Nuclear Envelope Permeability Barrier as a Fast-Response Intracellular Mechanostat

Understanding the role of mechanics in nucleocytoplasmic transport

Cell nuclei are submitted to mechanical forces, which in turn affect nuclear and cell functions. Recent evidence shows that a crucial mechanically regulated nuclear function is nucleocytoplasmic transport, mediated by nuclear pore complexes (NPCs). Mechanical regulation occurs at two levels: first, by force application to the nucleus, which increases NPC permeability likely through NPC stretch. Second, by the mechanical properties of the transported proteins themselves, as mechanically labile proteins translocate through NPCs faster than mechanically stiff ones. In this perspective, we discuss this evidence and the associated mechanisms by which mechanics can regulate the nucleo-cytoplasmic partitioning of proteins. Finally, we analyze how mechanical regulation of nucleocytoplasmic transport can provide a systematic approach to the study of mechanobiology and open new avenues both in fundamental and applied research.

Biophysical Approaches for Applying and Measuring Biological Forces

Over the past decades, increasing evidence has indicated that mechanical loads can regulate the morphogenesis, proliferation, migration, and apoptosis of living cells. Investigations of how cells sense mechanical stimuli or the mechanotransduction mechanism is an active field of biomaterials and biophysics. Gaining a further understanding of mechanical regulation and depicting the mechanotransduction network inside cells require advanced experimental techniques and new theories. In this review, the fundamental principles of various experimental approaches that have been developed to characterize various types and magnitudes of forces experienced at the cellular and subcellular levels are summarized. The broad applications of these techniques are introduced with an emphasis on the difficulties in implementing these techniques in special biological systems. The advantages and disadvantages of each technique are discussed, which can guide readers to choose the most suitable technique for their questions. A perspective on future directions in this field is also provided. It is anticipated that technical advancement can be a driving force for the development of mechanobiology.

Atomic Force Microscopy for Structural and Biophysical Investigations on Nuclear Pore Complexes

Atomic force microscopy (AFM) enables simultaneous generation of topographical and biophysical maps of surfaces of biological samples at nanoresolution in physiologically relevant environments. Here, we describe the application of AFM to study nuclear pore complexes from structural and biophysical aspects.

Impact of the Nuclear Envelope on Malignant Transformation, Motility, and Survival of Lung Cancer Cells

Nuclear pore complexes (NPCs) selectively mediate all nucleocytoplasmic transport and engage in fundamental cell-physiological processes. It is hypothesized that NPCs are critical for malignant transformation and survival of lung cancer cells, and test the hypothesis in lowly and highly metastatic non-small human lung cancer cells (NSCLCs). It is shown that malignant transformation is paralleled by an increased NPCs density, and a balanced pathological weakening of the physiological stringency of the NPC barrier. Pharmacological interference using barrier-breaking compounds collapses the stringency. Concomitantly, it induces drastic overall structural changes of NSCLCs, terminating their migration. Moreover, the degree of malignancy is found to be paralleled by substantially decreased lamin A/C levels. The latter provides crucial structural and mechanical stability to the nucleus, and interacts with NPCs, cytoskeleton, and nucleoskeleton for cell maintenance, survival, and motility. The recent study reveals the physiological importance of the NPC barrier stringency for mechanical and structural resilience of normal cell nuclei. Hence, reduced lamin A/C levels in conjunction with controlled pathological weakening of the NPC barrier stringency may facilitate deformability of NSCLCs during the metastasis steps. Modulation of the NPC barrier presents a potential strategy for suppressing the malignant phenotype or enhancing the effectiveness of currently existing chemotherapeutics.

Nanomechanical properties of enucleated cells: contribution of the nucleus to the passive cell mechanics

Background

The nucleus, besides its functions in the gene maintenance and regulation, plays a significant role in the cell mechanosensitivity and mechanotransduction. It is the largest cellular organelle that is often considered as the stiffest cell part as well. Interestingly, the previous studies have revealed that the nucleus might be dispensable for some of the cell properties, like polarization and 1D and 2D migration. Here, we studied how the nanomechanical properties of cells, as measured using nanomechanical mapping by atomic force microscopy (AFM), were affected by the removal of the nucleus.

Methods

The mass enucleation procedure was employed to obtain cytoplasts (enucleated cells) and nucleoplasts (nuclei surrounded by plasma membrane) of two cell lines, REF52 fibroblasts and HT1080 fibrosarcoma cells. High-resolution viscoelastic mapping by AFM was performed to compare the mechanical properties of normal cells, cytoplasts, and nucleoplast. The absence or presence of the nucleus was confirmed with fluorescence microscopy, and the actin cytoskeleton structure was assessed with confocal microscopy.

Results

Surprisingly, we did not find the softening of cytoplasts relative to normal cells, and even some degree of stiffening was discovered. Nucleoplasts, as well as the nuclei isolated from cells using a detergent, were substantially softer than both the cytoplasts and normal cells.

Conclusions

The cell can maintain its mechanical properties without the nucleus. Together, the obtained data indicate the dominating role of the actomyosin cytoskeleton over the nucleus in the cell mechanics at small deformations inflicted by AFM.