Lamins A and C but Not Lamin B1 Regulate Nuclear Mechanics

Abstract

Mutations in the nuclear envelope proteins lamins A and C cause a broad variety of human diseases, including Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy, and Hutchinson-Gilford progeria syndrome. Cells lacking lamins A and C have reduced nuclear stiffness and increased nuclear fragility, leading to increased cell death under mechanical strain and suggesting a potential mechanism for disease. Here, we investigated the contribution of major lamin subtypes (lamins A, C, and B1) to nuclear mechanics by analyzing nuclear shape, nuclear dynamics over time, nuclear deformations under strain, and cell viability under prolonged mechanical stimulation in cells lacking both lamins A and C, cells lacking only lamin A (i.e."lamin C-only" cells), cells lacking wild-type lamin B1, and wild-type cells. Lamin A/C-deficient cells exhibited increased numbers of misshapen nuclei and had severely reduced nuclear stiffness and decreased cell viability under strain. Lamin C-only cells had slightly abnormal nuclear shape and mildly reduced nuclear stiffness but no decrease in cell viability under strain. Interestingly, lamin B1-deficient cells exhibited normal nuclear mechanics despite having a significantly increased frequency of nuclear blebs. Our study indicates that lamins A and C are important contributors to the mechanical stiffness of nuclei, whereas lamin B1 contributes to nuclear integrity but not stiffness.Lamins are type V intermediate filament proteins that form the nuclear lamina, a filamentous network underlying the inner nuclear membrane of eukaryotic cells. Lamins form stable structures in the nuclear lamina and the nucleoplasm, determine nuclear shape and size, resist nuclear deformation, and position nuclear pore complexes (reviewed in Refs. 1-3). In addition, lamins recruit and anchor, either directly or indirectly, several nuclear envelope proteins (e.g. nesprin-1␣, emerin, and the lamin B receptor) to the inner nuclear membrane (3).Mammalian cells express two types of lamins, the A and B types. Both share a common structural organization: a globular N-terminal domain separated from a larger C-terminal globular domain by a central helical rod domain that allows lamins to form parallel coiled-coil dimers, which in turn assemble into stable strings and higher order networks. Lamins A and C, the major A-type lamins, are alternatively spliced isoforms of a single gene, LMNA. The expression of A-type lamins is developmentally regulated, beginning midway through embryonic development (4). A-type lamins are expressed in most but not all differentiated cells (5). B-type lamins (lamins B1 and B2) are encoded by separate genes, LMNB1 and LMNB2, respectively (6). Unlike A-type lamins, B-type lamins are expressed in all cells and throughout development (7,8), although it is not clear if they are always coexpressed at equivalent levels in the same cell.

Mouse models of the laminopathies

“…Comparative studies on cells derived from the lamin A-deficient mice and lamin B1 gene trap mice showed that the A-type lamins are the principal contributors to the biophysical properties of the lamina, providing mechanical strength and stiffness to the nuclei [37]. These observations would be consistent with the notion that mutations in LMNA weaken the nucleus, making it more prone to damage arising from physical stress, such as would occur in the contracting heart or skeletal musculature.…”

“…However, the mechanical properties of the nuclei of Lmnb1 −/− mouse embryonic fibroblasts (MEFs) in response to defined nuclear strain were normal [37]. These experiments revealed that loss of lamin B1 may cause local disturbances in NE structure without causing generalized defects in nuclear organization, stiffness, and shape stability.…”

A Chemomechanical Model for Nuclear Morphology and Stresses during Cell Transendothelial Migration

“…It has been shown that the levels of the NE proteins lamin A and C (lamin A/C) determine the stiffness of the nucleus (20,(31)(32)(33), and that lower levels of lamin A/C facilitate cell migration through tight spaces (8,10,34). We studied the influence of lamin A/C on transmigration by varying the modulus of the NE in our model (Fig.…”

An historical perspective on cell mechanics

“…Conceptually, this work is related to modern directions towards investigating multi-cellular assemblies, monolayers and tissues [16,24,25,141]. Micropipette aspiration [142][143][144][145] has also come into use to study whole-cell mechanics by examining cellular and nuclear deformations in response to suction [146][147][148][149]. Microplates [150,151] have been employed to measure cellular deformation and elasticity in response to force.…”