The Roles of Cytoskeleton in Huntington’s Disease

Jiayin Wu

School of Laboratory Medicine and Biotechnology, Southern Medicine University, Guangzhou, China

Keywords: Huntington’s Disease, Kinesin, Dynein, Actinin, Tau.

Abstract: This article indicates partly roles of cytoskeleton in Huntington’s Disease. First, mutant huntingtin may

change the vesicle transport due to the fact that phosphorylation of huntingtin as a switch to regulate the

anterograde/retrograde transport in neurons. Secondly, dysfunction of cellular morphology regulated by

huntingtin, actinin and growth factor may be related in the occurrence of the Huntington’s Disease. Finally,

the alterations of Tau in total level, imbalance of isoforms produced by alternative splicing or by post-

translational modifications imply that Huntington’s Disease and Alzheimer’s Disease may have similar

occurrence mechanism. According to specific description of these, this article hopes to provide new treatments

for Huntington’s Disease or new research orientations for diseases with similar characteristic of Huntington’s

Disease.

1 INTRODUCTION

With the continuous improvement of medical level

and health environment, the average life expectancy

of the population is gradually increasing and some

diseases associated with the aging of the population

are also gradually highlighted, especially in

neurodegenerative diseases such as Alzheimer's

disease and Huntington's disease, which cause huge

economic burden to the society and family.

Huntington’s Disease is a rare genetic autosomal-

dominant neurodegenerative disease that results from

expansion of a CAG trinucleotide repeat (>35) in the

HTT(Huntingtin) gene on the short (p) arm of

chromosome 4 at position 16.3 and first involves

basal ganglia (caudate nucleus and putamen) (Taran

et al. 2020). In neuropathology, Huntington’s Disease

is characterized by neuron death, primarily a

progressive atrophy of the basal ganglia produced by

medium-sized spiny neurons of the striatum, and the

presence of spherical inclusions due to aggregation of

mutant Huntingtin (Htt) in the neuronal nucleus and

cytoplasm (Marta et al. 2020). The clinical

manifestations of this disease are movement

disorders, cognitive decline and a range of somatic

symptoms. Progressive worsening comes with a

bedridden state and patients finally die in 20 years

after the onset of symptoms (Anne-Catherine et al.

2019). Huntington's disease affects approximately 1

in 10,000 people worldwide, and the average age of

onset is between 40 and 50 years (An et al. 2018) .

Although etiology of Huntington’s Disease is

clear, there is no radical treatment for it currently and

the mechanism of its occurrence and development is

unequivocal. In recent years, through the further

understanding of the cytoskeleton, more and more

studies have proposed that the cytoskeleton plays an

important role in the pathogenesis of Huntington's

disease.

This review is designed to integrate the

remarkable recent advances that have led to new

insights into the possible pathogenesis of

Huntington’s disease from the perspective of

cytoskeleton.

2 THE CYTOSKELETON IN

HUNTINGTON’S DISEASE

2.1 Huntingtin, Kinesins and Dyneins

Dynein is a minus end-directed microtubule motor

protein, while kinesin is a plus end-directed

microtubule motor protein. The property of motor

protein that carry vesicles along MTs determines the

correct intracellular transport of membranous

organelles and cargoes. In neurons, kinesins are

responsible for anterograde transprot of vesicles from

572

Wu, J.

The Roles of Cytoskeleton in Huntington’s Disease.

DOI: 10.5220/0011374400003438

In Proceedings of the 1st International Conference on Health Big Data and Intelligent Healthcare (ICHIH 2022), pages 572-576

ISBN: 978-989-758-596-8

cell centre to the end of neurites. On the contrary,

dyneins are responsible for the retrograde transport of

vesicles and organelles back towards the cell centre.

Dynactin is a dynein activator that binds to both

dynein and MT to form the complex which plays a

critical role in intracellular transport, such as

vesicular transport from the endoplasmic reticulum to

the Golgi and lysosomal motility. Besides, Juliane et

al found that proper localization of huntingtin in the

cell depends on functional dynein/dynactin complex

(Caviston et al. 2007).

Huntingtin, a protein with 3,144 amino acids, is

present in all cells especially neurons and has a large

number of huntingtin partner proteins (Taran et al.

