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1. Membrane stretching activates calcium-permeability of a putative channel Pkd2 during fission yeast cytokinesis

   https://doi.org/10.1091/mbc.E22-07-0248  

   Poddar, Abhishek; Hsu, Yen-Yu; Zhang, Faith; Shamma, Abeda; Kreais, Zachary; Muller, Clare; Malla, Mamata; Ray, Aniruddha; Liu, Allen; Chen, Qian (December 2022, Molecular Biology of the Cell)

   Martin, Sophie G (Ed.)

   Pkd2 is the fission yeast homolog of polycystins. This putative ion channel localizes to the plasma membrane. It is required for the expansion of cell volume during interphase growth and cytokinesis, the last step of cell division. However, the channel activity of Pkd2 remains untested. Here, we examined the calcium permeability and mechanosensitivity of Pkd2 through in vitro reconstitution and calcium imaging of the pkd2 mutant cells. Pkd2 was translated and inserted into the lipid bilayer of giant unilamellar vesicles using a cell-free expression system. The reconstituted Pkd2 permeated calcium when the membrane was stretched via hypo-osmotic shock. In vivo, inactivation of Pkd2 through a temperature-sensitive mutation pkd2-B42 reduced the average intracellular calcium level by 34%. Compared to the wild type, the hypomorphic mutation pkd2-81KD reduced the amplitude of hypo-osmotic shock-triggered calcium spikes by 59%. During cytokinesis, mutations of pkd2 reduced the calcium spikes accompanying cell separation and the ensuing membrane stretching by 60%. We concluded that fission yeast polycystin Pkd2 allows calcium influx when activated by membrane stretching, representing a likely mechanosensitive channel that contributes to the cytokinetic calcium spikes. 

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2. Coordinating septum formation and the actomyosin ring during cytokinesis in Schizosaccharomyces pombe

   https://doi.org/10.1111/mmi.14387  

   Hercyk, Brian S.; Onwubiko, Udo N.; Das, Maitreyi E. (September 2019, Molecular Microbiology)

   Summary During cytokinesis, animal and fungal cells form a membrane furrow via actomyosin ring constriction. Our understanding of actomyosin ring‐driven cytokinesis stems extensively from the fission yeast model system. However, unlike animal cells, actomyosin ring constriction occurs simultaneously with septum formation in fungi. While the formation of an actomyosin ring is essential for cytokinesis in fission yeast, proper furrow formation also requires septum deposition. The molecular mechanisms of spatiotemporal coordination of septum deposition with actomyosin ring constriction are poorly understood. Although the role of the actomyosin ring as a mechanical structure driving furrow formation is better understood, its role as a spatiotemporal landmark for septum deposition is not widely discussed. Here we review and discuss the recent advances describing how the actomyosin ring spatiotemporally regulates membrane traffic to promote septum‐driven cytokinesis in fission yeast. Finally, we explore emerging questions in cytokinesis, and discuss the role of extracellular matrix during cytokinesis in other organisms. 

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3. Cdc42 prevents precocious Rho1 activation during cytokinesis in a Pak1-dependent manner

   https://doi.org/10.1242/jcs.261160  

   Onwubiko, Udo N.; Kalathil, Dhanya; Koory, Emma; Pokharel, Sahara; Roberts, Hayden; Mitoubsi, Ahmad; Das, Maitreyi (April 2023, Journal of Cell Science)

   ABSTRACT During cytokinesis, a series of coordinated events partition a dividing cell. Accurate regulation of cytokinesis is essential for proliferation and genome integrity. In fission yeast, these coordinated events ensure that the actomyosin ring and septum start ingressing only after chromosome segregation. How cytokinetic events are coordinated remains unclear. The GTPase Cdc42 promotes recruitment of certain cell wall-building enzymes whereas the GTPase Rho1 activates these enzymes. We show that Cdc42 prevents early Rho1 activation during fission yeast cytokinesis. Using an active Rho probe, we find that although the Rho1 activators Rgf1 and Rgf3 localize to the division site in early anaphase, Rho1 is not activated until late anaphase, just before the onset of ring constriction. We find that loss of Cdc42 activation enables precocious Rho1 activation in early anaphase. Furthermore, we provide functional and genetic evidence that Cdc42-dependent Rho1 inhibition is mediated by the Cdc42 target Pak1 kinase. Our work proposes a mechanism of Rho1 regulation by active Cdc42 to coordinate timely septum formation and cytokinesis fidelity. 

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4. Callose deposition is essential for the completion of cytokinesis in the unicellular alga, Penium margaritaceum

   https://doi.org/10.1242/jcs.249599  

   Davis, Destiny J.; Wang, Minmin; Sørensen, Iben; Rose, Jocelyn K.; Domozych, David S.; Drakakaki, Georgia (December 2020, Journal of Cell Science)

   null (Ed.)

   Cytokinesis in land plants involves the formation of a cell plate that develops into the new cell wall. Callose, a β-1,3 glucan accumulates at later stages of cell plate development presumably to stabilize this delicate membrane network during expansion. Cytokinetic callose is considered specific to multicellular plant species, as it has not been detected in unicellular algae. Here we present callose at the cytokinesis junction of the unicellular charophyte, P. margaritaceum. Callose deposition at the division plane of P. margaritaceum showed distinct, spatiotemporal patterns likely representing distinct roles of this polymer in cytokinesis. Pharmacological inhibition by Endosidin 7 resulted in cytokinesis defects, consistent with the essential role for this polymer in P. margaritaceum cell division. Cell wall deposition at the isthmus zone was also affected by the absence of callose, demonstrating the dynamic nature of new wall assembly in P. margaritaceum. The identification of candidate callose synthase genes provides molecular evidence for callose biosynthesis in P. margaritaceum. The evolutionary implications of cytokinetic callose in this unicellular Zygnematopycean alga is discussed in the context of the conquest of land by plants. 

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5. The Mechanosensitive Pkd2 Channel Modulates the Recruitment of Myosin II and Actin to the Cytokinetic Contractile Ring

   https://doi.org/10.3390/jof10070455  

   Chowdhury, Pritha; Sinha, Debatrayee; Poddar, Abhishek; Chetluru, Madhurya; Chen, Qian (July 2024, Journal of Fungi)

   Cytokinesis, the last step in cell division, separates daughter cells through mechanical force. This is often through the force produced by an actomyosin contractile ring. In fission yeast cells, the ring helps recruit a mechanosensitive ion channel, Pkd2, to the cleavage furrow, whose activation by membrane tension promotes calcium influx and daughter cell separation. However, it is unclear how the activities of Pkd2 may affect the actomyosin ring. Here, through both microscopic and genetic analyses of a hypomorphic pkd2 mutant, we examined the potential role of this essential gene in assembling the contractile ring. The pkd2-81KD mutation significantly increased the counts of the type II myosin heavy chain Myo2 (+18%), its regulatory light chain Rlc1 (+37%) and actin (+100%) molecules in the ring, compared to the wild type. Consistent with a regulatory role of Pkd2 in the ring assembly, we identified a strong negative genetic interaction between pkd2-81KD and the temperature-sensitive mutant myo2-E1. The pkd2-81KD myo2-E1 cells often failed to assemble a complete contractile ring. We conclude that Pkd2 modulates the recruitment of type II myosin and actin to the contractile ring, suggesting a novel calcium-dependent mechanism regulating the actin cytoskeletal structures during cytokinesis. 

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