neurons overexpressing bcl-2 are capable of surviving axotomy and sprout when faced with an environment in which inhibition of regeneration has been reduced. Nevertheless, extensive regeneration does not occur, possibly because other factors act by preventing it.

The major cell populations of the mouse retina.

We report a quantitative analysis of the major populations of cells present in the retina of the C57 mouse. Rod and cone photoreceptors were counted using differential interference contrast microscopy in retinal whole mounts. Horizontal, bipolar, amacrine, and Müller cells were identified in serial section electron micrographs assembled into serial montages. Ganglion cells and displaced amacrine cells were counted by subtracting the number of axons in the optic nerve, learned from electron microscopy, from the total neurons of the ganglion cell layer. The results provide a base of reference for future work on genetically altered animals and put into perspective certain recent studies. Comparable data are now available for the retinas of the rabbit and the monkey. With the exception of the monkey fovea, the inner nuclear layers of the three species contain populations of cells that are, overall, quite similar. This contradicts the previous belief that the retinas of lower mammals are "amacrine-dominated", and therefore more complex, than those of higher mammals.

Protection of retinal ganglion cells and preservation of function after optic nerve lesion in bcl-2 transgenic mice.

Multicellular organisms face the necessity of removing superfluous or injured cells during normal development, tissue turn-over and in response to damaging conditions. These finalised killings occur throughout a process, commonly called programmed cell death (PCD), which is placed under strict cellular control. PCD is regulated by the products of the expression of a number of genes. This fact raises the intriguing possibility of inhibiting such degenerative processes by operating on some of the controlling genes. Central neurons of transgenic mice overexpressing bcl-2, a powerful inhibitor of PCD, are remarkably resistant to degeneration induced by noxious stimuli. We have explored the ate of retinal ganglion cells and of their axons, when such transgenic animals have been challenged by a lesion of the optic nerve. These results have direct bearing on the possibility of attaining functional restoration of the injured pathway.

Axonal transport blockade in the neonatal rat optic nerve induces limited retinal ganglion cell death.

Optic nerve section in the newborn rat results in a rapid apoptotic degeneration of most axotomized retinal ganglion cells (RGCs). This massive process of neuronal death has been ascribed mainly to the interruption of a trophic factor supply from target structures rather than to the axonal damage per se. To distinguish between these two possibilities, we induced a reversible axonal transport blockade in the developing optic nerve by topical application of a local anesthetic (lidocaine). Light and electron microscopy showed no alterations in the fine structure of treated optic nerves. Retinae of treated and control rats were stained with cresyl violet and examined at different times after surgery. We found that axonal transport blockade induced only a limited number of pyknotic RGCs. Degeneration of these cells was completely prevented by inhibiting protein synthesis during lidocaine application. We conclude that the rapid degeneration of RGCs after axotomy can be ascribed only partly to the loss of retrogradely transported trophic factors.

Cell physiology of the pineal body.

The results from recent experiments on the cellular physiology of the trout pineal photoreceptors are briefly reviewed. The arguments are mainly concerned with pineal phototransduction. These studies have stimulated further research on melatonin, a molecule produced in pineal as well as in retinal photoreceptors. A discussion follows on our actual research object, that is a study of the influences of endogenous melatonin upon retinal receptor cells activities.

The number of unidentified amacrine cells in the mammalian retina.

The three largest known populations of amacrine cells in the rabbit retina were stained with fluorescent probes in whole mounts and counted at a series of retinal eccentricities. The retinas were counterstained using a fluorescent DNA-binding molecule and the total number of nuclei in the inner nuclear layer were counted in confocal sections. From the total number of inner nuclear layer cells and the known fraction of them occupied by amacrine cells, the fraction of amacrine cells made up by the stained populations could be calculated. Starburst cells made up 3%, indoleamine-accumulating cells made up 4%, and AII cells made up 11% of all amacrine cells. By referring four smaller populations of amacrine cells to the number of indoleamine-accumulating cells, they were estimated to make up 4% of all amacrine cells. Thus, 78% of all amacrine cells in the rabbit's retina are known only from isolated examples, if at all. This proportion is similar in the retinas of the mouse, cat, and monkey. It is likely that a substantial fraction of the local circuit neurons present in other regions of the central nervous system are also invisible as populations to current techniques.

