Integrins establish dendrite-substrate relationships that promote dendritic self-avoidance and patterning in drosophila sensory neurons

Abstract

Dendrites achieve characteristic spacing patterns during development to ensure appropriate coverage of territories. Mechanisms of dendrite positioning via repulsive dendrite-dendrite interactions are beginning to be elucidated, but the control, and importance, of dendrite positioning relative to their substrate is poorly understood. We found that dendritic branches of Drosophila dendritic arborization sensory neurons can be positioned either at the basal surface of epidermal cells, or enclosed within epidermal invaginations. We show that integrins control dendrite positioning on or within the epidermis in a cell autonomous manner by promoting dendritic retention on the basal surface. Loss of integrin function in neurons resulted in excessive self-crossing and dendrite maintenance defects, the former indicating a role for substrate interactions in self-avoidance. In contrast to a contact-mediated mechanism, we find that integrins prevent crossings that are noncontacting between dendrites in different three-dimensional positions, revealing a requirement for combined dendrite-dendrite and dendrite-substrate interactions in self-avoidance.

Figure 1. Integrins are required cell autonomously for sensory dendrite morphogenesis

(A) Wild-type (FRT19A) MARCM clone of the class I neuron ddaE. (B) mys^XG43 ddaE MARCM clone shows a simplified dendritic arbor. (C) Similar dendrite phenotype is seen in neurons mutant for two α integrin subunits, mew (αPS1) and if (αPS2). (D) rhea13-8 ddaE mutant clone has dendrite defects that are similar to mys ddaE clones. (E) Quantification of branch points for wild-type (+/+) FRT19A, mys¹, mys^XG43, and mew if ddaE MARCM clones (all generated with FRT19A). n values are indicated in bars. (F) Quantification of total dendrite length for FRT19A control, mys¹, mys^XG43, and mew if ddaE MARCM clones. n values for each genotype are the same as in (E). (G) Quantification of branch points for wild-type (FRT2A) and FRT2A rhea ddaE MARCM clones. n values are indicated in bars. (H) Quantification of total dendrite length for FRT2A and FRT2A rhea ddaE MARCM clones. n values are the same as indicated in (G). (I) Quantification of branch points for ddaE neurons expressing UAS-dicer2, UAS-mys-RNAi, UAS-mew RNAi, or UAS-if RNAi (all with UAS-dicer2) under the control of 221-Gal4, UAS-mCD8::GFP. n values are indicated below boxplots. (J) Quantification of total dendrite length for ddaE neurons expressing UAS-dicer2, UAS-mys-RNAi, UAS-mew RNAi, or UAS-if RNAi. (K) Wild-type FRT19A control class IV neuron clone. Arrowheads indicate dendritic crossing points. (L) mys^XG43 class IV MARCM clone. Arrowheads indicate dendritic crossing points. (M) Quantification of the number of branch points for class IV ddaC neurons in wild-type (+/+) and mys mutant clones. n values are indicated in bars. (N) Quantification of sister dendrite overlaps in wild-type and mys clones standardized to dendrite length. n values are indicated in bars. Scale bars = 50 µm. Barplots show mean + standard deviation (S.D.). p values are indicated as: *=p < 0.05, ** = p < 0.01, and *** = p < 0.001 as assessed by pairwise t-tests with Bonferroni correction. Boxplots show median (thick line), quartiles Q1–Q3 (25%–75% quantiles; grey box), and data in the 1.5× quartile range (dashed bars). *=p < 0.05, ** = p < 0.01, and n.s. = not significant, as assessed by Wilcoxon rank-sum test (I) or pairwise t-tests with Bonferroni correction (J). Anterior is to the left and dorsal top for these and subsequent confocal images. See also Figure S1.

Figure 2. Electron microscopy of the dendrite-epidermal interface

(A) Electron microscopy of a branched process containing multiple parallel microtubules in third instar larva cut parallel to the epidermal surface. Key: d = dendrite. (B) Schematic of transverse sectioning for transmission electron microscopy (TEM). (C–D) Representative electron micrographs of dendrites of varying size located at the basal surface of the epidermis. Key: d = dendrite, ECM = extracellular matrix, * = junction. Apical-basal orientation of epidermis is indicated in (C). (E–F) Electron micrographs of dendrites that are enclosed by epidermal membrane. Key is the same as in (C). Arrowheads indicate membrane segments that extend between the dendrite and the basal surface of the epidermis. Note apparent junctions at the basal end of these membrane segments (asterisk). (G) Schematic of surface dendrite. (H) Schematic of enclosed dendrite. Scale bars = 200 nm.

