-210 Nottingham - Nottingham eTheses - The University of Nottingham

Oocyte maturation is generally controlled by three major mediators: gonadotropic
hormone (GTH, e. g. luteinising hormone and follicle stimulating hormone; or
gonad-stimulating substance in starfish, GSS) secreted from the (central) nervous
system, maturation-inducing hormone (MIH; or maturation-inducing substance, MIS)
secreted from somatic (follicle) cells surrounding the oocytes and MPF activated
within the maturing oocytes (Figure 1.2).
MPF kinase activity is maximal at metaphase of the meiotic divisions. During oocyte
maturation, it declines after MI and then increases reaching a maximum level at MII,
and then remains high (Campbell, 2002b). As long as MPF within the oocyte is high,
the chromosomes remain condensed and the nuclear envelope is unable to reform so
that meiosis is unable to proceed further. After fertilisation, MPF activity declines and
meiosis resumes and runs to completion, when the second polar body (PBII) is then
extruded, and the pronuclei are formed (Campbell et al., 1993; Campbell et al., 1994)
(Figure 1.3). MPF activity is stabilised at MII by cytostatic factor (CSF), which is
c-mos protooncogene product, pp39"' (Mos), (Sagata et al., 1989).
1.2.1.3 MAPK in oocytes
MAPK was first discovered as an insulin-stimulated kinase in 3T3-L1 fibroblasts
(Sturgill and Ray, 1986). MAP was initially selected for the specific substrate
microtubule-associated protein-2 and now stands for mitogen-activated protein
kinase (MAPK) to characterise the activation of the enzyme by diverse mitogens
(Rossomando, et al., 1989). MAPK is also called extracellular signal-regulated
kinase (ERK) because of its various extracellular signals (Boulton, et al., 1990). It is a
family of serine/threonine protein kinases. In MAPK cascade, a MAP kinase kinase
kinase (MAPKKK) phosphorylates and activates a MAP kinase kinase (MAPKK),
which phosphorylates a MAP kinase (MAPK). The MAPK signaling cascade plays
an important role in both mitosis and meiosis.
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