The first five years of the Wnt targetome.

The canonical Wnt pathway controls cell differentiation, proliferation and apoptosis by regulating the expression of a high number of target genes. The first target gene of the Wnt pathway was discovered nearly 20 years ago, when analysing gene expression patterns in the Drosophila embryo. Since the year 2002 entire transcriptomes have been screened by microarray analysis in order to identify genes, which are differentially expressed in cells with activated Wnt pathway. Recently, novel genome-based screening methods have been developed, which are less error-prone and independent from RNA. The exemplified methods STAGE (Sequence Tag Analysis Of Genomic Enrichment), ChIP-PET (chromatin immunoprecipitation with paired-end ditag), ChIP-Seq (ChIP followed by direct sequencing) and the bioinformatics approach EEL (Enhancer Element Locator) will be introduced shortly. The high number of potential target genes and regulated functions left questions unanswered, for instance how the Wnt pathway controls such a high number of genes and how it is able to regulate so many different cellular functions. In order to answer these questions we ordered the genes of the published Wnt target screenings according to their functions, and summarized the pathways, which are regulated by the Wnt pathway. This review focuses on the totality of Wnt target genes, the Wnt targetome, which is the clue to understand the manifold roles of the Wnt pathway within the cellular context.

Chronological expression of Wnt target genes Ccnd1, Myc, Cdkn1a, Tfrc, Plf1 and Ramp3.

The canonical Wnt pathway regulates several biological processes including development, cell growth and proliferation via consecutive gene regulation. A high number of target genes of the Wnt pathway has been identified, but the chronological order of target gene expression is still elusive. This order is supposed to be crucial for the controlled course of events downstream of the activated Wnt pathway. Here we present the expression chronologies of the target genes Ccnd1 (encoding for cyclin D1), Myc (c-Myc), Cdkn1a (p21CIP1/WAF1), Tfrc (Transferrin receptor 1), Plf1 (Proliferin-1) and Ramp3 (Receptor activity-modifying protein 3) in C57MG cells after stimulation with Wnt-3a. We discriminated between immediate (below 1 h), early (between 1 and 6 h), intermediate (between 6 and 12 h) and late (after 12 h) targets. According to this classification Myc and Tfrc belong to the immediate target genes; Ccnd1, Plf1 and Ramp3 are early target genes and Cdkn1a is an intermediate target gene.

The alpha and beta subunits of the metalloprotease meprin are expressed in separate layers of human epidermis, revealing different functions in keratinocyte proliferation and differentiation.

The zinc endopeptidase meprin (EC 3.4.24.18) is expressed in brush border membranes of intestine and kidney tubules, intestinal leukocytes, and certain cancer cells, suggesting a role in epithelial differentiation and cell migration. Here we show by RT-PCR and immunoblotting that meprin is also expressed in human skin. As visualized by immunohistochemistry, the two meprin subunits are localized in separate cell layers of the human epidermis. Meprin alpha is expressed in the stratum basale, whereas meprin beta is found in cells of the stratum granulosum just beneath the stratum corneum. In hyperproliferative epidermis such as in psoriasis vulgaris, meprin alpha showed a marked shift of expression from the basal to the uppermost layers of the epidermis. The expression patterns suggest distinct functions for the two subunits in skin. This assumption is supported by diverse effects of recombinant meprin alpha and beta on human adult low-calcium high-temperature keratinocytes. Here, beta induced a dramatic change in cell morphology and reduced the cell number, indicating a function in terminal differentiation, whereas meprin alpha did not affect cell viability, and may play a role in basal keratinocyte proliferation.