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Nonneuronal Control of the Differential Distribution of Myelin Along Retinal Ganglion Cell Axons in the Mouse

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Nonneuronal Control of the Differential Distribution of Myelin Along Retinal Ganglion Cell Axons in the Mouse Anatomy and Pathology Nonneuronal Control of the Differential Distribution of Myelin Along Retinal Ganglion Cell Axons in the Mouse Xiaoli Yang,*,1 Haidong Zou,†,1 Gila Jung,1 Gisbert Richard,1 Stephan J. Linke,1 Marius Ader,2 and Udo Bartsch1 1Department of Ophthalmology, University Medical Centre Hamburg-Eppendorf, Hamburg, Germany 2Center for Regenerative Therapies Dresden, TU Dresden, Germany Correspondence: Udo Bartsch, De- PURPOSE. In most mammalian species, retinal ganglion cell (RGC) axons are myelinated in the partment of Ophthalmology, Univer- optic nerve, but remain nonmyelinated in the retinal nerve fiber layer and the most proximal sity Medical Centre Hamburg- (i.e., retina-near) region of the nerve. Here we analyzed whether RGCs are involved in the Eppendorf, Martinistr. 52, 20246 control of this characteristic distribution of oligodendrocytes and myelin in the primary visual Hamburg, Germany; pathway of mice. [email protected]. METHODS. Neurospheres were enriched in oligodendrocyte progenitor cells (OPCs) by a short- XY and HZ contributed equally to the work presented here and should term exposure to platelet-derived growth factor (PDGF) and grafted into the retina of young therefore be regarded as equivalent postnatal mice close to the optic disc. Immunohistochemistry was performed to study the authors. 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 Current affiliation: *Department of retrobulbar region of the optic nerve. Ophthalmology, North Sichuan Medi- cal College, Nanchong Sichuan, China. RESULTS. Intraretinal transplantations of small-sized PDGF-treated neurospheres into young †Department of Ophthalmology, postnatal mice resulted in extensive integration of the grafted cells into host retinas. A Shanghai First People’s Hospital, significant fraction of the donor cells differentiated into oligodendrocytes that myelinated the Shanghai Jiao Tong University, Shang- nerve fiber layer. Importantly, RGC axon segments within the normally nonmyelinated hai, China. intrabulbar and most proximal retrobulbar region of the nerve also became myelinated in a Submitted: June 13, 2013 fraction of animals. Accepted: October 29, 2013 CONCLUSIONS. This is the first report demonstrating that the normally nonmyelinated Citation: Yang X, Zou H, Jung G, et al. intrabulbar and retrobulbar segments of RGC axons are competent to become myelinated. Nonneuronal control of the differen- Results support the view that the differential distribution of myelin and oligodendrocytes in tial distribution of myelin along retinal the primary visual pathway is controlled by nonneuronal factors rather than by the RGCs ganglion cell axons in the mouse. themselves. Invest Ophthalmol Vis Sci. 2013;54:7819–7827. DOI:10.1167/ Keywords: myelination, neurosphere cultures, oligodendrocytes, retina, transplantation iovs.13-12596 ligodendrocyte progenitor cells (OPCs) are generated in dendrocyte lineage cells, or neural progenitor cells into Othe preoptic area from where they colonize the develop- developing or adult mice or rats.23–29 All these reports ing optic nerve from its chiasmal toward its retinal end.1–3 In unequivocally demonstrate that the normally nonmyelinated most mammalian species, including rats and mice, OPCs stop intraretinal segments of RGC axons will become myelinated migrating and differentiate into myelinating oligodendrocytes once myelinogenic cells gain access to the retina. It was shortly before the optic nerve enters the retina.4,5 In addition, therefore hypothesized that nonneuronal factors located at the the retina is incapable of generating oligodendrocytes even retinal end of the optic nerve, rather than the RGCs themselves, under culture conditions that favor oligodendrogenesis.6,7 As a control the differential distribution of oligodendrocytes and result, the retina is devoid of oligodendrocytes and remains myelin in the primary visual pathway by preventing OPCs from nonmyelinated throughout life.6,8–10 Vision is thus not impaired entering the retina.4,6,8,24 by a myelinated retinal nerve fiber layer that would impede Among the nonneuronal factors implicated in preventing light reaching the photoreceptor cells. intraretinal myelination is a well-developed lamina cribrosa, a Abnormal myelination of the retinal nerve fiber layer has distinct band of connective tissue formed by collagen fibers of been found in various mammalian species that normally lack the sclera.30 In humans, retinal ganglion cell axons remain intraretinal myelin, including the mouse, rat, guinea pig, cat, nonmyelinated proximal to the lamina