Function, expression and localization of annexin A7 in platelets and red blood cells: insights derived from an annexin A7 mutant mouse.
BACKGROUND
Annexin A7 is a Ca2+- and phospholipid-binding protein expressed as a 47 and 51 kDa isoform, which is thought to be involved in membrane fusion processes. Recently the 47 kDa isoform has been identified in erythrocytes where it was proposed to be a key component in the process of the Ca2+-dependent vesicle release, a process with which red blood cells might protect themselves against an attack by for example complement components.


RESULTS
The role of annexin A7 in red blood cells was addressed in erythrocytes from anxA7-/- mice. Interestingly, the Ca2+-mediated vesiculation process was not impaired. Also, the membrane organization appeared not to be disturbed as assessed using gradient fractionation studies. Instead, lack of annexin A7 led to an altered cell shape and increased osmotic resistance of red blood cells. Annexin A7 was also identified in platelets. In these cells its loss led to a slightly slower aggregation velocity which seems to be compensated by an increased number of platelets. The results appear to rule out an important role of annexin A7 in membrane fusion processes occurring in red blood cells. Instead the protein might be involved in the organization of the membrane cytoskeleton. Red blood cells may represent an appropriate model to study the role of annexin A7 in cellular processes.


CONCLUSIONS
We have demonstrated the presence of both annexin A7 isoforms in red blood cells and the presence of the small isoform in platelets. In both cell types the loss of annexin A7 impairs cellular functions. The defects observed are however not compatible with a crucial role for annexin A7 in membrane fusion processes in these cell types.
Herr
Claudia
C
Center for Biochemistry, Medical Faculty, University of Cologne, Joseph-Stelzmann-Str, 52, 50931 Cologne, Germany. Claudia.Herr@uni-koeln.de
Clemen
Christoph S
CS
Lehnert
Gisela
G
Kutschkow
Rüdiger
R
Picker
Susanne M
SM
Gathof
Birgit S
BS
Zamparelli
Carlotta
C
Schleicher
Michael
M
Noegel
Angelika A
AA
D000818:Animals; D017310:Annexin A7 / Q000032:analysis / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D001792:Blood Platelets / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D048429:Cell Size; D004910:Erythrocyte Membrane / Q000737:chemistry; D004912:Erythrocytes / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009997:Osmotic Pressure; D010974:Platelet Aggregation; D022142:Secretory Vesicles / Q000378:metabolism
D017310:Annexin A7; C480504:Anxa7 protein, mouse