Revisiting Actin's role in early centrosome separation

The two principal non-muscle actin isoforms found in mammalian cells are β-actin and γ-actin. Their relative localization and function are intriguing and not fully coincident.¹ In general, γ-actin is associated with cytoplasmic and cortical actin meshworks while β-actin is enriched in contractile elements such as stress fibers and cleavage furrows. Recent work has specifically implicated the γ-actin isoform as a tumor promotor.² Further insight into the etiology behind this activity is provided in the present work by Po'uha and Kavallaris.³

Acto-myosin is widely celebrated for its role in conducting the final bars of the mitotic symphony: cytokinesis. However, actin dynamics may play a role in setting the stage for mitosis as well. Actin filaments are necessary for the earliest stages of centrosome separation during interphase but can be dispensable for prophase spindle assembly when the microtubule cytoskeleton can take over.⁴ However, delayed centrosome separation in the absence of actin filaments leaves the cell in a precarious position. At the onset of mitosis the cell must simultaneously separate its centrosomes while assembling the bipolar spindle and making chromosome attachments. This hurried pathway for spindle assembly, known as the “prometaphase pathway,” is demonstrably more error-prone than mitotic spindle assembly that utilizes centrosomes that have separated prior to nuclear envelope breakdown.⁵

Cells depleted of actin filaments that enter mitosis with unseparated centrosomes require more time to build a spindle, establish microtubule connections and align their chromosomes and, as a result, progress more slowly through prophase and prometaphase.⁴ In the present study by Po'uha and Kavallaris, γ-actin depletion increased time spent in mitosis and live imaging confirmed that the cells had difficulty in aligning and segregating chromosomes.³ Although Po'uha and Kavallaris did not score centrosome separation in this study, previous work from their lab established a role for γ-actin in centrosome reorientation during cell migration.¹ Taken together, these data implicate γ-actin in centrosome dynamics, and the mitotic defects and delays reported in the present study³ are consistent with those that appear when acto-myosin-dependent centrosome separation is compromised.⁶

Depletion of γ-actin also impaired interphase cell cycle progression, leading to unusually high levels of cyclin E.³ Cyclin E is highest during the transition from G1/S phase and suspiciously close to the time at which centrosomes are “licensed” to replicate by centriole disengagement. Could it be that, in addition to the well-known separase/plk1 pathways, the dynamic cytoplasmic γ-actin meshwork supplies mechanical forces required for centriole disengagement in G1? Live imaging from numerous labs suggests that centrosomes experience mechanical forces throughout interphase, although the source of these forces is not always clear.

A significant G1/S cell cycle delay arising from delays in centriole disengagement could explain why γ-actin-depleted cells exhibit resistance to mitotic inhibitors as has been previously demonstrated by the Kavallaris lab. Fewer cells would be entering mitosis at any one time in γ-actin-depleted cells. Consistent with this, is the observation that overexpression of γ-actin, but not β-actin, accelerates cell proliferation.² As hypothesized above, γ-actin-dependent acceleration of proliferation could arise from an acceleration of centrosome disjunction, licensing and maturation utilizing γ-actin filament mechanics. Alternatively, γ-actin filaments may serve as a scaffold for second messenger signaling, such as the activation of ERK1/2,² whose many downstream effects might accelerate the cell cycle. Whether a primarily downstream mechanical or upstream signaling mechanism predominates as a means by which γ-actin participates in cell cycle timing remains an open question.

Early stage centrosome separation requires an intact, dynamic, actin cytoskeleton,^4,7 whereas later stage centrosome separation in mammalian cells responds to cortical acto-myosin flow⁶ and a requires a dynamic microtubule array.⁵ Depletion of γ-actin could deliver a “one-two punch” to timely cell cycle progression leading to (i) interphase cell cycle delays at the G1/S boundary; (ii) late separation of duplicated centrosomes that culminates in prometaphase delays and mitotic errors. In the future, it will be interesting to quantify the length of each cell cycle stage in γ-actin-depleted cells in conjunction with centrosome duplication, centrosome-associated signaling proteins and centrosome position to test this idea.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.