Joseph G. Culotti

DELETED IN COLORECTAL CANCER (DCC) ENCODES A NETRIN RECEPTOR

The guidance of developing axons in the nervous system is mediated partly by diffusible chemoattractants secreted by axonal target cells. Netrins are chemoattractants for commissural axons in the vertebrate spinal cord, but the mechanisms through which they produce their effects are unknown. We show that Deleted in Colorectal Cancer (DCC), a transmembrane protein of the immunoglobulin superfamily, is expressed on spinal commissural axons and possesses netrin-1-binding activity. Moreover, an antibody to DCC selectively blocks the netrin-1-dependent outgrowth of commissural axons in vitro. These results indicate that DCC is a receptor or a component of a receptor that mediates the effects of netrin-1 on commissural axons, and they complement genetic evidence for interactions between DCC and netrin homologs in C. elegans and Drosophila.

The unc-5, unc-6, and unc-40 genes guide circumferential migrations of pioneer axons and mesodermal cells on the epidermis in C. elegans

Three known genes guide circumferential migrations of pioneer axons and mesodermal cells on the nematode body wall. unc-5 affects dorsal migrations, unc-40 primarily affects ventral migrations, and unc-6 affects migrations in both directions. Circumferential movements still occur, but are misdirected whereas longitudinal movements are normal in these mutants. Pioneer growth cones migrating directly on the epidermis are affected; growth cones migrating along established axon fascicles are normal. Thus these genes affect cell guidance and not cell motility per se. We propose that two opposite, adhesive gradients guide circumferential migrations on the epidermis. unc-5, unc-6, and unc-40 may encode these adhesion molecules or their cellular receptors. Neurons have access to the basal lamina and the basolateral surfaces of the epidermis, but mesodermal cells contact only the basal lamina. These genes probably identify molecular cues on the basal lamina that guide mesodermal migrations. The same basal lamina cues, or perhaps related molecules on the epidermal cell surfaces, guide pioneer neurons.

UNC-40, a C. elegans Homolog of DCC (Deleted in Colorectal Cancer), Is Required in Motile Cells Responding to UNC-6 Netrin Cues

UNC-6 netrin, a laminin-related protein secreted from neuroglia and neurons along the ventral midline, orients migrating cells and pioneering growth cones on the nematode epidermis. UNC-5, a cell surface protein expressed on motile cells and pioneer axons, orients movements away from UNC-6 sources. UNC-40, a homolog of the cell surface proteins DCC (Deleted in Colorectal Cancer) and neogenin, is also expressed on motile cells and pioneer neurons. UNC-40 acts cell autonomously to orient movement toward UNC-6 sources. For cells coexpressing UNC-5, it helps orient movement away from UNC-6 sources. Finally, UNC-40 helps determine the dorsoventral position of cells undergoing purely longitudinal migrations. Together with the recent report that DCC is a netrin receptor in vertebrates, our results suggest that UNC-40 is a component of UNC-6 receptors on motile cells.

UNC-6, a laminin-related protein, guides cell and pioneer axon migrations in C. elegans

The unc-6 gene is required for the guidance of pioneer axons and migrating cells along the body wall in C. elegans. In mutants, dorsal and ventral migrations are disrupted, but longitudinal movements are largely unaffected. The gene was tagged for molecular cloning by two independent transposon insertions. Based on genomic and cDNA sequencing, the gene encodes a novel laminin-related protein, UNC-6 (591 amino acids). The N-terminus is homologous to the N-termini (i.e., domains V1, V-1, V-2, and V-3) of laminin subunits, while the C-terminus is a unique domain. We propose that UNC-6 is a component of an extracellular matrix cue that guides dorsoventral migrations on the epidermis.

UNC-5, a transmembrane protein with immunoglobulin and thrombospondin type 1 domains, guides cell and pioneer axon migrations in C. elegans

The unc-5 gene is required for guiding pioneering axons and migrating cells along the body wall in C. elegans. In mutants, dorsal migrations are disrupted, but ventral and longitudinal movements are largely unaffected. The gene was tagged for molecular cloning by transposon insertions. Based on genomic and cDNA sequencing, the gene encodes UNC-5, a transmembrane protein of 919 aa. The predicted extracellular N-terminus comprises two immunoglobulin and two thrombospondin type 1 domains. Except for an SH3-like motif, the large intracellular C-terminus is novel. Mosaic analysis shows that unc-5 acts in migrating cells and pioneering neurons. We propose that UNC-5 is a transmembrane receptor expressed on the surface of motile cells and growth cones to guide dorsal movements.

