Báo cáo khoa học: Huntington’s disease: roles of huntingtin-interacting protein 1 (HIP-1) and its molecular partner HIPPI in the regulation of apoptosis and transcription pptx

In the present review, we present evidence that Htt-inter-acting protein 1 HIP-1, an endocytic protein, together with its interHtt-inter-acting partner HIPPI, regulates apoptosis and gen

Huntington’s disease: roles of huntingtin-interacting

protein 1 (HIP-1) and its molecular partner HIPPI in the

regulation of apoptosis and transcription

Nitai P Bhattacharyya, Manisha Banerjee and Pritha Majumder*

Crystallography and Molecular Biology Division and Structural Genomics Section, Saha Institute of Nuclear Physics, Kolkata, India

Huntington’s disease (HD, OMIM 143100) is an

auto-somal dominant progressive neurodegenerative disease

caused by the expansion of polymorphic CAG (coding

for glutamine) repeats beyond 36 at exon 1 of the

huntingtin (htt) gene, localized at chromosome 4p16.3

Age at onset (AO) of the disease varies widely

(1–90 years, mean 35 years) There is an inverse

cor-relation between AO and expanded CAG repeat

numbers, but it is not the only determinant of

variation in AO [1] HD is fatal within 10–15 years after appearance of the first symptom The symptoms include uncontrolled movement, emotional distur-bances, psychiatric abnormalities, cognitive deficits, and dementia The gene htt encodes a protein [hunting-tin (Htt),  348 kDa] with a polyglutamine stretch starting from the 18th amino acid Also, two proline-rich regions adjacent to the polyglutamine domain and several HEAT repeats, known to be involved in

Keywords

apoptosis; HIP-1; HIPPI;

huntingtin-interacting proteins; transcription

Correspondence

N P Bhattacharyya, Crystallography and

Molecular Biology Division and Structural

Genomics Section, Saha Institute of Nuclear

Physics, 1 ⁄ AF Bidhan Nagar, Kolkata

700 064, India

Fax: +91 033 23374637

Tel: +91 033 23375345–49 (5 lines),

ext 1301

E-mail: nitaipada.bhattacharya@saha.ac.in or

nitai_sinp@yahoo.com

*Present address

Roswell Park Cancer Institute, Cell Stress

Biology Department, Buffalo, New York,

USA

(Received 29 February 2008, revised 15

May 2008, accepted 18 June 2008)

doi:10.1111/j.1742-4658.2008.06563.x

Huntingtin protein (Htt), whose mutation causes Huntington’s disease (HD), interacts with large numbers of proteins that participate in diverse cellular pathways This observation indicates that wild-type Htt is involved

in various cellular processes and that the mutated Htt alters these processes

in HD The roles of these interacting proteins in HD pathogenesis remain largely unknown In the present review, we present evidence that Htt-inter-acting protein 1 (HIP-1), an endocytic protein, together with its interHtt-inter-acting partner HIPPI, regulates apoptosis and gene expression, both processes being implicated in HD Further studies are necessary to establish whether the HIPPI–HIP-1 complex or other interacting partners of HIPPI regulate apoptosis and gene expression that are relevant to HD

Abbreviations

ANTH, AP180 N-terminal homology domain; AO, age at onset; AP, adaptor protein; AR, androgen receptor; BLOC1S2, biogenesis of lysosome-related organelles complex-1 subunit 2; CLH1, clathrin heavy chain 1; CLH2, clathrin heavy chain 2; CLTA, clathrin light chain A; CLTB, clathrin light chain B; ENTH, Epsin N-terminal homology; HD, Huntington’s disease; HIP-1, huntingtin-interacting protein 1; HIPPI, huntingtin-interacting protein 1 interactor; Htt, huntingtin protein; NMDAR, N-methyl- D -aspartate receptor; pDED, pseudo-death effector domain; Shh, Sonic hedgehog.

