Characterization of a mouse model with complete RPE loss and its use for RPE cell transplantation

PURPOSE

Age-related macular degeneration (AMD) is a major leading cause of visual impairment and blindness with no cure currently established. Cell replacement of RPE is discussed as a potential therapy for AMD. Previous studies were performed in animal models with severe limitations in recapitulating the disease progression. In detail, we describe the effect of systemic injection of sodium iodate in the mouse retina. We further evaluate the usefulness of this animal model to analyze cell-specific effects following transplantation of human embryonic stem cell (hESC)-derived RPE cells.

METHODS

Morphologic, functional, and behavioral changes following sodium iodate injection were monitored by histology, gene expression analysis, electroretinography, and optokinetic head tracking. Human embryonic stem cell-derived RPE cells were transplanted 1 week after sodium iodate injection and experimental retinae were analyzed 3 weeks later.

RESULTS

Injection of sodium iodate caused complete RPE cell loss, photoreceptor degeneration, and altered gene and protein expression in outer and inner nuclear layers. Retinal function was severely affected by day 3 and abolished from day 14. Following transplantation, donor hESC-derived RPE cells formed extensive monolayers that displayed wild-type RPE cell morphology, organization, and function, including phagocytosis of host photoreceptor outer segments.

CONCLUSIONS

Systemic injection of sodium iodate has considerable effects on RPE, photoreceptors, and inner nuclear layer neurons, and provides a model to assay reconstitution and maturation of RPE cell transplants. The availability of an RPE-free Bruch's membrane in this model likely allows the unprecedented formation of extensive polarized cell monolayers from donor hESC-derived RPE cell suspensions.

Modeling human development in 3D culture

Recently human embryonic stem cell research has taken on a new dimension - the third dimension. Capitalizing on increasing knowledge on directing pluripotent cells along different lineages, combined with ECM supported three-dimensional culture conditions, it has become possible to generate highly organized tissues of the central nervous system, gut, liver and kidney. Each system has been used to study different aspects of organogenesis and function including physical forces underlying optic cup morphogenesis, the function of disease related genes in progenitor cell control, as well as interaction of the generated tissues with host tissue upon transplantation. Pluripotent stem cell derived organoids represent powerful systems for the study of how cells self-organize to generate tissues with a given shape, pattern and form.

Subretinal transplantation of MACS purified photoreceptor precursor cells into the adult mouse retina

Vision impairment and blindness due to the loss of the light-sensing cells of the retina, i.e. photoreceptors, represents the main reason for disability in industrialized countries. Replacement of degenerated photoreceptors by cell transplantation represents a possible treatment option in future clinical applications. Indeed, recent preclinical studies demonstrated that immature photoreceptors, isolated from the neonatal mouse retina at postnatal day 4, have the potential to integrate into the adult mouse retina following subretinal transplantation. Donor cells generated a mature photoreceptor morphology including inner and outer segments, a round cell body located at the outer nuclear layer, and synaptic terminals in close proximity to endogenous bipolar cells. Indeed, recent reports demonstrated that donor photoreceptors functionally integrate into the neural circuitry of host mice. For a future clinical application of such cell replacement approach, purified suspensions of the cells of choice have to be generated and placed at the correct position for proper integration into the eye. For the enrichment of photoreceptor precursors, sorting should be based on specific cell surface antigens to avoid genetic reporter modification of donor cells. Here we show magnetic-associated cell sorting (MACS) - enrichment of transplantable rod photoreceptor precursors isolated from the neonatal retina of photoreceptor-specific reporter mice based on the cell surface marker CD73. Incubation with anti-CD73 antibodies followed by micro-bead conjugated secondary antibodies allowed the enrichment of rod photoreceptor precursors by MACS to approximately 90%. In comparison to flow cytometry, MACS has the advantage that it can be easier applied to GMP standards and that high amounts of cells can be sorted in relative short time periods. Injection of enriched cell suspensions into the subretinal space of adult wild-type mice resulted in a 3-fold higher integration rate compared to unsorted cell suspensions.

