The LIM and SH3 domain-containing protein, lasp-1, may link the cAMP signaling pathway with dynamic membrane restructuring activities in ion transporting epithelia.
Lasp-1 is a unique LIM and src homology 3 (SH3) domain-containing protein that was initially identified as a 40 kDa cAMP-dependent phosphoprotein in the HCl-secreting gastric parietal cell. Because cAMP is a potent stimulator of parietal cell acid secretion, we have hypothesized that changes in lasp-1 phosphorylation might be involved in the regulation of ion transport-related activities, perhaps by modulating interactions among cytoskeletal and/or vesicle-associated proteins. In this study, we demonstrate that the cAMP-dependent acid secretory agonist, histamine, induces a rapid, sustained rise in parietal cell lasp-1 phosphorylation and this increase in phosphorylation is closely correlated with the acid secretory response. In addition, elevation of intracellular cAMP concentrations appear to induce a partial redistribution of lasp-1 from the cell cortex, where it predominates along with the gamma-isoform of actin in unstimulated cells, to the beta-actin enriched, apically-directed intracellular canalicular region, which is the site of active proton transport in the parietal cell. Additional studies demonstrate that although lasp-1 mRNA and protein are expressed in a wide range of tissues, the expression is specific for certain actin-rich cell types present within these tissues. For example, gastric chief cells, which contain relatively little F-actin and secrete the enzyme, pepsinogen, by regulated exocytosis, do not appear to express lasp-1. Similarly, lasp-1 was not detected in pancreatic acinar cells, which secrete enzymes by similar mechanisms and also contain relatively low levels of F-actin. Lasp-1 also was not detectable in proximal tubules in the kidney, in gastrointestinal smooth muscle, heart or skeletal muscle. In contrast, expression was prominent in the cortical regions of ion-transporting duct cells in the pancreas and in the salivary parotid gland as well as in certain F-actin-rich cells in the distal tubule/collecting duct. Interestingly, moderate levels of expression were also detected in podocytes present in renal glomeruli and in vascular endothelium. In primary cultures of gastric fibroblasts, lasp-1 was present mainly within the tips of lamellipodia and at the leading edges of membrane ruffles. Taken together these results support the hypothesis that the lasp-1 plays an important role in the regulation of dynamic actin-based, cytoskeletal activities. Agonist-dependent changes in lasp-1 phosphorylation may also serve to regulate actin-associated ion transport activities, not only in the parietal cell but also in certain other F-actin-rich secretory epithelial cell types.
Chew
C S
CS
Institute of Molecular Medicine and Genetics, Medical College of Georgia, Augusta, GA USA. cchew@immagene.mcg.edu.
Parente
J A
JA
Chen
X
X
Chaponnier
C
C
Cameron
R S
RS
D000199:Actins / Q000032:analysis; D000818:Animals; D001692:Biological Transport / Q000502:physiology; D015153:Blotting, Western; D019872:Chief Cells, Gastric / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism*; D003599:Cytoskeleton / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D018398:Homeodomain Proteins / Q000378:metabolism*; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D009687:Nuclear Proteins; D010295:Parietal Cells, Gastric / Q000378:metabolism; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D011506:Proteins; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism; D011817:Rabbits; D015290:Second Messenger Systems / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D018909:src Homology Domains / Q000502:physiology*
D000199:Actins; D018398:Homeodomain Proteins; D008565:Membrane Proteins; D009687:Nuclear Proteins; D011506:Proteins; D016271:Proto-Oncogene Proteins c-myc; C074370:cysteine-rich protein, mammalian; D000242:Cyclic AMP