Purification of chick retinal ganglion cells for molecular analysis: combining retrograde labeling and immunopanning yields 100% purity

Purification of retinal ganglion cells using low-pressure flow cytometry

Purified Retinal Ganglion Cells (RGCs) for in vitro study have been a valuable tool in the study of neural regeneration and in the development of therapies to treat glaucoma. Traditionally, RGCs have been isolated from early postnatal rats and mice, and more recently from human in vitro derived retinal organoids using a two-step immunopanning technique based upon the expression of Thy-1. This technique, however, limits the time periods from which RGCs can be isolated, missing the earliest born RGCs at which time the greatest stage of axon growth occurs, as well as being limited in its use with models of retinal degeneration as Thy-1 is downregulated following injury. While fluorescence associated cell sorting (FACS) in combination with new optogenetically labeled RGCs would be able to overcome this limitation, the use of traditional FACS sorters has been limited to genomic and proteomic studies, as RGCs have little to no survival post-sorting. Here we describe a new method for RGC isolation utilizing a combined immunopanning-fluorescence associated cell sorting (IP-FACS) protocol that initially depletes macrophages and photoreceptors, using immunopanning to enrich for RGCs before using low-pressure FACS to isolate these cells. We demonstrate that RGCs isolated via IP-FACS when compared to RGCs isolated via immunopanning at the same age have similar purity as measured by antibody staining and qRT-PCR; survival as measured by live dead staining; neurite outgrowth; and electrophysiological properties as measured by calcium release response to glutamate. Finally, we demonstrate the ability to isolate RGCs from early embryonic mice prior to the expression of Thy-1 using Brn3b-eGFP optogenetically labeled cells. This method provides a new approach for the isolation of RGCs for the study of early developed RGCs, the study of RGC subtypes and the isolation of RGCs for cell transplantation studies.

Conventional kinesin-I motors participate in the anterograde axonal transport of neurotrophins in the visual system

Retinal ganglion cells (RGCs) anterogradely transport neurotrophins to the midbrain tectum/superior colliculus with significant downstream effects. The molecular mechanism of this type of axonal transport of neurotrophins is not well characterized. We identified kinesin-I proteins as a motor participating in the anterograde axonal movement of vesicular structures containing radiolabeled neurotrophins along the optic nerve. RT-PCR analysis of purified murine RGCs showed that adult RGCs express all known members of the kinesin-I family. After intraocular injection of (125)I-brain-derived neurotrophic factor (BDNF) into the adult mouse or (125)I-neurotrophin-3 (NT-3) into the embryonic chicken eye, radioactivity was efficiently immunoprecipitated from the optic nerve lysates by anti-kinesin heavy chain and anti-kinesin light chain monoclonal antibodies (H2 and L1). Immunoreactivity for the BDNF receptor trkB is also present in the immunoprecipitates obtained by the anti-kinesin-I antibodies. The delivery of the H2 antibody in vivo into the mouse RGCs substantially reduced anterograde axonal transport of (125)I-BDNF. Anterograde transport of BDNF was not diminished in kinesin light chain 1 (KLC1) knockout mice. However, this may be due to redundancy in functions between two different isoforms of KLC present in the RGCs, as it was described previously for kinesin heavy chains (Kanai et al. [ 2000] J Neurosci 20:6374-6384). These data indicate that kinesin-I is a protein motor that participates in the anterograde axonal transport of neurotrophins in the chicken and mouse visual pathways.

Regional variations in the glial influence on synapse development in the mouse CNS

There is increasing evidence that synapse function depends on interactions with glial cells, namely astrocytes. Studies on specific neurons of the central nervous system (CNS) indicated that glial signals also control synapse development, but it remained unclear whether this is a general principle that applies to other neuronal cell types. To address this question, we developed new methods to immunoisolate neurons from different brain regions of postnatal mice and to culture them in a chemically defined medium. Electrophysiological recordings and immunocytochemical staining revealed vigorous synaptogenesis in hippocampal and cerebellar neurons, but not in retinal ganglion cells (RGCs) in the absence of glial cells. Co-culture with glia promoted synapse formation in RGCs as indicated by a strong increase in the incidence and frequency of action potential-independent miniature synaptic currents, but showed no such effects in hippocampal or cerebellar neurons. On the other hand, glial signals promoted the efficacy of excitatory synapses in all regions as indicated by an increase in the size of spontaneous synaptic events in cerebellar cultures and of miniature synaptic currents in hippocampal neurons and RGCs. Inhibitory synaptic currents remained largely unaffected by glia. Our results indicate that in the mammalian CNS, the way that glial signals promote the development of excitatory synapses depends on the type of neuron.

