Biology Reference
In-Depth Information
induced signaling molecule, sonic hedgehog, is required for BMP-4 induc-
tion. Functional studies have supported these results suggesting that
upregulation of the TGF-
b
pathway is important for the remodeling taking
place at the climax of metamorphosis (
Ishizuya-Oka, Hasebe, Shimizu,
Suzuki, & Ueda, 2006
).
As with other tissues, downregulated genes made up approximately half
of the genes regulated by TH. Strongly downregulated GO categories of
genes were glucose and fatty acid metabolism and the electron transport
chain. Metabolic pathways such as glycolysis, digestion, and the complexes
that transfer electrons and synthesize ATP in the mitochondrial inner mem-
brane all appear to shut down at metamorphic climax. Such downregulation
of genes, nearly 30, involved with energy consumption was temporary dur-
ing the remodeling period associated with cell death and lack of prolifera-
tion. Later developmental stages saw the expression levels of these genes
return to those found in the tadpole to achieve a functional absorptive organ,
including intestinal fatty acid binding protein allowing nutrient absorption.
An additional analysis from the natural metamorphosis data set was the
identification of larval-specific and adult-specific genes. In all, 17 larval- and
52 adult-specific genes were identified, 11 and 30 of which could be associated
with human homologs. The larval-specific genes varied in their potential
functions and included GO categories associated with catalytic activity and
RNA processing. The larval-specific genes such as UDP-glucose dehydroge-
nase and solute carrier 22A6 may play a role in signal transduction and cell
migration, while mucin, cytochrome P450, thioredoxin reductase, keratin 8,
and lactotransferrin are described as potential molecular markers for colon,
breast, and other cancers, suggesting that inappropriate expression of larval-
specific genes in adult intestine may cause or be indicative of cancer formation.
Many adult-specific genes are associated with digestion. For example, dietary
enzymes, including the serine proteinases PRSS2 and PRSS3, are significantly
increased in the adult intestine. Trypsin is essential for food digestion, but is also
involved in other physiological and pathological processes, such as inflammation
and tumor invasion. Two other adult-specific genes evaluated, ELA3 and NFI-
X2, were predominantly expressed in the epithelium. These genes may provide
good adult-specific markers to evaluate adult epithelial regeneration.
Similar processes are occurring in the tail and intestine at the beginning of
remodeling, where 100% of the cells in the tail and the vast majority of cells
in the intestine, the larval epithelial cells, are destined to die. The early re-
sponse to TH in the limb is proliferation and the same is true for the brain,
even though, like the intestine, the brain undergoes extensive remodeling.
The difference between early responses between brain and intestine is that
