Hy. Qi et Pm. Wassarman, Secretion of zona pellucida glycoprotein mZP2 by growing oocytes from mZP3(+/+) and mZP3(-/-) mice, DEV GENET, 25(2), 1999, pp. 95-102
The mouse egg extracellular coat, or zone pellucida (ZP), is composed of th
ree glycoproteins, celled mZP1-3, which are synthesized and secreted concom
itantly by growing oocytes. Disruption of the mZP3 gene by targeted mutagen
esis yields mice that are homozygous nulls (mZP3(-/-)). Growing oocytes fro
m mZP3(-/-) mice do not synthesize mZP3 mRNA or protein and, as a result, d
o not assemble a ZP. Here, we examined secretion of mZP2 by growing oocytes
and eggs from mZP3-/- mice, as well as incorporation of mZP2 into the ZP o
f oocytes from mZP3(+/+) mice. Laser scanning confocal microscopy (LSCM) of
antibody-labeled samples showed that, indeed, mZP2 was synthesized and sec
reted by oocytes isolated from mZP3(-/-) mice and cultured in vitro. Nascen
t mZP2 was Found in the culture medium, associated with the surface of the
plasma membrane of growing oocytes, and in the oocyte cytoplasm. By contras
t, mZP2 was barely detectable at any of these sites when ovulated eggs from
mZP3(-/-) mice were examined. Examination of oocytes from wild-type (mZP3(
+/+)) mice showed that, while a portion of nascent mZP2 was assembled into
the ZP (approximately 40%), here too a significant fraction was secreted in
to the culture medium (approximately 60%). Similar results also were obtain
ed when intact pre-antral follicles were isolated from mZP3(+/+) mice and c
ultured in vitro. Several of these observations are consistent with previou
s results obtained with oocytes from heterozygous null mice (mZP3(+/-)). Fu
rthermore, the results suggest that ZP assembly from nascent glycoproteins
may be a stochastic process that requires the presence of both mZP2 and mZP
3 and occurs completely outside the growing oocyte. Dev. Genet. 25:95-102,
1999. (C) 1999 Wiley-Liss, Inc.