Monocarboxylate transporter genes in the mammary gland of lactating cows

Effect of sodium butyrate treatment at the basolateral membranes on the tight junction barrier function via a monocarboxylate transporter in goat mammary epithelial cells

In lactating mammary glands, tight junctions (TJs) prevent blood from mixing with milk and maintain epithelial cell polarity, which is important for milk production. This study aimed to investigate the effect of sodium acetate and sodium butyrate (SB) stimulation direction on the TJ barrier function, which is measured with regard to transepithelial electrical resistance and fluorescein flux, in goat mammary epithelial cells. The expression and localization of the TJ proteins claudin-3 and claudin-4 were examined using Western blotting and immunofluorescence. SB treatment in the lower chamber of cell culture inserts adversely affected the TJ barrier function, whereas sodium acetate barely had any effect, regardless of stimulation direction. In addition, SB treatment in the lower chamber significantly upregulated claudin-3 and claudin-4, whereas TJ proteins showed intermittent localization. Moreover, SB induced endoplasmic reticulum (ER) stress. ARC155858, a monocarboxylate transporter-1 inhibitor, alleviated the adverse impact of SB on TJs and the associated ER stress. Interestingly, sodium β-hydroxybutyrate, a butyrate metabolite, did not affect the TJ barrier function. Our findings indicate that sodium acetate and SB influence the TJ barrier function differently, and excessive cellular uptake of SB can disrupt TJs and induce ER stress.

Acetate Upregulates GPR43 Expression and Function via PI3K-AKT-SP1 Signaling in Mammary Epithelial Cells during Milk Fat Synthesis

This study investigated the mechanism underlying acetate-induced orphan G-protein-coupled receptor 43 (GPR43) expression and milk fat production. The mammary epithelial cells of dairy cows were treated with acetate, and the effects of GPR43 on acetate uptake and the expression of lipogenesis-related genes were determined by gas chromatography and quantitative polymerase chain reaction (qPCR), respectively. RNAi, inhibitor treatment, and luciferase assay were used to determine the effect of phosphoinositide 3-kinase-protein kinase B-specificity protein 1 (PI3K-AKT-SP1) signaling on acetate-induced GPR43 expression and function. The results showed that GPR43 was highly expressed in lactating cow mammary tissues, which was related to milk fat synthesis. 12 mM acetate significantly increased the GPR43 expression in mammary epithelial cells of dairy cows. In acetate-treated cells, GPR43 overexpression significantly increased the cellular uptake of acetate, the intracellular triacylglycerol (TAG) content, and acetate-induced lipogenesis gene expression. Acetate activated PI3K-AKT signaling and promoted SP1 translocation from the cytosol into the nucleus, where SP1 bound to the GPR43 promoter and upregulated GPR43 transcription. Moreover, the activation of PI3K-AKT-SP1 by acetate facilitated the trafficking of GPR43 from the cytosol to the plasma membrane. In conclusion, acetate upregulated GPR43 expression and function via PI3K-AKT-SP1 signaling in mammary epithelial cells, thereby increasing milk fat synthesis. These results provide an experimental strategy for improving milk lipid synthesis, which is important to the dairy industry.

Renal localization and regulation by dietary phosphate of the MCT14 orphan transporter

MCT14 is an orphan transporter belonging to the SLC16 transporter family mediating the transport of monocarboxylates, aromatic amino acids, creatine, and thyroid hormones. The expression, tissue localization, regulation, and function of MCT14 are unknown. In mouse MCT14 mRNA abundance is highest in kidney. Using a newly developed and validated antibody, MCT14 was localized to the luminal membrane of the thick ascending limb of the loop of Henle colocalizing in the same cells with uromodulin and NKCC2. MCT14 mRNA and protein was found to be highly regulated by dietary phosphate intake in mice being increased by high dietary phosphate intake at both mRNA and protein level. In order to identify the transport substrate(s), we expressed MCT14 in Xenopus laevis oocytes where MCT14 was integrated into the plasma membrane. However, no transport was discovered for the classic substrates of the SLC16 family nor for phosphate. In summary, MCT14 is an orphan transporter regulated by phosphate and highly enriched in kidney localizing to the luminal membrane of one specific nephron segment.

Histological characterization of orphan transporter MCT14 (SLC16A14) shows abundant expression in mouse CNS and kidney

BACKGROUND

MCT14 (SLC16A14) is an orphan member of the monocarboxylate transporter (MCT) family, also known as the SLC16 family of secondary active transmembrane transporters. Available expression data for this transporter is limited, and in this paper we aim to characterize MCT14 with respect to tissue distribution and cellular localization in mouse brain.

