InteractiveFly: GeneBrief

Spindly: Biological Overview | References

BIOLOGICAL OVERVIEW

The eukaryotic spindle assembly checkpoint (SAC) monitors microtubule attachment to kinetochores and prevents anaphase onset until all kinetochores are aligned on the metaphase plate. In higher eukaryotes, cytoplasmic dynein is involved in silencing the SAC by removing the checkpoint proteins Mad2 and the Rod-Zw10-Zwilch complex (RZZ) from aligned kinetochores (Howell, 2001; Wojcik, 2001). Using a high throughput RNA interference screen in Drosophila melanogaster S2 cells, a new protein (Spindly) has been identified that accumulates on unattached kinetochores and is required for silencing the SAC. After the depletion of Spindly, dynein cannot target to kinetochores, and, as a result, cells arrest in metaphase with high levels of kinetochore-bound Mad2 and RZZ. A human homologue of Spindly serves a similar function. However, dynein's nonkinetochore functions are unaffected by Spindly depletion. These findings indicate that Spindly is a novel regulator of mitotic dynein, functioning specifically to target dynein to kinetochores (Griffis, 2007).

The spindle assembly checkpoint (SAC) is critical for preventing the onset of anaphase until all chromosomes are aligned on the metaphase plate. A single misaligned kinetochore is sufficient to generate a wait anaphase signal, thereby ensuring that all sister chromatids segregate to opposite ends of the spindle and are equally distributed to the daughter cells. Failure of the SAC can lead to premature anaphase onset and aneuploidy. Such defects can have consequences for a whole organism; mice that lack a full complement of SAC genes have more frequent DNA segregation errors and are more susceptible to tumor development (Griffis, 2007).

The presence of the SAC was initially inferred from observations that cells delay in metaphase when meiotic sex chromosomes fail to pair and align or after the spindle is perturbed by either microtubule poisons or microsurgery. Molecules responsible for the SAC were later identified in yeast genetic screens and named Mad1, -2, and -3 (Mad for mitotic arrest deficient) and Bub1, -2, and -3 (Bub for budding unperturbed by benzimidazole). Subsequent work showed that these proteins together with the MPS1 kinase form distinct complexes that target to the kinetochore. Two additional metazoan checkpoint proteins, Zw10 and Rough Deal (Rod), were later isolated as cell cycle mutants in Drosophila melanogaster. These two proteins, together with a third protein called Zwilch (for review see Karess, 2005), form a complex (Rod-Zw10-Zwilch complex [RZZ]) that regulates the levels of Mad1 and Mad2 on the kinetochore (Griffis, 2007).

Ultimately, the SAC pathway must lead to inhibition of the anaphase-promoting complex (APC), a multisubunit ubiquitin E3 ligase that targets multiple mitotic regulators (e.g., mitotic cyclins as well as the securin protein that inhibits the cleavage of cohesin molecules) for proteosome degradation to allow mitotic exit (Acquaviva, 2006). Several studies have shown that localization of the checkpoint proteins to misaligned kinetochores is essential for establishing the SAC and keeping the APC inhibited, most likely by generating a diffusible signal that inhibits the APC. The nature of the diffusible signal is still subject to debate. However, a current model (for review see Musacchio, 2007) suggests that the kinetochore-bound Mad1-Mad2 complex acts as a template that coverts the free, inactive Mad2 to an active form that can diffuse away from the kinetochore and bind to and sequester Cdc20, a regulatory component of the APC (Griffis, 2007).

The capture of microtubules by the kinetochore and the downstream activity of two different microtubule motors are required for silencing the SAC in metazoans. One of these motors is the kinesin centromere protein (CENP) E, which may act as a tension sensor that, when stretched, inactivates the BubR1-dependent inhibition of Cdc20 (Chan, 1999; Mao, 2005). The second motor is dynein, which transports Mad1, Mad2, and RZZ from the kinetochore to the spindle pole. Dynein-based removal of Mad1 and Mad2 from the kinetochore may disrupt the template mechanism that generates the active Mad2 that inhibits the APC (De Antoni, 2005; for review see Musacchio, 2007). After inhibition or depletion of dynein or its cofactors, metazoan cells arrest in metaphase with correctly aligned chromosomes and high levels of kinetochore-bound Mad1, Mad2, and RZZ (Griffis, 2007).

Resolving the mechanism of dynein recruitment to kinetochores is important for understanding how kinetochore-microtubule binding ultimately leads to inactivation of the SAC. Currently, it is thought that dynein is brought to the kinetochore by binding directly to dynactin (a multisubunit complex required for multiple dynein functions; Schroer, 2004), which, in turn, binds to the Zw10 subunit of the RZZ complex (Starr, 1998). Lis1, another dynein cofactor, also has been proposed to play a role in targeting dynein to kinetochores (Dzhindzhev, 2005). Dynactin, Lis1, and Zw10 are not kinetochore-specific factors, as they are involved in targeting dynein to multiple other locations in the cell. It has not been clearly established whether dynactin and Lis1 are sufficient for targeting dynein to kinetochores or whether other proteins might be involved (Griffis, 2007).

To find new proteins that might participate in the SAC, an automated 7,200 gene mitotic index RNAi screen was undertaken in S2 cells. This screen uncovered a novel gene, which was also identified in an independent screen of genes involved in S2 cell spreading and morphology. This protein (termed Spindly) localizes to microtubule plus ends in interphase and to kinetochores during mitosis. Cells depleted of Spindly arrest in metaphase with high levels of Mad2 and Rod on aligned kinetochores, a defect caused by a failure to recruit dynein to the kinetochore. However, Spindly is not required for other dynein functions during interphase and mitosis. A human homologue of Spindly, which is similarly involved in recruiting dynein to kinetochores, was identifed. Thus, these results have uncovered a novel conserved dynein regulator that is involved specifically in dynein's function in silencing the SAC (Griffis, 2007).

