Conserved miR-8/miR-200 defines a glial niche that controls neuroepithelial expansion and neuroblast transition

Abstract

Neuroepithelial cell proliferation must be carefully balanced with the transition to neuroblast (neural stem cell) to control neurogenesis. Here, we show that loss of the Drosophila microRNA mir-8 (the homolog of vertebrate miR-200 family) results in both excess proliferation and ectopic neuroblast transition. Unexpectedly, mir-8 is expressed in a subpopulation of optic-lobe-associated cortex glia that extend processes that ensheath the neuroepithelium, suggesting that glia cells communicate with the neuroepithelium. We provide evidence that miR-8-positive glia express Spitz, a transforming growth factor α (TGF-α)-like ligand that triggers epidermal growth factor receptor (EGFR) activation to promote neuroepithelial proliferation and neuroblast formation. Further, our experiments suggest that miR-8 ensures both a correct glial architecture and the spatiotemporal control of Spitz protein synthesis via direct binding to Spitz 3' UTR. Together, these results establish glial-derived cues as key regulatory elements in the control of neuroepithelial cell proliferation and the neuroblast transition.

Figure 1. The Conserved MicroRNA miR-8 Is Expressed in Glia

(A–D) miR-8 labels a subset of Repo (red) glial cells. (A) Whole-mount view of a L3 brain and (B–D) details of L1, L2, and L3 brains from flies carrying mir-8-Gal4 driving a nuclear UAS-H2B::RFP (green). The neuroepithelial cells (NE) are labeled by DE-Cadherin (DE-Cad, blue). (E–H) Magnification and serial optical sections from anterior (F) to posterior (H) showing miR-8⁺ cortex glia structure visualized by mCD8::GFP (green). NE is labeled for DE-Cad (blue). (I) Scheme to generate GFP-labeled mir-8-Gal4-positive glial clones in hsp70-FLP/+; mir-8-Gal4/UAS (FRT) CD2, y+ (FRT) mCD8::GFP. (J–L) Examples shown are the distinct classes of FLP-out clones recovered. (J) Single cell clones (glial nuclei visualized by Repo label) of surface-associated glia overlying the NE. (K–L) Single or few cell-clones of optic-lobe-associated cortex glia (K) and central brain cortex glia that ensheath neuronal cell bodies (K) and neuroblast (arrowheads, L). CB, central brain; IPC, inner proliferation center; OL, optic lobe; OPC, outer proliferation center. Scale bars represent 50 μm (A, F–H) and 20 μm (B–E, J–L). See also Figure S1.

Figure 2. Surface-Associated miR-8 Glia Layer Lies Underneath the Subperineurial Glia

(A–C) Images show miR-8⁺ cells (mir-8>H2B::RFP, A or mCD8::GFP, B–C) constitute a distinct class of surface-associated glia from that of the sub-perineurial glia as is visualized by Moody-β antibody (green, B and C). Arrowhead points to miR-8⁺ glia and arrow to subperineurial (miR-8⁻ Moody⁺) glia. (D–F) Subperineurial glial membranes are labeled by moody-Gal4 driving UAS-DsRed (red, arrow) and glia nuclei by Repo. Single (E) and double channel confocal image (Repo and moo-dy>DsRed: F) are shown. (G) Expression pattern of mir-8>DsRed in an adult medulla cortex (MeC). Arrowheads point cortex glia. (H) Expression pattern of moody>DsRed in an adult medulla cortex. Arrows point to subperineurial glia. Neuropil and glia in (G) and (H) are visualized by DN-Cad (blue) and Repo (green). Insets show whole-mount views of adult optic lobes. (I) Schematic overview of the three layers of surface-associated in a L3 brain lobe: miR-8⁺ (this study), subperineurial (SPG) and perineurial (PG) glial layers as single cells and highlighted in different colors. NE, neuroepithelial cells. Scale bar represents 20 μm. See also Figure S2.

Figure 3. Glial miR-8 Expression Controls Neuroepithelial Expansion

(A–C) Whole-mount views of L3 brains in WT (A), mir-8^Δ2 (B) and animals overexpressing mir-8 by repo-Gal4 (C). The NE are visualized by DE-Cad (blue) and Patj (green) and the location of neuroblasts by Mira (red). (B) Larger neuroepithelium in mir-8^Δ2 brains (arrowheads) and precocious neuroblast transitions (arrows). (C) Image illustrates the smaller neuroepithelial size (as measured in E) in animals overexpressing mir-8 by repo-Gal4. (D) Glia-specific depletion of mir-8 by expressing two copies of UAS-mir-8-SP::GFP with mir-8-Gal4. (E) Histogram shows NE area of the indicated genotypes measured using ImageJ (n = 13 [repo >], n = 14 [mir-8^Δ2] and n = 12 [repo>mir-8] brain lobes scored). Shown are mean ± SEM and p values were calculated by two-tailed unpaired t test. Scale bar represents 50 μm. See also Figure S3.