Long-term survival of retinal ganglion cells following optic nerve section in adult bcl-2 transgenic mice.

The bcl-2 gene codes for a protein that acts as a powerful inhibitor of active cell death. Since the transection of the optic nerve in adult mammalians starts a massive process of degeneration in retinal ganglion cells, we investigated whether the overexpression of bcl-2 in adult transgenic mice can protect the axotomized ganglion cells. We performed intracranial optic nerve transection on both wild type and transgenic adult mice, and we tested cell survival 2 or 3.5 months after axotomy. The percentage of surviving ganglion cells after optic nerve section was computed by combining the counts of the optic nerve fibres in intact nerves with the cell density measures of the ganglion cell layer of axotomized retinae. From these data we found that in transgenic mice approximately 65% of ganglion cells survived 3.5 months after axotomy. In contrast, 2 months after surgery, < 10% of ganglion cells were left in wild type retinae. We have also examined the morphology and fine structure of the proximal stump of the sectioned optic nerves by light and electron microscopy. In the transgenic mice a very large number of axons survived after surgery and they still exhibited fairly normal morphology and ultrastructure. On the other hand the wild type transected nerves had only a few visible axons that displayed clear signs of degeneration. We conclude that the overexpression of Bcl-2 protein in central neurons is a very effective strategy to ensure long-term survival in axotomized cells.

Protection of retinal ganglion cells from natural and axotomy-induced cell death in neonatal transgenic mice overexpressing bcl-2.

Approximately half of the retinal ganglion cells (RGCs) present in the rodent retina at birth normally die during early development. Overexpression of the photo-oncogene bcl-2 recently has been shown to rescue some neuronal populations from natural cell death and from degeneration induced by axotomy of nerves within the peripheral nervous system. Here we study in vivo the role of the overexpression of bcl-2 in the natural cell death of RGCs and in the degenerative process induced in these cells by transection of the optic nerve. We find that in newborn bcl-2 transgenic mice, the number of RGCs undergoing natural cell death is considerably lower than in wild-type pups. Consistently, a vast majority (90%) of the ganglion cells found in the retina of neonatal transgenics are maintained in adulthood, whereas only 40% survive in wild-type mice. After transection of the optic nerve, the number of degenerating ganglion cells, determined by counting pyknotic nuclei or nuclei with fragmented DNA, is substantially reduced in transgenic mice. In wild-type animals, almost 50% of ganglion cells degenerate in the 24 hr after the lesion, whereas almost the entire ganglion cell population survives axotomy in transgenic mice. Therefore, overexpression of bcl-2 is effective in preventing degeneration of this neuronal population, raising the possibility that ganglion cells are dependent on the endogenous expression of bcl-2 for survival. The remarkable rescue capacity of bcl-2 overexpression in these neurons makes it an interesting model for studying natural cell death and responses to injury in the CNS.

The organization of the inner nuclear layer of the rabbit retina.

The initial goal of this study was to establish an accounting of the major classes of cells present in the inner nuclear layer (INL) of the rabbit's retina. Series of 80-100 radial sections 1 micron thick were cut from retinal blocks dissected at intervals along the vertical meridian. They were photographed at high magnification in the light microscope. By visualizing the initial segments of processes leaving the somata, we could identify each cell as a bipolar, amacrine, horizontal, or Müller cell. The identifications made by light microscopy were confirmed by electron microscopy of alternating ultrathin sections. On average, bipolar cells made up 41% of the total INL cells, amacrine cells 32%, horizontal cells 1.5%, and Müller cells 24%. These fractions varied relatively little across the retina or among different animals. We next immunolabeled the rod bipolar cells of whole-mounted retinas with antibodies against protein kinase C, using FITC as the visualizing agent. The same retinas were counterstained with a DNA-binding probe that fluoresces at longer wavelengths. Serial optical horizontal sections of the double-labeled wholemounts were made by confocal microscopy. On average, rod bipolars accounted for 10% of the total INL cells. By subtraction, the cone bipolars made up 31% of the total cells. We conclude that cone bipolars substantially outnumber rod bipolars, even in a retina in which rods outnumber cones by more than 20:1. Using the base of reference created here, a similar analysis can be carried out for other subclasses of retinal neuron. Because the analysis does not depend on absolute cell densities or corrections for shrinkage, data acquired by different histochemical techniques may be combined.