Nonneuronal control of the differential distribution of myelin along retinal ganglion cell axons in the mouse

PURPOSE

In most mammalian species, retinal ganglion cell (RGC) axons are myelinated in the optic nerve, but remain nonmyelinated in the retinal nerve fiber layer and the most proximal (i.e., retina-near) region of the nerve. Here we analyzed whether RGCs are involved in the control of this characteristic distribution of oligodendrocytes and myelin in the primary visual pathway of mice.

METHODS

Neurospheres were enriched in oligodendrocyte progenitor cells (OPCs) by a short-term exposure to platelet-derived growth factor (PDGF) and grafted into the retina of young postnatal mice close to the optic disc. Immunohistochemistry was performed to study the integration and differentiation of the grafted cells, and the formation of donor-derived myelin in the normally nonmyelinated retinal nerve fiber layer and intrabulbar and most proximal retrobulbar region of the optic nerve.

RESULTS

Intraretinal transplantations of small-sized PDGF-treated neurospheres into young postnatal mice resulted in extensive integration of the grafted cells into host retinas. A significant fraction of the donor cells differentiated into oligodendrocytes that myelinated the nerve fiber layer. Importantly, RGC axon segments within the normally nonmyelinated intrabulbar and most proximal retrobulbar region of the nerve also became myelinated in a fraction of animals.

CONCLUSIONS

This is the first report demonstrating that the normally nonmyelinated intrabulbar and retrobulbar segments of RGC axons are competent to become myelinated. Results support the view that the differential distribution of myelin and oligodendrocytes in the primary visual pathway is controlled by nonneuronal factors rather than by the RGCs themselves.

Stress reaction in outer segments of photoreceptors after blue light irradiation

The retina is prone to oxidative stress from many factors which are also involved in the pathogenesis of degenerative diseases. In this study, we used the application of blue light as a physiological stress factor. The aim of this study was to identify the major source of intracellular ROS that mediates blue light-induced detrimental effects on cells which may lead to cytotoxicity. We hypothesized that outer segments are the major source of blue light induced ROS generation. In photoreceptors, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox) enzymes and the recently found respiratory chain complexes may represent a major source for reactive oxygen species (ROS), beside mitochondria and chromophores. Therefore, we investigated this hypothesis and analysed the exact localization of the ROS source in photoreceptors in an organotypic culture system for mouse retinas.

Whole eyeball cultures were irradiated with visible blue light (405 nm) with an output power of 1 mW/cm². Blue light impingement lead to an increase of ROS production (detected by H₂DCFDA in live retinal explants), which was particularly strong in the photoreceptor outer segments. Nox-2 and Nox-4 proteins are sources of ROS in blue light irradiated photoreceptors; the Nox inhibitor apocynin decreased ROS stimulated by blue light. Concomitantly, enzyme SOD-1, a member of the antioxidant defense system, indicator molecules of protein oxidation (CML) and lipid oxidation (MDA and 4-HNE) were also increased in the outer segments.

Interestingly, outer segments showed a mitochondrial-like membrane potential which was demonstrated using two dyes (JC-1 and TMRE) normally exclusively associated with mitochondria. As in mitochondria, these dyes indicated a decrease of the membrane potential in hypoxic states or cell stress situations.

The present study demonstrates that ROS generation and oxidative stress occurs directly in the outer segments of photoreceptors after blue light irradiation.

Three-dimensional neuroepithelial culture from human embryonic stem cells and its use for quantitative conversion to retinal pigment epithelium

A goal in human embryonic stem cell (hESC) research is the faithful differentiation to given cell types such as neural lineages. During embryonic development, a basement membrane surrounds the neural plate that forms a tight, apico-basolaterally polarized epithelium before closing to form a neural tube with a single lumen. Here we show that the three-dimensional epithelial cyst culture of hESCs in Matrigel combined with neural induction results in a quantitative conversion into neuroepithelial cysts containing a single lumen. Cells attain a defined neuroepithelial identity by 5 days. The neuroepithelial cysts naturally generate retinal epithelium, in part due to IGF-1/insulin signaling. We demonstrate the utility of this epithelial culture approach by achieving a quantitative production of retinal pigment epithelial (RPE) cells from hESCs within 30 days. Direct transplantation of this RPE into a rat model of retinal degeneration without any selection or expansion of the cells results in the formation of a donor-derived RPE monolayer that rescues photoreceptor cells. The cyst method for neuroepithelial differentiation of pluripotent stem cells is not only of importance for RPE generation but will also be relevant to the production of other neuronal cell types and for reconstituting complex patterning events from three-dimensional neuroepithelia.