An RNAi screen has identified Spindly as an essential factor for docking dynein to the kinetochore. Spindly is recruited to the kinetochore in an RZZ-dependent manner, and there, together with dynactin, Spindly recruits dynein to the outermost region of the kinetochore. The dynein motor complex then transports Spindly along with Mad2 and the RZZ complex to the spindle poles to inactivate the SAC. A Spindly homologue plays a similar role in human cells, revealing a conserved dynein kinetochore targeting mechanism in invertebrates and vertebrates. These data provide new insight into the mechanism and importance of recruiting dynein to the kinetochore to inactivate the SAC. Spindly also plays a role in maintaining S2 cell morphology during interphase and localizes to the growing ends of microtubules (Griffis, 2007).

The depletion of Spindly creates several mitotic defects that appear to reflect a loss of dynein activity exclusively at the kinetochore. Metaphase arrest is the most evident defect observed after the RNAi-mediated depletion of Spindly in Drosophila or human cells. This metaphase arrest phenotype is most likely explained by the absence of kinetochore-bound dynein in Spindly-depleted cells, and, indeed, the data support the model of Howell (2001), who proposes that kinetochore-bound dynein is required for transporting Mad2 from the kinetochore to inactivate the SAC. Nevertheless, the possibility that the mitotic delay seen after dynein or Spindly depletion is caused by another kinetochore aberration that keeps the checkpoint activated. However, Spindly-depleted cells ultimately overcome metaphase arrest, as seen in live cell imaging experiments and by the modest increases in the mitotic indices of Spindly-depleted S2 and HeLa cells (three- to seven-fold and two-fold, respectively). The mechanism of slippage from this metaphase arrest is not clear, but it might involve proteins (e.g., p31 comet) that silence the SAC by disrupting the interaction between Mad2 and Cdc20 (Griffis, 2007).

In addition to mitotic arrest, chromosomes in Spindly- and dynein-depleted S2 cells require a longer time to align on the metaphase plate. This result may be attributable either to the displacement of CLIP-190 (a microtubule tip-binding protein) from kinetochores after Spindly or dynein depletion (Dzhindzhev, 2005) or the loss of dynein-mediated lateral attachments to microtubules in early prometaphase. In HeLa cells, a defect in chromosome alignment was noticed after Hs Spindly depletion, which also has been observed after the depletion of dynein (perhaps mediated through a loss of kinetochore-bound CLIP-170) (Griffis, 2007).

Thus, the spectrum of mitotic defects observed in Spindly-depleted cells is consistent with a loss of dynein function specifically at the kinetochore. Spindly depletion does not produce any other defects seen after dynein depletion, such as centrosome detachment and spindle defocusing. Dynactin is another protein that is required for recruiting dynein to kinetochores, but it is important for other mitotic and interphase dynein functions. Depletion of the RZZ complex inhibits the kinetochore recruitment of dynein, but this also prevents Mad1 and Mad2 recruitment and reduces kinetochore tension to a greater degree than Spindly or dynein depletion alone. Thus, Spindly depletion appears to be the most specific means identified to date for interfering with dynein function only at the kinetochore (Griffis, 2007).

These findings provide new insight into how dynein localizes to kinetochores. Previous studies have led to a model in which dynactin binds to the RZZ complex and then, either alone or in collaboration with Lis1, recruits dynein to the kinetochore. Because it was found that both dynactin and Spindly are required for dynein localization to kinetochores, an updated model is proposed in which Spindly and dynactin target to the kinetochore independently and work together to recruit dynein (Griffis, 2007).

Thus, dynein recruitment to the kinetochore may involve multiple weak interactions. Consistent with the possibility of weak interactions, endogenous dynein, dynactin, and Rod did not coprecipitate with GFP in pull-down experiments, and Spindly did not coenrich with these proteins in sucrose gradient fractions. Lis1 is not included in the dynein localization model, since it was found that Lis1 RNAi does not block dynein recruitment to the kinetochore (using a colchicine treatment localization assay), although Lis1 depletion does cause a mitotic delay and substantial increase in GFP-Spindly on aligned kinetochores. Thus, a role is favored for Lis1 in dynein activity but not in recruiting dynein to the kinetochore (Griffis, 2007).

Spindly's role in the spreading morphology of S2 cells makes it unusual among proteins involved in silencing the SAC (including dynein and dynactin), which did not produce phenotypes in the interphase morphology screen. The Spindly RNAi interphase phenotype of defective actin morphology and the formation of extensive microtubule projections is still not understood. However, a clue may be Spindly's dynamic localization to the growing microtubule plus end. Other plus end-binding proteins (+TIPs) interact with signaling molecules that regulate cell shape, one example being the binding and recruitment of RhoGEF2 to the microtubule plus end by EB1. Spindly may similarly interact with and carry an actin regulatory molecule to the cortex, but this hypothesis will require identifying proteins that interact with Spindly during interphase (Griffis, 2007).

The mechanism of Spindly recruitment to the microtubule plus end also warrants further investigation. This interaction must be regulated by the cell cycle because GFP-Spindly no longer tracks along microtubule tips in prometaphase. Seven consensus CDK1 phosphorylation sites are present in the positively charged C-terminal repeats of Spindly, and phosphorylation of these sites could reverse the charge of these repeats and regulate the transition from microtubule tip binding to kinetochore binding at the onset of mitosis (Griffis, 2007).

Motor proteins must be guided to the correct subcellular site to execute their biological function. To carry out the multitude of transport activities required in eukaryotic cells, metazoans have evolved numerous kinesin motors (25 genes in Drosophila) with distinct domains that dictate their localization and regulation. In contrast, a single cytoplasmic DHC performs numerous roles in interphase and mitosis, suggesting that additional regulatory factors guide dynein to specific cargoes (e.g., organelles, mRNAs, and vesicles). The main dynein-associated proteins (the dynactin complex, Lis1, and NudEL) are involved in dynein function at many sites and, thus, do not appear to be cargo specific. Zw10 was initially thought to specifically regulate the recruitment of dynein-dynactin to the kinetochore, but it now also appears to play an essential role in targeting dynein to membrane-bound organelles. Bicaudal D is another multifunctional adaptor molecule that has a role in the dynein-based transport of multiple cargoes such as RNA, vesicles, and nuclei. Perhaps the most site-specific dynein recruitment factor is the Saccharomyces cerevisiae Num1 protein that binds to the DIC Pac11p to target the motor to the cortex of daughter cells, where it pulls the nucleus into the bud neck. However, dynein only serves this one function in yeast compared with its plethora of activities in metazoans, and Num1p homologues have yet to be identified in higher eukaryotes (Griffis, 2007 and references therein).

