Accumulation of nuclear ADAR2 regulates adenosine-to-inosine RNA editing during neuronal development.

Adenosine to inosine (A-to-I) RNA editing is important for a functional brain, and most known sites that are subject to selective RNA editing have been found to result in diversified protein isoforms that are involved in neurotransmission. In the absence of the active editing enzymes ADAR1 or ADAR2 (also known as ADAR and ADARB1, respectively), mice fail to survive until adulthood. Nuclear A-to-I editing of neuronal transcripts is regulated during brain development, with low levels of editing in the embryo and a dramatic increase after birth. Yet, little is known about the mechanisms that regulate editing during development. Here, we demonstrate lower levels of ADAR2 in the nucleus of immature neurons than in mature neurons. We show that importin-α4 (encoded by Kpna3), which increases during neuronal maturation, interacts with ADAR2 and contributes to the editing efficiency by bringing it into the nucleus. Moreover, we detect an increased number of interactions between ADAR2 and the nuclear isomerase Pin1 as neurons mature, which contribute to ADAR2 protein stability. Together, these findings explain how the nuclear editing of substrates that are important for neuronal function can increase as the brain develops.

ADAR2 regulates RNA stability by modifying access of decay-promoting RNA-binding proteins.

Adenosine deaminases acting on RNA (ADARs) catalyze the editing of adenosine residues to inosine (A-to-I) within RNA sequences, mostly in the introns and UTRs (un-translated regions). The significance of editing within non-coding regions of RNA is poorly understood. Here, we demonstrate that association of ADAR2 with RNA stabilizes a subset of transcripts. ADAR2 interacts with and edits the 3΄UTR of nuclear-retained Cat2 transcribed nuclear RNA (Ctn RNA). In absence of ADAR2, the abundance and half-life of Ctn RNA are significantly reduced. Furthermore, ADAR2-mediated stabilization of Ctn RNA occurred in an editing-independent manner. Unedited Ctn RNA shows enhanced interaction with the RNA-binding proteins HuR and PARN [Poly(A) specific ribonuclease deadenylase]. HuR and PARN destabilize Ctn RNA in absence of ADAR2, indicating that ADAR2 stabilizes Ctn RNA by antagonizing its degradation by PARN and HuR. Transcriptomic analysis identified other RNAs that are regulated by a similar mechanism. In summary, we identify a regulatory mechanism whereby ADAR2 enhances target RNA stability by limiting the interaction of RNA-destabilizing proteins with their cognate substrates.

Adenosine-to-inosine RNA editing affects trafficking of the gamma-aminobutyric acid type A (GABA(A)) receptor.

Recoding by adenosine-to-inosine RNA editing plays an important role in diversifying proteins involved in neurotransmission. We have previously shown that the Gabra-3 transcript, coding for the α3 subunit of the GABA(A) receptor is edited in mouse, causing an isoleucine to methionine (I/M) change. Here we show that this editing event is evolutionarily conserved from human to chicken. Analyzing recombinant GABA(A) receptor subunits expressed in HEK293 cells, our results suggest that editing at the I/M site in α3 has functional consequences on receptor expression. We demonstrate that I/M editing reduces the cell surface and the total number of α3 subunits. The reduction in cell surface levels is independent of the subunit combination as it is observed for α3 in combination with either the ß2 or the ß3 subunit. Further, an amino acid substitution at the corresponding I/M site in the α1 subunit has a similar effect on cell surface presentation, indicating the importance of this site for receptor trafficking. We show that the I/M editing during brain development is inversely related to the α3 protein abundance. Our results suggest that editing controls trafficking of α3-containing receptors and may therefore facilitate the switch of subunit compositions during development as well as the subcellular distribution of α subunits in the adult brain.

Recognition and coupling of A-to-I edited sites are determined by the tertiary structure of the RNA.

Adenosine-to-inosine (A-to-I) editing has been shown to be an important mechanism that increases protein diversity in the brain of organisms from human to fly. The family of ADAR enzymes converts some adenosines of RNA duplexes to inosines through hydrolytic deamination. The adenosine recognition mechanism is still largely unknown. Here, to investigate it, we analyzed a set of selectively edited substrates with a cluster of edited sites. We used a large set of individual transcripts sequenced by the 454 sequencing technique. On average, we analyzed 570 single transcripts per edited region at four different developmental stages from embryogenesis to adulthood. To our knowledge, this is the first time, large-scale sequencing has been used to determine synchronous editing events. We demonstrate that edited sites are only coupled within specific distances from each other. Furthermore, our results show that the coupled sites of editing are positioned on the same side of a helix, indicating that the three-dimensional structure is key in ADAR enzyme substrate recognition. Finally, we propose that editing by the ADAR enzymes is initiated by their attraction to one principal site in the substrate.

