Dominik Wiemuth

White matter degeneration in multiple sclerosis (MS) is characterized by oligodendrocyte dysfunction and subsequent demyelination. To determine whether acid-sensing ion channel subtype 1 (ASIC1) contributes to oligodendrocyte-associated pathology under non-autoimmune conditions, the cuprizone model was used in wild-type (WT) and ASIC1-deficient (KO) mice. Axonal degeneration, microgliosis (IBA1), demyelination, and oligodendrocyte abundance were assessed in the medial Corpus Callosum. Cuprizone increased IBA1 positivity, axonal damage, and demyelination, and these effects appeared more pronounced in ASIC1 KO animals than in WT animals. ASIC1 deficiency was associated with increased APP⁺ axonal spheroid formation and reduced myelination scores. However, oligodendrocyte marker analyses did not indicate an additional reduction in oligodendrocyte survival after 3 weeks. Overall, these data support a role of ASIC1 in preserving myelin integrity, consistent with myelin maintenance rather than direct regulation of oligodendrocyte survival.

Pancreatic α-cells play a key role in glucose homeostasis through glucagon secretion. Here, we discover that the bile acid-sensitive ion channel (BASIC), a Na ⁺ channel of the DEG/ENaC family, has a hitherto unrecognized role in α-cell function. We found that BASIC is mainly expressed in glucagon-positive α-cells of mouse islets. Genetic ablation of BASIC in mice reduced α-cell number, glucagon expression, and glucagon serum levels. These dramatic changes were accompanied by disturbed expression of α-cell specific transcription factors. Our findings identify BASIC as a previously unknown regulator of α-cell physiology.

The modulation of neurotransmitter receptor density is an important molecular mechanism underlying synaptic plasticity. Acid-sensing ion channel 1a (ASIC1a), a receptor for protons, plays an important role in synaptic transmission. In this study, we used high-resolution proteomic analysis to identify prenylcysteine oxidase 1 like (PCYOX1L) as a hitherto unknown interaction partner of ASIC1a. We found that PCYOX1L is a secreted protein that promotes ASIC1a assembly. PCYOX1L was indispensable for ASIC1a expression in the plasma membrane and synaptosomes, and the genetic deletion of PCYOX1L severely impaired hippocampal long-term potentiation. Endocytosed PCYOX1L promoted plasma membrane expression of ASIC1a, and PCYOX1L secreted from astrocytes induced ASIC1a activity in neurons. Our findings reveal that a secreted protein, PCYOX1L, is a checkpoint for the plasma membrane expression of a synaptic ion channel.

The bile acid-sensitive ion channel (BASIC) is a member of the Deg/ENaC family of ion channels that is activated by bile acids. Despite the identification of cholangiocytes in the liver and unipolar brush cells in the cerebellum as sites of expression, the physiological function of BASIC in these cell types is not yet understood. Here we used a cholangiocyte cell line, normal rat cholangiocytes (NRCs), which expresses BASIC to study the role of the channel in epithelial transport using Ussing chamber experiments. Apical application of bile acids induced robust and transient increases in transepithelial currents that were carried by Na⁺ and partly blocked by the BASIC inhibitor diminazene. Genetic ablation of the BASIC gene in NRC using a CRISPR-cas9 approach resulted in a decrease of the bile acid-mediated response that matched the diminazene-sensitive current in NRC WT cells, suggesting that cholangiocytes respond to bile acids with a BASIC-mediated Na⁺ influx. Taken together, we have identified BASIC as a component of the cholangiocyte transport machinery, which might mediate a bile acid-dependent modification of the bile and thus control bile flux and composition.