Spindly appears to be a highly selective dynein-recruiting factor, and, unlike other dynein cofactors, it does not appear to be involved in the motor's nonkinetochore functions in mitosis (e.g., pole focusing) or in interphase (e.g., endosome transport). However, the mechanism by which Spindly recruits dynein to the kinetochore remains to be elucidated. Observations that Spindly moves from kinetochores to the spindle poles as discrete punctae strongly suggests that it may incorporate into a large and somewhat stable particle that contains the RZZ complex, Mad1-Mad2, dynein, and likely additional proteins. Therefore, Spindly not only serves to recruit dynein to the kinetochore but also is part of a cargo that dynein transports. Future studies will be needed to better understand the protein composition of these transport particles and the contacts that Spindly makes within them (Griffis, 2007).

Polo regulates Spindly to prevent premature stabilization of kinetochore-microtubule attachments

Accurate chromosome segregation in mitosis requires sister kinetochores to bind to microtubules from opposite spindle poles. The stability of kinetochore-microtubule attachments is fine-tuned to prevent or correct erroneous attachments while preserving amphitelic interactions. Polo kinase has been implicated in both stabilizing and destabilizing kinetochore-microtubule attachments. However, the mechanism underlying Polo-destabilizing activity remains elusive. Resorting to an RNAi screen in Drosophila for suppressors of a constitutively active Polo mutant, this study has identified a strong genetic interaction between Polo and the Rod-ZW10-Zwilch (RZZ) complex, whose kinetochore accumulation has been shown to antagonize microtubule stability. Polo phosphorylates Spindly and impairs its ability to bind to Zwilch. This precludes dynein-mediated removal of the RZZ from kinetochores and consequently delays the formation of stable end-on attachments. It is proposed that high Polo-kinase activity following mitotic entry directs the RZZ complex to minimize premature stabilization of erroneous attachments, whereas a decrease in active Polo in later mitotic stages allows the formation of stable amphitelic spindle attachments. These findings demonstrate that Polo tightly regulates the RZZ-Spindly-dynein module during mitosis to ensure the fidelity of chromosome segregation (Barbosa, 2020).

To ensure the fidelity of chromosome segregation, sister kinetochores (KTs) mediate the attachment of chromosomes to microtubules (MTs) of opposite spindle poles (amphitelic attachments). However, the initial contact of KTs with MTs is stochastic and consequently erroneous attachments-syntelic (chromosome bound to MTs from the same spindle pole) or merotelic (same KT bound to MTs from opposite poles)-can be formed during early mitosis. Thus, accurate mitosis requires a tight regulation of KT-MT turnover so mistakes are prevented or corrected and amphitelic end-on interactions are stabilized. This relies heavily on the activity of two conserved mitotic kinases, Aurora B and Polo/Plk1. Aurora B promotes the destabilization of KT-MT interactions mainly through phosphorylation of proteins of the KMN network (KNL1/Spc105, Mis12 and Ndc80), which decreases their affinity for MTs. Interestingly, it has been shown that the RZZ complex (Rod, ZW10 and Zwilch) is able to interact with Ndc80 N-terminal tail and prevent the adjacent calponin homology (CH) domain from binding to tubulin (Cheerambathur, 2013). This Aurora B-independent destabilizing mechanism is proposed to prevent Ndc80-mediated binding when KTs are laterally attached, hence reducing the potential for merotely during early mitosis. The RZZ additionally recruits Spindly and the minus end-directed motor dynein to KTs, thus providing the means to relieve its inhibitory effect over KT-MT attachments, as well as to ensure the timely removal of spindle assembly checkpoint (SAC) proteins from KTs. However, it remains unclear how RZZ removal by Spindly-dynein is coordinated with end-on attachment formation (Barbosa, 2020).

Polo/Plk1 activity is implicated in both stabilization and destabilization of KT-MT attachments. While the contribution to the former function has been attributed to PP2A-B56 phosphatase recruitment through Plk1-dependent BubR1 phosphorylation, the mechanism underlying Polo/Plk1 destabilizing activity remains unclear. Interestingly, Polo/Plk1 KT localization and activity decrease from early mitosis to metaphase, concurrent with an increase in KT-MT stability. Moreover, high Plk1 activity at KTs was shown to correlate with decreased stability of KT-MT attachments during prometaphase, but the underlying molecular mechanisms have only been marginally addressed (Barbosa, 2020).

This study describes the mitotic effect of expressing a constitutively active Polo-kinase mutant (PoloT182D) in Drosophila neuroblasts and cultured S2 cells. The expression of PoloT182D causes persistent KT-MT instability and congression defects, extends mitotic timing associated with SAC activation and increases chromosome mis-segregation. A small-scale candidate-based RNAi screen was designed to identify partners/pathways that are affected by constitutive Polo activity in the Drosophila eye epithelium. The screen revealed that downregulation of the RZZ subunit Rod rescues the defects resulting from PoloT182D expression. PoloT182D causes permanent accumulation of the RZZ complex at KTs, which is associated with a delay in achieving stable biorientation. Accordingly, Rod depletion rescues the time required for establishing end-on KT-MT attachments and for chromosome congression. This study further demonstrates that Polo phosphorylates the dynein-adaptor Spindly to decrease its affinity for the RZZ. This in turn prevents dynein-dependent stripping of RZZ from KTs, hence causing a delay in the formation of stable end-on attachments. The findings provide a mechanism for the destabilizing action of Polo/Plk1 over KT-MT attachments. A model is proposed in which Polo/Plk1 activity fine-tunes the RZZ-Spindly-dynein module throughout mitosis to ensure the fidelity of KT-MT attachments and chromosome segregation (Barbosa, 2020).

