# 2009 The Authors

Journal compilation # 2009 Blackwell Munksgaard

doi: 10.1111/j.1600-0854.2009.00880.xTraffic 2009; 10: 472–481Blackwell Munksgaard

Review

Centrosome Function: Sometimes Less Is More

Nasser M. Rusan1,* and Gregory C. Rogers2

1Department of Biology, University of North Carolina atChapel Hill, CB#3280, Coker Hall, Chapel Hill,NC 27599, USA2Department of Cell Biology and Anatomy, ArizonaCancer Center, The University of Arizona, Tucson,AZ 85724, USA*Corresponding author: Nasser M. Rusan,nasser@unc.edu

Tight regulation of centrosome duplication is critical to

ensure that centrosome number doubles once and only

once per cell cycle. Superimposed onto this centrosome

duplication cycle is a functional centrosome cycle in which

they alternate between phases of quiescence and robust

microtubule (MT) nucleation and MT-anchoring activities.

In vertebrate cycling cells, interphase centrioles accumu-

late less pericentriolar material (PCM), reducing their MT

nucleation capacity. In mitosis, centrosomesmature, accu-

mulating more PCM to increase their nucleation and

anchoring capacities to form robust MT asters. Interest-

ingly, functional cycles of centrosomes can be altered to

suit the cell’s needs. Some interphase centrosomes func-

tion as a microtubule-organizing center by increasing their

ability to anchor MTs to form centrosomal radial arrays.

Other interphase centrosomes maintain their MT nucle-

ation capacity but reduce/eliminate their MT-anchoring

capacity. Recent work demonstrates that Drosophila cells

take this to the extreme, whereby centrioles lose all

detectable PCM during interphase, offering an explanation

as to how centrosome-deficient flies develop to adulthood.

Drosophila stem cells further modify the functional cycle

bydifferentially regulating their two centrioles – a situation

that seems important for stem cell asymmetric divisions,

as misregulation of centrosome duplication in stem/

progenitor cells can promote tumor formation. Here, we

review recent findings that describe variations in the

functional cycle of centrosomes.

Key words: asymmetric division, centriole, centrosome

maturation, Drosophila, spindle alignment, stem cells

Received 10 October 2008, revised and accepted for

publication 11 January 2009, uncorrected manuscript

published online 24 January 2009

In 1894, Sakugoro Hirase identified structures in pollen

cells of Ginkgo biloba that he called ‘attractive spheres’ (1).

However appropriate the naming was, these structures

were in fact ‘centrosomes’, a name coined by Theodor

Bovari in 1888 but first seen byWalther Flemming in 1875.

Today, we are still asking some of the same questions that

Bovari and others raised regarding these attractive

spheres. Of what are the centrosomes comprised? What

is the role(s) of a centrosome in cells? How does their role

change depending on the organism, cell type, develop-

mental stage and cell cycle phase? What happens in cells

with abnormal centrosome behavior, function or number?

Most importantly, could these changes be detrimental to

the health of the cell and organism? Researchers have

made several important advances in recent years to help

answer some of these questions, but we have only

scratched the surface of our understanding.

One fascinating aspect of centrosomes is their ability to

perform different tasks in different cells and at different

points in the cell cycle. This review focuses on how some

cells have modified the functional cycle of the centrosome

to complete cell-specific tasks. Cells can accomplish

this either by changing the role of the centrosome or by

reducing/eliminating its role completely. In particular, we

will discuss how Drosophila cells have adopted an inter-

esting twist to the canonical functional centrosome cycle.

The Canonical Centrosome Cycle:A Textbook View

The centrosome is a nonmembrane-bound organelle that

features two centrioles at its core. Centrioles are barrel-

shaped structures that lie orthogonal to each other and are

composed of nine doublet or triplet microtubule (MT)

bundles. The central role of centrioles is to recruit an

amorphous cloud of pericentriolar material (PCM) that

surrounds the centrioles and is used to nucleate and

anchor MTs, forming a functional MT-organizing center

(MTOC). Like the genome, centrosome number is inti-

mately coupled to the cell cycle and is duplicated once and

only once per cycle (Figure 1) (2,3). The conclusion of each

cell cycle culminates in the production of two genetically

identical cells, each receiving a single copy of the genome

and a single centrosome. As the cell exits mitosis, the

mother and daughter centriole pair disengage (3), breaking

their orthogonal arrangement but remaining in close prox-

imity. In the DNA synthesis phase (S-phase), each centri-

ole nucleates a procentriole (or daughter) along its wall,

and in the second gap phase (G2-phase), these two

centriole pair accumulate more PCM, maturing into the

two centrosomes needed for mitosis.