2020) involved in vesicales in neurons and endocytic

vesicles through directly binding to intermediate

chain of dyneins and to huntingtin-associated protein-

1(HAP1). HAP1 interacts with the subunit

p150Glued of dynactin and the heavy chain of

kinesin-1 by its coiled-coil domain. What is more, the

subunit p150Glued of dynactin can also interact with

kinesin-1. The fact that mutant huntingtin can cause

disturbance in axonal transport also indirectly

demonstrates the important role of Huntington

proteins in vesicle transport along microtubules. The

function of huntingtin to facilitate anterograde and

retrograde transport along MT in neurons is regulated

by phosphorylation of huntingtin at serine 421 by Akt

upon IGF-1 stimulation. When huntingtin S421 is

phosphorylated, it recruits kinesin-1 to vesicles and

MTs to facilitate anterograde transport. In contrast,

when huntingtin S421 is not phosphorylated,

huntingtin directly combine with intermediate chain

of dynein to facilitate dynein/dynactin-mediated

retrograde transport of vesicle along the axon like

Figure 1 shows. Furthermore, phosphorylation of

huntingtin does not affect basic characters of MT

such as nucleation, dynamics or stability (Colin et al.

2008).

Although phosphorylation of huntingtin S421

indicates no differences in binding between

huntingtin and HAP1 or between HAP1 and

p150Glued of dynactin or between HAP1 and

kinesin-1, the signal marked the interaction of

p150glued of dynactin and kinesin-1 increase. It

shows that the phosphorylation of huntingtin S421

leads to the stable interaction between dynactin and

kinesin-1 (Colin et al. 2008).

Figure 1. Participation of Huntingtin in vesicle transport

along microtubule (Taran et al. 2020).

Huntingtin as a scaffold for the dynein-dynactin

complex and its phosphorylation determines the

direction of vesicle transport along the microtubules.

2.2 Huntingtin and Actin

Huntingtin is essential for cellular adhesion and for

actin cytoskeleton in response to stimulation of

growth factor, like platelet derived growth factor

(PDGF), to change the normal morphology. The

deletion of huntingtin results in reduced adhesion and

altered morphology. Moreover, mutant huntingtin in

Huntington’s Disease inhibits the regulation of

growth factor stimulation in morphology changes and

increases numbers of vinculin-positive focal

adhesion. Immunoreactivity indicates huntingtin is

localized to actin stress fibers, vinculin-positive

adhesion contacts and membrane ruffles in

fibroblasts. Other studies further to show that first 14

amino acids of a purified fragment of huntingtin have

a direct interaction with actin in vitro. Besides,

huntingtin co-localizes with α-actinin and regulates

its localization at membranes to impact on

maintenance of adhesion and cellular morphologic

changes (Tousley et al. 2019). The Figure 2 shows the

co-localizations between huntingtin with α-actinin-1

in primary human fibroblasts (a and b).

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573

Figure 2: Co-localizations between huntingtin with α-actinin-1 in primary human fibroblasts (a and b) by double-label

immunofluorescence (Tousley et al. 2019).

Red shows huntingtin detected with Ab2527,

green shows α-actinin detected by a monoclonal

antibody and yellow show them co-localize at stress

fibers in serum-starved cells(top, short arrows). (b)

Green shows huntingtin detected with Ab1173, red

shows α-actinin detected by a monoclonal antibody

and yellow shows them co-localize in lamellipodia at

ruffled membranes(top panel, arrows and inset) and

in protrusions at the leading edge of

lamellipodia(bottom panel, arrow). 60x oil objective.

Scale bars = 10µm.

α-actinins are actin binding proteins including

different isoforms, like α-actinin-1 and α-actinin-2

(Tousley et al. 2019). Huntingtin can bind to α-

actinin-1, 2 and 4. The function of α-actinin is to

bundle and crosslink actin filaments in both

contractile and non-contractile cells and link actin

filaments to integrins in focal complexes and focal

adhesions that persists during hierarchical assembly.

Moreover, Actin and α-actinin-2 are concentrated in

the dendritic spines of neurons and play a role in

regulating the morphology of the spine and

stabilizing postsynaptic membrane proteins (Tousley

et al. 2019).

