The Neutrophil Nucleus: An Important Influence on Neutrophil Migration and Function

Abstract

Neutrophil nuclear morphology has historically been used in haematology for neutrophil identification and characterisation, but its exact role in neutrophil function has remained enigmatic. During maturation, segmentation of the neutrophil nucleus into its mature, multi-lobulated shape is accompanied by distinct changes in nuclear envelope composition, resulting in a unique nucleus that is believed to be imbued with extraordinary nuclear flexibility. As a rate-limiting factor for cell migration, nuclear morphology and biomechanics are particularly important in the context of neutrophil migration during immune responses. Being an extremely plastic and fast migrating cell type, it is to be expected that neutrophils have an especially deformable nucleus. However, many questions still surround the dynamic capacities of the neutrophil nucleus, and which nuclear and cytoskeletal elements determine these dynamics. The biomechanics of the neutrophil nucleus should also be considered for their influences on the production of neutrophil extracellular traps (NETs), given this process sees the release of chromatin "nets" from nucleoplasm to extracellular space. Although past studies have investigated neutrophil nuclear composition and shape, in a new era of more sophisticated biomechanical and genetic techniques, 3D migration studies, and higher resolution microscopy we now have the ability to further investigate and understand neutrophil nuclear plasticity at an unprecedented level. This review addresses what is currently understood about neutrophil nuclear structure and its role in migration and the release of NETs, whilst highlighting open questions surrounding neutrophil nuclear dynamics.

Keywords: NETs; lamin B receptor; lamins; leukocytes; migration; neutrophil extracellular traps; neutrophils; nucleus.

Figure 1

Neutrophil nuclear envelope composition. (A) A typical nuclear envelope comprises of the nuclear membrane bilipid layer (brown), which is embedded with membrane proteins like the LINC complex (yellow) and Lamin B Receptor (orange), and with nuclear pore complexes (blue). External to the nuclear membrane, the nuclear envelope interacts with the cytoskeleton (red). Directly beneath the inner nuclear membrane lies the nuclear lamina, a structual mesh formed of LaminA/C (pink) and B-type lamins (green). The lamina interacts with compact heterochromatin (purple). For simplicity, many nuclear membrane proteins are not shown, and LaminB2 and LaminB1 are considered together. (B) The nuclear envelope of mature neutrophils has very low levels of LaminA/C and LINC, but increased Lamin B receptor and peripheral heterochromatin, and relatively high levels of LaminB2.

Figure 2

Neutrophil nuclear dynamics during transmigration. When undergoing transmigration through the endothelium (brown) neutrophils undergo extreme cellular and nuclear deformation. Different components of the cell and nucleus are believed to play roles in mechanically enabling this process, at the rear uropod, constriction point, and front of the cell toward the leading edge. Some open questions in the field remain, but the consensus is that force generation and rear myosin-mediated contractility act to push the nucleus from behind, propelling the cell forward in concert with actin polymerisation at its leading edge.

Figure 3

Lamin and lamin B receptor expression in neutrophils related to neutrophil nuclear morphology. Changes in the expression of lamins and the lamin B receptor (LBR) during the transition from promyelocyte to mature neutrophil occurs in tandem with increasingly lobulated nuclear shape. This multi-lobulated nuclear shape is conserved across species. The specific roles of lamin or LBR in determining nuclear morphology have been assessed functionally only in the context of LaminB1 over-expression (hyper-lobulation), LaminA over-expression (hypo-lobulation), and LBR depletion or Pelger-Huët anomaly (PHA, hypo-lobulation).

Figure 4

Nuclear changes during NETosis. Prior to NETosis, a neutrophil carries a typical multi-lobulated nuclear envelope (dark purple), with distinct euchromatin (pink) and heterochromatin (yellow). Upon stimulation toward NETosis, the neutrophil rounds up, then adheres to the endothelium (brown) where chemotaxis is arrested, and the nuclear envelope begins to dilate and round. (A) For suicidal NETosis, nuclear vesicular budding begins early after stimulation, leading to nuclear envelope breakdown and chromatin decondensation. Decondensed chromatin (blue) swells and is no longer distinguishable as eu- or hetero-chromatin. The nuclear envelope completely breaks down, and decondensed chromatin fills the cytoplasm. NET release occurs via cell lysis as cell membrane (grey) ruptures, ultimately resulting in neutrophil cell death. (B) For ‘vital' NETosis, initially the nuclear envelope remains intact, with some release of nuclear vesicles containing DNA material. Nuclear chromatin condenses, and is no longer eu- or hetero-chromatin. DNA-containing vesicles fuse with the cell membrane, and NETs are released as these vesicles lyse in the extracellular space. As NET release continues via nuclear budding, nuclear chromatin decondenses and detaches, and the nuclear envelope breaks down. Completion of nuclear breakdown sees the post-NETosis neutrophil remain viable and functional as a cytoplast with some DNA material remaining in the cytoplasm.