Most relevant to this review is the centrosome functional/

maturation cycle. In vertebrate cycling cells, the amount of

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PCM that is associatedwith the centrioles varies throughout

the cell cycle (Figure 1). In S-phase, PCM is at its lowest

level; consequently, the MT nucleation and MT-anchoring

capacities of the centrosome are at its lowest (4). En

route to mitosis, centrosomes ‘mature’ by accumulating

more PCM. In late prophase–metaphase, PCM levels on

centrosomes reach a maximum level. Consequently, MT-

nucleating and MT-anchoring activities of centrosomes are

at their highest, forming robust asters that morph into the

bipolar mitotic spindle (5,6). Upon mitotic exit, PCM levels

on centrosomes drop (7). This functional cycle is super-

imposed onto, but independent of, the centriole duplication

cycle. This independence is best demonstrated during the

second meiotic division of Drosophila spermatocytes. Dur-

ing these reductional divisions, centriole duplication is

blocked, resulting in a single centriole comprising each pole

of the second meiotic spindle. In spite of the centriole

duplication block, each individual centriole is fully competent

in recruiting PCM and organizing spindle poles in mitosis.

Another example of this is seen in Caenorhabditis elegans

(C. elegans) embryos that are mutant for the protein kinase

zyg-1. In these mutants, centriole duplication is blocked, but

aster formation (and presumably PCM recruitment) is not

affected (8). Moreover, some centrosomal proteins are

involved in both duplication and PCM recruitment, and

these include C. elegans Spd-2 and its human homologue

Cep192 (9,10).

How is the centrosome maturation process regulated?

One known fact is that phosphorylation increases sub-

stantially at the centrosome in mitosis as shown by

immunofluorescence using the MPM2 antibody (11),

which recognizes mitotic phosphoepitopes. This centro-

some phosphorylation cycle perfectly matches the matu-

ration cycle of the centrosome (5,6,12), suggesting that

specific phosphorylation events lead to the recruitment of

PCM to the centrioles during mitotic entry. In fact, we

know of two mitotic kinases that are important for

centrosome maturation, Polo and Aurora A (13–15). These

two kinases phosphorylate their substrates, many of

which are unknown, which in turn leads to the recruitment

of PCM proteins such as g-tubulin, forming the so-called

amorphous cloud that surrounds the centrioles. Recently,

it has been shown that Aurora A phosphorylates Polo at

mitotic entry, suggesting that they both function in the

same centrosome maturation pathway (16–18). It is not

clear whether Aurora A can function independently in

centrosome maturation through a parallel mechanism

and whether Polo’s other roles, such as in specifying the

cytokinetic furrow, are reliant on Aurora A. A large

amount of work remains ahead to truly understand

how centrosome maturation is regulated and whether

or not the functional cycle of the centrosome is essential

for the centrosome’s role during both interphase and

mitosis.

Figure 1: The canonical cycle of

centrosome function. The cycle of

centrosome function is coupled to

the cell cycle. A) Cells in S-phase

duplicate their centrosomes and

begin to accumulate higher levels

of PCM. B) As cells near the end

of G2, the centrosomes continue

to mature by recruiting additional

PCM, nucleating more MTs and

anchoring their minus ends to form

robust asters. C) As PCM levels on

centrosomes continue to climb

through metaphase, so does their

MT-nucleating and MT-anchoring ca-

pacities. D) As the cell exits mitosis,

PCM levels drop and, in some cases,

PCM is shed in the form of mini MT

asters that eventually completely

disassemble (85). E) As cells pro-

ceed toward S-phase, PCM on the

centrosome drops to their lowest

level, concurrent with diminished

MTOC activity.

