Functional differentiation of ganglion cells from multipotent progenitor cells in sliced retina of adult goldfish

Live Cell Imaging of Dynamic Processes in Adult Zebrafish Retinal Cross-Section Cultures

Following retinal injury, zebrafish possess the remarkable capacity to endogenously regenerate lost retinal neurons from Müller glia-derived neuronal progenitor cells. Additionally, neuronal cell types that are undamaged and persist in the injured retina are also produced. Thus, the zebrafish retina is an excellent system to study the integration of all neuronal cell types into an existing neuronal circuit. The few studies that examined axonal/dendritic outgrowth and the establishment of synaptic contacts by regenerated neurons predominantly utilized fixed tissue samples. We recently established a flatmount culture model to monitor Müller glia nuclear migration in real time by two-photon microscopy. However, in retinal flatmounts, z-stacks of the entire retinal z-dimension have to be acquired to image cells that extend through parts or the entirety of the neural retina, such as bipolar cells and Müller glia, respectively. Cellular processes with fast kinetics might thus be missed. Therefore, we generated a retinal cross-section culture from light-damaged zebrafish to image the entire Müller glia in one z-plane. Isolated dorsal retinal hemispheres were cut into two dorsal quarters and mounted with the cross-section view facing the coverslips of culture dishes, which allowed monitoring Müller glia nuclear migration using confocal microscopy. Confocal imaging of cross-section cultures is ultimately also applicable to live cell imaging of axon/dendrite formation of regenerated bipolar cells, while the flatmount culture model will be more suitable to monitor axon outgrowth of ganglion cells.

Retinal Tissue Bioengineering, Materials and Methods for the Treatment of Glaucoma

BACKGROUND

Glaucoma, a characteristic type of optic nerve degeneration in the posterior pole of the eye, is a common cause of irreversible vision loss and the second leading cause of blindness worldwide. As an optic neuropathy, glaucoma is identified by increasing degeneration of retinal ganglion cells (RGCs), with consequential vision loss. Current treatments only postpone the development of retinal degeneration, and there are as yet no treatments available for this disability. Recent studies have shown that replacing lost or damaged RGCs with healthy RGCs or RGC precursors, supported by appropriately designed bio-material scaffolds, could facilitate the development and enhancement of connections to ganglion cells and optic nerve axons. The consequence may be an improved retinal regeneration. This technique could also offer the possibility for retinal regeneration in treating other forms of optic nerve ailments through RGC replacement.

METHODS

In this brief review, we describe the innovations and recent developments in retinal regenerative medicine such as retinal organoids and gene therapy which are specific to glaucoma treatment and focus on the selection of appropriate bio-engineering principles, biomaterials and cell therapies that are presently employed in this growing research area.

RESULTS

Identification of optimal sources of cells, improving cell survival, functional integration upon transplantation, and developing techniques to deliver cells into the retinal space without provoking immune responses are the main challenges in retinal cell replacement therapies.

CONCLUSION

The restoration of visual function in glaucoma patients by the RGC replacement therapies requires appropriate protocols and biotechnology methods. Tissue-engineered scaffolds, the generation of retinal organoids, and gene therapy may help to overcome some of the challenges in the generation of clinically safe RGCs.

Changes in somatic sodium currents of ganglion cells during retinal regeneration in the adult newt

Adult newts can regenerate their entire retinas following a complete removal of the original tissues. During retinal regeneration, ganglion cells differentiate first from the progenitor cells, and develop their capability of spike firing. In the present study, to understand the process of functional differentiation of ganglion cells, we investigated alterations of their voltage-gated sodium currents during retinal regeneration by a whole-cell patch-clamp technique. To minimize space clamp errors, sodium currents were recorded from neurite-free somata of presumptive ganglion cells that were mechanically isolated from living slices of regenerating retinas at different morphological stages. During retinal regeneration, the somatic sodium current density was increased 2.6-fold (48 to 123 pF/pA) and the half-activating voltage was shifted slightly to more hyperpolarizing membrane potentials (-10 to -13 mV), while steady-state inactivation was not changed obviously. Curve fitting analysis of currents revealed that the sodium current consists of two components with different inactivation time constants. During retinal regeneration, the ratio of slow to fast inactivating current component was increased 2.6-fold (0.11 to 0.29). These results suggest that the somatic sodium currents of ganglion cells may undergo modifications of their voltage dependence and kinetic properties during retinal regeneration. A small number of the presumptive ganglion cells in regenerating retinas with a segregating inner plexiform layer exhibited sodium currents comparable to those in the normal retina. This might suggest that maturational regulation of sodium channel function starts during a period of synaptic layer formation within the retina.