Cone bipolar cells as interneurons in the rod pathway of the rabbit retina.

In the mammalian retina, rod signals are transmitted by rod bipolars to the narrow-field, bistratified (AII) amacrine cell. This neuron, in turn, makes gap junctions with the axonal arborization of cone bipolar cells that reside in the vitreal half (sublamina b) of the inner plexiform layer (IPL). After examining rod bipolars and AII amacrines in the rabbit retina, we have now reconstructed from electron micrographs of continuous series of thin sections the synaptic connections of the axonal arborizations of cone bipolar cells that make the highest number of gap junctions with AII amacrines. These axonal arborizations were narrowly confined to stratum 4 (S4) of the IPL and made ribbon synapses to dyads of postsynaptic dendrites that belonged to either ganglion or amacrine cells. In the population of postsynaptic processes, 30% were ganglion cell dendrites. These dendrites were probably originating, at least in part, from on-center ganglion cells because their course was confined to sublamina b of the IPL. Of the remaining postsynaptic processes, 51.7% belonged to amacrine cells and 18.3% were not identified. Among the postsynaptic amacrine cell processes, 33.3% returned a reciprocal synapse onto the cone bipolar endings. These reciprocal synapses represented 21.3% of the total input onto the axonal arborizations, the remaining fraction (78.7%) arising from a heterogeneous population of amacrine dendrites that were purely presynaptic to the cone bipolars endings. Pre- and postsynaptic amacrines were part of several distinct microcircuits which suggest complex local processing of both rod and cone signals. Thus, the cone bipolars that make gap junctions with AII amacrines in sublamina b of the rabbit IPL exhibit a substantial output onto ganglion cells. This fact, in conjunction with our previous observations that in this sublamina ganglion cells receive negligible input from rod bipolars and AII amacrines, demonstrates that in the rabbit cone bipolars represent a necessary link in the pathway followed by rod signals to enter on-center ganglion cells. Thus, rod and cone signals ultimately share the same integrating mechanisms and converge onto the same set of ganglion cells.

The peripheral-type benzodiazepine receptor ligands [3H]Ro 5-4864 and [3H]PK 11195 bind to the retina of rabbit, but not of turtle.

The binding of [3H]flunitrazepam, [3H]Ro 5-4864, and [3H]PK 11195 to membrane preparations of the retina was studied in the turtle and rabbit. Only a single population of [3H]flunitrazepam binding sites was detected in the turtle, whereas two populations appeared to be present in the rabbit. No specific binding for [3H]Ro 5-4864 and [3H]PK 11195 could be detected in the turtle. In rabbit, both ligands bound with high affinity, revealing a significant population of binding sites (KD values of 24 +/- 2.3 and 2.2 +/- 0.8 nM, and Bmax values of 440 +/- 35 and 1,482 +/- 110 fmol/mg of protein, respectively). The binding was temperature- and protein-dependent. Displacement studies showed a similar rank order of potency of various unlabeled ligands against both [3H]Ro 5-4864 and [3H]PK 11195 (PK 11195 > Ro 5-4864 > flunitrazepam > flumazenil). These results suggest that peripheral-type benzodiazepine receptors are present in the retina of the rabbit, but not of the turtle.

Appearance of cGMP-phosphodiesterase immunoreactivity parallels the morphological differentiation of photoreceptor outer segments in the rat retina.

We have investigated by immunofluorescence the appearance of immunoreactive guanosine 3'-5' cyclic monophosphate phosphodiesterase (cGMP-PDE) during the postnatal development of the retina of the pigmented rat. We show that a sudden increase in immunoreactivity takes place during postnatal day five (P5), when rod outer segments begin to form; immunoreactivity develops rapidly in the following days. Labeling is restricted to the developing photoreceptor outer segments, sparing other retinal cells, as confirmed by electron microscopy immunocytochemistry. In addition, cGMP-PDE immunoreactivity follows a center-to-periphery gradient paralleling photoreceptor differentiation. It appears that cGMP-PDE is expressed when the photoreceptor subcellular compartments are already formed, and represents a specific marker of late photoreceptor differentiation. The appearance of cGMP-PDE during development is temporally correlated with the appearance of other proteins of the phototransduction machinery.