cribrosa. Rabbits, in and rhesus monkey,11–15 and occurs sporadically also in human contrast, lack a well-developed lamina cribrosa and RGC axons eyes.16–19 Furthermore, myelination of the nerve fiber layer by in this species are also myelinated intraretinally.30,31 However, Schwann cells is frequently seen in the Browman-Wyse rat, an rats and mice also lack a well-developed lamina cribrosa but inbred strain selected for microphthalmos,20,21 and can be have a nonmyelinated nerve fiber layer. Astrocytes have also induced in normal rats by lesioning the retina via the sclera and been suggested to act as a barrier for migrating OPCs at the choroid.22 In addition, transplantation studies have demonstrat- retinal end of the optic nerve. A dense astrocytic network is ed extensive myelination of the retinal nerve fiber layer present proximal to the myelinated part of the optic nerve in following intraocular transplantations of Schwann cells, oligo- rats and mice4,6 and several other species that lack intraretinal Copyright 2013 The Association for Research in Vision and Ophthalmology, Inc. www.iovs.org j ISSN: 1552-5783 7819 Downloaded from iovs.arvojournals.org on 09/23/2021 Nonneuronal Control of Myelin Distribution IOVS j December 2013 j Vol. 54 j No. 13 j 7820 myelin, while a similar concentration of astrocytes is absent (EGF) and 20 ng/mL fibroblast growth factor-2 (FGF-2; both from the optic nerve of rabbits and chicken, two species with from tebu-bio, Offenbach, Germany). After 7 days of culture, intraretinal myelination.6,32 In the Browman-Wyse rat, howev- cells had grown to free-floating neurospheres that were further er, the proximal part of the optic nerve remains nonmyelinated expanded in the same medium but lacking B27. After three over an abnormally long distance, but lacks a dense astrocytic passages, neurospheres were dissociated and cells were network at the transition zone between the myelinated and transferred to a medium containing 10 ng/mL FGF-2, 10 ng/ nonmyelinated region of the optic nerve.20 At the molecular mL PDGF (R&D Systems, Minneapolis, MN) and 10 lM level, the astrocyte-derived extracellular matrix glycoprotein forskolin (Sigma-Aldrich) to increase the percentage of tenascin-C has been suggested to prevent migration of OPCs oligodendrocyte lineage cells in the cultures. After an into the retina. Tenascin-C is expressed at elevated levels by additional culture period of 4 days, PDGF-treated neurosphere astrocytes in the most proximal region of the optic nerve cultures (in the following termed ‘‘PDGF-neurospheres’’) before the arrival of the first OPCs in this region, and has been consisted of small cellular aggregates that were used for in demonstrated to inhibit OPC adhesion and migration in vitro differentiation experiments and transplantation experi- vitro.4,33 Bone morphogenetic proteins (BMPs), expressed in ments (see below). the developing retina and at the retina-optic nerve junction, have also been implicated in preventing intraretinal myelina- Quantification of Oligodendrocyte Differentiation 7 tion. However, in vivo evidence for a critical role of any of In Vitro these factors in controlling the differential distribution of oligodendrocytes and myelin in the primary visual pathway has For in vitro differentiation experiments, neurospheres and not been provided. PDGF-neurospheres were plated onto coverslips coated with While the available data are in line with the view that poly-D-lysine (Sigma-Aldrich) and 1% Matrigel (Becton, Dick- nonneuronal factors in the most proximal region of the optic inson and Company, Heidelberg, Germany). Oligodendrocytic nerve prevent migration of OPCs into the retina and thus differentiation was induced by cultivating the cells for 3 days in intraretinal myelination, they do not exclude a neuronal DMEM/F12 supplemented with 1% N2, 30 ng/mL 3,30,5- control of the differential distribution of oligodendrocytes triiodothyronine (T3) and 200 lM ascorbic acid (both from and myelin along RGC axons. In fact, axon segments of RGCs Sigma-Aldrich). To compare the numbers of oligodendrocytes within the intrabulbar and most proximal retrobulbar region of in the differentiated cultures, cells were fixed in PBS (pH 7.3) the optic nerve may be refractory to myelination, thereby containing 4% paraformaldehyde (PA), blocked in PBS contain- restricting oligodendrocytes and myelin to the distal part of the ing 0.1% BSA and 0.3% Triton X-100 (both from Sigma-Aldrich), nerve. To address this possibility experimentally, we enriched and incubated with rat monoclonal antibodies to myelin basic neurosphere cultures in oligodendrocyte lineage cells by a protein (MBP, 1:100; Millipore, Billerica, MA) for 4 hours at short-term cultivation in a medium supplemented with room temperature to visualize oligodendrocytes. Oligodendro- platelet-derived growth
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