SOX5 postmitotically regulates migration, postmigratory differentiation, and projections of subplate and deep-layer neocortical neurons

Abstract

Neocortical projection neurons exhibit layer-specific molecular profiles and axonal connections. Here we show that the molecular identities of early-born subplate and deep-layer neurons are not acquired solely during generation or shortly thereafter but undergo progressive postmitotic refinement mediated by SOX5. Fezf2 and Bcl11b, transiently expressed in all subtypes of newly postmigratory early-born neurons, are subsequently downregulated in layer 6 and subplate neurons, thereby establishing their layer 5-enriched postnatal patterns. In Sox5-null mice, this downregulation is disrupted, and layer 6 and subplate neurons maintain an immature differentiation state, abnormally expressing these genes postnatally. Consistent with this disruption, SOX5 binds and represses a conserved enhancer near Fezf2. The Sox5-null neocortex exhibits failed preplate partition and laminar inversion of early-born neurons, loss of layer 5 subcerebral axons, and misrouting of subplate and layer 6 corticothalamic axons to the hypothalamus. Thus, SOX5 postmitotically regulates the migration, postmigratory differentiation, and subcortical projections of subplate and deep-layer neurons.neocortex development ͉ postmitotic mechanisms ͉ pyramidal neurons ͉ Sox genes ͉ transcriptional enhancer T he neocortex is composed of six distinct layers, each containing a unique subset of projection (pyramidal) neurons with specific molecular profiles and axonal connectivities (1-3). Subcortical axons arise solely from deep-layer (L5 and L6) neurons, whereas upper-layer (L2-L4) neurons project intracortically. The first projection neurons, born from progenitors in the ventricular zone (VZ) (4, 5), migrate radially and settle within the preplate (PP), where they form the nascent cortical plate (CP) (6). Incoming CP neurons, which split the PP into the marginal zone (MZ) and subplate (SP), are generated sequentially so that early-born neurons occupy the deep layers and later-born neurons migrate past older neurons to settle in more superficial layers. The molecular mechanisms that regulate the laminar position and identity of projection neurons are being unraveled (3, 7). Previous studies suggested that neurons are specified at the time of their birth (2, 8). However, the extent to which postmitotic events contribute to their laminar and molecular identity remains an open question.Fezf2 (Fezl or Zfp312) and Bcl11b (Ctip2) encode transcription factors enriched in and necessary for the development of L5 neurons (9-12). Here we show that the L5-enriched postnatal expression patterns of Fezf2 and Bcl11b are not acquired solely during neuronal generation but rather are the result of progressive refinement during postmigratory differentiation mediated by SOX5 (L-SOX5), a transcription factor that has been shown to regulate chondrogenesis, oligodendrogenesis, and the sequential generation of cortical neurons (13)(14)(15)(16). In this study, we show that Sox5 postmitotically controls the laminar positioning, molecular differentiation, and layer-specific pattern o...

Area-specific temporal control of corticospinal motor neuron differentiation by COUP-TFI

“…indicating that precise levels of CSMN-specific genes are a fundamental requisite for correct differentiation of corticofugal neurons. Finally, because COUP-TFI and Fezf2 are both expressed in postmitotic neurons, it is possible that abnormal differentiation of CSMN and corticothalamic neurons in the absence of COUP-TFI occurs at a postmitotic level, as recently shown for other cortical transcriptional regulators (8,30,31,32).…”

TBR1 directly represses Fezf2 to control the laminar origin and development of the corticospinal tract

“…First, we chose ZFPM2 (FOG2) to molecularly define L6 neurons, because it is a reliable marker of L6 corticothalamic neurons in the early postnatal neocortex (ref. 15 and Fig. S5) and, more importantly, because its expression level is unchanged in Tbr1 −/− neocortex (Fig.…”

“…These findings indicate that Fezf2 transcription is tightly regulated during development, and that the integrity of normal Fezf2 expression is critical to proper development of the CS tract. Interestingly, Fezf2 is transiently expressed in L6 neurons during early embryonic development, where its transcription is directly repressed by SOX5, thereby establishing a high-in-L5, low-in-L6 postnatal pattern (15,16). Paradoxically, in Sox5 null mutants, the number of axon projections reaching the brainstem and spinal cord is severely reduced despite increased cortical Fezf2 expression (15).…”

“…Interestingly, Fezf2 is transiently expressed in L6 neurons during early embryonic development, where its transcription is directly repressed by SOX5, thereby establishing a high-in-L5, low-in-L6 postnatal pattern (15,16). Paradoxically, in Sox5 null mutants, the number of axon projections reaching the brainstem and spinal cord is severely reduced despite increased cortical Fezf2 expression (15). This suggests that Sox5 is required for the formation of these connections independent of its regulation of Fezf2.…”

“…This suggests that Sox5 is required for the formation of these connections independent of its regulation of Fezf2. Furthermore, SOX5 is normally expressed in L5 Fezf2-expressing CS neurons (15). Therefore, the down-regulation of Fezf2 in L6 neurons, and hence the establishment of Fezf2's L5 pattern, likely requires additional molecules.…”