Multiple Novel Nesprin-1 and Nesprin-2 Variants Act as Versatile Tissue-Specific Intracellular Scaffolds

<div><h3>Background</h3><p>Nesprins (<u>N</u>uclear <u>e</u>nvelope <u>s</u>pectrin-<u>r</u>epeat <u>p</u>roteins) are a novel family of giant spectrin-repeat containing proteins. The nesprin-1 and nesprin-2 genes consist of 146 and 116 exons which encode proteins of ∼1mDa and ∼800 kDa is size respectively when all the exons are utilised in translation. However emerging data suggests that the nesprins have multiple alternative start and termination sites throughout their genes allowing the generation of smaller isoforms.</p> <h3>Results</h3><p>In this study we set out to identify novel alternatively transcribed nesprin variants by screening the EST database and by using RACE analysis to identify cDNA ends. These two methods provided potential hits for alternative start and termination sites that were validated by PCR and DNA sequencing. We show that these alternative sites are not only expressed in a tissue specific manner but by combining different sites together it is possible to create a wide array of nesprin variants. By cloning and expressing small novel nesprin variants into human fibroblasts and U2OS cells we show localization to actin stress-fibres, focal adhesions, microtubules, the nucleolus, nuclear matrix and the nuclear envelope (NE). Furthermore we show that the sub-cellular localization of individual nesprin variants can vary depending on the cell type, suggesting any single nesprin variant may have different functions in different cell types.</p> <h3>Conclusions</h3><p>These studies suggest nesprins act as highly versatile tissue specific intracellular protein scaffolds and identify potential novel functions for nesprins beyond cytoplasmic-nuclear coupling. These alternate functions may also account for the diverse range of disease phenotypes observed when these genes are mutated.</p> </div

Nesprins LINC the nucleus and cytoskeleton

Like other spectrin repeat proteins, nesprins co-ordinate and maintain cellular architecture by linking membranous organelles to the cytoskeleton. However nuclear envelope (NE) nesprins, uniquely hardwire the nuclear lamina to the cytoskeleton and molecular motors. Emerging evidence suggests that nesprins also form a continuous network linking the plasma membrane to the NE that potentially translates mechanical stimuli into nuclear reorganisation. Surprisingly, this network is also essential for cytoskeletal organisation and its disruption has dramatic effects on nuclear migration, centrosomal positioning, focal adhesion maturation and cell motility. Herein we review recent advances in our understanding of how nesprins couple to various filamentous systems within the cell and emphasise the importance of both KASH and KASH-less nesprin isoforms in these interactions

Novel Nuclear Nesprin-2 Variants Tether Active Extracellular Signal-regulated MAPK1 and MAPK2 at Promyelocytic Leukemia Protein Nuclear Bodies and Act to Regulate Smooth Muscle Cell Proliferation*

Nuclear and cytoplasmic scaffold proteins have been shown to be essential for temporal and spatial organization, as well as the fidelity, of MAPK signaling pathways. In this study we show that nesprin-2 is a novel extracellular signal-regulated MAPK1 and 2 (ERK1/2) scaffold protein that serves to regulate nuclear signaling by tethering these kinases at promyelocytic leukemia protein nuclear bodies (PML NBs). Using immunofluorescence microscopy, GST pull-down and immunoprecipitation, we show that nesprin-2, ERK1/2, and PML colocalize and bind to form a nuclear complex. Interference of nesprin-2 function, by either siRNA-mediated knockdown or overexpression of a dominant negative nesprin-2 fragment, augmented ERK1/2 nuclear signaling shown by increased SP1 activity and ELK1 phosphorylation. The functional outcome of nesprin-2 disruption and the resultant sustained ERK1/2 signal was increased proliferation. Importantly, these activities were not induced by previously identified nuclear envelope (NE)-targeted nesprin-2 isoforms but rather were mediated by novel nuclear isoforms that lacked the KASH domain. Taken together, this study suggests that nesprin-2 is a novel intranuclear scaffold, essential for nuclear ERK1/2 signaling fidelity and cell cycle progression

Identification of novel nesprin UTRs.

