Adenomatous Polyposis Coli Regulates Axon Arborization and Cytoskeleton Organization Via Its N-Terminus
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Adenomatous Polyposis Coli Regulates Axon Arborization and Cytoskeleton Organization via Its N-Terminus Youjun Chen1, Xu Tian2, Woo-Yang Kim3, William D. Snider1* 1 Department of Cell and Molecular Physiology and Neuroscience Center, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America, 2 Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, Hefei, Anhui Province, People’s Republic of China, 3 Department of Developmental Neuroscience, Munroe-Meyer Institute, University of Nebraska Medical Center, Omaha, Nebraska, United States of America Abstract Conditional deletion of APC leads to marked disruption of cortical development and to excessive axonal branching of cortical neurons. However, little is known about the cell biological basis of this neuronal morphological regulation. Here we show that APC deficient cortical neuronal growth cones exhibit marked disruption of both microtubule and actin cytoskeleton. Functional analysis of the different APC domains revealed that axonal branches do not result from stabilized b-catenin, and that the C-terminus of APC containing microtubule regulatory domains only partially rescues the branching phenotype. Surprisingly, the N-terminus of APC containing the oligomerization domain and the armadillo repeats completely rescues the branching and cytoskeletal abnormalities. Our data indicate that APC is required for appropriate axon morphological development and that the N-terminus of APC is important for regulation of the neuronal cytoskeleton. Citation: Chen Y, Tian X, Kim W-Y, Snider WD (2011) Adenomatous Polyposis Coli Regulates Axon Arborization and Cytoskeleton Organization via Its N-Terminus. PLoS ONE 6(9): e24335. doi:10.1371/journal.pone.0024335 Editor: Michael A. Fox, Virginia Commonwealth University Medical Center, United States of America Received June 15, 2011; Accepted August 4, 2011; Published September 6, 2011 Copyright: ß 2011 Chen et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by National Institutes of Health grant RO1 NS050968. Imaging was done in the Confocal and Multiphoton Imaging Core supported by National Institutes of Health grant P30 NS045892 (www.nih.gov). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: [email protected] Introduction binds to microtubules and microtubule plus end-binding protein 1 (EB1) [11] and the mammalian homolog of Discs large (DLG1) APC is an important tumor suppressor that regulates b-catenin binding region that is thought to be important for cell cycle and levels in the wnt signaling pathway [1]. In addition to the b-catenin cell polarity regulation [12,13]. It is plausible that all domains of binding region, APC contains multiple functional domains, each of APC are required to mediate its morphological regulation of which binds to proteins that regulate key cellular processes. APC is neurons. On the other hand, functions mediated by a specific thought to have important functions related to cell polarity, domain might be most important. microtubule stability and cell migration based on in vitro studies [2]. Prior investigations in neurons including our previous study Previously we and another group showed that conditional loss of have been directed at functions of the entire protein with shRNA APC in neural progenitor cells severely disrupted the structure of knockdown or dominant inhibitory approaches [14,15]. We took the developing cerebral cortex as well as axon projections in vivo advantage of the striking morphological abnormalities that we [3,4]. Further, we found that dissociated APC-deficient cortical have observed in neurons that lack APC altogether. We neurons exhibit exuberant axon branching in vitro [4]. Other introduced the various APC-domains into APC deficient neurons recent studies have demonstrated that APC is involved in axon and assessed their ability to rescue morphology. guidance of retinal ganglion cells by its differential distribution at We have found that both actin and microtubule organization the growth cone [5] and that knock down of APC in dorsal root are severely disrupted in the growth cones of APC deleted neurons ganglion (DRG) neurons leads to microtubule looping in the well prior to axon branch formation. Because of the microtubule growth cone [6]. However the cell biological basis of this APC growth cone regulation remains unclear and the APC domains abnormalities, we hypothesized that expression of the C-terminus required to regulate