Retinal ganglion cell survival in development: mechanisms of retinal growth hormone action

Several variants of growth hormone (GH) are found in the retina and vitreous of the chick embryo, where they appear to act as cell survival factors, having neuroprotective effects on retinal ganglion cells (RGCs). Here, we investigate the molecular mechanisms of the anti-apoptotic effect of GH in cultured RGCs. GH treatment increased Akt phosphorylation in these cells, which is an anti-apoptotic event. Whereas unphosphorylated Akt was detected in both nucleus and cytoplasm of RGCs by immunocytochemistry, the phosphorylated form of Akt (Akt-phos) was located primarily in the cytoplasm of both normal and apoptotic cells, although levels were markedly lower in the latter. It was found that GH treatment of RGCs reduced Akt levels, while concomitantly raising Akt-phos levels, consistent with a role for Akt signaling pathways in GH neuroprotective action. This was substantiated using Wortmannin, which, like GH antiserum, inhibited Akt phosphorylation and initiated apoptosis. The addition of Wortmannin to RGC cultures simultaneously with GH significantly reduced the anti-apoptotic effect of GH. The induction of apoptosis by GH antiserum was clearly accompanied by an increase in caspase-3 activation and PARP-1 cleavage, both of which were significantly reduced in the presence of the broad spectrum caspase inhibitor, Q-VD-OPh, which itself had a dramatic neuroprotective effect on cultured RGCs. Calpain activation appeared to be a major caspase-independent pathway to PARP-1 cleavage and apoptosis in these cells. Calpain inhibitor III (MDL 28170) was able to reduce PARP-1 cleavage and abrogate the apoptogenic effect of GH antiserum. The results support the view that caspase and calpain inhibitors are major neuroprotective agents for RGCs, and that pathways that activate both caspases and calpains are important for the anti-apoptotic actions of GH in these cells.

Immunomagnetic separation of adult human olfactory neural progenitors

Olfactory neuroepithelium (ONe) has lifelong regenerative capacity owing to the presence of mitotically active progenitors. The accessibility of ONe makes it a unique source of progenitors for cell replacement strategies in the CNS. We have established lines of neurosphere forming cells (NSFCs) from adult postmortem ONe and patients undergoing nasal sinus surgery by endoscopic biopsy. These heterogeneous lines are composed primarily of an immature neuronally restricted and a small glial restricted subpopulation. More homogeneous subpopulations of the NSFCs are essential for detailed study of factors influencing their lineage restriction. Immunomagnetic bead separation using an antibody against tyrosine kinase (Trk) receptors (Trk-pan, which recognizes Trk-A, B, C) resulted in viable, enriched positive and negative subpopulations that could be analyzed immunocytochemically. The positive cells remained positive for the first week after which the number of Trk-pan expressing cells decreased. The negative subpopulation began to express Trk-pan immunoreactivity after five days in vitro. Both subpopulations reverted to the heterogeneous composition after two weeks. Furthermore, most NSFCs were positive for Trk-B, a few for Trk-A, while no reactivity was observed for Trk-C. Because NSFCs produce brain derived neurotrophic factor (BDNF) and express Trk B, the specific receptor for BDNF, it is likely that population dynamics are under a paracrine and/or autocrine regulatory mechanism. Lineage restriction analysis demonstrated that the isolated subpopulation had a restriction potential equivalent to the original heterogeneous population. These studies characterize further the NSFCs and support the future potential therapeutic use of ONe-derived progenitors for CNS injury and neurodegenerative disorders.

Retinal growth hormone is an anti-apoptotic factor in embryonic retinal ganglion cell differentiation

