Cyclic-AMP initiates protein tyrosine phosphorylation independent of cholesterol efflux during ram sperm capacitation.
Unlike most other species, ram spermatozoa are difficult to capacitate in vitro. Bicarbonate and Ca(2+) are necessary, whereas bovine serum albumin does not appear to be obligatory. In the present investigation we have assessed (1) the ability of the cholesterol-sequestering agent, methyl-beta-cyclodextrin (M-beta-CD), to initiate protein tyrosine phosphorylation, and (2) the importance of phosphodiesterases (PDEs) in controlling the levels of cAMP. Results show that despite removing significant amounts of membrane cholesterol, as assessed by filipin staining, M-beta-CD treatment did not stimulate major increases in protein tyrosine phosphorylation. Addition of a cocktail of PDE inhibitors (theophylline and caffeine), a phosphatase inhibitor (okadaic acid) and dibutyryl-cAMP (db-cAMP), however, stimulated specific tyrosine phosphorylation of several proteins between 30 and 120 kDa. On their own, none of the above reagents were effective but a combination of db-cAMP + PDE inhibitors was sufficient to achieve a maximal response. H-89, a protein kinase-A inhibitor, suppressed tyrosine phosphorylation significantly. Immunofluorescence revealed that the newly-phosphorylated proteins localised mainly in the sperm tail. These findings suggest that in ram spermatozoa cAMP levels are too low to initiate tyrosine phosphorylation of flagellar proteins that are indicative of the capacitation state and that this is caused by unusually high levels of intracellular PDEs.
Colas
Carmen
C
Department of Biochemistry and Molecular and Cellular Biology, School of Veterinary Medicine, University of Zaragoza, Miguel Servet, Saragossa, Spain.
James
Peter
P
Howes
Liz
L
Jones
Roy
R
Cebrian-Perez
José A
JA
Muiño-Blanco
Teresa
T
D000818:Animals; D002784:Cholesterol / Q000378:metabolism*; D000242:Cyclic AMP / Q000494:pharmacology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D005372:Filipin / Q000737:chemistry / Q000378:metabolism; D008297:Male; D008856:Microscopy, Fluorescence; D010766:Phosphorylation / Q000187:drug effects; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D012756:Sheep / Q000378:metabolism* / Q000502:physiology; D013075:Sperm Capacitation / Q000187:drug effects*; D013094:Spermatozoa / Q000187:drug effects / Q000378:metabolism; D047392:beta-Cyclodextrins / Q000494:pharmacology
D047428:Protein Kinase Inhibitors; D047392:beta-Cyclodextrins; C108732:methyl-beta-cyclodextrin; D005372:Filipin; D002784:Cholesterol; D000242:Cyclic AMP; D011505:Protein-Tyrosine Kinases; D017868:Cyclic AMP-Dependent Protein Kinases