Limits...
Thrombopoietin-induced polyploidization of bone marrow megakaryocytes is due to a unique regulatory mechanism in late mitosis.

Nagata Y, Muro Y, Todokoro K - J. Cell Biol. (1997)

Bottom Line: It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication.We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B.Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis.

View Article: PubMed Central - PubMed

Affiliation: Tsukuba Life Science Center, The Institute of Physical and Chemical Research (RIKEN), Tsukuba, Ibaraki 305, Japan.

ABSTRACT
Megakaryocytes undergo a unique differentiation program, becoming polyploid through repeated cycles of DNA synthesis without concomitant cell division. However, the mechanism underlying this polyploidization remains totally unknown. It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication. We carried out immunohistochemical studies on mouse bone marrow megakaryocytes during thrombopoietin- induced polyploidization and found that during this process megakaryocytes indeed enter mitosis and progress through normal prophase, prometaphase, metaphase, and up to anaphase A, but not to anaphase B, telophase, or cytokinesis. It was clearly observed that multiple spindle poles were formed as the polyploid megakaryocytes entered mitosis; the nuclear membrane broke down during prophase; the sister chromatids were aligned on a multifaced plate, and the centrosomes were symmetrically located on either side of each face of the plate at metaphase; and a set of sister chromatids moved into the multiple centrosomes during anaphase A. We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B. Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis. These observations clearly indicate that polyploidization of megakaryocytes is not simply due to a skipping of mitosis, and that the megakaryocytes must have a unique regulatory mechanism in anaphase, e.g., factors regulating anaphase such as microtubule motor proteins might be involved in this polyploidization process.

Show MeSH
Nuclear membrane is broken down during polyploidization of megakaryocytes. Megakaryocytes in interphase (A) or in mitosis (B) were stained with DAPI (a), or probed with anti–α-tubulin antibody (b) or anti-RanBP2 antibody (anti-551) (c), followed by incubation with an FITC-labeled F(ab′)2 fragment (b) or a Cy3-conjugated F(ab′)2 fragment (c). d shows triple stainings of the same cells.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2139799&req=5

Figure 2: Nuclear membrane is broken down during polyploidization of megakaryocytes. Megakaryocytes in interphase (A) or in mitosis (B) were stained with DAPI (a), or probed with anti–α-tubulin antibody (b) or anti-RanBP2 antibody (anti-551) (c), followed by incubation with an FITC-labeled F(ab′)2 fragment (b) or a Cy3-conjugated F(ab′)2 fragment (c). d shows triple stainings of the same cells.

Mentions: As described above, megakaryocytes were found to enter mitosis. In normal mammalian cells, concomitant with this entry into mitosis, the nuclear envelope breaks down and disappears, and then upon exit from mitosis, the nuclear envelope reassembles to form the nucleus. It has been postulated, however, that polyploidization was caused by the skipping of mitosis after each round of DNA replication (Long, M.W. 1993. 8th Symposium of Molecular Biology of Hematopoiesis. 196; Datta et al., 1996), and thus polyploidization has been thought to occur within an intact nuclear envelope. Whether or not the nuclear membrane breaks down as megakaryocytes polyploidize has never been examined. We therefore stained the nuclear membrane in megakaryocytes with anti-RanBP2 antibody (anti-551). RanBP2 is a nuclear pore complex protein, and thus anti-551 antibody that specifically recognizes RanBP2 can clearly stain the nuclear envelopes as described (Yokoyama et al., 1995). As shown in Fig. 2 A, c, anti-551 antibody staining clearly showed a lobulated nuclear surface of a megakaryocyte in interphase. The whole cell including cytoplasm, however, was stained with anti-551 antibody in a mitotic megakaryocyte forming eight mitotic spindle poles (Fig. 2 B, c), indicating that the nuclear membrane was broken down as the megakaryocyte entered mitosis and was reassembled in interphase. These observations clearly indicate that polyploidization of megakaryocytes is not simply due to the skipping of mitosis and that it does not occur within an intact nuclear envelope.


