Viscoelastic Behavior of Human Lamin A Proteins in the Context of Dilated Cardiomyopathy Avinanda Banerjee 1. , Vikram Rathee 2. , Rema Krishnaswamy 3 , Pritha Bhattacharjee 1 , Pulak Ray 1 , Ajay K. Sood 2 , Kaushik Sengupta 1 * 1 Biophysics and Structural Genomics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata, West Bengal, India, 2 Department of Physics, Indian Institute of Science, Bangalore, Karnataka, India, 3 Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur Campus, Bangalore, Karnataka, India Abstract Lamins are intermediate filament proteins of type V constituting a nuclear lamina or filamentous meshwork which lines the nucleoplasmic side of the inner nuclear membrane. This protein mesh provides a supporting scaffold for the nuclear envelope and tethers interphase chromosome to the nuclear periphery. Mutations of mainly A-type lamins are found to be causative for at least 11 human diseases collectively termed as laminopathies majority of which are characterised by aberrant nuclei with altered structural rigidity, deformability and poor mechanotransduction behaviour. But the investigation of viscoelastic behavior of lamin A continues to elude the field. In order to address this problem, we hereby present the very first report on viscoelastic properties of wild type human lamin A and some of its mutants linked with Dilated cardiomyopathy (DCM) using quantitative rheological measurements. We observed a dramatic strain-softening effect on lamin A network as an outcome of the strain amplitude sweep measurements which could arise from the large compliance of the quasi-cross-links in the network or that of the lamin A rods. In addition, the drastic stiffening of the differential elastic moduli on superposition of rotational and oscillatory shear stress reflect the increase in the stiffness of the laterally associated lamin A rods. These findings present a preliminary insight into distinct biomechanical properties of wild type lamin A protein and its mutants which in turn revealed interesting differences. Citation: Banerjee A, Rathee V, Krishnaswamy R, Bhattacharjee P, Ray P, et al. (2013) Viscoelastic Behavior of Human Lamin A Proteins in the Context of Dilated Cardiomyopathy. PLoS ONE 8(12): e83410. doi:10.1371/journal.pone.0083410 Editor: Anindita Das, Virginia Commonwealth University, United States of America Received April 18, 2013; Accepted November 4, 2013; Published December 30, 2013 Copyright: ß 2013 Banerjee et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: AB thanks University Grants Council, Government of India for the fellowship. AKS thanks Council of Scientific and Industrial Research, (Government of India) Bhatnagar Fellowship for support. RK thanks support under Ramanujan Fellowship, Department of Science and Technology, Government of India. KSG thanks MMDDA and BARD projects of Department of Atomic Energy, Government of India. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: kaushik.sengupta@saha.ac.in . These authors contributed equally to this work. Introduction The ‘fibrous lamina’ [1] underlying the inner nuclear membrane (INM) of the nucleus of most metazoan cells provides mechanical rigidity to the nucleus thus ensuring proper size and shape. Lamin A (LA) a type V intermediate filament protein is one of the major constituent proteins of the lamina along with lamin C (LC), lamin B1 (LB1) and lamin B2 (LB2). LA & LC are alternate splice products of the gene LMNA and expressed in differentiated cells only whereas LB1 & LB2 encoded by LMNB1 & LMNB2 genes respectively are expressed in mostly all cell types throughout the process of development [2]. The lamin protein(s) organize into distinctive mesh like structure inside the nucleus. Lamins exhibit general characteristic of an intermediate filament protein com- prising of a central rod domain flanked by a short globular head at the N-terminal and a C-terminal tail domain. The central rod domain in turn consists of four coiled-coil domains (1a, 1b, 2a, 2b) interspersed with linker regions. In vitro, lamin assembly is triggered with the formation of dimers which elongate in a head-to-tail fashion into protofilaments at higher pH which further compact laterally to form paracrystal arrays under acidic pH [3]. Although lamins were isolated as detergent insoluble proteins being tightly associated with the nuclear envelope [4,5] many studies have also established their localizations in a more soluble form within the nucleoplasm and nuclear matrices [6,7,8,9]. The pool of A-type lamins within the nucleoplasm is more soluble than the peripheral lamin A [10]. Unpolymerized (hence soluble) lamin A and lamin C are distributed throughout the nucleoplasm at early G1 phase which subsequently get incorporated into the lamina over time forming partially interconnected network [11,12]. A recent report by Kolb et.al.(2011), have shown by indirect immunofluorescence that lamin A and lamin C are partially segregated in lamina [12]. Similar pools of more soluble B-type lamins are also found in the nucleoplasm. There is a distinct difference in the mobility between the A-type and B-type lamins indicating different states of organization which also suggest their difference in the state of aggregation inside the nucleus. For instance, Goldberg et.al., (2009) had shown that expression of somatic A- and B-type lamins in Xenopus oocyte produced distinctly different types of filaments – wavy and irregular bundles for LB2 and thick multi-layered ones for LA [13]. Furthermore, internal B-type lamins are relatively static whereas the A-type lamins are much more labile [14]. Silencing LA/C, LB1 or LB2 demonstrated the distinct compart- mentalization and roles of each type [14]. Additionally, Fluores- cence Resonance Energy Transfer (FRET) experiments have PLOS ONE | www.plosone.org 1 December 2013 | Volume 8 | Issue 12 | e83410