protein–protein interactions, are present at the

N-ter-minal region of the protein Wild-type Htt is localized

at the endoplasmic reticulum, Golgi complex,

mito-chondria, and synaptic vesicles Htt is ubiquitously

expressed, although the neurodegeneration caused by

the mutated Htt shows region specificity [2,3]

The expanded polyglutamine domain of mutant Htt

is highly self-associative, resulting in aggregates⁄

neuro-nal intranuclear inclusions Aggregates⁄ neuronal

intra-nuclear inclusions are observed in cell models, brains

of transgenic animals, and post-mortem brains of HD

patients [2] Aggregate formation is enhanced with the

increase in the number of glutamines in vitro and

in vivo, and is believed to cause neurodegeneration [4]

Although a contradictory finding, that visible

aggre-gates are protective to neurons, has also been made

[5] The autosomal dominant nature of the disease

sug-gests a toxic gain-of-function of the mutated protein

that disrupts normal cellular functions and causes

neuronal death [3] Loss-of-function of the wild-type

protein may also contribute, at least partially, to the

disease pathology [6] Over the years, various cellular

events, such as excitotoxicity, oxidative stress,

mito-chondrial dysfunction, stress in the endoplasmic

reticu-lum, formation of channels through membranes,

axonal transport, protein degradation, autophagy,

transcriptional dysregulation, and apoptosis, have been

implicated in HD These processes may not be

inde-pendent of each other Detailed descriptions of these

processes are beyond the scope of this review In

the present review, we specifically focus on the role of

Htt-interacting protein HIP-1 and its molecular

part-ner HIPPI in the regulation of apoptosis and

transcrip-tion, the two processes that are altered in HD [7,8]

HIP-1 – its interacting partners and

endocytosis

Large numbers of proteins have been identified, by

different techniques that interact with Htt [9–11]

These studies reveal that Htt may function as a

scaffold and coordinate diverse cellular functions [9– 13] Some of the Htt-interacting proteins also alter the pathogenicity in the Drosophila model of HD [13] Among  300 Htt-interacting proteins described so far, HIP-1 is one of the most studied The possible involvement of HIP-1 in various cancers has been reviewed recently [14] and will not be discussed here HIP-1 has been shown by yeast two-hybrid assays to interact with N-terminal Htt HIP-1 is orthologous to yeast Sla2p, which is known to be involved in endocy-tosis and regulation of the actin cytoskeleton HIP-1 and Htt colocalize in neuronal cells [15,16] The inter-action of HIP-1 with wild-type Htt is stronger than that observed with mutated Htt [17] In addition to Htt, HIP-1 interacts with its paralog HIP1-R, subunits

of clathrin-associated adaptor protein (AP) complex AP2A1 and AP2A2, clathrin heavy chain 1 (CLH1), and clathrin heavy chain 2 (CLH2), clathrin light chain A (CLTA), and clathrin light chain B (CLTB), and N-methyl-d-aspartate receptor (NMDAR) subun-its NR2A and NR2B Various domains, such as the AP180 N-terminal homology domain (ANTH), also known as the Epsin N-terminal homology (ENTH) domain, the central coiled-coil region and a C-terminal talin homology domain are present at HIP-1 The coiled-coil domain contains a leucine-zipper motif and mediates heterodimerization with HIP-1R Consensus binding sites for the endocytic adaptor protein AP2 (DPF motif), clathrin heavy chain (LMDMD clathrin-box motif) and a phosphatidylinositol 4,5-biphosphate-binding motif at its ANTH⁄ ENTH domain are also present [14,18] Various domains of HIP-1 are shown

in Fig 1 Direct evidence that HIP-1 is involved in endocytosis comes from HIP-1 knock-out (HIP-1) ⁄ )) mice, which show defects in assembly of endocytic protein complexes on liposomal membranes and a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor trafficking [19] The similarities in amino acid sequences and domains between HIP-1, HIP-1R and yeast ortholog Sla2p, the interacting partners of HIP-1 with known functions and results with knockout mice

