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 t hat was initially identified as a 40 kDa cAMP-dependent phosphoprotein in t he 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-rel ated 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 ri se in parietal cell lasp-1 phosphorylation and this increase in phosphoryla tion is closely correlated with the acid secretory response. In addition, e levation of intracellular cAMP concentrations appear to induce a partial re distribution of lasp-1 from the cell cortex, where it predominates along wi th the gamma-isoform of actin in unstimulated cells, to the beta-actin enri ched, apically-directed intracellular canalicular region, which is the site of active proton transport in the parietal cell, Additional studies demons trate that although lasp-l mRNA and protein are expressed in a wide range o f tissues, the expression is specific for certain actin-rich cell types pre sent 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 dete cted in pancreatic acinar cells, which secrete enzymes by similar mechanism s and also contain relatively low levels of F-actin. Lasp-1 also was not de tectable in proximal tubules in the kidney, in gastrointestinal smooth musc le, 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 dist al tubule/collecting duct. interestingly, moderate levels of expression wer e also detected in podocytes present in renal glomeruli and in vascular end othelium. In primary cultures of gastric fibroblasts, lasp-1 was present ma inly within the tips of lamellipodia and at the leading edges of membrane r uffles. Taken together these results support the hypothesis that the lasp-1 plays an important role in the regulation of dynamic actin-based, cytoskel etal activities. Agonist-dependent changes in lasp-1 phosphorylation may al so serve to regulate actin-associated ion transport activities, not only in the parietal cell but also in certain other F-actin-rich secretory epithel ial cell types.