Choline acetyltransferase and acetylcholinesterase in the normal, developing and regenerating newt retinas

Hematological- and Neurological-Expressed Sequence 1 Gene Products in Progenitor Cells during Newt Retinal Development

Urodele amphibians such as Japanese common newts have a remarkable ability to regenerate their injured neural retina, even as adults. We found that hematological- and neurological-expressed sequence 1 (Hn1) gene was induced in depigmented retinal pigment epithelial (RPE) cells, and its expression was maintained at later stages of newt retinal regeneration. In this study, we investigated the distribution of the HN1 protein, the product of the Hn1 gene, in the developing retinas. Our immunohistochemical analyses suggested that the HN1 protein was highly expressed in an immature retina, and the subcellular localization changed during this retinogenesis as observed in newt retinal regeneration. We also found that the expression of Hn1 gene was not induced in mouse after retinal removal. Our results showed that Hn1 gene can be useful for detection of undifferentiated and dedifferentiated cells during both newt retinal development and regeneration.

The role of bioactive compounds on the promotion of neurite outgrowth

Neurite loss is one of the cardinal features of neuronal injury. Apart from neuroprotection, reorganization of the lost neuronal network in the injured brain is necessary for the restoration of normal physiological functions. Neuritogenic activity of endogenous molecules in the brain such as nerve growth factor is well documented and supported by scientific studies which show innumerable compounds having neurite outgrowth activity from natural sources. Since the damaged brain lacks the reconstructive capacity, more efforts in research are focused on the identification of compounds that promote the reformation of neuronal networks. An abundancy of natural resources along with the corresponding activity profiles have shown promising results in the field of neuroscience. Recently, importance has also been placed on understanding neurite formation by natural products in relation to neuronal injury. Arrays of natural herbal products having plentiful active constituents have been found to enhance neurite outgrowth. They act synergistically with neurotrophic factors to promote neuritogenesis in the diseased brain. Therefore use of natural products for neuroregeneration provides new insights in drug development for treating neuronal injury. In this study, various compounds from natural sources with potential neurite outgrowth activity are reviewed in experimental models.

MEK-ERK and heparin-susceptible signaling pathways are involved in cell-cycle entry of the wound edge retinal pigment epithelium cells in the adult newt

The onset mechanism of proliferation in mitotically quiescent retinal pigment epithelium (RPE) cells is still obscure in humans and newts, although it can be a clinical target for manipulating both retinal diseases and regeneration. To address this issue, we investigated factors or signaling pathways involved in the first cell-cycle entry of RPE cells upon retinal injury using a newt retina-less eye-cup culture system in which the cells around the wound edge of the RPE exclusively enter the cell cycle. We found that MEK-ERK signaling is necessary for their cell-cycle entry, and signaling pathways whose activities can be modulated by heparin, such as Wnt-, Shh-, and thrombin-mediated pathways, are capable of regulating the cell-cycle entry. Furthermore, we found that the cells inside the RPE have low proliferation competence even in the presence of serum, suggesting inversely that a loss of cell-to-cell contact would allow the cells to enter the cell cycle.

The alpha1 isoform of the Na+/K+ ATPase is up-regulated in dedifferentiated progenitor cells that mediate lens and retina regeneration in adult newts

Adult newts are able to regenerate their retina and lens after injury or complete removal through transdifferentiation of the pigmented epithelial tissues of the eye. This process needs to be tightly controlled, and several different mechanisms are likely to be recruited for this function. The Na(+)/K(+) ATPase is a transmembrane protein that establishes electrochemical gradients through the transport of Na(+) and K(+) and has been implicated in the modulation of key cellular processes such as cell division, migration and adhesion. Even though it is expressed in all cells, its isoform composition varies with cell type and is tightly controlled during development and regeneration. In the present study we characterize the expression pattern of Na(+)/K(+) ATPase alpha1 in the adult newt eye and during the process of lens and retina regeneration. We show that this isoform is up-regulated in undifferentiated cells during transdifferentiation. Such change in composition could be one of the mechanisms that newt cells utilize to modulate this process.

