unc-44 Ankyrin and stn-2 γ-Syntrophin Regulate sax-7 L1CAM Function in Maintaining Neuronal Positioning in Caenorhabditis elegans

Abstract

The L1 family of single-pass transmembrane cell adhesion molecules (L1CAMs) is conserved from Caenorhabditis elegans and Drosophila to vertebrates and is required for axon guidance, neurite outgrowth, and maintenance of neuronal positions. The extracellular region of L1CAMs mediates cell adhesion via interactions with diverse cell-surface and extracellular matrix proteins. In contrast, less is known regarding the function of the intracellular domains in the L1CAM cytoplasmic tail. Previously, we identified a role of the C. elegans L1CAM homolog, SAX-7, in maintaining neuronal and axonal positioning. Here, we demonstrate that this function is dependent on three conserved motifs that reside in the SAX-7 cytoplasmic tail: (1) the FERM-binding motif, (2) the ankyrin-binding domain, and (3) the PDZ-binding motif. Furthermore, we provide molecular and genetic evidence that UNC-44 ankyrin and STN-2 g-syntrophin bind SAX-7 via the respective ankyrin-binding and PDZ-binding motifs to regulate SAX-7 function in maintaining neuronal positioning.

A Novel Nondevelopmental Role of the SAX-7/L1CAM Cell Adhesion Molecule in Synaptic Regulation inCaenorhabditis elegans

“…Indeed, examination by threefactor crosses (see Materials and Methods) revealed a second mutation, designated as eq3, to the right of sax-7 in the LH1 strain ( Figure 1A). Free of eq3, the sax-7(eq1) phenotype is generally similar to that described for other sax-7 alleles, including defective neuronal position maintenance (Sasakura et al 2005;Zhou et al 2008). The scrawny, Gro, Egl, Con, and lethargic phenotype of LH1 is attributed to the eq3 mutation ( Figure 1D).…”

“…In addition to the neuronal position maintenance defects observed in other sax-7 mutant backgrounds, LH1 animals are also scrawny, slow growing (Gro), egg-laying defective (Egl), constipated (Con), and lethargic with a tendency to coil their bodies, behaviors that are not exhibited by other sax-7 alleles (Wang et al 2005). One possible explanation for this difference is that eq1, unlike the other sax-7 alleles, is a null mutation, as revealed by molecular and genetic analyses and the lack of detectable SAX-7 protein in sax-7(eq1) animals immunostained with an anti-SAX-7 antibody; in contrast, the other sax-7 alleles still show residual SAX-7 protein (Zhou et al 2008). An alternative explanation is the presence of a second mutation that remained linked to sax-7 after the original isolation and outcrossing of the eq1 mutation.…”

“…LGIV: unc-24(e138), dpy-20(e1282), and unc-29(e193) (Brenner 1974); sax-7(eq1) (Wang et al 2005;Zhou et al 2008); sax-7(nj48) (Sasakura et al 2005); juIs1 (Hallam and Jin 1998); gtl-2(tm1463) (The Japanese National Bioresource Project); and gtl-2(eq3), which was outcrossed from the LH1 strain (Wang et al 2005) (see Three-factor cross analysis in Materials and Methods). LGX: oxIs12 (McIntire et al 1997).…”

“…Based on Zhou et al (2008), we injected sax-7 gtl-2 animals with co-injection marker P str-1 :: gfp (Troemel et al 1995) along with different sax-7 constructs that include 70 ng/ml pLC262 (P sax-7 ::sax-7) or 10 ng/ml pLC237 (P myo-3 ::sax-7L) or 20 ng/ml pLC227 (P unc-119 :: sax-7L) or 2.5 ng/ml pLC238 (P dpy-7 ::sax-7L). To generate the inducible sax-7 construct (pLC672), we inserted sax-7L into pPD49.78, which contains the HSP16-2 promoter (courtesy of A.…”

The novel intestinal filament organizer IFO-1 contributes to epithelial integrity in concert with ERM-1 and DLG-1

“…In case of the ERM-1/DAC pathway, the L1CAM SAX-7, a single-pass transmembrane cell adhesion receptor belonging to the immunoglobulin superfamily (Chen and Zhou, 2010), has the potential to interact with ERM-1 and DLG-1. The SAX-7 cytoplasmic tail contains three distinct consensus-binding sites, RGQNYPVSQR, SFIGQY and STFV, for cytoskeletal adaptor proteins (FERM: protein 4.1, ezrin, radixin, moesin), for ankyrin and for PDZ proteins (PSD95, DlgA, ZO-1), respectively (Zhou et al, 2008). Although loss of SAX-7 seems not to interfere with the junctional localization of the DAC (Bernadskaya et al, 2011), depletion of the DAC disturbs junctional localization of phosphorylated SAX-7 at the CeAJ in the intestine (our unpublished results).…”