EDITORIAL FOCUS
Ca2+-activated Clminus channels Focus on "Molecular cloning and transmembrane structure of hCLCA2 from human lung, trachea, and mammary gland"

John Cuppoletti and Danuta H. Malinowska

Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267-0576


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CHLORIDE CHANNELS, widely distributed in nature, play roles as diverse as maintaining membrane potential in muscle and movement of Cl- for fluid and electrolyte transport in epithelial tissues. The importance of Cl- channels to human health and disease is clear in myotonia and cystic fibrosis, where ClC Cl- channels (16, 17) and the cystic fibrosis transmembrane conductance regulator (CFTR) (1, 2, 14), respectively, are defective. In the current article in focus, Gruber et al. (Ref. 8, see page C1261 in this issue) present their latest findings on the human Ca2+-activated CLCA2 Cl- channel, a new member of another growing family of cloned Cl- channels, which includes lung endothelial cell adhesion molecule (Lu-ECAM) (4, 19), bovine CLCA1 (bCLCA1) (3), murine CLCA1 (mCLCA1) (5, 7) and human CLCA1 (hCLCA1) (6)

A great deal of work has gone toward molecular identification of the protein responsible for the Ca2+-activated Cl- current. This current is retained in epithelia in the absence of the CFTR (2, 17), and levels of Ca2+-activated Cl- currents correlate with the severity of disease at the organ level (2). Members of this family of channels have been shown to be activated both by Ca2+ (3, 5, 6, 8) and phorbol ester (10). This channel is also a target for D-myo-inositol 3,4,5,6-tetrakisphosphate, an agent that affects Ca2+ sensitivity of this channel (9). Whether these channels and/or others (11, 15) provide a physiologically significant alternative to CFTR in the lungs of cystic fibrosis patients remains to be elucidated.

Gruber et al. (8) have carried out a detailed biochemical analysis using scanning glycosylation and protease protection assays to determine the membrane topology of the protein. They have further demonstrated that the protein, like other members of the family, is cleaved from a 130-kDa precursor into two fragments. The amino-terminal fragment is an 86-kDa polypeptide with three transmembrane domains. The 34-kDa carboxy-terminal fragment has two transmembrane domains. By analogy with studies with bCLCA1 (10), channel activity may reside in the larger fragment.

The cloning of the CLCA2 Cl- channel and the structural studies represent important achievements based on a synthesis of the work of two groups of researchers working independently on different types of problems for nearly a decade. The Alabama transport group initially used biochemical approaches to purify the channel from bovine trachea (12, 13). This protein was then used to prepare an antibody for screening a library to obtain a cDNA clone for the channel (3). Other channel forms were obtained by homology cloning. The Cornell cancer group identified (19) and cloned (4) the cell adhesion protein Lu-ECAM-1 and found 88% amino acid similarity to bCLCA1, suggesting that it was also a member of the CLCA family (4). The relationship between cell adhesion and Cl- channel function has not been elucidated.

Unlike some ClC Cl- channels and CFTR, CLCA Cl- channels, with the exception of mouse CLCA1 (7), appear to be expressed with a high degree of tissue specificity (6, 8). Human CLCA1 is expressed exclusively in the intestine (6), whereas human CLCA2 is found only in the lung, trachea, and mammary tissue (8). In the lung and trachea, CLCA2 could therefore play a role in fluid and electrolyte transport in cystic fibrosis patients. It will be important to determine whether CLCA2 is in the same cells as CFTR in the lung, as appears to be the case for CLCA1 in the intestine (6). If the distribution of CLCA2 is found to be the same, activation of CLCA2 could compensate for defective CFTR.

A new family of Cl- channels has been identified. The present study represents a synthesis of work by two different groups working in different disciplines. The work raises several new questions. The relationship between function, structure, and pharmacology of the CLCA family of proteins in cell adhesion and fluid and electrolyte transport remains to be elucidated. In addition, CFTR, the CLCA and ClC families of channels, and other types of Cl- channels often appear together. The significance of multiple types of channel for the transport of Cl- must be clarified. Careful mapping (functional architecture) of the various Cl- channels within tissues and cells throughout development and in normal and diseased states will be required to fully understand the contributions of these proteins.


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1.   Clarke, L. L., B. R. Grubb, S. E. Gabriel, O. Smithies, B. H. Koller, and R. C. Boucher. Defective epithelial transport in a gene-targeted mouse model of cystic fibrosis. Science 257: 1125-1128, 1992[Medline].

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Am J Physiol Cell Physiol 276(6):C1259-C1260
0002-9513/99 $5.00 Copyright © 1999 the American Physiological Society