GABAergic amacrine cells and visual function are reduced in PAC1 transgenic mice

Relevance of Peptide Homeostasis in Metabolic Retinal Degenerative Disorders: Curative Potential in Genetically Modified Mice

The mammalian retina contains approximately 30 neuropeptides that are synthetized by different neuronal cell populations, glia, and the pigmented epithelium. The presence of these neuropeptides leaves a mark on normal retinal molecular processes and physiology, and they are also crucial in fighting various pathologies (e.g., diabetic retinopathy, ischemia, age-related pathologies, glaucoma) because of their protective abilities. Retinal pathologies of different origin (metabolic, genetic) are extensively investigated by genetically manipulated in vivo mouse models that help us gain a better understanding of the molecular background of these pathomechanisms. These models offer opportunities to manipulate gene expression in different cell types to help reveal their roles in the preservation of retinal health or identify malfunction during diseases. In order to assess the current status of transgenic technologies available, we have conducted a literature survey focused on retinal disorders of metabolic origin, zooming in on the role of retinal neuropeptides in diabetic retinopathy and ischemia. First, we identified those neuropeptides that are most relevant to retinal pathologies in humans and the two clinically most relevant models, mice and rats. Then we continued our analysis with metabolic disorders, examining neuropeptide-related pathways leading to systemic or cellular damage and rescue. Last but not least, we reviewed the available literature on genetically modified mouse strains to understand how the manipulation of a single element of any given pathway (e.g., signal molecules, receptors, intracellular signaling pathways) could lead either to the worsening of disease conditions or, more frequently, to substantial improvements in retinal health. Most attention was given to studies which reported successful intervention against specific disorders. For these experiments, a detailed evaluation will be given and the possible role of converging intracellular pathways will be discussed. Using these converging intracellular pathways, curative effects of peptides could potentially be utilized in fighting metabolic retinal disorders.

The Neuroprotective Peptide PACAP1-38 Contributes to Horizontal Cell Development in Postnatal Rat Retina

Purpose

PACAP1-38, a member of the secretin/glucagon superfamily, is expressed in the developing retina with documented neuroprotective effects. However, its function in retinal cell differentiation has yet to be elucidated. Our goals, therefore, were to identify PAC1 expressing cells morphologically, investigate the PACAP1-38 action functionally, and establish PACAP1-38 regulated events developmentally during the first postnatal week in rat retina.

Methods

P1 retinal sections or whole mounts of Wistar rats were used to reveal PAC1 and calbindin immunoreactive structures. P1, P3, or P7 pups were injected intravitreally with 100 pmol PACAP1-38. Tissues were harvested 24 hours post-treatment, then processed for calbindin immunohistochemistry to determine horizontal cell number, or 6, 12, 24 hours post-treatment for real-time PCR and immunoblots to detect PCNA expression. To localize proliferating cells, anti-PCNA antibody was applied.

Results

We showed various PAC1 expressing cells in RPE, NBL, and GCL in P1 retina including calbindin positive horizontal cells. We found that PACAP1-38 induced a marked cell number increase at P3 and P7 and showed upregulated cell proliferation as its mechanism; however, it was ineffective at P1. PACAP1-38 induced proliferative cells localized in the NBL, and double-marker studies demonstrated that the induced proliferative cells were horizontal cells.

Conclusions

PACAP1-38 appears to act in retinal differentiation by inducing mitosis selectively in a time and cell specific manner through PAC1. The control of horizontal cell proliferation raises the novel possibilities that (1) PACAP1-38 may be a major player in retinal patterning and (2) PACAP signaling may be critical in retinoblastoma.

Pleiotropic and retinoprotective functions of PACAP

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 27- or 38-amino acid neuropeptide, which belongs to the vasoactive intestinal polypeptide/glucagon/secretin family. PACAP and its three receptor subtypes are expressed in neural tissues of the eye, including the retina, cornea and lacrimal gland, and PACAP is known to exert pleiotropic effects throughout the central nervous system. This review provides an overview of current knowledge regarding the cell protective effects, mechanisms of action and therapeutic potential of PACAP in response to several types of eye injury.

