Supramolecular organization of the sperm plasma membrane during maturation and capacitation.
OBJECTIVE
In the present study, a variety of high resolution microscopy techniques were used to visualize the organization and motion of lipids and proteins in the sperm's plasma membrane. We have addressed questions such as the presence of diffusion barriers, confinement of molecules to specific surface domains, polarized diffusion and the role of cholesterol in regulating lipid rafts and signal transduction during capacitation.


METHODS
Atomic force microscopy identified a novel region (EqSS) within the equatorial segment of bovine, porcine and ovine spermatozoa that was enriched in constitutively phosphorylated proteins. The EqSS was assembled during epididymal maturation. Fluorescence imaging techniques were then used to follow molecular diffusion on the sperm head.


RESULTS
Single lipid molecules were freely exchangeable throughout the plasma membrane and showed no evidence for confinement within domains. Large lipid aggregates, however, did not cross over the boundary between the post-acrosome and equatorial segment suggesting the presence of a molecular filter between these two domains.


CONCLUSIONS
A small reduction in membrane cholesterol enlarges or increases lipid rafts concomitant with phosphorylation of intracellular proteins. Excessive removal of cholesterol, however, disorganizes rafts with a cessation of phosphorylation. These techniques are forcing a revision of long-held views on how lipids and proteins in sperm membranes are assembled into larger complexes that mediate recognition and fusion with the egg.
Jones
Roy
R
The Babraham Institute, Cambridge, CB2 4AT, UK. roy.jones@bbsrc.ac.uk
James
Peter S
PS
Howes
Liz
L
Bruckbauer
Andreas
A
Klenerman
David
D
D002462:Cell Membrane / Q000502:physiology / Q000648:ultrastructure*; D006801:Humans; D008297:Male; D008563:Membrane Lipids / Q000502:physiology; D018625:Microscopy, Atomic Force; D013075:Sperm Capacitation / Q000502:physiology*; D013079:Sperm Maturation / Q000502:physiology*; D013094:Spermatozoa / Q000166:cytology* / Q000502:physiology
D008563:Membrane Lipids