Alterations in the differentiation of chick retina caused by an intraocular injection of an alkaline phosphatase inhibitor

The Retinal TNAP

Accumulating evidence from recent literature underline the important roles of tissue non specific alkaline phosphatase (TNAP) in diverse functions as well as diseases of the nervous system. Exploration of TNAP in well characterized neural circuits such as the retina, might significantly advance our understanding regarding neural TNAP's roles. This chapter reviews the scarce literature as well as our findings on retinal TNAP. We found that retinal TNAP activity was preserved and followed diverse patterns throughout vertebrate evolution. We have consistently observed TNAP activity (1) in retinal vessels, (2) in photoreceptors and (3) in the majority of the studied species in the outer (OPL) and inner plexiform layers (IPL), where synaptic transmission occurs. Importantly, in some species the IPL exhibits several TNAP positive strata. These strata exactly corresponded those seen after quadruple immunohistochemistry with four canonical IPL markers (tyrosine hydroxylase, choline acetyltransferase, calretinin, protein kinase C α). Diabetes results in diminishing retinal TNAP activity before changes in canonical markers could be observed in a rat model. The presence of TNAP activity at critical sites of neurotransmission suggests its important and evolutionary conserved role in vision. In diabetes, the decreased TNAP activity indicates neurological alterations adding further evidence for the role of TNAP in brain diseases.

TNAP activity is localized at critical sites of retinal neurotransmission across various vertebrate species

Evidence is emerging with regard to the role of tissue non-specific alkaline phosphatase (TNAP) in neural functions. As an ectophosphatase, this enzyme might influence neural activity and synaptic transmission in diverse ways. The localization of the enzyme in known neural circuits, such as the retina, might significantly advance an understanding of its role in normal and pathological functioning. However, the presence of TNAP in the retina is scarcely investigated. Our multispecies comparative study (zebrafish, cichlid, frog, chicken, mouse, rat, golden hamster, guinea pig, rabbit, sheep, cat, dog, ferret, squirrel monkey, human) using enzyme histochemistry and Western blots has shown the presence of TNAP activity in the retina of several mammalian species, including humans. Although the TNAP activity pattern varies across species, we have observed the following trends: (1) in all investigated species (except golden hamster), retinal vessels display TNAP activity; (2) TNAP activity consistently occurs in the photoreceptor layer; (3) in majority of the investigated species, marked TNAP activity is present in the outer and inner plexiform layers. In zebrafish, frog, chicken, guinea pig, and rat, TNAP histochemistry has revealed several sublayers of the inner plexiform layer. Frog, golden hamster, guinea pig, mouse, and human retinas possess a subpopulation of amacrine cells positively staining for TNAP activity. The expression of TNAP in critical sites of retinal signal transmission across a wide range of species suggests its fundamental, evolutionally conserved role in vision.

Vascularization in the primate visual cortex during development

We studied the relationship between vascularization and neuronal activity in the visual cortex during postnatal development in the primate. Analyses were focused on layer IVC that displays a sequential pattern of maturation for the magno- and parvocellular systems in separate sublayers, respectively IVC alpha and IVC beta. Cytochrome oxidase and endogenous alkaline phosphatase histochemistry was used to analyse, on the same sections, the laminar patterns of cortical activity and vessel density in the primary visual cortex of the marmoset (Callithrix jacchus). Experiments were carried out in five young and two adult animals. We showed that the temporal pattern of angiogenesis differs in layer IVC alpha and IVC beta. During the first postnatal month, vessel density is higher in IVC alpha than in IVC beta and runs parallel to cytochrome oxidase intensity. In 2-month-old animals, both vessel densities and cytochrome oxidase activity are similar in IVC alpha and IVC beta. In adults, the vessel densities in IVC alpha and IVC beta are the reverse of those observed during the first postnatal month. Vessel diameter does not account for this evolution in vascular patterns. In the discussion, we suggest that such a developmental time-course of angiogenesis might be linked to the synaptogenesis requirements that proceed differently for the magno- and parvocellular systems in the primate striate cortex.

Histochemical observations on unique rod-like cells in the developing retina of the normal rat

Our previous immunohistochemical studies using specific anti-opsin antibodies have shown that in the normal postnatal rat retina, opsin-like immunoreactive materials can be seen not only in photoreceptor cells but also in some cells of the inner nuclear layer (INL). To characterize these ectopic opsin-like immunoreactive cells further, we have now studied developing rat retinae at the light and electron microscopic levels using both opsin immunocytochemistry and 5'-nucleotidase (5'-N) cytochemistry. Since photoreceptor cells are specifically stained in both methods, we investigated whether these methods would also stain some of the cells in the INL. Opsin-like immunoreactive cells in the INL always showed a characteristic nuclear structure, which was clearly distinguished from that of other INL cells and resembled rod cell nucleus because of the presence of conspicuous masses of condensed heterochromatin. Moreover, these opsin-like immunoreactive cells in the INL occasionally also displayed a ciliary process. They could only be seen in the INL until about day 25 of postnatal life. In the INL, 5'-N activity was found exclusively in cells showing rod-like nuclear features, and the timing of appearance and disappearance of 5'-N-positive cells in the INL was similar to that of the opsin-like immunoreactive cells. Numerous degenerating cells were observed in the INL at around day 10, some of which showed 5'-N reaction product at the cell surface. These results indicate that during postnatal development of the rat retina, some cells in the INL express several phenotypic properties characteristic of rod cells. It is suggested that these cells might undergo either cell degeneration or phenotypic conversion during the subsequent maturation of the retina.

Effects of alkaline phosphatase inhibitors on chick neural retinal cell differentiation in vitro: ultracytochemical studies

In the matured chick retina, alkaline phosphatase (ALPase) activity is specifically localized in the outer plexiform layer and in horizontal and Müller cells. In the developing chick retina, ALPase activity is first recognized in growing neurites from horizontal cells during the 13th day of incubation, when synaptogenesis begins in the outer plexiform layer. Intraocular administration of ALPase inhibitors to developing chick embryos resulted in developmental disturbances in differentiation of the outer plexiform layer and also of photoreceptor cells. We have now extended these studies to an in vitro system. ALPase activity was studied by ultracytochemistry in cultured retinal cells from chick embryos, and the effects of specific ALPase inhibitor on retinal development were also analyzed. Two cell types showed intense ALPase activity: 1) flat glial cell, which formed a multi-layered epithelial sheet and 2) neuronal cell found within cell aggregates. Some cellular processes forming a neuropil-like structure within these aggregates also showed ALPase activity. When the ALPase inhibitor bromotetramisole was present in the culture medium, there was delay in aggregate formation and the development of neuritic processes was also affected. Moreover, this treatment also caused a considerable reduction in the number of photoreceptor cells present in the culture. The present results indicate that ALPase activity plays a significant role in retinal cell differentiation.