cells Review Neurofibromin Structure, Functions and Regulation Mohammed Bergoug, Michel Doudeau, Fabienne Godin, Christine Mosrin, Béatrice Vallée and Hélène Bénédetti * Centre de Biophysique Moléculaire, CNRS, UPR 4301, University of Orléans and INSERM, CEDEX 2, 45071 Orléans, France; [email protected] (M.B.); [email protected] (M.D.); [email protected] (F.G.); [email protected] (C.M.); [email protected] (B.V.) * Correspondence: [email protected]; Tel.: +33-23-825-5584 Received: 1 October 2020; Accepted: 26 October 2020; Published: 27 October 2020 Abstract: Neurofibromin is a large and multifunctional protein encoded by the tumor suppressor gene NF1, mutations of which cause the tumor predisposition syndrome neurofibromatosis type 1 (NF1). Over the last three decades, studies of neurofibromin structure, interacting partners, and functions have shown that it is involved in several cell signaling pathways, including the Ras/MAPK, Akt/mTOR, ROCK/LIMK/cofilin, and cAMP/PKA pathways, and regulates many fundamental cellular processes, such as proliferation and migration, cytoskeletal dynamics, neurite outgrowth, dendritic-spine density, and dopamine levels. The crystallographic structure has been resolved for two of its functional domains, GRD (GAP-related (GTPase-activating protein) domain) and SecPH, and its post-translational modifications studied, showing it to be localized to several cell compartments. These findings have been of particular interest in the identification of many therapeutic targets and in the proposal of various therapeutic strategies to treat the symptoms of NF1. In this review, we provide an overview of the literature on neurofibromin structure, function, interactions, and regulation and highlight the relationships between them. Keywords: neurofibromin; structure; function; localization; interactions; post-translational modifications 1. Introduction Neurofibromatosis type 1 is an autosomal dominant disorder caused by inherited or de novo germline mutations in the NF1 tumor suppressor gene [1,2]. It is the most common tumor-predisposing disease in humans. It affects approximately one in 3000 live births and patients present widely heterogeneous clinical manifestations, even within the same family. Hallmark traits of the disease include pigmentary lesions and various types of peripheral nervous system tumors, cutaneous neurofibromas (cNFs), plexiform neurofibromas (pNFs), and malignant peripheral nerve sheath tumors (MPNSTs). However, some individuals develop other symptoms, such as skeletal abnormalities, brain tumors, learning disabilities, attention deficit, and social and behavioral problems. NF1 has been shown to be an essential gene for embryonic development and mice lacking a functional gene die in utero from cardiovascular defects [3]. To date, more than 3000 different germline mutations within the NF1 gene have been identified and shown to be pathogenic [4,5], but acquired somatic mutations in NF1 are also found in a wide variety of malignant neoplasms unrelated to NF1 [6]. NF1 encodes neurofibromin, a large multifunctional protein that is ubiquitously expressed but of which the highest levels are found in the neuronal cells of adults. Allelic variation, second-hit events in the NF1 gene, germline-specific genetic context, and epigenetic changes, as well as tissue-specific functions of neurofibromin may account for the profound degree of clinical heterogeneity in NF1 [7,8]. Detailed studies of the molecular and cellular properties of neurofibromin are required for a full understanding of the diverse phenotypes associated with the disease and their progression, as well as Cells 2020, 9, 2365; doi:10.3390/cells9112365 www.mdpi.com/journal/cells Cells 2020, 9, 2365 2 of 32 Cells 2020, 9, x 2 of 33 for thea full identification understanding of newof the therapeutic diverse phenotypes targets associated to develop with pharmacological the disease and their therapies progression, against as NF1. This reviewwell as for summarizes the identifi threecation decadesof new therapeutic of research targets on neurofibromin to develop pharmacological by focusing therapies on its structure against and molecularNF1. This aspects review of itssummarizes functions three and decades regulation. of research on neurofibromin by focusing on its structure and molecular aspects of its functions and regulation. 