How are Dynamic Microtubules Stably Tethered to Human Chromosomes?

Abstract

During cell division, microtubules capture and pull chromosomes apart into two equal sets. Without the establishment of proper chromosome-microtubule attachment, microtubules cannot impart the pulling forces needed to separate sister chromatid pairs. How are chromosomes captured along microtubule walls? How is the attachment of chromosomes to dynamic microtubule-ends achieved and monitored? We discuss these key questions by considering the roles of kinetochore-bound microtubule regulating proteins and also the complex regulatory loops of kinases and phosphatases that control chromosome-microtubule attachment and ensure the accurate segregation of chromosomes.

Kinetochores attached to microtubule-ends are stabilised by Astrin bound PP1 to ensure proper chromosome segregation

“…We showed that Astrin is selectively enriched at end-on attached kinetochores and is required to stabilise end-on attachments, independent of biorientation (Shrestha and Draviam, 2013; Shrestha et al, 2017). The enrichment of Astrin at kinetochores prior to biorientation points to a novel attachment stabilisation mechanism, independent of already known intra- or inter- kinetochore stretching mediated stabilisation of bioriented attachments (reviewed in Conti et al, 2017). To explore this hypothesis, we tracked Astrin recruitment and dynamics at the kinetochores of monopolar spindles that can not biorient kinetochores.…”

“…Chromosome segregation can not be initiated until all chromosomes are properly attached to microtubules from opposing spindle poles - a state called biorientation. To mediate and monitor chromosome-microtubule attachments, a macromolecular structure, the kinetochore (KT) assembles on centromeric DNA (reviewed in Conti et al, 2017; Musacchio and Desai, 2017). Kinetochores are first captured along microtubule-walls (immature lateral attachment) and then brought to microtubule-ends (mature end-on attachment), by a multi-step end-on conversion process (Tanaka et al, 2005; Shrestha and Draviam, 2013).…”

Cells protect chromosome-microtubule attachments, independent of biorientation, using an Astrin-PP1 and CyclinB-CDK1 feedback loop

“…However, this is unlikely to be the primary mechanism as stable monotelic kinetochores lacking lateral microtubule interaction have been observed in a variety of conditions (21,23,24). Other kinetochore-bound kinases, CDK1 and PLK1, have been implicated in stabilising chromosome-microtubule attachments (27) but their impact specifically on mono-oriented attachments (independent of microtubule-mediated opposing pull) remain unclear.…”

Human genetic variations reveal Chromosomal Instability aiding Variants (CIVa) in kinetochore-microtubule associated proteins

“…CIN can arise from errors in chromosome-microtubule attachment events, leading to the loss or gain of chromosomes, disorganisation of nuclear structure, transcriptional heterogeneity and replication and proteotoxic stress [2][3][4] . Chromosome-microtubule attachment is facilitated by a macromolecular protein structure, the kinetochore (reviewed in 5 ). Variations in kinetochore (KT) proteins are known to cause genetic disorders including primary microcephaly (MCPH, OMIM 604321, OMIM 616051 6 ) and mosaic variegated aneuploidy (MVA, OMIM 257300 7 ).…”

“…3S3 A and B). In mitotic cells expressing either SKA3 p.Q70Kfs*7 fusion proteins, we observed normal chromosome congression, and hence we probed if mature kinetochore attachments had formed using the end-on attachment marker, Astrin-SKAP complex 5 . Live-cell microscopy of cells coexpressing mKate2-Astrin and SKA3 showed normal Astrin localisation in both N-terminally and C-terminally tagged SKA3-WT and Q70Kfs*7 expressing cells (Fig.…”