Limits...
A mechanism for nuclear positioning in fission yeast based on microtubule pushing.

Tran PT, Marsh L, Doye V, Inoué S, Chang F - J. Cell Biol. (2001)

Bottom Line: The MT bundles are organized from medial MT-organizing centers that may function as nuclear attachment sites.After an average of 1.5 min of growth at the cell tip, MT plus ends exhibit catastrophe and shrink back to the nuclear region before growing back to the cell tip.Computer modeling suggests that a balance of these pushing MT forces can provide a mechanism to position the nucleus at the middle of the cell.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology, Columbia University, New York, New York 10032, USA. pt143@columbia.edu

ABSTRACT
The correct positioning of the nucleus is often important in defining the spatial organization of the cell, for example, in determining the cell division plane. In interphase Schizosaccharomyces pombe cells, the nucleus is positioned in the middle of the cylindrical cell in an active microtubule (MT)-dependent process. Here, we used green fluorescent protein markers to examine the dynamics of MTs, spindle pole body, and the nuclear envelope in living cells. We find that interphase MTs are organized in three to four antiparallel MT bundles arranged along the long axis of the cell, with MT plus ends facing both the cell tips and minus ends near the middle of the cell. The MT bundles are organized from medial MT-organizing centers that may function as nuclear attachment sites. When MTs grow to the cell tips, they exert transient forces produced by plus end MT polymerization that push the nucleus. After an average of 1.5 min of growth at the cell tip, MT plus ends exhibit catastrophe and shrink back to the nuclear region before growing back to the cell tip. Computer modeling suggests that a balance of these pushing MT forces can provide a mechanism to position the nucleus at the middle of the cell.

Show MeSH
Nuclear positioning in a computer-modeled cell. (A) Position of computer-modeled cell nucleus after release from a cell end. Nuclear position was calculated by computer based on an algorithm in which nuclear movement was solely dependent on MT pushing forces using parameters seen in living cells (see Materials and Methods). For a 14-μm cell with one antiparallel MT pair, the position of the nucleus was recorded every 30 s for a modeled period of 1 h. (B) Distribution of observed nuclear positions with different MT arrangements. Nuclear position was recorded at 1-s intervals for 1 h for 10 modeled cells for each MT arrangement. The nucleus of each cell was initially at the center of the 14-μm cell. The sum of total observed occurrences of nuclei within specified spatial intervals is reported: (•) 1 left MT, 1 right MT; (○) 4 left MTs, 4 right MTs; (▪) 2 left MTs, 1 right MT.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2169469&req=5

Figure 9: Nuclear positioning in a computer-modeled cell. (A) Position of computer-modeled cell nucleus after release from a cell end. Nuclear position was calculated by computer based on an algorithm in which nuclear movement was solely dependent on MT pushing forces using parameters seen in living cells (see Materials and Methods). For a 14-μm cell with one antiparallel MT pair, the position of the nucleus was recorded every 30 s for a modeled period of 1 h. (B) Distribution of observed nuclear positions with different MT arrangements. Nuclear position was recorded at 1-s intervals for 1 h for 10 modeled cells for each MT arrangement. The nucleus of each cell was initially at the center of the 14-μm cell. The sum of total observed occurrences of nuclei within specified spatial intervals is reported: (•) 1 left MT, 1 right MT; (○) 4 left MTs, 4 right MTs; (▪) 2 left MTs, 1 right MT.

Mentions: Our experimental results suggested a model for nuclear positioning based on simple MT pushing forces. To test if this simple model is sufficient to explain proper centering of the nucleus, we generated an iterative algorithm of this process (see Materials and Methods). The inputs were parameters of MT dynamics and organization as measured in this paper, with the nucleus having one or more bundle of leftward and rightward pairs of dynamic MTs attached to and pushing on the nucleus; the output from the algorithm was nuclear position. In the computer simulation, dynamic MTs were capable of centering an offset nucleus by a pushing mechanism. Starting at the left tip of the cell at time 0, a nucleus with one MT bundle moved towards a medial position after ∼10 min and then oscillated around the medial position (Fig. 9 A). In multiple simulations, 50% of nuclei were located within 10% of the cell length away from the medial position (Fig. 9 B). We also tested various parameters using this computer model. If the nucleus was attached to four MT bundles, the nucleus was more stable and exhibited less oscillatory deviation than a nucleus with one attached MT bundle (Fig. 9 B). To test if symmetry of MTs was important for centering, nuclei with two MTs on the left and one MT on the right were tested. The average positions of nuclei with asymmetric MTs were skewed toward the side with only one MT (Fig. 9 B). Thus, the best nuclear centering resulted from multiple attached MTs with equal numbers of MTs on each side of the nucleus. The amplitude of oscillation and deviation was greater in the computer simulation than observed in vivo, suggesting that additional forces in the cell, such as viscosity or membrane tension, may also exert effects on nuclear positioning. Addition of a conservative term for viscosity further improved centering (Marsh, L., unpublished observations). Nevertheless, these results show that a simple mechanism based upon MT pushing is sufficient to center the nucleus.


