untitled

Communication between the cytoskeleton

and the nuclear envelope to position the

nucleus

Daniel A. Starr

DOI: 10.1039/b703878j

In most eukaryotic cells, the nucleus is localized to a specific location. This

highlight article focuses on recent advances describing the mechanisms of

nuclear migration and anchorage. Central to nuclear positioning mechanisms is

the communication between the nuclear envelope and the cytoskeleton. All three

components of the cytoskeleton—microtubules, actin filaments and intermediate

filaments—are involved in nuclear positioning to varying degrees in different cell

types. KASH proteins on the outer nuclear membrane connect to SUN proteins on

the inner nuclear membrane. Together they transfer forces between the

cytoskeleton and the nuclear lamina. Once at the outer nuclear membrane, KASH

proteins can interact with the cytoskeleton. Nuclear migrations are a component

of many cellular migration events and defects in nuclear positioning lead to

human diseases, most notably lissencephaly.

Introduction

The human body contains a wide variety

of amazingly specialized cells. One

striking example is a skeletal muscle cell,

a myotube, which extends along the

entire length of a muscle (up to tens of

centimeters) and contains hundreds to

thousands of nuclei. The nuclei are

evenly spaced along the entire myotube

and are apposed to the plasma

membrane.

Exceptions include the neu-

romuscular junction where a cluster of

myonuclei form immediately below the

synapse, the myotendinous junction, and

in developing, injured, or pathological

myotubes where nuclei are mispositioned

to the interior of the myotube. A nuclear

positioning phenotype in a myotube,

from certain mutants described below,

where large aggregates of nuclei form, is

striking (see Fig. 1). The challenges of

positioning nuclei in most eukaryotic

cells are not as severe as those for a

skeletal muscle cell. Nonetheless, the

textbook model where the nucleus ran-

domly floats around in the middle of the

cytoplasm is overly simplistic. Many, if

not most, cell and developmental pro-

cesses, including fertilization, cell divi-

sion, cell migration, and establishment of

polarity, depend on positioning the

nucleus to a specific location within the

cell. In this review, while discussing some

of the better-understood mechanisms of

nuclear positioning, I will touch on

selected examples spread across all

eukaryotes.

Two related processes are required to

control the specific positioning of nuclei.

First, nuclei migrate through the cyto-

plasm to the appropriate location within

a cell, and then they are anchored at that

position to prevent drift. Different

mechanisms exist for nuclear migration

and anchorage and different mechanisms

function in different cell types and

organisms. In addition, the switch

between migration and anchorage is

predicted to be a carefully coordinated

event, for which no molecular mechan-

isms have been identified. Nonetheless,

common to all nuclear positioning

events, the nucleus must communicate

with the cytoskeleton. This is accom-

plished by two general mechanisms;

either the nuclear envelope actively

engages and manipulates the cytoskele-

ton or an active cytoskeletal network

passively positions the nucleus. All three

Section of Molecular and Cellular Biology,

University of California-Davis, Davis, CA,

95616. E-mail: [email protected];

Fax: (530)754-6083

Daniel Starr grew up in Minneapolis, MN and earned

a BA in Biology in 1992 from Colby College in

Waterville, ME. He earned a PhD in Genetics from

Cornell University in 1998. His thesis, under the

guidance of Michael Goldberg, focused on the role of

the kinetochore protein ZW10 and the recruitment of

dynein in Drosophila and human tissue culture. Dr

Starr completed a postdoctoral fellowship with Min

Han at the University of Colorado-Boulder from

1998–2003 where he focused on the molecular

characterization of unc-83 and anc-1 and their roles

in nuclear positioning in Caenorhabditis elegans. Dr

Starr joined the Section of Molecular and Cellular

Biology and the Center for Genetics and Development

at the University of California, Davis in 2003 as an

assistant professor where he runs a laboratory focusing on the role of KASH and SUN

proteins in nuclear positioning in C. elegans.

HIGHLIGHT www.rsc.org/molecularbiosystems | Molecular BioSystems

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, 2007, 3, 583–589 | 583

components of the cytoskeleton, micro-

tubules, actin filaments, and intermediate

filaments, function together to position

nuclei, although the role of each type of

filament varies in different cell types.

This review will first summarize mechan-

isms that bridge the nuclear envelope and

target proteins to the outer nuclear mem-

brane, and then focus on the role of the

three types of cytoskeletal filaments that

function during nuclear positioning.

Finally, I will discuss how defects in

nuclear positioning could block neuronal

migration events, leading to disease.

KASH and SUN proteins

bridge the nuclear envelope

Central to any nuclear positioning

mechanism, is communication between

the nuclear envelope and the cytoskele-

ton. The nuclear envelope is a specialized

extension of the endoplasmic reticulum

(ER) with two membranes and a peri-

nuclear space that together separate the

nucleoplasm from the cytoplasm (Fig. 2;

for review see ref. 2). The outer nuclear

membrane (ONM) is contiguous with the

ER. The ONM is also contiguous with

the inner nuclear membrane (INM) at

nuclear pores. Finally, a nuclear lamina,

which provides support to the nuclear

membrane in higher eukaryotes, lies

immediately below the INM. INM pro-

teins are targeted to the nucleoplasmic

face of the nuclear envelope by karyo-

pherins and retained in the INM by

interactions with the lamina.

