Age-related changes in expression of CTL2/SLC44A2 and its isoforms in the mouse inner ear

Slc44a2 deletion alters tetraspanin and N-cadherin expression: Reduced adhesion and enhanced proliferation in cultured mesenchymal lung cells

Slc44a2 is reported to interact with tetraspanins CD9 and CD81. To investigate how Slc44a2 affects adhesion protein expression, cells from wild-type (WT) Slc44a2+/+, heterozygous (HET) Slc44a2+/-, and knockout (KO) Slc44a2-/- mice were cultured from lung tissue. The cultured cells expressed vimentin, N-cadherin, p120 catenin, beta-catenin, actin, CD9, and CD81, but not E-cadherin. Vimentin expression with lack of E-cadherin indicated that the cultured cells were of mesenchymal origin. Slc44a2 KO cells and HET cells demonstrated lower adherence and faster proliferation than the WT cells. All three groups displayed dramatically altered intracellular distribution of N-cadherin, CD9, and CD81. The CD9 membrane foci observed in WT cell membranes were less frequent and diminished in size in HET cells and KO cells. N-cadherin was dispersed throughout both the cytoplasm and membrane in WT cells, with similar yet weaker distribution in HET cells; however, in KO cells, N-cadherin was densely aggregated in the perinuclear cytoplasm. CD81 had a distribution pattern in WT, HET, and KO cells similar to that of N-cadherin with dense cytoplasmic clusters in the cells. KO cells also exhibited reduced filamentous actin as compared to WT cells. These results suggest that Slc44a2 is necessary for proper cellular localization of adhesion proteins and growth regulation that may be related to altered adhesion signals.

Functional and Biochemical Consequences of Disease Variants in Neurotransmitter Transporters: A Special Emphasis on Folding and Trafficking Deficits

Neurotransmitters, such as γ-aminobutyric acid, glutamate, acetyl choline, glycine and the monoamines, facilitate the crosstalk within the central nervous system. The designated neurotransmitter transporters (NTTs) both release and take up neurotransmitters to and from the synaptic cleft. NTT dysfunction can lead to severe pathophysiological consequences, e.g. epilepsy, intellectual disability, or Parkinson’s disease. Genetic point mutations in NTTs have recently been associated with the onset of various neurological disorders. Some of these mutations trigger folding defects in the NTT proteins. Correct folding is a prerequisite for the export of NTTs from the endoplasmic reticulum (ER) and the subsequent trafficking to their pertinent site of action, typically at the plasma membrane. Recent studies have uncovered some of the key features in the molecular machinery responsible for transporter protein folding, e.g., the role of heat shock proteins in fine-tuning the ER quality control mechanisms in cells. The therapeutic significance of understanding these events is apparent from the rising number of reports, which directly link different pathological conditions to NTT misfolding. For instance, folding-deficient variants of the human transporters for dopamine or GABA lead to infantile parkinsonism/dystonia and epilepsy, respectively. From a therapeutic point of view, some folding-deficient NTTs are amenable to functional rescue by small molecules, known as chemical and pharmacological chaperones.

Hair Cell Loss, Spiral Ganglion Degeneration, and Progressive Sensorineural Hearing Loss in Mice with Targeted Deletion of Slc44a2/Ctl2

SLC44A2 (solute carrier 44a2), also known as CTL2 (choline transporter-like protein 2), is expressed in many supporting cell types in the cochlea and is implicated in hair cell survival and antibody-induced hearing loss. In mice with the mixed C57BL/6-129 background, homozygous deletion of Slc44a2 exons 3–10 (Slc44a2^Δ/Δ) resulted in high-frequency hearing loss and hair cell death. To reduce effects associated with age-related hearing loss (ARHL) in these strains, mice carrying the Slc44a2^Δ allele were backcrossed to the ARHL-resistant FVB/NJ strain and evaluated after backcross seven (N7) (99 % FVB). Slc44a2^Δ/Δ mice produced abnormally spliced Slc44a2 transcripts that contain a frameshift and premature stop codons. Neither full-length SLC44A2 nor a putative truncated protein could be detected in Slc44a2^Δ/Δ mice, suggesting a likely null allele. Auditory brain stem responses (ABRs) of mice carrying the Slc44a2^Δ allele on an FVB/NJ genetic background were tested longitudinally between the ages of 2 and 10 months. By 6 months of age, Slc44a2^Δ/Δ mice exhibited hearing loss at 32 kHz, but at 12 and 24 kHz had sound thresholds similar to those of wild-type Slc44a2^+/+ and heterozygous +/Slc44a2^Δ mice. After 6 months of age, Slc44a2^Δ/Δ mutants exhibited progressive hearing loss at all frequencies and +/Slc44a2^Δ mice exhibited moderate threshold elevations at high frequency. Histologic evaluation of Slc44a2^Δ/Δ mice revealed extensive hair cell and spiral ganglion cell loss, especially in the basal turn of the cochlea. We conclude that Slc44a2 function is required for long-term hair cell survival and maintenance of hearing.

