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

Abstract

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, or whether its perceived role stems from its spindle function, is 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 regulatory and structural hub: it prevents NuMA from binding chromosomes at mitosis, regulates its nuclear mobility and is essential for nuclear formation. Thus, NuMA plays a long-range structural role in building and maintaining an intact nucleus, as it does for the spindle, playing a protective role over the cell cycle.

The Nuclear Mitotic Apparatus (NuMA) Protein: A Key Player for Nuclear Formation, Spindle Assembly, and Spindle Positioning

“…On the other hand, the interphase role of the N-terminal region of NuMA is not clear. Although both N-terminal and C-terminal globular domains contain several S/TPXX motifs which are supposed to contribute to DNA binding ( Suzuki, 1989 ), the N-terminal NuMA fragments do not show clear DNA or chromatin binding compared to NuMA’s C-terminal ( Serra-Marques et al, 2020 ). However, the N-terminal domain may contribute to the lattice formation of NuMA oligomers in the nucleus ( Gueth-Hallonet et al, 1998 ; Harborth et al, 1999 ), as described below.…”

“…Recently, Serra-Marques et al (2020) found that the long coiled-coil is also required for the formation of a single, round nucleus, and that this role is independent from NuMA’s role in spindle formation. In addition, the authors revealed that a small portion of the coiled-coil 213–705 is sufficient to prevent NuMA’s C-terminal region from binding chromosomes during late metaphase and anaphase ( Serra-Marques et al, 2020 ).…”

“…Previously, human NuMA was reported to interact with defined DNA sequences called matrix attached regions (MARs) in vitro ( Luderus et al, 1994 ). Recently, two studies demonstrated that the C-terminal region of human NuMA interacts with DNA in vitro and chromatin in cells ( Rajeevan et al, 2020 ; Serra-Marques et al, 2020 ). Rajeevan et al (2020) showed that the C-terminus NuMA 2058–2115 fragment is sufficient to bind DNA in vitro , and the basic amino acids within the region are critical for its interaction with chromatin in cells ( Figure 2C ).…”

“…Furthermore, they showed that NuMA keeps the decondensing chromosome mass compact during mitotic exit ( Serra-Marques et al, 2020 ). Although the precise mechanisms are still unclear, evidence suggests that NuMA, like barrier-to-auto integration factor (BAF) at the chromosome ensemble surface ( Samwer et al, 2017 ), may offer structural support throughout the nucleus by cross-linking chromosomes and preventing the nuclear envelop from penetrating into the chromosome mass, which results in a decrease of multinucleation and abnormally shaped nuclei during mitotic exit ( Serra-Marques et al, 2020 ). Alternatively, NuMA may coordinate chromosome compaction and nuclear envelop assembly using its binding abilities to both chromosomes and membrane ( Serra-Marques et al, 2020 ), as well as its binding to importins which can recruit membrane vesicles and nucleoporins ( Lu et al, 2012 ).…”

“…In addition to its roles during mitotic exit, NuMA promotes the nucleus’ mechanical robustness ( Serra-Marques et al, 2020 ) and could contribute to several interphase events, including chromatin organization ( Abad et al, 2007 ), gene expression ( Harborth et al, 2000 ; Ohata et al, 2013 ), DNA repair ( Vidi et al, 2014 ; Moreno et al, 2019 ), and apoptosis ( Gueth-Hallonet et al, 1997 ; Kivinen et al, 2005 ; Lin et al, 2007 ). Since chromatin architecture is highly dynamic during different phases of the cell cycle ( Shoaib et al, 2020 ), acute protein depletion technologies, such as auxin-inducible degron (AID) ( Natsume et al, 2016 ; Yesbolatova et al, 2020 ; Tsuchiya et al, 2021 ), would be useful to precisely understand the interphase functions of NuMA in future studies.…”