The cerebral cortex develops through the coordinated generation of dozens of neuronal subtypes, but the mechanisms involved remain unclear. Here we show that mouse embryonic stem cells, cultured without any morphogen but in the presence of a sonic hedgehog inhibitor, recapitulate in vitro the major milestones of cortical development, leading to the sequential generation of a diverse repertoire of neurons that display most salient features of genuine cortical pyramidal neurons. When grafted into the cerebral cortex, these neurons develop patterns of axonal projections corresponding to a wide range of cortical layers, but also to highly specific cortical areas, in particular visual and limbic areas, thereby demonstrating that the identity of a cortical area can be specified without any influence from the brain. The discovery of intrinsic corticogenesis sheds new light on the mechanisms of neuronal specification, and opens new avenues for the modelling and treatment of brain diseases.

The study of human cortical development has major implications for brain evolution and diseases but has remained elusive due to paucity of experimental models. Here we found that human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), cultured without added morphogens, recapitulate corticogenesis leading to the sequential generation of functional pyramidal neurons of all six layer identities. After transplantation into mouse neonatal brain, human ESC-derived cortical neurons integrated robustly and established specific axonal projections and dendritic patterns corresponding to native cortical neurons. The differentiation and connectivity of the transplanted human cortical neurons complexified progressively over several months in vivo, culminating in the establishment of functional synapses with the host circuitry. Our data demonstrate that human cortical neurons generated in vitro from ESC/iPSC can develop complex hodological properties characteristic of the cerebral cortex in vivo, thereby offering unprecedented opportunities for the modeling of human cortex diseases and brain repair.

␤-Secretase (BACE1) is the rate-limiting protease for the generation of the amyloid ␤-peptide (A␤) in Alzheimer disease. Mice in which the bace1 gene is inactivated are reported to be healthy. However, the presence of a homologous gene encoding BACE2 raises the possibility of compensatory mechanisms. Therefore, we have generated bace1, bace2, and double knockout mice. We report here that BACE1 mice display a complex phenotype. A variable but significant number of BACE1 offspring died in the first weeks after birth. The surviving mice remained smaller than their littermate controls and presented a hyperactive behavior. Electrophysiologically, subtle alterations in the steady-state inactivation of voltage-gated sodium channels in BACE1-deficient neurons were observed. In contrast, bace2 knockout mice displayed an overall healthy phenotype. However, a combined deficiency of BACE2 and BACE1 enhanced the bace1 ؊/؊ lethality phenotype. At the biochemical level, we have confirmed that BACE1 deficiency results in an almost complete block of A␤ generation in neurons, but not in glia. As glia are 10 times more abundant in brain compared with neurons, our data indicate that BACE2 could indeed contribute to A␤ generation in the brains of Alzheimer disease and, in particular, Down syndrome patients. In conclusion, our data challenge the general idea of BACE1 as a safe drug target and call for some caution when claiming that no major side effects should be expected from blocking BACE1 activity. Alzheimer disease (AD)1 is the most common cause of dementia for which neither a good diagnostic test nor an effective treatment is available yet. The most widely accepted hypothesis states that AD is initially triggered by the abnormal accumulation and possibly deposition of the small amyloid ␤-peptide (A␤) in different brain regions, which in turn initiates a pathogenic cascade that ultimately leads to neuronal death, AD pathology, and dementia. A␤ is cleaved from a long membranebound precursor, the amyloid precursor protein (APP), by two consecutive cleavages. ␤-and ␥-secretases are the enzymes that liberate the N and C termini of A␤, respectively, and are the subject of intense investigation because of their relevance as candidate therapeutic targets to treat AD.BACE1 and BACE2 are two highly homologous membranebound aspartyl proteases that can process APP at the ␤-secretase site (1-8). Although both enzymes exhibit many of the characteristics expected for ␤-secretase, it has been quite convincingly demonstrated that BACE1 is in fact the major ␤-secretase responsible for A␤ generation in brain (9 -11). Contrary to BACE1, BACE2 is more highly expressed in peripheral tissues, but also to some extent in brain (2,8,12,13), raising the question of whether BACE2 could contribute to the generation of the brain A␤ pool. Both BACE1 and BACE2 can cleave APP in vitro not only at Asp 1 (numbering considering the first amino acid of A␤ as position 1), but also at internal sites within the A␤ region. BACE1 cleaves between amino acids 10 and 11 o...

Human pluripotent stem cells (PSCs) provide a unique entry to study species-specific aspects of human disorders such as Alzheimer's disease (AD). However, in vitro culture of neurons deprives them of their natural environment. Here we transplanted human PSC-derived cortical neuronal precursors into the brain of a murine AD model. Human neurons differentiate and integrate into the brain, express 3R/4R Tau splice forms, show abnormal phosphorylation and conformational Tau changes, and undergo neurodegeneration. Remarkably, cell death was dissociated from tangle formation in this natural 3D model of AD. Using genome-wide expression analysis, we observed upregulation of genes involved in myelination and downregulation of genes related to memory and cognition, synaptic transmission, and neuron projection. This novel chimeric model for AD displays human-specific pathological features and allows the analysis of different genetic backgrounds and mutations during the course of the disease.