Biology Reference
In-Depth Information
A phosphorylated protein complex has been reported that localizes to ODFs,
SC, and C. This complex may regulate sperm centrosomal function through ODF
dephosphorylation and connecting piece disassembly since it has been reported
that dephosphorylation of sperm midpiece antigens initiate aster formation in
rabbit zygotes (Pinto-Correia et al.
1994
; Long et al.
1997
; Schalles et al.
1998
).
Long et al. (
1997
), Rawe et al. (
2008
) and Hoyer-Fender (
2010
) have noted that
the sperm basal body-centriole must first disengage from the connecting piece to
be able to organize the zygote centrosome by recruiting oocyte-derived PCM.
Proteasomes localized to the sperm neck are probably necessary for normal
centriolar release (Wójcik et al.
2000
; Wójcik and DeMartino
2003
; Rawe et al.
2008
). The organization of centrioles (basal bodies) docking to cell membranes
and giving rise to microtubular axonemes is an evolutionary conserved mechanism
common to ciliated and flagellated cells. Vashishtha et al. (
1996
) have studied in
Chlamydomonas the role of KHP1, a kinesin-homologous protein that localizes to
basal bodies and centrioles and possibly acts as a transporter of protein compo-
nents to their distal site of assembly in axonemes or aster microtubules. Prior to
mitosis, flagella are resorbed and basal bodies duplicate to become centrosomes
that occupy Chlamydomonas spindle poles from where aster microtubules radiate.
These observations point to the dual function of basal bodies/centrioles in flagellar
assembly and mitotic spindle formation. Similar phenomena occur after fertil-
ization in humans: flagella detach from sperm heads, and basal bodies (proximal
centrioles) recruit PCM to become the zygote MTOC, from which the sperm aster
and mitotic spindle will assemble. Sutovsky et al. (
1996
) have reported that after
sperm incorporation into oocytes connecting pieces break down and microtubules
first associate with proximal centrioles to form sperm asters that direct pronuclear
migration and fusion. During this process, capitulum and SC move away and
disintegrate in the cytoplasm. After syngamy, sperm centrioles form the zygote
centrosome that subsequently duplicates and migrates to both poles of the cell to
assemble the mitotic spindle as the embryo enters its first cell cycle.
The need for a functional centriolar complement was demonstrated by Palermo
et al. (
1997
) and Colombero et al. (
1999
) who showed that injection of separated
sperm components (head only, separated head and tail, isolated tail) is followed by
oocyte activation and bipronuclear formation, but ultimately results in abnormal
centrosomal function and embryonic mosaicism. They concluded that the integrity
of the sperm head-neck region is essential for human early embryogenesis.
Experimental evidence presented by Comizzoli et al. (
2006
) points to the
importance of complete centriolar maturation, since aster formation was reduced
after injection of testicular immature spermatozoa when compared to that obtained
with fully mature ejaculated sperm. Recent investigations have shown that the
pericentrosomal area is enriched in proteasomes and may function as a proteolytic
center of the cell. Under conditions of cell stress or proteasome inhibition
increased numbers of proteasomes and ubiquitinated proteins concentrate around
the centrosome forming ''aggresomes'' (Wójcik
1997a
,
b
; Fabunmi et al.
2000
;
Wójcik and DeMartino
2003
; Rawe et al.
2008
). These evidences support an active
proteasome
involvement
in
centrosomal
function
during
early
zygote