3,4

The

structure of the nuclear envelope presents

two problems for nuclear positioning.

First, a mechanism must exist to transfer

forces across both membranes and the

intermembrane space to physically link

the cytoskeleton to the nuclear matrix.

Second, the cell needs to target proteins

specifically to the ONM to distinguish it

from the ER.

A genetic approach in C. elegans

identified three genes required for

nuclear positioning (for review see

ref. 5,6). Mutations in unc-83 or unc-84

disrupt nuclear migration events in at

least three tissues

7–9

while mutations in

anc-1 or unc-84 disrupt nuclear ancho-

rage in a wide variety of cells.

8,10

The

UNC-83, UNC-84, and ANC-1 proteins

were all shown to localize to the nuclear

envelope; localization of either UNC-83

or ANC-1 requires UNC-84 at the

nuclear envelope.

8,9,11

UNC-84 con-

tains a conserved SUN domain (for

S_ad1, U_N_C-84 homology; see below).

UNC-83 and ANC-1 both have KASH

domains at their C-termini (for

K_larsicht, A_NC-1, S_yne h_omology; see

below).

11,12

Genetic and cell biological

studies on UNC-83, UNC-84, and

ANC-1 implicate for the first time

an essential role for integral nuclear

envelope proteins in the mechanisms of

nuclear positioning.

KASH and SUN proteins have since

been identified across eukaryotes (Fig. 2;

for review see

6,13–15

). In general, SUN

proteins contain a conserved SUN

domain at their C-termini and at least

one transmembrane domain. The rest of

the protein is not obviously conserved at

the sequence level, although it is likely

conserved at a functional level.

15–17

KASH proteins play multiple functions.

All KASH proteins contain a C-terminal

KASH domain consisting of a predicted

trans-membrane domain followed by

10–30 conserved residues before the stop

Fig. 1 Nuclear anchorage is aberrant in desmin knockout mouse myotubes. A skeletal muscle

fiber from a desmin knockout mouse is stained with Hoechst to detect nuclei. Arrows indicate

areas lacking nuclei and arrowheads mark large nuclear aggregates. One cluster is magnified

in the inset. A portion of a wildtype myotube is shown on the left.

John Wiley & Sons, Inc. Reprinted with premission of Wiley-Liss, Inc., a subsidiary of John

Wiley & Sons, inc

Fig. 2 KASH and SUN proteins bridge the nuclear envelope and connect the cytoskeleton to

the nucleus. Proteins with SUN domains (red) localize to the inner nuclear membrane (INM) and

proteins with KASH domains (purple) localize to the outer nuclear membrane (ONM). Various

KASH/SUN interactions from different organisms are depicted here. From left to right: the

ANC-1/MSP-300/Syne family connects actin to the ONM, C. elegans UNC-83 and UNC-84

function during nuclear migration by unknown mechanisms, Drosophila Klar mediates dynein

and is necessary to link the centrosome to the ONM, unrelated C. elegans ZYG-12 dimerizes to

connect the centrosome to the ONM, S. pombe Sad1p and Kms1p transfer the forces from

cytoplasmic dynein to move telomeres at the INM, and human Nesprin-3 connects intermediate

filaments to the ONM. For more details, see text.

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codon.

The KASH domain is necessary

and sufficient for nuclear envelope loca-

lization.

The large N-terminal domains

of KASH proteins play multiple roles,

suggesting that the KASH domain func-

tions as a targeting module for different

proteins.

Adding more complexity to

the role of KASH proteins is that in

examined cases, multiple transcripts

exist, some without KASH domains.

19,20

A model has emerged where KASH

and SUN proteins function together to

bridge the nuclear envelope, effectively

connecting the nuclear lamina to the

cytoskeleton (see Fig. 2; for review see

ref. 6,13–15). The nuclear envelope brid-

ging model has been refined by four

groups who have each concluded that

SUN proteins reside in the INM where

they function to target and retain KASH

proteins to the ONM through a direct

interaction.

12,21–23

The nuclear envelope

bridging model predicts that the

N-termini of SUN proteins face the

nucleoplasm. Consistent with the model,

SUN proteins have been shown to

interact with lamins and to serve as

linkers to telomeres during meio-

sis.

22,24,25

The large N-terminal domains

of KASH proteins are predicted to

extend into the cytoplasm. In support,

KASH proteins have been shown to

interact with actin filaments, inter-

mediate filaments and centrosomes (see

below for examples). Finally, the nuclear

envelope bridging model addresses two

major cell biological problems. First,

KASH and SUN proteins likely transfer

forces from the cytoskeleton to the

nucleoskeleton through a direct inter-

action in the perinuclear space. Second,

SUN proteins recruit KASH proteins

specifically to the ONM, marking this

unique membrane domain apart from

the contiguous rough endoplasmic

reticulum.

The role of microtubules,

centrosomes and dynein in

nuclear migration

The role of microtubules in nuclear

positioning is relatively well understood

and thoroughly reviewed.

