The closely related Sox5 and Sox6 turn out to perform distinct functions during brain development. Maintenance of the ventricular zone neuroepithelium requires Sox6 in the dorsal and Sox5 in the ventral telencephalon. A portion of the interneurons born in ventral telencephalon then express Sox6, which is necessary for their correct migration and phenotypic differentiation. The neuronal diversity of the CNS emerges largely from controlled spatial and temporal segregation of cell type-specific molecular regulators. We found that the transcription factor SOX6 controls the molecular segregation of dorsal (pallial) from ventral (subpallial) telencephalic progenitors and the differentiation of cortical interneurons, regulating forebrain progenitor and interneuron heterogeneity. During corticogenesis in mice, SOX6 and SOX5 were largely mutually exclusively expressed in pallial and subpallial progenitors, respectively, and remained mutually exclusive in a reverse pattern in postmitotic neuronal progeny. Loss of SOX6 from pallial progenitors caused their inappropriate expression of normally subpallium-restricted developmental controls, conferring mixed dorsal-ventral identity. In postmitotic cortical interneurons, loss of SOX6 disrupted the differentiation and diversity of cortical interneuron subtypes, analogous to SOX5 control over cortical projection neuron development. These data indicate that SOX6 is a central regulator of both progenitor and cortical interneuron diversity during neocortical development.

Little is known about the molecular development and heterogeneity of callosal projection neurons (CPN), cortical commissural neurons that connect homotopic regions of the two cerebral hemispheres via the corpus callosum and that are critical for bilateral integration of cortical information. Here we report on the identification of a series of genes that individually and in combination define CPN and novel CPN subpopulations during embryonic and postnatal development. We usedin situhybridization analysis, immunocytochemistry, and retrograde labeling to define the layer-specific and neuron-type-specific distribution of these newly identified CPN genes across different stages of maturation. We demonstrate that a subset of these genes (e.g.,Hspb3andLpl) appear specific to all CPN (in layers II/III and V–VI), whereas others (e.g.,Nectin-3,Plexin-D1, andDkk3) discriminate between CPN of the deep layers and those of the upper layers. Furthermore, the data show that several genes finely subdivide CPN within individual layers and appear to label CPN subpopulations that have not been described previously using anatomical or morphological criteria. The genes identified here likely reflect the existence of distinct programs of gene expression governing the development, maturation, and function of the newly identified subpopulations of CPN. Together, these data define the first set of genes that identify and molecularly subcategorize distinct populations of callosal projection neurons, often located in distinct subdivisions of the canonical cortical laminae.

Studies of the choroid plexuses lag behind those of, the more widely known, blood brain barrier in spite of a much longer history. This review has two overall aims. The first is to outline longstanding areas of research where there are unanswered questions, such as control of cerebrospinal fluid (CSF) secretion and blood flow. The second part reviews research over the past ten years where the focus has shifted to the idea that the choroid plexuses make specific contributions to brain development and function through molecules they generate and circulate throughout the CSF; these appear to be particularly important for aspects of normal brain growth. Most research in the 20^th Century dealt with the choroid plexuses as one of the brain barrier interfaces that make an important contribution to the composition and stability of the internal environment of the brain in the adult and during its development. More recent research has shown the importance of choroid plexus generated CSF in neurogenesis, influence of sex and other hormones on plexus function, and their role in circadian rhythms and sleep. Of clinical importance are attempts to develop methods to deliver brain-specific drugs via the CSF and understanding the implications of drug entry into developing brain when administered to pregnant women.

Cerebrospinal fluid (CSF) provides vital support for the brain. Abnormal CSF accumulation, such as hydrocephalus, can negatively affect perinatal neurodevelopment. The mechanisms regulating CSF clearance during the postnatal critical period are unclear. Here, we show that CSF K⁺, accompanied by water, is cleared through the choroid plexus (ChP) during mouse early postnatal development. We report that, at this developmental stage, the ChP showed increased ATP production and increased expression of ATP-dependent K⁺transporters, particularly the Na⁺, K⁺, Cl⁻, and water cotransporter NKCC1. Overexpression of NKCC1 in the ChP resulted in increased CSF K⁺clearance, increased cerebral compliance, and reduced circulating CSF in the brain without changes in intracranial pressure in mice. Moreover, ChP-specific NKCC1 overexpression in an obstructive hydrocephalus mouse model resulted in reduced ventriculomegaly. Collectively, our results implicate NKCC1 in regulating CSF K⁺clearance through the ChP in the critical period during postnatal neurodevelopment in mice.