Full-length nucleotide sequences of 30 common SLC44A2 alleles encoding human neutrophil antigen-3

BACKGROUND

Human neutrophil antigen-3a (HNA-3a) alloantibodies can cause severe transfusion-related acute lung injury. The frequencies of the single-nucleotide polymorphisms (SNPs) indicative of the two clinically relevant HNA-3a/b antigens are known in many populations. In this study, we determined the full-length nucleotide sequence of common SLC44A2 alleles encoding the choline transporter-like protein-2 that harbors HNA-3a/b antigens.

STUDY DESIGN AND METHODS

A method was devised to determine the full-length coding sequence (CDS) and adjacent intron sequences from genomic DNA by eight polymerase chain reaction amplifications covering all 22 SLC44A2 exons. Samples from 200 African American, 96 Caucasian, two Hispanic, and four Asian blood donors were analyzed. We developed a decision tree to determine alleles (confirmed haplotypes) from the genotype data.

RESULTS

A total of 10 SNPs were detected in the SLC44A2 CDS. The noncoding sequences harbored an additional 28 SNPs (one in the 5'-untranslated region [UTR]; 23 in the introns; and four in the 3'-UTR). No SNP indicative of a nonfunctional allele was detected. The nucleotide sequences for 30 SLC44A2 alleles (haplotypes) were confirmed. There may be 66 haplotypes among the 604 chromosomes screened.

CONCLUSIONS

We found 38 SNPs, including one novel SNP, in 8192 nucleotides covering the CDS of the SLC44A2 gene among 302 blood donors. Population frequencies of these SNPs were established for African Americans and Caucasians. Because alleles encoding HNA-3b are more common than non-functional SLC44A2 alleles, we confirmed our previous postulate that African American donors are less likely to form HNA-3a antibodies compared to Caucasians.

Choline transporter-like proteins CTLs/SLC44 family as a novel molecular target for cancer therapy

Choline is essential for the synthesis of the major membrane phospholipid phosphatidylcholine (PC), the methyl donor betaine and the neurotransmitter acetylcholine (ACh). Elevated levels of choline and up-regulated choline kinase activity have been detected in various cancers. Thus, the intracellular accumulation of choline through choline transporters is the rate-limiting step in phospholipid metabolism and a prerequisite for cancer cell proliferation. Previous studies have demonstrated abnormalities in choline uptake and choline phospholipid metabolism in cancer cells using the imaging of cancer with positron emission tomography (PET) and magnetic resonance spectroscopy (MRS). The aberrant choline metabolism in cancer cells is strongly correlated with their malignant progression. Using quantitative real-time PCR, the mRNA expression of choline transporters was measured, and it was found that choline transporter-like proteins CTLs/SLC44 family are highly expressed in various cancer cell lines. Choline uptake through CTLs is associated with cell viability, and the functional inhibition of CTLs could promote apoptotic cell death. Furthermore, non-neuronal cholinergic systems that include CTLs-mediated choline transport are associated with cell proliferation and their inhibition promotes apoptotic cell death in colon cancer, small cell lung cancer and human leukemic T-cells. The identification of this new CTLs-mediated choline transport system provides a potential new target for cancer therapy.

The choline transporter-like family SLC44: properties and roles in human diseases

The Na(+)-independent, high affinity choline carrier system proposed to supply choline for the synthesis of cell membrane phospholipids was recently associated with SLC44 family members (SLC44A1-5) also called choline-like transporter family. SLC44A1 is widely expressed throughout the nervous system in both neurons and oligodendrocytes, while SLC44A2-4 are mainly detected in peripheral tissues. The subcellular localization of the proteins was mainly addressed for SLC44A1 through the development of specific antibodies. SLC44A1 is detected in both the plasma and mitochondrial membranes where the protein is able to transport choline at high affinity and in a Na(+)-independent manner. The physiological relevance of SLC44A1 as a choline carrier is indicated by its likely involvement in membrane synthesis for cell growth or repair, and also by its role in phospholipid production for the generation of lung surfactant. Moreover, an autoimmune disease has been related to the blockade of SLC44A2 function, which results in the alteration of hair cells in the inner ear and leads to autoimmune hearing loss. In the alloimmune syndrome called transfusion-related acute lung injury, antibodies to SLC44A2 cause a deleterious aggregation of granulocytes. Therefore transporters of the SLC44 family represent attractive and promising targets for therapeutic and diagnostic applications regarding both immune and degenerative diseases.