KT-MT attachments at metaphase must be sufficiently stable to satisfy the spindle assembly checkpoint and sustain chromatid segregation during anaphase. On the other hand, during prometaphase, MTs must be able to rapidly detach from KTs to allow efficient correction of erroneous attachments. How KTs regulate the balance of MTs stabilizing and destabilizing forces during successive mitotic stages has remained unclear. This study shows that Polo kinase plays a critical role in this process through control of the RZZ-Spindly-dynein module at KTs. Polo-mediated phosphorylation of Spindly on Ser499 results in a transient increase in RZZ accumulation at KTs, which inhibits stable end-on attachments and likely minimizes merotely in early mitosis. However, permanent Spindly Ser499 phosphorylation is deleterious for mitotic fidelity since it prevents stable KT biorientation and timely chromosome congression (Barbosa, 2020).

Polo/Plk1 has been implicated in the stabilization of KT-MT attachments. Intriguingly, however, attachments are most stable during metaphase, when Polo/Plk1 activity is reduced. Maintaining Polo active in Drosophila larval neuroblasts markedly decreases the stability of KT-MT interactions, which is in line with previous observations in RPE-cultured cells. It is important to mention that insc-GaL4- driven expression of PoloWT and PoloT182D consistently yielded higher levels of the latter protein. This, however, unlikely explains the different phenotypic consequences observed in these neuroblasts, as analogous experiments with S2 cells expressing equivalent levels of PoloWT and PoloT182D mimic the decrease in the efficiency of chromosome congression and KT-MT stability when constitutively active Polo is expressed. Moreover, previous work has shown that Drosophila S2 cells depleted of Polo accumulated hyperstable attachments and that this phenotype was not exclusively attributed to reduced Aurora B activity. A requirement for Polo in fine-tuning the RZZ-Spindly-dynein axis offers a mechanistic explanation for these observations. During early mitosis, high levels of active Polo at KTs ensure that as soon as Spindly is recruited to RZZ, it is efficiently phosphorylated on Ser499. This promptly reduces Spindly affinity towards Zwilch, sensitizing RZZ-uncoupled Spindly for dynein-mediated transport away from KTs. As a result, the RZZ complex is retained at KTs to levels that normally inhibit the formation of stable end-on attachments by the Ndc80 complex (Cheerambathur, 2013) and maintain the SAC signalling active . Rod interacts with the basic tail of Ndc80 and, in this way, precludes binding of MTs to the calponin homology domain of Ndc80 (Cheerambathur, 2013). Thus, the conversion of lateral attachments preferentially formed at early stages of mitosis into stable amphitelic interactions that are essential for faithful chromosome segregation requires the relief of this Rod-mediated inhibitory mechanism. Evidence is provided that a decrease in Polo activity and, consequently, in Spindly phosphorylation, is critical for this transition by allowing the RZZ to fully engage with the Spindly-dynein complex and to be stripped from KTs. This raises the question of how and when Polo activity and Ser499 phosphorylation are antagonized to allow timely formation of stable end-on attachments. PP2A-B56 phosphatase may have a role in this process, since impairing its association with BubR1 was recently shown to dramatically increase the frequency of laterally attached KTs in human cells. However, because BubR1-PP2A-B56 is already present at high levels on early mitotic KTs, it was reason that additional mechanisms must operate to prevent premature end-on conversion. It is plausible that the switch is determined by the levels of cyclin A, which have been shown to function as a timer in prometaphase to destabilize attachments and facilitate error correction. Since Cdk1/CycA is able to phosphorylate human Spindly in vitro, it is hypothesized that this phosphorylation primes Spindly for Polo binding and increases Ser499 phosphorylation to levels that surpass the opposing phosphatase activity. As mitosis progresses, degradation of Cyclin A tips the balance towards Ser499 dephosphorylation, hence favouring stabilization of end-on attachments. This concurs with an increase in tension across KTs that allows the recruitment of PP1, whose role in Polo T-loop dephosphorylation has been described (Barbosa, 2020).

Although the Polo-phosphorylation site in Drosophila Spindly is not conserved in vertebrates, additional residues conforming to Polo/Plk1 consensus signature are present within the same domain, hinting that an analogous regulatory mechanism may take place in these organisms. Interestingly, Ser499 lies within motif that is conserved among different dynein-adaptors. Two other conserved domains have also been recently described for a number of adaptors and shown to act as regulatory modules involved in the interaction with dynein. Thus, it is envision that the motif identified in this study might provide an additional level of regulation in controlling dynein-adaptor complex formation (Barbosa, 2020).

The results suggest that Polo-mediated phosphorylation of Spindly on Ser499 uncouples dynein-mediated transport of the RZZ complex from Spindly. Moreover, it is proposed that phosphorylation of Ser499 causes Spindly C-terminal domain to elicit a negative regulatory action over the N-terminus Zwilch binding domain. In line with these results, it has been recently shown that intramolecular interactions occur within Spindly, causing it to fold on itself at different regions (Sacristan, 2018). Spindly C-terminal region could be involved in facilitating these interactions since it is thought to be of disordered nature (Sacristan, 2018). This structural organization resembles that of BicD/BicD2, a dynein-adaptor which is predicted to share with Spindly a similar mechanism of interaction with dynein. It is therefore noteworthy that Polo has been shown to activate BicD-dynein transport during oogenesis. Furthermore, several point mutations in BicD/BicD2 were shown to hyperactivate dynein for cargo transport. It will be interesting to establish whether Spindly Ser499 phosphorylation could also impact on dynein complex motility/processivity (Barbosa, 2020).