α-actinin-1 and huntingtin co-localized to stress

fibers, membrane and ruffles and lamellar protrusions

in fibroblasts through double-label

immunofluorescence. Proximity ligation assays

indicate that α-actinin-1 have a close interaction with

huntingtin in human fibroblasts and neurons (Tousley

ICHIH 2022 - International Conference on Health Big Data and Intelligent Healthcare

574

et al. 2019). Adelaide et al. found that huntingtin is

responsible for regulation of α-actinin-1 proper

localization on the membrane and combination of

growth factor with actin polymerization at new sites

of adhesion (Taran et al. 2020).

α-actinin-2 interacts with 399-969 amino acids

region of huntingtin. However, full interaction

between α-actinin-2 and huntingtin demands

additional amino acids N-terminal to huntingtin

residue 399. Highly dynamic α-actinin-2 is

concentrated in dendritic spines of neurons in brain,

where it regulates morphology and maturation of

dendritic spines and the transport of the AMPA

subtype of glutamate receptors to post-synaptic.

Huntingtin is also essential for development of

excitatory synapses in the cortical-striatal pathways

in brain. Hence, the interaction between huntingtin

and α-actinin-2 may induce maturation and function

of excitatory synapses on neurons (Taran et al. 2020).

On one hand, IP 3-kinase in respond to growth

factor stimulation activates Akt and produces PI

(3,4,5) P3 and PI (3,4) P2. Activated Akt can

phosphorylate huntingtin at serines 419 and 421 to

interfere the combination between huntingtin and α-

actinin-2 or facilitate their dissociation. On the other

hand, α-actinins can bind to both PI (4,5) P2 and PI

(3,4,5) P3. Huntingtin binds to PI (4,5) P2 with a low

affinity while binds to PI (3,4) P2 and PI (3,4,5) P3

with a high affinity. As the result of the fact that PI

(4,5) P2 is more abundant in the membrane than PI

(3,4,5) P3, α-actinins bind to PI (4,5) P2 with lack of

huntingtin. With the interaction of huntingtin, α-

actinins bind to PI (3,4,5) P3 with a high affinity at

highly specialized regions. These may be pathways

for huntingtin interacting with actin and actinin to

impact the cellular morphology, induced adhesion

and neuronal maturation and they may alter in

Huntington’s Disease. Nevertheless, it still requires

more relevant experiments and studies to prove

(Taran et al. 2020).

It is worth noting that α-actinin-2 and dynein have

the same region S421 of interaction on huntingtin and

their competitive binding to huntingtin perhaps play

a critical role in regulating the transport of vesicle

from MTs to actin filaments (Tousley et al. 2019).

2.3 Alternation of Tau in Huntington's

Disease

Recently, with further studies of Huntington's

Disease, increasing evidences of multiple alterations

of Tau have been found in brains of Huntington's

Disease patients, which implies that abnormal

alterations of Tau is likely to pathogenic for

contributing to the process of Huntington's Disease.

Tau, a microtubule-associated protein, is encode

by the MAPT gene that is located in the long(q) arm

of chromosome 17 at position 21.31 and contains 16

exons. Multiple Tau isoforms is generated by

alternative splicing. For instance, the exclusion of

exon 10 results in 3R isoform of Tau while inclusion

of exon 10 results in 4R isoform. The difference

between 3R and 4R is in the C-terminal region of Tau,

where exon 10 encodes a 31 amino acid sequence and

provides one of the four probable tubulin-binding

repeats. The proportion of Tau isoforms as well as

post-translational modifications such as

phosphorylation and acetylation influence the affinity

of Tau for microtubules (Marta et al. 2020).

The MAPT gene is mainly expressed in neurons

of the central nervous system, which is related to its

function of maintaining neuronal polarity by

regulating microtubule assembly and stability. In

general, Tau is almost exclusively located in the axon

of healthy neurons. The N-terminal region of Tau

binds to plasma membrane components and

participates in the formation of microtubule bundles

as a spacer between microtubule bundles while the C-

terminal region binds to microtubules to regulate their

dynamic assembly. Besides, Tau is involved in the

transport of mRNA and proteins along axons in

intracellular, neurite extension and synaptic plasticity

(Marta et al. 2020).

