Stephen Osmani

Focus

Cell Cycle Regulation and Nuclear Structure

Research interests

The cell cycle is a fundamental process by which two cells are generated from one during growth and development. The cell cycle is under complex regulatory control to ensure that all cellular constituents are first duplicated and then segregated equally to produce two viable cells from one. This regulation is fundamental to all life and has significance to cancer biology as cancerous cells result from unbalanced cell cycle regulation.

My laboratory is conducting research aimed at elucidating how the cell cycle is regulated. We utilize the model filamentous fungus Aspergillus nidulans in these studies. This enables us to use the power of classical genetics, molecular genetics, cell biology, genomics and biochemistry to identify and study novel proteins involved in cell cycle regulation. These findings will provide insights to how mitosis is regulated in filamentous fungi, a poorly understood group of organisms which have far-reaching beneficial and detrimental impacts on mankind. The findings should also provide insights to how mitosis is regulated in higher eukaryotes.

Recent work in the lab has focused on understanding how nuclear transport is regulated during mitosis. In higher eukaryotes the nuclear envelope is dismantled during mitosis (open mitosis) by ill defined mechanisms. However in fungi, such as A. nidulans, mitosis occurs within intact nuclei (closed mitosis). During closed mitosis proteins, such as tubulin which forms the mitotic spindle, enter nuclei only during mitosis. It had long been assumed that specific transport pathways through the nuclear pore complex (NPC, the structure providing regulated conduits across the nuclear envelope between the cytoplasm and the nucleoplasm) were modified during closed mitosis. However, our recent data has demonstrated that in A. nidulans the NPC is opened to allow diffusion between the inside and outside of nuclei during mitosis rather than specific transport pathways being modified. This is achieved by the release of over half of the 27 NPC proteins from theNPC specifically during mitosis (See Figure, taken from Osmani et al., Molecular Biology of the Cell 2006). The NPC proteins that remain form a core conduit between the inside and outside of nuclei. During exit from mitosis the dispersed NPCproteins return back to the core NPC structure and reestablish regulated transport. These findings provide a newparadigm for how mitosis can be regulated and indicate A. nidulans mitosis is an evolutionary intermediate between closed and open mitoses.

In addition to explaining how nuclear transport is regulated during A. nidulans mitosis these new findings provide a framework to help decipher how the massive NPC structure is first dismantled then reassembled during mitosis. We know that protein phosphorylation plays a critical role and that the NimA and Cdk1 mitotic kinases promote disassembly of the NPC. The challenge now is to define which NPC proteins are phosphorylated then dephosphorylated to reversibly disassemble the NPC during mitosis.

 

 

Publications

• Liu H-L, De Souza CP, Osmani AH, Osmani SA. The three fungal transmembrane nuclear pore complex proteins of Aspergillus nidulans are dispensable in the presence of an intact An-Nup84-120 complex. Mol. Biol. Cell. 20, 616-30. 2009
• Son S and Osmani SA. Analysis of all protein phosphatase genes in Aspergillus nidulans identifies a new mitotic regulator, Fcp1. Eukaryotic Cell. Jan 30. PMID: 19181872 (Cover article) 2009
• Ukil, L, De Souza CP, Liu HL, Osmani SA. Nucleolar separation from chromosomes during Aspergillus nidulans mitosis can occur without spindle forces. Mol. Biol. Cell. In Press 2009
• De Souza CP, Hashmi SB, Nayak T, Oakley BR Osmani SA. Mlp1 acts as a mitotic scaffold to spatially regulate spindle assembly checkpoint proteins in Aspergillus nidulans. Mol. Biol. Cell. In Press 2009
• Taheri-Talesh N, Horio T, Araujo-Bazan L, Dou X, Espeso E, Penalva M, Osmani SA, Oakley BR. The tip growth apparatus of Aspergillus nidulans. Mol. Biol. Cell. 19, 1439-49. 2008
• Ukil L, Varadaraj A, Liu HL Osmani SA. Copy number suppressors of the Aspergillus nidulans nimA1 mitotic kinase display distinctive and highly dynamic cell cycle regulated locations. Eukaryotic Cell. 7,2087-99. 2008
• Wortman JR, Gilsenan JM, Joardar V, Deegan J, Clutterbuck J et al. The 2008 update of the Aspergillus nidulans genome annotation: A community effort. Fungal Genet. Biol. Dec 25. PMID: 19146970. 2008
• De Souza CP, Osmani SA. Mitosis, not just open or closed. Eukaryotic Cell. 6, 1521-7. Review. 2007
• Nayak T, Szewczyk E, Oakley CE, Osmani A, Ukil L, Murray SL, Hynes MJ, Osmani SA, Oakley BR. A versatile and efficient gene targeting system for Aspergillus nidulans. Genetics. 172,1557-66. 2006
• De Souza CP, Hashmi SB, Horn KP, Osmani SA. A point mutation in the Aspergillus nidulans sonBNup98 nuclear pore complex gene causes conditional DNA damage sensitivity. Genetics. 174, 1881-93. 2006
• Osmani AH, Davies J, Liu HL, Nile A, Osmani SA. Systematic deletion and mitotic localization of the nuclear pore complex proteins of Aspergillus nidulans. Mol. Biol. Cell. 17, 4946-61. 2006
• Osmani AH, Oakley BR, Osmani SA. Identification and analysis of essential Aspergillus nidulans genes using the heterokaryon rescue technique. Nature Protocols 1, 2517-2526, 2006
• Szewczyk E, Nayak T, Oakley CE, Edgerton H, Xiong Y, Taheri-Talesh N, Osmani SA, Oakley B. Fusion PCR and gene targeting in Aspergillus nidulans. Nature Protocols 1, 3111 – 3120, 2006