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Centrosome Function in Interphase

Centrosome Function in Mitotic andInterphase Cells

The centrosome is a critical MT-nucleating center and

MTOC in animal cells. In mitosis, two centrosomes

nucleate and anchor the MTs used to assemble a bipolar

spindle, capture kinetochores (19) and reel in any remain-

ing interphase noncentrosomal MTs (20). Centrosomes

then form the cornerstones of the bipolar mitotic spindle,

supplying with the MTs it needs to complete the task of

separating the duplicated genome. In interphase, centro-

somes are thought to anchor MTs to form a radial array

that provides cells with support and rigidity. This array

also establishes intracellular polarity, with MTminus ends

concentrated at the MTOC in the middle of the cell and

plus ends extending toward the cell cortex. MT polarity

then provides directionality to traffic a variety of cargo

toward and away from the nucleus. Other examples of

centrosome-based intracellular polarity include the apical

to basal MTs found in columnar epithelial cells and theMT

arrays oriented toward the leading edge of migrating

cells.

However, this ‘centrosomal array’ of MTs is an overly

simplistic view of how MTs are organized and is not used

by all cell types; in fact, the interphase array can be

remodeled to become ‘noncentrosomal’ depending on

the cell’s needs (21,22). For example, noncentrosomal

MTs are arranged along the axons of neurons to facilitate

trafficking of mitochondria (23), messenger RNA (24) and

essential proteins to and from the growth cone. In cells

with noncentrosomal MT arrays, centrosomes are thought

to serve as the main MT nucleation site, as is the case in

neurons, producing MTs and then releasing or severing

them to allow for their transport (25). Centrosomes have

also been implicated in signaling pathways, such as cell

cycle progression and cell abscission at the end of mitosis

(26). It would seem centrosomes are of such great

importance that they are absolutely indispensable in all

phases of the cell cycle . . . right?

Flies Without CentrosomesChallenge This Model

In 2001, a study by Megraw et al. showed for the first time

that Drosophila mutants that lack functional centrosomes

complete zygotic development to produce viable adults

(27). This involved flies with zygotic null mutations in

Centrosomin (Cnn), a PCM component required to prop-

erly recruit other important PCM proteins such as

g-tubulin, D-TACC and Mini spindles (28). These findings

left researchers both fascinated and skeptical, as cnn

mutants still possess centrioles, and thus may retain some

semblance of centrosome function. This was later cor-

roborated by examining flies mutant for either D-PLP or

SAS-4, each vital for centriole assembly. These mutants

also developed to adulthood (29,30). Although centro-

somes play such an important role in nucleating MTs and

organizing bipolar mitotic spindles, dividing cells could still

build functional spindles in the complete absence of

centrosomes (27,28). Likely, this is because of a Ran-

dependent chromatin-induced spindle assembly pathway,

which is masked by the more dominant centrosome-

induced pathway in normal cells (31).

This significantly altered our view of the critical nature of

centrosomes. However, centrosomes remain quite impor-

tant. For example, mutations in the C. elegans centroso-

mal protein Spd-5 lead to defects in centrosome function

and spindle formation, indicating the importance of cen-

trosomes in C. elegans embryos (32). In addition, these

results suggest that noncentrosomal spindle assembly

pathways cannot compensate for defects in centrosome

function in this system (32), although we cannot exclude

a role for Spd-5 in these other pathways. Another example

is the essential role of centrosomes for progression

through the early syncytial blastoderm stage of Drosophila

development. During this stage, nuclei and their attached

centrosomes migrate to the cortex in these large cells and

undergo a series of very rapid mitotic events (cleavage

divisions 10–13) that occur without intervening cytokinesis

(33). The hundreds of spindles that orchestrate these

divisions assemble as a two-dimensional array that lies

beneath the plasma membrane. Centrosomes and the

astral arrays they produce play a critical role in maintaining

the proper spacing of the spindles. Loss of centrosomes

compromises this spacing, which induces severe spindle

assembly defects and errors in chromosome segregation,

culminating in a block to further development (34). Cen-

trioles also form the basal bodies required to assemble

axonemes, which give rise to cilia and flagella. Indeed,

centriole-deficient adult flies are severely uncoordinated

and die soon after eclosion because of a lack of cilia in their

type I mechanosensory neurons (29). In addition, centriole-

deficient males produce sperm that lack flagella (29).

