The complexity of the calretinin-expressing progenitors in the human cerebral cortex

Abstract

The complex structure and function of the cerebral cortex critically depend on the balance of excitation and inhibition provided by the pyramidal projection neurons and GABAergic interneurons, respectively. The calretinin-expressing (CalR+) cell is a subtype of GABAergic cortical interneurons that is more prevalent in humans than in rodents. In rodents, CalR+ interneurons originate in the caudal ganglionic eminence (CGE) from Gsx2+ progenitors, but in humans it has been suggested that a subpopulation of CalR+ cells can also be generated in the cortical ventricular/subventricular zone (VZ/SVZ). The progenitors for cortically generated CalR+ subpopulation in primates are not yet characterized. Hence, the aim of this study was to identify patterns of expression of the transcription factors (TFs) that commit cortical stem cells to the CalR fate, with a focus on Gsx2. First, we studied the expression of Gsx2 and its downstream effectors, Ascl1 and Sp8 in the cortical regions of the fetal human forebrain at midgestation. Next, we established that a subpopulation of cells expressing these TFs are proliferating in the cortical SVZ, and can be co-labeled with CalR. The presence and proliferation of Gsx2+ cells, not only in the ventral telencephalon (GE) as previously reported, but also in the cerebral cortex suggests cortical origin of a subpopulation of CalR+ neurons in humans. In vitro treatment of human cortical progenitors with Sonic hedgehog (Shh), an important morphogen in the specification of interneurons, decreased levels of Ascl1 and Sp8 proteins, but did not affect Gsx2 levels. Taken together, our ex-vivo and in vitro results on human fetal brain suggest complex endogenous and exogenous regulation of TFs implied in the specification of different subtypes of CalR+ cortical interneurons.

Diversity of Cortical Interneurons in Primates: The Role of the Dorsal Proliferative Niche

“…Furthermore, human RGCs express growth factor PDGFD and its receptor, which is not observed in mice ( Lui et al, 2014 ). Supporting evidence is also obtained in vitro from modeling human cortical development using induced pluripotent stem cells ( Mariani et al, 2012 ), human embryonic stem cells ( Reinchisi et al, 2013 ), or enriched RGCs ( Radonjić et al, 2014b ; Yu and Zecevic, 2011 ). Recently, large-scale studies of genes involved in cortical development have shown considerable species-specific differences between humans and mice ( Kang et al, 2011 ; Miller et al, 2014 ).…”

“…Tissue was processed as previously described ( Radonjić et al, 2014b ). For detailed protocol and a list of antibodies, see the Supplemental Experimental Procedures and Table S2 .…”

Expression of ventral telencephalon transcription factors ASCL1 and DLX2 in the early fetal human cerebral cortex

“…Therefore, it remains a possibility that a small number of ASCL1 + progenitors may give rise to GABAergic neurons, particularly at later stages, where it has been reported that mRNA levels of ASCL1 increase dramatically between 16 and 19 PCW along with GSX2, a transcription factor important in the differentiation of calretinin‐positive interneurons of CGE origin (Radonjić et al. 2014b). Our RNAseq study only extended as far as 17 PCW but showed no trend towards a dramatic increase in ASCL1 expression towards mid‐gestation, although we did see a large increase in ASCL1 immunoreactive cells in the VZ in parts of the cortex at 19 PCW.…”

Secretagogin is Expressed by Developing Neocortical GABAergic Neurons in Humans but not Mice and Increases Neurite Arbor Size and Complexity

“…However, there is no molecular explanation for morphological differences in VIP + /CR + neocortical GABAergic neurons between rodents and primates. Indeed, although several studies have examined the origins and development of human GABAergic neurons (Bayatti et al 2008;Jakovcevski et al 2011;Radonjic et al 2014b;Al-Jaberi et al 2015), to our knowledge, no molecular differences between GABAergic neurons of rodents and primates have been described. Given the increasing appreciation for the role of GABAergic dysfunction in a variety of human neurodevelopmental disorders (Marin 2012;Southwell et al 2014), it is critical to understand the molecular bases of GABAergic neuron specification and maturation in the human brain (Clowry et al 2010;Molnar and Clowry 2012).…”