Fig 1 Various domains of HIP-1 The ANTH ⁄ ENTH domain (38–160), coiled-coil domain (371–610), and talin-like domain (814–1112) were predicted with the SMART tool (http://smart.embl-heidelberg.de/) Binding sites for HIPPI (422–503), AP2 (262–266 and 358–360), CLH1, CLH2 (332–336), CLTA, CLTB (484–489) and other domains are taken from the published literature and mentioned in the text The positions of the amino acids are not to scale.

show that HIP-1 participates in the regulation of

cytoskeletal and endocytic processes

HIP-1 and its interacting partners –

roles in apoptosis and survival

Various pathways followed during apoptosis have been

reviewed recently [20] In the ‘extrinsic pathway’,

acti-vation of caspase-8⁄ caspase-10, mostly through

trans-membrane death receptors, leads to activation of

downstream caspase-3 and cleavage of other

down-stream substrates, leading to nucleosomal ladders, a

hallmark of apoptosis In the ‘intrinsic pathway’,

signal factors released from mitochondria activate

caspase-9 and then caspase-3, leading to cell death

These two pathways may crosstalk via

caspase-8-medi-ated cleavage of Bid In the caspase-independent

path-way, apoptosis-inducing factors or endonuclease G,

normally present in the mitochondria, are released and

translocated to the nucleus, where they cleave the

genome into nucleosomal ladders directly

Several experimental findings indicate that HIP-1 is

a proapoptotic protein Exogenous expression of

HIP-1 in neuronal and non-neuronal cells induces apoptosis

following the intrinsic pathway [17,21] The

pseudo-death effector domain (pDED) of HIP-1 (Fig 1) alone

is able to induce apoptosis; Phe398 of HIP-1 (within

the pDED) is critical for increased apoptosis

Coex-pression of wild-type N-terminal Htt (encoded by

exon 1 of htt) and HIP-1 reduces HIP-1-induced

apop-tosis [17,21] Wild-type N-terminal Htt, being able to

interact with HIP-1 strongly, may reduce the amount

of HIP-1 that is available to interact with other

pro-tein(s) and reduce apoptosis In rat cells, HIP-1 is

cleaved in response to drugs that are known to induce

apoptosis, as well as in cells expressing exogenous

HIP-1, although the relevance of such cleavages in

apoptosis remains unknown HIP-1 interacts directly

with procaspase-9 and activates it Direct interaction

of HIP-1 with Apaf1 increases recruitment of

cyto-chrome c to the apotosome complex, resulting in

increased apoptosis [21] Depending on the status of

phosphorylation of HIP-1 by Dyrk1, HIP-1 interacts

with caspase-3 and enhances apoptosis, in a condition

where interaction and phosphorylation of HIP-1 by

Dyrk1 are reduced [22]

Exogenous expression of HIPPI (HIP-1 protein

interactor), a molecular partner of HIP-1, increases

apoptosis through the extrinsic pathway The HIP-1–

HIPPI heterodimer recruits procaspase-8 and activates

it [23] Enhancement of apoptosis by exogenous HIPPI

in the presence of endogenous HIP-1 is mediated

through activation of caspase-8, caspase-1, caspase-9⁄

caspase-6, and caspase-3 Cleavage of Bid and release

of cytochrome c and apoptosis-inducing factors from the mitochondria are also observed Coexpression of wild-type htt exon 1 and Hippi decreases apoptosis and increases survival in comparison with that obtained in cells expressing Hippi only In such a condition, inter-action of HIPPI with HIP-1 is reduced This result fur-ther shows that freely available HIP-1 is necessary to induce apoptosis [24]