Finally, larval neuroblasts (NBs) that lack centrosomes

show occasional defects in asymmetric division

(27,29,35,36). Thus, centrosomes are important organelles

for proper fly development and physiology.

Alternate Centrosome Functional Cycles andMT Nucleation Pathways in Interphase Cells

Still, the fact that centriole-deficient flies can progress

through zygotic development raises an interesting para-

dox. Textbooks describe the centrosome as the MT-

nucleating center and MTOC of the cell. By recruiting

g-tubulin, the major MT-nucleating factor in cells, the

centrosome builds a polarized MT array used to position

organelles and mediate numerous intracellular transport

events (37). Without the polarized MT array that centro-

somes provide, how do centriole-deficient interphase

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Drosophila cells survive? Recent work revealed a simple

but surprising answer. Normal Drosophila cells do not

adhere to the canonical functional centrosome cycle, in

that they do not assemble functional centrosomes during

interphase (Figure 2A) (38). For example, culturedDrosophila

S2 and D16 cells, as well as embryonic leading edge and

amnioserosal cells, do not have functional interphase

centrosomes; instead, their centrioles lack PCM and,

consequently, do not nucleate MTs. Drosophila NBs,

however, represent an exception to this rule, as they

contain two distinct centrioles that display a unique asym-

metric centrosome-forming behavior during interphase

(we discuss these observations in more detail below).

Although Drosophila centrioles undergo a normal duplica-

tion program, this functionally dormant behavior in most

cells differs from vertebrate somatic cycling cells. Verte-

brate centrioles recruit less PCM during interphase com-

pared with mitosis. Apparently, fly interphase centrioles

take this behavior to the extreme and completely lack PCM

components such as g-tubulin, Cnn, CP60 and CP190

during interphase (Figure 2A) (38). As a consequence,

centrioles do not nucleate MT growth and wander seem-

ingly aimlessly within the interphase cytoplasm. Only as

cells near mitosis do fly centrioles recruit PCM and

become dominant MTOCs that assist in (but are not

required for) spindle formation. The revelation that centro-

somes do not normally form in interphase Drosophila cells

helps explain why the absence of centrioles has little

impact on development. Perhaps centrosomes are only

functional during cell division to ensure their own proper

segregation so that they are available to build axonemes

when later called upon (39). The question remains: is it

possible for organisms that do have functional centro-

somes in interphase to develop without centrosomes?

Interestingly, this highly specialized functional centrosome

cycle is not observed in all fly cells. For example, during the

early embryonic syncytial divisions, functional centro-

somes are observed during both alternating M-phase and

S-phase. Perhaps centrioles during these rapid divisions

(10–15 min apart) cannot afford the time to completely

disassemble and assemble their PCM. Furthermore, cen-

trosomes ensure robust MT asters to maintain proper

spacing of nuclei and spindles. It remains unclear when the

functional centrosome cycle changes during embryogen-

esis, but loss of functional centrosomes is observed in

interphase cells during the dorsal closure stage (38). This

may happen during the incorporation of G2-phase in the

cell cycle, which occurs after cellularization (cell cycle 14).

This will require further investigation to resolve.

Acentrosomal MT arrays are observed in a variety of animal

and fungal cells (22). Examples include human myotubes

(40), Schizosaccharomyces pombe (S. pombe) (41), axonal

MTs and many more (22). In Drosophila, acentrosomal cells

include embryonic amnioserosal and leading edge cells

(38,42), wing epidermal cells (43,44), ommatidial cone

cells (45), epidermal tendon cells (46) as well as several

immortalized cell lines (38,47,48). Acentrosomal MTs in

cultured fly cells (Figure 2A) appear as nonradial networks

that lack an obvious MTOC and grow apparently randomly

in the cytoplasm (38,49). Without centrosomes, how are

interphase MT arrays generated and then organized to

provide the proper polarity needed for general cell homeo-

stasis? Although the answers are not clear, Drosophila

offers an excellent system to address these questions.