26–30

Most of

the data implicating microtubules and

the dynein motor in nuclear positioning

come from genetic studies in fungi. The

growing hyphae of filamentous fungi

undergo multiple divisions of the nucleus

without cytokinesis (for review see

ref. 26,29,30). The nuclei migrate

throughout hyphal growth by a micro-

tubule dependent mechanism, which

allows them to remain evenly spaced.

Molecular characterization of a series of

nud mutations in Aspergillus (for nuclear

distribution) have demonstrated a role

for dynein and associated proteins in

nuclear migration and distribution (for

review see ref. 30). Microtubules and

dynein are also essential for nuclear

migration events in budding and fission

yeast. During mitosis, the nucleus must

migrate to the proper position to ensure

accurate chromosomal segregation to

daughter cells of appropriate size. Micro-

tubules and dynein function to position

fission yeast nuclei to the middle of the

cells and to control migration of budding

yeast nuclei to the bud neck prior to

mitosis.

30,32,33

During meiosis in S.

pombe the nucleus rapidly migrates back

and forth along the long axis of the cell in

a motion termed the horsetail movement,

which is thought to induce recombina-

tion between homologous chromosomes.

Horsetail movement is led by the spindle

pole body (the yeast version of a centro-

some) and requires microtubules and

dynein.

Many advances in understanding the

contributions of microtubules in nuclear

positioning in animal cells come from

studies in C. elegans and Drosophila.

Dynein and microtubules play essential

roles in pronuclear migration, nuclear

rotation, and asymmetric spindle posi-

tioning in the early C. elegans

embryo.

28,35–37

In addition, dynein and

microtubules clearly function during

nuclear migration in the developing

Drosophila eye and nuclear positioning

in the syncytial embryo.

38,39

How does the microtubule network

interact with the nucleus? KASH and

SUN proteins function at the nuclear

envelope to communicate with microtu-

bules and centrosomes. A striking exam-

ple is in the C. elegans zygote. ZYG-12

(a KASH protein) and SUN-1 (a SUN

protein also known as MTF-1) were

identified through genetic screens for

mutants defective in centrosome attach-

ment to the nuclear envelope (Fig. 2).

ZYG-12 exists as both a KASH-less

isoform that binds to the centrosome

and a full-length KASH isoform that

localizes to the outer nuclear membrane.

Localization of ZYG-12 to the nuclear

envelope requires SUN-1, which func-

tions analogously to UNC-84 to specify

the outer nuclear membrane. The centro-

some and nuclear envelope isoforms of

ZYG-12 bind to one another to physi-

cally connect the centrosome to the

nuclear envelope.

ZYG-12 also recruits

dynein to the nuclear envelope where it

likely plays multiple roles in nuclear

positioning.

A second example of the

role of KASH proteins and microtubules

in nuclear positioning is in the Drosophila

eye disk. The KASH protein Klarsicht

functions to connect the centrosome to

the migrating nucleus (Fig. 2).

13,41

Klarsicht function requires lamin and

the SUN protein Klaroid; Klarsicht is

thought to coordinate opposing forces

from dynein and kinesins.

17,41

One final

example is the role of the SUN protein

Sad1p in S. pombe meiosis (Fig. 2).

Sad1p interacts with telomeres at the

nuclear envelope and transports them

to the spindle pole body.

Telomere

localization requires dynein and micro-

tubules in the cytoplasm. The forces

of the microtubule motors are thought

to be transmitted into the nucleus

through Kms1p, a KASH protein at

the outer nuclear membrane and

Sad1p.

Recently two Drosophila

nuclear envelope proteins have been

shown to play an important role in

nuclear positioning during blastoderm

cellularization. Mutations in charleston/

kugelkern or kurzkern disrupt nuclear

shape and nuclear anchoring to the

apical side of the cell. The Char/Kuk

and Kur proteins are thought to act

in the nuclear matrix.

42,43

Microtubules

also function in this process,

although

the mechanism and interactions with any

KASH or SUN proteins remain to be

elucidated.

Actin filaments and the Syne/

ANC-1/MSP-300 family in

nuclear positioning

A family of huge KASH proteins, Syne-1

and -2 in mammals (also known as

nesprin-1 and -2), MSP-300 in

Drosophila, and ANC-1 in C. elegans,

functions to anchor nuclei to the actin

network (Fig. 2). Dictyostelium inter-

aptin may also be a member of this

family, although it is significantly smaller

and its role in nuclear positioning has not

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been described.

Full-length isoforms

of ANC-1/MSP-300/Syne each contain

a conserved N-terminal actin-binding

domain of the calponin family and a

C-terminal KASH domain that targets

the protein to the outer nuclear mem-

brane. The KASH and calponin domains

are connected by a huge (over 8000

residues) spectrin-like domain. The

ANC-1/MSP-300/Syne proteins are

related to Dystrophin, which functions

to connect actin filaments to the plasma

membrane. The major difference is that

the C-terminus of Dystrophin does not

contain a KASH domain and instead

interacts with the plasma membrane.

45,46

The ANC-1/MSP-300/Syne proteins are

thought to act as a rope to connect actin

to the nuclear membrane (for review see

ref. 6,13).

Null mutations that affect the KASH

isoforms of Syne-1 in mice, MSP-300

in Drosophila or ANC-1 in C. elegans

disrupt nuclear anchorage. Mutations in

C. elegans anc-1 were originally isolated

in screens for nuclear positioning

mutants in the syncytial hypodermis.

