Formation of functional retinotectal connections in co-cultures of fetal mouse explants

Optimized Culture System to Induce Neurite Outgrowth From Retinal Ganglion Cells in Three-Dimensional Retinal Aggregates Differentiated From Mouse and Human Embryonic Stem Cells

PURPOSE

To establish a practical research tool for studying the pathogenesis of retinal ganglion cell (RGC) diseases, we optimized culture procedures to induce neurite outgrowth from three-dimensional self-organizing optic vesicles (3D-retinas) differentiated in vitro from mouse and human embryonic stem cells (ESCs).

MATERIALS AND METHODS

The developing 3D-retinas isolated at various time points were placed on Matrigel-coated plates and cultured in media on the basis of the 3D-retinal culture or the retinal organotypic culture protocol. The number, length, and morphology of the neurites in each culture condition were compared.

RESULTS

First, we confirmed that Venus-positive cells were double-labeled with a RGC marker, Brn3a, in the 3D-retina differentiated from Fstl4::Venus mouse ESCs, indicating specific RGC-subtype differentiation. Second, Venus-positive neurites grown from these RGC subsets were positive for beta-III tubulin and SMI312 by immunohistochemistry. Enhanced neurite outgrowth was observed in the B27-supplemented Neurobasal-A medium on Matrigel-coated plates from the optic vesicles isolated after 14 days of differentiation from mouse ESCs. For the differentiated RGCs from human ESCs, we obtained neurite extension of >4 mm by modifying Matrigel coating and the culture medium from the mouse RGC culture.

CONCLUSION

We successfully optimized the culture conditions to enhance lengthy and high-frequency neurite outgrowth in mouse and human models. The procedure would be useful for not only developmental studies of RGCs, including maintenance and projection, but also clinical, pathological, and pharmacological studies of human RGC diseases.

Extrinsic GABAergic innervation of developing neocortical layer 1 in organotypic slice co-cultures

Afferents from the zona incerta (ZI) of the ventral thalamus contribute to the dense, transient gamma-aminobutyric acid (GABA)ergic fiber plexus in layer 1 of the developing rodent somatosensory cortex. Incertocortical axons contact the distal apical dendrites of postmigratory cortical pyramidal cells. Although recent work has shown that these GABAergic incertocortical fibers are likely to provide widespread fast synaptic excitation of pyramidal cells in layers 2-6 during peak periods of cortical synaptogenesis, little is known about the mechanisms by which these axons project to the neocortex and are confined to layer 1. Here we characterize organotypic slice co-cultures in which a region of embryonic diencephalon containing the ZI is maintained adjacent to a region of embryonic somatosensory cortex. Diencephalic explants from transgenic mice expressing enhanced green fluorescent protein (EGFP) enabled direct visualization of diencephalocortical connections. Isochronic co-cultures exhibited diencephalocortical fiber ingrowth immunoreactive for both GABA and the presynaptic vesicle-associated protein synaptophysin that was restricted to neocortical layer 1. This pattern of lamina-specific diencephalocortical ingrowth occurred irrespective of placement of the afferent explant, and persisted in the absence of action potential activity and GABA(A) receptor activation. Heterochronic co-cultures containing older cortex demonstrated that the cortical explants remain permissive for lamina-specific ingrowth through the first postnatal week. Organotypic slice cocultures provide a system in which to study the mechanisms underlying the layer 1-specific ingrowth of extrinsic GABAergic inputs to the perinatal neocortex.

Development of specific synaptic network functions in organotypic central nervous system (CNS) cultures: implications for transplantation of CNS neural cells in vivo

This article provides a broad overview of the significant roles that morphophysiologic analyses of organotypic cultures of neural tissues explanted in vitro-initiated during the 1950s-have played in stimulating the more recent development of techniques for transplantation of neural cells and tissues into specific regions of the central nervous system (CNS) in vivo. The demonstrations by Crain and co-workers in the 1950s and 1960s that fetal rodent and human CNS neurons can continue to develop a remarkable degree of mature structure and function during many months of complete isolation in culture provided crucial evidence that development of many organotypic properties of nerve cells is regulated by epigenetic factors that ensure rather stereotyped expression despite wide variations in environmental conditions. These in vitro studies strongly suggested that fetal neural cells should, indeed, be capable of even more highly organotypic development after transplantation in vivo, as has been elegantly demonstrated by many of the successful CNS transplantation studies reviewed here.