Because g-tubulin is the principal MT nucleation factor in all

eukaryotic cells, it is expected to generate the bulk of tubulin

polymer within acentrosomal arrays. Surprisingly, however,

RNA interference-mediated depletion of both fly g-tubulin

isotypes (23C and 37C) has no effect on tubulin polymer

levels at steady state in interphase cultured Schneider (S2)

cells (38). These results suggest that either MT self-

assembly, an inherent property of ab-tubulin, drives

the production of acentrosomal arrays or alternative MT

nucleation factors remain to be identified (Figure 2B). How-

ever, g-tubulin does appear to play some role in MT

assembly during interphase, as cells depleted of g-tubulin

display a delay in the kinetics of MT regrowth after cold-

induced depolymerization (38). Cells that fully recover from

this treatment show equivalent MT polymer levels com-

pared with control cells. Similar results were observed in

human U2OS cells (50), suggesting a conserved g-tubulin-

independent mechanism in nucleating acentrosomal

MT arrays. Thus, many Drosophila interphase cells lack

Figure 2: The functional centrosome cycle in Drosophila. A)

Observations ofDrosophila cells at steady state, both cultured and

in vivo, differ from the canonical functional cycle by reducing PCM

at the centrioles below detectable levels during interphase.

Furthermore, live imaging shows that centrioles are both highly

mobile and do not nucleate MTs. In contrast, centrioles recruit

PCM and assemble functional centrosomes only during cell

division (not shown). Interestingly, NBs represent an exception

as they do have an interphase centrosome. B) MT regrowth after

a cold-inducedMTdepolymerization treatment in culturedDrosophila

cells revealed the formation of discrete MT asters (38). Some

nucleation is observed at the Golgi and at centrioles (not observed

at steady state) as well as other unidentified sites throughout the

cytoplasm. These findings suggest the presence of an unknown

nucleation pathway that may involve additional organelles.

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Centrosome Function in Interphase

centrosomes and do not require g-tubulin to build their MT

arrays, challenging the canonical model of MT assembly in

animal cells.

Interestingly, MTOCs, which are not seen in S2 cells at

steady state, form throughout the cell during the cold

recovery period of anMT regrowth assay, appearing as small

MT foci (Figure 2B) (38,48,49). As MT regrowth continues,

these foci morph into an extensive interconnecting network.

Importantly, most of these MT foci are not associated with

centrioles. Given these findings, it is enticing to speculate

that MT nucleation occurs from a different organelle. Indeed,

acentrosomal MT nucleation is observed off of the trans

Golgi network in mammalian cells (51). However, many MT

foci in cold-recovering S2 cells are not associated with Golgi

(38). Recently, new MT nucleation factors have been iden-

tified that assist in assembling both interphase and mitotic

MT arrays. These include MT-associated plus-end tracking

proteins and motors, most of which apparently work in

concert with g-tubulin to generateMT growth. These include

a Golgi-associated population of mammalian CLIP-associated

protein (CLASP), Xenopus XMAP215 (52), its yeast homo-

logue Stu2p (53) and its fly homologue Mini spindles (54),

cytoplasmic Dynein (55), EB1 and CLIP190 (the Drosophila

homologue of CLIP170) (38). In addition, a provocative study

identified a role for Aurora A in assembling a g-tubulin-

independent population of MTs from mitotic asters during

early C. elegans embryogenesis (56), although this assembly

appears to still occur at the centrosomes. Thus, identifying

newMT nucleation factors and unraveling their mechanisms

of action are exciting areas of cytoskeletal research.

The Functional Centrosome Cycle inDrosophila Stem and Progenitor Cells

The hallmark of a stem cell is its ability to divide to produce

a self-renewing stem cell and a daughter cell that will

undertake a path of differentiation. They are vital for the

development and homeostasis of organs. In Drosophila,

stem cells have been identified in tissues such as the

midgut (57), hindgut (58), nervous system and reproduc-

tive system. The best characterized stem cells are the

Drosophilamale and female germ line stem cells (mGSCs/

fGSCs) and the neural precursor cells, NBs. Both serve as

wonderful models to ask questions regarding polarity

establishment, mitotic spindle orientation and its regula-

tion as well as the asymmetric distribution of cell-fate

determinants. Recently, the germ line stem cells (GSCs)

and NBs have revealed new insights into the roles of

centrosomes in stem cells (36,59–61). These studies also

uncovered a pivotal role for the centrosome in preventing

malignant transformation (62,63) by establishing a new

twist to the conventional functional cycle (36,60,61).