Figure 3. Identifying markers of enclosed dendrites

(A–B) Labeling of third instar ppk-Gal4, UAS-mCD8-GFP larva with anti-HRP in Triton. Anti-GFP and anti-Coracle channels are omitted for clarity. (C–C") Labeling of third instar ppk-Gal4, UAS-mCD8-GFP larva with anti-GFP and anti-HRP. Anti-Coracle labeling is omitted for clarity. Anti-HRP labeling was performed prior to Triton X-100 exposure. HRP channel shows different labeling intensities along major branches of class I, III, and IV neurons (arrows). GFP labeling, performed after Triton exposure, is fairly uniform along class IV dendrites. (D–D") Labeling of third instar ppk-Gal4, UAS-mCD8-GFP larva with anti-GFP and anti-HRP. Anti-Coracle labeling is omitted for clarity. Anti-HRP labeling was performed prior to Triton X-100 exposure. (D') HRP channel shows differential labeling intensities along higher order branches of class IV neurons (arrowheads). (D") GFP labeling, performed after Triton exposure, is fairly uniform along class IV dendrites. (E–E"') Labeling of third instar ppk-Gal4, UAS-mCD8-GFP larva with anti-GFP, HRP, and Coracle, all after detergent treatment, reveals uniform labeling with GFP and HRP, and intermittent labeling of Coracle along a class IV dendrite. A dashed arrow lies next to the dendritic region quantified in the linescan (G). (F) Schematic of labeling patterns shown in (E). (G) Linescan of dendrite marked with a dashed arrow in (E) showing labeling intensity along length of the dendrite for GFP (green), HRP (red), and Coracle (blue). Note that while the Coracle intensity varies, the trajectories of GFP and HRP linescans are fairly uniform and equivalent. A high-Coracle region is indicated by a horizontal blue bar. (H–H"') Dendrites were labeled with anti-HRP in non-detergent conditions, treated with detergent (Triton), and then labeled with anti-GFP and anti-Coracle. GFP immunoreactivity is fairly uniform, but anti-HRP immunoreactivity is variable. Low HRP labeling is observed along a high-Coracle dendritic region and stronger HRP labeling is observed along low-Coracle dendritic region. A dashed arrow lies next to the dendritic region quantified in the linescan (J). (I) Schematic of labeling patterns shown in (H) (J) Linescan of the dendrite marked by a dashed arrow in (H) shows that HRP intensity is dampened relative to GFP intensity where Coracle labeling is high (blue horizontal bars), and that anti-HRP fluorescence intensity increases where Coracle levels drop. Scale bars = 25µm See also Figure S2.

Figure 4. Loss of integrins leads to dendritic enclosure

(A–A") mys+/− class I ddaE neuron, acquired from the same animal as the MARCM clone in (C), labeled for anti-GFP (ddaE is not labeled), anti-HRP, and anti-Coracle. Isolated GFP and Coracle channels are shown to the right of the merged image. ddaE arbor is indicated by small yellow arrowheads. Class IV ddaC dendrites are indicated by red arrows. Immunolabeling was performed in the presence of Triton throughout. Coracle enrichments show little co-localization with class I ddaE dendrites (small yellow arrowheads) but more extensive co-localization with class IV ddaC dendrites (red arrows). Large yellow arrowheads indicate location of ddaE cell body. (B) Schematic drawing of labeling pattern shown in (A) with class I ddaE neuron drawn with thick lines and class IV dendrites drawn with thin lines. (C–C") mys¹ mutant MARCM clone labeled with anti-GFP, anti-HRP, and anti-Coracle. Isolated GFP and Coracle channels are shown to the right of the merged image. Immunolabeling was performed in the presence of Triton throughout. Coracle is enriched along the trajectories of mys mutant class I dendrites (yellow arrowheads). Coracle labeling along class IV neuron dendrites is indicated by red arrows. (D) Schematic drawing of labeling patterns shown in (C). (E) Quantification of mean length of ddaE arbors showing Coracle enrichments in matched heterozygous (mys+/−; n=5) and mys¹ neurons (n=5). (F–F") mys^XG43 mutant MARCM clone showing that enrichment of Coracle correlates with decreased HRP immunofluorescence when anti-HRP labeling is performed prior to detergent treatment. Isolated HRP and Coracle channels are shown to the right of the merged image. A dashed arrow in (F) lies adjacent to the region of dendrite quantified in the linescan shown in (H). (G) Schematic drawing of labeling patterns shown in (F). (H) Linescan of class I mys MARCM clone dendrite labeled with dashed arrow in (F) showing HRP (red line) and Coracle (blue line) fluorescence intensities (in arbitrary units, A.U.). GFP labeling is omitted from linescan for clarity. Anti-HRP intensities are lowest in regions of high Coracle, and increase where Coracle levels decline. Scale bars = 50 µm. Barplot shows mean + S.D. p values are indicated as ** = p < 0.01 as assessed by Student’s t-test.

