Lamprey Metamorphosis - Metamorphosis in Fish - page 60

In gnathostomes all members of the POMC family of peptides are
derived from the preprohormone, POMC, which is encoded for by a single
gene. POMC is post-translationally cleaved in a cell-specifi c manner to
produce the bioactive hormones adrenocorticotropin (ACTH), melanotropin
(MSH; γ-MSH and β-MSH), endorphin (β-END), and lipotropin (LPH)
(Smith and Funder, 1988). Unlike all other vertebrates, members of the
lamprey POMC family are encoded for by two distinct genes: proopiocortin
(POC) and proopiomelanotropin (POM) (Heinig et al . , 1995; Takahashi et
al . , 1995; Youson et al., 2006). POC encodes for ACTH, one MSH, β-END
and nasohypophysial factor (NHF), and POM encodes for MSH-A, MSH-B
and a different β-END (Takahashi et al . , 1995). The POMC gene is expressed
throughout the adenohypophysis as well as in the hypothalamus and
other brain regions in gnathostomes, but POM and POC expression in
lampreys is restricted to the pars intermedia (PI) and the pars distalis (PD),
respectively (Ficele et al . , 1998). Likewise, immunohistochemical data for
ACTH and MSH-like peptides indicates that they are restricted to the PD
and PI, respectively (Nozaki et al . , 1995, 2008). Thus, the spatial distribution
of POM and POC in the lamprey adenohypophysis is consistent with the
localization of POMC-derived peptides in gnathostomes where MSH
and ACTH peptides are restricted to the melanotropes of the PI and the
corticotropes of the PD, respectively (see Sower, 1998).
POM and POC expression is regulated both temporally and spatially
in the adenohypophysis of P. marinus . Northern blotting data indicates that
the mRNA levels of both transcripts are elevated following the completion
of metamorphosis, and in the parasitic juvenile and prespawning adult
(Heinig et al . , 1999; Youson et al . , 2006). Transcript levels in larvae are
lower, but tend to increase later in metamorphosis. In the non-parasitic
L. appendix , a similar temporal expression pattern was observed with the
increase in expression during metamorphosis perhaps beginning slightly
earlier than in P. marinus (Youson et al . , 2006). Youson et al . (2006) suggest
that this difference in timing may be related to the earlier onset of gonadal
maturation in L. appendix .
Detailed spatial and temporal analysis with in situ hybridization
shows uniform POC expression in the rostral pars distalis (RPD) in larvae,
throughout metamorphosis and into the spawning adult (Ficele et al . ,
1998). At stage 5 of metamorphosis, POC-expressing cells are also scattered
throughout the caudal pars distalis (CPD) with expression becoming
restricted to dorsally located cells of the CPD in prespawning adults (Fig.
16). Expression of POC in the RPD, as determined by quantitative analysis of
signal density and volume, is low in larvae, increases gradually throughout
metamorphosis, peaks in post-metamorphic individuals, and remains
elevated into adulthood (Ficele et al . , 1998). The requirement of an intact
PD for metamorphosis was demonstrated for G. australis whereby removal