Outer segment formation of transplanted photoreceptor precursor cells

Transplantation of photoreceptor precursor cells (PPCs) into the retina represents a promising treatment for cell replacement in blinding diseases characterized by photoreceptor loss. In preclinical studies, we and others demonstrated that grafted PPCs integrate into the host outer nuclear layer (ONL) and develop into mature photoreceptors. However, a key feature of light detecting photoreceptors, the outer segment (OS) with natively aligned disc membrane staples, has not been studied in detail following transplantation. Therefore, we used as donor cells PPCs isolated from neonatal double transgenic reporter mice in which OSs are selectively labeled by green fluorescent protein while cell bodies are highlighted by red fluorescent protein. PPCs were enriched using CD73-based magnetic associated cell sorting and subsequently transplanted into either adult wild-type or a model of autosomal-dominant retinal degeneration mice. Three weeks post-transplantation, donor photoreceptors were identified based on fluorescent-reporter expression and OS formation was monitored at light and electron microscopy levels. Donor cells that properly integrated into the host wild-type retina developed OSs with the formation of a connecting cilium and well-aligned disc membrane staples similar to the surrounding native cells of the host. Surprisingly, the majority of not-integrated PPCs that remained in the sub-retinal space also generated native-like OSs in wild-type mice and those affected by retinal degeneration. Moreover, they showed an improved photoreceptor maturation and OS formation by comparison to donor cells located on the vitreous side suggesting that environmental cues influence the PPC differentiation and maturation. We conclude that transplanted PPCs, whether integrated or not into the host ONL, are able to generate the cellular structure for effective light detection, a phenomenon observed in wild-type as well as in degenerated retinas. Given that patients suffering from retinitis pigmentosa lose almost all photoreceptors, our findings are of utmost importance for the development of cell-based therapies.

Differentiation of chromaffin progenitor cells to dopaminergic neurons

The differentiation of dopamine-producing neurons from chromaffin progenitors might represent a new valuable source for replacement therapies in Parkinson's disease. However, characterization of their differentiation potential is an important prerequisite for efficient engraftment. Based on our previous studies on isolation and characterization of chromaffin progenitors from adult adrenals, this study investigates their potential to produce dopaminergic neurons and means to enhance their dopaminergic differentiation. Chromaffin progenitors grown in sphere culture showed an increased expression of nestin and Mash1, indicating an increase of the progenitor subset. Proneurogenic culture conditions induced the differentiation into neurons positive for neural markers β-III-tubulin, MAP2, and TH accompanied by a decrease of Mash1 and nestin. Furthermore, Notch2 expression decreased concomitantly with a downregulation of downstream effectors Hes1 and Hes5 responsible for self-renewal and proliferation maintenance of progenitor cells. Chromaffin progenitor-derived neurons secreted dopamine upon stimulation by potassium. Strikingly, treatment of differentiating cells with retinoic and ascorbic acid resulted in a twofold increase of dopamine secretion while norepinephrine and epinephrine were decreased. Initiation of dopamine synthesis and neural maturation is controlled by Pitx3 and Nurr1. Both Pitx3 and Nurr1 were identified in differentiating chromaffin progenitors. Along with the gained dopaminergic function, electrophysiology revealed features of mature neurons, such as sodium channels and the capability to fire multiple action potentials. In summary, this study elucidates the capacity of chromaffin progenitor cells to generate functional dopaminergic neurons, indicating their potential use in cell replacement therapies.