Acid-sensing ion channels (ASICs) are ionotropic receptors that are directly activated by protons. Although protons have been shown to act as a neurotransmitter and to activate ASICs during synaptic transmission, it remains a possibility that other ligands directly activate ASICs as well. Neuropeptides are attractive candidates for alternative agonists of ASICs, because related ionotropic receptors are directly activated by neuropeptides and because diverse neuropeptides modulate ASICs. Recently, it has been reported that the neuropeptide nocistatin directly activates ASICs, including ASIC1a. Here we show that nocistatin does not directly activate ASIC1a expressed in Xenopus oocytes or CHO cells. Moreover, we show that nocistatin acidifies the bath solution to an extent that can fully explain the previously reported activation by this highly acidic peptide. In summary, we conclude that nocistatin only indirectly activates ASIC1a via acidification of the bath solution.

Extracellular adenosine triphosphate (ATP) regulates a broad variety of physiological functions in a number of tissues partly via ionotropic P2X receptors. Therefore, P2X receptors are promising targets for the development of therapeutically active molecules. Bile acids are cholesterol-derived amphiphilic molecules; their primary function is the facilitation of efficient nutrient fat digestion. However, bile acids have also been shown to serve as signaling molecules and as modulators of different membrane proteins and receptors including ion channels. In addition, some P2X receptors are sensitive to structurally related steroid hormones. In this study, we systematically analyzed whether rat P2X receptors are affected by micromolar concentrations of different bile acids. The taurine-conjugated bile acids TLCA, THDCA, and TCDCA potently inhibited P2X2, whereas other P2X receptors were only mildly affected. Furthermore, stoichiometry and species origin of the P2X receptors affected the modulation by bile acids: in comparison to rat P2X2, the heteromeric P2X2/3 receptor was less potently modulated and the human P2X2 receptor was potentiated by TLCA. In summary, bile acids are a new class of P2X receptor modulators, which might be of physiological relevance.

Despite the identification of cholangiocytes in the liver and unipolar brush cells in the cerebellum as sites of expression, the physiological function of the bile acid-sensitive ion channel (BASIC) remains unknown. Rat BASIC (rBASIC) and mouse BASIC (mBASIC) share 97% of their amino acid sequence but show strikingly different biophysical properties. rBASIC is inactive at rest while mBASIC is constitutively active, when expressed in Xenopus oocytes. This conundrum rendered the identification of the physiological function even more difficult. In this study, we investigated the electrophysiological and pharmacological properties of BASIC from rat, mouse, and human in Hek293 cells using the patch clamp technique. Surprisingly, in Hek293 cells, rBASIC and mBASIC showed almost completely identical properties. Both are blocked by extracellular Ca²⁺ and thus are inactive at rest; both are selective for Na⁺, show similar affinities for extracellular Ca²⁺, were inhibited by diminazene, and activated by various bile acids. This is in contrast to previous results derived from Xenopus oocytes as expression system and suggests that the cell type is important for shaping the biophysical properties of BASIC. Furthermore, we compared hBASIC with rBASIC and mBASIC and observed similar properties between these channels with one exception: the bile acid sensitivity profile of hBASIC is different from rBASIC and mBASIC; hBASIC is more sensitive to bile acids which are abundant in human bile but not in rodent bile. Taken together, these results suggest similar physiological roles for BASIC in different species.

The bile acid-sensitive ion channel (BASIC) is a member of the DEG/ENaC family of ion channels. Channels of this family are characterized by a common structure, their physiological functions and modes of activation, however, are diverse. Rat BASIC is expressed in brain, liver and intestinal tract and activated by bile acids. The physiological function of BASIC and its mechanism of bile acid activation remain a puzzle. Here we addressed the question whether amphiphilic bile acids activate BASIC by directly binding to the channel or indirectly by altering the properties of the surrounding membrane. We show that membrane-active substances other than bile acids also affect the activity of BASIC and that activation by bile acids and other membrane-active substances is non-additive, suggesting that BASIC is sensitive for changes in its membrane environment. Furthermore based on results from chimeras between BASIC and ASIC1a, we show that the extracellular and the transmembrane domains are important for membrane sensitivity.