Musashi-1, an RNA-binding protein, is indispensable for survival of photoreceptors

Musashi-1 (Msi1), an RNA-binding protein (RBP), has been postulated to play important roles in the maintenance of the stem-cell state, differentiation, and tumorigenesis. However, the expression and function of Msi1 in differentiated cells remain obscure. Here we show that Msi1 is expressed in mature photoreceptors and retinal pigment epithelium (RPE) cells, and is indispensable for the survival of photoreceptors. We found in the adult newt eye that Msi1 is expressed in all photoreceptors and RPE cells as well as in the retinal stem/progenitor cells in the ciliary marginal zone (CMZ). We found in the analyses of the newt normal and regenerating retinas that the expression profiles of the Msi1 transcripts and protein isoforms in the photoreceptors are different from those in the retinal stem/progenitor cells. Furthermore, we found that all photoreceptors and RPE cells of the adult mice also express Msi1, and that Msi1 knockout (Msi1-KO) results in degeneration of photoreceptors and a lack of a visual cycle protein RPE65 in the microvilli of RPE cells. Taken together, our current results demonstrate that the expression of Msi1 in mature photoreceptors and RPE cells is evolutionarily conserved, and that Msi1 bears essential functions for vision. Considering such an Msi1-KO phenotype in the retina, it is now reasonable to address whether defects of the Msi1 functions are responsible for inherited retinal diseases. Studying the regulation of Msi1 and the target RNAs of Msi1 in photoreceptors and RPE cells might contribute to fundamental and clinical studies of retinal degeneration.

Role of acetylcholine in nitric oxide production in the salamander retina

Although acetylcholine is one of the most widely studied neurotransmitters in the retina, many questions remain about its downstream signaling mechanisms. In this study we initially characterized the cholinergic neurotransmitter system in the salamander retina by localizing a variety of cholinergic markers. We then examined the link between both muscarinic and nicotinic receptor activation and nitric oxide production by using immunocytochemistry for cyclic guanosine monophosphate (cGMP) as an indicator. We found a large increase in cGMP-like immunoreactivity (cGMP-LI) in the inner retina in response to muscarinic (but not nicotinic) receptor activation. Based on the amplification of mRNA transcripts, receptor immunocytochemistry, and the use of selective antagonists, we identified these receptors as M2 muscarinic receptors. Using double-labeling techniques, we established that these increases in cGMP-LI were seen in GABAergic but not cholinergic amacrine cells, and that the increases were blocked by inhibitors of nitric oxide production. The creation of nitric oxide in response to cholinergic receptor activation may provide a mechanism for modulating the well-known mutual interactions of acetylcholine-glycine-GABA in the inner retina. As GABA and glycine are the primary inhibitory neurotransmitters in the retina, signaling pathways that modulate their levels or release will have major implications for the processing of complex stimuli by the retina.

MEK mediates in vitro neural transdifferentiation of the adult newt retinal pigment epithelium cells: Is FGF2 an induction factor?

Adult newts can regenerate their entire retinas through transdifferentiation of the retinal pigment epithelium (RPE) cells. As yet, however, underlying molecular mechanisms remain virtually unknown. On the other hand, in embryonic/larval vertebrates, an MEK [mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase] pathway activated by fibroblast growth factor-2 (FGF2) is suggested to be involved in the induction of transdifferentiation of the RPE into a neural retina. Therefore, we examined using culture systems whether the FGF2/MEK pathway is also involved in the adult newt RPE transdifferentiation. Here we show that the adult newt RPE cells can switch to neural cells expressing pan-retinal-neuron (PRN) markers such as acetylated tubulin, and that an MEK pathway is essential for the induction of this process, whereas FGF2 seems an unlikely primary induction factor. In addition, we show by immunohistochemistry that the PRN markers are not expressed until the 1-3 cells thick regenerating retina, which contains retinal progenitor cells, appears. Our current results suggest that the activation of an MEK pathway in RPE cells might be involved in the induction process of retinal regeneration in the adult newt, however if this is the case, we must assume complementary mechanisms that repress the MEK-mediated misexpression of PRN markers in the initial process of transdifferentiation.