Labeling of ultrathin resin sections for correlative light and electron microscopy

Correlative microscopy combines the versatility of the light microscope with the excellent spatial resolution of the electron microscope. Here, we describe fast and simple methods for correlative immunofluorescence and immunogold labeling on the very same ultrathin section. The protocols are demonstrated on sections of tissue samples embedded in the methacrylate Lowicryl K4M. Ultrathin sections are mounted on electron microscopy (EM) grids and stained simultaneously with fluorescent and gold markers. For the detection of primary antibodies, we applied either protein A gold or immunoglobulin G (IgG) gold in combination with secondary antibodies coupled to Alexa488 or Alexa555. Alternatively, the correlative marker FluoroNanogold was used, followed by silver enhancement. The samples have to be analyzed first at the light microscope and then in the transmission electron microscope (TEM), because the fluorescence is bleached by the electron beam. Labeled structures selected at the fluorescence microscope can be identified in the TEM and analyzed at high resolution. This way, fluorescent signals can be directly correlated to the corresponding subcellular structures in the area of interest.

Increased integration of transplanted CD73-positive photoreceptor precursors into adult mouse retina

PURPOSE. Retinal degeneration initiated by loss of photoreceptors is the prevalent cause of visual impairment and blindness in industrialized countries. Transplantation of photoreceptor cells represents a possible replacement strategy. This study determined that identification of cell surface antigens can assist in enriching photoreceptor precursors for transplantation. METHODS. The expression profile of rod photoreceptors at postnatal day 4 was investigated by microarray analysis to identify photoreceptor-specific cell surface antigens. For enrichment of transplantable photoreceptors, neonatal retinas from rod photoreceptor-specific reporter mice were dissociated, and the rods were purified by magnetic associated cell sorting (MACS) with CD73 antibodies. MAC-sorted cell fractions were transplanted into the subretinal space of adult wild-type mice. The number of rod photoreceptors contained in unsorted, CD73-negative, and CD73-positive cell fractions were quantified in vitro and after grafting in vivo. RESULTS. Microarray analysis revealed that CD73 is a marker for rod photoreceptors. CD73-based MACS resulted in enrichment of rods to 87%. Furthermore, in comparison with unsorted cell fractions, CD73-positive MAC-sorted cells showed an approximately three-fold increase in the number of integrated, outer segment-forming photoreceptors after transplantation. CONCLUSIONS. CD73-based MACS is a reliable method for the enrichment of integrating photoreceptors. Purification via cell surface markers represents a new tool for the separation of transplantable photoreceptor precursors from a heterogeneous cell population, avoiding the need of reporter gene expression in target cells.

Influence of blue light on photoreceptors in a live retinal explant system

PURPOSE

The present study was performed to investigate the early effects of blue light irradiation of photoreceptors in retinal explant cultures.

METHODS

Murine retinal explant cultures were irradiated with visible blue light (405 nm) with an output power of 1 mW/cm2. Dihydroethidium was used to determine the production of reactive oxygen species. Morphological alterations of photoreceptor outer segments were determined by live imaging microscopy with mitochondrial dye JC-1. Transmission and scanning electron microscopy were used for ultrastructural evaluations. Cell death in the retina was assessed by the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick end labeling (TUNEL) assay method.

RESULTS

Live retinal explants displayed an increase in reactive oxygen species production, as revealed by fluorescent dihydroethidium products in photoreceptor cells after 30 min of blue light exposure. After 3 h of exposure, blue light caused disorganization of the normally neatly stacked outer segments of living photoreceptors. Ultrastructural analysis revealed breaks in the cell membrane surrounding the outer segments, especially in the middle section. The outer segments appeared tortuous, and the lamellar structures had been disrupted. TUNEL-staining revealed that long-term blue light exposure induced photoreceptor cell death.

CONCLUSIONS

In vitro blue light irradiation of retinal explants is a suitable model system for investigating early ultrastructural changes, as well as damage that leads to cell death in photoreceptor cells.