UNC-73 Activates the Rac GTPase and Is Required for Cell and Growth Cone Migrations in C. elegans

unc-73 is required for cell migrations and axon guidance in C. elegans and encodes overlapping isoforms of 283 and 189 kDa that are closely related to the vertebrate Trio and Kalirin proteins, respectively. UNC-73A contains, in order, eight spectrin-like repeats, a Dbl/Pleckstrin homology (DH/PH) element, an SH3-like domain, a second DH/PH element, an immunoglobulin domain, and a fibronectin type III domain. UNC-73B terminates just downstream of the SH3-like domain. The first DH/PH element specifically activates the Rac GTPase in vitro and stimulates actin polymerization when expressed in Rat2 cells. Both functions are eliminated by introducing the S1216F mutation of unc-73(rh40) into this DH domain. Our results suggest that UNC-73 acts cell autonomously in a protein complex to regulate actin dynamics during cell and growth cone migrations.

C. elegans Slit Acts in Midline, Dorsal-Ventral, and Anterior-Posterior Guidance via the SAX-3/Robo Receptor

Robo receptors interact with ligands of the Slit family. The nematode C. elegans has one Robo receptor (SAX-3) and one Slit protein (SLT-1), which direct ventral axon guidance and guidance at the midline. In larvae, slt-1 expression in dorsal muscles repels axons to promote ventral guidance. SLT-1 acts through the SAX-3 receptor, in parallel with the ventral attractant UNC-6 (Netrin). Removing both UNC-6 and SLT-1 eliminates all ventral guidance information for some axons, revealing an underlying longitudinal guidance pathway. In the embryo, slt-1 is expressed at high levels in anterior epidermis. Embryonic expression of SLT-1 provides anterior-posterior guidance information to migrating CAN neurons. Surprisingly, slt-1 mutants do not exhibit the nerve ring and epithelial defects of sax-3 mutants, suggesting that SAX-3 has both Slit-dependent and Slit-independent functions in development.

Dopamine modulates the plasticity of mechanosensory responses in Caenorhabditis elegans

Dopamine‐modulated behaviors, including information processing and reward, are subject to behavioral plasticity. Disruption of these behaviors is thought to support drug addictions and psychoses. The plasticity of dopamine‐mediated behaviors, for example, habituation and sensitization, are not well understood at the molecular level. We show that in the nematode Caenorhabditis elegans, a D1‐like dopamine receptor gene (dop‐1) modulates the plasticity of mechanosensory behaviors in which dopamine had not been implicated previously. A mutant of dop‐1 displayed faster habituation to nonlocalized mechanical stimulation. This phenotype was rescued by the introduction of a wild‐type copy of the gene. The dop‐1 gene is expressed in mechanosensory neurons, particularly the ALM and PLM neurons. Selective expression of the dop‐1 gene in mechanosensory neurons using the mec‐7 promoter rescues the mechanosensory deficit in dop‐1 mutant animals. The tyrosine hydroxylase‐deficient C. elegans mutant (cat‐2) also displays these specific behavioral deficits. These observations provide genetic evidence that dopamine signaling modulates behavioral plasticity in C. elegans.

DCC AND NETRINS

Recent advances highlight the versatility and complexity of this highly conserved axon and cell migration guidance system. Characterization of netrin mutant phenotypes in worm, fly and mouse all suggest that netrins play local as well as long-range roles in guidance. Evidence from multiple sources also indicates that the netrin receptor DCC can mediate both attractive and repulsive responses to netrins.

The mec-7 beta-tubulin gene of Caenorhabditis elegans is expressed primarily in the touch receptor neurons.

Mutants of the mec‐7 beta‐tubulin gene of Caenorhabditis elegans lack the large diameter 15‐protofilament microtubules normally found only in the set of six touch receptor neurons. Both a mec‐7‐lacZ reporter gene and affinity‐purified anti‐mec‐7 antibodies were used to show that mec‐7 is expressed primarily in the touch neurons. These data are consistent with a possible instructive role for the mec‐7 tubulin in determining microtubule protofilament number. The antibodies and the mec‐7‐lacZ transgene were also used to examine mec‐7 expression in mutants affecting the generation, differentiation or maintenance of the touch neurons. Decreased expression was observed in mutants of unc‐86 and mec‐3, genes that encode transcription factors essential for touch receptor neuron generation and differentiation, respectively.