Long-lasting Polo activation or permanent Spindly Ser499 phosphorylation stalls KTs in labile interactions with MTs. The data confirm a destabilizing role for Polo in KT-MT attachments which has also been shown to operate through the control the kinase exerts over the recruitment and activation of Aurora B and the MT depolymerizing motor Kif2b . Hence, high levels of active Polo in early mitosis ensure efficient correction of merotelic and syntelic attachments, errors that typically occur upon nuclear envelope breakdown as a result of stochastic interactions between KTs and spindle MTs. Paradoxically, Plk1 activity has also been implicated in stabilization of KT-MT attachments through phosphorylation of BubR1. A model is envisioned where these apparently antagonistic Polo-directed inputs are not mutually exclusive but rather cooperate to establish proper attachments. Phosphorylation of BubR1 by Polo/Plk1 in prometaphase promotes the accumulation of PP2A-B56, which opposes Aurora B destabilizing phosphorylations on Ndc80. This is important to allow binding of MTs to the Ndc80 complex during the end-on conversion process, tipping the balance against the KT-MT destabilizing environment, particularly when Cyclin A levels drop. The observation that disrupting Plk1 activity rescues the attachment defects otherwise generated by depletion of PP2A-B56 strongly argues in favour of this integrated model for Polo-regulated stabilizing and destabilizing forces (Barbosa, 2020).

In summary, these findings demonstrate that the RZZ-Spindly-dynein module is tightly regulated by Polo kinase to ensure accurate chromosome segregation. Spindly phosphorylation by Polo on early mitotic KTs ensures RZZ-mediated inhibition of end-on interactions, hence preventing premature stabilization of erroneous attachments. As mitosis progresses, decreased Polo-kinase activity and concurrent Spindly dephosphorylation render the RZZ prone for removal from KTs by Spindly-dynein. This alleviates RZZ antagonism of MT binding by the Ndc80 complex, thus allowing timely conversion of labile lateral interactions into stable amphitelic attachments ensuring proper sister chromatid segregation (Barbosa, 2020).

Kinetochore protein Spindly controls microtubule polarity in Drosophila axons

Microtubule polarity in axons and dendrites defines the direction of intracellular transport in neurons. Axons contain arrays of uniformly polarized microtubules with plus-ends facing the tips of the processes (plus-end-out), while dendrites contain microtubules with a minus-end-out orientation. It has been shown that cytoplasmic dynein, targeted to cortical actin, removes minus-end-out microtubules from axons. This study has identified Spindly, a protein known for recruitment of dynein to kinetochores in mitosis, as a key factor required for dynein-dependent microtubule sorting in axons of Drosophila neurons. Depletion of Spindly affects polarity of axonal microtubules in vivo and in primary neuronal cultures. In addition to these defects, depletion of Spindly in neurons causes major collapse of axonal patterning in the third-instar larval brain as well as severe coordination impairment in adult flies. These defects can be fully rescued by full-length Spindly, but not by variants with mutations in its dynein-binding site. Biochemical analysis demonstrated that Spindly binds F-actin, suggesting that Spindly serves as a link between dynein and cortical actin in axons. Therefore, Spindly plays a critical role during neurodevelopment by mediating dynein-driven sorting of axonal microtubules (Del Castillo, 2020).

Several works using different model systems have demonstrated that uniform polarity of microtubules in axons requires activity of cytoplasmic dynein recruited to cortical actin filaments. However, the mechanism that targets dynein to cortical actin remains unknown. In the search for adaptors involved in the recruitment of dynein to F-actin, a targeted RNAi screen was performed, and Spindly, a well-characterized protein that recruits dynein to kinetochores in mitosis, was shown to be required in postmitotic neurons for dynein-dependent organization of microtubules in axons. Depletion of Spindly in Drosophila neurons impairs axonal microtubule sorting; brain of Spindly-depleted third-instar larvae showed severe defects in axonal patterning. These neurodevelopmental defects result in impairment of coordination and locomotion and reduced life span of adult flies. These phenotypes are not caused by reduction of the dynein level or inhibition of dynein-driven organelle transport upon Spindly knockdown. Spindly RNAi defects found in the Drosophila brain are fully rescued by expression of full-length Spindly or the variant deficient in kinetochore binding, but not by variants with mutations in its dynein-binding domain. Together, these data suggest that Spindly plays a role in neurodevelopment through a dynein-dependent pathway (Del Castillo, 2020).

Spindly was originally identified as a mitotic component recruited to the kinetochore in a RZZ-dependent pathway. In mitosis, the formation of stable interactions between kinetochores and dynein in the metaphase plate is required to silence spindle assembly checkpoint, allowing the progression of the cell cycle to anaphase. In Spindly-depleted cells, dynein motors fail to be recruited to the outer plate of the kinetochores, and the lack of stable kinetochore-microtubule contacts results in cell-cycle arrest in metaphase. It is proposed that in postmitotic neurons Spindly is also important for dynein recruitment. However, in the case of neurons, Spindly recruits dynein to the actin cortex in axons. Importantly, two types of experiments show that this postmitotic role of Spindly is independent of its canonical role in mitosis. First, depletion of Rod1, one of the kinetochore components that interacts with Spindly during cell division, did not affect the microtubule polarity. More directly, expression of the Spindly variant lacking its kinetochore-binding domain (Spindly-ΔC) rescues the Spindly RNAi defects in Drosophila neurons. Both observations together suggest that the neuronal Spindly-dependent pathway of dynein recruitment and microtubule organization is different from its canonical mitotic pathway (Del Castillo, 2020).

It has been shown recently that, in addition to its role in cell division, Spindly functions in interphase cells. Both in mammalian cells and in Drosophila, changes of Spindly level negatively impact cell migration. Remarkably, both Spindly and the dynein/dynactin complex are found at the cell cortex at the leading edge of migrating human cells. In good agreement with this observation, biochemical and cellular assays showed that Spindly interacts with F-actin. However, at this point it is unknown whether Spindly interacts with actin directly or whether this interaction is mediated by other proteins. The lack of known/predicted actin-binding domains in Spindly favors the second scenario. It will be very interesting to identify the proteins that form a complex with Spindly in interphase and find components of this complex that are involved in the recruitment of Spindly and dynein to F-actin (Del Castillo, 2020).