It is absolutely crucial for Drosophila GSCs and NBs, both

embryonic and larval, to divide along a highly specific axis

to reach the desired outcome of a self-renewing stem cell

and a differentiating daughter cell. In NBs, spindle axes

are determined by the polarity of the stem cell, which are

initiated by intertwining Par3/Par6/aPKC and Lgl/Dlg/

Scribble polarity pathways (64,65). The mitotic spindle

is then aligned along the cell polarity axis, which in turn

feeds back into the system to maintain polarity (66,67).

This process of alignment is thought to involve the attach-

ment of astral MTs to apical polarity proteins (details are

not discussed here). Instead, we discuss the behavior of

the centrosome and its role in spindle alignment within

stem cells.

Germ Line Stem Cells

Stem cells of the male and female germ line reside in

a highly organized cellular niche within their respective

organs. The mGSCs are located at the tip of the testis

where a cluster of somatic cells, called the hub, anchors

6–12 GSCs (Figure 3A). mGSCs undergo a highly stereo-

typed division along an axis perpendicular to the hub and

parallel to the established polarity axis (Figure 3A) –

dividing asymmetrically to produce a gonialblast daughter

cell (68). Proper asymmetric division is highly reliant on

centrosome function and astral MT–cortical interactions.

mGSCs mutant for Cnn contain centrosomes of highly

reduced function, leading to failed alignment of the mitotic

spindle toward the hub (30% of the time) and, conse-

quently, a symmetric division that increases the number of

mGSCs (59). Correct spindle orientation is thought to

be achieved by astral MT attachment to the interface

between the mGSC and the hub, possibly through an

MT–cell junction interaction by the tumor suppressor gene

adenomatous polyposis coli (59). Likely, this secure attach-

ment of the centrosome near the hub forces the spindle

into the correct alignment (Figure 3A).

Recent work revealed that mGSCs retain the same centro-

some through multiple divisions. This ‘mother centrosome’

is stably attached to the hub and thus is continuously

inherited by the mGSC. Consequently, the newly duplicated

‘daughter centrosome’ is always segregated into the gonial-

blast (68). Could modulating the functional cycles of the

mother and daughter centrosomes be used as amechanism

to promote asymmetric division, spindle alignment and

centrosomal inheritance?

NBs: The Neural Progenitor Cells

Drosophila larval central brain NBs also undergo a stereo-

typed asymmetric division. Each NB divides to self-renew

and to produce a daughter ganglionmother cell (GMC). The

GMCs remain attached to the NB, as an asymmetric

cluster (or cap), and their daughters differentiate as

neurons (Figure 3B). Together, the GMCs act as a single

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functional unit by sending out individual axons that then

join to form a fasciculate axon. This grouping of the GMCs

on one side of the NB is achieved through a persistent

division axis, always placing the newest daughter GMC

next to the previous GMC (Figure 3B).

Centrosomes are important for spindle alignment in NBs

because they produce astral MTs that are thought to

interact with the cell cortex through protein complexes

such as Lis/Dynein and Mud/Pins/GaI (69–72). Mutations

in the protein DSAS-4 or Asterless, which eliminate

centrioles/centrosomes completely, can lead to defects

in spindle alignment and a symmetric division (29,36) –

producing two NBs and no GMC. Recent live cell imaging

uncovered a novel functional centrosome cycle in these

stem cells that may play a significant role in correctly

aligning the spindle (36,61). As the NB exits mitosis, its

centriole pair disengage and completely detach from each

other. One centriole remains stationary opposite the GMC

cap (apically) and, surprisingly, retains a sufficient amount

of PCM to organize an interphase MTOC (the ‘dominant

centrosome’, Figure 3B, early interphase). We hypothe-

size that the dominant centrosome remains in the NB by

anchoring itself to an unknown apical–cortical cue that

maintains its position opposite the GMC cap throughout

the cell cycle. The second centriole adheres to the

canonical Drosophila functional centrosome cycle, shed-

ding all associated PCM and MTs and moving freely

throughout the cell (Figure 3B, early interphase).