neuronal morphology have not been specified. of APC might be sufficient to suppress branch formation in APC Extensive domain analysis of the APC protein has been carried deleted neurons. However, surprisingly, neither the microtubule out in non-neuronal cells [7]. In addition to the b-catenin binding binding domain, nor the EB1 binding domain, nor both together domain, the key structural motifs include the oligomerization fully rescued the phenotype. Instead, expression of the amino domain, the armadillo repeats that bind to the IQ-motif- terminus, containing the oligomerization domain and the containing GTPase-activating protein (IQGAP), the APC-stimu- armadillo repeats, mediated complete rescue. We conclude that lated guanine nucleotide exchange factors (ASEFs) and kinesin N-terminal region of APC has important functions in the associated protein 3 (KAP3) [8,9,10], the carboxyl-terminus that regulation of neuronal cytoskeleton. PLoS ONE | www.plosone.org 1 September 2011 | Volume 6 | Issue 9 | e24335 APC Regulates Axon Morphogenesis Materials and Methods ogy, Beverly, MA), or a-tubulin (mouse monoclonal, 1:10000, Sigma, St. Louis, MO) at 4 degrees overnight. After three times of Mice washing with PBS, blots were incubated in horseradish peroxidase- All procedures were carried out according to an animal protocol conjugated secondary anti-mouse (1:10000, Dako, Carpinteria, CA) (protocol number: 11-029.0) approved by the Institutional Animal or anti-rabbit (1:2000, Cell Signaling Technology, Beverly, MA) Care and Use Committee (IACUC) at University of North antibodies and were developed using enhanced chemiluminescence lox/lox + Carolina-Chapel Hill. APC Nestin-Cre embryos [4] were (GE Healthcare, Waukesha, WI). generated by mating mice harboring an APC floxed allele [16] with Nestin-Cre mice [17]. Since no differences were observed between Immunocytochemistry and Antibodies heterozygote embryos (APClox/+ Nestin-Cre+) and wild type lox/lox 2 Neurons were fixed with 4% parafomaldehyde in 0.1 M sodium littermates (APC Nestin-Cre or APClox/+ Nestin cre-), both phosphate buffer (pH 7.2) for 20 minutes, and permeabilized and heterozygous and wild-type littermates were used as controls. blocked with 2% BSA in PBS containing 0.1% triton X-100 and Genotypes were determined by PCR using primers specific for the NaN3. Then neurons were incubated with primary antibody at APC floxed allele, the APC wild type allele, and cre. 4uC overnight, followed by secondary antibody incubation for 1 hour. DNA constructs Primary antibodies used were APC (C-20, rabbit polyclonal, DNA constructs were amplified and purified with by EndoFree 1:50, Sana Cruz biotechnology, Santa Cruz, CA), a-tubulin Plasmid Maxi kits (QIAGEN Sciences). (mouse monoclonal, 1:1000, Sigma, St. Louis, MO), Tau1 (mouse The N-terminal truncation mutant APC-N, and the C-terminal monoclonal, 1:500, Millipore, Billerica, MA), MAP2 (rabbit truncation mutants APC-C, APC-C1, and APC-C2 expression polyclonal, 1:500, Millipore, Billerica, MA), GFP (rabbit poly- vectors were generated as described previously [15]. APC-N1 and clonal, 1:1000, Invitrogen, Eugene, OR). All fluorescence-labeled APC-N2 expression vectors were kindly provided by Dr Inke secondary antibodies (1:1000) were from Molecular Probes Nathke [18]. The stabilized b-catenin expression vector was kindly (Eugene, OR). Fluorescence-labeled phalloidin (1:40) was from provided by Dr Fengquan Zhou. The inhibitor of b-catenin and Molecular Probes (Eugene, OR). TCF-4 (ICAT) expression vectors were kindly provided by Dr Anjen Chenn [19]. To obtain the APC-N1-C2 expression vector, Acquisition of images we first subcloned the APC-N1 cDNA fragment into Hind III and Images of cortical neurons stained with indicated antibodies Sal I sites of pEGFP-C1 vector. The APC-C2 cDNA fragment was were acquired either by Metamorph under a Nikon SMZ1500 then subcloned into the (using Sal I and BamH I sites) of pEGFP- wide field fluorescence microscope (for branching analysis) or C1-APC-N1 construct. LSM (Laser Scanning Microscopy) software under the Zeiss 510 confocal microscope (for cytoskeleton analysis). When neurons Primary neuronal cultures were visualized by the wide field microscope, around 50 Cerebral cortices dissected from mouse E14-E16 embryos were consecutive fields for each experimental condition were imaged treated with trypsin for 5 min and washed with minimum essential to avoid selection bias. When neurons were visualized by confocal medium (MEM) containing 10% fetal bovine serum before microscope, around 30 consecutive fields for each