Normally nuclei of the hypodermis are

evenly spaced apart, but in an anc-1

mutant animal, the nuclei are moved

back and forth by underlying muscles

and aggregate in large clusters

(Fig. 3A–

B). Mutations in Drosophila msp-300

lead to embryonic lethality because of

muscle morphology defects.

To inves-

tigate the role of msp-300 in nuclear

positioning, germ-line clones were exam-

ined. Developing oocyte cysts in female

germline clones of homozygous msp-300

null mutations have a strong dumping

phenotype caused by a lack of nurse cell

nuclear anchorage.

Normally the nuclei

of the 15 nurse cells remain anchored by

an actin-dependent process while they

contract and ‘‘dump’’ their cytoplasm

into the oocyte through narrow ring

canals; nurse cell nuclei must remain

anchored to prevent their movement into

ring canals.

msp-300 mutant nurse cells

have a dumping phenotype resulting in

small eggs. Nurse cell nuclei are mis-

placed in oocytes, and others block ring

canals. The oocyte nucleus is also unan-

chored and floats freely in the oocyte

cytoplasm (Fig. 3C–D).

To determine if

the roles of ANC-1 and MSP-300 are

conserved in the human homologues

Syne-1 and Syne-2, Zhang et al.

created

mice with the KASH domains of Syne-1

and/or Syne-2 knocked out. Disruption

of both Syne-1 and Syne-2 was lethal

shortly after birth, as the pups never

breathed.

Syne-1 KASH knockout

mice were viable, but had severe nuclear

positioning defects in the muscle

(Fig. 2E–F); the Syne-2 KASH knockout

mice had no obvious phenotype.

The

Syne-1 knockout was more severe than

the previously described dominant nega-

tive Syne mice where overexpression of

the KASH domain in skeletal muscles

displaced endogenous Syne proteins and

disrupted the positioning of nuclei at the

neuromuscular junction.

50,51

The Syne-1

KASH knockout mice had no nuclei

clustering under the neuromuscular

junction. The non-synaptic nuclei in the

myotube were also displaced, often

aggregating in large clusters.

Addi-

tionally, non-synaptic nuclei were

occasionally observed in the center of a

cross-section of a myotube, a phenotype

associated with regeneration and disease

pathology.

Together these genetic

experiments show that the ANC-1/

MSP-300/Syne family of proteins func-

tion to anchor nuclei and that the

mechanism is conserved from nematodes

to insects and mammals.

The importance of actin networks in

nuclear migration has recently been

described for an unexpected situation. It

had been known that NIH 3T3 fibro-

blasts in tissue culture polarize in re-

sponse to wounding, which results in the

centrosomes facing the wound.

It was

believed that this was due to an active

movement of the centrosome towards the

wound edge via microtubules. However,

Fig. 3 The ANC-1/MSP-300/Syne-1 family of proteins function to anchor nuclei. (A–B)

Normarski microscopy images of C. elegans syncytial hypodermal cells from wildtype (A) and

anc-1 mutant (B) adults. Normally, the nuclei (black arrowheads) are evenly spaced apart

whereas in the anc-1 mutant animals nuclei float freely through the syncytium. (Reproduced with

permission from ref.11.) (C–D) Fluorescent images showing GFP-histone marked nuclei in stage

10A developing Drosophila oocytes from wildtype (C) and msp-300 mutant (D) ovaries. In the

msp-300 mutant ovary, nurse cells are misplaced; one (arrow) is in the oocyte cytoplasm. The

oocyte nucleus (arrowhead) is also mispositioned. Reproduced with permission from ref. 48,

fiber from heterozygous control siblings (E) or homozygous Syne-1KASH knockout mice (F). In

control fibers the nuclei are evenly spaced apart while in the mutant fiber the nuclei cluster.

(Reproduced with permission of the Company of Biologists from ref. 50.)

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video microscopy has demonstrated that

the centrosomes remain anchored in the

middle of the cell while the nucleus

actively migrates behind the centro-

some.

This rearward nuclear movement

requires actin, myosin, and the small

G-protein cdc42 and is coupled with

retrograde actin flow.

It is not known

how the nuclear envelope interacts with

the actin network during rearward

nuclear migration, but Syne-1 and

Syne-2 are prime candidates to anchor

the nucleus to the retrograde actin

flow.

There are also some less well under-

stood examples of roles for actin fila-

ments in nuclear positioning.

Drosophila early embryonic nuclear

migrations towards the periphery of the

syncytial blastoderm, the actin gel-like

network around migrating nuclei

depolymerizes, allowing the nucleus to

passively move forward along the depo-

lymerizing front.

Additionally, actin

depolymerizing drugs completely abol-

ished the rapid intracellular nuclear

migration events in Arabidopsis root

hairs through unknown mechanisms,

whereas drugs that disrupted microtu-

bules had no effect.

