Mechanics of the Cell Nucleus as a Function of Lamin Expression in Granulocyte Differentiation

The ability of cells to deform through narrow spaces is critical for processes ranging from immune function to metastasis. Neutrophils are the most abundant white blood cell, which are required to transit through spaces less than 1/5 of the cell's diameter. As the nucleus is typically the stiffest organelle in the cell, the lobulated shape of the neutrophil nucleus is thought to facilitate its transit. However, neither the mechanical properties of the nuclei, nor the mechanism underlying the transition from ovoid to lobulated nuclear shape, are fully understood. We used HL60 cells as a differentiable model system to study nuclear shape transitions and the effects on cell mechanics. To elucidate the effects of the nuclear envelope protein, lamin A, we genetically modified the cells to generate subpopulations of cells with well-defined lamin A levels. Quantitative image analysis revealed that increased lamin A expression inhibits the nuclear shape transition to the typical lobulated morphology. To determine the effect on whole-cell mechanics, we measured cell deformability by flowing cells through channels of a microfluidic device, monitoring transit time and nuclear deformation. In addition, we performed functional assays to test if the impaired transition in nuclear morphology is also associated with defects in phagocytotic function, and thus reflect overall impairment of differentiation due to increased lamin levels. These results help to elucidate the molecular mechanism of granulocyte differentiation, and may have possible implications for understanding reduced immune function in aging, where lamin A has been reported to accumulate at the nuclear envelope.