Contradictory findings that HIP-1 may act as a prosurvival⁄ antiapoptotic protein and may not influ-ence apoptosis at all are also available Expression of full-length HIP-1 does not increase apoptosis, whereas deletion of the N-terminal ANTH⁄ ENTH domain increases apoptosis [25] Deletion of murine HIP-1

in vivo increases testicular degeneration by apoptosis, indicating a protective role of HIP-1 in apoptosis [26] Mice deficient in both HIP-1 and its paralog HIP-1R exhibit neurodegeneration at adulthood and can be rescued by human HIP-1 [27] Reduced sperm count and defects in reproduction have been observed in HIP-1) ⁄ ) mice, due to apparent loss of postmeiotic spermatids [28] These results show that HIP-1 in dif-ferent conditions may act as an antiapoptotic protein Overexpression of HIP-1 in brain tumors is correlated with the increased expression of epidermal growth factor receptor and platelet-derived growth factor b-receptor [29] The ANTH⁄ ENTH domain of HIP-1 interacts with 3-phosphate containing inositol lipids and stabilizes the growth factor receptor tyrosine kinases by increasing their half-life following ligand induced endocytosis Such interaction affects cell growth and survival [30] This observation supports the contention that the ANTH⁄ ENTH domain of HIP-1 protects cells from death by apoptosis, as mentioned earlier [25] Taken together, these results show that HIP-1 may act as a prosurvival protein in different conditions

Contradictory results showing that HIP-1 is a proa-poptotic protein [17,21–24] or an antiaproa-poptotic protein [25–30] in different conditions could be due to the presence or absence of HIP-1-interacting partners The decrease in HIP-1–HIPPI-mediated apoptosis, either

by overexpression of Homer 1c, an interactor of

HIP-PI (for details see the next section), or by the wild-type N-terminal Htt, which strongly interacts with HIP-1 [24,31], supports this contention In such cases, the amounts of freely available HIPPI or HIP-1 may decrease, resulting in reduced apoptosis Exogenous expression of HIP-1 (cloned in pcDNA3 and kindly provided to us by T S Ross, Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, MI, USA) in HeLa cells, where

endoge-nous HIPPI is undetectable [24], did not increase

apoptosis However, in Neuro2A and K562 cells,

where endogenous HIPPI is present [24], exogenous

expression of HIP-1 increased apoptosis Additionally,

expression of HIPPI in HeLa cells, where HIP-1 was

knocked down, decreased apoptosis (M Banerjee and

N P Bhattacharyya, unpublished observations)

