AtoI RNA Editing and ADARs

RNA editing is one of posttranscriptional events, and this modifies pre-mRNA sequences, resulted in the change of genomically encoded information (Bass 2002;

Laboratory for Molecular Dynamics of Mental Disorders, RIKEN Brain Science Institute,

2-1 Hirosawa, Wako, Saitama Japan, 351-0198

Department of Molecular Psychiatry, Graduate School of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo Japan, 113-8655 e-mail: [email protected]

G. Di Giovanni et al. (eds.), 5-HT2C Receptors in the Pathophysiology of CNS Disease, 157

The Receptors 22, DOI 10.1007/978-1-60761-941-3_8, © Springer Science+Business Media, LLC 2011

Keegan et al. 2001; Maas et al. 2003). Consequence of RNA editing is dependent on the context of target site, and affects splice pattern, intracellular localization, and protein function. In mammals, there are two dominant types of RNA editing: C-to-U and A-to-I. The C-to-U RNA editing includes conversion of the cytidine (C) to uri-dine (U) in apolipoprotein B pre-mRNA by APOBEC1 cytidine deaminases (Navaratnam and Sarwar 2006). The latter, A-to-I RNA editing includes conversion of adenosine (A) to inosine (I), and is mediated by adenosine deaminases acting on RNA (ADARs). Because translation machinery recognizes inosine in pre-mRNA as guanosine, change of amino acid sequences can occur. The target RNA for A-to-I RNA editing included neurotransmitter receptors such as glutamate and serotonin receptors, although recent studies revealed its more frequent targets would be repetitive sequence regions in the mRNAs and noncoding RNAs (Nishikura 2006).

In mammals, three ADARs (ADAR1, ADAR2, and ADAR3) have been identified (Fig. 8.1) (Keegan et al. 2001; Nishikura 2006). They all have conserved double-stranded RNA-binding domain and deaminase domain in the C-terminal of the protein. In the N-terminal, there are specific domains of each ADAR such as presence or absence of Z-DNA-binding domain and single-stranded RNA-binding domain. ADAR1 and ADAR2 express ubiquitously, while ADAR3 showed restricted expression of the subregions of brain. Some reports showed that ADAR3 have no deaminase activity (Chen et al. 2000; Melcher et al. 1996) and biological function of ADAR3 remains to be defined.

The locus of ADAR2 (22q22.3) has been suggested to be associated with familial BD by the linkage studies (Aita et al. 1999; Detera-Wadleigh et al. 1996). Amore et al. examined cDNA sequences of ADAR2 in seven patients with familial BDs (Amore et al. 2004), and Kostyrko et al. performed mutation screening in 60 patients with BD (Kostyrko et al. 2006). Neither study identified polymorphisms or mutations specific to BD patients. On the other hand, ADAR1 (1q21.3) has been reported to be involved in dyschromatosis symmetrica hereditaria, which is characterized by hyperpigmented and hypopigmented macules on extremities (Miyamura et al. 2003). However, there is no attempt to examine the relationship between genetic variations of ADAR1 and mental disorders further.

Fig. 8.1 Structure of human adenosine deaminases acting on RNA (ADAR) proteins. Domain structure and their genomic locations were shown. Alternative translation site of ADAR1 is denoted by arrow (Z Z DNA-binding domain, dsRBD double-stranded RNA-binding domain)

8.2 RNA Editing of HTR2C

In HTR2C, A-to-I RNA editing occurs at five positions (termed sites A to E), and resulted in the amino acid change at three positions (Fig. 8.2). Despite the high sequence homologies across 14 identified serotonin receptors, including other members of 5-HT2 subfamily (HTR2A and HTR2B), HTR2C is the only receptor that undergoes RNA editing. The nonedited HTR2C isoform, which contains a genomically encoded sequence, showed the amino acids isoleucine at position 156 (I156), asparagine at position 158 (N158), and isoleucine at position 160 (I160, abbreviate as INI). If A-to-I RNA editing occurs at A and D sites, the resultant pre-mRNA would encode the VNV isoform. Theoretically, there are 32 transcript combinations in total, and they could generate up to 24 distinctive iso-forms (Table 8.1). The distribution of HTR2C isoforms was differed across brain regions (Burns et al. 1997) and has been believed to have physiological and pathophysiological roles in vivo. The editing site is located within the second intracellular loop of the receptor, which is important for the association with G proteins. In addition, to activate phospholipase C via coupling with Gq protein (Berg et al. 1994), the nonedited INI isoform also activates phospholipase D (McGrew et al. 2002) and ERK1/2 (Werry et al. 2005) via coupling with Ga 12/13. It has been established that RNA editing generally results in the reduction of receptor activity by modulating serotonin potency and G-protein-coupling

