Editorial: At the top of the interneuronal pyramid—calretinin expressing cortical interneurons

(2015) Editorial: At the top of the interneuronal pyramid—calretinin expressing cortical interneurons. Intracellular protein calretinin (CR) acquired its name based on structural similarity to calbindin D28k and the site of first detection—retina (Rogers, 1987). Soon after this discovery, a basic description of the distribution of CR in rodent brain emerged from the work of several authors (reviewed by Baimbridge et al., 1992; Andressen et al., 1993). In the upcoming years, calcium-binding proteins (CaBP) calretinin, parvalbumin (PV), and calbindin (CB) could be shown to be expressed in largely non-overlapping interneuronal populations. The usefulness of utilizing PV, CB, and CR for categorization of interneuronal subpopulations was validated by gene cluster analysis (Toledo-Rodriguez et al., 2004). While extensive literature on PV expressing (PV+) interneurons' physiological functions and changes in various pathological conditions have been published, we still have a limited view about the CR+ (and CB+) interneurons. This is quite disappointing, considering their increasing number in primates and various unique features they possess. In the Research Topic " At the top of the interneuronal pyramid—calretinin expressing cortical interneurons, " we intended to provide a summary of current knowledge on CR+ cortical neurons and especially to emphasize and discuss those " unique features " of CR+ neurons mentioned above. In the first article of this compilation, the story begins with a mini-review on protein calretinin from Beat Schwaller (Schwaller, 2014). The Author not only offers a summary of what is known about the expression and physiological role of calretinin in cellular homeostasis, but, to mention one of many interesting aspects, the reader also learns why calretinin " may be a potential therapeutic target for treatment of Huntington disease " (see Dong et al., 2012 for original research report). Further two original research papers focus on the development of CR+ neurons during the ontogenesis of human cerebral cortex. Miriam Gonzáles-Gómez and Gundela Meyer (González-Gómez and Meyer, 2014) put emphasis on the expression of CR in neuronal progenitors during the early period (between 6 and 14 gestational weeks, gw) of human corticogenesis. At the early embryonic time, the expression of CR is unrelated to the GABAergic neurons and could be seen also in glutamatergic neurons of the preplate and " pioneer " cortical plate. Complementary to this paper, Nevena V. Radonji´c with colleagues (Radonjic et al., 2014) describes the pattern of expression of a transcription factor (Gsx2) and its two downstream effectors, (Ascl1 and Sp8) proposed …