rela-tive to that obtained in HeLa cells with endogenous

HIP-1 [24] The proapoptotic activity of HIPPI or

HIP-1 may thus be dependent on the presence or

absence of its interacting partner HIP-1 and HIPPI

respectively

HIPPI and its interacting partners –

regulation of apoptosis

HIPPI, also known as estrogen-related receptor

b-like 1, is a homolog of Chlamydomonas intraflagellar

transport 57 HIPPI does not have any known domain

except a pDED and a myosin-like domain Interaction

of HIPPI with HIP-1 takes place through the pDED,

specifically through 409 K, present in helix 5 of HIPPI,

although other regions have influence over such

inter-actions [23] We mentioned above that HIP-1 and

HIPPI together induce apoptosis [23,24] Identification

of additional proteins such as Homer1c⁄ Homer1 [31],

BAR⁄ BFAR [32], RybP [33], BLOC1S2 [34] and

apop-tin [35] that interact with HIPPI further indicates that

HIPPI may regulate apoptosis Homer1c⁄ Homer1

belongs to the homer family of proteins and is known

to participate, in neuronal signaling HIPPI interacts

with Homer1c and colocalizes in the postsynaptic

region of hippocampus It has been shown that

Homer1c completely abolishes HIP-1–HIPPI-mediated

apoptosis in striatal neurons, the specific region of

neuronal loss in HD [31] The bifunctional apoptosis

inhibitor BAR, also known as BFAR, is expressed

pre-dominantly in neurons, and interacts with HIP-1 as

well as HIPPI BAR inhibits neuronal apoptosis in

response to diverse stimuli [32] It is not known

whether BAR can regulate HIP-1–HIPPI-mediated

apoptosis Recently, Rybp has been shown to interact

with HIPPI and increase HIPPI-mediated apoptosis

through the caspase-8-mediated pathway Rybp also

interacts with ubiquitin-binding protein, procaspase-8,

procaspase-10, and the HIPPI interactor apotin

Inter-action of HIPPI with Rybp is involved in murine

neural development, although the significance of such

an interaction in apoptosis regulation or HD

patho-genesis remains elusive [33] Apotin, a chicken anemia

virus-encoded protein, has been shown to colocalize

with HIPPI in the cytoplasm of normal cells, whereas

in tumor cells, they localize separately in the nucleus

and cytoplasm The HIPPI–apoptin interaction may suppress apoptosis [35] The functional relevance of such interactions in HIP-1- or HIPPI-mediated apop-tosis also remains unknown Biogenesis of lysosome-related organelles complex-1 subunit 2 (BLOC1S2) specifically interacts with HIPPI, but not with HIP-1 Coexpression of HIPPI and BLOC1S2 does not increase apoptosis but sensitizes apoptosis induction by stauro-sporin or death ligand In addition, the expression of BLOC1S2 is increased in some tumors [34]

Information on the interacting partners of HIPPI such as HIP-1, Homer 1c, BAR and apoptin indicates that HIPPI may also be a proapoptotic protein Rybp, an interactor of HIPPI, interacts with caspase-8 and caspase-10, indicating that HIPPI might also be involved in the regulation of apoptosis Even though the exact function of HIPPI remains unknown, knockout mice for Hippi (HIPPI) ⁄ )) exhi-bit downregulation of the Sonic hedgehog (Shh) pathway and developmental abnormalities [36] However, the exact molecular defects in the Shh pathway in HIPPI) ⁄ ) mice are unknown It may be worthwhile to mention that Dyrk1, an HIP-1 inter-actor, is also involved in the Shh pathway by regu-lating Gli1 [37] The specific role of the Shh pathway

in apoptosis or HD remains unknown

Role of HIP-1 and HIPPI in transcriptional regulation There are not many reports on the transcriptional activity of HIP-1 or HIPPI It has been shown that HIP-1 interacts with androgen receptor (AR) through its coiled-coil domain and increases the transcriptional activity of AR on known AR-inducible promoter Treatment with androgen increases the nuclear fraction

of AR as well as that of HIP-1, indicating that forma-tion of the HIP-1–AR heterodimer is involved in trans-location of AR to the nucleus Facilitation of nuclear transport of AR by HIP-1 depends on its C-terminal nuclear localization signal for HIP-1 In addition to that of AR, mediation of transcriptional activity of genes by other nuclear hormone receptors, such as estrogen and glucocorticoid receptors, is also enhanced

by HIP-1 All these results demonstrate a nonconven-tional function of HIP-1 as a transcripnonconven-tional regulator [38], in addition to its endocytosis and proapoptotic or prosurvival⁄ antiapoptotic functions described in the preceding sections

The evidence that HIPPI directly or indirectly alters gene expression comes from the observations that caspase-1, caspase-3, caspase-7, caspase-8 and caspase-10 expression is increased in cells expressing exogenous