Importantly, the loss of dynein activity in Drosophila sensory neurons did not affect microtubule polarity in dendrites, indicating that the microtubule-sorting activity of dynein is restricted to axons. The apparent restriction of the dynein-recruiting Spindly activity to axons is yet to be determined. The data support that Spindly primary localizes in the cell body and axon, although a small fraction of the protein can be found in dendrites. It has been reported that Spindly is posttranslationally modified and that modifications affect its localization and/or functions. For example, farnesylation of Cys602 of human Spindly is essential for Spindly accumulation at prometaphase kinetochores. Spindly can also be phosphorylated by cyclin-dependent kinases during cell division, and S515 of human Spindly is the major phosphorylation site. Interestingly, this modification seems to regulate ZW10 function rather than Spindly localization. It is hypothesized that posttranslational modifications in the amino-terminal domain of Spindly may regulate its role in neurodevelopment. Recently, it has been reported that other kinetochore proteins are important for neurodevelopment. For example, depletion of Mis12, Knl1, and Ndc80 (other kinetochore components) results in abnormal neuromuscular junctions and central nervous system development both in Drosophila and in Caenorhabditis elegans. However, the precise roles of these kinetochore proteins in neurodevelopment are as yet unknown (Del Castillo, 2020).

Is cortical dynein the only factor that sorts microtubule polarity in axons? Data from a number of groups using different model systems support the idea of dynein being the universal motor that sorts axonal microtubules. However, depletion of other proteins also results in microtubule polarity defects in axons. For example, it has been reported that TRIM46, a microtubule-associated protein anchored to the AIS through AnkG, is able to organize uniformly oriented microtubule bundles. Obviously, the S2 screen is not comprehensive, and it is not even possible to exclude that the targets that gave negative results in the S2 screen could in fact be involved in microtubule organization in neurons as S2 cell processes are a very crude model of microtubule organization in neurons. It is likely that the development of a nonpolarized neurite into a fully functional axon is a complex process that requires cooperation of multiple components including dynein, dynein adaptors, other microtubule-binding proteins and components of the AIS. The data shown in this study simply demonstrate that Spindly belongs to this group of proteins and is an important factor in the recruitment of dynein to F-actin. Future work will show how these 'mitotic' components work together to properly organize axonal microtubules (Del Castillo, 2020).

A possible indirect role of RIN in the regulation of cRP mRNAs relates to their upregulation by the Ragulator complex. This study shows that RIN binds mRNAs encoding all five subunits of the Ragulator complex. Thus, positive regulation by RIN of Ragulator subunit synthesis could indirectly promote cRP mRNA translation (Laver, 2020).

Direct and indirect regulation of other aspects of gene expression by RIN are also likely. Potentiation of production of core components of the transcription, splicing, and translation machinery might ensure that none of them becomes rate limiting for gene expression in rapidly developing early embryos. Likewise, adequate ATP production would be ensured by potentiation of expression of mitochondrial ribosomal proteins and components of the electron transport chain (Laver, 2020).

That said, the data suggest that the direct effects of RIN are much more pronounced than its indirect effects. Specifically, this study has shown that, in rin mutants, levels of several target mRNAs are significantly reduced, whereas co-expressed non-targets do not change significantly. It should be noted, however, that these analyses were of a small subset of targets; future global analyses might reveal that indirect targets also change significantly (Laver, 2020).

The data have implications for understanding of how cells respond to stress and the role of SGs in that response. A theme in the cellular stress response is a general downregulation of mRNA expression. For example, stress triggers eukaryotic translation initiation factor 2 subunit alpha (eIF2a) phosphorylation, which prevents translation initiation. This, in turn, triggers polysome disassembly, resulting in SG assembly. The storage of long mRNAs in SGs serves as a further mechanism to downregulate their translation. Based on the current results, it is proposed that the recruitment of RIN/G3BPs into SGs would sequester these proteins from their short target mRNAs in the cytoplasm, serving to downregulate the expression of these transcripts. This could indirectly downregulate global gene expression by limiting the production of proteins involved in translation, transcription, and splicing. Likewise, if RIN/G3BPs serve to upregulate mitochondrial function and ATP production, sequestration could attenuate this aspect of cellular metabolism in stressed cells (Laver, 2020).

A transient helix in the disordered region of dynein light intermediate chain links the motor to structurally diverse adaptors for cargo transport

All animal cells use the motor cytoplasmic dynein 1 (dynein) to transport diverse cargo toward microtubule minus ends and to organize and position microtubule arrays such as the mitotic spindle. Cargo-specific adaptors engage with dynein to recruit and activate the motor, but the molecular mechanisms remain incompletely understood. HThis study used structural and dynamic nuclear magnetic resonance (NMR) analysis to demonstrate that the C-terminal region of human dynein light intermediate chain 1 (LIC1) is intrinsically disordered and contains two short conserved segments with helical propensity. NMR titration experiments reveal that the first helical segment (helix 1) constitutes the main interaction site for the adaptors Spindly (SPDL1), bicaudal D homolog 2 (BICD2), and Hook homolog 3 (HOOK3). In vitro binding assays show that helix 1, but not helix 2, is essential in both LIC1 and LIC2 for binding to SPDL1, BICD2, HOOK3, RAB-interacting lysosomal protein (RILP), RAB11 family-interacting protein 3 (RAB11FIP3), ninein (NIN), and trafficking kinesin-binding protein 1 (TRAK1). Helix 1 is sufficient to bind RILP, whereas other adaptors require additional segments preceding helix 1 for efficient binding. Point mutations in the C-terminal helix 1 of Caenorhabditis elegans LIC, introduced by genome editing, severely affect development, locomotion, and life span of the animal and disrupt the distribution and transport kinetics of membrane cargo in axons of mechanosensory neurons, identical to what is observed when the entire LIC C-terminal region is deleted. Deletion of the C-terminal helix 2 delays dynein-dependent spindle positioning in the one-cell embryo but overall does not significantly perturb dynein function. It is concluded that helix 1 in the intrinsically disordered region of LIC provides a conserved link between dynein and structurally diverse cargo adaptor families that is critical for dynein function in vivo (Celestino, 2019).

