VSI: The anoctamins: Structure and function: "Intracellular" anoctamins

Plasma membrane localized anoctamin 1, 2 and 6 (TMEM16A, B, F) have been examined in great detail with respect to structure and function, but much less is known about the other seven intracellular members of this exciting family of proteins. This is probably due to their limited accessibility in intracellular membranous compartments, such as the endoplasmic reticulum (ER) or endosomes. However, these so-called intracellular anoctamins are also found in the plasma membrane (PM) which adds to the confusion regarding their cellular role. Probably all intracellular anoctamins except of ANO8 operate as intracellular phospholipid (PL) scramblases, allowing for Ca2+-activated, passive transport of phospholipids like phosphatidylserine between both membrane leaflets. Probably all of them also conduct ions, which is probably part of their physiological function. In this brief overview, we summarize key findings on the biological functions of ANO3, 4, 5, 7, 8, 9 and 10 (TMEM16C, D, E, G, H, J, K) that are gradually coming to light. Compartmentalized regulation of intracellular Ca2+ signals, tethering of the ER to specific PM contact sites, and control of intracellular vesicular trafficking appear to be some of the functions of intracellular anoctamins, while loss of function and abnormal expression are the cause for various diseases.

Autosomal Recessive Spinocerebellar Ataxia Type 10: A Report of a New Case in Japan

Autosomal recessive spinocerebellar ataxia of type 10 (SCAR10) is a very rare neurodegenerative disease caused by mutations in the TMEM16K (ANO10) gene. This disorder is characterized by slowly progressive cerebellar ataxia and pyramidal signs inconstantly associated with cognitive decline, polyneuropathy, epilepsy, and vesicorectal dysfunction. To date, more than 40 cases have been reported in Europe. In contrast, only three cases have been identified in Asian countries. We herein report the third Japanese case of SCAR10 harboring a novel homozygous deletion mutation (c.616delG, p.Glu206Lysfs*17). This case presented with adult-onset slowly progressive spastic ataxia with cerebellar atrophy and mild cognitive decline.

ANO10 Function in Health and Disease

Anoctamin 10 (ANO10), also known as TMEM16K, is a transmembrane protein and member of the anoctamin family characterized by functional duality. Anoctamins manifest ion channel and phospholipid scrambling activities and are involved in many physiological processes such as cell division, migration, apoptosis, cell signalling, and developmental processes. Several diseases, including neurological, muscle, blood disorders, and cancer, have been associated with the anoctamin family proteins. ANO10, which is the main focus of the present review, exhibits both scrambling and chloride channel activity; calcium availability is necessary for protein activation in either case. Additional processes implicating ANO10 include endosomal sorting, spindle assembly, and calcium signalling. Dysregulation of calcium signalling in Purkinje cells due to ANO10 defects is proposed as the main mechanism leading to spinocerebellar ataxia autosomal recessive type 10 (SCAR10), a rare, slowly progressive spinocerebellar ataxia. Regulation of the endolysosomal pathway is an additional ANO10 function linked to SCAR10 aetiology. Further functional investigation is essential to unravel the ANO10 mechanism of action and involvement in disease development.

A new survival model based on ion channel genes for prognostic prediction in hepatocellular carcinoma

Accumulating studies revealed the vital role of ion channels in cancers, but the prognosis role of ion channels in hepatocellular carcinoma (HCC) remains limited. Here, we developed and validated an ion channel signature for prognostic prediction of HCC patients. In total, 35 differential expressed ion channel genes (DEChannelGs) were identified in HCC and a novel ion channel risk model was established for HCC prognosis prediction using the TCGA cohort, which was validated using the ICGC cohort. Moreover, this risk model was an independent prognostic factor and was associated with the immune microenvironment in HCC. Finally, the mRNA and protein levels of ANO10 and CLCN2 were prominently up-regulated and were related to the poor prognosis of HCC patients. Taken together, these results indicated a novel ion channel risk model as a prognostic biomarker for HCC patients and provided further insight into its immunoregulatory mechanism in HCC progression.

Targeting of Intracellular TMEM16 Proteins to the Plasma Membrane and Activation by Purinergic Signaling