Neuritogenesis: a model for space radiation effects on the central nervous system

Pivotal to the astronauts' functional integrity and survival during long space flights are the strategies to deal with space radiations. The majority of the cellular studies in this area emphasize simple endpoints such as growth related events which, although useful to understand the nature of primary cell injury, have poor predictive value for extrapolation to more complex tissues such as the central nervous system (CNS). In order to assess the radiation damage on neural cell populations, we developed an in vitro model in which neuronal differentiation, neurite extension, and synaptogenesis occur under controlled conditions. The model exploits chick embryo neural explants to study the effects of radiations on neuritogenesis. In addition, neurobiological problems associated with long-term space flights are discussed.

Formation of functional synapses by regenerating adult rat retinal ganglion cell axons in midbrain target regions in vitro

The ability of adult rat retinal ganglion cell (RGC) axons to reinnervate normal target regions was examined in vitro. In co-culture experiments, adult rat retinal explants were placed adjacent to fetal rat midbrain sections that contained the superior colliculus (SC) which is the main target for RGC axons. Adult rat RGCs regrew axons over more than 500 microns on a polylysine-laminin substrate to reach the co-cultured explants. By using neurofilament immunohistochemistry and the fluorescent dye DiI for anterograde and retrograde tracing, it was shown that (1) adult rat RGCs with a stereotyped morphology survived in explant cultures for more than 4 weeks in the presence of fetal midbrain explants, (2) regenerating RGC axons preferentially terminated within midbrain target regions, and (3) RGCs formed functional synapses. In addition, the maturation of the SC region in midbrain explants was examined histologically and ultrastructurally to demonstrate appropriate target development.

Cultured fetal tectal tissue grafted to the midbrain of newborn rats: morphology of grafts and innervation by host retinal and cortical axons

It has been shown previously that tectal tissue obtained from young embryos can be successfully transplanted to the neonatal rat brain. In the present study, tecta from E15 rat embryos were maintained as free-floating explants for 3-14 days in vitro (DIV) before being transplanted to the midbrain of newborn rats. We wished to determine how explant culture affected (i) graft survival, (ii) the subsequent morphological and histochemical development of tectal grafts and (iii) the specificity with which host retinal and cortical axons grew into and innervated the graft neuropil. Grafts were examined 6-40 weeks posttransplantation. Host retinal input was assessed by injecting the host eyes with either [3H]proline, horseradish peroxidase (HRP) or wheat-germ agglutinin conjugated HRP. The host cortical projection was examined using anterograde degeneration techniques. Frozen tissue sections were also stained for Nissl, neurofibrils or reacted for acetylcholinesterase (AChE). All 3 DIV and 7 DIV explants survived transplantation and many grew considerably in size within the host brain. 14 DIV grafts were smaller and were found in only 50% of host brains. The cellular organization, fibre architecture and pattern of AChE staining in cultured grafts was similar to that found in non-cultured tectal transplants. Furthermore, host retina and cortex projected into the grafts in a manner similar to their innervation of non-cultured tectal tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular and cellular aspects of nerve regeneration

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state; second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and eventually cease their elongation.

Chemical neurotoxicity: detection and analysis in organotypic cultures of sensory and motor systems

Screening of chemical substances for human neurotoxic (and therapeutic) properties may be carried out with the aid of organotypic tissue cultures composed of foetal explants of mouse sensory and neuromuscular tissues that develop in vitro their characteristic cytoarchitectural and functional organization. Supporting this statement is a wealth of studies describing a range of specific, chemically-induced responses in organotypic neural cultures that parallel changes induced in the nervous system of humans and animals.

