Mitochondrial Populations Exhibit Differential Dynamic Responses to Increased Energy Demand during Exocytosis In Vivo

Abstract

Mitochondria are dynamic organelles undergoing fission, fusion, and translocation. These processes have been studied in cultured cells; however, little is known about their regulation in cells within tissues in vivo. We applied four-dimensional intravital microscopy to address this in secretory cells of the salivary gland. We found that mitochondria are organized in two populations: one juxtaposed to the basolateral plasma membrane and the other dispersed in the cytosol. Under basal conditions, central mitochondria exhibit microtubule-dependent motility and low fusion rate, whereas basolateral mitochondria are static and display high fusion rate. Increasing cellular energy demand by β-adrenergic stimulation of regulated exocytosis selectively enhanced motility and fusion of central mitochondria. Inhibition of microtubule polymerization led to inhibition of central mitochondrial motility and fusion and a marked reduction in exocytosis. This study reveals a conserved heterogeneity in mitochondrial positioning and dynamics in exocrine tissues that may have fundamental implications in organ pathophysiology.

Keywords: Cell Biology; Functional Aspects of Cell Biology; Optical Imaging.

Graphical abstract

Figure 1

Exocytosis Is an Energy-Dependent Process (A) 4D-IVM of regulated exocytosis induced by injection of 10 μg/kg ISO in cytosolic-GFP mice. Arrowheads indicate granules undergoing exocytosis. (B) Number of exocytic events per frame (Video S1). (C) Single granule undergoing exocytosis. (D) SGs in cytosolic-GFP mice treated for 20 min with DMSO, 10 μM rotenone, or 1 μg/mL oligomycin followed by saline or ISO injection (30 min). Arrowheads indicate fused granule; arrows indicate large vacuoles. (E) Exocytosis was evaluated by plotting the normalized number of granules, expressed as percent of the number of granules under basal conditions, 30 min after stimulation under the various treatments. (F) Fold change in pAMPK/total AMPK 5 min after ISO stimulation. Results are shown as mean ± SEM in three experiments; *p < 0.05, **p < 0.005; unpaired t test. Scale bar, 5 μm.

Figure 2

Mitochondrial Distribution in SGs (A) Optical section of an acinus from cytosolic-GFP/mTom mouse (green and red, respectively) labeled with MitoTracker (white). Arrowheads show mitochondria along the PM (Videos S2 and S3). (B) Probability ratio was calculated as described in Transparent Methods. (C) Mitochondria-positive pixels are plotted as a function of distance from the PM. Mitochondria within 2 μm from the PM are labeled in blue and the rest in yellow. (D) Diagram of an acinar cell illustrating color-coded mitochondrial distribution. (E) Immunofluorescence labeling of NKCC1 (white, BL) and AQP5 (red, apical) in Mito-Dendra2 mouse (green) (Video S4). (F) FIB-SEM of mitochondrial distribution and connectivity. Left, section of an acinar cell; center, segmentation of mitochondria (blue and yellow) and nuclei (green) (Video S5); right, volume rendering of the stack (Video S6). Results are shown as mean ± SEM for N number of cells in three mice, *p < 0.05 unpaired t test. Scale bar, 5 μm.

Figure 3

Mitochondrial Dynamics under Basal Conditions (A–F) 4D-IVM of mitochondria labeled with MitoTracker (white) in cytosolic-GFP (green) mice, either untreated (A–C and Video S7) or treated with 33 μm nocodazole (NZ; D–F and Video S8). Time in min: sec. Dashed red line marks outlines of cells. Insets at different time points over 3 min. (B and E) Temporal color coding and projection of whole frames and insets (last inset at the bottom; temporal projection of static mitochondria will show white mitochondria, whereas moving mitochondria will show color). Red and white arrowheads point CEN and static BL mitochondria, respectively. (C) Quantification of BL (blue) and CEN (yellow) mitochondrial motility (Figures S2A–S2G). (F) BL (blue) and CEN (yellow) motility in the presence of NZ. Representative experiments are shown as mean ± SEM for N mitochondrial clusters; significance was calculated using unpaired t test *p < 0.05; ns, p > 0.05. Scale bar, 5 μm. (G–J) Photo conversion of CEN and BL mitochondria in Mito-Dendra2 mice. (G) Converted areas are marked by a white rectangle, and insets are shown for each population. (H) Converted fluorescence is shown as volumes (CEN in yellow and BL in blue) or merged image of converted (red) and MitoDendra2 (white). Arrowheads show mitochondria containing converted Dendra2 outside of the original converted region (white line). Time is shown in hours: min (see Video S10). (I and J) Fold change in fusion over time in CEN (I; yellow) and BL (J; blue) mitochondria. (K) Table summarizing mitochondrial dynamics under basal conditions. Scale bar, 5 μm.

Figure 4

Increased Motility and Volume in CEN Mitochondria during Exocytosis (A) 4D-IVM of mitochondria labeled with MitoTracker (white), under basal and ISO stimulation (Video S11). Maximal projection images shown; red dotted lines highlight basolateral membrane, along which BL mitochondria align. Right panels show temporal color coding as described in Transparent Methods. (B and C) (B) Fold change in BL (blue) and CEN (yellow) motility following stimulation of exocytosis (C). Photo conversion of CEN or BL mitochondria (white rectangle) in Mito-Dendra2 mice. (D) Fold change in CEN mitochondrial fusion 5 min before (orange) and 15 min after (red) ISO stimulation. Right y axis, number of exocytic events, quantified using cytosolic-GFP mice (white circles). (E) Converted CEN mitochondria from (C) before (0:15) and after (10:34) ISO stimulation. Arrowheads show CEN mitochondrial fusion (Video S12). (F) Fluorescence intensity (red: converted Dendra2; green: non-converted Dendra2) in line scan from (E) before (0:15) and after (10:34) ISO stimulation. (G) Fold change in BL mitochondrial fusion 5 min before (orange) and 15 min after (red) stimulation with ISO. (H) Fold change in CEN mitochondrial fusion calculated by comparing CEN mitochondria converted Dendra2 volume before and 30 min after ISO (see Transparent Methods). (I) Table summarizing mitochondrial motility and fusion under basal and stimulated conditions. (J–M) Effect of NZ treatment on CEN mitochondrial dynamics and exocytosis. (J) Fold change in BL (blue) and CEN (yellow) motility following ISO in NZ-treated mice (33 μM). (K) Inset of converted CEN mitochondria from NZ-treated mice (Video S13) are shown before (0:15) and after (10:34) ISO showing inhibition of CEN mitochondrial fusion. (L) Fold change in CEN mitochondrial fusion calculated by comparing CEN mitochondria converted Dendra2 volume, in NZ-treated mice, before and 30 min after ISO (see Transparent Methods). (M) SGs in cytosolic-GFP mice were treated with DMSO or 33 μM NZ followed by injection of saline or ISO. Exocytosis was evaluated by plotting the normalized number of granules, expressed as percent of the number of granules under basal conditions, 30 min after stimulation. Representative experiments are shown as mean ± SEM for N number of mitochondrial clusters (B and J) or mice (H, L, and M). Significance was calculated using unpaired t test *p < 0.05; ***p < 0.0005; ns, p > 0.05. Scale bar, 5 μm.