Lamprey Metamorphosis - Metamorphosis in Fish - page 45

Changes in the lipogenic and lipolytic enzymes fl uctuated appropriately
with the increases and decreases in plasma fatty acid levels. For instance,
elevations in fatty acids following SST-14 injection coincided with increases
in TG lipase activity and decreases in DAGT activity in both larval kidney
and larval liver (Kao et al . , 1998). At stage 6 of metamorphosis a small, non-
signifi cant increase in TG lipase activity was detected but DAGT activity
in muscle was signifi cantly suppressed (Kao et al . , 1998). Intraperitoneal
injections of insulin suppressed TG lipase activity in most larval tissues as
well as stage 6 liver, and elevated ACC and DGAT activity levels in some
tissues of larvae and stage 6 animals.
In addition to directly modulating lipid metabolism during the lamprey
life cycle, insulin and somatostatin also appear to modulate lipid metabolism
in concert with TH. The lipogenic and lipolytic patterns associated with
KClO 4 -induced metamorphosis closely approximate those observed in
spontaneous metamorphosis (Kao et al . , 1999b). Moreover, these metabolic
effects can be reversed following treatment with exogenous TH which
upregulate a suite of lipogenic processes. One possible theory is that the
high TH levels late in the larval phase are essential to drive the lipogenic
processes and fat accumulation necessary for metamorphosis and that these
processes play a key role in preparing the animal for metamorphosis long
before any external signs of metamorphosis are realized.
The lipogenic phase, which occurs prior to and in the fi rst stages of
metamorphosis, is required to ensure suffi cient energy reserves for the
long and energetically demanding non-trophic period of metamorphosis.
However, it is possible that this accumulation of lipids serves a secondary
purpose in lamprey metamorphosis, either related to nutrition and
metabolism or some other aspect of metamorphosis. In mammals it is well
established that leptin is involved in the regulation of food consumption
and body weight but can also infl uence reproduction, growth, the stress
response, and thyroid function (Harvey and Ashford, 2003; Ahima and Osei,
2004, 2008). Leptin and its receptor are also expressed during embryonic
development and have been shown to correlate with birth weight and
fat accumulation prior to birth (Cetin, 2000; Lepercq et al., 2001). These
findings prompted the search for a lamprey leptin and its potential
function in lamprey metamorphosis. Using a polyclonal antibody against
the C-terminal end of human leptin, four immunoreactive proteins were
identifi ed in lamprey tissue including: a 65 kDa protein in sera, 100 and 50
kDa proteins in muscle and fat column, and 50 and 16 kDa proteins in the
nephric fold (Yaghoubian et al . , 2001). Of particular interest is the presence
of an immunoreactive protein, similar in size to the mammalian leptin
(16 kDa), in the nephric fold early in metamorphosis (Yaghoubian et al . ,
2001). The nephric fold is the primary site of fat storage in larval and early
metamorphic P. marinus (Youson et al . , 1979) and, therefore, it is feasible