Projections of growth-cone-bearing fibers of retinal ganglion cells within co-cultured tectal explants: early branching depends on age of target tissue

The projections of retinal ganglion cell axons within co-cultured tectal explants were analyzed in order to investigate some of the factors that determine the earliest responses of retinal axons to cues present in an isolated target tissue. Half retinas and superior colliculi (tecta) from the embryonal mouse were explanted, separated by a 0.5 mm gap. After 5 days in vitro retinal ganglion cells were labeled by extracellular ionophoresis of HRP into the optic nerve head region. Cleared co-cultures were studied as whole mounts. Growth-cone-bearing retinal fibers were studied in standard tectal co-cultures, and in cases where tectum had been explanted 2 weeks prior to retina. The heterochronously prepared cultures had a higher proportion of fibers with complex branching patterns than the synchronous explants. Cultures in which retinas were explanted 1 week after tecta exhibited intermediate proportions of such fibers. These observations suggest that older tecta facilitate branching of ingrowing retinal fibers, although other alterations during in vitro development must be evaluated. The growth patterns of axons originating in nasal and temporal hemi-retinas were analyzed in terms of possible positional cues provided by the target tecta. Axons originating in temporal hemi-retinas did not show evidence of preferential branching in, or growth toward, appropriate anterior regions of co-cultured tectal explants. In contrast, the majority of nasal retinal axons showed enhanced terminations and complex branching in, and bending towards, the posterior tectum.

Radiosensitivity and differentiation of ganglion cells within fetal mouse retinal explants in vitro

Fetal mouse retinas were explanted at 13-14 days of gestation, and exposed to gamma radiation in vitro. Not all regions of the retina were equally susceptible to radiation-induced necrosis; when exposed to 5000 rads soon after explanation, each explant had a single small radioresistant nubbin of apparently intact tissue, located near the optic nerve-head. This region of radioresistant tissue was larger when the dose of radiation was reduced and when the explants were exposed at later times in vitro, indicating the existence of a gradient of radioresistance across retinal explants which spread outward through at least the first week in vitro, the period examined. Based upon the extensive in situ literature which has correlated the emergence of radioresistance with the differentiation of retinal neurons, we conclude that the in situ central-to-peripheral sequence of cellular differentiation continues in vitro within our retinal explants. Whereas the ganglion cell axonal outgrowth from control retinas grown in isolation on collagen substrates underwent a gradual disintegration over 3 weeks in vitro, the sparse axonal outgrowth from explants exposed to 5000 rads disintegrated abruptly at 5-7 days in vitro. This did not appear to be due to direct damage from radiation, but instead reflected the fact that axons in irradiated cultures arose from central retinal regions only, while many axons in control cultures emerged from later-differentiating peripheral regions. We suggest that disintegration of individual axons in the outgrowth may occur rapidly and in a central-to-peripheral sequence. These findings should be useful in designing assays for trophic factors which may prevent ganglion cell axon disintegration in this in vitro model system.

Catecholamine-containing neurons in cultures of fetal rat hypothalamus: distribution, morphology, and maturation

The distribution, morphology, and maturation of catecholamine (CA) neurons have been studied in hypothalamic explants from late-gestation rats. CA-containing neurons were identified using the glyoxylic acid technique. CA-containing processes were present from all hypothalamic areas except the preoptic region. Several fiber types were identified. CA neurons in vitro resemble CA neurons in adult hypothalamus. This tissue culture system may be useful in the study of a number of properties of hypothalamus CA neurons.