Extreme 15N-enrichments in 2.72-Gyr-old sediments: evidence for a turning point in the nitrogen cycle

Although nitrogen is a key element in organic molecules such as nucleic acids and proteins, the timing of the emergence of its modern biogeochemical cycle is poorly known. Recent studies on the antiquity of the nitrogen cycle and its interaction with free oxygen suggests the establishment of a complete aerobic N biogeochemical cycle with nitrification, denitrification, and nitrogen fixation at about 2.68 Gyr. Here, we report new bulk nitrogen isotope data for the 2.72 billion-year-old sedimentary succession of the Tumbiana Formation (Pilbara Craton, Western Australia). The nitrogen isotopic compositions vary widely from +8.6‰ up to +50.4‰ and are inversely correlated with the very low δ(13)C values of associated organic matter defining the Fortescue excursion (down to about -56‰). We propose that this (15)N-enrichment records the onset of nitrification coupled to the continuous removal of its derivatives (nitrite and nitrate) by denitrification. This finding implies an increase in the availability of electron acceptors and probably oxygen in the Tumbiana depositional environment, 300 million years before the oxygenation of the Earth's atmosphere.

Therapeutic benefit derived from RNAi-mediated ablation of IMPDH1 transcripts in a murine model of autosomal dominant retinitis pigmentosa (RP10)

Mutations within the inosine 5'-monophosphate dehydrogenase 1 (IMPDH1) gene cause the RP10 form of autosomal dominant retinitis pigmentosa (adRP), an early-onset retinopathy resulting in extensive visual handicap owing to progressive death of photoreceptors. Apart from the prevalence of RP10, estimated to account for 5-10% of cases of adRP in United States and Europe, two observations render this form of RP an attractive target for gene therapy. First, we show that while recombinant adeno-associated viral (AAV)-mediated expression of mutant human IMPDH1 protein in the mouse retina results in an aggressive retinopathy modelling the human counterpart, expression of a normal human IMPDH1 gene under similar conditions has no observable pathological effect on retinal function, indicating that over-expression of a therapeutic replacement gene may be relatively well tolerated. Secondly, complete absence of IMPDH1 protein in mice with a targeted disruption of the gene results in relatively mild retinal dysfunction, suggesting that significant therapeutic benefit may be derived even from the suppression-only component of an RNAi-based gene therapy. We show that AAV-mediated co-expression in the murine retina of a mutant human IMPDH1 gene together with short hairpin RNAs (shRNA) validated in vitro and in vivo, targeting both human and mouse IMPDH1, substantially suppresses the negative pathological effects of mutant IMPDH1, at a point where, in the absence of shRNA, expression of mutant protein in the RP10 model essentially ablates all photoreceptors in transfected areas of the retina. These data strongly suggest that an RNAi-mediated approach to therapy for RP10 holds considerable promise for human subjects.

Retinal cells integrate into the outer nuclear layer and differentiate into mature photoreceptors after subretinal transplantation into adult mice

Vision impairment caused by degeneration of photoreceptors, termed retinitis pigmentosa, is a debilitating condition with no cure presently available. Cell-based therapeutic approaches represent one treatment option by replacing degenerating or lost photoreceptors. In this study the potential of transplanted primary retinal cells isolated from neonatal mice to integrate into the outer nuclear layer (ONL) of adult mice and to differentiate into mature photoreceptors was evaluated. Retinal cells were isolated from retinas of transgenic mice ubiquitously expressing enhanced green fluorescence protein (EGFP) at either postnatal day (P) 0, P1 or P4 and transplanted into the subretinal space of adult wild-type mice. One week to 11 months post-transplantation experimental retinas were analyzed for integration and differentiation of donor cells. Subsequent to transplantation some postnatal retinal cells integrated into the ONL of the host and differentiated into mature photoreceptors containing inner and outer segments as confirmed by immunohistochemistry and electron microscopy. Notably, the appearance of EGFP-positive photoreceptors was not the result of fusion between donor cells and endogenous photoreceptors. Retinal cells isolated at P4 showed a significant increase in their capacity to integrate into the ONL and to differentiate into mature photoreceptors when compared with cells isolated at P0 or P1. As cell suspensions isolated at P4 are enriched in cells committed towards a rod photoreceptor cell fate it is tempting to speculate that immature photoreceptors may have the highest integration and differentiation potential and thus may present a promising cell type to develop cell replacement strategies for diseases involving rod photoreceptor loss.