Ascl1 as a novel player in the Ptf1a transcriptional network for GABAergic cell specification in the retina

In contrast with the wealth of data involving bHLH and homeodomain transcription factors in retinal cell type determination, the molecular bases underlying neurotransmitter subtype specification is far less understood. Using both gain and loss of function analyses in Xenopus, we investigated the putative implication of the bHLH factor Ascl1 in this process. We found that in addition to its previously characterized proneural function, Ascl1 also contributes to the specification of the GABAergic phenotype. We showed that it is necessary for retinal GABAergic cell genesis and sufficient in overexpression experiments to bias a subset of retinal precursor cells towards a GABAergic fate. We also analysed the relationships between Ascl1 and a set of other bHLH factors using an in vivo ectopic neurogenic assay. We demonstrated that Ascl1 has unique features as a GABAergic inducer and is epistatic over factors endowed with glutamatergic potentialities such as Neurog2, NeuroD1 or Atoh7. This functional specificity is conferred by the basic DNA binding domain of Ascl1 and involves a specific genetic network, distinct from that underlying its previously demonstrated effects on catecholaminergic differentiation. Our data show that GABAergic inducing activity of Ascl1 requires the direct transcriptional regulation of Ptf1a, providing therefore a new piece of the network governing neurotransmitter subtype specification during retinogenesis.

PAC1-expressing structures of neural retina alter their PAC1 isoform splicing during postnatal development

Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the secretin/glucagon/vasoactive intestinal peptide family, exerts various effects on neuronal development as mediated by the differential expression of PAC1 receptor (PAC1-R) isoforms. The expression changes of PAC1-R isoforms (Hip, Hop1) reported in correlation with retinal development suggest an isoform switch during the second postnatal week. Our aim is to determine the exact period of the isoform shift and to describe the PAC1-R-immunoreactive structures appearing from postnatal day 5 (P5) to P10 in the rat retina. The ratio of Hip and Hop1 receptors was assessed and changes in their expression were followed by Taqman and SybrGreen-based quantitative polymerase chain reaction. For the detection of PAC1-R-expressing retinal structures, anti-PAC1-R, anti-calbindin, anti-protein kinase C, anti-glutamine synthetase, anti-HPC1 and anti-Brn3a antibodies were utilized. At the transcript level, a marked decrease to an undetectable level was measured in Hip mRNA expression from P6 to P9. Hop1 expression appeared to be unchanged from P6 to P9, followed by a significant elevation at P10. A Hip/Hop1 isoform shift occurred between P6 and P7. Immunostaining showed strong PAC1-R labeling from P5 to P10 in ganglion, amacrine, horizontal and rod bipolar neurons and in glial Muller cell processes. The Hop1 isoform was predominantly expressed in various types of retinal cell beginning at P7, because of a dramatic reduction in Hip mRNA level. As the Hop1 receptor is coupled to different signaling cascades, this isoform shift might alter the physiological role of PACAP during this particular period.

Pituitary adenylate cyclase activating polypeptide in the retina: focus on the retinoprotective effects

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurotrophic and neuroprotective peptide that has been shown to exert protective effects against different neuronal injuries, such as traumatic brain and spinal cord injury, models of neurodegenerative diseases, and cerebral ischemia. PACAP and its receptors are present in the retina. In this study, we summarize the current knowledge on retinal PACAP with focus on the retinoprotective effects. Results of histological, immunohistochemical, and molecular biological analysis are reviewed. In vitro, PACAP shows protection against glutamate, thapsigargin, anisomycin, and anoxia. In vivo, the protective effects of intravitreal PACAP treatment have been shown in the following models of retinal degeneration in rats: excitotoxic injury induced by glutamate and kainate, ischemic injury, degeneration caused by UV-A light, optic nerve transection, and streptozotocin-induced diabetic retinopathy. Studying the molecular mechanism has revealed that PACAP acts by activating antiapoptotic and inhibiting proapoptotic signaling pathways in the retina in vivo. These studies strongly suggest that PACAP is an excellent candidate retinoprotective agent that could be a potential therapeutic substance in various retinal diseases.