Development of starburst cholinergic amacrine cells in the retina of Tupaia belangeri

"Starburst" cholinergic amacrines specify the response of direction-selective ganglion cells to image motion. Here, development of cholinergic amacrines was studied in the tree shrew Tupaia belangeri (Scandentia) by immunohistochemistry with antibodies against choline acetyltransferase (ChAT) and neurofilament proteins. Starburst amacrines expressed ChAT much earlier than previously thought. From embryonic day 34 (E34) onward, orthotopic and displaced subpopulations segregated from a single cluster of immunoreactive precursor cells. Orthotopic starburst amacrines rapidly took up positions in the inner nuclear layer. Displaced starburst amacrines were first arranged in a monocellular row in the inner plexiform layer, and, with a delay of 1 week, they descended to the ganglion cell layer. Conversely, dendritic stratification of displaced amacrines slightly preceded that of orthotopic ones. Starburst amacrines expressed the medium-molecular-weight neurofilament protein (NF-M) from E34 to postnatal day 11 (P11) and coexpressed alpha-internexin from E36.5 to P11. Consequently, neurofilaments composed of alpha-internexin and NF-M may stabilize developing dendrites of starburst amacrines. During the first 2 postnatal weeks, subpopulations of anti-NF-M-labeled ganglion cells costratified with the preexisting dendritic strata of starburst amacrines in the ON sublamina, OFF sublamina, or both. Hence, anti-NF-M-labeled ganglion cells may include direction-selective ones. Thereafter, NF-M and alpha-internexin proteins disappeared from starburst amacrines, and NF-M immunoreactivity was lost in the dendrites of ganglion cells. Our findings suggest that NF-M and alpha-internexin are important for starburst amacrines and ganglion cells to recognize each other and, thus, contribute to the formation of early developing retinal circuits in the inner plexiform layer.

Evidence for Notch signaling involvement in retinal regeneration of adult newt

Involvement of Notch signaling in retinal regeneration by transdifferentiation of pigment epithelium cells was investigated using the adult newt Cynops pyrrhogaster. During retinal regeneration, cells expressing Notch-1 first appeared in the regenerating retina one to two cells thick (stage E-3) originated from the retinal pigment epithelium (RPE) cells, and increased in number as the regenerating retina increased in thickness. Notch-1 expression was decreased in the central retina in association with cell differentiation and became restricted to the peripheral retina. Administration of a Notch signaling blocker DAPT resulted in the appearance of a cluster of neurons, earlier than in normal regeneration, along the regenerating retina 1-3 cells thick (stage E-3 to I-1). Immunoblot analysis suggested that DAPT could perturb the processing of Notch-1. Similar results were obtained in the newt embryonic retinal development. These results suggest that the Notch-1 signaling system may be reset to regulate neurogenesis during retinal regeneration. However, PCR analysis revealed that the adult newt RPE cells express Hes-1, neurogenin1 and sometimes Delta-1 Hes-1, neurogenin1 and sometimes Delta-1 all of which are differently regulated in association with retinal regeneration, implying that Notch signaling might also be involved early in the process of transdifferentiation.

Visual cycle protein RPE65 persists in new retinal cells during retinal regeneration of adult newt

Adult newts can regenerate their entire retina through transdifferentiation of the retinal pigment epithelium (RPE). The objective of this study was to redescribe the retina regeneration process by means of modern biological techniques. We report two different antibodies (RPE-No.112 and MAB5428) that recognize the newt homolog of RPE65, which is involved in the visual cycle and exclusively label the RPE cell-layer in the adult newt eye. We analyzed the process of retinal regeneration by immunohistochemistry and immunoblotting and propose that this process should be divided into nine stages. We found that the RPE65 protein is present in the RPE-derived new retinal rudiment at 14 days postoperative (po) and in the regenerating retinas at the 3-4 cell stage (19 days po). These observations suggest that certain characteristics of RPE cells overlap with those of retinal stem/progenitor cells during the period of transdifferentiation. However, RPE65 protein was not detected in either retinal stem/progenitor cells in the ciliary marginal zone (CMZ) of adult eyes or in neuroepithelium present during retina development, where it was first detected in differentiated RPE. Moreover, the gene expression of RPE65 was drastically downregulated in the early phase of transdifferentiation (by 10 days po), while those of Connexin43 and Pax-6, both expressed in regenerating retinas, were differently upregulated. These observations suggest that the RPE65 protein in the RPE-derived retinal rudiment may represent the remainder after protein degradation or discharge rather than newly synthesized protein.