Intermediate filaments and

nuclear anchorage

The role of intermediate filaments in

nuclear positioning is less well under-

stood. Since there are no motors that

move along intermediate filaments and

because they do not have the polar

dynamic growth properties of microtu-

bules or actin filaments, intermediate

filaments are relegated to a more static

scaffolding role to anchor nuclei in place

or to recruit components that can inter-

act with other members of the cytoske-

leton. Desmin is the major component of

intermediate filaments in mammalian

muscle fibers and the desmin knockout

mouse has a striking nuclear anchorage

phenotype

(Fig. 1). Normally the hun-

dreds of nuclei in a mouse skeletal muscle

are evenly spaced apart and at the

periphery of the cell. In desmin knockout

muscles, nuclei cluster together in dense

aggregates reminiscent of the mouse

Syne-1 or C. elegans anc-1 mutant

phenotypes. Additionally, the oval nuclei

fail to align normally with the long axis

of the muscle fiber.

The mispositioned

nuclei in the desmin null muscles do,

however, retain their preferential associa-

tion with blood vessels, suggesting that

desmin and blood vessels play indepen-

dent roles in anchoring nuclei.

remains to be determined whether Syne-

1 functions in conjunction with either

desmin or blood vessels. In other cells,

intermediate filaments might function

in part to connect the plasma mem-

brane to the nucleus. For example,

vimentin is found closely associated

with the nucleus and mutant forms of

vimentin exhibit nuclear morphology

defects.

A newly identified KASH protein,

nesprin-3, likely functions to connect

the outer nuclear membrane to inter-

mediate filaments. Nesprin-3 localizes to

the nuclear envelope where it interacts

with plectin in a yeast two-hybrid assay

(Fig. 2).

Plectin, a plakin family mem-

ber, consists of an actin-binding domain,

an extended coiled-coil domain, and an

intermediate filament-binding domain

and can crosslink actin filaments to

intermediate filaments.

The actin-bind-

ing domain can alternatively interact

with other proteins including integrins.

Nesprin-3 was found to bind to the actin-

binding domain of plectin in a two-

hybrid screen.

It is hypothesized that

nesprin-3 and plectin together could

extend from the outer nuclear membrane

into the cytoplasm to interact with

intermediate filaments; this would be

analogous to how the ANC-1/MSP-300/

Syne proteins connect the outer nuclear

membrane with actin filaments.

Lamin, a type V intermediate filament,

is the major structural component of the

nuclear matrix immediately underlying

the inner nuclear membrane (Fig. 2). It

plays both structural and scaffolding

roles during nuclear positioning

(reviewed in ref. 2). Some, but not all,

SUN proteins require lamins to localize

to the inner nuclear membrane.

22,61

Since

SUN proteins recruit KASH proteins to

the outer nuclear membrane, lamins

function in a variety of the mechanisms

for nuclear positioning discussed above.

The role of nuclear migration

in neuronal migration and

disease

Nuclear migration and anchorage

events have been described for a wide

variety of cellular processes. Thus, it is

not surprising that defects in nuclear

positioning would lead to disease.

Characterization of migrating neurons

has implicated both microtubules and

actin–myosin contraction in nuclear

migration. The growth cone, the leading

edge of a migrating neuron, extends at

a constant rate while the nucleus

moves through the cytoplasm in jumps

and pauses. Each nuclear migration

translocation of about 10 microns is

preceded by a movement of the centro-

some away from the nucleus and into a

swelling of the cellular process at the site

the nucleus will jump to.

62–64

Nuclear

translocation requires myosin II con-

traction at the rearward side to propel

the nucleus forward.

Thus neuronal

migration is an example of how the

microtubule and actin networks can

function together to control nuclear

positioning.

Lissencephaly (Greek for smooth

brain) is a neuro-developmental disease

where neurons fail to properly migrate

to the cortex. Mutations in any of

three proteins, LIS1, Doublecortin or

a-tubulin, have been linked to lissence-

phaly.

65–67

LIS1 regulates dynein and is a

homologue of Aspergillus NudF, which is

required for nuclear positioning (for

review see ref. 68). Doublecortin is a

microtubule-associated protein

and the

mutation in a-tubulin blocks GTP bind-

ing, slowing microtubule dynamics.

Recently developed techniques to study

neuronal migration in siRNA-treated

living brain and mouse knockout systems

have implicated the lissencephaly muta-

tions in nuclear migration.

63,67,69

Thus

microtubule and dynein driven nuclear

migration play an essential role in human

brain development.

One KASH protein has been directly

implicated in a disease. Mutations in

Syne-1 lead to autosomal cerebellar

recessive ataxia type 1 (ARCA1), also

known as recessive ataxia of Beauce,

a late-onset ataxia with a slow progres-

sion.

Further studies are required to

understand if and how nuclear

positioning contributes to this neuro-

degenerative disease or if the ataxia

represents a defect independent of

KASH function. It also remains to be

seen if other human KASH or SUN

proteins are also involved in the patho-

logy of disease.

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Conclusions and directions for

the field

The position of the nucleus within the

cell is essential to a wide variety of

cellular and developmental events across

eukaryotes. In the past five years, great

progress has been made in understanding

some of the mechanisms of nuclear

positioning. The main focus of these

studies was on the role KASH and

SUN proteins play to position the

nucleus. KASH proteins specify the

outer nuclear membrane and together

with SUN proteins, transfer forces across

the nuclear envelope. KASH and SUN

proteins act as the central link to connect

all three components of the cytoskeleton

to the nuclear matrix. There are certainly

more players and mechanisms to be

identified in nuclear positioning. Proteins

remain to be identified to explain how

nuclear migration is controlled, and

many unanswered questions remain.

