Biology Reference
In-Depth Information
P. marinus
sera throughout development, thus, it is very unlikely that the
decline in serum TH concentrations during metamorphosis is a result of a
decrease in serum transport or binding capacity (Gross and R.G. Manzon,
unpublished). The cDNA for a fi fth THDP, transthyretin (TTR) was isolated
from
L. appendix
and
P. marinus
and its mRNA transcript was shown to be
developmentally regulated in
P. marinus
liver and widely expressed in other
tissues (R.G. Manzon
et al
.
, 2007). TTR is a dominant serum THDP in other
fi sh, anurans, birds and mammals; however, it was not identifi ed as a THDP
in lamprey serum (Gross and R.G. Manzon, unpublished). Collectively,
these studies are among the fi rst data on THDP in lampreys.
Peripheral deiodination of TH appears to play a particularly important
role in the regulation of TH homeostasis in fi shes, including “primitive”
fi shes (Youson, 2007), and in many instances may be a more signifi cant
regulator of thyroid status than the hypothalamic-pituitary axis (Eales and
Brown, 1993). Three classes of deiodinases designated type 1, 2 and 3 (D1,
D2, and D3, respectively) have been identifi ed in most vertebrates studied
to date. D2 is an outer ring deiodinase (ORD) responsible for the conversion
of T
4
to the more biologically active T
3
and, thus, is an activating enzyme.
Conversely, D3 is an inner ring deiodinase (IRD) that inactivates both T
4
and T
3
via their conversion to reverse T
3
(rT
3
) and T
2
(diiodothyronine),
respectively. Mammalian D1 is capable of both IRD and ORD reactions
(see Bianco
et al
.
, 2002).
TH deiodinases may contribute to the precipitous decline in serum TH
levels during lamprey metamorphosis (Eales
et al
.
, 2000). Contrary to other
vertebrates, the intestine and not the liver is the primary site of T
4
ORD in
P. marinus
larvae, but lower activities can be detected in the liver, kidney and
muscle. This fi nding is consistent with the hypothesis that the primary route
of entry of TH into the circulation of lampreys is by intestinal absorption
(see Eales, 1997). The larval lamprey endostyle produces not only TH, but
also large quantities of mucopolysaccarides that aid in the trapping of
nutrients during fi lter feeding; both of these compounds are secreted into
the pharynx and transported through the digestive tract where absorption
takes place (see Barrington, 1972; Eales, 1997; Youson, 2007). Only T
4
ORD
activity was detected in larval intestines, while parasitic-phase juveniles
and upstream-migrant adults have T
4
ORD, T
4
IRD and T
3
IRD activities
(Eales
et al
.
, 1997). T
4
ORD and T
4
IRD (i.e., TH activation and inactivation
reactions, respectively) show a reciprocal relationship throughout lamprey
development. Intestinal T
4
ORD activity is low in
P. marinus
larvae, increases
in pre-metamorphic larvae, peaks at stage 1 of metamorphosis (Eales et al
.
,
2000) and then declines to very low levels that are sustained through to
the upstream-migrant period. In contrast, T
4
IRD activity levels are at or
below the limits of detection in larvae and during the fi rst two stages of
metamorphosis, but increase signifi cantly at stage 3 and reach peak levels at