Anoctamins such as TMEM16A and TMEM16B are Ca²⁺-dependent Cl⁻ channels activated through purinergic receptor signaling. TMEM16A (ANO1), TMEM16B (ANO2) and TMEM16F (ANO6) are predominantly expressed at the plasma membrane and are therefore well accessible for functional studies. While TMEM16A and TMEM16B form halide-selective ion channels, TMEM16F and probably TMEM16E operate as phospholipid scramblases and nonselective ion channels. Other TMEM16 paralogs are expressed mainly in intracellular compartments and are therefore difficult to study at the functional level. Here, we report that TMEM16E (ANO5), -H (ANO8), -J (ANO9) and K (ANO10) are targeted to the plasma membrane when fused to a C-terminal CAAX (cysteine, two aliphatic amino acids plus methionin, serine, alanin, cystein or glutamin) motif. These paralogs produce Ca²⁺-dependent ion channels. Surprisingly, expression of the TMEM16 paralogs in the plasma membrane did not produce additional scramblase activity. In contrast, endogenous scrambling induced by stimulation of purinergic P2X7 receptors was attenuated, in parallel with reduced plasma membrane blebbing. This could suggest that intracellular TMEM16 paralogs operate differently when compared to plasma membrane-localized TMEM16F, and may even stabilize intracellular membranes. Alternatively, CAAX tagging, which leads to expression in non-raft compartments of the plasma membrane, may antagonize phosphatidylserine exposure by endogenous raft-located TMEM16F. CAAX-containing constructs may be useful to further investigate the molecular properties of intracellular TMEM16 proteins.

A Postural Tremor Highly Responsive to Transcranial Cerebello-Cerebral DCS in ARCA3

Background and objectives

Cerebellar ataxias are disabling disorders that impact the quality of life of patients. In many cases, an effective treatment is missing. Despite the increasing knowledge on the pathogenesis of cerebellar disorders including genetic aspects, there is currently a gap in the therapeutical management of cerebellar deficits. Cerebellar ataxia associated with ANO10 mutation (ARCA3) presents a disabling cerebellar syndrome. The aim of this study is to report a patient with a marked postural tremor responding to transcranial cerebello-cerebral direct current stimulation (tCCDCS).

Methods

We applied tCCDCS using anodal stimulation over the cerebellum with a return electrode on the contralateral motor cortex. We performed a clinical rating, accelerometry studies, and recordings of voluntary movements at baseline, after sham, and after active tCCDCS.

Results

A dramatic response of postural tremor was observed after tCCDCS, with a major drop of the power spectral density to 26.12% of basal values.

Discussion

The postural tremor of cerebellar ataxia associated with ANO10 mutation was highly responsive to tCCDCS in our patient. This case illustrates that tCCDCS is a novel therapeutic option in the treatment of cerebellar deficits and might represent a promising tool to reduce tremor in ARCA3.

Cellular defects by deletion of ANO10 are due to deregulated local calcium signaling

TMEM16K (ANO10) belongs to a family of ion channels and phospholipid scramblases. Mutations in ANO10 cause neurological and immunological defects, and abrogated ion transport. Here we show that Ano10 knockout in epithelial cells leads to defective ion transport, attenuated volume regulation and deranged Ca²⁺ signaling. Intestinal epithelial cells from Ano10 null mice are reduced in size and demonstrate an almost abolished spontaneous and TNFα-induced apoptosis. Similar defects were found in mouse peritoneal Ano10 null macrophages and in human THP1 macrophages with reduced ANO10 expression. A cell cycle dependent colocalization of Ano10 with acetylated tubulin, centrioles, and a submembranous tubulin containing compartment was observed in Fisher rat thyroid cells. Axs, the Drosophila ortholog of ANO10 is known for its role in mitotic spindle formation and association with the endoplasmic reticulum and Ca²⁺ signaling. We therefore propose that mutations in ANO10 cause cellular defects and genetic disorders through deranged local Ca²⁺ signaling.

Expression of anoctamins in retinal pigment epithelium (RPE)

The anoctamin (ANO, TMEM16) family of Ca²⁺-activated Cl^- channels consists of ten members with different cellular functions (ANO1-10). ANO1 is a Ca²⁺-activated Cl^- channel in secretory epithelial cells of exocrine pancreas, salivary glands, or enterocytes. Expression of ANO1 also promotes cell proliferation and migration of tumor cells. ANO6 is essential for Ca²⁺-dependent scrambling of membrane phospholipids in platelets, red blood cells, and lymphocytes. ANO10 modulates Ca²⁺ signals in macrophages and has a role in cerebellar ataxia and other neurological disorders. All three anoctamins have been proposed to control intracellular Ca²⁺ signals. Anoctamins may also form the basolateral Ca²⁺-activated Cl^- channel in the retinal pigment epithelium (RPE). We show that native human, bovine, porcine, and mouse RPEs express ANO1, ANO6, and ANO10. Growth arrested and confluent RPR cells expressed ANO1 in the plasma membrane, whereas ANO6 and ANO10 were found in the primary cilium. Ussing chamber experiments showed that the application of ATP to the apical (retinal) side of porcine RPE induced a Ca²⁺-activated Cl^- secretion. Activation was inhibited by basolateral (choroidal) administration of the ANO inhibitors AO1, niflumic acid (NFA), and 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS). The results suggest that ANO1 is responsible for basolateral Ca²⁺-dependent Cl^- secretion in RPE, whereas ANO6 and ANO10 may have different functions, such as modulating Ca²⁺ signals.