Hippi Also, expression of the mitochondrial-coded

genes ND1 and ND4, the nuclear genome-coded

mitochondrial genes SDHA and SDHB and the

antia-poptotic genes BCL-2 and survivin is decreased in

Hippi-expressing cells [24] Decreased expression of

ND1, ND4, SDHA, SDHB and BCL-2 may cause

mitochondrial dysfunctions and contribute towards the

increased apoptosis by HIPPI as mentioned earlier

Decreased expression of the antiapoptotic gene survivin

may also enhance apoptosis

HIPPI interacts with the putative promoter of

caspase-1 in vitro and in vivo [39] On the basis of

in vitrointeractions of various mutants of the sequence

5¢-AAAGACATG-3¢ ()101 to )93) present at the

caspase-1putative promoter sequence, where HIPPI can

bind, it has been predicted that HIPPI will interact with

AAAGA[GC][ATC][TG] [40] The presence of other

sequence motifs around the HIPPI binding site where

transcription factors p53, p73 and ETS1 can bind and

influence the expression of caspase-1 [41–43] indicate

that these or other unknown transcription factors may

cooperate with HIPPI for the regulation of caspase-1

expression HIPPI also interacts with putative promoter

sequences of caspase-8 and caspase-10 [40] and PARP-1

()579 to )149) (P Majumder and N P Bhattacharyya,

unpublished results) in vitro However, to determine

whether HIPPI binds with a similar motif present at the

putative promoters will require further studies It will be

of interest to determine whether such a motif is also

present at the putative promoter regions of other genes,

especially those that are altered in HD models, as

observed in several high-throughput gene expression

studies [8], and investigate whether HIPPI alters the

expression of some of them

It is not clear how HIPPI, being a cytoplasmic protein, enters into the nucleus and interacts with the putative promoter sequences and eventually increases the expression of the genes A similar mechanism to that described above for AR translocation by HIP-1 [38] may also operate for HIPPI translocation We are presently investigating whether a similar mechanism of translocation of HIPPI may take place, using HIP-1 knocked down or HIP-1 overexpressed cells However, indirect evidence that HIP-1 is necessary for the increased expression of caspase-1 has been obtained Exogenous expression of Hippi and exon 1 of htt with

16 CAG repeats reduces the interaction of HIP-1 with HIPPI and reduces apoptosis [24] In such a condition, expression of caspase-1 was decreased, as shown in Fig 2 This result indicates that a similar mechanism

of translocation of HIPPI by HIP-1 as observed with

AR and HIP-1 may also act in this condition, but this requires further confirmation Rybp, also known as death effector domain-associated factor, belongs to a family of small zinc finger-containing proteins that participate in transcriptional regulation by binding with other transcription factors such as YY1 and E2F

or transcription repressors [44] Additionally, the Rybp-related protein Yaf2 interacts with HIPPI Inter-action of HIPPI with Rybp is proposed to be involved

in murine neural development, although the signifi-cance of such an interaction in HD pathogenesis remains elusive [33] It is speculative that Rybp, a molecular interactor of HIPPI, cooperates with HIPPI

to augment transcription of caspase-1, and this war-rants further studies A summary of the findings that HIPPI increases apoptosis and alters gene expression is shown in Fig 3

wH16-Hi Pro-caspase-1

Caspase-1

Beta actin

45 kDa

20 kDa

42 kDa

Hi

Fig 2 Western blot analysis for the expression of caspase-1 in HeLa cells expressing green fluorescent protein (GFP)-tagged HIPPI (GFP– Hippi, lane denoted by Hi) and HeLa cells coexpressing GFP–HIPPI and the red fluorescent protein-tagged wild-type exon 1 of the htt gene with 16 CAG repeats (DsRed–wH16, lane denoted by wH16-Hi) The lower panel shows the result with antibody to b-actin (42 kDa) as load-ing control The sizes of procaspase-1 (45 kDa) and the activated caspase-1 (20 kDa) are shown by the arrows on the left The bar diagram (right panel) shows the average (n = 3) of integrated optical density (IOD) of the bands obtained with antibody to caspase-1 in western blot analysis using GFP–HIPPI-expressing cells (unfilled) and cells coexpressing GFP–HIPPI and DsRed–wH16 (filled bar).

Possible role of HIP-1 and HIPPI in the

pathogenesis of HD

Various cellular processes, such as apoptosis and

tran-scriptional dysregulation, are altered in HD, leading

to neuronal dysfunction and⁄ or neurodegenaration

HIP-1 and HIPPI may participate in some of these

processes HIP-1 modulates aggregate formation, and

mutant Htt induced neuronal dysfunction in the

Caenorhabditis elegans model of HD [45] The

increased functional role of NMDAR in HD and the

involvement of HIP-1 in NMDAR-mediated

phos-phorylation of Htt [18] further indicate the possible

participation of HIP-1 in HD pathogenesis HIP-1–

HIPPI-mediated apoptosis is observed in the striatal

neuron, the specific target for neurodegeneration in

HD The role of HIPPI, if any, in either the increased

apoptosis observed in animal and cell models and

post-mortem brains of HD patients [3,7] or in the

increased expression of caspase-1, caspase-3 and

PARP-1[46–48] remains unclear There is are

similari-ties in the apoptotic pathway and altered gene expres-sion observed in Hippi-expressing cells and cellular models, animal models or brains of HD patients The transcriptional deregulation observed in a variety of

