Expression and localization of FRMD7 in human fetal brain, and a role for F-actin

FERM domain containing 7 (FRMD7) is a member of the four-point-one, ezrin, radixin, moesin (FERM) family of proteins, and has been reported to cause X-linked idiopathic congenital nystagmus (ICN), a disease which affects ocular motor control. There have been over 30 mutations reported for FRMD7, however, their role in the pathogenesis of ICN remains unclear. The purpose of this study is to perform the expression distributes of protein FRMD7 from human fetal brain during development and to understand the relationship with cytoskeletal protein F-actin between wild-type and mutation-type FRMD7.

Expression of protein FRMD7 from developing human fetal brain was tested by immunohistochemistry. Enhanced green fluorescent protein (EGFP)-tagged recombinant plasmids DNA encoding the normal or mutant FRMD7 were used to transiently transfect the mouse neuroblastoma cells (Neuro-2a) and human embryonic kidney 293 cells (HEK293T). Further, confocal microscopic analysis was used to determine the subcellular localization of the fusion proteins. To visualize F-actin, fixed HEK293T cells were stained with rhodamine-phalloidin.

We show that expression of FRMD7 was mainly detected in the brainstem (a region associated with ocular motor control), while limited level was observed in the cortex. The COOH-terminus of FRMD7 was found to play a key role in the subcellular localization of FRMD7 in mouse neuroblastoma cells (Neuro-2a) and human embryonic kidney 293 cells (HEK293T). While no differences in the co-localization of F-actin between the wild-type and missense mutation-type (c.781C>G and c.886G>C) proteins was observed, an additional mutant, c.1003C>T, which results in a COOH-terminally truncated protein, exhibited a nuclear localization pattern which did not co-localize with the cytoplasmic distribution of F-actin.

The results of the present study indicate that FRMD7 may play an important role in the brainstem in the early stages of development of the human fetal brain, and provides clues for the mechanism of mutation FRMD7, which may be involved in influencing F-actin dynamics.