Downregulation of Otx2 in the dedifferentiated RPE cells of regenerating newt retina

Cynops pyrrhogaster (the Japanese common newt) regenerates neural retina from retinal pigmented epithelium (RPE) cells. Otx2 is a transcription factor that is involved in RPE cell differentiation. To understand the role of Otx2 during transdifferentiation of RPE cells, we cloned a Cynops Otx2 cDNA, and explored its expression by RT-PCR, immunohistochemistry and in situ hybridization. The expression of Otx2 was compared with the localization of a proliferating cell marker (PCNA), RPE cell markers (RPE65, CRBP) and an RPE and Muller glial cell marker (CRALBP). At the early stage of regeneration, 2 to 3 cell layered regenerating retina consisting of pigmented cells uniformly expressed Otx2 and other markers. Following this stage, 4-cell layered regenerating retina consisted of two distinct layers, pigmented monolayer (the outer layer) attached to Bruch's membrane and presumptive neural retina (the inner layers). In the outer layer, Otx2 and CRBP expression was maintained and majority of cells lost PCNA expression. Some of cells maintained RPE65. In the inner layers, expression of Otx2, CRBP and RPE65 was downregulated, but a majority of those cells maintained PCNA expression. These results indicate that spatiotemporal regulation of Otx2 expression is consistent with those of RPE markers. Otx2 may play a pivotal role in maintenance and specification of RPE cells during neural retina regeneration. In contrast to RPE cell markers, CRALBP was expressed in both the pigmented and the de-pigmented layers. This observation implicates the appearance of Muller glial cells in an early phase of regenerating retina.

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.

Nardostachys chinensis Glycoside Induces Characteristics of Neuronal Differentiation in Rat Pheochromocytoma PC12 Cells

Rat pheochromocytoma PC12 cells undergo neuronal differentiation in response to nerve growth factor. We show here that exposure of PC12 cells to Nardostachys chinensis glycoside induces the outgrowth of neurites, increases the activity of AChE, triggers cell cycle arrest in G1 and enhances the expression of growth associated protein 43 (GAP-43). Both the outgrowth of neurites and the increase in AChE activity are prevented partly by PD98059, a specific inhibitor of MEK1. These results suggest that N. chinensis glycoside induces the characteristics of neuronal differentiation in PC12 cells via the mitogen-activated protein kinase (MAPK)-related signal cascade.

GABAA-receptor-mediated increase in intracellular Ca2+ concentration in the regenerating retina of adult newt

We used optical recording with the Ca(2+)-sensitive dye, fura-2, in living slice preparations from the newt retina at different stages of regeneration. gamma-Aminobutyric acid (GABA) induced pronounced [Ca(2+)](i) rise in progenitor cells and differentiating ganglion cells in the 'intermediate' stage of retinal regeneration. This [Ca(2+)](i) rise became less pronounced at the beginning of synapse formation in the late regenerating retina. At the late period of the late regenerating retina with the IPL thickness comparable to that of the control retina, GABA-induced [Ca(2+)](i) rise became undetectable or sometimes a small decrease in [Ca(2+)](i) was observed in regenerated ganglion cells. In contrast, N-methyl-d-aspartate (NMDA)-induced [Ca(2+)](i) rise appeared in premature ganglion cells and became prominent gradually as the regeneration proceeded. The [Ca(2+)](i) rise to GABA was mediated by GABA(A) receptors. This was shown by inhibition of GABA-induced Ca(2+) response with the preincubation of the GABA(A) receptor antagonist, bicuculline. The [Ca(2+)](i) rise due to GABA was suppressed in the absence of extracellular Ca(2+) or in the presence of the L-type voltage-gated Ca(2+) channel blocker, verapamil, suggesting that Ca(2+) may be entered through L-type Ca(2+) channels. Transient appearance of [Ca(2+)](i) rise to GABA during regeneration and origin of GABA-induced [Ca(2+)](i) rise were similar to those in the developing retina [J. Neurobiol. 24 (1993) 1600]. These similarities may suggest that common mechanisms may control neurogenesis and/or synaptogenesis during development and regeneration.