Self-assembly of the RZZ complex into filaments drives kinetochore expansion in the absence of microtubule attachment

The kinetochore is a dynamic multi-protein assembly that forms on each sister chromatid and interacts with microtubules of the mitotic spindle to drive chromosome segregation. In animals, kinetochores without attached microtubules expand their outermost layer into crescent and ring shapes to promote microtubule capture and spindle assembly checkpoint (SAC) signaling. Kinetochore expansion is an example of protein co-polymerization, but the mechanism is not understood. This study presents evidence that kinetochore expansion is driven by oligomerization of the Rod-Zw10-Zwilch (RZZ) complex, an outer kinetochore component that recruits the motor dynein and the SAC proteins Mad1-Mad2. Depletion of ROD in human cells suppresses kinetochore expansion, as does depletion of Spindly, the adaptor that connects RZZ to dynein, although dynein itself is dispensable. Expansion is also suppressed by mutating ZWILCH residues implicated in Spindly binding. Conversely, supplying cells with excess ROD facilitates kinetochore expansion under otherwise prohibitive conditions. Using the C. elegans early embryo, this study demonstrates that ROD-1 has a concentration-dependent propensity for oligomerizing into micrometer-scale filaments, and the ROD-1 beta-propeller was identified as a key regulator of self-assembly. Finally, it was shown that a minimal ROD-1-Zw10 complex efficiently oligomerizes into filaments in vitro. These results suggest that RZZ's capacity for oligomerization is harnessed by kinetochores to assemble the expanded outermost domain, in which RZZ filaments serve as recruitment platforms for SAC components and microtubule-binding proteins. Thus, it is proposed that reversible RZZ self-assembly into filaments underlies the adaptive change in kinetochore size that contributes to chromosome segregation fidelity (Pereira, 2018).

Spindly is required for rapid migration of human cells

Dynein is the sole processive minus-end-directed microtubule motor found in animals. It has roles in cell division, membrane trafficking, and cell migration. Together with dynactin, dynein regulates centrosomal orientation to establish and maintain cell polarity, controls focal adhesion turnover and anchors microtubules at the leading edge. In higher eukaryotes, dynein/dynactin requires additional components such as Bicaudal D to form an active motor complex and for regulating its cellular localization. Spindly is a protein that targets dynein/dynactin to kinetochores in mitosis and can activate its motility in vitro However, no role for Spindly in interphase dynein/dynactin function has been found. This study shows that Spindly binds to the cell cortex and microtubule tips and colocalizes with dynein/dynactin at the leading edge of migrating U2OS cells and primary fibroblasts. U2OS cells that lack Spindly migrated slower in 2D than control cells, although centrosome polarization appeared to happen properly in the absence of Spindly. Re-expression of Spindly rescues migration, but the expression of a mutant, which is defective for dynactin binding, failed to rescue this defect. Taken together, these data demonstrate that Spindly plays an important role in mediating a subset of dynein/dynactin's function in cell migration (Conte, 2018a).

USP45 and Spindly are part of the same complex implicated in cell migration

Ubiquitylation is a protein modification implicated in several cellular processes. This process is reversible by the action of deubiquinating enzymes (DUBs). USP45 is a ubiquitin specific protease about which little is known, aside from roles in DNA damage repair and differentiation of the vertebrate retina. In this study, by using mass spectrometry, Spindly was identified as a new target of USP45. The data show that Spindly and USP45 are part of the same complex and that their interaction specifically depends on the catalytic activity of USP45. In addition, the type of ubiquitin chains associated with the complex are described that can be cleaved by USP45, with a preferential activity on K48 ubiquitin chain type and potentially K6. This study also shows that Spindly is mono-ubiquitylated and this can be specifically removed by USP45 in its active form but not by the catalytic inactive form. Lastly, a new role was identified for USP45 in cell migration, similar to that which was recently described for Spindly (Conte, 2018b).

Localization and function of mSpindly during mouse oocyte meiotic maturation

Spindly was first identified in Drosophila; its homologues are termed SPDL-1 in Caenorhabditis elegans and Hs Spindly/hSpindly in humans. In all species, Spindly and its homologues function by recruiting dynein to kinetochores and silencing spindle assembly checkpoint (SAC) in mitosis of somatic cells. Depletion of Spindly causes an extensive metaphase arrest during somatic mitoses in Drosophila, C. elegans and humans. In Drosophila, Spindly is required for shedding of Rod and Mad2 from the kinetochores in metaphase; in C. elegans, SPDL-1 presides over the recruitment of dynein and MDF-1 to the kinetochores; in humans, Hs Spindly is required for recruiting both dynein and dynactin to kinetochores but it is dispensable for removal of checkpoint proteins from kinetochores. The present study was designed to investigate the localization and function of the Spindly homologue (mSpindly) during mouse oocyte meiotic maturation by immunofluorescent analysis, and by overexpression and knockdown of mSpindly. mSpindly was found typically localized to kinetochores when chromatin condensed into chromosomes after GVBD. In metaphase of both first meiosis and second meiosis, mSpindly was localized not only to kinetochores but also to the spindle poles. Overexpression of mSpindly did not affect meiotic progression, but its depletion resulted in an arrest of the pro-MI/MI stage, failure of anaphase entry and subsequent polar body emission, and in abnormal spindle morphology and misaligned chromosomes. These data suggest that mSpindly participates in SAC silencing and in spindle formation as a recruiter and/or a transporter of kinetochore proteins in mouse oocytes, but that it needs to cooperate with other factors to fulfill its function (Zhang, 2010).

Mitotic control of kinetochore-associated dynein and spindle orientation by human Spindly

Mitotic spindle formation and chromosome segregation depend critically on kinetochore-microtubule (KT-MT) interactions. A new protein, termed Spindly in Drosophila and SPDL-1 in C. elegans, was recently shown to regulate KT localization of dynein, but depletion phenotypes revealed striking differences, suggesting evolutionarily diverse roles of mitotic dynein. By characterizing the function of Spindly in human cells, this study identified specific functions for KT dynein. Localization of human Spindly (hSpindly) to KTs is controlled by the Rod/Zw10/Zwilch (RZZ) complex and Aurora B. hSpindly depletion results in reduced inter-KT tension, unstable KT fibers, an extensive prometaphase delay, and severe chromosome misalignment. Moreover, depletion of hSpindly induces a striking spindle rotation, which can be rescued by co-depletion of dynein. However, in contrast to Drosophila, hSpindly depletion does not abolish the removal of MAD2 and ZW10 from KTs. Collectively, these data reveal hSpindly-mediated dynein functions and highlight a critical role of KT dynein in spindle orientation (Chan, 2009).