Figure 4. Structure of the Surface Glia and Their Effects on the Neuroepithelium

(A) Histogram represents the means ± SEM of the glial nuclear area of wild-type perineurial (red bar), Moody⁺ subperineurial (green), miR-8⁺ surface cortex glia (dark blue), and inner glia (gray); the glial nuclear area of non-miR-8 surface (green), miR-8 surface cortex glia (dark blue) and inner (gray) glia upon mir-8 overexpression, and the glial nuclear area of Moody-subperineurial glia (green) and inner (gray) glia upon dup/cdt1 depletion by RNAi (dup-IR) by repo-Gal4. (B and C) Illustrative examples of 3D reconstructions of whole-mount WT (B) and repo>mir-8 (C) L3 brains. (D–G) Changed size in glia cells of control WT (D–E) and repo>mir-8>CD8::GFP (F–G) L3 brain lobes. Glia nuclei were visualized with Repo (D, F, red) and neuroepithelia stained with DE-Cad (D, F) and Dlg (E, blue). Arrowheads in (E) and (G) point to miR-8⁺ glia (green). Subperineurial (SPG) glia are visualized by Moody (red, arrow in E and G). (H) APC^Fzr/Cdh1-lacZ (green) labels a subset of glial cells that by position agrees with miR-8⁺ glia. (I) miR-8⁺ glia (mir-8>H2B::RFP, green) are always negative for pH3 mitotic marker (red). (J and K) Depletion of dup by RNAi (dup-IR) in repo cells. Arrowheads in (J) point to disrupted surface glia, which are all positive for Moody (red; arrow in K). (L–O) The glial undergrowth defect of repo>dup-IR is rescued by mir-8 overexpression (repo>dup-IR>mir-8). Glial nuclei were visualized by Repo (red; L, M), the NE by Patj (green, N, O) and DE-Cad (blue, L–O) and neuroblasts by Mira (red, N and O). CB, central brain; IPC, inner proliferation center; OPC, outer proliferation center; VNC, ventral nerve cord. Scale bars represent 100 μm (B and C), 50 μm (D, F, H–J, L), and 20 μm (E, G, K, M–O). See also Figure S4 and Table S1.

Figure 5. Optic-Lobe-Associated Cortex Glia Is a Source of the EGFR Ligand Spitz

(A–D) Whole brain (A and C) and a magnification of an area (B and D) from a L3 WT (A and B) and homozygous aos^Δ7 animals (C and D). The NE is visualized by DE-Cad (blue) and neuroblasts by Mira (red). (E–G) Enhancer-traps showing the transcription of aos-lacZ (E), rho-lacZ (F) (green staining) occurs in glia (Repo⁺, red). Coexpression of spi-lacZ (green) and mir-8>H2B::RFP (G) in optic lobe-associated cortex glia. Scale bars represent 50 μm (A and C) and 20 μm (B, D, E–G). See also Figure S5.

Figure 6. Surface Cortex Glia-Derived Spitz Promotes Neuroepithelial Expansion and Neuroblast Generation

(A and B) L3 brains shown are of larvae raised at 18°C (A), or shifted to 31°C during the early to late L3 larval stage (B) to drive expression of sSpi::GFP by mir-8-Gal4. (C) Inhibition of endogenous spi in miR-8 glia by UAS-spi-IR. NE, neuroepithelial cells; Nbs, neuroblasts. Scale bar represents 50 μm. See also Figure S6.

Figure 7. Spitz Is a Direct Target of miR-8

(A) Computer prediction of miR-8-binding site in the spi 3′ UTR (seed sequence in red). (B) Luciferase assay in S2 cells cotransfected with tub-mir-8 or the tub-empty vector, together with a firefly luciferase vector containing the spi 3′ UTR (tub-luc::spi-3′UTR^wt) or a luciferase vector with mutations (tub-luc::spi-3′UTR^mut) in the seed sequence (underlined bases in A). (C and D) Wing discs showing overexpression of UAS-mir-8 and UAS-DsRed by en-Gal4 (red) and of GFP in a tub-eGFP::spi-3′UTR^wt (C) and mutated tub-eGFP::spi-3′UTR^mut (D) sensors. (E) Histogram shows the relative mRNA levels of spi determined by qRT-PCR in three independent experiments (mean + SEM) in the brains of controls, mir-8^Δ2 mutants and flies expressing mir-8>sSpi::GFP and tub-Gal80^ts. (F and G) Rescue of the effects of mir-8 overexpression by a Spi transgene lacking its 3′ UTR. Flies reared at 18°C (F) or shifted to 31°C during the early to late L3 larval stage (G). (H and I) NE and neuroblast transitions in controls (H) and in larvae expressing repo>mir-8>spi-IR (I). Glial cells (Repo⁺, green) and NE cells [(DE-Cad⁺, blue) or (Patj⁺, yellow)]. DAPI counterstaining highlights all nuclei. Scale bar represents 20 μm (F–I).

Figure 8. Intrinsic Alteration of EGFR Signaling Mimics the Defects Induced by Altered miR-8 Expression

(A and B) Control (A) and c855a-Gal4>EGFR^Act (B) neuroepithelia stained with Dlg (blue) and with the neuroblast transition markers L(1)sc(red) and Mira (green). (C and D) c855a-Gal4 MARCM GFP (green) control clones (C) and those overexpressing UAS-Ras^V12 (D). Neuroblasts are visualized by Mira (red) and NE by DE-Cad (blue). (E) Intrinsic inactivation of EGFR signaling via EGFR^DN. (F) Brains in which Notch receptor is inactivated via RNAi by c855a-Gal4 and stained with Dlg (blue), L(1)sc (red), and Mira (green). (G) Model of miR-8-Spitz regulatory interactions and glial-derived Spitz in coordinating neuroepithelial expansion and neuroblast transition. miR-8 increases, in an autocrine manner, cortex glial cellular size that facilitates the formation of long cellular processes that ensheath the underlying neuroepithelium. miR-8 also negatively regulates, in a paracrine manner, neuroepithelial growth, and neuroblast transition via direct inhibition of Spitz protein translation. Blocking endoreplication and/or loss of miR-8 in cortex glia releases miR-8 inhibition of Spitz levels, thereby resulting in inappropriate or enhanced EGFR signaling. Feedback signaling via EGFR may also stimulate the expression of rho and aos in miR-8⁺ positive cortex glia, thereby contributing to the fine-tuning of Spitz protein activation and secretion. Glial-to-neuroepithelial cells signaling via secondary signal (X) is not ruled out. Scale bar represents 20 μm. See also Figure S7.