Zika virus infection has been linked to an increase in the number of infants born with microcephaly in Brazil, but direct experimental proof that Zika virus causes birth defects was lacking. Here Alysson Muotri and colleagues show that the Brazilian Zika virus strain can cross the placenta and cause intrauterine growth restriction, including signs of microcephaly, in the SJL strain of mice. They also show that the virus can infect human brain organoids, inducing cell death by apoptosis and disrupting cortical layers. Zika virus (ZIKV) is an arbovirus belonging to the genus Flavivirus (family Flaviviridae) and was first described in 1947 in Uganda following blood analyses of sentinel Rhesus monkeys1. Until the twentieth century, the African and Asian lineages of the virus did not cause meaningful infections in humans. However, in 2007, vectored by Aedes aegypti mosquitoes, ZIKV caused the first noteworthy epidemic on the Yap Island in Micronesia2. Patients experienced fever, skin rash, arthralgia and conjunctivitis2. From 2013 to 2015, the Asian lineage of the virus caused further massive outbreaks in New Caledonia and French Polynesia. In 2013, ZIKV reached Brazil, later spreading to other countries in South and Central America3. In Brazil, the virus has been linked to congenital malformations, including microcephaly and other severe neurological diseases, such as Guillain–Barré syndrome4,5. Despite clinical evidence, direct experimental proof showing that the Brazilian ZIKV (ZIKVBR) strain causes birth defects remains absent6. Here we demonstrate that ZIKVBR infects fetuses, causing intrauterine growth restriction, including signs of microcephaly, in mice. Moreover, the virus infects human cortical progenitor cells, leading to an increase in cell death. We also report that the infection of human brain organoids results in a reduction of proliferative zones and disrupted cortical layers. These results indicate that ZIKVBR crosses the placenta and causes microcephaly by targeting cortical progenitor cells, inducing cell death by apoptosis and autophagy, and impairing neurodevelopment. Our data reinforce the growing body of evidence linking the ZIKVBR outbreak to the alarming number of cases of congenital brain malformations. Our model can be used to determine the efficiency of therapeutic approaches to counteracting the harmful impact of ZIKVBR in human neurodevelopment.

L1 retrotransposons are dynamically regulated and active genomic elements that affect gene expression and neuronal function throughout brain development. According to a new study by Alysson Muotri and colleagues, the absence of MeCP2, a modulator of DNA methylation implicated in several neurodevelopmental disorders, increases L1 retrotransposon activity in rodent models. This increase in susceptibility to L1 retrotransposition is duplicated in iPS cells derived from patients with Rett syndrome. These data correlations suggest that disease-related genetic mutations may influence L1 retrotransposon activity, adding another layer of complexity to our understanding of molecular neurological disorders. Long interspersed nuclear elements-1 (L1) retrotransposons affect gene expression and neuronal function throughout brain development. These authors show that the absence of methyl-CpG-binding protein 2, a modulator of DNA methylation implicated in several neurodevelopmental disorders, increases L1 retrotransposon activity in rodent models, with this increase in susceptibility duplicated in patients with Rett syndrome. These correlations suggest that disease-related genetic mutations may influence L1 retrotransposon activity. Long interspersed nuclear elements-1 (LINE-1 or L1s) are abundant retrotransposons that comprise approximately 20% of mammalian genomes1,2,3. Active L1 retrotransposons can impact the genome in a variety of ways, creating insertions, deletions, new splice sites or gene expression fine-tuning4,5,6. We have shown previously that L1 retrotransposons are capable of mobilization in neuronal progenitor cells from rodents and humans and evidence of massive L1 insertions was observed in adult brain tissues but not in other somatic tissues7,8. In addition, L1 mobility in the adult hippocampus can be influenced by the environment9. The neuronal specificity of somatic L1 retrotransposition in neural progenitors is partially due to the transition of a Sox2/HDAC1 repressor complex to a Wnt-mediated T-cell factor/lymphoid enhancer factor (TCF/LEF) transcriptional activator7,10. The transcriptional switch accompanies chromatin remodelling during neuronal differentiation, allowing a transient stimulation of L1 transcription7. The activity of L1 retrotransposons during brain development can have an impact on gene expression and neuronal function, thereby increasing brain-specific genetic mosaicism11,12. Further understanding of the molecular mechanisms that regulate L1 expression should provide new insights into the role of L1 retrotransposition during brain development. Here we show that L1 neuronal transcription and retrotransposition in rodents are increased in the absence of methyl-CpG-binding protein 2 (MeCP2), a protein involved in global DNA methylation and human neurodevelopmental diseases. Using neuronal progenitor cells derived from human induced pluripotent stem cells and human tissues, we revealed that patients with Rett syndrome (RTT), carrying MeCP2 mutations, have increased susceptibility for L1 retrotransposition. Our data demonstrate that L1 retrotransposition can be controlled in a tissue-specific manner and that disease-related genetic mutations can influence the frequency of neuronal L1 retrotransposition. Our findings add a new level of complexity to the molecular events that can lead to neurological disorders.

Autism spectrum disorders (ASD) are common, complex and heterogeneous neurodevelopmental disorders. Cellular and molecular mechanisms responsible for ASD pathogenesis have been proposed based on genetic studies, brain pathology and imaging, but a major impediment to testing ASD hypotheses is the lack of human cell models. Here, we reprogrammed fibroblasts to generate induced pluripotent stem cells, neural progenitor cells (NPCs) and neurons from ASD individuals with early brain overgrowth and non-ASD controls with normal brain size. ASD-derived NPCs display increased cell proliferation because of dysregulation of a β-catenin/BRN2 transcriptional cascade. ASD-derived neurons display abnormal neurogenesis and reduced synaptogenesis leading to functional defects in neuronal networks. Interestingly, defects in neuronal networks could be rescued by insulin growth factor 1 (IGF-1), a drug that is currently in clinical trials for ASD. This work demonstrates that selection of ASD subjects based on endophenotypes unraveled biologically relevant pathway disruption and revealed a potential cellular mechanism for the therapeutic effect of IGF-1.