Synaptic connections of the narrow-field, bistratified rod amacrine cell (AII) in the rabbit retina.

The synaptic connections of the narrow-field, bistratified rod amacrine cell (AII) in the inner plexiform layer (IPL) of the rabbit retina were reconstructed from electron micrographs of continuous series of thin sections. The AII amacrine cell receives a large synaptic input from the axonal endings of rod bipolar cells in the most vitreal region of the IPL (sublamina b, S5) and a smaller input from axonal endings of cone bipolar cells in the scleral region of the IPL (sublamina a, S1-S2). Amacrine input, localized at multiple levels in the IPL, equals the total number of synapses received from bipolar cells. The axonal endings of cone bipolar cells represent the major target for the chemical output of the AII amacrine cell: these synapses are established by the lobular appendages in sublamina a (S1-S2). Ganglion cell dendrites represent only 4% of the output of the AII amacrine and most of them are also postsynaptic to the cone bipolars which receive AII input. The AII amacrine is not presynaptic to other amacrine cells. Finally, the AII amacrine makes gap junctions with the axonal arborizations of cone bipolars that stratify in sublamina b (S3-S4) as well as with other AII amacrine cells in S5. Therefore, in the rabbit retina 1) the rod pathway consists of five neurons arranged in series: rod-->rod bipolar-->AII amacrine-->cone bipolar-->ganglion cell; 2) it seems unlikely that a class of ganglion cells exists that is exclusively devoted to scotopic functions. In ventral, midperipheral retina, about nine rod bipolar cells converge onto a single AII amacrine, but one of them establishes a much higher proportion of synaptic contacts than the rest. Conversely, each rod bipolar cell diverges onto four AII amacrine cells, but one of them receives the largest fraction of synapses. Thus, within the pattern of convergence and divergence suggested by population studies, preferential synaptic pathways are established.

Synaptic connections of rod bipolar cells in the inner plexiform layer of the rabbit retina.

We have reconstructed from electron micrographs of a continuous series of thin sections the synaptic connections of the axonal arborizations of all the rod bipolar cells contained in a small region of the retina of the rabbit. We observed that all rod bipolars share the same pattern of connectivity and are probably functionally equivalent. As a rule, they do not contact ganglion cells. Their prevalent synaptic output is on narrow-field, bistratified, and indoleamine-accumulating amacrine cells. Their dominant inputs are the reciprocal synapses from the indoleamine-accumulating amacrines, but they also receive a sizable number of synaptic contacts from other, non-reciprocal, amacrine cells. The lateral spread of scotopic signals at the synapse between rod bipolars and narrow-field, bistratified amacrines is small. Finally, in the rabbit, as in the cat, a narrow-field, bistratified amacrine is inserted in series along the rod pathway.

Involvement of D1 and D2 Dopamine Receptors in the Control of Horizontal Cell Electrical Coupling in the Turtle Retina.

We studied the actions of D1 and D2 dopamine agonists and antagonists on the coupling of horizontal cell axons in the turtle retina by a combination of pharmacological and electrophysiological methods. Both D1 and D2 receptors were identified in membrane fractions by radioligand binding using [3H]-SCH 23390 and [3H]-spiperone, respectively. The KD of both receptor classes were identical (0.21 nM) but D1 receptor density exceeded that of D2 receptors by more than four-fold. D1 agonists increased the activity of adenylate cyclase in a dose-dependent manner, whereas D2 agonists were without significant effect by themselves, nor did D2 antagonists block the D1-mediated increase in adenylate cyclase activity. Intracellular recordings and Lucifer Yellow dye injections were used to characterize the modifications of the receptive field profile of horizontal cell axons (H1AT) exposed to different pharmacological agents. Dopamine or D1 agonists (0.05 - 10 microM) induced a marked constriction of the H1AT receptive field, whereas D2 agonists elicited a small expansion of the receptive field. However, in the presence of a D1 antagonist, as well as IBMX to inhibit phosphodiesterase, D2 agonists (10 - 70 microM) induced a marked increase in the receptive field profile. These results indicate that both D1 and D2 dopamine receptors play a role in shaping the receptive field profile of the horizontal cell axon terminal in the turtle retina.