RNA interference-mediated suppression and replacement of human rhodopsin in vivo

Mutational heterogeneity represents a significant barrier to development of therapies for many dominantly inherited diseases. For example, >100 mutations in the rhodopsin gene (RHO) have been identified in patients with retinitis pigmentosa (RP). The development of therapies for dominant disorders that correct the primary genetic lesion and overcome mutational heterogeneity is challenging. Hence, therapeutics comprising two elements--gene suppression in conjunction with gene replacement--have been investigated. Suppression is targeted to a site independent of the mutation; therefore, both mutant and wild-type alleles are suppressed. In parallel with suppression, a codon-modified replacement gene refractory to suppression is provided. Both in vitro and in vivo validation of suppression and replacement for RHO-linked RP has been undertaken in the current study. RNA interference (RNAi) has been used to achieve ~90% in vivo suppression of RHO in photoreceptors, with use of adeno-associated virus (AAV) for delivery. Demonstration that codon-modifed RHO genes express functional wild-type protein has been explored transgenically, together with in vivo expression of AAV-delivered RHO-replacement genes in the presence of targeting RNAi molecules. Observation of potential therapeutic benefit from AAV-delivered suppression and replacement therapies has been obtained in Pro23His mice. Results provide the first in vivo indication that suppression and replacement can provide a therapeutic solution for dominantly inherited disorders such as RHO-linked RP and can be employed to circumvent mutational heterogeneity.

RNAi-based suppression and replacement of rds-peripherin in retinal organotypic culture

Extensive mutational heterogeneity presents a significant barrier to the development of therapeutics for RDS-peripherin-linked autosomal-dominant retinitis pigmentosa (RP), for which more than 50 disease-related mutations have been identified to date. Mutation-independent suppression, using RNA interference (RNAi), together with simultaneous expression of a replacement rds gene (r-rds, which has been altered to escape suppression but nevertheless encodes wild-type protein) has been explored in COS-7 cells and mouse retinal explants. The efficacy of small interfering and short hairpin RNAs (si/shRNAs) silencing mouse rds, and the function of r-rds (containing degenerate substitutions in the RNAi target sequence) were analyzed at transcript (RT-PCR) and protein (ELISA) levels in COS-7 cells. "Dual-" and "triple-expression" constructs carrying the shRNA suppressor and the marker EGFP with or without the r-rds cassette were electroporated in vitro into retinal explants from 1-day-old pups. The retinae were dissociated at day 14, and transduced cells were FACS-sorted using the coexpressed EGFP marker and analyzed by RT-PCR. si/shRNAs decreased rds mRNA and protein expression by up to 82%, while r-rds was protected from suppression in COS-7 cells. Similarly, efficient RNAi-mediated suppression of endogenous rds was detected in retinal explants, while concomitant rescue of r-rds was also achieved. These data validate the concept of RNAi-based suppression coupled with replacement technology for the development of therapies targeting RDS-linked autosomal-dominant RP, and suggest that such approaches could potentially be used for other autosomal-dominant diseases with similarly extensive intragenic heterogeneity.

Toward a gene therapy for dominant disease: validation of an RNA interference-based mutation-independent approach

The intragenic heterogeneity encountered in many dominant disease-causing genes represents a significant challenge with respect to development of economically viable therapeutics. For example, 25% of autosomal dominant retinitis pigmentosa is caused by over 100 different mutations within the gene encoding rhodopsin, each of which could require a unique gene therapy. We describe here an RNA interference (RNAi)-based mutation-independent approach, targeting as an example murine rhodopsin. Native transcripts are suppressed by a single RNAi molecular species, whereas transcripts from replacement genes engineered at degenerate third-codon wobble positions are resistant to suppression. We demonstrate suppression of murine rhodopsin transcript by up to 90% with full concomitant expression of replacement transcript and establish the validity of this approach in cell culture, retinal explants, and mouse liver in vivo.