A decay of gap junctions associated with ganglion cell differentiation during retinal regeneration of the adult newt

Changes in the gap junctional coupling and maturation of voltage-activated Na(+) currents during regeneration of newt retinas were examined by whole-cell patch-clamping in slice preparations. Progenitor cells in regenerating retinas did not exhibit Na(+) currents but showed prominent electrical and tracer couplings. Cells identified by LY-fills were typically slender. Na(+) currents were detected in premature ganglion cells with round somata in the 'intermediate-II' regenerating retina. No electrical and tracer couplings were observed between these cells. Mature ganglion cells did not exhibit electrical coupling, but showed tracer coupling. On average, the maximum Na(+) current amplitude recorded from premature ganglion cells was roughly 2.5-fold smaller than that of mature ganglion cells. In addition, the activation threshold of the Na(+) current was nearly 11 mV more positive than that of mature cells. We provide morphological and physiological evidence showing that loss of gap junctions between progenitor cells is associated with ganglion cell differentiation during retinal regeneration and that new gap junctions are recreated between mature ganglion cells. Also we provide evidence suggesting that the loss of gap junctions correlates with the appearance of voltage-activated Na(+) currents in ganglion cells.

Maturation of spiking activity in trout retinal ganglion cells coincides with upregulation of Kv3.1- and BK-related potassium channels

Developmental changes in membrane excitability and the potassium channel profile were monitored in acutely isolated trout retinal ganglion cells by patch-clamp recording in combination with single-cell RT-PCR. During embryonic development in the egg, a sustained above-threshold stimulation of ganglion cells elicited in most cases only a single spike response. After hatching, the proportion of multiply spiking cells increased strongly and the ability of spike frequency coding was acquired. This was accompanied by the occurrence of a highly tetraethylammonium (TEA)- and quinine-sensitive delayed rectifier current, which gradually masked a rapidly inactivating A-type potassium current that was predominant at earlier stages. Pharmacology of the delayed rectifier current closely matched those of recombinant Traw1, a Kv3.1-related potassium channel in trout. The appearance of this current correlated closely with initial expression of Traw1 and Traw2 channel transcripts, as revealed by multiplex single-cell RT-PCR, whereas mRNA, encoding Shaker-related channel genes in trout (termed Tsha1-Tsha4), were already detectable at early embryonic stages. Iberiotoxin-sensitive, calcium-activated potassium currents (BK) were extremely low before hatching, but increased significantly thereafter. These developmental changes in potassium channel expression occurred after the arrival of retinal fibers in the optic tectum and the initiation of synapse formation in the visual center. It is suggested that early expressed Shaker-related potassium channels could act to influence neuronal differentiation, whereas proper neuronal signaling requires expression of Kv3.1- and BK-related potassium channels.

Gap junctional coupling between progenitor cells at the retinal margin of adult goldfish

We prepared living slice preparations of the peripheral retina of adult goldfish to examine electrical membrane properties of progenitor cells at the retinal margin. Cells were voltage-clamped near resting potential and then stepped to either hyperpolarizing or depolarizing test potentials using whole-cell voltage-clamp recordings. Electrophysiologically examined cells were morphologically identified by injecting both Lucifer Yellow (LY) and biocytin. All progenitor cells examined (n = 37) showed a large amount of passively flowing currents of either sign under suppression of the nonjunctional currents flowing through K(+) and Ca(2+) channels in the cell membrane. They did not exhibit any voltage-gated Na(+) currents. Cells identified by LY fills were typically slender. As the difference between the test potential and the resting potential increased, 13 out of 37 cells exhibited symmetrically voltage- and time-dependent current decline on either sign at the resting potential. The symmetric current profile suggests that the current may be driven and modulated by the junctional potential difference between the clamping cell and its neighbors. The remaining 24 cells did not exhibit voltage dependency. A gap junction channel blocker, halothane, suppressed the currents. A decrease in extracellular pH reduced coupling currents and its increase enhanced them. Dopamine, cAMP, and retinoic acid did not influence coupling currents. Injection of biocytin into single progenitor cells revealed strong tracer coupling, which was restricted in the marginal region. Immature ganglion cells closely located to the retinal margin exhibited voltage-gated Na(+) currents. They did not reveal apparent tracer coupling. These results demonstrate that the marginal progenitor cells couple with each other via gap junctions, and communicate biochemical molecules, which may subserve or interfere with cellular differentiation.