Biology Reference
In-Depth Information
(NF61, beginning of adult epithelial proliferation), and end of metamorpho-
sis (NF66, tail resorption). For induced metamorphosis, the guts from
premetamorphic tadpoles treated with 10 nM TH for 0, 1, 3, and 6 days
were analyzed. The intestine of tadpoles treated with TH for 3-6 days had
extensive cell death and adult cell proliferation but no adult epithelial differ-
entiation, and thus resembled stages at climax of natural metamorphosis
(NF61). Despite this morphological similarity, only 20.9% and 33% of the
genes upregulated at prometamorphosis (stage 58) overlapped with genes
upregulated after 3 and 6 days of T3 treatment, respectively. A similar percent-
age overlap was observed for downregulated genes. Thus, even though similar
histological changes occur during induced and spontaneous metamorphosis,
the majority of gene expression changes between natural and induced intes-
tine remodeling are not congruent, likely due to different levels of TH and/or
presence of additional hormones during natural metamorphosis.
The functional activities of many genes altered during early metamorphic
stages or early TH treatment time points are consistent with the histological
epithelial tissue degeneration. Genes transiently up- and downregulated have
roles consistent with larval epithelial degeneration, the predominant early
event in TH-induced remodeling. The proteolysis gene ontology (GO) cat-
egory had significantly regulated genes, such as stromelysin-3 in larval cell ap-
optosis and collagenase 3 in tissue breakdown. Also, genes associated with the
cell cycle were significantly downregulated. After the initial wave of apoptosis,
adult cells proliferate and differentiate, accompanied by the proliferation of
connective tissue andmuscle cells, whichwould presumably include extensive
changes in gene regulation. Indeed, a significant number of transcription fac-
tors have altered expression at later time points or developmental stages, as
well as development genes and extracellular matrix genes. In late stages of
remodeling, genes for cell proliferation, signal transduction, and cell-cell
signaling are significantly upregulated. Therefore, the two waves of morpho-
logical remodeling, namely cell death then cell proliferation and differenti-
ation, are reflected by corresponding waves of gene expression. One of the
pathways strongly upregulated during this period is TGF-
b
signaling
including TGF-
b
ligands (e.g., BMP-1, -2, -3, -4, -7), type I and II recep-
tors, and SMADs. As the tadpole progressed through prometamorphosis,
four genes in this pathway were upregulated. By climax, 15 genes were
upregulated, and by stage 66, the number of genes upregulated compared
to premetamorphosis was only 5, and one gene was downregulated.
TGF-
b
pathway genes were also identified after 6 days of TH treatment.
Many of these genes may be indirect response genes because another