Thrombopoietin-induced polyploidization of bone marrow megakaryocytes is due to a unique regulatory mechanism in late mitosis.

Nagata Y, Muro Y, Todokoro K - J. Cell Biol. (1997)

Nuclear membrane is broken down during polyploidization of megakaryocytes. Megakaryocytes in interphase (A) or in mitosis (B) were stained with DAPI (a), or probed with anti–α-tubulin antibody (b) or anti-RanBP2 antibody (anti-551) (c), followed by incubation with an FITC-labeled F(ab′)2 fragment (b) or a Cy3-conjugated F(ab′)2 fragment (c). d shows triple stainings of the same cells.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2139799&req=5

Figure 2: Nuclear membrane is broken down during polyploidization of megakaryocytes. Megakaryocytes in interphase (A) or in mitosis (B) were stained with DAPI (a), or probed with anti–α-tubulin antibody (b) or anti-RanBP2 antibody (anti-551) (c), followed by incubation with an FITC-labeled F(ab′)2 fragment (b) or a Cy3-conjugated F(ab′)2 fragment (c). d shows triple stainings of the same cells.
Mentions: As described above, megakaryocytes were found to enter mitosis. In normal mammalian cells, concomitant with this entry into mitosis, the nuclear envelope breaks down and disappears, and then upon exit from mitosis, the nuclear envelope reassembles to form the nucleus. It has been postulated, however, that polyploidization was caused by the skipping of mitosis after each round of DNA replication (Long, M.W. 1993. 8th Symposium of Molecular Biology of Hematopoiesis. 196; Datta et al., 1996), and thus polyploidization has been thought to occur within an intact nuclear envelope. Whether or not the nuclear membrane breaks down as megakaryocytes polyploidize has never been examined. We therefore stained the nuclear membrane in megakaryocytes with anti-RanBP2 antibody (anti-551). RanBP2 is a nuclear pore complex protein, and thus anti-551 antibody that specifically recognizes RanBP2 can clearly stain the nuclear envelopes as described (Yokoyama et al., 1995). As shown in Fig. 2 A, c, anti-551 antibody staining clearly showed a lobulated nuclear surface of a megakaryocyte in interphase. The whole cell including cytoplasm, however, was stained with anti-551 antibody in a mitotic megakaryocyte forming eight mitotic spindle poles (Fig. 2 B, c), indicating that the nuclear membrane was broken down as the megakaryocyte entered mitosis and was reassembled in interphase. These observations clearly indicate that polyploidization of megakaryocytes is not simply due to the skipping of mitosis and that it does not occur within an intact nuclear envelope.

Bottom Line: It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication.We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B.Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis.

View Article: PubMed Central - PubMed

Affiliation: Tsukuba Life Science Center, The Institute of Physical and Chemical Research (RIKEN), Tsukuba, Ibaraki 305, Japan.

ABSTRACT
Megakaryocytes undergo a unique differentiation program, becoming polyploid through repeated cycles of DNA synthesis without concomitant cell division. However, the mechanism underlying this polyploidization remains totally unknown. It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication. We carried out immunohistochemical studies on mouse bone marrow megakaryocytes during thrombopoietin- induced polyploidization and found that during this process megakaryocytes indeed enter mitosis and progress through normal prophase, prometaphase, metaphase, and up to anaphase A, but not to anaphase B, telophase, or cytokinesis. It was clearly observed that multiple spindle poles were formed as the polyploid megakaryocytes entered mitosis; the nuclear membrane broke down during prophase; the sister chromatids were aligned on a multifaced plate, and the centrosomes were symmetrically located on either side of each face of the plate at metaphase; and a set of sister chromatids moved into the multiple centrosomes during anaphase A. We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B. Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis. These observations clearly indicate that polyploidization of megakaryocytes is not simply due to a skipping of mitosis, and that the megakaryocytes must have a unique regulatory mechanism in anaphase, e.g., factors regulating anaphase such as microtubule motor proteins might be involved in this polyploidization process.

Show MeSH