Lamins and Tissue Stiffness: Microenvironment can influence cell fate and behavior; for example, extracellular matrix (ECM) stiffness increases cell proliferation, and ECM rigidity induces disorders in tissue morphogenesis by increasing cell tension. Swift et al. ( 1240104 ; see the Perspective by Bainer and Weaver ) used proteomics to identify molecules that are mechanical sensors for tissue elasticity in the nucleus and discovered that expression of lamin-A levels apparently functions as a “mechanostat.”

Cell migration through solid tissue often involves large contortions of the nucleus, but biological significance is largely unclear. The nucleoskeletal protein lamin-A varies both within and between cell types and was shown here to contribute to cell sorting and survival in migration through constraining micropores. Lamin-A proved rate-limiting in 3D migration of diverse human cells that ranged from glioma and adenocarcinoma lines to primary mesenchymal stem cells (MSCs). Stoichiometry of A- to B-type lamins established an activation barrier, with high lamin-A:B producing extruded nuclear shapes after migration. Because the juxtaposed A and B polymer assemblies respectively conferred viscous and elastic stiffness to the nucleus, subpopulations with different A:B levels sorted in 3D migration. However, net migration was also biphasic in lamin-A, as wild-type lamin-A levels protected against stress-induced death, whereas deep knockdown caused broad defects in stress resistance. In vivo xenografts proved consistent with A:B-based cell sorting, and intermediate A:B-enhanced tumor growth. Lamins thus impede 3D migration but also promote survival against migration-induced stresses.

On rigid surfaces, the cytoskeleton of migrating cells is polarized, but tissue matrix is normally soft. We show that nonmuscle MIIB (myosin-IIB) is unpolarized in cells on soft matrix in 2D and also within soft 3D collagen, with rearward polarization of MIIB emerging only as cells migrate from soft to stiff matrix. Durotaxis is the tendency of cells to crawl from soft to stiff matrix, and durotaxis of primary mesenchymal stem cells (MSCs) proved more sensitive to MIIB than to the more abundant and persistently unpolarized nonmuscle MIIA (myosin-IIA). However, MIIA has a key upstream role: in cells on soft matrix, MIIA appeared diffuse and mobile, whereas on stiff matrix, MIIA was strongly assembled in oriented stress fibers that MIIB then polarized. The difference was caused in part by elevated phospho-S1943–MIIA in MSCs on soft matrix, with site-specific mutants revealing the importance of phosphomoderated assembly of MIIA. Polarization is thus shown to be a highly regulated compass for mechanosensitive migration.

Significance: Comparing human blood cell types, nuclear diversity is visually striking but unexplained: quasi-spherical nuclei in stem/progenitor cells and T cells contrast with multilobed nuclei in neutrophils, giant nuclei in megakaryocytes, and anuclear erythrocytes. We hypothesized broad roles for the major nuclear structure proteins—lamins—and developed mass spectrometry-calibrated intracellular flow cytometry to quantify lamin-A:B ratios. This ratio controls both nuclear viscoelasticity and cell trafficking across microporous barriers. High A:B rigidifies erythroblast nuclei to favor marrow retention and also enhances erythropoiesis. Intermediate A:B enhances thrombopoiesis and opposes cell division to favor marrow anchorage of megakaryocytes. Human stem/progenitor cells have moderate lamin levels and reside in marrow whereas white cells are favored by low lamins and predominantly circulate.