2. The NF1 Gene 2. The NF1 Gene Neurofibromatosis type 1 is caused by mutations within the NF1 gene. Early genetic linkage analysis located theNeurofibromatosisNF1 gene near the type centromere 1 is caused on by the mutations long arm within of chromosome the NF1 gene. 17 Early [9]. Subsequentgenetic linkage studies analysis located the NF1 gene near the centromere on the long arm of chromosome 17 [9]. Subsequent that showed NF1 patients to have translocations t(1;17) and t(17;22) [2] and deletions and point studies that showed NF1 patients to have translocations t(1;17) and t(17;22) [2] and deletions and NF1 NF1 mutationspoint mutations [1,10] led [1,10] to mapping led to mapping of the of the NF1gene gene to theto the 17q11.2 17q11.2 locus. locus. NF1 is isa large a large gene gene that that spansspans 350kb 350 of kb genomic of genomic DNA DNA sequence sequence [ 1[1]].. Its Its transcripttranscript is is 11 11 to to 13 13 kb kb long, long, with with an 8454 an 8454-bp-bp open open readingreading frame frame [11] [11] and and 3.5 3.5 kb kb of of 3’ 3 untranslated’ untranslated regionregion [[12]12].. It It contains contains 60 60 exons exons and and is ubiquitously is ubiquitously expressedexpressed [2] (Figure [2] (Figure1). 1). Figure 1. Schematic representation of the NF1 gene and its mRNA transcript; kb: kilo bases Figure 1. Schematic representation of the NF1 gene and its mRNA transcript; kb: kilo bases Three active genes lie within intron 35 (27b in the previous numbering) of NF1 and are transcribed Three active genes lie within intron 35 (27b in the previous numbering) of NF1 and are fromtranscribed the opposite from strand: the oppositeEVI2A strand:(ecotropic EVI2A ( viralecotropic integration viral integration site) [13 site)], EVI2B [13], EVI2B[14], [14] and, andOMGP (oligodendrocyteOMGP (oligodendrocyte myelin glycoprotein) myelin glycoprotein) [15] (Figure [15]1). (FigureEVI2A 1).and EVI2AEVI2B anared EVI2B the human are the homologues human of thehomologues murine Evi-2A of the murineand Evi-2B Evi-2Agenes, and Evi which-2B genes, encode which proteins encode proteins involved involved in retrovirus-induced in retrovirus- murineinduced myeloid murine leukemogenesis myeloid leukemogenesis [13,14]. OMGP [13,14]encodes. OMGP a glycoproteinencodes a glycoprotein specifically specifically expressed in oligodendrocytesexpressed in oligodendrocytes [16]. None of these [16] genes. None have of these been genes found have to bebeen mutated found into NF1be mutated patients in [NF113–15 ]. Thepatients presence [13–15 of]. multiple tumors in affected patients and the identification of somatic mutations within theTheNF1 presencegene in of sporadic multiple tumors tumors independentlyin affected patients of NF1and the disease identification has led toof NF1somaticbeing mutations designated within the NF1 gene in sporadic tumors independently of NF1 disease has led to NF1 being as a tumor suppressor gene [17]. designated as a tumor suppressor gene [17]. 3. Neurofibromin Protein 3. Neurofibromin Protein The NF1Thegene NF1 encodesgene encodes a large a large protein protein of 2818 of amino2818 amino acids acids called called neurofibromin neurofibromin [11]. [11] Sequence. Sequence analysis has shownanalysis homologyhas shown homology between betweenNF1 and NF1 theand Saccharomycesthe Saccharomyces cerevisiae cerevisiae IRA1 IRA1 andand IRA2IRA2 genes,genes, whichwhich negatively negatively regulate regulate the the RAS-cyclic RAS-cyclic AMP AMP pathway pathway [ 18[18]]..In In addition,addition, a a 360 360 amino amino-acid-acid portion portion of neurofibrominof neurofibromin appears appears to be homologous to be homologous to the catalytic to the catalytic domains domains of IRA gene of IRA products gene products and mammalian and p120GAPmammalian (GTPase-activating p120GAP (GTPase protein),-activating suggesting protein), that suggesting neurofibromin that neur possessesofibromin a Ras-GAP possesses function a Ras- [19]. This wasGAP confirmed function [19] by. This the capacitywas confirmed of this by portion the capacity of the of NF1this portiongene to of complement the NF1 gene theto complement function of the IRA1 theand functionIRA2 genes of the andIRA1 restoreand IRA2 a wildtypegenes and phenotyperestore a wildtype when phenotype expressed when in IRA expressed-mutated in IRA yeast- by mutated yeast by stimulating the intrinsic GTPase activity of the yeast Ras protein or the GTPase stimulating the intrinsic GTPase activity of the yeast Ras protein or the GTPase activity of human Ras activity of human Ras expressed in yeast. These studies established neurofibromin as a GTPase- expressed in yeast. These studies established neurofibromin as a GTPase-activating protein for Ras activating protein for Ras (Ras-GAP). The neurofibromin domain carrying the GAP function was (Ras-GAP).named TheGRD neurofibromin for GAP-related domain domain carrying[18,20–23] the. The GAP development function of was specific named antibodies GRD for against GAP-related the domainNF1 [18 gene,20 –product23]. The allowed development the identification of specific of antibodiesneurofibromin