Mechanisms that regulate morphogenesis of a highly branched neuron in C. elegans

Abstract

The shape of an individual neuron is linked to its function with axons sending signals to other cells and dendrites receiving them. Although much is known of the mechanisms for axonal outgrowth, the striking complexity of dendritic architecture has hindered efforts to uncover pathways that direct dendritic branching. Here we review the results of an experimental strategy that exploits the power of genetic analysis and live cell imaging of the PVD sensory neuron in C. elegans to reveal key molecular drivers of dendrite morphogenesis.

Fig. 1.

PVD dendritic branching and fasciculation with a neighboring neuron. (A) Tracings of PVD (red) and FLP (green) along the body of the worm, anterior to left, ventral down. (B) Post embryonic development of PVD dendrites during successive L2, L3 and L4 larval stages. (C) The PVD 1⁰ dendrite is located in the lateral nerve cord with ALA and CAN neurons. In the wild type, the PVD 1⁰ dendrite fasciculates with ALA and throughout its length. In sax-7 and sax-3 mutants, the PVD 1⁰ dendrite fails to fasciculate with ALA. SAX-7 in ALA and SAX-3 in PVD interact through their extracellular domains to facilitate fasciculation of ALA with the PVD 1⁰ dendrite. SAX-7 may also mediate additional homophilic interactions between PVD and ALA.

Fig. 2.

The DMA-1 receptor complex promotes PVD branching. (A) Tracing of disrupted PVD dendrite branching in various mutant backgrounds. Self-avoidance defects are indicated by green arrowheads. Alleles are sax-7(nj48), mnr-1(wy758), dma-1(wy686), lect-2(wy953), tiam-1(tm1556), gex-3(zu196), kpc-1(gk8) (null allele) and kpc-1(xr58) (hypomorphic allele), (B) Receptor complex (DMA-1, SAX-7, MNR-1, LECT-2, HPO-30) that mediates PVD interaction with the epidermis and downstream effectors, TIAM-1 and the WRC (Wave-Regulatory Complex), that direct F-actin polymerization.

Fig. 3.. Regions of expression for proteins involved in dendrite morphogenesis.

(A) Cut-away of the PVD neuron with 4⁰ dendrites sandwiched between body-wall muscles and the epidermis. PVD dendrites grow out on the inner surface of the skin with 4⁰ branches aligned with SAX-7/L1CAM stripes (green) adjacent to underlying body muscles. Adapted from O’Brien et al. (2016). (B) SAX-7/L1CAM (green) is localized in narrow bands flanking seam cells, in bilateral longitudinal stripes beneath 3⁰ dendrites and in circumferential lines adjacent to body muscle cells. UNC-52/Perlecan (pink) is localized to the body wall muscles and is denoted by stripes between the PVD 4° dendrites. Dashed lines outline the location of body muscle cells (C) Schematic depicting PVD 4⁰ dendrites and location of SAX-7/L1CAM in conjunction with the basement membrane (BM) and body muscles. UNC-52/Perlecan is expressed in body muscle cells and localized to the basement membrane where it links muscle dense bodies to intermediate filaments (IF/MUA-6) anchored in the epidermis. Adapted from Liang et al. (2015). (D) PVD 2⁰ dendrite outgrowth depends on soluble cues UNC-52/Perlecan (from muscle) and UNC-6/Netin from ventrally located cells and cell-autonomous UNC-40/DCC function in PVD.

Fig. 4.. Opposing pathways promote actin polymerization to define dendrite length.

(A) Representative tracings of PVD morphology in an unc-6(ev400) background and a cartoon summarizing self-avoidance in 3° PVD dendrites denoting growth, contact and retraction. (B) Representative tracings of PVD morphology for mutants of genes that encode regulators of actin polymerization (UNC-34/ENA/VASP, MIG-10/Lamellipodin, UNC-73/Trio) and for RNAi of Arp2/3 complex component, ARX-5/p21. Alleles are unc-34(gm104), mig-10(ct41) and unc-73(rh40). Arrowheads denote self-avoidance defects. (C) Model showing components for 3⁰ branch outgrowth vs retraction. KPC-1 antagonizes surface expression of DMA-1 which interacts with epidermal membrane-associated proteins, SAX-7/L1CAM and MNR-1, and the soluble ligand LECT-2 to function with HPO-30 to promote actin polymerization and dendrite outgrowth via TIAM-1 and the WRC (WAVE Regulatory Complex). UNC-6/Netrin is captured by UNC-40/DCC for interaction with UNC-5 and activation of UNC-34/Ena/VASP which functions with WSP-1/WASP and the Arp2/3 complex to drive F-actin assembly for NMY-1/myosin-dependent dendrite retraction in the self-avoidance response. UNC-73/Trio and MIG-10/Lamellipodin are not shown. Homophilic MIG-14/Wntless interaction also mediates self-avoidance.

Fig. 5.

Microtubule orientation and motors in PVD dendrites. Microtubule (MT) orientation in PVD 1⁰ dendrite with anterior minus-end (−) and posterior plus-end (+). MTs are oriented plus-end (+) out in the axon. Inset shows directionality of the plus-end microtubule motor, UNC-116/Kinesin-1, and minus-end motor, DHC-1/Dynein in the 1⁰ dendrite.

Fig. 6.

Transcriptional regulation of PVD morphogenesis (A) Schematics of mechanosensory neurons on the right and left sides of the worm and corresponding with the cell lineages. Unbranched touch neurons are AVM, ALMR and PLMR on the right and PVM, ALML and PLML on the left. Note PVDR on the right and PVDL on the left. (B) A model for AHR-1-dependent transcriptional regulation of PVD and touch neuron fate (C) Tracings of a converted AVM (cAVM) and PVD neuron in an ahr-1 mutant background. (D) Schematic of pioneer PVD 2⁰ dendrites (green) that grow on the inside surface of the epidermis vs commissural 2⁰ dendrites that also fasciculate with motor neuron commissures (red). The MEC-3 transcription factor promotes expression of the transcription factor EGL-46/TFIIA Zinc finger and HPO-30/Claudin. EGL-46 is required for outgrowth of 2⁰ dendrites that fasciculate with motor neuron commissures and HPO-30/Claudin mediates extension of pioneer 2⁰ dendrites. MEC-3 also regulates expression of additional branching factors (?).