The BTB-Kelch protein KLEIP controls endothelial migration and sprouting angiogenesis.

Sprouting and invasive migration of endothelial cells are important steps of the angiogenic cascade. Vascular endothelial growth factor (VEGF) induces angiogenesis by activating intracellular signal transduction cascades, which regulate endothelial cell morphology and function. BTB-kelch proteins are intracellular proteins that control cellular architecture and cellular functions. The BTB-kelch protein KLEIP has been characterized as an actin-binding protein that interacts with the nucleotide exchange factor ECT2. We report that KLEIP is preferentially expressed in endothelial cells, suggesting that it may play a critical role in controlling the functions of migrating, proliferating, and invading endothelial cells during angiogenesis. KLEIP mRNA level in endothelial cells is strongly regulated by hypoxia which is controlled by hypoxia-inducible factor-1alpha. Functional analysis of KLEIP in endothelial cells revealed that it acts as an essential downstream regulator of VEGF- and basic fibroblast growth factor-induced migration and in-gel sprouting angiogenesis. Yet, it is not involved in controlling VEGF- or basic fibroblast growth factor-mediated proliferative responses. The depletion of KLEIP in endothelial cells blunted the VEGF-induced activation of the monomeric GTPase RhoA but did not alter the VEGF-stimulated activation of extracellular signal-regulated kinase 1/2. Moreover, VEGF induced a physical association of KLEIP with the guanine nucleotide-exchange factor ECT2, the depletion of which also blunted VEGF-induced sprouting. We conclude that the BTB-kelch protein KLEIP is a novel regulator of endothelial function during angiogenesis that controls the VEGF-induced activation of Rho GTPases.

An ancient Wnt-Dickkopf antagonism in Hydra.

The dickkopf (dkk) gene family encodes secreted antagonists of Wnt signalling proteins, which have important functions in the control of cell fate, proliferation, and cell polarity during development. Here, we report the isolation, from a regeneration-specific signal peptide screen, of a novel dickkopf gene from the fresh water cnidarian Hydra. Comparative sequence analysis demonstrates that the Wnt antagonistic subfamily Dkk1/Dkk2/Dkk4 and the non-modulating subfamily Dkk3 separated prior to the divergence of cnidarians and bilaterians. In steady-state Hydra, hydkk1/2/4-expression is inversely related to that of hywnt3a. hydkk1/2/4 is an early injury and regeneration responsive gene, and hydkk1/2/4-expressing gland cells are essential for head regeneration in Hydra, although once the head has regenerated they are excluded from it. Activation of Wnt/beta-Catenin signalling leads to the complete downregulation of hydkk1/2/4 transcripts. When overexpressed in Xenopus, HyDkk1/2/4 has similar Wnt-antagonizing activity to the Xenopus gene Dkk1. Based on the corresponding expression patterns of hydkk1/2/4 and neuronal genes, we suggest that the body column of Hydra is a neurogenic environment suppressing Wnt signalling and facilitating neurogenesis.

The BTB-kelch protein LZTR-1 is a novel Golgi protein that is degraded upon induction of apoptosis.

Members of the BTB-kelch superfamily play important roles during fundamental cellular processes, such as the regulation of cell morphology, migration, and gene expression. The BTB-kelch protein LZTR-1 is deleted in the majority of DiGeorge syndrome patients and is believed to act as a transcriptional regulator. However, functional and expression profiling studies of LZTR-1 have not been performed thus far. Therefore, we examined the subcellular localization and function of LZTR-1 to gain insights into its biological role. Analysis of the primary structure of the protein revealed six N-terminal kelch motifs and two BTB/POZ domains at the C terminus within LZTR-1. Confocal analysis of the subcellular distribution of LZTR-1 using the Golgi markers GM130, Golgin-97, and TGN46 identified a localization of LZTR-1 exclusively on the cytoplasmic surface of the Golgi network that is mediated by its second BTB/POZ domain. In contrast to most other BTB-kelch proteins, LZTR-1 did not co-localize with actin. Treatment with brefeldin A did not lead to redistribution of LZTR-1 to the endoplasmic reticulum but caused its relocalization in dispersed, punctuated structures that were also positive for GM130. These data demonstrate that LZTR-1 is a Golgi matrix-associated protein. Upon induction of apoptosis, LZTR-1 was phosphorylated on tyrosine residues and subsequently degraded; that could be rescued partially by the addition of the caspase inhibitor Z-VAD-fmk and the proteasome inhibitors lactacystin and MG132. Taken together, our experiments identify LZTR-1 as the first BTB-kelch protein that exclusively localizes to the Golgi network, and the binding of LZTR-1 to the Golgi complex is mediated by its second BTB/POZ domain.