Choline acetyltransferase-immunoreactive neurons in the retina of adult and developing lampreys

The presence of choline acetyltransferase-immunoreactive (ChATir) amacrine cells is reported for the first time in the retinas of three species of lamprey (Lampetra fluviatilis, Ichthyomyzon unicuspis, and Petromyzon marinus). In the three species, the ChATir cells were mainly distributed in the inner plexiform layer (IPL), which in lampreys extends from the inner nuclear layer (INL) to the inner limiting membrane. These cells had a bipolar, triangular or stellate appearance, and gave rise to processes coursing in the inner plexiform layer. In transforming lampreys, ChATir processes formed two asymmetrical inner and outer subplexuses in the inner plexiform layer, which is reminiscent of the distribution of processes of ChATir cells in the On and Off sublaminae reported in jawed vertebrates. The larval retina lacked ChAT immunoreactivity, and ChATir cells and processes appeared at early metamorphosis throughout the retina, exhibiting in late transforming stages an organization similar to that of adults. This first report of ChATir cells in the lamprey retina indicates that the appearance of cholinergic circuits in the retina of vertebrates occurred before the divergence of the agnathan and gnathostome lines.

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.

Muscarinic calcium mobilization in the regenerating retina of adult newt

We used optical recording with a Ca(2+)-sensitive dye, fura2, in living slice preparations from the newt retina at different stages of regeneration. ACh produced the most pronounced [Ca2+]i rise in progenitor cells and premature ganglion cells of the earlier stage of retinal regeneration, but less pronounced Ca2+ response in ganglion cells just before, or at the beginning of, synaptogenesis. The [Ca2+]i rise to ACh was mediated by mAChRs. This was shown by inhibition of the ACh-induced Ca2+ response with a preincubation of the mAChR antagonist atropine as well as with direct stimulation of the [Ca2+]i rise by the mAChR agonist muscarine. This muscarine-induced [Ca2+]i rise was more greatly suppressed by the M1 and/or M3 preferring mAChR antagonists than by the M2 preferring mAChR antagonist. The [Ca2+]i rise due to muscarine was not suppressed in the absence of extracellular Ca2+, but suppressed in part in the presence of the L-type voltage-gated Ca2+ channel blockers, verapamil or nicardipine. Furthermore, thapsigargin (TG), a Ca-ATPase inhibitor, abolished the muscarine-induced [Ca2+]i rise in the absence of extracellular Ca2+. These results suggest that the mAChR-mediated [Ca2+]i rise is mainly a result of a release of Ca2+ from intracellular stores. TG produced a slow rise in the resting level of [Ca2+]i. This [Ca2+]i raise was suppressed as extracellular Ca2+ was omitted, whereas a rapid rise in [Ca2+]i occurred when extracellular Ca2+ was reintroduced, suggesting the occurrence of the capacitative Ca2+ influx in the progenitor cells and premature ganglion cells of the regenerating newt retina.

The appearance and maturation of excitatory and inhibitory neurotransmitter sensitivity during retinal regeneration of the adult newt

Using living slice preparations from newt retinas at different stages of regeneration, we examined the time course of appearance and maturation of neurotransmitter-induced currents with whole-cell patch-clamp methods. Neurons from which currents were recorded were identified by Lucifer Yellow fills. All progenitor cells examined at the regenerating retinas did not express any voltage-gated Na+ currents and responsiveness to excitatory amino acid analogues (AMPA and NMDA) and inhibitory amino acids (GABA and glycine). Voltage-gated Na+ currents were first detected in premature ganglion cells with round cell body located at the most proximal level of the 'intermediate-II' regenerating retina. AMPA- GABA- and glycine-induced currents were simultaneously observed in many premature ganglion cells expressing Na+ channels, but not all, suggesting that the onset of the Na+ channels is slightly earlier than that of excitatory and inhibitory amino acid receptors in regeneration. NMDA-evoked currents were first observed in the 'intermediate-III' regenerating retina just before the synaptogenesis. Pharmacological properties and reversal potential values of the excitatory and inhibitory amino acid responses did not change substantially between regenerating ganglion cells and mature ganglion cells, while rectification properties of current-voltage relations for AMPA and NMDA responses were somewhat different between them.

