Progressive loss of PAX6, TBR2, NEUROD and TBR1 mRNA gradients correlates with translocation of EMX2 to the cortical plate during human cortical development

The refinement of the critical region for the 2q31.2q32.3 deletion syndrome indicates candidate genes for mental retardation and speech impairment

Current literature provides more than 30 patients with interstitial deletions in chromosome 2q31q33. Only a few of them were studied using high-resolution methods. Among these, two patients had presented with a particular consistence of some clinical features associated to a deletion between bands q31.2 and q32.3 of chromosome 2. This clinical pattern, labeled as "2q31.2q32.3 syndrome," consists of multiple dysmorphisms, developmental delay, mental retardation and behavioural disturbances. We report an adult female patient with a 4.4 Mb deletion in the 2q31.2q32.3 region, showing facial dysmorphisms, mental retardation and absence of speech. The region overlaps with the deletion found in the two cases previously reported. The critical region points to a few genes, namely NEUROD1, ZNF804A, PDE1A, and ITGA4, which are good candidates to explain the cognitive and behavioural phenotype, as well as the severe speech impairment associated with the 2q31.2q32.3 deletion.

Renewed focus on the developing human neocortex

Many specifically human psychiatric and neurological conditions have developmental origins. Rodent models are extremely valuable for the investigation of brain development, but cannot provide insight into aspects that are specifically human. The human brain, and particularly the cerebral cortex, has some unique genetic, molecular, cellular and anatomical features, and these need to be further explored. Cortical expansion in human is not just quantitative; there are some novel types of neurons and cytoarchitectonic areas identified by their gene expression, connectivity and functions that do not exist in rodents. Recent research into human brain development has revealed more elaborated neurogenetic compartments, radial and tangential migration, transient cell layers in the subplate, and a greater diversity of early-generated neurons, including predecessor neurons. Recently there has been a renaissance of the study of human brain development because of these unique differences, made possible by the availability of new techniques. This review gives a flavour of the recent studies stemming from this renewed focus on the developing human brain.

Investigating gradients of gene expression involved in early human cortical development

The division of the neocortex into functional areas (the cortical map) differs little between individuals, although brain lesions in development can lead to substantial re-organization of regional identity. We are studying how the cortical map is established in the human brain as a first step towards understanding this plasticity. Previous work on rodent development has identified certain transcription factors (e.g. Pax6, Emx2) expressed in gradients across the neocortex that appear to control regional expression of cell adhesion molecules and organization of area-specific thalamocortical afferent projections. Although mechanisms may be shared, the human neocortex is composed of different and more complex local area identities. Using Affymetrix gene chips of human foetal brain tissue from 8 to 12.5 post-conceptional weeks [PCW, equivalent to Carnegie stage (CS) 23, to Foetal stage (F) 4], human material obtained from the MRC-Wellcome Trust Human Developmental Biology Resource (http://www.hdbr.org), we have identified a number of genes that exhibit gradients along the anterior-posterior axis of the neocortex. Gene probe sets that were found to be upregulated posteriorally compared to anteriorally, included EMX2, COUPTFI and FGF receptor 3, and those upregulated anteriorally included cell adhesion molecules such as cadherins and protocadherins, as well as potential motor cortex markers and frontal markers (e.g. CNTNAP2, PCDH17, ROBO1, and CTIP2). Confirmation of graded expression for a subset of these genes was carried out using real-time PCR. Furthermore, we have established a dissociation cell culture model utilizing tissue dissected from anteriorally or posteriorally derived developing human neocortex that exhibits similar gradients of expression of these genes for at least 72 h in culture.

Beta-catenin signaling negatively regulates intermediate progenitor population numbers in the developing cortex

Intermediate progenitor cells constitute a second proliferative cell type in the developing mammalian cerebral cortex. Little is known about the factors that govern the production of intermediate progenitors. Although persistent expression of stabilized β-catenin was found to delay the maturation of radial glial progenitors into intermediate progenitors, the relationship between β-catenin signaling and intermediate progenitors remains poorly understood. Using a transgenic reporter mouse for Axin2, a direct target of Wnt/β-catenin signaling, we observed that β-catenin signaling is decreased in intermediate progenitor cells relative to radial glial progenitors. Conditional deletion of β-catenin from mouse cortical neural progenitors increased intermediate progenitor numbers, while conditional expression of stabilized β-catenin reduced the intermediate progenitor population. Together, these findings provide evidence that β-catenin signaling in radial progenitors negatively regulates intermediate progenitor cell number during cortical development.

Unravelling the development of the visual cortex: implications for plasticity and repair

The visual cortex comprises over 50 areas in the human, each with a specified role and distinct physiology, connectivity and cellular morphology. How these individual areas emerge during development still remains something of a mystery and, although much attention has been paid to the initial stages of the development of the visual cortex, especially its lamination, very little is known about the mechanisms responsible for the arealization and functional organization of this region of the brain. In recent years we have started to discover that it is the interplay of intrinsic (molecular) and extrinsic (afferent connections) cues that are responsible for the maturation of individual areas, and that there is a spatiotemporal sequence in the maturation of the primary visual cortex (striate cortex, V1) and the multiple extrastriate/association areas. Studies in both humans and non-human primates have started to highlight the specific neural underpinnings responsible for the maturation of the visual cortex, and how experience-dependent plasticity and perturbations to the visual system can impact upon its normal development. Furthermore, damage to specific nuclei of the visual cortex, such as the primary visual cortex (V1), is a common occurrence as a result of a stroke, neurotrauma, disease or hypoxia in both neonates and adults alike. However, the consequences of a focal injury differ between the immature and adult brain, with the immature brain demonstrating a higher level of functional resilience. With better techniques for examining specific molecular and connectional changes, we are now starting to uncover the mechanisms responsible for the increased neural plasticity that leads to significant recovery following injury during this early phase of life. Further advances in our understanding of postnatal development/maturation and plasticity observed during early life could offer new strategies to improve outcomes by recapitulating aspects of the developmental program in the adult brain.

Oligodendrocyte development and the onset of myelination in the human fetal brain

Oligodendrocytes are cells that myelinate axons, providing saltatory conduction of action potentials and proper function of the central nervous system. Myelination begins prenatally in the human, and the sequence of oligodendrocyte development and the onset of myelination are not thoroughly investigated. This knowledge is important to better understand human diseases, such as periventricular leukomalacia, one of the leading causes of motor deficit in premature babies, and demyelinating disorders such as multiple sclerosis (MS). In this review we discuss the spatial and temporal progression of oligodendrocyte lineage characterized by the expression of specific markers and transcription factors in the human fetal brain from the early embryonic period (5 gestational weeks, gw) until midgestation (24 gw). Our in vitro evidence indicated that a subpopulation of human oligodendrocytes may have dorsal origin, from cortical radial glia cells, in addition to their ventral telencephalic origin. Furthermore, we demonstrated that the regulation of myelination in the human fetal brain includes positive and negative regulators. Chemokines, such as CXCL1, abundant in proliferative zones during brain development and in regions of remyelination in adult, are discussed in the view of their potential roles in stimulating oligodendrocyte development. Other signals are inhibitory and may include, but are not limited to, polysialic acid modification of the neural cell adhesion molecule on axons. Overall, important differences in temporal and spatial distribution and regulatory signals for oligodendrocyte differentiation exist between human and rodent brains. Those differences may underlie the unique susceptibility of humans to demyelinating diseases, such as MS.