Genetic and Infectious Causes of Microcephaly: NDE1 Mutations Compared to the Zika Virus

Brain development is an exquisitely coordinated process of progenitor cell proliferation followed by the migration of progeny to their final location in the developing brain. There are a myriad of points at which this process can be disturbed, and the examination of these perturbations help us further understand basic science, as well as epidemics sweeping through the world around us. Microcephaly, which is defined as a head circumference greater than 2 standard deviations below the mean, can occur through genetic, infectious, vascular, or metabolic etiologies, and the studies herein examine two forms by which microcephaly occurs. First, we investigate the role of the dynein regulatory protein Nde1 in the development of the neocortex, which is the outer region of the forebrain. NDE1 mutations are associated with severe microcephaly, and we find that unlike most microcephaly genes whose products have one role in the cell cycle, Nde1 is required at three discrete points in neuronal progenitors, termed radial glia progenitors (RGPs). We initially find that Nde1 is required to recruit dynein to the nuclear envelope to allow for interkinetic nuclear migration (INM) during G2. Additionally, Nde1 helps to initiate primary cilia resorption at the G1-to-S transition. Finally, there is a necessity for Nde1 at the G2-to-M transition after the completion of INM and prior to nuclear envelope breakdown. These three distinct roles for Nde1 illustrate the breadth of functions that the protein has during RGP proliferation, and help to explain why patients with NDE1 mutations have such severe microcephaly. As this work was ongoing there was a global outbreak of a new pathogen that had previously been dormant throughout Africa and Asia, only to emerge at epidemic proportions in the Western Hemisphere. This pathogen, the Zika Virus (ZIKV), is particularly alarming because of its subclinical course in adults but devastating consequences for fetal development, with the hallmark symptom being microcephaly. Using our organotypic brain slice model system, we demonstrate the ability of a variety of ZIKV isolates to infect and replicate in embryonic brain tissue. All ZIKV isolates that infect the organotypic slices lead to increases in apoptosis, though these increases are particularly pronounced in isolates from the Asian/American lineages. Notably, one isolate from a patient in Nigeria (termed 30656) does not replicate in mouse neuronal tissue, but electroporation of the 30656 ZIKV genome allows for a single cycle replication, suggesting that this isolate is unable to enter RGPs. All infectious isolates are pathogenic in early- and mid- gestation embryonic tissue, but only one isolate infects and replicates in late- gestation embryonic tissue. This was the most recently isolated sample tested, and it demonstrates a predilection for neurons, suggesting that ZIKV may be mutating as it spreads. These results provide foundational insight into the pathogenesis of ZIKV- associated microcephaly, and illustrate how studies of genetic forms of microcephaly can enhance and facilitate our understanding of infectious causes of the disease

Severe NDE1-mediated microcephaly results from neural progenitor cell cycle arrests at multiple specific stages

Microcephaly is a cortical malformation disorder characterized by an abnormally small brain. Recent studies have revealed severe cases of microcephaly resulting from human mutations in the NDE1 gene, which is involved in the regulation of cytoplasmic dynein. Here using in utero electroporation of NDE1 short hairpin RNA (shRNA) in embryonic rat brains, we observe cell cycle arrest of proliferating neural progenitors at three distinct stages: during apical interkinetic nuclear migration, at the G2-to-M transition and in regulation of primary cilia at the G1-to-S transition. RNAi against the NDE1 paralogue NDEL1 has no such effects. However, NDEL1 overexpression can functionally compensate for NDE1, except at the G2-to-M transition, revealing a unique NDE1 role. In contrast, NDE1 and NDEL1 RNAi have comparable effects on postmitotic neuronal migration. These results reveal that the severity of NDE1-associated microcephaly results not from defects in mitosis, but rather the inability of neural progenitors to ever reach this stage

VGF (TLQP-62)-induced neurogenesis targets early phase neural progenitor cells in the adult hippocampus and requires glutamate and BDNF signaling

Peer reviewe

Distinct roles for dynein light intermediate chains in neurogenesis, migration, and terminal somal translocation

Cytoplasmic dynein participates in multiple aspects of neocortical development. These include neural progenitor proliferation, morphogenesis, and neuronal migration. The cytoplasmic dynein light intermediate chains (LICs) 1 and 2 are cargo-binding subunits, though their relative roles are not well understood. Here, we used in utero electroporation of shRNAs or LIC functional domains to determine the relative contributions of the two LICs in the developing rat brain. We find that LIC1, through BicD2, is required for apical nuclear migration in neural progenitors. In newborn neurons, we observe specific roles for LIC1 in the multipolar to bipolar transition and glial-guided neuronal migration. In contrast, LIC2 contributes to a novel dynein role in the little-studied mode of migration, terminal somal translocation. Together, our results provide novel insight into the LICs' unique functions during brain development and dynein regulation overall.This project was supported by National Institutes of Health grants HD40182 and GM105536 to R.B. Vallee and the Fundação para Ciência e a Tecnologia MDPhD Scholarship PD/BD/113766/2015 to J.C. Gonçalves. During the final year, T.J. Dan-tas was supported by the Porto Neurosciences and Neurologic Disease Research Initiative at Instituto de Investigação e Inovação em Saúde (Norte-01-0145-FED ER-000008

LIS1 determines cleavage plane positioning by regulating actomyosin-mediated cell membrane contractility

Heterozygous loss of human PAFAH1B1 (coding for LIS1) results in the disruption of neurogenesis and neuronal migration via dysregulation of microtubule (MT) stability and dynein motor function/localization that alters mitotic spindle orientation, chromosomal segregation, and nuclear migration. Recently, human induced pluripotent stem cell (iPSC) models revealed an important role for LIS1 in controlling the length of terminal cell divisions of outer radial glial (oRG) progenitors, suggesting cellular functions of LIS1 in regulating neural progenitor cell (NPC) daughter cell separation. Here we examined the late mitotic stages NPCs in vivo and mouse embryonic fibroblasts (MEFs) in vitro from Pafah1b1-deficient mutants. Pafah1b1-deficient neocortical NPCs and MEFs similarly exhibited cleavage plane displacement with mislocalization of furrow-associated markers, associated with actomyosin dysfunction and cell membrane hyper-contractility. Thus, it suggests LIS1 acts as a key molecular link connecting MTs/dynein and actomyosin, ensuring that cell membrane contractility is tightly controlled to execute proper daughter cell separation

The Findings in the Flare

We present a case of papilledema in a young patient