Effect of Cuscuta chinensis glycoside on the neuronal differentiation of rat pheochromocytoma PC12 cells

Exposure of rat pheochromocytoma PC12 cells to Cuscuta chinensis glycoside induced neuronal differentiation with resulting outgrowth of neurites and increase of acetylcholinesterase activity. A specific inhibitor of mitogen-activated protein kinase (MAPK) kinase, PD98059, prevented this effect of C. chinensis on PC12 cells. These results suggested that C. chinensis glycoside induced neuronal differentiation in PC12 cells linked to the mitogen-activated protein kinase signaling cascade.

Morphological and functional organization of ON and OFF pathways in the adult newt retina

Morphological and functional organization of ON and OFF pathways in the adult newt retina were examined by intracellular recording and staining techniques and immunohistochemistry. Synaptotagmin immunoreactivity discriminated three broad bands within the IPL: the distal band (sublamina I), the middle band (sublamina II) consisting of two dense punctate bands (sublaminae II(a) and II(b)), and proximal band (sublamina III). The Lucifer-yellow labeled OFF amacrine and ganglion cells send their processes mainly in sublamina I and/or II(a) where OFF bipolar cells extend their axon terminals, while ON amacrine and ganglion cells send their processes in sublamina III and/or II(b) where ON bipolar cells extend their axon terminals. Processes of ON-OFF amacrine and ganglion cells ramify broadly in the whole thickness of the IPL. Many bipolar cells responded to light spot with a transient hyperpolarization at both light onset and offset. They are probably subtypes of ON bipolar cells, because their axon terminals branch mainly in sublaminae III and/or II(b), although a few cells ramified the axon at both sublaminae II(a) and III. Two immunohistochemical markers for bipolar cells, PKC and RB-1, identified axon terminals in sublaminae III and/or II(b). From the ramification pattern of axon terminal, they are probably subtypes of ON bipolar cells. ChAT-ir amacrine cells ramified their dendrites in either sublamina I or II(b). Altogether, present studies support the general idea of segregation of ON and OFF pathways in sublaminae a and b of the IPL.

Choline acetyltransferase is expressed by non-starburst amacrine cells in the ground squirrel retina

We have used immunostaining techniques to reveal a new type of amacrine cell that is immunoreactive for choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, in the Ground Squirrel (Spermophilus beecheyi) retina. Cryostat sections and double immunostained wholemount preparations were examined by confocal microscopy. This new ChAT type III cell is distinct in morphology and neurotransmitter content from the well know 'starburst' amacrine cells (types I and II) that are so well represented in the ground squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. The type III cell colocalizes glycine with the acetylcholine and does not appear to be GABAergic or exhibit calcium-binding proteins like the well-known starburst type. As well, type III cells do not occur as a mirror-symmetric pair with normally placed and displaced varieties. The type III cell is probably a small field amacrine type branching broadly in upper sublamina b of the inner plexiform layer, and is most likely A6 of the Ground Squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. Type III cells are ideally placed in the architecture of the Ground Squirrel retina to influence ON directionally selective ganglion cell types.

Spatial and temporal patterns of distribution of the gap junctional protein connexin43 during retinal regeneration of adult newt

Newts possess the ability to regenerate a functional retina after complete removal of the original retina. We performed immunoblot and immunohistochemical analyses of newt retinas at different stages of regeneration by using an antibody against a gap junction channel protein, connexin43 (Cx43). The specificity of the antibody was shown on immunoblots as well as immunohistochemical staining pattern in the normal retina. Punctate Cx43 immunolabeling was detected intensely between proliferating cell nuclear antigen-immunoreactive progenitor cells in the regenerating retinas, and the amount of this labeling tended to be prominent along both scleral and vitreal sides. The amount of Cx43 became less abundant as regeneration proceeded. This temporal loss of Cx43 during regeneration was also shown on the immunoblot analysis. Furthermore, the loss of Cx43 was observed in a spatial manner in the peripheral retina, where progenitor cells clustered at the ciliary marginal zone (CMZ) are adding new cells of all types in order toward the central retina. Immunolabeling often extended longitudinally throughout the retina when regenerating retinas became thick. Double immunolabeling with Cx43 and glial fibrillary acidic protein indicated the overlapping between the Cx43 and Müller cell processes. At the beginning of the synaptic formation, immunolabeling almost disappeared in the entire retina. However, in the completely regenerated retina, Cx43 reappeared in the distal end of Müller cells and pigment epithelial cells in the same pattern as in the normal retina. The above observations lead us to speculate that Cx43-mediated gap junctions may play an important role in regenerating events. Possible roles of Cx43 during regeneration are discussed.