Cells of the neural retina in the chick embryo undergo several waves of apoptosis during development, including peaks at approximately embryonic day (ED) 7 and 12. Prominent among the cells involved in these phases of cell death are the retinal ganglion cells (RGCs). We have previously shown that growth hormone (GH) is expressed in the neural retina, and particularly, in the RGCs. Here we study the ability of GH to rescue retinal cells from apoptosis, both in vitro and in vivo. When retinas from embryos at ED 6-8 are explanted on collagen gels, the application of recombinant GH, at 10(-6)m, significantly reduced the incidence of apoptotic cells in the cultures as judged by terminal deoxynucleotide transferase-mediated dUTP-biotin nick end labelling (TUNEL). GH was delivered to neural retinas in ovo, by microinjection into the eye cup at ED 2. When these embryos were examined at ED 6-8, no reduction in cell death was observed below the normal low control levels. However, when antibodies to GH were microinjected, the incidence of cell death increased significantly at ED 6, providing evidence that in vivo immunoneutralization of endogenous GH results in triggering of apoptotic signaling pathways. Evidence that RGCs are a particular target of this neuroprotective effect of GH was provided by examination of cultures enriched for RGCs by immunopanning. In serum-free culture, RGCs, identified by anti-Islet 1 immunolabelling, were found to be susceptible to the effect of GH immunoneutralization, which approximately quadrupled the incidence of apoptosis in the cultures. We propose that GH is a naturally occurring autocrine and/or paracrine neuroprotective agent in the developing retina which is involved in the regulation of retinal cell numbers during early embryogenesis.

Anterograde axonal transport of the exogenous cellular isoform of prion protein in the chick visual system

The cellular isoform of endogenous, newly synthesized prion protein (PrPc) can be transported by axons in the anterograde direction. To determine whether a mechanism exists for secreted PrPc to be internalized and then axonally transported, we analyzed internalization and anterograde axonal transport of radiolabeled recombinant PrPc after its intraocular injection in chick embryos. Internalization and axonal transport of exogenous PrPc to the midbrain by retinal ganglion cells (RGCs) is efficient, saturable and likely receptor-mediated. Ultrastructural quantitative localization of radiolabeled PrPc within RGC soma showed significant labeling of vesicular/endosomal compartments and much less labeling present over the Golgi apparatus and lysosomes, which indicates slow degradation of exogenous PrPc in this system. These data show that a mechanism exists to internalize a secreted form of PrPc and then to axonally transport such PrPc in an anterograde direction. This may provide an additional, novel mechanism for prion protein to spread among neurons.

Anterograde axonal transport of BDNF and NT-3 by retinal ganglion cells: roles of neurotrophin receptors

Retinal ganglion cells (RGCs) transport exogenous neurotrophins anterogradely to the midbrain tectum/superior colliculus with significant downstream effects. We determined contributions of neurotrophin receptors for anterograde transport of intraocularly injected radiolabeled neurotrophins. In adult rodents, anterograde transport of brain-derived neurotrophic factor (BDNF) was receptor-mediated, and transport of exogenous BDNF and neurotrophin-3 (NT-3) was more efficient, per RGC, in rodents than chicks. RT-PCR and Western blot analysis of purified murine RGCs showed that adult RGCs express the p75 receptor. Anterograde transport of BDNF or NT-3 was not diminished in p75 knock-out mice (with unaltered final numbers of RGCs), but BDNF transport was substantially reduced by co-injected trkB antibodies. In chick embryos, however, p75 antisense or co-injected p75 antibodies significantly attenuated anterograde transport of NT-3 by RGCs. Thus, neither BDNF nor NT-3 utilizes p75 for anterograde transport in adult rodent RGCs, while anterograde NT-3 transport requires the p75 receptor in embryonic chicken RGCs.

Cryptochromes and neuronal-activity markers colocalize in the retina of migratory birds during magnetic orientation

Migratory birds can use a magnetic compass for orientation during their migratory journeys covering thousands of kilometers. But how do they sense the reference direction provided by the Earth's magnetic field? Behavioral evidence and theoretical considerations have suggested that radical-pair processes in differently oriented, light-sensitive molecules of the retina could enable migratory birds to perceive the magnetic field as visual patterns. The cryptochromes (CRYs) have been suggested as the most likely candidate class of molecules, but do CRYs exist in the retina of migratory birds? Here, we show that at least one CRY1 and one CRY2 exist in the retina of migratory garden warblers and that garden-warbler CRY1 (gwCRY1) is cytosolic. We also show that gwCRY1 is concentrated in specific cells, particularly in ganglion cells and in large displaced ganglion cells, which also showed high levels of neuronal activity at night, when our garden warblers performed magnetic orientation. In addition, there seem to be striking differences in CRY1 expression between migratory and nonmigratory songbirds at night. The difference in CRY1 expression between migrants and nonmigrants is particularly pronounced in the large displaced ganglion cells known to project exclusively to a brain area where magnetically sensitive neurons have been reported. Consequently, cytosolic gwCRY1 is well placed to possibly be the primary magnetic-sensory molecule required for light-mediated magnetoreception.

