Critical Roles of Α-Type Lamins in Neutrophil Differentiation Revealed By Analyses of Lmna Trancriptional Regulation and Myeloid Models with Aberrant Lamin Α Expression

Neutrophils differentiate from common myeloid progenitors in the bone marrow where they form uniquely lobulated nuclei while acquiring multiple functions critical to their innate immune responses. We are investigating the roles of nuclear envelope (NE)-associated proteins, including the lamin B receptor (Lbr) and the nuclear lamina proteins lamin A/C, in regulating these nuclear changes, along with determining if these proteins are important to neutrophil functions including chemotaxis, phagocytosis, or reactive oxygen species (ROS) production. We previously reported that Lbr expression increases during neutrophil differentiation while lamin A/C expression decreases. We also used immortalized mouse myeloid progenitors to show that either loss of Lbr or increased lamin A/C expression caused nuclear hypolobulation similar to Pelger-Huët anomaly in humans, plus aberrant chemotaxis. Interestingly, cells lacking Lbr produced less ROS, whereas those overexpressing the lamins exhibited deficient phagocytosis. These studies suggest that the functions of these nuclear proteins go beyond simply supporting nuclear structural changes. Since our previous studies relied on the use of immortalized cell lines, which may not exhibit the full spectrum of normal neutrophil functional responses, we have focused on using techniques to derive and characterize neutrophils with aberrant NE protein expression from ex vivo cultured bone marrow. We also examined whether loss of both A-type lamins affects myeloid progenitor growth and differentiation.

Here we show that functionally active neutrophils can be generated by ex vivo culture of bone marrow progenitors from cryopreserved stocks. Imaging flow cytometry, which combines the spatial resolution of fluorescence microscopy with the high throughput format of flow cytometry, was used to demonstrate that the ex vivo cultures at progressive stages of neutrophil maturation exhibit characteristic changes to cell surface marker expression. Customized gating and masking strategies were applied using the image analysis software to quantitatively measure phagocytosis of Escherichia coli particles and release of neutrophil extracellular traps (NETs). We also identified a method to evaluate NET release as a function of cell activation during phagocytosis.

We next transduced our ex vivo cultured progenitors with retroviral vectors providing ectopic lamin A expression, and found that derived neutrophils exhibited hypolobulated nuclei morphologically resembling our in vitro models. We also knocked out Lmna expression in the immortalized myeloid progenitors by CRISPR-Cas9 mediated gene disruption, and identified defective growth of the progenitors plus aberrant ROS production in derived neutrophils, similar to the phenotypes observed in cells lacking Lbr.

The transcriptional control of the Lmna gene during neutrophil differentiation also was investigated. In contrast to our previous report showing that the ETS factor GABP cooperates with PU.1 or C/EBPε to regulate the Lbr promoter, none of these factors affected background levels of Lmna promoter activities in COS cell reporter assays despite identified ETS and C/EBP consensus binding sites. However, addition of C/EBPα expression caused a greater than 50% decrease in Lmna promoter activities. Furthermore, quantitative ChIP assays demonstrated that C/EBPα most efficiently bound to identified C/EBP binding sequences in the Lmna promoter, indicating that C/EBPα inhibits Lmna expression in differentiating neutrophils.

Combined our results demonstrate that precisely regulated levels of NE-associated proteins are critical to neutrophil nuclear maturation and their acquisition of certain functional responses. Our continued studies will utilize imaging flow cytometry to characterize the functions of ex vivo culture-derived neutrophils with aberrant A-type lamin expression, as well as identify roles of additional NE-associated proteins during neutrophil maturation. We are also poised to further characterize the control of genes encoding such NE proteins by various combinations of ETS and C/EBP transcription factors known to regulate myelopoiesis. We predict these studies will provide important insight into the molecular mechanisms governing neutrophil nuclear changes, and why aberrant lobulation is commonly associated with certain hematopoietic disorders.