Nuclear envelope proteins including the various lamins are expressed in all adult tissues, but lamins that are implicated in aging, cancer, and various dystrophies often affect specific tissues for unknown reasons. We have shown that the level of lamin-A,C and additional components of the nuclear envelope proteome that link the nucleus to the cytoskeleton scale systematically with tissue elasticity, while physical manipulation has demonstrated that nuclear stiffness scales with lamin-A,C. For example, brain tissue, which is relatively unaffected in the lamin-A,C-based aging disease progeria, has an elasticity about ten-fold softer than striated muscle with proportionately less lamin-A,C and a dominant amount of constitutive B-type lamins. Evidence suggests a mechano-sensitive regulation of the lamina, but the process of mechanical stimulation of molecular processes is poorly understood. Cysteine-shotgun mass spectrometry (CS-MS), a method capable of mapping the exposure of cysteine residues as proteins are stressed in complex biological systems such as isolated nuclear or whole cells, allowed us to identify stress sensitive proteins. We discovered a number of stress-sensitive proteins in isolated nuclei - including lamin-A,C - consistent with cell and tissue evidence that the nucleus transduces physical stress. Further work enquires whether substrate stiffnesses representative of soft and stiff tissue are reflected in changes to protein conformation.

Tissues can be soft like fat, which bears little stress, or stiff like bone, which sustains high stress, but whether there is a systematic relationship between tissue mechanics and differentiation is unknown. Here, proteomics analyses revealed that levels of the nucleoskeletal protein lamin-A scaled with tissue elasticity, E, as did levels of collagens in the extracellular matrix that determine E. Stem cell differentiation into fat on soft matrix was enhanced by low lamin-A levels, whereas differentiation into bone on stiff matrix was enhanced by high lamin-A levels. Matrix stiffness directly influenced lamin-A protein levels, and, although lamin-A transcription was regulated by the vitamin A/retinoic acid (RA) pathway with broad roles in development, nuclear entry of RA receptors was modulated by lamin-A protein. Tissue stiffness and stress thus increase lamin-A levels, which stabilize the nucleus while also contributing to lineage determination.

A solid tissue can be soft like fat or brain, stiff like striated muscle and heart, or rigid like bone. Proteomic profiling of tissue nuclei shows that Lamin-A/C expression increases more than 30-fold and in near-proportion to micro-elasticity of tissue, while other nuclear envelope components such as Lamin-B exhibit small variations. Lamin-A/C has been implicated in aging syndromes that affect muscle and fat but not brain, and we find nuclei in brain-derived cells are indeed dominated by Lamin-B and are much softer than nuclei derived from muscle cells with predominantly Lamin-A/C. In vitro, matrix elasticity can affect expression of nuclear envelope components in adult stem cells, and major changes in Lamin-A/C are indeed shown to direct lineage with lower levels favoring soft tissue and higher levels promoting rigid tissue lineage. At a molecular level, tagging of cryptic sites while physically stressing isolated nuclei reveals stress-driven, mass spectrometry-mapped changes in various nuclear proteins including Lamin-A/C, consistent with cell and tissue evidence that the nucleus transduces physical stress.