Neurotropic influence of mesenchymal limb target tissue on spinal cord neurite growth in vitro

Peripheral nerve fiber outgrowth from developing spinal cord is proposed to be under the influence of the limb bud target which, at the time of nerve fiber invasion, is in an essentially premuscular, mesenchymal condition. Thus, the true target for elongating spinal nerve fibers in early development is mesenchyme rather than differentiated skeletal muscle. Spinal cord explants derived from stage V larval Rana pipiens were cultured in a defined medium in the presence or absence of mesenchymal limb tissue or limb-conditioned medium (LCM). Analysis of quantified neuritic outgrowth under these conditions demonstrated a dependency on the target tissue for enhanced nerve fiber density and oriented growth. The characteristics of neuritic growth in the presence of limb mesenchyme or LCM changed from the relatively sparse and straight outgrowth of control cords to dense, wavy arborizations. Areas of the cord explants nearest the limb tissue exhibited the greatest increases in nerve fiber density and morphologic complexity. Additionally, an inverse relationship existed between growth enhancement and the cord-to-target distance. Regulation of directed nerve growth in vitro is suggested to result from a diffusible, target-originated growth factor that binds to the attachment substratum as a concentration gradient guidance pathway with implications for mechanisms of in vivo nerve growth.

Positional specificity tests in co-cultures of retinal and tectal explants

In previous studies we demonstrated that HRP-labeled ganglion-cell axons arising from fetal mouse retinal explants can distinguish appropriate (tectum) from inappropriate (spinal cord) CNS target regions in vitro. Retinal fibers preferentially invade the tectum where they ramify, develop terminal arbors, and make functional connections. The present study attempts to determine if fibers from a half-retina prefer the 'appropriate' half-region of a tectal explant. In one series, nasal or temporal halves were placed near the medial edge of the tectum, to test turning toward or away from the appropriate (posterior or anterior) half-tectal region. Several co-cultures showed apparent preferences, especially those with many ingrowing retinal fibers; but in other co-cultures the data were equivocal. A second paradigm placed the half-retinas near anterior or posterior tectal edges, and simply scored presence vs absence of retinal fiber ramifications within the nearest tectal half-region. This identified entry of retinal fibers into appropriate vs inappropriate half-tectal regions in 8 out of 8 blind scorings. These data encourage further, more critical analyses of retinotectal co-cultures to explore the basis of the specific neuritic connections which form in situ.

Neurites from mouse retina and dorsal root ganglion explants show specific behavior within co-cultured tectum or spinal cord

We have utilized extracellular microiontophoretic injections of horseradish peroxidase into fetal mouse retinal explants to label retinal ganglion cell axons innervating co-cultured tectal explants in a solid Golgi-like manner. Using dorsal root ganglia-tectum and retina-spinal cord co-cultures as controls, our results indicate that retinal neurites show selective growth and arborizations within their appropriate tectal, target tissue. Retinotectal explant co-cultures may be a useful model system for studying aspects of neuronal specificity.

Development of ganglion cells and their axons in organized cultures of fetal mouse retinal explants

Retinas from 13-15 day fetal mice were explanted alone, with adjacent eyeball tissue, or with nearby superior colliculus explants. The organotypic structure of the retina developed in situ, including photoreceptors, interneurons, plexiform layers, ganglion cells, and an optic fibre layer. Electrophysiologic recordings demonstrated that functional synaptic networks developed resembling bioelectric response patterns seen in situ. Within half-retinas, arrays of optic fibers converged to the optic nerve head; in co-cultures with tectum they could become myelinated. Large bundles of long, naked neurites--1 degree primary retinal fibers--emerged from the explant in the first few days in vitro; these could often be traced back to the optic nerve head and a detailed survey of their properties using horseradish peroxidase (HRP) tracing methods identified tham as ganglion cell axons. When growing upon collagen substrata, 1 degree fibers began to disintegrate during the second week in vitro; however, many 1 degree fibers that grew into superior colliculus explants were maintained for at least 5 weeks in vitro, where they formed elaborate, functional terminal arborizations. In a few cases, 1 degree fibers grew across skeletal muscle fibers and appeared to induce them to contract. A second type of neuritic outgrowth pattern appeared after the first week in vitro: 2 degrees retinal fibers. This was composed of a mixed population of interneuronal neurites; a small percentage was catecholaminergic. Our characterization of the morphologic properties of retinal ganglion cells and their axons in organotypic cultures provides the necessary background to interpret electrophysiologic mapping and neural-specificity analyses of retino-CNS co-cultures. This in vitro model system may have biological relevance to understanding the cues that control the development of the retinotectal projection in situ.