Choline acetyltransferase immunoreactivity in the developing brain of Xenopus laevis

The spatiotemporal sequence of the appearance of cholinergic structures in the brain of Xenopus laevis during development was studied by means of choline acetyltransferase (ChAT) immunohistochemistry. The first ChAT labeling in the central nervous system of Xenopus was obtained at late embryonic stages in the spinal motoneurons, the cranial nerve motor nuclei of the brainstem, and in amacrine cells of the retina. During premetamorphosis, these cholinergic structures maturated significantly and new ChAT-immunoreactive cells were observed in several other nuclei such as the solitary tract nucleus, isthmic nucleus, laterodorsal and pedunculopontine tegmental nuclei, epiphysis, dorsal habenular nucleus, medial amygdala, bed nucleus of the stria terminalis, and dorsal pallidum. Further maturation continued through prometamorphosis and the climax of the metamorphosis together with the appearance of new cell groups in the efferent octaval nucleus, ventral hypothalamic nucleus, anterior preoptic area, suprachiasmatic nucleus, and medial septum. Transient expression of ChAT was only seen in the large Mauthner cells that showed moderate ChAT labeling during pre- and prometamorphosis but became immunonegative at the end of the metamorphosis. The gradual appearance, in general from caudal to rostral brain levels, of ChAT immunoreactivity in Xenopus, was correlated with other developmental events to get insight into the possible roles of acetylcholine during ontogeny. Comparison with the developmental pattern of cholinergic systems in other vertebrates shows that Xenopus possesses abundant features in common with amniotes, suggesting a conservative developmental plan for tetrapods.

Localization of choline acetyltransferase in the developing and adult retina of Xenopus laevis

In the present study the presence of choline acetyltransferase (ChAT) immunoreactive cells in the adult retina of Xenopus laevis was demonstrated and their appearance and maturation during development were examined. Two cholinergic amacrine cell types were identified in the retina. They were located in the deepest row of cells in the inner nuclear layer and in the ganglion cell layer, respectively. Cell processes from these cells organized distinct laminae within the inner plexiform layer. ChAT immunoreactivity was first observed at embryonic stage 35 coinciding with the onset of vision and increased rapidly in premetamorphosis as synaptogenesis and growth proceeded. The development of both ChAT cell populations occurred simultaneously and cells that expressed ChAT transiently were not observed. Our results contrast with previous studies that suggested a late involvement of acetylcholine in the retina of Xenopus and support the notion that, like in mammals, this transmitter is involved in early phases of neurogenesis, cell migration, neuronal growth, and synaptogenesis.

Opsin expression in adult, developing, and regenerating newt retinas

Japanese common newts (Cynops pyrrhogaster) have an ability to regenerate their neural retina even as adults. Although extensive research has been carried out attempting to understand this retinal regeneration, the molecules characterized in newt retina are limited. We isolated cDNAs encoding three putative opsins (Cp-Rh, -LWS and -SWS1), in addition to Cp-SWS2 [Takahashi et al., FEBS Lett. 501 (2001) 151-155] from a cDNA library of adult newt retina. Our immunohistochemical and in situ hybridization studies demonstrated that Cp-Rh is selectively expressed in rods, whereas the other opsins are expressed in cones. The distribution of opsin mRNAs in normal and regenerated retinas is very similar. In both developing and regenerating retinas, Cp-Rh and its mRNA first appeared in immature rods at the beginning or just after the formation of plexiform layers. Cp-Rh was initially found isotropically in the plasma membrane, and then translocalized to the apical region along with the maturation of regenerating rods. This suggests that reorganization of the intracellular structure takes place during maturation of the regenerating newt photoreceptors.

