Mutually repressive interaction between Brn1/2 and Rorb contributes to the establishment of neocortical layer 2/3 and layer 4

Abstract

Although several molecules have been shown to play important roles in subtype specification of neocortical neurons, the entire mechanism involved in the specification, in particular, of upper cortical plate (UCP) neurons still remains unclear. The UCP, which is responsible for intracortical connections in the neocortex, comprises histologically, functionally, and molecularly different layer 2/3 (L2/3) and L4. Here, we report the essential interactions between two types of transcription factors, Rorb (RAR-related orphan receptor beta) and Brn1/2 (Brain-1/Brain-2), for UCP specification. We found that Brn2 expression was detected in all upper layers in the immature UCP, but was subsequently restricted to L2/3, accompanied by up-regulation of Rorb in L4, suggesting demarcation of L2/3 and L4 during cortical maturation. Rorb indeed inhibited Brn2 expression and the expression of other L2/3 characteristics, revealed by ectopic expression and knockdown studies. Moreover, this inhibition occurred through direct binding of Rorb to the Brn2 locus. Conversely, Brn1/2 also inhibited Rorb expression and the expression of several L4 characteristics. Together, these results suggest that a mutually repressive mechanism exists between Brn1/2 and Rorb expression and that the established expression of Brn1/2 and Rorb further specifies those neurons into L2/3 and L4, respectively, during UCP maturation.cerebral cortex | cell fate | transcription factor | layer formation T he mammalian neocortex consists of six anatomically distinct layers, each of which contains one or more subtype of neurons, and is characterized by a specific cell morphology, birth date, molecular identity, and connectivity to other regions of the central nervous system. The cortical plate (CP) can be categorized into two populations: lower and upper cortical plate (LCP and UCP). Neurons in the LCP, which consists of layer 5 (L5) and L6, send axons to subcortical targets such as the thalamus, pons, and spinal cord, whereas the UCP, which is known to be much thicker in primates than in rodents, is characterized by corticocortical (intracortical) connections (1-3). The UCP, in rodents, can be further divided into L2/3 and L4. Many L2/3 neurons show a typical pyramidal morphology and send axons to the contralateral cortex, whereas L4 neurons, showing a round shape and granular morphology, receive inputs from the thalamus and transmit the thalamic inputs to local cortical networks (4). Thus, although UCP neurons share at least some features (e.g., intracortical connections), there are apparent differences (e.g., long vs. local projections and cell morphologies). However, how these differences arise during development is poorly understood.Previous studies have revealed the important roles of subtype-or lineage-specific transcription factors (TFs) in the specification of different neuronal subtypes (5-11). One of the intriguing features of how TFs specify neuronal subtypes is that a crucial TF for a given subtype sometimes suppresses other subtypes (3,12). This mechan...

A translaminar genetic logic for the circuit identity of intracortically-projecting neurons

“…Consistent with acquisition of SSN-type local axonal projections, long-range projections were lacking in ICPNRORB, as previously reported ( Supplementary Fig. 4) 18 . Focusing on local microcircuit properties, we next examined the local connectivity of ICPNRORB.…”

“…Finally, we sought to identify a functional molecular counterpart to the circuit-based classification identified above, using Rorb as a proof-of-principle transcript. Indeed, this orphan receptor shows a 3-fold enrichment in SSN vs. CPNSL ( Supplementary Table 1) and has been implicated in circuit assembly within and beyond the cortex [17][18][19] . Here, we directly examined the function of RORB in intracortical circuit assembly by assessing whether targeted overexpression in ICPN induces acquisition of SSNtype morphology, electrophysiology, and circuit connectivity.…”

Progenitor Hyperpolarization Regulates the Sequential Generation of Neuronal Subtypes in the Developing Neocortex

“…1C, left) (Jabaudon et al, 2012; Oishi et al, 2016). Consistent with a graded acquisition of a L2/3-type molecular identity, E14.5-born neurons in the Kir2.1 condition repressed RORB expression, and induced BRN2 expression (Figure 1C, middle and right).…”

Extensive transcriptional and chromatin changes underlie astrocyte maturationin vivoand in culture

“…By reconstituting the expression of transcription factors that are induced in astrocytes maturing in vivo but are missing from cultured astrocytes, we could demonstrate a maturation-promoting activity of four factors, Rorb, Dbx2, Lhx2 and Fezf2. These factors have mainly been studied for their roles in neuronal subtype specification (Chen et al, 2008;Muralidharan et al, 2017;Oishi et al, 2016;Pierani et al, 1999). Interestingly, their roles in neurons seem to be mediated at least partially by cross-repression of alternative neuronal subtype programmes, suggesting that their expression in astrocytes might contribute to the global suppression of neuronal programmes.…”