TMEM16F Regulates Spinal Microglial Function in Neuropathic Pain States

Abstract

Neuropathic pain is a widespread chronic pain state that results from injury to the nervous system. Spinal microglia play a causative role in the pathogenesis of neuropathic pain through secretion of growth factors and cytokines. Here, we investigated the contribution of TMEM16F, a protein that functions as a Ca(2+)-dependent ion channel and a phospholipid scramblase, to microglial activity during neuropathic pain. We demonstrate that mice with a conditional ablation of TMEM16F in microglia do not develop mechanical hypersensitivity upon nerve injury. In the absence of TMEM16F, microglia display deficits in process motility and phagocytosis. Moreover, loss of GABA immunoreactivity upon injury is spared in TMEM16F conditional knockout mice. Collectively, these data indicate that TMEM16F is an essential component of the microglial response to injury and suggest the importance of microglial phagocytosis in the pathogenesis of neuropathic pain.

Graphical abstract

Figure 1

TMEM16 Expression and Generation of Conditional TMEM16F Knockout Mice (A) RT-PCR analysis of TMEM16 transcripts in microglia, dorsal root ganglia (DRGs), kidney, and brain; p < 0.0001; n = 3. (B) Representative images of injured sciatic nerve from indicated genotypes, 7 days after injury. (C) Iba1 (green) and RFP (red) immunofluorescence in the lumbar spinal cord LysM^Cre/Rosa26^tdRFP mouse, 3 days after injury. (D) Iba1 (green) and RFP (red) immunofluorescence of microglia in the hippocampus from LysM^Cre/Rosa26^tdRFP mouse. (E) In situ hybridization for TMEM16F (red) and Iba1 (green) immunofluorescence of microglia in spinal cord, 7 days after injury. (F) High-magnification representative images of TMEM16F in situ hybridization in control and cKO mice. (G) In situ hybridization for TMEM16F in peritoneal macrophages cells from control and cKO mice. Values are mean ± SEM. Scale bars, 100 and 40 μm (B), 100 μm (C), 20 μm (D, F, and G), 300 μm (E). See also Figure S1.

Figure 2

Microglial TMEM16F Is Required for the Development of Mechanical Hypersensitivity in the PNL Neuropathic Pain Model (A) Paw withdrawal thresholds of TMEM16F^fl/fl (control) and LysM^cre/TMEM16F^fl/fl(cKO) mice showing ipsilateral (ipsi) and contralateral (contra) paw withdrawals before and after partial nerve ligation (PNL). n = 9; p < 0.05. (B) From left to right: microglial Iba1 immunofluorescence in the lumbar spinal cord after injury (left side), magnified images from control and cKO tissues 2 days after PNL. Bar graphs show microglial densities in injured (PNL) and non-injured (sham) tissue 2 (left) and 7 (right) days after injury. n = 8/9; p < 0.05. (C) Neuronal marker NeuN immunofluorescence in the ipsilateral dorsal horn from control (top) and cKO (bottom) mice; bar graph showing neuronal density in the imaged volume. n = 10/7; p < 0.05. (D) CD68 (red) and Iba1 (green) immunofluorescence in the ipsilateral dorsal horn from control and cKO mice. CD68 immunoreactivity in Iba1 positive cells at 2 and 7 days after injury. n = 6; p < 0.01. (E) Top: Iba1 (red) and DAPI (blue) immunolabeling in the ipsilateral dorsal horn; 3D image segmentation using IMARIS Bitplane surface (middle) and filament (bottom) algorithms. (F) Frequency distribution of microglial volume in the ipsilateral dorsal horn from control and cKO mice, n = 79/277 cells; p = 0.005. (G) Frequency distribution of microglial total branch length in the ipsilateral dorsal horn from control and cKO mice; p = 0.0293. Values are mean ± SEM. Scale bars represent 100 μm (B), 30 μm (C), 20 μm (D), 40 μm, and 7 μm (E). See also Figures S2 and S3.

Figure 3

Histological and Biochemical Analysis of Macrophages Recruited at the Nerve Injury (A) Iba1 (red), CD68 (green), and DAPI (blue) immunofluorescence in the ipsilateral nerve cryosections from control and cKO mice, 7 days after injury. Scale bar, 20 μm. (B) Density of Iba1 positive macrophages at the distal side of the nerve injury at 3 and 7 days after injury. n = 3, p < 0.05. (C) Size of Iba1 positive macrophages on the distal side of the nerve injury at 3 and 7 days after injury; n = 3; p < 0.05. (D) CD68 intensity in Iba1 positive macrophages, 7 days after PNL; n = 3; p > 0.05. (E) Cytokine/chemokine array blots incubated with contralateral and ipsilateral sciatic nerve extracts (130 μg) from control and cKO mice, 4 days after injury. (F) Pixel densities of selected cytokine spots in the contralateral and ipsilateral sciatic nerve lysates from control mice. p < 0.05, p < 0.01. n = 3 sets of pooled samples from four mice each. (G) Pixel densities of selected cytokine spots in the ipsilateral sciatic nerve lysates from control and cKO mice 4 days after injury, p > 0.05. Values are mean ± SEM.

Figure 4

TMEM16F Regulates Microglia Branch Motility and Engulfment of Neuronal Material (A) Top: fluorescence changes in a Control^GFP hippocampal slice near the tip (green squares). Bottom: relative fluorescence increase measured in a 10-μm radius from the ATP-containing (3 mM) pipette in acute hippocampal slices from mice of indicated genotypes, n = 9/24 fields in three of seven mice; p < 0.05 from 25 to 34 min. (B) Top: ipsilateral dorsal horn sections from mice of indicated genotypes. The selected segmented GFP positive microglia (yellow) is surrounded by a bounding box (red), which was used for branch motility analysis. Bottom: SD of the bounding box from its mean value over time to quantify microglia branch motility in spinal cord slices, from mice of indicated genotypes.; n = 44/45 cells; p < 0.0001. (C) Segmentation of GFP positive microglia (green) and Alexa-647-labeled neurons, external to microglia (blue) and internalized (pink). Ratio of internalized over total labeled neuronal material to quantify engulfed neuronal terminals by microglia over time. (D) Top: image segmentation: GABA positive neuron (red) colocalizes (white) within Iba1 positive microglia (green) after injury. Bottom: Iba1 (green) and GABA (red) immunofluorescence in the ipsilateral and contralateral dorsal horns from control and cKO mice, 3 days after injury, with zoomed images from lamina I–III in the insets. (E) Density of GABA-positive neurons in the lamina I–III of the dorsal horn of control and cKO mice, 3 days after injury. p < 0.05, n = 5. Values are mean ± SEM. Scale bar, 50 μm (B and C) and 60 μm (D). See Movies S1, S2, S3, and S4.