Biology Reference
In-Depth Information
feeding, thus, other factors are likely important for the completion of normal
metamorphosis (Fig. 1). Although PTU toxicity has not been eliminated
as a factor contributing to the absence of metamorphosis following PTU
treatment and KClO
4
treatment at more moderately cool temperatures
remain to be tested, these studies indicate that other factors such as water
temperature, lipid reserves (size), and other components of the thyroid
system (i.e., peripheral regulators) are important for the initiation and
progression of lamprey metamorphosis, at least in
P. marinus
.
The peripheral regulators of the lamprey thyroid system which have
received some attention include the serum TH distributor proteins (THDP),
TH deiodinases, and TH nuclear receptors (TRs) and their heterodimeric
partners the retinoid-X receptors (RXRs). Other components, including the
TH transmembrane cellular uptake transporters, the cytosolic TH binding
(transport) proteins (CTBPs), and transmembrane receptors, have yet to
be investigated in lampreys but should be the focus of future studies.
The observations that larval lamprey serum has a tremendous capacity to
store and transport TH, and treatment with exogenous TH can result in
elevations in serum T
4
and T
3
concentrations that are 5-20 fold and 3-10
fold greater than control levels, respectively, (R.G. Manzon and Youson,
1997; R.G. Manzon et al
.
, 1998) raises several questions. Is the decline in
TH at the onset of metamorphosis due to a combination of a decrease in
TH synthetic capacity of the transforming endostyle (discussed above),
reduced TH-binding and transport capacity of the serum, increased TH
uptake by cells, increased cytosolic TH deiodination, and/or increased
TH nuclear binding?
Serum THDP are important since they allow the serum to function as a
reservoir for TH and ensure a uniform distribution to all cells and tissues.
In the absence of serum THDP, TH will readily partition into the fi rst cells
they encounter (Mendel
et al
.
, 1987). An investigation of serum THDP in
P. marinus
has identifi ed at least four distinct proteins capable of binding
T
4
and T
3
(Gross and R.G. Manzon, unpublished). Included among these
THDP are the albumin-like proteins AS and SDS-1, the glycolipoprotein
band CB-III and an unidentifi ed protein band designated Spot-5 (Gross and
R.G. Manzon, unpublished). TH binding studies indicate that these THDP
are developmentally regulated: AS is the dominant THDP in larvae and
throughout metamorphosis; AS is replaced by SDS-1 and CB-III beginning
at stage 7 of metamorphosis; SDS-1 and CB-III are the primary TH-binding
proteins in juveniles and upstream-migrant adults (Gross and R.G. Manzon,
unpublished). This shift from AS to SDS-1 and CB-III using TH-binding as a
metric is consistent with other reports on these proteins using immunometric
methodologies (Filosa
et al
.
, 1982, 1986). Despite the change in the type and
number of THDP in lamprey sera during the life cycle, there did not appear
to be any dramatic change in the overall total of TH binding capacity of
