Biology Reference
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to levels more typical of other vertebrates (0.78 and 0.12 pg T
3
/µg DNA,
respectively) (Lintlop and Youson, 1983a). These data must be interpreted
with some caution as the increase in binding capacity at metamorphosis was
not statistically signifi cant, sample size was limited, and the data are derived
from isolated nuclei and, thus, do not account for cellular uptake and
cytosolic or nuclear transport mechanisms. However, they do suggest that
T
3
may exert some infl uence on liver morphogenesis. It will be interesting
to determine if similar increases in nuclear binding capacity are observed
in other tissues during metamorphosis and whether cellular uptake and
transport mechanism are likewise elevated during metamorphosis.
TH action on vertebrate development is predominately mediated by
interaction with nuclear receptors that are TH-regulated transcription factors
and are, therefore, directly responsible for modulation of the associated gene
expression cascades. To elaborate on the hepatocyte nuclear binding data
of Lintlop and Youson (1983b), information on lamprey TRs and RXRs was
sought. Two TR (PmTR1 and PmTR2) and three RXR (PmRXR 1, PmRXR 2,
and PmRXR 3) cDNAs were cloned from
P. marinus
(L.A. Manzon, 2006).
Interestingly, although these receptors are highly conserved, phylogenetic
analyses suggest that the lamprey TRs diverged from the gnathostome
lineage prior to the TRα and TRβ split, hence their designation as PmTR1
and PmTR2 (Escriva
et al
.
, 2002). Although the vertebrate RXR phylogenetic
tree is much more diffi cult to resolve, the fact that all three PmRXRs are
equally identical to the vertebrate RXRα, RXRβ and RXRγ strongly suggests
they would not group with individual RXRs but rather would form a
separate group (L.A. Manzon, 2006). Preliminary developmental expression
analyses for TRs indicate that PmTR2 is constitutively expressed throughout
metamorphosis, but PmTR1 is up-regulated at times of tissue morphogenesis
and is expressed in an organ- and tissue- specifi c manner throughout
metamorphosis in the liver, kidney, intestine and gill (L.A. Manzon, 2006).
These data are consistent with the tissue-specifi c upregulation of TRβ and
the constitutive expression of TRα in
X. laevis
(reviewed in Shi, 2000). In the
Japanese fl ounder (
Paralichthys olivaceus
), TRβ is expressed constitutively
whereas TRα expression correlates with tissue morphogenesis (Yamano and
Miwa, 1998) and in the Senegalese sole (
Solea senegalensis
) both TRα and
TRβ expression correlate with metamorphosis (Isorna
et al
.
, 2009). When
the relatively low TH levels during metamorphosis are viewed in light of
increased nuclear T
3
binding and the upregulation of PmTR2 they indicate
that the components necessary for gene regulation are present and, thus,
TH might function to drive morphogenesis in a fashion similar to that
observed in other vertebrates.
When all data are considered it is feasible that TH have a dual role in
lamprey development (R.G. Manzon, In Press). During the larval period
high TH levels promote feeding, larval growth and lipid accumulation,