Donor islet endothelial cells participate in formation of functional vessels within pancreatic islet grafts.

Pancreatic islet transplantation has emerged as a therapy for type 1 diabetes and is today performed using both freshly isolated and cultured islets. Islet blood vessels are disrupted during islet isolation; therefore, proper revascularization of the transplanted islets is of great importance for islet graft function and survival. We have studied intraislet endothelial cells after islet isolation, during islet culture, and following islet transplantation. By isolating islets from the transgenic Tie2-GFP (green fluorescent protein) mouse, characterized by an endothelial cell-specific expression of GFP, living endothelial cells could be studied in intact islets utilizing two-photon laser-scanning microscopy (TPLSM). Intraislet endothelial cells were found to survive islet transplantation but to rapidly disappear during islet culture. By transplanting freshly isolated Tie2-GFP islets and applying a novel ex vivo model for simultaneous perfusion and TPLSM imaging of the graft-bearing kidneys, GFP fluorescent endothelial cells were found to extensively contribute to vessels within the islet graft vasculature. Real-time imaging of the flow through the islet graft vasculature confirmed that the donor-derived vessels were functionally integrated. Hence, intraislet endothelial cells have the capability of participating in revascularization of pancreatic islets subsequent to transplantation. Therefore, preservation of intraislet endothelial cell mass may improve long-term graft function.

Site-selective versus promiscuous A-to-I editing.

RNA editing by adenosine deamination is acting on polymerase II derived transcripts in all metazoans. Adenosine-to-inosine (A-to-I) editing is mediated by the adenosine deaminase that acts on RNA (ADAR) enzymes. Two types of adenosine to inosine (A-to-I) RNA editing have been defined: site selective and hyper-editing. Typically, in site selectively edited substrates, one or a few A-to-I sites are edited in double-stranded RNA structures, frequently interrupted by single-stranded bulges and loops. Hyper-editing occurs in long stretches of duplex RNA where multiple adenosines are subjected to deamination. In this review, recent findings on editing within noncoding RNA as well as examples of site selective editing within coding regions are presented. We discuss how these two editing events have evolved and the structural differences between a site selective and hyper-edited substrate.

Large-scale mRNA sequencing determines global regulation of RNA editing during brain development.

RNA editing by adenosine deamination has been shown to generate multiple isoforms of several neural receptors, often with profound effects on receptor function. However, little is known about the regulation of editing activity during development. We have developed a large-scale RNA sequencing protocol to determine adenosine-to-inosine (A-to-I) editing frequencies in the coding region of genes in the mammalian brain. Using the 454 Life Sciences (Roche) Amplicon Sequencing technology, we were able to determine even low levels of editing with high accuracy. The efficiency of editing for 28 different sites was analyzed during the development of the mouse brain from embryogenesis to adulthood. We show that, with few exceptions, the editing efficiency is low during embryogenesis, increasing gradually at different rates up to the adult mouse. The variation in editing gave receptors like HTR2C and GABA(A) (gamma-aminobutyric acid type A) a different set of protein isoforms during development from those in the adult animal. Furthermore, we show that this regulation of editing activity cannot be explained by an altered expression of the ADAR proteins but, rather, by the presence of a regulatory network that controls the editing activity during development.

SmeA, a small membrane protein with multiple functions in Streptomyces sporulation including targeting of a SpoIIIE/FtsK-like protein to cell division septa.

Sporulation in aerial hyphae of Streptomyces coelicolor involves profound changes in regulation of fundamental morphogenetic and cell cycle processes to convert the filamentous and multinucleoid cells to small unigenomic spores. Here, a novel sporulation locus consisting of smeA (encoding a small putative membrane protein) and sffA (encoding a SpoIIIE/FtsK-family protein) is characterized. Deletion of smeA-sffA gave rise to pleiotropic effects on spore maturation, and influenced the segregation of chromosomes and placement of septa during sporulation. Both smeA and sffA were expressed specifically in apical cells of sporogenic aerial hyphae simultaneously with or slightly after Z-ring assembly. The presence of smeA-like genes in streptomycete chromosomes, plasmids and transposons, often paired with a gene for a SpoIIIE/FtsK- or Tra-like protein, indicates that SmeA and SffA functions might be related to DNA transfer. During spore development SffA accumulated specifically at sporulation septa where it colocalized with FtsK. However, sffA did not show redundancy with ftsK, and SffA function appeared distinct from the DNA translocase activity displayed by FtsK during closure of sporulation septa. The septal localization of SffA was dependent on SmeA, suggesting that SmeA may act as an assembly factor for SffA and possibly other proteins required during spore maturation.