Microtubules, actin filaments, and

intermediate filaments all play important

roles in positioning nuclei. However,

little is known about how they function

together or why some components of the

cytoskeleton play more important roles

than others in different tissue types. For

example, in the early C. elegans embryo,

the microtubule system plays the major

role in positioning the nucleus through

the KASH protein ZYG-12.

However,

other systems function to migrate nuclei

later in development through the KASH

protein UNC-83.

9,61

The identity of the

cytoskeleton components that UNC-83

interacts with is being actively pursued.

Most likely, multiple cytoskeletal com-

ponents function together to position

nuclei. This is best described in budding

yeast where the actin and microtubule

networks function together.

Some of the questions to be addressed

include: How are polar cues incorporated

to move the nucleus directionally? What

happens to the ER during migration, is it

dragged behind or does it actively

rearrange during nuclear migration?

And, how do nuclei switch from an

anchored position to a migrating one?

The switch must be carefully coordi-

nated; this could be accomplished by

controlling the specificity of KASH SUN

interactions. UNC-84 interacts with one

KASH protein, UNC-83, during migra-

tion and a different KASH protein,

ANC-1, during anchorage. Nothing is

known about how this switch is made.

Furthermore, a sub-molecular in vitro

characterization of the KASH/SUN

interaction should help determine how

these proteins transfer force across the

nuclear envelope. During the next few

years the field will likely identify addi-

tional nuclear envelope and cytoskeletal

interactions that function to position the

nucleus. Both genetic and biochemical

approaches are likely to clarify the

mechanisms of nuclear migration.

Finally, the link between nuclear posi-

tioning and disease needs further exam-

ination. There is an exciting possibility

that proteins involved in nuclear posi-

tioning are also involved in disease. The

list of top candidates today include

Syne-1, implicated in an ataxia,

Lis1,

one cause of lissencephaly,

and lamins,

which may act through SUN proteins

to cause a variety of diseases termed

laminopothies (reviewed in ref. 2).

Furthermore, since nuclear migration

is an important component of many

cellular migration events, it is exciting

to speculate that nuclear migration might

play a role in all cellular migration events

including metastasis. It is likely that

defects in KASH and SUN proteins lead

to disease, although further studies are

required to determine if they play causal

or correlative roles in cancer and other

diseases. Therefore, KASH and SUN

proteins may be good targets for chemi-

cal genetics to both better understand the

processes of nuclear and cellular migra-

tion and ultimately as potential medical

treatments.

Acknowledgements

I am grateful to Lesilee Rose and the

members of my laboratory for comments

on this manuscript and thoughtful dis-

cussions. I apologize to those whose

work was not included because of space.

The studies in my lab are supported by a

grant from the NIH (GM073874) and a

March of Dimes Basil O’Conner award.

References

1 J. C. Bruusgaard, K. Liestol, M. Ekmark,

K. Kollstad and K. Gundersen, J. Physiol.

(Oxford, UK), 2003, 551, 467–478.

2Y.Gruenbaum,A.Margalit,R.D.

Goldman, D. K. Shumaker and K. L.

Wilson, Nat. Rev., 2005, 6, 21–31.

3 M. C. King, C. P. Lusk and G. Blobel,

Nature, 2006, 442, 1003–1007.

4 T. Ohba, E. C. Schirmer, T. Nishimoto

and L. Gerace, J. Cell Biol., 2004, 167,

1051–1062.

5 D. A. Starr and M. Han, Novartis Found.

Symp., 2005, 264, 208–219; discussion

219–230.

6 K. Wilhelmsen, M. Ketema, H. Truong

and A. Sonnenberg, J. Cell Sci., 2006, 119,

5021–5029.

7 H. R. Horvitz and J. E. Sulston, Genetics,

1980, 96, 435–454.

8 C. J. Malone, W. D. Fixsen, H. R. Horvitz

and M. Han, Development (Cambridge,

UK), 1999, 126, 3171–3181.

9 D. A. Starr, G. J. Hermann, C. J. Malone,

W. Fixsen, J. R. Priess, H. R. Horvitz and

M. Han, Development (Cambridge, UK),

2001, 128, 5039–5050.

10 E. M. Hedgecock and J. N. Thomson,

Cell, 1982, 30, 321–330.

11 D. A. Starr and M. Han, Science, 2002,

298, 406–409.

12 M. D. McGee, R. Rillo, A. S. Anderson

and D. A. Starr, Mol. Biol. Cell, 2006, 17,

1790–1801.

13 D. A. Starr and J. A. Fischer, Bioessays,

2005, 27, 1136–1146.

14 Y. B. Tzur, K. L. Wilson and

Y. Gruenbaum, Nat. Rev., 2006, 7,

782–788.

15 H. J. Worman and G. G. Gundersen,

Trends Cell Biol., 2006, 16, 67–69.

16 S. L. Jaspersen, A. E. Martin, G. Glazko,

T. H. Giddings, Jr., G. Morgan,

A. Mushegian and M. Winey, J. Cell

Biol., 2006,

174, 665–675.

