Integration of GPCR Signaling and Sorting from Very Early Endosomes via Opposing APPL1 Mechanisms

Abstract

Endocytic trafficking is a critical mechanism for cells to decode complex signaling pathways, including those activated by G-protein-coupled receptors (GPCRs). Heterogeneity in the endosomal network enables GPCR activity to be spatially restricted between early endosomes (EEs) and the recently discovered endosomal compartment, the very early endosome (VEE). However, the molecular machinery driving GPCR activity from the VEE is unknown. Using luteinizing hormone receptor (LHR) as a prototype GPCR for this compartment, along with additional VEE-localized GPCRs, we identify a role for the adaptor protein APPL1 in rapid recycling and endosomal cAMP signaling without impacting the EE-localized β2-adrenergic receptor. LHR recycling is driven by receptor-mediated Gαs/cAMP signaling from the VEE and PKA-dependent phosphorylation of APPL1 at serine 410. Receptor/Gαs endosomal signaling is localized to microdomains of heterogeneous VEE populations and regulated by APPL1 phosphorylation. Our study uncovers a highly integrated inter-endosomal communication system enabling cells to tightly regulate spatially encoded signaling.

Keywords: APPL1; G-protein-coupled receptor; cAMP; endosome; recycling.

Graphical abstract

Figure 1

APPL1 Is Essential for LHR Recycling from VEEs (A) Confocal images of FLAG-LHR (green) and endogenous APPL1 (red) in cells with or without stimulation with LH (15 min). Scale bar, 5 μm; scale bar in inset, 1 μm. (B) Western blot of total cellular levels of APPL1 from cells treated with scramble or APPL1 siRNA. GAPDH was used as a loading control. (C and D) Flow cytometry analysis of cells expressing FLAG-LHR or FLAG-B2AR for ligand-induced internalization (15 min) (C) and recycling (1-hr ligand washout) (D) in cells treated with scramble or APPL1 siRNA. n = 4 independent experiments. ^∗p < 0.05; ^∗∗p < 0.01. (E) Recycling of SEP-LHR and SEP-B2AR was measured in real time, via TIR-FM, in cells treated with scramble or APPL1 siRNA 5 min after ligand addition. n = 16 cells per condition for LHR and 13 cells per condition for B2AR across at least 3 independent experiments. ^∗∗∗p < 0.001. (F) Confocal images of FLAG-LHR (red) and endogenous APPL1 (green) in primary hESCs with or without stimulation with LH (15 min). Ligand-treated cells were “stripped” by PBS/EDTA (to remove surface-bound FLAG antibody). Scale bars, 5 μm; scale bar in inset, 1 μm. (G) Western blot of total cellular levels of APPL1 from hESC lysates following transfection with scramble or APPL1 siRNA. GAPDH was used as loading control. (H) SEP-LHR recycling in hESCs following siRNA-mediated knockdown of APPL1 was analyzed as in (E) . n = 29 cells per condition collected across 3 independent experiments. ^∗∗∗p < 0.001. Data indicate mean ± SE. See also Figures S1–S3 and Movies S1 and S2.

Figure 2

APPL1-Dependent Recycling of LHR Is Driven by cAMP/PKA Signaling and APPL1 S410 (A) SEP-LHR recycling was measured in real time by TIR-FM in the presence of LH in HEK293 cells pre-treated with either DMSO, PKA inhibitor KT5720 (10 μM, 15 min), or PKA activator 8-Br-cAMP (0.5 mM, 15 min). n = 16 cells per condition collected across 3 independent experiments. ^∗p < 0.05; ^∗∗∗p < 0.001. (B) Confocal images of FLAG-LHR (green) and endogenous APPL1 (red) in cells stimulated with LH (15 min) with or without KT5720 pre-treatment (10 μM, 15 min). Scale bars in insets, 1 μm. (C) Quantification of LHR endosomes positive for endogenous APPL1 from (B). n = 15 cells per condition, collected across 3 independent experiments. ^∗∗p < 0.01. (D) Confocal images of FLAG-LHR (green) and either mCherry-WT, -S410A (S/A) or -S410D (S/D) APPL1 in cells stimulated with LH (15 min) Scale bars, 5 μm; scale bars in insets, 1 μm. (E) Quantification of (B) (endog) and (D) (WT, S/A, and S/D); n = 15 cells per condition, collected across 3 independent experiments. (F) Western blot analysis of total cellular levels of APPL1 from cells expressing SEP-LHR and transfected with mock (endog), siAPPL1 (-), siAPPL1 + mCherry-WT or mCherry-S410A (S/A), or mCherry-S410D (S/D) APPL1. GAPDH was used as loading control. (G) SEP-LHR recycling measured by TIR-FM in cells transfected as in (F). n ≥ 16 cells per condition imaged across at least 3 independent experiments. ^∗∗∗p < 0.001. Data indicate mean ± SE.

