Mutations in ARFGEF2 implicate vesicle trafficking in neural progenitor proliferation and migration in the human cerebral cortex

Abstract

Disruption of human neural precursor proliferation can give rise to a small brain (microcephaly), and failure of neurons to migrate properly can lead to an abnormal arrest of cerebral cortical neurons in proliferative zones near the lateral ventricles (periventricular heterotopia). Here we show that an autosomal recessive condition characterized by microcephaly and periventricular heterotopia 1 maps to chromosome 20 and is caused by mutations in the gene ADP-ribosylation factor guanine nucleotide-exchange factor-2 (ARFGEF2). By northernblot analysis, we found that mouse Arfgef2 mRNA levels are highest during embryonic periods of ongoing neuronal proliferation and migration, and by in situ hybridization, we found that the mRNA is widely distributed throughout the embryonic central nervous system (CNS). ARFGEF2 encodes the large (>200 kDa) brefeldin A (BFA)-inhibited GEF2 protein (BIG2), which is required for vesicle and membrane trafficking from the trans-Golgi network (TGN). Inhibition of BIG2 by BFA, or by a dominant negative ARFGEF2 cDNA, decreases cell proliferation in vitro, suggesting a cell-autonomous regulation of neural expansion. Inhibition of BIG2 also disturbed the intracellular localization of such molecules as E-cadherin and β-catenin by preventing their transport from the Golgi apparatus to the cell surface. Our findings show that vesicle trafficking is an important regulator of proliferation and migration during human cerebral cortical development.Autosomal recessive periventricular heterotopia with microcephaly (ARPHM) is a severe malformation of the cerebral cortex, characterized by severe developmental delay and recurrent infections 1 . No anomalies extrinsic to the CNS, such as dysmorphic features or grossly abnormal endocrine or other conditions, have been observed in individuals with ARPHM. Magnetic resonance imaging of the brains of affected individuals shows notable periventricular heterotopia, resulting from the failure of neurons to migrate normally from the lateral ventricular proliferative zone, where they are formed, to the cerebral cortex ( Fig. 1a-d). Abnormal magnetic resonance image signals in subcortical white matter and elsewhere also suggest a delay in the normal myelination by glial cells.An initial genome-wide screen at 10-cM intervals of two Turkish families with ARPHM identified shared homozygosity at a single locus on chromosome 20q11.21-13.2, which we refined by further marker analysis (Fig. 1e,f). Because the two families had indistinguishable clinical and radiographic features, we assumed that they had an allelic disorder and summed their linkage results. The ARPHM region (66.16-77.75 cM on chromosome 20) that was identical by descent in each of the two families yielded a summed multipoint lod score of 4.41 (Fig. 1g). Two-point analyses gave combined maximal lod scores of 3.28 (pedigree 1 = 1.53 and pedigree 2 = 1.75) at marker D20S109 (Fig. 1h).We sequenced several candidate genes in the minimal linked region and found a different deleterious mutation in ARFGEF2 (encod...

The hyh mutation uncovers roles for αSnap in apical protein localization and control of neural cell fate

“…Regulation of cell fate by αSnap may affect other organs, but previous analysis has not shown hypoplasia of other organs 1 in hyh mice, perhaps because other tissues regulate size postnatally, whereas the postnatal brain contains largely postmitotic neurons. Other genes that affect the size of the central nervous system in humans commonly do not affect other organs, even when they are ubiquitously expressed 27,28 29 , analysis of the hyh mutant supports this role and provides an opportunity to examine the role of apical vesicle trafficking in other aspects of neural development. The hyh mutant has additional defects in cerebellar development, neuronal migration and axonal pathfinding (data not shown), suggesting roles for apical trafficking in these processes.…”

Interaction of BIG2, a brefeldin A-inhibited guanine nucleotide-exchange protein, with exocyst protein Exo70

“…Wang et al (25) found that overexpression of GFP-Exo70 (but not GFP-Exo84) resulted in the disruption of microtubule architecture in the area of cytoplasm under regions of plasma membrane protrusions, and microtubule polymerization in vitro was inhibited by recombinant Exo70. The interaction of BIG2 with Exo70 in these processes suggests a mechanism for the congenital defects described in two families with mutations in the BIG2 gene (7).…”

“…They appear to colocalize at the trans-Golgi network (4,5), but move independently among intracellular compartments, e.g., after serum starvation of HepG2 cells, BIG1, but not BIG2, is present in nuclear structures (6). BIG2 mutations were related to an autosomal recessive periventricular heterotopia with microcephaly, a congenital human developmental disorder (7). No disease involving BIG1 has been reported.…”

Mutations in a Highly Conserved Region of the Arf1p ActivatorGEA2Block Anterograde Golgi Transport but Not COPI Recruitment to Membranes

“…In yeast, Gea1p and Gea2p are functionally redundant in that one or the other can be deleted with no detectable effect on growth rate or rates of transport to the cell surface and vacuole, whereas a gea1⌬ gea2⌬ double mutant is inviable (Peyroche et al, 1996. A similar situation is likely true of the BIG1-BIG2 pair in mammalian cells, given their extensive sequence homology (74% identity; Togawa et al, 1999;Yamaji et al, 2000) and the fact that a human disease (autosomal recessive periventricular heterotopia) is caused by almost complete deletion of the BIG2 gene (Sheen et al, 2004). The fact that these affected individuals with no BIG2 function are born and live several years (albeit with severe abnormalities) indicates that at the cellular level, there is a high level of redundancy between BIG1 and BIG2 for essential functions.…”