17 M. P. Kracklauer, S. M. L. Banks, X. Xie,

Y. Wu and J. A. Fischer, Fly, 2007, 1,

75–85.

18 Q. Zhang, J. N. Skepper, F. Yang, J. D.

Davies, L. Hegyi, R. G. Roberts, P. L.

Weissberg, J. A. Ellis and C. M. Shanahan,

J. Cell Sci., 2001, 114, 4485–4498.

19 Y. Guo, S. Jangi and M. A. Welte, Mol.

Biol. Cell, 2005, 16, 1406–1416.

20 D. T. Warren, Q. Zhang, P. L. Weissberg

and C. M. Shanahan, Expert Rev. Mol.

Med., 2005, 7, 1–15.

21 M. Crisp, Q. Liu, K. Roux, J. B. Rattner,

C. Shanahan, B. Burke, P. D. Stahl and

D. Hodzic, J. Cell Biol., 2006, 172, 41–53.

22 F. Haque, D. J. Lloyd, D. T. Smallwood,

C. L. Dent, C. M. Shanahan, A. M. Fry,

R. C. Trembath and S. Shackleton, Mol.

Cell. Biol., 2006, 26, 3738–3751.

23 V. C. Padmakumar, T. Libotte, W. Lu,

H. Zaim, S. Abraham, A. A. Noegel,

J. Gotzmann, R. Foisner and I.

Karakesisoglou, J. Cell Sci., 2005, 118,

3419–3430.

24 Y. Chikashige, C. Tsutsumi, M. Yamane,

K. Okamasa, T. Haraguchi and Y.

Hiraoka, Cell, 2006, 125, 59–69.

25 J. Schmitt, R. Benavente, D. Hodzic,

C.Hoog,C.L.StewartandM.

Alsheimer, Proc. Natl. Acad. Sci. U. S. A.,

2007.

26 N. R. Morris, J. Cell Biol., 2000, 148,

1097–1101.

27 N. R. Morris, Curr. Opin. Cell Biol., 2003,

15, 54–59.

28 S. Reinsch and P. Gonczy, J. Cell Sci.,

1998, 111, 2283–2295.

588 |

Mol. BioSyst.

, 2007, 3, 583–589 This journal is

The Royal Society of Chemistry 2007

29 X. Xiang and R. Fischer, Fungal Genet.

Biol., 2004, 41, 411–419.

30 A. Yamamoto and Y. Hiraoka, J. Cell

Sci., 2003, 116, 4501–4512.

31 B. R. Oakley and N. R. Morris, Cell, 1980,

19, 255–262.

32 R. E. Palmer, D. S. Sullivan, T. Huffaker

and D. Koshland, J. Cell Biol., 1992, 119,

583–593.

33 P. T. Tran, L. Marsh, V. Doye, S. Inoue

and F. Chang, J. Cell Biol., 2001, 153,

397–411.

34 D. Q. Ding, Y. Chikashige, T. Haraguchi

and Y. Hiraoka, J. Cell Sci., 1998,

111(Pt. 6), 701–712.

35 P. Gonczy, S. Pichler, M. Kirkham and A.

A. Hyman, J. Cell Biol., 1999, 147,

135–150.

36 A. R. Skop and J. G. White, Curr. Biol.,

1998, 8, 1110–1116.

37 B. Sonnichsen, L. B. Koski, A. Walsh,

P. Marschall, B. Neumann, M. Brehm,

A. M. Alleaume, J. Artelt, P. Bettencourt,

E. Cassin, M. Hewitson, C. Holz, M.

Khan, S. Lazik, C. Martin, B. Nitzsche,

M. Ruer, J. Stamford, M. Winzi, R.

Heinkel, M. Roder, J. Finell, H. Hantsch,

S. J. Jones, M. Jones, F. Piano, K. C.

Gunsalus, K. Oegema, P. Gonczy, A.

Coulson, A. A. Hyman and C. J.

Echeverri, Nature, 2005, 434, 462–469.

38 S. S. Fan and D. F. Ready, Development

(Cambridge, UK), 1997, 124, 1497–1507.

39 J. L. Whited, A. Cassell, M. Brouillette

and P. A. Garrity, Development (Cambridge,

UK),2004,131, 4677–4686.

40 C. J. Malone, L. Misner, N. Le Bot,

M. C. Tsai, J. M. Campbell, J. Ahringer

and J. G. White, Cell, 2003, 115, 825–836.

41 K. Patterson, A. B. Molofsky, C.

Robinson, S. Acosta, C. Cater and

J. A. Fischer, Mol. Biol. Cell, 2004, 15,

600–610.

42 A. Brandt, F. Papagiannouli, N. Wagner,

M. Wilsch-Brauninger, M. Braun, E. E.

Furlong, S. Loserth, C. Wenzl, F. Pilot,

N. Vogt, T. Lecuit, G. Krohne and

J. Grosshans, Curr. Biol., 2006, 16,

543–552.

43 F. Pilot, J. M. Philippe, C. Lemmers,

J. P. Chauvin and T. Lecuit, Development

(Cambridge, UK), 2006, 133

, 711–723.

44F.Rivero,A.Kuspa,R.Brokamp,

M. Matzner and A. A. Noegel, J. Cell

Biol., 1998, 142, 735–750.