Interestingly, work in mGSCs suggests that an identical

functional centrosome cycle exists in mGSC (60). At some

point in interphase, themother centrosome, retained in the

mGSC, forms a more robust MT aster compared with the

aster formed by the daughter centrosome. It is possible

that mGSCs use a similar functional cycle as observed in

NBs. Although challenging, live cell imaging of MTs,

centrioles and PCM through multiple divisions will be

necessary to uncover the details of centrosome behavior

in these cells. If asymmetric functional centrosome cycles

are observed in mGSCs, then it raises the question of

whether this unorthodox twist to the cycle is a general

feature of stem cells.

In both mGSCs and NBs, anchoring of an active dominant

centrosome could essentially ensure the proper alignment

of the mitotic spindle prior to its assembly. How could this

happen mechanistically? As the centriole pair separate at

the end of mitosis, one centriole remains an active MTOC

and presumably anchored through MT–cortical interac-

tions, while the second centriole is inactivated. Blocking

the ability of one of the centrioles to recruit PCM would

eliminate two potential pitfalls. First, it prevents the

anchoring of both centrosomes to the apical cortex.

Figure 3: Drosophila stem cells differentially regulate theMTOC activity of their two centrosomes. A) mGSCs require centrosomes

for proper spindle alignment and asymmetric division. In late interphase, one centrosome forms a robust aster that is anchored by astral

MTs to the hub through adherens junctions (red block). The second centrosome forms a lesser MT aster and travels away from the

anchored centrosome prior to mitosis. B) Central brain NBs also require centrosomes for alignment and asymmetric division. In early

interphase, one centrosome remains active to form an MTOC and is possibly anchored to unknown apical–cortical cue (pink). The other

centriole sheds its PCM to allow its mobility and only matures as the cell enters prophase. In metaphase, the spindle is aligned along the

cell polarity axis (red and blue) as instructed by the position of the dominant centrosome during interphase.

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Centrosome Function in Interphase

Second, it would eliminate the effect of MT-based centro-

some clustering, known to be quite robust in Drosophila

cells (73,74). Thus, conceivably, inactivation of one centri-

ole would result in its mobility [similar to the behavior of

centrioles in cultured fly cells (38)], whereas the active

centriole would remain stationary. Although the mecha-

nism of centriole movement is unknown, MT-based motor

proteins may play a role in transporting centrioles as cargo.

It remains to be shownwhat would happen if maturation of

the two centrosomes and aster formation occurred con-

currently in NBs as it does in most animal cells [further

discussion and speculation on the matter were presented

by Rusan and Peifer (36)].

fGSCs present another stem cell type to study centrosomal

behaviors. Similar to mGSCs, fGSCs are also anchored

through adherens junctions by a group of somatic cells

called cap cells. fGSCs asymmetrically divide perpendicular

to this attachment to produce a cystoblast daughter cell.

Interestingly, the story is quite different in fGSCs than in

mGSCs and NBs. In wild-type fGSCs, no functional asym-

metry is observed between the two centrosomes. Further-

more, centrioles/centrosomes appear to be dispensable for

proper spindle orientation and asymmetric division (75). It

remains to be shown how spindles are oriented in fGSCs,

but it is clear that not all stem cells share asymmetric

centrosome function as a common behavior. Live cell

imaging will be critical to unambiguously identify the

functional centrosome cycle used in fGSC.

Lessons Learned from Stem Cells andSimilarities to Other Systems

The behavior of centrosomes in mGSC and NB reveals two

interesting features of the functional centrosome cycle in

Drosophila stem cells. First, an interphase MTOC does

exist unlike in other Drosophila interphase cells (38). This

MTOC appears to be important in properly aligning the

mitotic spindle and ensuring asymmetric division. MT–

cortical interactions are also important for spindle align-

ment of Drosophila embryonic NBs (72); however, it is still

not known whether centrosomes, per se, are crucial for

alignment in this cell type. It is also not known whether

a true interphase MTOC is found in embryonic NBs; live

imaging of two consecutive cell cycles using centriole and

PCM markers should address this issue. Second, the

asymmetric activation of two centrosomes within the

same cytoplasm presents a fascinating and challenging

regulatory mechanism to unravel. Future work should

focus on how this asymmetry is achieved. Possibly, the

asymmetry is inherent within the centrioles themselves.

