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1. The mitotic protein NuMA plays a spindle-independent role in nuclear formation and mechanics

   https://doi.org/10.1083/jcb.202004202  

   Serra-Marques, Andrea; Houtekamer, Ronja; Hintzen, Dorine; Canty, John T.; Yildiz, Ahmet; Dumont, Sophie (December 2020, Journal of Cell Biology)

   null (Ed.)

   Eukaryotic cells typically form a single, round nucleus after mitosis, and failures to do so can compromise genomic integrity. How mammalian cells form such a nucleus remains incompletely understood. NuMA is a spindle protein whose disruption results in nuclear fragmentation. What role NuMA plays in nuclear integrity, and whether its perceived role stems from its spindle function, are unclear. Here, we use live imaging to demonstrate that NuMA plays a spindle-independent role in forming a single, round nucleus. NuMA keeps the decondensing chromosome mass compact at mitotic exit and promotes a mechanically robust nucleus. NuMA’s C terminus binds DNA in vitro and chromosomes in interphase, while its coiled-coil acts as a central regulatory and structural element: it prevents NuMA from binding chromosomes at mitosis, regulates its nuclear mobility, and is essential for nuclear formation. Thus, NuMA plays a structural role over the cell cycle, building and maintaining the spindle and nucleus, two of the cell’s largest structures. 

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2. Ccp1-Ndc80 switch at the N terminus of CENP-T regulates kinetochore assembly

   https://doi.org/10.1073/pnas.2104459118  

   Dong, Qianhua; Liu, Xue-lei; Wang, Xiao-hui; Zhao, Yu; Chen, Yu-hang; Li, Fei (November 2021, Proceedings of the National Academy of Sciences)

   Kinetochores, a protein complex assembled on centromeres, mediate chromosome segregation. In most eukaryotes, centromeres are epigenetically specified by the histone H3 variant CENP-A. CENP-T, an inner kinetochore protein, serves as a platform for the assembly of the outer kinetochore Ndc80 complex during mitosis. How CENP-T is regulated through the cell cycle remains unclear. Ccp1 (counteracter of CENP-A loading protein 1) associates with centromeres during interphase but delocalizes from centromeres during mitosis. Here, we demonstrated that Ccp1 directly interacts with CENP-T. CENP-T is important for the association of Ccp1 with centromeres, whereas CENP-T centromeric localization depends on Mis16, a homolog of human RbAp48/46. We identified a Ccp1-interaction motif (CIM) at the N terminus of CENP-T, which is adjacent to the Ndc80 receptor motif. The CIM domain is required for Ccp1 centromeric localization, and the CIM domain–deleted mutant phenocopies ccp1 Δ. The CIM domain can be phosphorylated by CDK1 (cyclin-dependent kinase 1). Phosphorylation of CIM weakens its interaction with Ccp1. Consistent with this, Ccp1 dissociates from centromeres through all stages of the cell cycle in the phosphomimetic mutant of the CIM domain, whereas in the phospho-null mutant of the domain, Ccp1 associates with centromeres during mitosis. We further show that the phospho-null mutant disrupts the positioning of the Ndc80 complex during mitosis, resulting in chromosome missegregation. This work suggests that competitive exclusion between Ccp1 and Ndc80 at the N terminus of CENP-T via phosphorylation ensures precise kinetochore assembly during mitosis and uncovers a previously unrecognized mechanism underlying kinetochore assembly through the cell cycle. 

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3. Endoreduplication during embryogenesis in Megacyclops gigas (Crustacea: Cyclopoida): A rare example of depolyploidization

   Jordan, Kathryn; Sheveleva, Natalya; Hołyńska, Maria; Wyngaard, Grace A (February 2025, Invertebrate biology)

