Search

The interphase nucleus and nuclear envelope can acquire a myriad of shapes in normal or pathological cell states. There exist a wide variety of indentations and invaginations, of protrusions and evaginations. It has been difficult to classify and name all of these nuclear shapes and, consequently, a barrier to understanding the biochemical and biophysical causes. This review focuses upon one type of nuclear envelope shape change, named "nuclear envelope-limited chromatin sheets" (ELCS), which appears to involve exaggerated nuclear envelope growth, carrying with it one or more layers of approximately 30 nm diameter heterochromatin. A hypothesis on the formation of ELCS is proposed, relating higher order heterochromatin structure in an interphase nucleus, nuclear envelope growth, and nuclear envelope-heterochromatin interactions.

The human blood granulocyte (neutrophil) is adapted to find and destroy infectious agents. The nucleus of the human neutrophil has a segmented appearance, consisting of a linear or branched array of three or four lobes. Adequate levels of lamin B receptor (LBR) are necessary for differentiation of the lobulated nucleus. The levels of other components of the nuclear envelope may also be important for nuclear shape determination. In the present study, immunostaining and immunoblotting procedures explored the levels of various components of the nuclear envelope and heterochromatin, comparing freshly isolated human neutrophils with granulocytic forms of HL-60 cells, a tissue culture model system. In comparison to granulocytic HL-60 cells, blood neutrophil nuclear envelopes contain low-to-negligible amounts of LBR, lamins A/C, B1 and B2, LAP2beta and emerin. Surprisingly, a "mitotic" chromosome marker, H3(S10)phos, is elevated in neutrophil nuclei, compared to granulocytic HL-60 cells. Furthermore, neutrophil nuclei appear to be more fragile to methanol fixation, than observed with granulocytic HL-60 cells. Thus, the human neutrophil nucleus appears to be highly specialized, possessing a paucity of nuclear envelope-stabilizing proteins. In consequence, the neutrophil nucleus appears to be very malleable, supporting rapid migration through tight tissue spaces.

Background: Mammalian blood neutrophilic granulocytes are terminally differentiated cells, possessing extensive heterochromatin and lobulated (or ring-shaped) nuclei. Despite the extensive amount of heterochromatin, neutrophils are capable of increased gene expression, when activated by bacterial infection. Understanding the mechanisms of transcriptional repression and activation in neutrophils requires detailing the chromatin epigenetic markers, which are virtually undescribed in this cell type. Much is known about the heterochromatin epigenetic markers in other cell-types, permitting a basis for comparison with those of mature normal neutrophilic granulocytes., Results: Immunostaining and immunoblotting procedures were employed to study the presence of repressive histone modifications and HP1 proteins in normal human and mouse blood neutrophils, and in vitro differentiated granulocytes of the mouse promyelocytic (MPRO) system. A variety of repressive histone methylation markers were detectable in these granulocytes (di- and trimethylated H3K9; mono-, di- and trimethyl H3K27; di- and trimethyl H4K20). However, a paucity of HP1 proteins was noted. These granulocytes revealed negligible amounts of HP1 alpha and beta, but exhibited detectable levels of HP1 gamma. Of particular interest, mouse blood and MPRO undifferentiated cells and granulocytes revealed clear co-localization of trimethylated H3K9, trimethylated H4K20 and HP1 gamma with pericentric heterochromatin., Conclusion: Mature blood neutrophils possess some epigenetic heterochromatin features that resemble those of well-studied cells, such as lymphocytes. However, the apparent paucity of HP1 proteins in neutrophils suggests that heterochromatin organization and binding to the nuclear envelope may differ in this cell-type. Future investigations should follow changes in epigenetic markers and levels of HP1 proteins during granulopoiesis and bacterial activation of neutrophils.

Abstract: Mature blood neutrophils (polymorphonuclear granulocytes) have characteristically complex nuclear shapes. The human neutrophil nucleus generally possesses 3–4 lobes; the mouse neutrophil nucleus frequently resembles a twisted toroid with a central hole. Myeloid tissue culture systems (e.g., human HL-60 and murine MPRO) can be induced to differentiate in vitro towards neutrophils by addition of retinoic acid, exhibiting the characteristic nuclear shape changes. Confocal immunostaining and thin-section transmission electron microscopic image data from differentiated HL-60 and MPRO cells clearly demonstrate proximity of the centrosomal region (containing dynein, γ-tubulin and C-Nap1) to regions of granulocytic nuclear indentations. In addition, the centrosomal region, flanked by the Golgi apparatus, is shown to be present within the central hole of the toroidal mouse granulocyte nucleus. A role for the centrosomal region and associated microtubules in molding granulocytic nuclear shape is suggested. [Copyright &y& Elsevier]

Background: During granulopoiesis in the bone marrow, the nucleus differentiates from ovoid to lobulated shape. Addition of retinoic acid (RA) to leukemic HL-60 cells induces development of lobulated nuclei, furnishing a convenient model system for nuclear differentiation during granulopoiesis. Previous studies from our laboratory have implicated nuclear envelope composition as playing important roles in nuclear shape changes. Specifically noted were: 1) a paucity of lamins A/C and B1 in the undifferentiated and RA treated cell forms; 2) an elevation of lamin B receptor (LBR) during induced granulopoiesis. Results: The present study demonstrates that perturbation of cytoskeletal elements influences nuclear differentiation of HL-60 cells. Because of cytotoxicity from prolonged exposure to cytoskeleton-modifying drugs, most studies were performed with a Bcl-2 overexpressing HL-60 subline. We have found that: 1) nocodazole prevents RA induction of lobulation; 2) taxol induces lobulation and micronuclear formation, even in the absence of RA; 3) cytochalasin D does not inhibit RA induced nuclear lobulation, and prolonged exposure induces nuclear shape changes in the absence of RA. Conclusions: The present results, in the context of earlier data and models, suggest a mechanism for granulocytic nuclear lobulation. Our current hypothesis is that the nuclear shape change involves factors that increase the flexibility of the nuclear envelope (reduced lamin content), augment connections to the underlying heterochromatin (increased levels of LBR) and promote distortions imposed by the cytoskeleton (microtubule motors creating tension in the nuclear envelope). [ABSTRACT FROM AUTHOR]