Age could be an important determinant of asymmetry.

In fact, mGSCs selectively retain an ‘immortal’ centriole,

which forms the mother centrosome (60). Likely, this

centriole contains modifications not present in the younger

daughter centrioles that it produces, thereby allowing it to

recruit PCM more readily. Although this feature has not

been shown in other stem cells, the inheritance and age of

centrioles will be exciting areas of future exploration.

Drosophila stem cells have proven to be a powerful

system to investigate asymmetric centrosome function,

a behavior that is also observed in several other organisms.

For example, after budding yeasts duplicate their spindle

pole body (SPB), an organelle analogous to the centro-

some, they display asymmetric activity prior to entering

mitosis. The older SPB is capable of nucleating and

anchoring MTs, while the new daughter SPB is inactive,

lacking the MT motor Dynein and MTOC activity (76).

While embedded in the nuclear envelope, the inactive SPB

moves 1–1.5 mm away from the active SPB and, roughly

10 min postseparation, it then acquiresMTOC activity (76).

Amazingly, this delay is quite similar to what occurs in NB.

In fact, budding yeast could be equated to a stem cell in the

sense that the mother yeast cell is constantly self-renewing

and budding off daughter cells. Other examples include the

inactivation of the paternal centrosome and activation of

the maternal centrosome during meiosis I of Spisula

solidissima oocytes (77). Furthermore, mother and daugh-

ter centrioles in interphase cultured mammalian cells also

display a differential behavior. Mother centrioles recruit

both the PCM and the putative MT minus-end anchor

protein Ninein, whereas daughter centrioles possess PCM

but lack Ninein. Consequently, both centrioles can nucle-

ate MTs, but only the mother can anchor them (78). This

differential regulation can be attributed to clear structural

differences between the two centrioles; mother centrioles

display distal and subdistal appendages not observed in

their daughters. Although structural differences are not

seen betweenmother and daughter centrioles inDrosophila

cells, modifications likely exist between the two that have

not yet been identified, accounting for their differences in

MTOC activity.

The Edge of Centrosome Biology

Over the past few years, we have witnessed an exciting

surge in centrosome research. Work from several model

systems has shed light on long-standing questions regard-

ing centrosome function and regulation. The field of

centrosome biogenesis has gained an incredible amount

of momentum, thanks to genetic screens in C. elegans

(8,9,79–81) and functional genomic screens in Drosophila

(82,83). In particular, C. elegans screens have made sig-

nificant contributions by identifying several new conserved

genes required for centriole duplication and centrosome

maturation, which include zyg-1, spd-2, sas-4, sas-5 and

sas-6. The roles of these proteins continue to be dissected

in various systems including mammals, Drosophila and

C. elegans. Recent work in Drosophila cells reveals that

the functional cycles of centrosomes do not adhere to the

conventional model. In fact, the cycles of centrosome

478 Traffic 2009; 10: 472–481

Rusan and Rogers

function display unexpected diversity among different cell

types throughout fly development. Some cells assemble

functional centrosomes throughout the cell cycle, while

others only build centrosomes for mitosis. Of particular

interest is the evolution of the novel alterations that

Drosophila stem cells display in their centrosome func-

tional cycles. Strikingly, larval NBs with too many or too

few centrosomes can develop into tumors in transplanta-

tion assays (62,63). Thus, tumorigenesis might not be

caused through genomic instability; instead, it may be

triggered by symmetric divisions, altering cell fate. There is

mounting evidence supporting this model (84); mutations

that affect cell polarity, spindle orientation (including cen-

trosome function) and cell fate in NBs can all lead to

neoplasia (84). Many exciting questions remain. Is the

asymmetric activation of centrosomes a conserved fea-

ture of stem cells? Does asymmetric activation play

a universal and essential role in tumor suppression? It will

be worthwhile to explore centrosome behaviors and their

functional cycles within stem/progenitor cells in an array of

vertebrate tissues.

Acknowledgments

We would like to thank Mark Peifer, Steve Rogers and Edward Rogers for

their helpful discussion and comments.

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