Identification and functional characterization of the novel isoforms of bovine norepinephrine transporter produced by alternative splicing

Global detection and identification of developmental stage specific transcripts in mouse brain using subtractive cross-screening algorithm

BACKGROUND

Pre-mRNA splicing is a crucial step for genetic regulation and accounts largely for downstream translational diversity. The current time of biological research is characterized by advances in functional genomics study and the understanding of the pre-mRNA splicing process has thus become a major portal for biologists to gain insights into the complex gene regulatory mechanism. The intranuclear alternative splicing process can form a variety of genomic transcripts that modulate the growth and development of an organism, particularly in the immune and neural systems.

METHODS

In the current study, we investigated and identified alternative splicing transcripts at different stages of embryonic mouse brain morphogenesis using subtractive cross-screening algorithm.

RESULTS

A total of 195 candidate transcripts were found during organogenesis; 1629 identified at fetus stage, 116 in juvenile and 148 transcripts from adulthood. To document our findings, we developed a database named DMBAS, which can be accessed through the link: http://173.234.48.5/DMBAS. We further investigated the alternative splicing products obtained in our experiment and noted the existence of chromosome preference between prenatal and postnatal transcripts. Additionally, the distribution of splicing sites and the splicing types were found to have distinct genomic features at varying stages of brain development. The majority of identified alternative splices (72.3%) at fetus stage were confirmed later using separate RNA-seq data sets.

CONCLUSION

This study is a comprehensive profiling of alternative splicing transcripts of mouse brain morphogenesis using advanced computational algorithm. A series of developmental stage specific transcripts, as well as their splicing sites and chromosome preferences were revealed in the current study. Our findings and the related online database would form a solid foundation for studies of broader biological significance and paved the way for future investigations in relevant human brain diseases.

C-terminal region regulates the functional expression of human noradrenaline transporter splice variants

The NET [noradrenaline (norepinephrine) transporter], an Na+/Cl--dependent neurotransmitter transporter, has several isoforms produced by alternative splicing in the C-terminal region, each differing in expression and function. We characterized the two major isoforms of human NET, hNET1, which has seven C-terminal amino acids encoded by exon 15, and hNET2, which has 18 amino acids encoded by exon 16, by site-directed mutagenesis in combination with NE (noradrenaline) uptake assays and cell surface biotinylation. Mutants lacking one third or more of the 24 amino acids encoded by exon 14 exhibited neither cell surface expression nor NE uptake activity, with the exception of the mutant lacking the last eight amino acids of hNET2, whose expression and uptake resembled that of the WT (wild-type). A triple alanine replacement of a candidate motif (ENE) in this region mimicked the influences of the truncation. Deletion of either the last three or another four amino acids of the C-terminus encoded by exon 15 in hNET1 reduced the cell surface expression and NE uptake, whereas deletion of all seven residues reduced the transport activity but did not affect the cell surface expression. Replacement of RRR, an endoplasmic reticulum retention motif, by alanine residues in the C-terminus of hNET2 resulted in a similar expression and function compared with the WT, while partly recovering the effects of the mutation of ENE. These findings suggest that in addition to the function of the C-terminus, the common proximal region encoded by exon 14 regulates the functional expression of splice variants, such as hNET1 and hNET2.

Alternative splicing in the nervous system: an emerging source of diversity and regulation

Alternative splicing is emerging as a major mechanism of functional regulation in the human genome. Previously considered to be an unusual event, it has been detected by many genomics studies in 40%-60% of human genes. Moreover, it appears to be of central importance for neuronal genes and other genes involved in "information processing" functions. In this review, we will summarize alternative splicing's effects on mRNA transcripts, protein products, biological function, and human disease, focusing on genes of neuropsychiatric interest. We will also describe the latest experimental methods and database resources that can help neuroscientists make use of alternative splicing in their own research.

Norepinephrine transporter splice variants and their interaction with substrates and blockers

Norepinephrine transporter (NET), a member of the Na+/Cl--dependent neurotransmitter transporter family, displays species-specific isoforms produced by alternative RNA splicing. This occurs at 3'-flanking coding and noncoding regions, resulting in different carboxy-terminals. When these NET splice variants were expressed in cultured cell lines, the characteristics of substrate transport and the sensitivity to uptake inhibitors differed between isoforms. The different functional expression suggests the physiological importance of the action and interaction of NET splice variants in synaptic transmission.

[Pharmacology of monoamine neurotransmitter transporters]

Following exocytotic release, the biogenic amine neurotransmitters, norepinephrine, dopamine, and serotonin are removed from the synaptic cleft by the respective transporter, NET, DAT, and SERT, located on the plasma membrane and then re-stored into synaptic vesicles by vesicular monoamine transporter, VMAT. The molecular cloning of these transporters revealed that NET, DAT, and SERT are members of a sodium-dependent neurotransmitter transporter gene family, while VMATs arise from proton-dependent transporter gene family. Structural features common to NET, DAT, and SERT reveal a putative 12 transmembrane-spanning domain structure with cytosolic N- and C-terminal regions. Recent evidence suggest the regulation of the functional expression of these transporters via phosphorylation, which include direct phosphorylation of transporter proteins and/or of associated proteins that may control transporter function/expression. In addition, the substrates and inhibitors for these transporters appear capable of regulating transporter cell surface expression, thereby suggesting both activity-dependent and pharmacological regulatory mechanisms for transporter expression. Analyses of the genes provide new insight into their relation to neuronal diseases since NET, DAT and SERT are the molecular targets for many antidepressants as well as drugs of abuse such as cocaine and amphetamine. The availability of cDNAs of these and vesicular transporters has permitted detailed pharmacological studies in heterologous expression systems, and thus would promise the development of novel drugs with diverse chemical structures.