Figure 3

LHR Activation Induces PKA-Dependent Phosphorylation of APPL1, which Includes S410 Cells expressing FLAG-LHR were transfected with either WT or S/A GFP-APPL1 with or without stimulation with LH (5 and 15 min). Cells expressing WT GFP-APPL1 were also pre-treated with KT5720 (10 μM, 15 min). After collection of lysates, GFP-APPL1 was immunoprecipitated, and both phosphoserine and APPL1 levels were determined by western blot. (A) Representative immunoblot of phosphoserine (pSer) and total APPL1 (totAPPL1). (B) Densitometry analysis of APPL1 serine phosphorylation levels normalized to total APPL1. (C) Data are expressed as percentage of maximal response quantified (WT, 15 min LH). n = 3 independent experiments. ^∗p < 0.05; ^∗∗p < 0.01; ^∗∗∗p < 0.001. Data indicate mean ± SE. See also Figure S4.

Figure 4

APPL1 Negatively Regulates LH-Induced cAMP Production (A) Intracellular levels of cAMP measured in cells stably expressing FLAG-LHR following transfection with either scramble (control), APPL1 siRNA (siAPPL1), or APPL1 siRNA and mCherry-APPL1 WT (siAPPL1 + WT). Cells were either not stimulated or stimulated with LH (5 min). n = 3 independent experiments. ^∗∗∗p < 0.001. (B) Intracellular levels of cAMP were measured in cells stably expressing FLAG-B2AR following transfection with scramble or APPL1 siRNA and with or without stimulation with isoproterenol, (ISO; 5 min). n = 4 independent experiments. (C) Phosphorylation of ERK 1/2 was determined by western blot at stated time points after LH stimulation in FLAG-LHR cells treated with scramble or APPL1 siRNA. Total ERK was used as a loading control. (D) Densitometry analysis of ERK 1/2 phosphorylation was normalized to the 5-min control stimulation. n = 4 independent experiments. Data indicate mean ± SE. See also Figures S5 and S6.

Figure 5

Regulation of cAMP Signaling via the Phosphorylation Status of APPL1 (A) Intracellular levels of cAMP measured in cells expressing FLAG-LHR following stimulation with LH (5 min) following pre-treatment with either DMSO or KT5720 (10 μM, 15 min); n = 3. ^∗p < 0.05. (B) Intracellular levels of cAMP measured in FLAG-LHR cells following transfection with mCherry-WT (WT), S410A (S/A), or S410D (S/D) APPL1 and with or without stimulation with LH (5 min). n = 4 independent experiments. ^∗p < 0.05; ^∗∗p < 0.01. Data indicate mean ± SE.

Figure 6

LHR Activates Gαs/cAMP Signaling from VEEs Heterogenous for APPL1 (A) Intracellular levels of cAMP measured in cells expressing FLAG-LHR with or without stimulation with LH (5 and 15 min) and pre-treatment with either DMSO or Dyngo-4a (30 μM, 45 min). n = 4 independent experiments. ^∗∗∗p < 0.001. Data are expressed as cAMP levels normalized to 5 min of LH treatment (DMSO). (B) TIR-FM images of FLAG-LHR (red) and Nb37-GFP with or without stimulation with LH (15 min). Scale bars, 5 μm. (C) SIM images of FLAG-LHR (red) and Nb37-GFP following stimulation with LH. Scale bar, 5 μm. Inset shows the microdomain organization of Nb37 within individual LHR endosomes (scale bar, 500 nm). Line intensity analysis is shown for two endosomes (i and ii). Representative images are from n = 30 endosomes. (D) TIR-FM images of FLAG-LHR (red), Nb37-GFP (green), and endogenous APPL1 (pink) in cells stimulated with LH (5 or 15 min). Arrows indicate LHR endosome positive for Nb37 only; circles indicate LHR endosome positive for Nb37 and APPL1; squares indicate LHR endosome positive for APPL1 only. Scale bar, 1 μm. (E) Quantification of LHR endosomes positive for either APPL1 or Nb37 (i), LHR-Nb37 endosomes positive for APPL1 (ii), and LHR-APPL1 endosomes positive for Nb37 (iii) after 5 or 15 min of LH stimulation. n = 15 cells per condition from Figure 5D that were quantitated across 3 independent experiments. ^∗p < 0.05; ^∗∗p < 0.01; ^∗∗∗p < 0.001. Data indicate mean ± SE.