Figure 5. Integrin overexpression can suppress markers of dendritic enclosure

(A–A") Labeling of ppk-Gal4, UAS-mCD8-GFP/+; ppk-eGFP/+ third instar larva with anti-mCD8 (green), anti-HRP, and anti-Coracle reveals Coracle labeling along class IV neuron dendrites (yellow arrowheads). Labeling is also seen at junctions between epidermal cells. Anti-HRP labeling was performed in PBS without detergent. A red asterisk indicates a dorsal external sensory neuron. (B) Schematic of class IV dendrites (green) and Coracle labeling (blue). (C–C") Labeling of ppk-Gal4, UAS-mCD8-GFP/+; ppk-eGFP/UAS-αPS2, UAS-βPS third instar larva with anti-mCD8 (green), anti-HRP, and anti-Coracle reveals diminished anti- Coracle labeling of class IV neuron dendrites. Labeling is retained at epidermal cell-cell junctions. Anti-HRP labeling was performed in PBS without detergent. Yellow arrowheads indicate areas of dendrite-associated Coracle labeling. A red asterisk indicates a dorsal external sensory neuron. (D) Schematic of class IV dendrites (green) and Coracle labeling (blue). Scale bars = 50 µm.

Figure 6. Defective dendritic maintenance in class I integrin mutant clones

(A) Wild-type MARCM clone of class I neuron ddaE imaged in second and third instar larval stages. Very little addition or retraction of branches is observed, however branches elongate and maintain territory coverage. (B) mys¹ MARCM clone of class I neuron ddaE imaged in second and third instar larval stages. Arrowheads indicate areas of dendritic arbor shown enlarged in (E). (C) Plot of branch lengths in wild-type ddaE MARCM clones in second and third instar stages (n=4 clones). Blue circles represent non-terminal branch segments and yellow circles represent terminal branches. The dashed line represents the isolength line. (D) Plot of branch lengths in mys ddaE MARCM clones in second and third instar stages (n=4 clones). Blue circles represent non-terminal branch segments, yellow circles represent terminal branches, and red circles represent terminal branches that were identified as having Coracle tails. The dashed line represents the isolength line. (E) Dendrites of mys MARCM clone in third instar larva that have regressed since second instar. Coracle tails are indicated by red arrows. Branches correspond to branches with arrowheads in (B). Scale bars = 50 µm (A, B), 25 µm (E).

Figure 7. Dendritic enclosure and basis for self-crossing in integrin and Dscam1 deficient neurons

(A–A"') Isoneuronal overlaps can occur where one branch is labeled strongly by anti-Coracle and another branch is unlabeled. Yellow arrow indicates point of overlap. Note low levels of anti-HRP in one of the crossing branches (A"). (B) Schematic drawing of labeling patterns shown in (A). (C–C"') Heteroneuronal overlap between class III and class IV neuron. Yellow arrowhead indicates point of overlap. Note lack of Coracle enrichment along either branch. (D) Schematic drawing of labeling patterns shown in (C). (E) Summary of Coracle immunohistochemical signatures of crossing dendrites in ppk-Gal4, UAS-mCD8::GFP third instar larvae. Contacting crossings are scored by absence of Coracle along both dendrites (left column). Non-contacting crossings are scored by presence of Coracle along at least one dendrite at crossing (right column). The predicted dendritic arrangements are schematized as two crossing dendrites on the basal surface of the epidermis, or one surface dendrite and one enclosed dendrite. The proportions of isoneuronal crossings (n=28 crossings; 10 cells) and heteroneuronal/heterotypic crossings (n=154 crossings from 5 cells) in each category are shown. (F) Stacked barchart showing percentages of dendrite crossings in mys and Dscam1 clones scored as contacting crossing (red bar) or non-contacting crossing (blue bar) as assessed by Coracle labeling. (G) Fluorescence intensity plots of Coracle labeling (measured in arbitrary units, A.U.) in 17 dendrite-dendrite crossings in mys clones in which relative depth of crossing dendrites could be discriminated. Coracle labeling intensity along the more apical branch is indicated by a blue box and paired with the associated basal branch by an “X”. In each crossing the apical-most branch shows higher levels of Coracle. (H) Quantification of fluorescence intensity along crossing dendrites that could be resolved as “basal” and “apical” in 1µm confocal Z-steps (n=17 pairs). Shown are median of median values of Coracle immunofluorescence in linescans. (I–I') Confocal image of Dscam1 class IV MARCM clone showing two isolated non-contacting crossings (arrows) and an area of dendrite crossings that includes putative contacting crossings (arrowhead). Anti-GFP is in green and anti-Coracle is in blue. Coracle channel is shown in isolation (I') with arrows and arrowheads indicating regions of non-contacting and contacting crossing, respectively. (J) Schematic drawing of labeling patterns shown in (I). Datasets are presented in boxplots as median (thick line), quartiles Q1–Q3 (25%–75% quantiles; blue box), and data in 1.5× quartile range (dashed bars). *** = p < 0.001 by Wilcoxon rank-sum test. Scale bars = 12.5 µm. See also Figure S3