Asbestos Fibers Enhance the TMEM16A Channel Activity in Xenopus Oocytes

Our plain-English summary of Asbestos Fibers Enhance the TMEM16A Channel Activity in Xenopus Oocytes, by A. Bernareggi, M. Zangari, A. Constanti, et al (Membranes, 2023).

The study

• Authors: A. Bernareggi, M. Zangari, A. Constanti, P. Zacchi, V. Borelli, A. Mangogna, P. Lorenzon, G. Zabucchi
• Journal: Membranes
• Published: 2023;13(2)
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: The interaction of asbestos fibers with target cell membranes is still poorly investigated. Here, we detected and characterized an enhancement of chloride conductance in Xenopus oocyte cell membranes induced by exposure to crocidolite (Croc) asbestos fibers.

Methods: A two-microelectrode voltage clamp technique was used to test the effect of Croc fiber suspensions on outward chloride currents evoked by step membrane depolarization. Calcium imaging experiments were also performed to investigate the variation of 'resting' oocyte [Ca2+]i following asbestos exposure.

Results: The increase in chloride current after asbestos treatment, was sensitive to [Ca2+]e, and to specific blockers of TMEM16A Ca2+-activated chloride channels, MONNA and Ani9. Furthermore, asbestos treatment elevated the 'resting' [Ca2+]i likelihood by increasing the cell membrane permeability to Ca2 in favor of a tonic activation of TMEME16A channels. Western blot analysis confirmed that TMEME16A protein was endogenously present in the oocyte cell membrane and absorbed by Croc.

Conclusion: the TMEM16A channels endogenously expressed by Xenopus oocytes are targets for asbestos fibers and represent a powerful tool for asbestos-membrane interaction studies. Interestingly, TMEM16A channels are highly expressed in many types of tumors, including some asbestos-related cancers, suggesting them, for the first time, as a possible early target of crocidolite-mediated tumorigenic effects on target cell membranes.

In plain English

This was a laboratory study on cells, not a study of people. The authors exposed egg cells (oocytes) from female African clawed frogs (Xenopus laevis) to crocidolite asbestos at 15 µg/mL for 5 to 30 minutes and measured electrical currents across the cell membrane. Crocidolite lowered average membrane resistance from 0.82 to 0.53 megaohms and increased outward currents. These effects were reduced when part of the chloride in the bathing solution was replaced, and when two blockers of the TMEM16A chloride channel, MONNA and Ani9, were applied. Raising calcium outside the cells strengthened the crocidolite effect. Crocidolite-treated oocytes had 21.99% higher resting calcium inside the cell than untreated oocytes, but not in calcium-free solution. In membrane extracts from both oocytes and a human mesothelial cell line, crocidolite bound more TMEM16A protein than chrysotile, and wollastonite gave a very weak signal. The authors concluded that TMEM16A channels are key participants in crocidolite's effects on cell membranes. They also stated that a link between these electrical changes and tumor formation is presently difficult to draw.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment on isolated frog oocytes (electrical recordings, calcium imaging) plus protein-binding (Western blot) tests on membrane extracts
• Who was studied: Stage VI oocytes from adult female Xenopus laevis frogs (University of Trieste, Italy). Electrical recordings used cells from 5 frog donors in normal solution and 3 further donors in high-calcium solution. Membrane extracts were made from 750 oocytes and from the MeT5a human mesothelial cell line. Study dates not given.
• Exposure: UICC standard crocidolite (South African) at 15 µg/mL for 5 to 30 minutes in the electrical experiments. In the binding experiments, 0.18 mg each of crocidolite, chrysotile ('A' Rhodesian) or wollastonite (a non-asbestos control) was incubated with membrane extracts.
• Main results:
  • In oocytes, crocidolite treatment reduced average membrane resistance from 0.82 ± 0.02 MΩ (n = 39, untreated) to 0.53 ± 0.03 MΩ (n = 33, treated), p less than 0.001.
  • Replacing half of the outside chloride with aspartate partially abolished the crocidolite effect on the evoked currents.
  • The TMEM16A blocker MONNA (10 µM) substantially reduced evoked currents only in crocidolite-treated cells. It reversed the crocidolite effect on membrane resistance, from 0.41 ± 0.07 MΩ to 0.71 ± 0.11 MΩ (n = 5, p less than 0.05).
  • The blocker Ani9 (1 µM) partially recovered membrane resistance in crocidolite-treated cells, from 0.52 ± 0.10 MΩ to 0.75 ± 0.12 MΩ (n = 8, p less than 0.05).
  • At +40 mV, crocidolite-treated cells had currents 175 ± 51% more than untreated cells in normal Ringer solution (n = 31). In high-calcium (11 mM) solution the increase was 339 ± 31% (n = 20), p less than 0.001.
  • Resting calcium inside oocytes exposed to crocidolite in normal Ringer solution was 21.99 ± 1.99% higher than in untreated oocytes (untreated n = 17; treated n = 9). It was 31.83 ± 1.77% higher when exposure was in high-calcium solution (n = 14).
  • In calcium-free solution, crocidolite-treated oocytes did not differ significantly from untreated oocytes in internal calcium (untreated 99.51%, n = 13; treated 91.22%, n = 10; p greater than 0.05).
  • In oocyte membrane extracts, crocidolite bound TMEM16A consistently more than chrysotile did, and wollastonite showed a very weak signal. Similar results were obtained with membrane extracts of MeT5a human mesothelial cells.
• The authors' conclusion: The authors concluded that TMEM16A channels are key molecular participants in crocidolite's effects on cell membranes in this model. They suggested that TMEM16A channels may have a role in tumor development in asbestos-related human tumors.
• Limitations the authors state:
  • The authors stated that a relationship between the electrical changes seen in their model and tumor formation is presently difficult to draw.
  • The authors stated they cannot exclude other pathways, such as internal cell structures or direct fiber–protein interaction.
  • The effect of chrysotile on channel activity was not tested; the authors stated it is under investigation.
  • The authors noted variability among oocytes from different frog donors.

Topics

Medical terms in this study, explained in our Lexicon: Crocidolite, Asbestos Exposure.

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