Electroporation-based gene transfer for efficient transfection of neural precursor cells

Transplantation of neural precursor cells (NPCs) is a potential tool to replace dysfunctional or degenerated neuronal or glial cell types in the central nervous system. Furthermore, transplantation of genetically engineered neural precursor cells might provide a strategy to target therapeutic gene products to the diseased nervous system. Here, we describe a novel and highly efficient electroporation-based transfection protocol for mitogen-expanded mouse NPCs. Transfection of NPCs with the reporter gene enhanced green fluorescent protein (EGFP) or the neural adhesion molecule L1 revealed transfection efficacies of more than 70% as estimated by the number of EGFP-positive or L1-immunoreactive cells 1 day after transfection in vitro. The percentage of EGFP- or L1-positive cells decreased with increasing time in culture. Positive cells were detectable for up to 3 weeks after transfection. When EGFP- or L1-transfected NPCs were grafted into the retina of adult wild-type or L1-deficient mice, they differentiated into glial cells some of which expressed EGFP and L1 for up to 2 and 3 weeks, respectively, the longest post-transplantation periods investigated.

Integration and differentiation of neural stem cells after transplantation into the dysmyelinated central nervous system of adult mice

Mutant mice deficient in the myelin-associated glycoprotein (MAG) and the nonreceptor-type tyrosine kinase Fyn are characterized by a severely hypomyelinated central nervous system (CNS) and morphologically abnormal myelin sheaths. Despite this pronounced phenotype, MAG/Fyn-deficient mice have a normal longevity. In the present study, we took advantage of the normal life expectancy of this myelin mutant and grafted neural stem cells (NSCs) into the CNS of MAG/Fyn-deficient mice to study in short- and long-term experiments the fate of NSCs in adult dysmyelinated brains. Neural stem cells were isolated from spinal cords of transgenic mouse embryos ubiquitously expressing enhanced green fluorescent protein. Cells were expanded in vitro in the presence of mitogens for up to 5 weeks before they were grafted into the lateral ventricles or injected into white matter tracts. Analysis of mutant brains 3-15 weeks after intracerebroventricular transplantation of NSCs revealed only limited integration of donor cells into the host brains. However, injection of NSCs directly into white matter tracts resulted in widespread distribution of donor cells within the host tissue. Donor cells survived for at least 15 weeks in adult host brains. The majority of grafted cells populated white matter tracts and differentiated into oligodendrocytes that myelinated host axons. Results suggest that intraparenchymal transplantation of NSCs might be a strategy to reconstruct myelin in dysmyelinated adult brains.

erbB3 is dispensable for oligodendrocyte development in vitro and in vivo

During development and in the adult, erbB2, erbB3, and erbB4 are expressed in many tissues and as heterodimers (B2/B3, B2/B4) serve as receptors for neuregulins. The general importance of neuregulin receptors for development is underlined by the observed embryonic (erbB2, erbB4) or perinatal (erbB3) lethality in mouse mutants. These mutants further revealed the fundamental role of the erbB2/erbB3 heterodimer for proper Schwann cell development, the ensheathing glia of the peripheral nervous system. However, only little is known about the functions of neuregulins and their receptors during postnatal development and in the adult. erbB2 and erbB3 during late embryogenesis and postnatally are expressed in different areas and cell types of the central nervous system, including oligodendrocytes, the ensheathing glia of the central nervous system. As terminal differentiation of oligodendrocytes peaks during postnatal development, it is not possible to use the neuregulin receptor mouse mutants to study terminal differentiation of oligodendrocytes in their absence in vivo. In order to investigate possible functions of the erbB3 gene in oligodendrocytes, we employed two different techniques. First, we directed the differentiation of erbB3-deficient embryonic stem cells into neural cell types to analyze the development of oligodendrocytes in the absence of erbB3 in vitro. Second, we grafted neural stem cells from spinal cords of erbB3 mutants into the retina of young mice to monitor oligodendrocyte differentiation and myelination in vivo. Results of both experimental approaches clearly show that erbB3 is not required for normal oligodendrocyte development and myelination.