Overlapping expression of ARFGEF2 and Filamin A in the neuroependymal lining of the lateral ventricles: Insights into the cause of periventricular heterotopia

Abstract

Periventricular heterotopia (PH) is a malformation of cortical development characterized by nodules of neurons, ectopically located along the lateral ventricles of the brain. Mutations in the vesicle transport ADP-ribosylation factor guanine exchange factor 2 gene (ARFGEF2) or the actin-binding Filamin A (FLNA) gene cause PH. Previous studies have shown that FLNA expression is developmentally regulated, with strongest expression observed along the ventricular zone (VZ) and to a lesser degree in postmitotic neurons in the cortex. Here we characterize the expression patterns for ARFGEF2 within the central nervous systems of human and mouse in order to better understand their potential roles in causing PH. ARFGEF2 mRNA was widely expressed in all cortical layers, especially in the neural precursors of the ventricular and subventricular zones (SVZ) during development, with persistent but diminished expression in adulthood. ARFGEF2 encodes for the protein brefeldin-inhibited guanine exchange factor 2 (BIG2). BIG2 protein immunoreactivity was most strongly localized to the neural progenitors along the neuroependymal lining of the VZ during development, with decreased expression in adulthood. Furthermore, overlapping BIG2 and FLNA expression was greatest in these same neuroependymal cells of human embryonic brain and was co-expressed in progenitors by Western blot. Finally, transfection of a dominant-negative construct of ARFGEF2 in SHSY5Y neuroblastoma cells partially blocked FLNA transport from the Golgi apparatus to the cell membrane. These results suggest that mutations in ARFGEF2 may impair targeted transport of FLNA to the cell surface within neural progenitors along the neuroependyma and that disruption of these cells could contribute to PH formation.

Filamin A controls matrix metalloprotease activity and regulates cell invasion in human fibrosarcoma cells.

“…Little is known about regulation of TIMP-2 secretion making it difficult to place our finding in a larger framework; we can hypothesize, however, that FLN directly regulates TIMP-2 secretion. The idea that FLNa could regulate membrane trafficking, while not completely new (Liu et al, 1997), is intriguing especially considering that null mutations in either FLNa or the trafficking related gene ARFGEF2 both cause periventricular heterotopias (Lu et al, 2006). Nonetheless, FLNa deficiency clearly does not result in a general membrane trafficking defect as MMP2 and MMP9 secretion and b1 and b3 integrin surface expression are normal in filamin knockdown cells.…”

Malformations of Cortical Development

“…So far, X-linked dominant mutations in FLNA (Filamin A) and autosomal recessive mutations in ARFGEF2 (ADP-ribosylation factor guanine exchange factor 2) are associated with PH (Table 1). FLNA is an actin-binding phosphoprotein that stabilizes the cytoskeleton and mediates focal adhesions along the ventricular epithelium 34 . ARFGEF2 encodes for the BIG2 protein which converts guanine diphosphate (GDP) to guanine triphosphate (GTP) and thereby activates the ADPribosylation factors (ARFs).…”

Genetic Basis of Brain Malformations

“…FLNA encodes a large actin-binding phosphoprotein that stabilizes the cytoskeleton and contributes to the formation of focal adhesions along the ventricular epithelium ]. FLNA may be required for the initial attachment of neurons onto the radial glial scaffolding before migration from the ventricular zone [Lu et al, 2006]. Failure of migrating neurons to attach to radial glia is one likely mechanism leading to the formation of heterotopia.…”