Nesprin-2 mediated nuclear trafficking and its clinical implications

Abstract

Nuclear translocation of proteins has a crucial role in the pathogenesis of cancer, Alzheimer disease and viral infections. A complete understanding of nuclear trafficking mechanisms is therefore necessary in order to establish effective intervention strategies. Here we elucidate the role of Nesprin-2 in Ca2+/Calmodulin mediated nuclear transport. Nesprin-2 is an actin-binding nuclear envelope (NE) protein with roles in maintaining nuclear structure and location, regulation of transcription and mechanotransduction. Upon depletion of Nesprin-2 using shRNA, HaCaT cells show abnormal localization of the shuttling proteins BRCA1 and NF-κB. We show that their nuclear transport is unlikely due to the canonical RAN mediated nuclear import, but rather to a RAN independent Ca2+/Calmodulin driven mechanism involving Nesprin-2. We report novel interactions between the actin-binding domain of Nesprin-2 and Calmodulin and between the NLS containing region of BRCA1 and Calmodulin. Strikingly, displacing Nesprins from the NE resulted in increased steady state Ca2+ concentrations in the cytoplasm suggesting a previously unidentified role of Nesprins in Ca2+ regulation. On comparing Nesprin-2 and BRCA1 localization in the ovarian cancer cell lines SKOV-3 and Caov-3, Nesprin-2 and BRCA1 were localized to the NE envelope and the nucleus in SKOV-3, respectively, and to the cytoplasm in Caov-3 cells. Fibroblasts obtained from EDMD5 (Emery Dreifuss muscular dystrophy) patients showed loss of Nesprin-2 from the nuclear envelope, corresponding reduced nuclear localization of BRCA1 and enhanced cytoplasmic Ca2+. Taken together, the data suggests a novel role of Nesprin-2 in Ca2+/Calmodulin mediated nuclear trafficking and provides new insights which can guide future therapies.

Emery‐Dreifuss muscular dystrophy

“…EDMD typically results from a structural or functional defect of one or more proteins comprising the nuclear envelope (Figure ), thus giving rise to the term “nuclear envelopathy” . A potential unifying disease mechanism may be loss of protein importation into the nucleus . The nuclear envelope is composed of an inner and outer nuclear membrane as well as a nuclear lamina, which, collectively, form a structural framework for the nucleus.…”

“…49 A potential unifying disease mechanism may be loss of protein importation into the nucleus. 50,51 The nuclear envelope is composed of an inner and outer nuclear membrane as well as a nuclear lamina, which, collectively, form a structural framework for the nucleus. A deficiency or mutation affecting any of the proteins providing this framework can result in a loss of the structural integrity of the nucleus, which can be particularly problematic for tissues that are frequently under stress, including cardiac and skeletal muscle.…”

Functions of nuclear actin-binding proteins in human cancer (Review)

“…Nesprin-2 is a nuclear membrane protein of the nuclear envelope spectrin-repeat (nesprin) family, which contains an actin-binding domain. Loss of Nesprin-2 is associated with less nuclear accumulation of c-Fos, mothers against decapentaplegic homolog (SMAD) 2, 3 and 4, holding back the course of nuclear translocation (65,66). Furthermore, the latest studies imply that Nesprin-2 is a prerequisite for the nuclear transport of certain proteins, for example, BRCA1 and NF-κB (Fig.…”

“…1). The nuclear localization of BRCA1 is regulated by a RAN-independent Ca 2+ /CaM mediated machinery in which Nesprin-2 is necessary (66). Additionally, abnormality in Nesprin-2 nuclear trafficking results in impaired nuclear translocation and mislocalization of BRCA1.…”

“…Downregulation of Nesprin-2 is revealed in breast cancer tissue (67). Therefore, disturbance of nuclear translocation of certain proteins by the ABPs may be associated to a number of diseases, including cancer (66).…”