Abstract
Calretinin is a calcium-binding protein often used as a marker for a subset of inhibitory interneurons in the mammalian neocortex. We studied the labeled cells in offspring from a cross of a Cre-dependent reporter line with the CR-ires-Cre mice, which express Cre-recombinase in the same pattern as calretinin. We found that in the mature visual cortex, only a minority of the cells that have expressed calretinin and Cre-recombinase during their lifetime is GABAergic and only about 20% are immunoreactive for calretinin. The reason behind this is that calretinin is transiently expressed in many cortical pyramidal neurons during development. To determine whether neurons that express or have expressed calretinin share any distinct functional characteristics, we recorded their visual response properties using GCaMP6s calcium imaging. The average orientation selectivity, size tuning, and temporal and spatial frequency tuning of this group of cells, however, match the response profile of the general neuronal population, revealing the lack of functional specialization for the features studied.
Highlights
Calretinin is a calcium-binding protein that took its name from its structural similarity to calbindin and to the location where it was first discovered, the retina
OVERLAP OF tdTOMATO AND CALRETININ EXPRESSION To study the function in visual processing of neurons that express calretinin, we used offspring from a cross of two mouse lines: one expressing the tdTomato red fluorescent protein inside the Rosa26 locus preceded by a floxed stop cassette; and the CR-ires-Cre line in which the Cre-recombinase coding sequence follows the Calb2 promoter and an IRES-element
It was possible that even for the very low levels of labeling, there was some calretinin expression, it did suggest a natural classification of CR-IR positive and negative cells
Summary
Calretinin (gene symbol Calb2) is a calcium-binding protein that took its name from its structural similarity to calbindin and to the location where it was first discovered, the retina. The exclusively GABAergic nature of cortical CR-IR neurons is not conserved across mammals, though, as in monkey prefrontal and human temporal neocortex about a quarter of the CR-IR neurons are not positive for GABA (del Río and DeFelipe, 1996; Melchitzky et al, 2005). Still, in both monkey and rodent, the class of CR-IR neurons contains morphologically similar groups, in particular the double-bouquet neurons of layer II/III and a subpopulation of Cajal-Retzius neurons in layer I (Condé et al, 1994; see Barinka and Druga, 2010 for a review)
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