Retinal ganglion cells are autonomous circadian oscillators synthesizing N-acetylserotonin during the day

Retinal ganglion cells send visual and circadian information to the brain regarding the environmental light-dark cycles. We investigated the capability of retinal ganglion cells of synthesizing melatonin, a highly reliable circadian marker that regulates retinal physiology, as well as the capacity of these cells to function as autonomous circadian oscillators. Chick retinal ganglion cells presented higher levels of melatonin assessed by radioimmunoassay during both the subjective day in constant darkness and the light phase of a light-dark cycle. Similar changes were observed in mRNA levels and activity of arylalkylamine N-acetyltransferase, a key enzyme in melatonin biosynthesis, with the highest levels of both parameters during the subjective day. These daily variations were preceded by the elevation of cyclic-AMP content, the second messenger involved in the regulation of melatonin biosynthesis. Moreover, cultures of immunopurified retinal ganglion cells at embryonic day 8 synchronized by medium exchange synthesized a [3H]melatonin-like indole from [3H]tryptophan. This [3H]indole was rapidly released to the culture medium and exhibited a daily variation, with levels peaking 8 h after synchronization, which declined a few hours later. Cultures of embryonic retinal ganglion cells also showed self-sustained daily rhythms in arylalkylamine N-acetyltransferase mRNA expression during at least three cycles with a period near 24 h. These rhythms were also observed after the application of glutamate. The results demonstrate that chick retinal ganglion cells may function as autonomous circadian oscillators synthesizing a melatonin-like indole during the day.

Highly specific interactions between bHLH transcription factors and chromatin during retina development

Basic helix-loop-helix (bHLH) transcription factors such as atonal homolog 5 (ATH5) and neurogenin 2 (NGN2) determine crucial events in retinogenesis. Using chromatin immunoprecipitation, we demonstrate that their interactions with target promoters undergo dynamic changes as development proceeds in the chick embryo. Chick ATH5 associates with its own promoter and with the promoter of the β3 nicotinic receptor specifically in retinal ganglion cells and their precursors. NGN2 binds to the ATH5 promoter in retina but not in optic tectum, suggesting that interactions between bHLH factors and chromatin are highly tissue specific. The transcriptional activations of both promoters correlate with dimethylation of lysine 4 on histone H3. Inactivation of the ATH5 promoter in differentiated neurons is accompanied by replication-independent chromatin de-methylation. This report is one of the first demonstrations of correlation between gene expression, binding of transcription factors and chromatin modification in a developing neural tissue.

Presynaptic neurotrophin-3 increases the number of tectal synapses, vesicle density, and number of docked vesicles in chick embryos

To determine whether presynaptically derived neurotrophins may contribute to synaptic plasticity, we examined whether neurotrophin-3 (NT-3) changed the number, size, vesicle content, or vesicle distribution of synapses within the retinorecipient layers of the chick optic tectum. In this system, endogenous NT-3 derives presynaptically from retinal ganglion cell axons. Retinotectal synapses comprise the majority of synapses in superficial tectal layers, as demonstrated by destruction of retinotectal input by intraocular application of the drug monensin. To examine the effect of increased or decreased levels of NT-3, either exogenous NT-3 or monoclonal NT-3 blocking antibodies were injected into the optic tectum of 19-day-old chick embryos, spiked with radiolabeled protein to verify the success of injections and estimate effective concentrations. After 48 hours, the ultrastructure of superficial tectal layers was analyzed and compared with samples from control tecta injected with cytochrome C. NT-3 increased the number of synapses, synaptic vesicles/profile, synaptic vesicle densities, the number of docked vesicles, and the length of the synaptic profile. Deprivation of anterogradely transported endogenous NT-3 with NT-3 antibodies resulted in the opposite effect: decreased numbers of synapses, decreased vesicle densities, and decreased numbers of docked vesicles. Brain-derived neurotrophic factor (BDNF) had a largely different effect than NT-3. BDNF increased the density of vesicles and deprivation of endogenous TrkB ligands with TrkB fusion protein reduced the density of vesicles in the synapses, without effects on synapse number or docked vesicles. We conclude that anterogradely transported NT-3 affects synapse strength in a way that differs from that of presumably postsynaptic-derived BDNF.

Isolation of total-RNA from formalin-fixed rat retina

The aim of this project was to establish a method for the purification of total-RNA from fixed rat-retina. Two different established methods were used for RNA purification, and successful isolation was verified with RT-PCR for amplification of beta-actin (two different product-lengths) and subsequent gel-electrophoresis. Total-RNA was successfully isolated from fixed rat-retina. The house keeping gene, beta-actin could be detected after fixing the retina either with 1% formalin or with 4% paraformaldehyde (PFA). Hexamer-primer based RT-PCR gave better results than the oligo-d(T)-primer based RT-PCR method. Both the 698 and 225 bp beta-actin-fragments could be successfully amplified, where amplification of the latter was more efficient. This approach shows that tissue fixation prior to RNA-isolation facilitates the rapid isolation of undamaged RNAs in tissues such as the retina, which are known to yield low levels of RNA and are vulnerable to RNases.

