Identification and tissue distribution of two differentially spliced variants of the rat carnitine transporter OCTN2

Methods and platforms for the quantification of splice variants' expression

The relatively limited number of human protein encoding genes highlights the importance of the diversity generated at the level of the mRNA transcripts. As alternative RNA splicing plays a key role in mediating this diversity, it becomes critical to develop the tools and platforms that will deliver quantitative information on the specific expression levels associated with splice isoforms. This chapter describes the constraints generated by this global transcriptome analysis and the state-of-the-art techniques and products available to the scientific community.

Expression and distribution of OCTN2 in mouse epididymis and its association with obstructive azoospermia in juvenile visceral steatosis mice

AIM

L-carnitine, an essential cofactor for mitochondrial, beta-oxidation of long-chain fatty acids, is known to play important roles in sperm maturation and metabolism when spermatozoa pass and acquire motility in the epididymis. We reported that obstructive azoospermia occurred in the epididymis in the juvenile visceral steatosis (JVS) mice, which are OCTN2 dysfunction mice caused by mutations in the gene encoding OCTN2, have been used for animal models of primary systemic carnitine deficiency. The aim of present study is to investigate the expression of OCTN2 protein in the mouse epididymis and its relation between the localization of OCTN2 and obstructive azoospermia in JVS mice as animal models for human male infertility.

METHODS

Animals used in this study were wild-type (C57BL/6 J) mice (n = 4) and JVS mice (n = 4). We made a specific polyclonal antibody against OCTN2 and examined immunohistochemically the localization of OCTN2 in the mouse epididymis.

RESULTS

OCTN2 was localized on the apical membrane of the principal cells of distal corpus and cauda epididymides. Immunocytochemistry demonstrated that OCTN2 was localized on the surface of microvillus upon the principal cells. In JVS mice, immunoreactivity started in a region immediately distal to where the sperm obstruction occurred.

CONCLUSIONS

Our results suggest that OCTN2 functions as a carnitine transporter between the epithelium and the lumen in distal corpus and cauda epididymides and provides a clue as to why obstructive azoospermia is induced in distal parts of epididymis.

Organic cation transporters

Over the last 15 years, a number of transporters that translocate organic cations were characterized functionally and also identified on the molecular level. Organic cations include endogenous compounds such as monoamine neurotransmitters, choline, and coenzymes, but also numerous drugs and xenobiotics. Some of the cloned organic cation transporters accept one main substrate or structurally similar compounds (oligospecific transporters), while others translocate a variety of structurally diverse organic cations (polyspecific transporters). This review provides a survey of cloned organic cation transporters and tentative models that illustrate how different types of organic cation transporters, expressed at specific subcellular sites in hepatocytes and renal proximal tubular cells, are assembled into an integrated functional framework. We briefly describe oligospecific Na(+)- and Cl(-)-dependent monoamine neurotransmitter transporters ( SLC6-family), high-affinity choline transporters ( SLC5-family), and high-affinity thiamine transporters ( SLC19-family), as well as polyspecific transporters that translocate some organic cations next to their preferred, noncationic substrates. The polyspecific cation transporters of the SLC22 family including the subtypes OCT1-3 and OCTN1-2 are presented in detail, covering the current knowledge about distribution, substrate specificity, and recent data on their electrical properties and regulation. Moreover, we discuss artificial and spontaneous mutations of transporters of the SLC22 family that provide novel insight as to the function of specific protein domains. Finally, we discuss the clinical potential of the increasing knowledge about polymorphisms and mutations in polyspecific organic cation transporters.

Quantification of NF1 transcripts reveals novel highly expressed splice variants

Previously, we have shown that the NF1 gene gives rise to multiple novel splice variants. In the present study, nine NF1 variants were quantified by real-time PCR in various human tissues. Some of these variants were expressed at low to moderate low levels and possible implications of these findings are discussed. Interestingly, two variants (NF1-DeltaE4b and NF1-DeltaE43) were shown to be highly expressed in specific tissues. NF1-DeltaE43 lacks a nuclear targeting sequence and might be functionally different from full-length NF1. These novel NF1 splice variants might expand our understanding of the role of neurofibromin.

Organic cation/carnitine transporter, OCTN2, is differentially expressed in the adult rat epididymis

L-Carnitine must be transported against a substantial concentration gradient across the epididymal epithelium to achieve high intraluminal levels, approximately 50 mM in the cauda. Recently, an organic cation transporter, OCTN2, was cloned from rat intestinal epithelium and shown to transport L-carnitine in a sodium-dependent manner. To test the hypothesis that OCTN2 was present in the epididymis, primers were designed based on the published OCTN2 mRNA sequence. A 1.9-kilobase OCTN2 cDNA from rat epididymis was amplified by reverse transcription polymerase chain reaction (RT-PCR) and cloned. Northern analysis demonstrated the presence of OCTN2 transcripts in the epididymis, with highest expression in the distal caput and corpus. To localize the protein, an antibody raised against a carboxy-terminal peptide of OCTN2 was produced in rabbits and used for Western blot analysis and immunohistochemistry. The antibody recognized a band of approximately 65 kDa in Western blots using epididymal lysates. Immunohistochemical studies demonstrate that OCTN2 is present in the basolateral membrane of epithelial cells in the distal caput, corpus, and proximal cauda epididymides. In conclusion, OCTN2 is present in the rat epididymis in a region-dependent manner and is likely to be responsible for the transport of L-carnitine into the cells of the epididymal epithelium.