Biology Reference
In-Depth Information
tissue degeneration is not as extensive in the brain. Comparison of gene
expression data across these tissues indicates that tissues with similar cellular
responses to TH have the most overlap in TH-induced genes. Thus, the ini-
tial proliferative responses of the limb and brain to TH explain their similar
sets of regulated genes, which share little overlap with genes similarly in-
duced in tail and intestine associated with cell death occurring in the initial
response to TH. Despite the association between morphology and altered
gene expression among tissues, a core set of 59 genes similarly expressed
among all these tissues was identified. How these similarly expressed genes
are involved in the mechanisms of gene regulation underlying tissue-specific
remodeling has yet to be determined.
2.5. Skin
The larval skin is composed of three main cell types, apical cells, skein cell, and
basal cells, organized in two layers (
Suzuki, Utoh, Kotani, Obara, &Yoshizato,
2002
). During metamorphosis or precocious exposure to TH, basal cells dif-
ferentiate into adult-type skin stem cells and replace the apoptotic apical and
skein cells with cornified, multilayered adult skin with mucus, and serous
glands, the latter producing antimicrobial peptides among other proteins.
Global gene expression during TH-induced metamorphosis in body skin
was analyzed after 0 versus 1, 3, 5, and 7 days of 5 nM T3 (
Suzuki et al.,
2009
). They found 240 upregulated genes and 161 downregulated genes
organized into nine functional groups. Within the first 24 h, the T3-response
genes, TR
b
,TH/bZIP,KLF9,andST3,wereinducedintheskin,as
observed in all other organs from previous studies. Transcription and prote-
olysis were also functional categories recognized by 24 h, followed by cell
cycle genes by 3 days, and then defense genes (including antimicrobial
peptides) by 7 days. Adult keratin genes and antimicrobial peptide genes were
among the most upregulated genes, and genes 17-20 (apical cell-specific genes
identified by subtractive hybridization;
Wang & Brown, 1991
)wereamong
the most downregulated. A major change in gene expression unanticipated
by histology was the gradual down regulation of immune response genes
(mostly MHC genes). Explanation for this change awaits further work.
2.6. Direct response genes
Because TH induces all metamorphic changes and its effects are mediated
through transcriptional regulation by TRs, the identification and character-
ization of the TH early, direct target genes are of critical importance in elu-
cidating mechanisms underlying the tissue-specific responses
to TH.
