Saccharomyces cerevisiae: First Steps to a Suitable Model System To Study the Function and Intracellular Transport of Human Kidney Anion Exchanger 1

Heterologous (Over) Expression of Human SoLute Carrier (SLC) in Yeast: A Well-Recognized Tool for Human Transporter Function/Structure Studies

For more than 20 years, yeast has been a widely used system for the expression of human membrane transporters. Among them, more than 400 are members of the largest transporter family, the SLC superfamily. SLCs play critical roles in maintaining cellular homeostasis by transporting nutrients, ions, and waste products. Based on their involvement in drug absorption and in several human diseases, they are considered emerging therapeutic targets. Despite their critical role in human health, a large part of SLCs' is 'orphans' for substrate specificity or function. Moreover, very few data are available concerning their 3D structure. On the basis of the human health benefits of filling these knowledge gaps, an understanding of protein expression in systems that allow functional production of these proteins is essential. Among the 500 known yeast species, S. cerevisiae and P. pastoris represent those most employed for this purpose. This review aims to provide a comprehensive state-of-the-art on the attempts of human SLC expression performed by exploiting yeast. The collected data will hopefully be useful for guiding new attempts in SLCs expression with the aim to reveal new fundamental data that could lead to potential effects on human health.

A Deep Insight Into Regulatory T Cell Metabolism in Renal Disease: Facts and Perspectives

Kidney disease encompasses a complex set of diseases that can aggravate or start systemic pathophysiological processes through their complex metabolic mechanisms and effects on body homoeostasis. The prevalence of kidney disease has increased dramatically over the last two decades. CD4⁺CD25⁺ regulatory T (Treg) cells that express the transcription factor forkhead box protein 3 (Foxp3) are critical for maintaining immune homeostasis and preventing autoimmune disease and tissue damage caused by excessive or unnecessary immune activation, including autoimmune kidney diseases. Recent studies have highlighted the critical role of metabolic reprogramming in controlling the plasticity, stability, and function of Treg cells. They are also likely to play a vital role in limiting kidney transplant rejection and potentially promoting transplant tolerance. Metabolic pathways, such as mitochondrial function, glycolysis, lipid synthesis, glutaminolysis, and mammalian target of rapamycin (mTOR) activation, are involved in the development of renal diseases by modulating the function and proliferation of Treg cells. Targeting metabolic pathways to alter Treg cells can offer a promising method for renal disease therapy. In this review, we provide a new perspective on the role of Treg cell metabolism in renal diseases by presenting the renal microenvironment、relevant metabolites of Treg cell metabolism, and the role of Treg cell metabolism in various kidney diseases.

The Bicarbonate Transporter (MoAE4) Localized on Both Cytomembrane and Tonoplast Promotes Pathogenesis in Magnaporthe oryzae

Bicarbonate (HCO₃⁻) transporter family including the anion exchanger (AE) group is involved in multiple physiological processes through regulating acid-base homeostasis. HCO₃⁻ transporters have been extensively studied in mammals, but fungal homologues of AE are poorly understood. Here, we characterized the AE group member (MoAE4) in Magnaporthe oryzae. MoAE4 exhibits more sequence and structure homologies with the reported AE4 and BOR1 proteins. In addition to the common sublocalization on cytomembrane, MoAE4 also localizes on tonoplast. Yeast complementation verified that MoAE4 rescues boron sensitivity and endows NaHCO₃ tolerance in the BOR1 deleted yeast. MoAE4 gene is bicarbonate induced in M. oryzae; and loss of MoAE4 (ΔMoAE4) resulted in mycelial growth inhibited by NaHCO₃. Lucigenin fluorescence quenching assay confirmed that ΔMoAE4 accumulated less HCO₃⁻ in vacuole and more HCO₃⁻ in cytosol, revealing a real role of MoAE4 in bicarbonate transport. ΔMoAE4 was defective in conidiation, appressorium formation, and pathogenicity. More H₂O₂ was detected to be accumulated in ΔMoAE4 mycelia and infected rice cells. Summarily, our data delineate a cytomembrane and tonoplast located HCO₃⁻ transporter, which is required for development and pathogenicity in M. oryzae, and revealing a potential drug target for blast disease control.

Boosting endoplasmic reticulum folding capacity reduces unfolded protein response activation and intracellular accumulation of human kidney anion exchanger 1 in Saccharomyces cerevisiae

Human kidney anion exchanger 1 (kAE1) facilitates simultaneous efflux of bicarbonate and absorption of chloride at the basolateral membrane of α-intercalated cells. In these cells, kAE1 contributes to systemic acid-base balance along with the proton pump v-H⁺ -ATPase and the cytosolic carbonic anhydrase II. Recent electron microscopy analyses in yeast demonstrate that heterologous expression of several kAE1 variants causes a massive accumulation of the anion transporter in intracellular membrane structures. Here, we examined the origin of these kAE1 aggregations in more detail. Using various biochemical techniques and advanced light and electron microscopy, we showed that accumulation of kAE1 mainly occurs in endoplasmic reticulum (ER) membranes which eventually leads to strong unfolded protein response (UPR) activation and severe growth defect in kAE1 expressing yeast cells. Furthermore, our data indicate that UPR activation is dose dependent and uncoupled from the bicarbonate transport activity. By using truncated kAE1 variants, we identified the C-terminal region of kAE1 as crucial factor for the increased ER stress level. Finally, a redistribution of ER-localized kAE1 to the cell periphery was achieved by boosting the ER folding capacity. Our findings not only demonstrate a promising strategy for preventing intracellular kAE1 accumulation and improving kAE1 plasma membrane targeting but also highlight the versatility of yeast as model to investigate kAE1-related research questions including the analysis of structural features, protein degradation and trafficking. Furthermore, our approach might be a promising strategy for future analyses to further optimize the cell surface targeting of other disease-related PM proteins, not only in yeast but also in mammalian cells.