   Megacyclops gigas, a freshwater microcrustacean (Copepoda: Cyclopoida), undergoes endoreduplication during early embryogenesis, a process during which DNA synthesis during the cell cycle occurs without subsequent mitosis and cytokinesis. This kind of cell cycle, an endocycle, results in a doubling of the nuclear DNA content. By the 4-cell embryonic stage, two endocycles have occurred, increasing the initial 2C level of DNA at 7.5 pg DNA per nucleus to the 8C level at 30 pg DNA per nucleus. These 8C cells proliferate to produce additional cells at the 8C level in the 8-cell embryo. DNA contents were measured using Feulgen Image Analysis Densitometry. Re-entry into the typical 2C – 4C cell cycle has begun by the 6th cleavage division, with some cell lineages containing 2C amounts while others contain 4C amounts. This mixture of cells with different DNA contents persists until the ~ 500-cell stage. All adult somatic cells contain the 2C amount of DNA. Some half anaphase chromosome figures at the 64-cell stage contain ~ 5 or ~ 9 pg DNA per nucleus, indicative of aneuploidy resulting from imperfect mitoses. The large clutch sizes of M. gigas may compensate for such an error prone process. Endoreduplication of the kind found in M. gigas is rare in that (1) it involves endocycles in all cells of the organism in a particular developmental stage, rather than being limited to specific tissues and (2) the endoreduplicated cells undergo depolyploidization. Overall, genomic stability in the M. gigas population appears to be unaffected. We hypothesize that the role of endoreduplication in this species is associated with a combination of increased demands for cellular differentiation and protein transcription related to the unusually large body size of M. gigas and its habitat type characterized by extremely low temperatures. 

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4. Endoreduplication during embryogenesis in Megacyclops gigas (Crustacea: Cyclopoida): A rare example of depolyploidization

   Jordan, Kathryn; Sheveleva, Natalya; Hołyńska, Maria; Wyngaard, Grace A (January 2031, Invertebrate biology)

   Megacyclops gigas, a freshwater microcrustacean (Copepoda: Cyclopoida), undergoes endoreduplication during early embryogenesis, a process during which DNA synthesis during the cell cycle occurs without subsequent mitosis and cytokinesis. This kind of cell cycle, an endocycle, results in a doubling of the nuclear DNA content. By the 4-cell embryonic stage, two endocycles have occurred, increasing the initial 2C level of DNA at 7.5 pg DNA per nucleus to the 8C level at 30 pg DNA per nucleus. These 8C cells proliferate to produce additional cells at the 8C level in the 8-cell embryo. DNA contents were measured using Feulgen Image Analysis Densitometry. Re-entry into the typical 2C – 4C cell cycle has begun by the 6th cleavage division, with some cell lineages containing 2C amounts while others contain 4C amounts. This mixture of cells with different DNA contents persists until the ~ 500-cell stage. All adult somatic cells contain the 2C amount of DNA. Some half anaphase chromosome figures at the 64-cell stage contain ~ 5 or ~ 9 pg DNA per nucleus, indicative of aneuploidy resulting from imperfect mitoses. The large clutch sizes of M. gigas may compensate for such an error prone process. Endoreduplication of the kind found in M. gigas is rare in that (1) it involves endocycles in all cells of the organism in a particular developmental stage, rather than being limited to specific tissues and (2) the endoreduplicated cells undergo depolyploidization. Overall, genomic stability in the M. gigas population appears to be unaffected. We hypothesize that the role of endoreduplication in this species is associated with a combination of increased demands for cellular differentiation and protein transcription related to the unusually large body size of M. gigas and its habitat type characterized by extremely low temperatures. 

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5. Emergence of Escherichia coli critically buckled motile helices under stress

   https://doi.org/10.1073/pnas.1809374115  

   Phan, Trung V.; Morris, Ryan J.; Lam, Ho Tat; Hulamm, Phuson; Black, Matthew E.; Bos, Julia; Austin, Robert H. (November 2018, Proceedings of the National Academy of Sciences)

   Bacteria under external stress can reveal unexpected emergent phenotypes. We show that the intensely studied bacteriumEscherichia colican transform into long, highly motile helical filaments poized at a torsional buckling criticality when exposed to minimum inhibitory concentrations of several antibiotics. While the highly motile helices are physically either right- or left-handed, the motile helices always rotate with a right-handed angular velocity $\overrightarrow{\omega }$, which points in the same direction as the translational velocity ${\overrightarrow{v}}_T$of the helix. Furthermore, these helical cells do not swim by a “run and tumble” but rather synchronously flip their spin $\overrightarrow{\omega }$and thus translational velocity—backing up rather than tumbling. By increasing the translational persistence length, these dynamics give rise to an effective diffusion coefficient up to 20 times that of a normalE. colicell. Finally, we propose an evolutionary mechanism for this phenotype’s emergence whereby the increased effective diffusivity provides a fitness advantage in allowing filamentous cells to more readily escape regions of high external stress. 

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