Fig. 8.2 RNA editing of HTR2C. Top: Predicted secondary structure of the human HTR2C pre-mRNA. Double-stranded RNA structure is recognized by adenosine deaminases acting on RNAs (ADARs). In addition to the five sites, it has been reported that the sixth editing site (site F) is in the intoronic sequence (Flomen et al. 2004). Functional significance remains to be elucidated, though. Bottom: Pattern of amino acid change

Fig. 8.2 RNA editing of HTR2C. Top: Predicted secondary structure of the human HTR2C pre-mRNA. Double-stranded RNA structure is recognized by adenosine deaminases acting on RNAs (ADARs). In addition to the five sites, it has been reported that the sixth editing site (site F) is in the intoronic sequence (Flomen et al. 2004). Functional significance remains to be elucidated, though. Bottom: Pattern of amino acid change

Table 8.1 HTR2C isoform distribution in the human and rat prefrontal cortex8

Isoform

Human (%)

Rat (%)

VSV

25.2

14.8

VNV

18.0

41.8

VSI

12.6

11.7

INI

9.0

6.3

ISV

6.3

1.1

VDV

5.4

1.6

VGV

5.4

0.5

VNI

5.4

17

INV

3.6

1.4

VGI

3.6

0.8

MNI

1.8

1.2

IDI

0.9

nd

IGV

0.9

0.5

ISI

0.9

nd

MNV

0.9

nd

IDV

nd

nd

IGI

nd

nd

MDI

nd

nd

MDV

nd

nd

MGI

nd

nd

MGV

nd

nd

MSI

nd

nd

MSV

nd

nd

VDI

nd

1.2

nd not detected aData were obtained by vidual bacterial clone tained HTR2C cDNA unpublished data)

sequencing of indi-(N > 100) that consequence (Iwamoto, nd not detected aData were obtained by vidual bacterial clone tained HTR2C cDNA unpublished data)

sequencing of indi-(N > 100) that consequence (Iwamoto, activity (Berg et al. 2008; Marion et al. 2004; Niswender et al. 1999). In addition, RNA editing may affect the cellular localization of HTR2C (Marion et al. 2004). While nonedited INI isoform showed agonist-independent internalization, fully edited VGV resides in membrane.

Molecular mechanism that contributes to differential RNA editing level across different brain regions remains elucidated. Expression level of ADAR1 and ADAR2 in a given sample will be one of important factors, because in vivo and in vitro studies revealed that ADARs has differential editing activities in the editing sites. For example, A and B sites were preferentially edited by ADAR1, whereas D site was exclusively edited by ADAR2 (Hartner et al. 2004; Higuchi et al. 2000; Liu et al. 1999). In addition, it has been suggested that splice event of HTR2C was closely linked to RNA editing (Flomen et al. 2004). It has been also reported that HBII-52, one of small nucleolar RNAs (snoRNAs), regulated alternative splicing of

HTR2C (Cavaille et al. 2000; Kishore and Stamm 2006) and influenced RNA editing level (Vitali et al. 2005). Therefore, regulation of splicing and related factors may also have important role in the RNA editing level of HTR2C.

Defeat Drugs and Live Free

Defeat Drugs and Live Free

Being addicted to drugs is a complicated matter condition that's been specified as a disorder that evidences in the obsessional thinking about and utilization of drugs. It's a matter that might continue to get worse and become disastrous and deadly if left untreated.

Get My Free Ebook


Post a comment