Search PubMed for articles about Drosophila Spindly

Acquaviva, C. and Pines, J. (2006). The anaphase-promoting complex/cyclosome: APC/C. J. Cell Sci. 119: 2401-2404. PubMed ID: PubMed ID; Online text

Barbosa, J., Martins, T., Bange, T., Tao, L., Conde, C. and Sunkel, C. (2020). Polo regulates Spindly to prevent premature stabilization of kinetochore-microtubule attachments. EMBO J 39(2): e100789. PubMed ID: 31849090

Celestino, R., Henen, M. A., Gama, J. B., Carvalho, C., McCabe, M., Barbosa, D. J., Born, A., Nichols, P. J., Carvalho, A. X., Gassmann, R. and Vogeli, B. (2019). A transient helix in the disordered region of dynein light intermediate chain links the motor to structurally diverse adaptors for cargo transport. PLoS Biol 17(1): e3000100. PubMed ID: 30615611

Chan, G. K., et al. (1999). Human BUBR1 is a mitotic checkpoint kinase that monitors CENP-E functions at kinetochores and binds the cyclosome/APC. J. Cell Biol. 146: 941-954. PubMed ID: 10477750

Chan, Y. W., Fava, L. L., Uldschmid, A., Schmitz, M. H., Gerlich, D. W., Nigg, E. A. and Santamaria, A. (2009). Mitotic control of kinetochore-associated dynein and spindle orientation by human Spindly. J Cell Biol 185(5): 859-874. PubMed ID: 19468067

Cheerambathur, D. K., Gassmann, R., Cook, B., Oegema, K. and Desai, A. (2013). Crosstalk between microtubule attachment complexes ensures accurate chromosome segregation. Science 342(6163): 1239-1242. PubMed ID: 24231804

Conte, C., Baird, M. A., Davidson, M. W. and Griffis, E. R. (2018a). Spindly is required for rapid migration of human cells. Biol Open 7(5). PubMed ID: 29685992

Conte, C., Griffis, E. R., Hickson, I. and Perez-Oliva, A. B. (2018b). USP45 and Spindly are part of the same complex implicated in cell migration. Sci Rep 8(1): 14375. PubMed ID: 30258100

De Antoni, A., et al. (2005). The Mad1/Mad2 complex as a template for Mad2 activation in the spindle assembly checkpoint. Curr. Biol. 15: 214-225. PubMed ID: 15694304

Del Castillo, U., Muller, H. J. and Gelfand, V. I. (2020). Kinetochore protein Spindly controls microtubule polarity in Drosophila axons. Proc Natl Acad Sci U S A 117(22): 12155-12163. PubMed ID: 32430325

Dzhindzhev, N. S., Rogers, S. L., Vale, R. D. and Ohkura. H. (2005). Distinct mechanisms govern the localisation of Drosophila CLIP-190 to unattached kinetochores and microtubule plus-ends. J. Cell Sci. 118: 3781-3790. PubMed ID: PubMed ID; Online text

Griffis, E. R., Stuurman, N. and Vale, R. D. (2007). Spindly, a novel protein essential for silencing the spindle assembly checkpoint, recruits dynein to the kinetochore. J. Cell Biol. 177(6): 1005-15. PubMed ID: 17576797

Howell, B. J., et al. (2001). Cytoplasmic dynein/dynactin drives kinetochore protein transport to the spindle poles and has a role in mitotic spindle checkpoint inactivation. J. Cell Biol. 155: 1159-1172. PubMed ID: PubMed ID; Online text

Karess, R. (2005). Rod-Zw10-Zwilch: a key player in the spindle checkpoint. Trends Cell Biol. 15: 386-392. PubMed ID: 15922598

Mao, Y., Desai, A. and Cleveland, D. W. (2005). Microtubule capture by CENP-E silences BubR1-dependent mitotic checkpoint signaling. J. Cell Biol. 170: 873-880. PubMed ID: 16144904

Musacchio, A. and Salmon, E. D. (2007). The spindle-assembly checkpoint in space and time. Nat. Rev. Mol. Cell Biol. 8: 379-393. PubMed ID: 17426725

Pereira, C., Reis, R. M., Gama, J. B., Celestino, R., Cheerambathur, D. K., Carvalho, A. X. and Gassmann, R. (2018). Self-assembly of the RZZ complex into filaments drives kinetochore expansion in the absence of microtubule attachment. Curr Biol 28(21): 3408-3421. PubMed ID: 30415699

Sacristan, C., Ahmad, M. U. D., Keller, J., Fermie, J., Groenewold, V., Tromer, E., Fish, A., Melero, R., Carazo, J. M., Klumperman, J., Musacchio, A., Perrakis, A. and Kops, G. J. (2018). Dynamic kinetochore size regulation promotes microtubule capture and chromosome biorientation in mitosis. Nat Cell Biol 20(7): 800-810. PubMed ID: 29915359

Schroer, T. A. (2004). Dynactin. Annu. Rev. Cell Dev. Biol. 20: 759-779. PubMed ID: 15473859

Starr, D. A., Williams, B. C., Hays, T. S. and Goldberg, M. L. (1998). ZW10 helps recruit dynactin and dynein to the kinetochore. J. Cell Biol. 142: 763-774. PubMed ID: 9700164

Wojcik, E., et al. (2001). Kinetochore dynein: its dynamics and role in the transport of the Rough deal checkpoint protein. Nat. Cell Biol. 3: 1001-1007. PubMed ID: 11715021

Zhang, Q. H., Wei, L., Tong, J. S., Qi, S. T., Li, S., Ou, X. H., Ouyang, Y. C., Hou, Y., An, L. G., Schatten, H., Schatten, H. and Sun, Q. Y. (2010). Localization and function of mSpindly during mouse oocyte meiotic maturation. Cell Cycle 9(11): 2230-2236. PubMed ID: 20505367

date revised: 27 December 2020

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