RESULTS

Using qPCR, we found that Slc16a14 mRNA was highly abundant in mouse kidney and moderately in central nervous system, testis, uterus and liver. Using immunohistochemistry and in situ hybridization, we determined that MCT14 was highly expressed in excitatory and inhibitory neurons as well as epithelial cells in the mouse brain. The expression was exclusively localized to the soma of neurons. Furthermore, we showed with our phylogenetic analysis that MCT14 most closely relate to the aromatic amino acid- and thyroid-hormone transporters MCT8 (SLC16A2) and MCT10 (SLC16A10), in addition to the carnitine transporter MCT9 (SLC16A9).

CONCLUSIONS

We provide here the first histological mapping of MCT14 in the brain and our data are consistent with the hypothesis that MCT14 is a neuronal aromatic-amino-acid transporter.

Proteomic Analysis of Isolated Plasma Membrane Fractions from the Mammary Gland in Lactating Cows

The mammary gland of dairy cows is a formidable lipid-synthesizing machine for lactation. This unique function depends on the activities of plasma membrane (PM) proteins in mammary cells. Little information is known about the expression profiles of PM proteins and their functions during the lactating process. This study investigated the proteome map of PM fractions of mammary gland in lactating cows using 1D-Gel-LC-MS/MS and identified 872 nonredundant proteins with 141 unknown proteins, wherein 215 were PM-associated proteins. Most of the PM-associated proteins were binding, transport, and catalytic proteins such as annexin proteins, heat shock proteins, integrins, RAS oncogene family members, and S100 calcium binding proteins. The PM-associated pathways such as caveolae-mediated endocytosis, leukocyte extravasation, aldosterone signaling in epithelial cells, and remodeling of epithelial adherens junctions were also significantly over-represented. Proteomic analysis revealed the characteristics and predicted functions of PM proteins isolated from the lactating bovine mammary gland. These results further provide experimental evidence for the presence of many proteins predicted in the annotated bovine genome. The data generated here also provide a reference for the PM-related functional research in the mammary gland of lactating cows.

Lactate-H⁺ transport is a significant component of the in vivo corneal endothelial pump

PURPOSE

To confirm the expression of monocarboxylate transporters (MCT) 1, 2, and 4 in rabbit CE and to test the hypothesis that cellular buffering contributed by HCO₃⁻, NBCe1, and carbonic anhydrase (CA) activity facilitates lactate-H⁺ efflux thereby controlling corneal hydration in vivo.

METHODS

MCT1-4 expression of rabbit endothelium was examined by Western blotting and immunofluorescence staining. Lactate-induced acidification (LIA) was measured in perfused CE in the presence and absence of HCO₃⁻ and acetazolamide (ACTZ) using tissue treated with siRNA specific to MCT1, 2, and 4. Corneal thickness and lactate concentration were measured in New Zealand White rabbits treated with the topical CA inhibitor Azopt, and from eyes that were injected intracamerally with ouabain, disodium 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS), and shRNA specific to the 1Na⁺:2HCO₃⁻ cotransporter NBCe1.

RESULTS

MCT1 and MCT4 are localized to the lateral membrane, while MCT2 is apical. Cell pH measurements showed LIA in response to 40 mM lactate in bicarbonate free (BF) Ringer's that was inhibited by niflumic acid and by MCT siRNA knockdown, and significantly reduced in the presence of HCO₃⁻. Lactate-dependent proton flux in vitro was not significantly greater in the presence of HCO₃⁻ or reduced by ACTZ. However, when active transport, NBCe1, or CA activity was disrupted in vivo, corneal edema ensued and was associated with significant corneal lactate accumulation.

CONCLUSIONS

MCT1, 2, and 4 are expressed in rabbit CE on both the apical and basolateral surfaces and function to transport lactate-H⁺. Lactate-H⁺ flux is facilitated by active transport, HCO₃⁻ transport and CA activity, disruption of which causes corneal edema in vivo and indicates that facilitation of lactate efflux is a component of the endothelial pump.

Extending the knowledge in histochemistry and cell biology

Central to modern Histochemistry and Cell Biology stands the need for visualization of cellular and molecular processes. In the past several years, a variety of techniques has been achieved bridging traditional light microscopy, fluorescence microscopy and electron microscopy with powerful software-based post-processing and computer modeling. Researchers now have various tools available to investigate problems of interest from bird's- up to worm's-eye of view, focusing on tissues, cells, proteins or finally single molecules. Applications of new approaches in combination with well-established traditional techniques of mRNA, DNA or protein analysis have led to enlightening and prudent studies which have paved the way toward a better understanding of not only physiological but also pathological processes in the field of cell biology. This review is intended to summarize articles standing for the progress made in "histo-biochemical" techniques and their manifold applications.