The alterations of Tau are mainly reflected in

increased total levels, imbalance of isoforms

produced by alternative splicing or by post-

translational modifications and the presence of Tau

nuclear rods (TNRs) or Tau-positive nuclear

indentations (TNIs) (Marta et al. 2020).

Marked by Tau-5 antibody, Tau showed a high

increase in the cortex of Huntington’s Disease

patients while no changes were found in the striatum.

Moreover, elevated Tau total mRNA levels in the

putamen of Huntington’s Disease patients and

attenuate motor abnormalities by Tau knock-down in

an HD mouse model also demonstrate that excess of

Tau contributes to the process of Huntington’s

Disease (Marta et al. 2020).

In Huntington’s Disease patients, another

prominent manifestation of alteration of Tau is an

increase in the ratio 4R-Tau/3R-Tau isoforms, which

is regulated by alternative splicing of exon 10. It

shows an increase of the level of 4R-Tau protein in

the cortex while in the striatum, an increase of the

level of 4R-Tau is accompanied by a decrease of the

level of 3R-Tau. Alternative splicing of exon 10 is

mainly regulated by the family of the serine- and

The Roles of Cytoskeleton in Huntington’s Disease

575

arginine-rich (SR) proteins, especially SRSF6. Post-

translational modifications of SR proteins, like

phosphorylation of serine and threonine residues,

regulate their activity and localization. In general,

phosphorylation of SR proteins is benefit to its

translocation from the cytoplasm to the nucleus. In

Huntington’s Disease, increased levels of

phosphorylation of SRSF6 in the striatum and cortex,

as well as sequestration by mutant Huntingtin

inclusions result in a decrease of SRSF6 activity. In

addition, SRSF6 regulates alternative splicing of

MAP-2, which also alters in Huntington’s Disease

(Marta et al. 2020).

Marking by AT-8 antibody, it found that an

increase of phosphorylation of Tau at Ser396, 404,

199 and Thr205 epitopes in the putamen of

Huntington’s Disease patients. GSK-3 is one of the

main kinases to phosphorylate Tau. Further studies

found that the level and activity of GSK-3 decreased

and the phosphorylation of GSK-3β Ser9, inactive

form of the kinase, increased in Huntington’s

Disease. What’s more, a decrease of phosphatases

(PP1, PP2A and PP2B) implicated in

dephosphorylation of Tau is detected in the R6/2

mouse model. All of these implicate that the reason

of Tau hyperphosphorylation in Huntington’s Disease

is the deficiency of dephosphorylation of Tau (Marta

et al. 2020).

Finally, some research has found the presence of

TNIs, known as TNRs, in the striatum and cortex of

Huntington’s Disease patients by antibodies that

recognize 4R-Tau isoforms, 3R-Tau isoforms, total

Tau or Tau oligomers. The ordered filamentous

ultrastructure of TNIs/TNRs fills neuronal

invaginations of nuclear envelope and partially or

totally span the neuronal nuclear space (Marta et al.

2020).

3 CONCLUSIONS

All in all, this review describes the roles of

cytoskeleton in Huntington's Disease from three

aspects. When huntingtin S421 is phosphorylated, it

recruits kinesin-1 to vesicles and MTs to facilitate

anterograde transport. In contrast, when huntingtin

S421 is not phosphorylated, huntingtin directly

combine with intermediate chain of dynein to

facilitate dynein/dynactin-mediated retrograde

transport of vesicle along the axon. Huntingtin

interacts with actin and actinin to impact the cellular

morphology, induced adhesion and neuronal

maturation. Moreover, the competitive binding of α-

actinin-2 and dynein to huntingtin S421 may be the

switch on vesicle transport from MT to actin. As a

result, mutant huntingtin in Huntington's Disease

may changes its function above. In addition, the

alterations of Tau like total level, alternative splicing

and post-translational modification suggest that Tau

may be an independent factor that results in

Huntington's Disease, or together with Huntingtin

leads to the occurrence and development of the

disease.

This article through to partly roles of cytoskeleton

in Huntington’s Disease, on the one hand, hopes to

provide new targets for clinical treatment and

method. On the other hand, such as Alzheimer's and

Huntington's part neurodegenerative diseases are

characterized by accumulation of protein misfolding,

which may have a certain similarity in pathogenesis,

this article may provide help for researches on these

diseases with similar characteristics.

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