ZENON, a Novel POZ Kruppel-Like DNA Binding Protein Associated with Differentiation and/or Survival of Late Postmitotic Neurons

Abstract

The rat tyrosine hydroxylase gene promoter contains an E-box/dyad motif and an octameric and heptameric element that may be recognized by classes of transcription factors highly expressed during nervous system development. In a one-hybrid genetic screen, we used these sites as targets to isolate cDNAs encoding new transcription factors present in the brain. We identified ZENON, a novel rat POZ protein that contains two clusters of Kruppel-like zinc fingers and that presents several features of a transcription factor. ZENON is found in nuclei following transient transfection with the cDNA. The N-terminal zinc finger cluster contains a DNA binding domain that interacts with the E box. Cotranfection experiments revealed that ZENON induces tyrosine hydroxylase promoter activity. Unlike other POZ proteins, the ZENON POZ domain is not required for either activation of transcription or self-association. In the embryonic neural tube, ZENON expression is restricted to neurons that have already achieved mitosis and are engaged in late stages of neuronal differentiation (late postmitotic neurons). ZENON neuronal expression persists in the adult brain; therefore, ZENON can be considered a marker of mature neurons. We propose that ZENON is involved in the maintenance of panneuronal features and/or in the survival of mature neurons.The two major cell types in the nervous system, neurons and glia, both comprise a large number of subtypes. The generation and maintenance of this phenotypic diversity require extracellular signals that are converted into tightly regulated transcriptional cascades. Many transcription factors involved in these cascades-often basic helix-loop-helix (bHLH), homeodomain, or zinc finger-containing proteins-remain to be identified. Description of these factors would help elucidate the molecular events leading to the differentiation of the various cell types in the nervous system.One way to identify these factors is to isolate novel proteins that interact with sequences promoting the neural expression of a highly developmentally regulated gene. A good candidate is the gene encoding tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis (44, 56). The expression of the TH gene is a common feature of all neurons and neuroendocrine cells that synthesize and release catecholamines (dopamine, norepinephrine, and epinephrine). These cells are extremely diverse in their functional, morphological, and anatomical properties and have different embryonic origins. Depending on their locations in the adult central nervous system (CNS) and peripheral nervous system (PNS), catecholaminergic cells originate from different parts of the neural tube and neural crest (6,73,74). In the embryonic neural tube, catecholaminergic cell groups arise from various neuromeres (64, 78), suggesting that a large number of extracellular signals and transcription factors govern the specification and maintenance of catecholaminergic identity.The production and functional analysis of knockout mice revealed the essen...

A Family of Human Zinc Finger Proteins That Bind Methylated DNA and Repress Transcription

“…A BLAST search based on a 78-amino-acid sequence of human Kaiso identified two human proteins with Kaiso-like zinc fingers: ZBTB4 and ZBTB38. ZBTB4 is uncharacterized, while ZBTB38 is the human orthologue of the rat protein Zenon, which was recently cloned in a one-hybrid screen using the tyrosine hydroxylase gene promoter as a target sequence (24). ZBTB4 has 62% identity to Kaiso over the region used for BLAST, while ZBTB38 has 65% identity (Fig.…”

“…5B) but do not interact with the zinc fingers of Kaiso. This probably explains the observation that Zenon, that rat orthologue of ZBTB38, can homodimerize independently of the BTB/POZ domain (24).…”

“…The brain was dissected, postfixed with PFA, and embedded in bovine serum albumin-gelatin, and 250-m sections were cut with a Vibratome. The sections were then processed for hybridization with a riboprobe as described previously (24). The probe was synthesized by in vitro transcription of a fragment of the mouse ZBTB4 cDNA containing nucleotides 1938 to 2949.…”

Kaiso-Deficient Mice Show Resistance to Intestinal Cancer

“…This suggests either that these genes are not targets of Kaiso-mediated repression in the mouse or that there is a level of redundancy in their control. It is not possible to predict which proteins might substitute for the absence of mouse Kaiso, but the related protein CIBZ/Zenon may be a potential candidate (21,42). It is clear that deficiency of NCoR is not equivalent to loss of Kaiso, as Kaiso-null mice show none of the abnormalities observed in N-CoR-null embryos, which die before birth (16).…”

Many paths to one goal? The proteins that recognize methylated DNA in eukaryotes

“…A second difference between Kaiso and ZBTB4/ZBTB38 is their cellular distribution: in transfected mouse fibroblasts ZBTB4 and ZBTB38 primarily localize to pericentromeric heterochromatin whereas Kaiso shows a diffuse nuclear abundance pattern (Filion et al, 2006). Thirdly, the BTB domain of Kaiso is known to homodimerize (Daniel and Reynolds, 1999), whereas ZBTB4 and ZBTB38 cannot homodimerize via the BTB domains, but homodimerize or heterodimerize each other via zinc fingers (Kiefer et al, 2005, Filion et al, 2006. To better understand the functional differences among Kaiso family proteins, it would be useful to determine whether all three proteins bind the same target sites, or have different binding preferences.…”