<p>A) cDNA ends identified by 3′ and 5′ RACE from Brain, Skeletal Muscle (SkeMus) and HeLa cDNA libraries. B) DNA sequencing results suggest that nesprin isoforms terminate with unique C-terminal ends absent from the giant isoforms as a result of intron retention. For example, isoforms utilising the N1-3′E90 UTR terminate with ‘AGAGYPHQ’ amino acids, giving it a unique fingerprint. Blue sequences show the coding regions of exons 90 and 91, black sequences show intronic regions and red sequence indicates a stop codon. C) Validation and tissue specificity of nesprin-1 UTRs identified on online databases and by RACE were confirmed by PCR amplification from a multiple tissue cDNA panel and DNA sequencing. Nesprin-1 PCRs were carried out when UTRs were identified on cDNA panels available at the time and are therefore organised into 3 separate sections. D) Validation and tissue specificity of nesprin-2 UTRs identified on online databases and by RACE were confirmed by PCR amplification from a multiple tissue cDNA panel and DNA sequencing. Nesprin-2 PCRs were carried out when UTRs were identified on cDNA panels available at the time and are therefore organised into 3 separate sections. Small Intestine and Peripheral Blood Lymphocytes have been abbreviated as ‘SI’ and ‘PBL’ respectively for all cDNA panels.</p

UTRs identified through online databases.

<p>Table listing all potential UTRs identified through available online databases.</p

Cloning and expression of novel Nesprin KASH and CH isoforms.

<p>A) Schematic representation of p53KASH^Nesp1 (Accession numberJQ754366) and p56CH^Nesp1 (Accession number JQ740783) relative to the nesprin-1 giant. B) p53KASH^Nesp1 localizes to the NE when transfected into U2OS cells. C) Nesprin-1 Flag-p56CH^Nesp1 localized to the nucleolus when transfected into U2OS cells. D) Nesprin-1 Flag-p56CH^Nesp1 localizes to actin stress fibres and with Focal Adhesion Kinase (FAK) at focal adhesions when transfected into Human Dermal Fibroblasts (HDFs). E) Nesprin-2 Flag-p32CH^Nesp2 (Accession numberJQ754367) co-localized with FAK at focal adhesions when transfected into U2OS cells. F) p53KASH^Nesp1 expression was not detected by PCR in U2OS, Human Dermal Fibroblasts (HDFs), Vascular Smooth Muscle Cells (VSMCs) or Myoblasts (MBs), however it was detected in the heart, spleen and peripheral blood leukocytes (PBL). p56CH^Nesp1 was detected in all cells and tissues examined whereas p32CH^Nesp2 was limited to U2OS cells, MBs and PBL.</p

Potential nesprin-1 isoforms.

<p>A) Genomic map of the nesprin-1 gene highlighting the positions of the nesprin-1 UTRs identified to date. B) Proposed nesprin-1 isoforms created by alternative transcription. SRs are numbered and shown according to the scheme of <i>Simpson and Roberts 2008</i> and are shown to scale.</p

Potential nesprin-2 isoforms.

<p>A) Genomic map of the nesprin-2 gene highlighting the positions of the nesprin-1 UTRs identified to date. B) Proposed nesprin-2 isoforms created by alternative transcription. SRs are numbered and shown according to the scheme of <i>Simpson and Roberts 2008</i> and are shown to scale.</p

Cassette exons identified through online databases.

<p>An online scan of the EST and nucleotide databases indicated that the nesprin-1 and nesprin-2 genes underwent extensive alternative splicing and this was verified using PCR (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0040098#pone-0040098-g007" target="_blank">Figure 7A,B</a>).</p>*<p>Represents a stop codon for nesprin-1 exon 145 and nesprin-2 exons 110–113.</p

Nesprin-1 expression is highly adaptable.

<p>Expression levels of N1-3′E87, N1-3′E90 and nesprin-1 KASH domain were monitored post-siRNA knockdown using siRNAs targeting exons 90 and 136 of the nesprin-1 gene. As demonstrated si-136 increased expression of N1-3′E87 whereas si-90 reduced it’s expression. *p<0.01, ANOVA analysis, 95% confidence interval.</p