Expression pattern of a newt Notch homologue in regenerating newt retina

We isolated part of a newt Notch homologue, N-Notch, from regenerating newt retina. The spatio-temporal pattern of N-Notch expression was studied by in situ hybridization at different stages of newt retinal regeneration. Proliferating cells were confirmed by the injection of bromodeoxyuridine (BrdU). In the early stage of regeneration, when the retina was one to two cells thick, all proliferating retinal progenitors expressed N-Notch. As the thickness of the retina increased with regeneration, N-Notch expression decreased in BrdU-positive cells on the vitreal side of the retina. Subsequently, presumptive retinal ganglion cells that were BrdU-negative cells appeared at the vitreal edge of the regenerating retina. These differentiating cells did not express N-Notch. Later, N-Notch expression decreased in the BrdU-positive cells on the scleral surface of the retina. Subsequently, presumptive photoreceptor cells that were BrdU-negative cells appeared in this region. These differentiating cells also did not express N-Notch. The proliferating retinal progenitors ceased expressing N-Notch and then stopped dividing during the differentiation of ganglion cells and photoreceptor cells. It was found that retinal regeneration involves the expression of an important developmental signaling molecule, Notch, in retinal progenitors and the expression of Notch ceased as cell differentiation proceeded during retinal regeneration.

Muscarinic acetylcholine receptors in the normal, developing and regenerating newt retinas

Immunoreactivity for m2 and m4 muscarinic acetylcholine receptors (mAChRs) was demonstrated in the adult newt retina. The m2 mAChR was localized to somata on either side of the inner plexiform layer (IPL), especially ganglion cells, and also distributed into two bands within the IPL. The distal band at a depth of 0-15% IPL co-localized with one of two choline acetyltransferase (ChAT) immunoreactive bands, while the proximal band at 85-100% depth did not overlap with either of the ChAT-ir bands. The m4 mAChR was localized to somata closely apposed to either side of the IPL, probably amacrine cell somata, and no immunoreactivity was detectable throughout the IPL. The time course of appearance of the m2 and m4 mAChRs was examined in both developing and regenerating retinas. Like acetylcholinesterase (AChE), the m2 was first detected in somata located at the most proximal level of the retina well before ChAT-ir cholinergic neurons appeared, while the m4 was detected at the time of appearance of ChAT, in both developing and regenerating retinas. When the outer plexiform layer (OPL) began to form, somata in the horizontal cell layer became transiently immunoreactive to the m2. The discrepancy in distribution of the m2 and ChAT in the IPL suggests that mAChR may play a role other than cholinergic neurotransmission. Furthermore, the similarity in time course of appearance of the m2 and m4, as well as other cholinergic system components [4], in both developing and regenerating retinas would suggest that the mechanisms that control neuronal differentiation during retinal development and regeneration are similar.

Expression of cholinergic system molecules during development of the chick nervous system

There are suggestions of the participation of nicotinic acetylcholine receptors (nAChRs), the acetylcholine degradation enzyme, acetylcholinesterase (AChE), and the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT), in the development of the nervous system. In this study, we aimed at comparing the development of some subunits of the nAChRs, AChE, and ChAT in the chick nervous system by standard immunohistochemical methods. The expression of all molecules investigated here appeared very early in ganglia (embryonic day 3.5-4), persisting into posthatching, except for ChAT, which is not detected after hatching in ganglia. A differential development was observed for nAChR subunits, with these receptors appearing around embryonic day 6 in some sites. The time-course of development of different nAChR subunits revealed several instances of transient expression (such as in the cerebellum), increasing expression (such as in the nucleus spiriformis lateralis), and diminishing expression into posthatching stages (such as in the oculomotor and throclear nuclei). Expression of AChE and ChAT also starts around embryonic day 6 in some structures and follows mainly increasing time-courses in the chick brain. The results of this study reveal a developmentally regulated expression of cholinergic system-related molecules in the chick nervous system and characterize differential time-courses of expression for nAChR subunits, AChE, and ChAT during development.