Science China Life Sciences

, Volume 54, Issue 8, pp 686–690

The plasma membrane calcium pump in the hearing process: physiology and pathology

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DOI: 10.1007/s11427-011-4200-z

Cite this article as:
Carafoli, E. Sci. China Life Sci. (2011) 54: 686. doi:10.1007/s11427-011-4200-z

Abstract

Mammalian cells express four different plasma membrane Ca2+ ATPases. Two of them (PMCA1 and PMCA4) are expressed ubiquitously, and are considered housekeeping isoforms. Two (PMCA2 and PMCA4) have tissue restricted distribution. They are abundantly expressed in the brain and in nervous tissue-derived cell types. The primary transcripts of all PMCAs undergo alternative splicing, generating a large number of additional isoforms. Splicing occurs at site A, in the N-terminal moiety of the pump, and at site C, within the C-terminal calmodulin binding domain: The pumps are canonical targets of calmodulin stimulation. The site C insertion leads to a truncation of the pump about 50 residues short of the original C-terminal. One of the pumps (PMCA2) has special properties: It displays high activity even in the absence of the natural activator calmodulin, and has a particularly complex pattern of alternative splicing at both sites A and C. A variant of the PMCA2 pump containing an insert at site A and truncated C-terminally is the resident isoform of the pump in the stereocilia of hair cells of the inner ear. It exports Ca2+ to the endolymph that bathes the stereocilia less efficiently than the full length, non-inserted PMCA2 pump. The proper functioning of hair cells demands the precise maintenance of the Ca2+ balance between hair cells and the endolymph. Disturbances in the balance affect the process of mechano-electrical transduction, which depends on the ability of the stereociliar bundle to deflect in response to sound waves. The tip links that organize the bundle are formed by the Ca2+ binding protein cadherin 23 and by protocadherin 15: Disturbances of the Ca2+ binding by cadherin 23 and/or of the ability of the PMCA2 variant of the stereocilia to export Ca2+ to the endolymph generate hearing loss phenotypes. Such phenotypes have now been described in mice and humans. In some cases they are linked to mutations of both cadherin 23 and the PMCA2 pump, but in other cases they may be generated by mutations of particular severity in only one of the two proteins. The PMCA2 defect that leads to deafness has now been analyzed molecularly: It affects the long range, unstimulated ability of PMCA2 to export Ca2+.

Keywords

calcium ATPases PMCA2 hereditary deafness calmodulin hair cells 
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© The Author(s) 2011

This article is published under license to BioMed Central Ltd. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.

Authors and Affiliations

  1. 1.Venetian Institute of Molecular MedicineUniversity of PadovaPadovaItaly

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