Cellular distribution of the calcium-binding proteins parvalbumin, calbindin, and calretinin in the neocortex of mammals: phylogenetic and developmental patterns

Abstract

The three calcium-binding proteins parvalbumin, calbindin, and calretinin are found in morphologically distinct classes of inhibitory interneurons as well as in some pyramidal neurons in the mammalian neocortex. Although there is a wide variability in the qualitative and quantitative characteristics of the neocortical subpopulations of calcium-binding protein-immunoreactive neurons in mammals, most of the available data show that there is a fundamental similarity among the mammalian species investigated so far, in terms of the distribution of parvalbumin, calbindin, and calretinin across the depth of the neocortex. Thus, calbindin- and calretinin-immunoreactive neurons are predominant in layers II and III, but are present across all cortical layers, whereas parvalbumin-immunoreactive neurons are more prevalent in the middle and lower cortical layers. These different neuronal populations have well defined regional and laminar distribution, neurochemical characteristics and synaptic connections, and each of these cell types displays a particular developmental sequence. Most of the available data on the development, distribution and morphological characteristics of these calcium-binding proteins are from studies in common laboratory animals such as the rat, mouse, cat, macaque monkey, as well as from postmortem analyses in humans, but there are virtually no data on other species aside of a few incidental reports. In the context of the evolution of mammalian neocortex, the distribution and morphological characteristics of calcium-binding protein-immunoreactive neurons may help defining taxon-specific patterns that may be used as reliable phylogenetic traits. It would be interesting to extend such neurochemical analyses of neuronal subpopulations to other species to assess the degree to which neurochemical specialization of particular neuronal subtypes, as well as their regional and laminar distribution in the cerebral cortex, may represent sets of derived features in any given mammalian order. This could be particularly interesting in view of the consistent differences in neurochemical typology observed in considerably divergent orders such as cetaceans and certain families of insectivores and metatherians, as well as in monotremes. The present article provides an overview of calcium-binding protein distribution across a large number of representative mammalian species and a review of their developmental patterns in the species where data are available. This analysis demonstrates that while it is likely that the developmental patterns are quite consistent across species, at least based on the limited number of species for which ontogenetic data exist, the distribution and morphology of calcium-binding protein-containing neurons varies substantially among mammalian orders and that certain species show highly divergent patterns compared to closely related taxa. Interestingly, primates, carnivores, rodents and tree shrews appear closely related on the basis of the observed patterns, marsupials show some affinities with that group, whereas prototherians have unique patterns. Our findings also support the relationships of cetaceans and ungulates, and demonstrates possible affinities between carnivores and ungulates, as well as the existence of common, probably primitive, traits in cetaceans and insectivores.

Introduction

Calcium-binding proteins are intracellular calcium acceptors that belong to two different families: the EF-hand proteins and the annexins. The annexin family is characterized by proteins that bind calcium in the presence of phospholipid-containing membranes. The former family consists of proteins showing a general structural principle in the calcium-binding domain called the EF-hand, which is a stretch of amino acids forming a typical helix-loop-helix structure (Andressen et al., 1993). The EF-hand family of calcium-binding proteins contains about forty known calcium-regulated proteins, of which several are found in the central nervous system. The EF-hand proteins may function either as ‘triggers’, starting a cascade of reactions or as calcium ‘buffers’, decreasing the free cytoplasmic concentration of this ion (Dalgarno et al., 1984). The prototype of a ‘trigger’ protein is the ubiquitous calmodulin that activates at least twenty different enzymes. The ‘buffer’ proteins, such as parvalbumin (PV), calbindin (CB), and calretinin (CR), represent a more passive system responsible for decreasing the amplitude of calcium signals. PV was originally purified from fish muscle, and is present in high levels in the central nervous system where it is observed in a large number of neurons belonging to several functional systems (Heizmann, 1984, Celio, 1990). CB was extracted from the chick duodenum where it was thought to facilitate calcium transport across the mucosa (Wassermann and Taylor, 1966), and was later detected and mapped in the brain (Jandé et al., 1981). CR is a recently described protein specific to the nervous system, with several amino acid sequence homologies to CB (Jacobowitz and Winsky, 1991, Rogers, 1992).

Although the function of many of these proteins is not yet known (see reviews by Baimbridge et al., 1992, Andressen et al., 1993), these molecules are interesting from a neuroanatomical point of view, since they are specifically observed in well-defined subpopulations of neurons belonging to multiple functional systems in a large number of vertebrate species, including birds, reptiles, amphibians, and mammals (Rogers, 1989, Celio, 1990, Baimbridge et al., 1992, Martı́nez-Guijarro and Freund, 1992, Résibois and Rogers, 1992Glezer et al., 1998, Glezer et al., 1999Dávila et al., 1997). They are useful to study the development of specific systems as well as their evolution, as these calcium-binding proteins exhibit preferential distribution within functionally distinct pathways (Rausell et al., 1992, Glezer et al., 1993, Glezer et al., 1999, Hashikawa et al., 1995, Molinari et al., 1995, Jones, 1998). In the cerebral cortex, they are powerful markers for studying the complexity of the GABAergic systems, as each of these three calcium-binding proteins is mostly colocalized with GABA, in distinct subpopulations of non-pyramidal cells (Kosaka et al., 1987 Hendry et al., 1989; for review see DeFelipe, 1997).