Ahcyl2 upregulates NBCe1-B via multiple serine residues of the PEST domain-mediated association

Both IRBIT and long-IRBIT bind to and coordinately regulate Cl-/HCO3- exchanger AE2 activity through modulating the lysosomal degradation of AE2

Anion exchanger 2 (AE2) plays crucial roles in regulating cell volume homeostasis and cell migration. We found that both IRBIT and Long-IRBIT (L-IRBIT) interact with anion exchanger 2 (AE2). The interaction occurred between the conserved AHCY-homologous domain of IRBIT/L-IRBIT and the N-terminal cytoplasmic region of AE2. Interestingly, AE2 activity was reduced in L-IRBIT KO cells, but not in IRBIT KO cells. Moreover, AE2 activity was slightly increased in IRBIT/L-IRBIT double KO cells. These changes in AE2 activity resulted from changes in the AE2 expression level of each mutant cell, and affected the regulatory volume increase and cell migration. The activity and expression level of AE2 in IRBIT/L-IRBIT double KO cells were downregulated if IRBIT, but not L-IRBIT, was expressed again in the cells, and the downregulation was cancelled by the co-expression of L-IRBIT. The mRNA levels of AE2 in each KO cell did not change, and the downregulation of AE2 in L-IRBIT KO cells was inhibited by bafilomycin A1. These results indicate that IRBIT binding facilitates the lysosomal degradation of AE2, which is inhibited by coexisting L-IRBIT, suggesting a novel regulatory mode of AE2 activity through the binding of two homologous proteins with opposing functions.

Integrated Transcriptomic Analysis Reveals the Molecular Mechanism of Meningiomas by Weighted Gene Coexpression Network Analysis

Meningiomas are the most common primary intracranial tumor in adults. However, to date, systemic coexpression analyses for meningiomas fail to explain its pathogenesis. The aim of the present study was to construct coexpression modules and identify potential biomarkers associated with meningioma progression. Weighted gene coexpression network analysis (WGCNA) was performed based on GSE43290, and module preservation was tested by GSE74385. Functional annotations were performed to analyze biological significance. Hub genes were selected for efficacy evaluations and correlation analyses using two independent cohorts. A total of 14 coexpression modules were identified, and module lightcyan was significantly associated with WHO grades. Functional enrichment analyses of module lightcyan were associated with tumor pathogenesis. The top 10 hub genes were extracted. Ten biomarkers, particularly AHCYL2, FGL2, and KCNMA1, were significantly related to grades and prognosis of meningioma. These findings not only construct coexpression modules leading to the better understanding of its pathogenesis but also provide potential biomarkers that represent specific on tumor grades and identify recurrence, predicting prognosis and progression of meningiomas.

Activation of mouse NBCe1-B by Xenopus laevis and mouse IRBITs: Role of the variable Nt appendage of IRBITs

The IP3 receptor binding protein released with inositol 1,4,5-trisphosphate (IRBIT) plays important roles in the regulation of intracellular Ca²⁺ signaling and intracellular pH. The mammals express two IRBIT paralogs, i.e., IRBIT1 (encoded by AHCYL1) and IRBIT2 (encoded by AHCYL2). The clawed frog Xenopus laevis oocyte is widely used for biophysical studies on ion channels and transporters. It remains unknown whether endogenous IRBIT is expressed in Xenopus oocytes. Here, we cloned from frog oocyte irbit2.L and irbit2.S, orthologs of mammalian IRBIT2. When over-expressed, the frog IRBITs powerfully stimulate the electrogenic Na⁺/HCO₃^- cotransporter NBCe1-B as mouse IRBIT2-V2 does. Expression of an isolated Nt fragment of NBCe1-B containing the IRBIT-binding domain greatly decreases NBCe1-B activity in oocytes, suggesting that the basal activity of NBCe1-B contains a large component derived from the stimulation by endogenous frog IRBIT. The frog IRBITs are highly homologous to the mammalian ones in the carboxyl-terminal region, but varies greatly in the amino-terminal (Nt) appendage. Interestingly, truncation study showed that the Nt appendage of IRBIT1 and the long Nt appendage of IRBIT2-V2 modestly enhance, whereas the short Nt appendage of IRBIT2-V4 greatly inhibits the functional interaction between IRBIT and NBCe1-B. Finally, Ala-substitution of Ser68, a key phosphorylation site in the PEST domain of IRBIT, causes distinct functional consequences depending on the structural context of the Nt appendage in different IRBIT isoforms. We conclude that the Nt appendage of IRBITs is not necessary for, but plays an important regulatory role in the functional interaction between IRBIT and NBCe1-B.

Expression of the B splice variant of NBCe1 (SLC4A4) in the mouse kidney

Sodium-coupled bicarbonate transporters are critical for renal electrolyte transport. The electrogenic, sodium-coupled bicarbonate cotransporter, isoform 1 (NBCe1), encoded by the SLC4A4 geneencoded by the SLC4A4 gene has five multiple splice variants; the A splice variant, NBCe1-A, is the primary basolateral bicarbonate transporter in the proximal convoluted tubule. This study's purpose was to determine if there is expression of additional NBCe1 splice variants in the mouse kidney, their cellular distribution, and their regulation by metabolic acidosis. In wild-type mice, an antibody reactive only to NBCe1-A showed basolateral immunolabel only in cortical proximal tubule (PT) segments, whereas an antibody reactive to all NBCe1 splice variants (pan-NBCe1) showed basolateral immunolabel in PT segments in both the cortex and outer medulla. In mice with NBCe1-A deletion, the pan-NBCe1 antibody showed basolateral PT immunolabel in both the renal cortex and outer stripe of the outer medulla, and immunoblot analysis showed expression of a ~121-kDa protein. RT-PCR of mRNA from NBCe1-A knockout mice directed at splice variant-specific regions showed expression of only NBCe1-B mRNA. In wild-type kidney, RT-PCR confirmed expression of mRNA for the NBCe1-B splice variant and absence of mRNA for the C, D, and E splice variants. Finally, exogenous acid loading increased expression in the proximal straight tubule in the outer stripe of the outer medulla. These studies demonstrate that the NBCe1-B splice variant is present in the PT, and its expression increases in response to exogenous acid loading, suggesting it participates in the PT contribution to acid-base homeostasis.

Splicing variation of Long-IRBIT determines the target selectivity of IRBIT family proteins

IRBIT [inositol 1,4,5-trisphosphate receptor (IP₃R) binding protein released with inositol 1,4,5-trisphosphate (IP₃)] is a multifunctional protein that regulates several target molecules such as ion channels, transporters, polyadenylation complex, and kinases. Through its interaction with multiple targets, IRBIT contributes to calcium signaling, electrolyte transport, mRNA processing, cell cycle, and neuronal function. However, the regulatory mechanism of IRBIT binding to particular targets is poorly understood. Long-IRBIT is an IRBIT homolog with high homology to IRBIT, except for a unique N-terminal appendage. Long-IRBIT splice variants have different N-terminal sequences and a common C-terminal region, which is involved in multimerization of IRBIT and Long-IRBIT. In this study, we characterized IRBIT and Long-IRBIT splice variants (IRBIT family). We determined that the IRBIT family exhibits different mRNA expression patterns in various tissues. The IRBIT family formed homo- and heteromultimers. In addition, N-terminal splicing of Long-IRBIT changed the protein stability and selectivity to target molecules. These results suggest that N-terminal diversity of the IRBIT family and various combinations of multimer formation contribute to the functional diversity of the IRBIT family.