Calcium plays a fundamental role in the cell as second messenger and is principally regulated by calcium-binding proteins. Although these proteins share in common their ability to bind calcium, they belong to different subfamilies. They present, in general, specific developmental and distribution patterns. Most Purkinje cells express the fast and slow calcium buffer proteins calbindin-D28k and parvalbumin, whereas basket, stellate and Golgi cells the slow buffer parvalbumin only. They are, almost all, calretinin negative. Granule, Lugaro and unipolar brush cells present an opposite immunoreactivity profile, most of them being calretinin positive while lacking calbindin-D28k and parvalbumin. The developmental pattern of appearance of these proteins seems to follow the maturation of neurons. Calbindin-D28k appears early, shortly after cessation of mitosis when neurons become ready to start migration and differentiation while parvalbumin is expressed later in parallel with an increase in neuronal activity. The other proteins are generally detected later. During development, some of these proteins, like calretinin, are transiently expressed in specific cellular subpopulations. The function of these proteins is not fully understood, although strong evidence supports a prominent role in physiological settings with altered calcium concentrations. These proteins regulate and are regulated by intracellular calcium level. For example, they may directly or indirectly enable sensitization or desensitization of calcium channels, and may further block calcium entry into the cells, like the calcium-sensor proteins, that have been shown to be potent and specific modulators of ion channels, which may allow for feedback control of current function and hence signaling. The absence of calcium buffer proteins results in marked abnormalities in cell firing; with alterations in simple and complex spikes or transformation of depressing synapses into facilitating synapses. Calcium-binding protein implication in resistance to degeneration is still a controversial issue. Neurons rich in calcium-binding proteins, especially calbindin-D28k and parvalbumin, seem to be relatively resistant to degeneration in a variety of acute and chronic disorders. However other data support that an absence of calcium-binding proteins may also have a neuroprotective effect. It is not unlikely that neurons may face a dual action mechanism where a decrease in calcium-binding proteins has a first short-term beneficial effect while it becomes detrimental for the cell over the long term.
Calmodulin is abundant in the central nervous system, including the retina. However, the localization of calmodulin in the retina has not been described in detail. We therefore decided to investigate calmodulin localization in retinae from six vertebrate species, by using immunohistochemical labeling with four different rabbit polyclonal antibodies against calmodulin. The localization of calbindin-D28k, another calcium-binding protein already well described in retina, was compared. We found that calmodulin distribution is more highly conserved among species, contrasting with calbindin variability. The most striking result emerging is that calmodulin could not be detected in photoreceptors although other layers are intensely calmodulin-immunoreactive, casting doubt about a direct role of calmodulin in phototransduction. Horizontal cells are weakly calmodulin-immunoreactive, bipolar cells are calmodulin-immunoreactive except in turtle retina, numerous amacrine and ganglion cells are labeled in all species, and the fiber layer is always labeled. These data demonstrate that, while the calmodulin distribution in retina is similar among vertebrate species, selective differences in localization can be detected not only among the same cell types in different species but also among different cell types in the same species. The results showing differences in calmodulin immunoreactivity among cell types also provide further evidence that calmodulin expression in eukaryotes is not constitutive, in the sense that not every cell expresses similar levels of calmodulin.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.