HD model systems is possibly due to interactions of transcription factors⁄ repressors with the mutated Htt [8]; whether HIP-1–HIPPI contributes, at least for the subset of genes altered in HD, needs further investiga-tions The mechanism by which caspase-1 expression is increased in HD is not well understood, although the protein is implicated in the progression of HD [47,48] The regulation of caspase-1 by HIPPI observed in cul-tured cells provides an explanation for the increased caspase-1 expression in HD In HD, owing to weaker interactions of HIP-1 with the mutated Htt, the free HIP-1 pool might increase, and this in turn would lead to the formation of more HIP1–HIPPI, initiating apoptosis by caspase-8 activation and its downstream pathway, and might also increase the transcription of caspase-1 Further studies using animal models are necessary to confirm this

HIPPI Freely available HIP1 2

HIPPI

1

interaction HIP1

6 4

5

Caspase-8

Nucleus

Weak interaction

pDED of HIPPI N-terminal of HIPPI Mutant Htt

Caspase1

Pro-caspase 8

Pro-caspase 3 Caspase 3

1 Heterodimerization of HIPPI and HIP1

2 Recruitment of Pro-caspase 8

Nuclear pore complex HIP1

2.

3 Activation of caspase 8 that leads to activation of

caspase 3 either by extrinsic or intrinsic pathways

4 Activation of caspase 3

5 Entry of caspase 3 to nucleus

6 Entry of the HIP-1-HIPPI heterodimer into the nucleus

7 Regulation of gene expression by HIPPI

Fig 3 Possible mechanisms of regulation of transcription and apoptosis by HIPPI and HIP-1 in HD Interaction of HIP-1 with the wild-type Htt allele is stronger than that of the mutated Htt [17] In HD, one of the alleles of Htt is mutated and thus likely to release free HIP-1 Free HIP-1 then interacts with HIPPI through its C-terminal pDED domain and recruits caspase-8, and activates caspase-8 and its downstream effector proteins, resulting in apoptosis On the other hand, HIPPI–HIP-1 heterodimer may translocate to the nucleus, interact with the putative promoters of caspase-1, caspase-8 and caspase-10, and increase their expressions In turn, increased pro-caspase-8 is recruited to HIPPI–HIP-1 heterodimer and increases apoptosis The role of caspase-1 in apoptosis is not known, but in some conditions it may increase apoptosis Different symbols representing different proteins are shown in the box Numbers representing different processes are also shown.

HIP-1 and its interacting partner HIPPI together

induce apoptosis by the intrinsic and extrinsic

pathways Homer 1c, an interactor of HIPPI, and the

wild-type N-terminal Htt, which interacts strongly

with HIP-1, reduce HIP-1–HIPPI-mediated apoptosis

In the presence of Homer 1c, HIPPI may interact

preferentially with it, resulting in a decrease of the

amount of HIP-1–HIPPI heterodimer and apoptosis

induction The effect of HIP-1 may depend not only

on the amount of the interacting proteins but also on

the affinities of interacting proteins The uncreased

expression of caspase-1 observed in HD may be

medi-ated through HIPPI The role of HIP-1 in

transloca-tion of HIPPI into the nucleus and that of other

transcriptional regulators cooperating with HIPPI are

yet to be determined If the observations in cell

culture are replicated in HD models or post-mortem

brains, an explanation of the increased expression of

caspase-1 and other subset of genes altered in HD

may be available

Acknowledgements

We acknowledge Professors A Mukherjee, D

Mukho-padhyay and M S Moumita Datta for critically

read-ing the manuscript and their valuable suggestions

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