A mechanism for nuclear positioning in fission yeast based on microtubule pushing.

Tran PT, Marsh L, Doye V, Inoué S, Chang F - J. Cell Biol. (2001)

Nuclear positioning in a computer-modeled cell. (A) Position of computer-modeled cell nucleus after release from a cell end. Nuclear position was calculated by computer based on an algorithm in which nuclear movement was solely dependent on MT pushing forces using parameters seen in living cells (see Materials and Methods). For a 14-μm cell with one antiparallel MT pair, the position of the nucleus was recorded every 30 s for a modeled period of 1 h. (B) Distribution of observed nuclear positions with different MT arrangements. Nuclear position was recorded at 1-s intervals for 1 h for 10 modeled cells for each MT arrangement. The nucleus of each cell was initially at the center of the 14-μm cell. The sum of total observed occurrences of nuclei within specified spatial intervals is reported: (•) 1 left MT, 1 right MT; (○) 4 left MTs, 4 right MTs; (▪) 2 left MTs, 1 right MT.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2169469&req=5

Figure 9: Nuclear positioning in a computer-modeled cell. (A) Position of computer-modeled cell nucleus after release from a cell end. Nuclear position was calculated by computer based on an algorithm in which nuclear movement was solely dependent on MT pushing forces using parameters seen in living cells (see Materials and Methods). For a 14-μm cell with one antiparallel MT pair, the position of the nucleus was recorded every 30 s for a modeled period of 1 h. (B) Distribution of observed nuclear positions with different MT arrangements. Nuclear position was recorded at 1-s intervals for 1 h for 10 modeled cells for each MT arrangement. The nucleus of each cell was initially at the center of the 14-μm cell. The sum of total observed occurrences of nuclei within specified spatial intervals is reported: (•) 1 left MT, 1 right MT; (○) 4 left MTs, 4 right MTs; (▪) 2 left MTs, 1 right MT.
Mentions: Our experimental results suggested a model for nuclear positioning based on simple MT pushing forces. To test if this simple model is sufficient to explain proper centering of the nucleus, we generated an iterative algorithm of this process (see Materials and Methods). The inputs were parameters of MT dynamics and organization as measured in this paper, with the nucleus having one or more bundle of leftward and rightward pairs of dynamic MTs attached to and pushing on the nucleus; the output from the algorithm was nuclear position. In the computer simulation, dynamic MTs were capable of centering an offset nucleus by a pushing mechanism. Starting at the left tip of the cell at time 0, a nucleus with one MT bundle moved towards a medial position after ∼10 min and then oscillated around the medial position (Fig. 9 A). In multiple simulations, 50% of nuclei were located within 10% of the cell length away from the medial position (Fig. 9 B). We also tested various parameters using this computer model. If the nucleus was attached to four MT bundles, the nucleus was more stable and exhibited less oscillatory deviation than a nucleus with one attached MT bundle (Fig. 9 B). To test if symmetry of MTs was important for centering, nuclei with two MTs on the left and one MT on the right were tested. The average positions of nuclei with asymmetric MTs were skewed toward the side with only one MT (Fig. 9 B). Thus, the best nuclear centering resulted from multiple attached MTs with equal numbers of MTs on each side of the nucleus. The amplitude of oscillation and deviation was greater in the computer simulation than observed in vivo, suggesting that additional forces in the cell, such as viscosity or membrane tension, may also exert effects on nuclear positioning. Addition of a conservative term for viscosity further improved centering (Marsh, L., unpublished observations). Nevertheless, these results show that a simple mechanism based upon MT pushing is sufficient to center the nucleus.

Bottom Line: The MT bundles are organized from medial MT-organizing centers that may function as nuclear attachment sites.After an average of 1.5 min of growth at the cell tip, MT plus ends exhibit catastrophe and shrink back to the nuclear region before growing back to the cell tip.Computer modeling suggests that a balance of these pushing MT forces can provide a mechanism to position the nucleus at the middle of the cell.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology, Columbia University, New York, New York 10032, USA. pt143@columbia.edu

ABSTRACT
The correct positioning of the nucleus is often important in defining the spatial organization of the cell, for example, in determining the cell division plane. In interphase Schizosaccharomyces pombe cells, the nucleus is positioned in the middle of the cylindrical cell in an active microtubule (MT)-dependent process. Here, we used green fluorescent protein markers to examine the dynamics of MTs, spindle pole body, and the nuclear envelope in living cells. We find that interphase MTs are organized in three to four antiparallel MT bundles arranged along the long axis of the cell, with MT plus ends facing both the cell tips and minus ends near the middle of the cell. The MT bundles are organized from medial MT-organizing centers that may function as nuclear attachment sites. When MTs grow to the cell tips, they exert transient forces produced by plus end MT polymerization that push the nucleus. After an average of 1.5 min of growth at the cell tip, MT plus ends exhibit catastrophe and shrink back to the nuclear region before growing back to the cell tip. Computer modeling suggests that a balance of these pushing MT forces can provide a mechanism to position the nucleus at the middle of the cell.

Show MeSH