45 T. A. Rando, Muscle Nerve, 2001, 24,

1575–1594.

46 D. A. Starr and M. Han, J. Cell Sci., 2003,

116, 211–216.

47 Y. Rosenberg-Hasson, M. Renert-Pasca

and T. Volk, Mech. Dev., 1996, 60, 83–94.

48 J. Yu, D. A. Starr, X. Wu, S. M. Parkhurst,

Y. Zhuang, T. Xu, R. Xu and M. Han,

Dev. Biol., 2006, 289, 336–345.

49 D. N. Robinson and L. Cooley, Annu. Rev.

Cell Dev. Biol., 1997, 13, 147–170.

50 X. Zhang, R. Xu, B. Zhu, X. Yang,

X. Ding, S. Duan, T. Xu, Y. Zhuang and

M. Han, Development (Cambridge, UK),

2007, 134, 901–908.

51 R. M. Grady, D. A. Starr, G. L. Ackerman,

J. R. Sanes and M. Han, Proc. Natl. Acad.

Sci. U. S. A., 2005.

52 A. Kupfer, D. Louvard and S. J. Singer,

Proc. Natl. Acad. Sci. U. S. A., 1982, 79,

2603–2607.

53 E. R. Gomes, S. Jani and G. G. Gundersen,

Cell, 2005, 121, 451–463.

54 G. von Dassow and G. Schubiger, J. Cell

Biol., 1994, 127, 1637–1653.

55 E. Chytilova, J. Macas, E. Sliwinska,

S. M. Rafelski, G. M. Lambert and

D. W. Galbraith, Mol. Biol. Cell, 2000,

11, 2733–2741.

56 E. Ralston, Z. Lu, N. Biscocho, E.

Soumaka, M. Mavroidis, C. Prats, T.

Lomo, Y. Capetanaki and T. Ploug, J.

Cell. Physiol., 2006, 209, 874–882.

57 D. M. Toivola, G. Z. Tao, A. Habtezion,

J. Liao and M. B. Omary, Trends Cell

Biol., 2005, 15, 608–617.

58 K. Wilhelmsen, S. H. Litjens, I. Kuikman,

N. Tshimbalanga, H. Janssen, I. van den

Bout, K. Raymond and A. Sonnenberg,

J. Cell Biol., 2005, 171, 799–810.

59 E. Fuchs and I. Karakesisoglou, Genes

Dev., 2001, 15, 1–14.

60 D. Geerts, L. Fontao, M. G. Nievers,

R. Q. Schaapveld, P. E. Purkis, G. N.

Wheeler, E. B. Lane, I. M. Leigh and

A. Sonnenberg, J. Cell Biol.

, 1999, 147,

417–434.

61 K. K. Lee, D. A. Starr, M. Cohen, J. Liu,

M. Han, K. L. Wilson and Y. Gruenbaum,

Mol. Biol. Cell, 2002, 13, 892–901.

62 A. Bellion, J. P. Baudoin, C. Alvarez,

M. Bornens and C. Metin, J. Neurosci.,

2005, 25, 5691–5699.

63 C. Kappeler, Y. Saillour, J. P. Baudoin,

F. P. Tuy, C. Alvarez, C. Houbron,

P. Gaspar, G. Hamard, J. Chelly,

C. Metin and F. Francis, Hum. Mol.

Genet., 2006, 15, 1387–1400.

64 B. T. Schaar and S. K. McConnell, Proc.

Natl. Acad. Sci. U. S. A., 2005, 102,

13652–13657.

65 O. Reiner, R. Carrozzo, Y. Shen, M.

Wehnert, F. Faustinella, W. B. Dobyns,

C. T. Caskey and D. H. Ledbetter, Nature,

1993, 364, 717–721.

66 J. G. Gleeson, K. M. Allen, J. W. Fox,

E. D. Lamperti, S. Berkovic, I. Scheffer,

E.C.Cooper,W.B.Dobyns,S.R.

Minnerath, M. E. Ross and C. A. Walsh,

Cell, 1998, 92, 63–72.

67 D. A. Keays, G. Tian, K. Poirier, G. J.

Huang, C. Siebold, J. Cleak, P. L. Oliver,

M. Fray, R. J. Harvey, Z. Molnar,

M. C. Pinon, N. Dear, W. Valdar, S. D.

Brown, K. E. Davies, J. N. Rawlins,

N. J. Cowan, P. Nolan, J. Chelly and

J. Flint, Cell, 2007, 128, 45–57.

68 R. B. Vallee and J. W. Tsai, Genes Dev.,

2006, 20, 1384–1393.

69 J. W. Tsai, Y. Chen, A. R. Kriegstein and

R. B. Vallee, J. Cell Biol., 2005, 170,

935–945.

70 F. Gros-Louis, N. Dupre, P. Dion, M. A.

Fox, S. Laurent, S. Verreault, J. R. Sanes,

J. P. Bouchard and G. A. Rouleau, Nat.

Genet., 2007, 39, 80–85.

71 C. G. Pearson and K. Bloom, Nat. Rev.,

2004, 5, 481–492.

This journal is

The Royal Society of Chemistry 2007

Mol. BioSyst.

, 2007, 3, 583–589 | 589