Anterograde axonal transport of internalized GDNF in sensory and motor neurons

Endogenous glial cell line-derived neurotrophic factor (GDNF) has been shown to be anterogradely transported in sensory and motor neurons which do not express detectable levels of GDNF mRNA. The source of the anterograde axonal GDNF in these neurons has remained unclear. Here we show that radiolabeled GDNF is internalized by dorsal root ganglion (DRG) and hypoglossal motor neurons at their cell bodies and/or their dendrites and that a mechanism exists to transport the internalized GDNF anterogradely in their axons. Since adjacent glial cells express GDNF mRNA, these data support the concept that Schwann cell- or oligodendrocyte-derived GDNF is taken up by DRG and motor neurons, respectively, and transported anterogradely along the axons for release from terminals, resulting in neuronal transcytosis.

Sorting of internalized neurotrophins into an endocytic transcytosis pathway via the Golgi system: Ultrastructural analysis in retinal ganglion cells

Subcellular pathways and accumulation of internalized radiolabeled neurotrophins NGF, BDNF, and NT-3 were examined in retinal ganglion cells (RGCs) of chick embryos by using quantitative electron microscopic autoradiography. All three neurotrophins accumulated in endosomes and multivesicular bodies. BDNF and NGF also concentrated at the plasma membrane, whereas NT-3 accumulated transiently in the Golgi system. The enhanced targeting of NT-3 to the Golgi system correlated with the anterograde axonal transport of this neurotrophin. Anterograde transport of NT-3, but not its internalization, was significantly attenuated by the tyrosine kinase (trk) inhibitor K252a. Abolishment of trk activity with K252a shifted NT-3 (and BDNF) away from the Golgi system and into a lysosomal pathway, indicating that trk activity regulated sorting of the ligand-receptor complex. Cross-linking of neurotrophins and immunoprecipitation with antibodies to the neurotrophin receptors p75, trkA, trkB, and trkC showed that the large majority of exogenous, receptor-bound NT-3 was bound to trkC in RGC somata, but during anterograde transport in the optic nerve most receptor-bound NT-3 was associated with p75, and after arrival and release in the optic tectum transferred to presumably postsynaptic trkC. These results reveal remarkable and unexpected differences in the intracellular pathways and fates of different neurotrophins within the same cell type. They provide first evidence for an endocytic pathway of internalized neurotrophic factors via the Golgi system before anterograde transport and transcytosis. The results challenge the belief that after internalization all neurotrophins are rapidly degraded in lysosomes.

Expression of neurotrophin-3 (NT-3) and anterograde axonal transport of endogenous NT-3 by retinal ganglion cells in chick embryos

Anterograde axonal transport of neurotrophins has been demonstrated recently, but to date such transport has only been shown for brain-derived neurotrophic factor and no other endogenous neurotrophin. Endogenous neurotrophin-3 (NT-3) protein is present in the ganglion cell layer of the chicken retina, as well as the superficial layers of the optic tectum. NT-3 immunolabel in these tectal layers is largely reduced or abolished after treatment of the eye with colchicine or monensin, demonstrating that endogenous NT-3 is transported to the optic tectum by retinal ganglion cells (RGCs). Reverse transcription-PCR analysis of RGCs purified to 100% shows that RGCs, but not tectal cells, express NT-3 mRNA. Blockade of the intercellular transfer of NT-3 within the retina does not reduce the anterograde transport of endogenous NT-3 to the tectum, indicating that a major fraction of the anterogradely transported NT-3 is produced by RGCs rather than taken up from other retinal cells. Immunolabel for the neurotrophin receptor p75, but not trkB or trkC, in the superficial tectum coincides with the NT-3 label. The p75 label in the neuropil of superficial tectal layers is largely reduced or eliminated by injection of monensin in the eye, indicating that p75 protein is exported along RGC axons to the retinotectal terminals and may act as a neurotrophin carrier. These results show that NT-3 is produced by RGCs and that some of this NT-3 is transported anterogradely along the axons to the superficial layers of the tectum, possibly to regulate the survival, synapse formation, or dendritic growth of tectal neurons.