Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267-0576
CHLORIDE CHANNELS, widely distributed in nature, play
roles as diverse as maintaining membrane potential in muscle and
movement of Cl A great deal of work has gone toward molecular identification of the
protein responsible for the
Ca2+-activated
Cl 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 Unlike some ClC Cl A new family of Cl
ARTICLE
TOP
ARTICLE
REFERENCES
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)
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.
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.
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.
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].
2.
Clarke, L. L.,
B. R. Grubb,
J. R. Yankaskas,
C. U. Cotton,
A. McKensie,
and
R. C. Boucher.
Relationship of a non-cystic fibrosis transmembrane conductance regulator-mediated chloride conductance to organ level disease in CFTR (/
) mice.
Proc. Natl. Acad. Sci. USA
91:
479-483,
1994[Abstract].
3.
Cunningham, S. A.,
M. S. Awayda,
J. K. Bubien,
I. I. Ismailov,
M. P. Arrate,
B. K. Berdiev,
D. J. Benos,
and
C. M. Fuller.
Cloning of an epithelial channel from bovine trachea.
J. Biol. Chem.
270:
31016-31026,
1995
4.
Elble, R.,
J. Widom,
A. D. Gruber,
M. Abdel-Ghany,
R. Levine,
A. Goodwin,
H. C. Cheng,
and
B. U. Pauli.
Cloning and characterization of lung-endothelial cell adhesion molecule-1 suggest it is an endothelial chloride channel.
J. Biol. Chem.
272:
27853-27861,
1997
5.
Gandhi, R.,
R. C. Elble,
A. D. Gruber,
K. D. Schreur,
H.-L. Ji,
C. M. Fuller,
and
B. U. Pauli.
Molecular and functional characterization of a calcium-sensitive chloride channel from mouse lung.
J. Biol. Chem.
273:
32096-32101,
1998
6.
Gruber, A. D.,
R. C. Elble,
H.-L. Ji,
K. D. Schreur,
C. M. Fuller,
and
B. U. Pauli.
Genomic cloning, molecular characterization, and functional analysis of human CLCA1, the first human member of the family of Ca2+-activated Cl channel proteins.
Genomics
54:
200-214,
1998[Medline].
7.
Gruber, A. D.,
R. Gandhi,
and
B. U. Pauli.
The murine calcium-sensitive chloride channel (mCaCC) is widely expressed in secretory epithelia and in other select tissues.
Histochem. Cell Biol.
110:
43-49,
1998[Medline].
8.
Gruber, A. D.,
K. D. Schreur,
H.-L. Ji,
C. M. Fuller,
and
B. U. Pauli.
Molecular cloning and transmembrane structure of hCLCA2 from human lung, trachea, and mammary gland.
Am. J. Physiol.
276 (Cell Physiol. 45):
C1261-C1270,
1999
9.
Ismailov, I. I.,
C. M. Fuller,
B. K. Berdiev,
V. G. Shlyonsky,
D. J. Benos,
and
K. E. Barrett.
A biologic function for an orphan messenger: D-myo-inositol 3,4,5,6-tetrakisphosphate selectively blocks epithelial calcium-activated chloride channels.
Proc. Natl. Acad. Sci. USA
93:
10505-10509,
1996
10.
Ji, H.-L.,
M. D. DuVall,
H. K. Patton,
C. L. Satterfield,
C. M. Fuller,
and
D. J. Benos.
Functional expression of a truncated Ca2+-activated Cl channel and activation by phorbol ester.
Am. J. Physiol.
274 (Cell Physiol. 43):
C455-C464,
1998
11.
Jovov, B.,
G. Vadim,
B. K. Shlyonsky,
and
D. J. Benos.
Purification and reconstitution of an outwardly rectified Cl channel from tracheal epithelia.
Am. J. Physiol.
275 (Cell Physiol. 44):
C449-C458,
1998
12.
Ran, S.,
and
D. J. Benos.
Isolation and functional reconstitution of a 38 kDa chloride channel protein from bovine tracheal membranes.
J. Biol. Chem.
266:
4782-4788,
1991
13.
Ran, S.,
and
D. J. Benos.
Immunopurification and structural analysis of a putative epithelial chloride channel protein isolated from bovine trachea.
J. Biol. Chem.
267:
3618-3625,
1992
14.
Riordan, J. R.,
J. M. Rommens,
B. Kerem,
N. Alon,
R. Rozmahel,
Z. Grzelczak,
J. Zielenski,
S. Lok,
N. Plavsic,
J.-L. Chou,
M. L. Drumm,
M. C. Iannuzzi,
F. S. Collins,
and
L.-C. Tsui.
Identification of the cystic fibrosis gene: cloning and characterization of complementary DNA.
Science
245:
1066-1073,
1989[Medline].
15.
Sherry, A. M.,
K. Stroffekova,
L. M. Knapp,
E. Y. Kupert,
J. Cuppoletti,
and
D. H. Malinowska.
Characterization of the human pH- and PKA-activated ClC-2G (2) Cl
channel.
Am. J. Physiol.
273 (Cell Physiol. 42):
C384-C393,
1997
16.
Steinmeyer, K.,
R. Klocke,
C. Ortland,
M. Gronemeier,
H. Jockusch,
S. Grunder,
and
T. J. Jentsch.
Inactivation of muscle chloride channel by transposon insertion in myotonic mice.
Nature
354:
304-308,
1991[Medline].
17.
Steinmeyer, K.,
C. Ortland,
and
T. J. Jentsch.
Primary structure and functional expression of a developmentally regulated skeletal muscle chloride channel.
Nature
354:
301-304,
1991[Medline].
18.
Willumsen, N. J.,
and
R. C. Boucher.
Activation of an apical Cl conductance by Ca2+ ionophores in cystic fibrosis airway epithelia.
Am. J. Physiol.
256 (Cell Physiol. 25):
C266-C235,
1989.
19.
Zhu, D. Z.,
C. F. Cheng,
and
B. U. Pauli.
Mediation of lung metastasis of murine melanomas by a lung-specific endothelial cell adhesion molecule.
Proc. Natl. Acad. Sci. USA
88:
9568-9572,
1991[Abstract].