Astrocytic vesicle mobility in health and disease

Abstract

Astrocytes are no longer considered subservient to neurons, and are, instead, now understood to play an active role in brain signaling. The intercellular communication of astrocytes with neurons and other non-neuronal cells involves the exchange of molecules by exocytotic and endocytotic processes through the trafficking of intracellular vesicles. Recent studies of single vesicle mobility in astrocytes have prompted new views of how astrocytes contribute to information processing in nervous tissue. Here, we review the trafficking of several types of membrane-bound vesicles that are specifically involved in the processes of (i) intercellular communication by gliotransmitters (glutamate, adenosine 5'-triphosphate, atrial natriuretic peptide), (ii) plasma membrane exchange of transporters and receptors (EAAT2, MHC-II), and (iii) the involvement of vesicle mobility carrying aquaporins (AQP4) in water homeostasis. The properties of vesicle traffic in astrocytes are discussed in respect to networking with neighboring cells in physiologic and pathologic conditions, such as amyotrophic lateral sclerosis, multiple sclerosis, and states in which astrocytes contribute to neuroinflammatory conditions.

Figure 1

Distinct types of mobility of pro-ANP-Emd labeled vesicles. (A) Magnification of part of a rat astrocyte with fluorescently labeled vesicles. Bar: 2.5 μm; (B) An example of a two-dimensional Gaussian curve fitted to a single vesicle [see area outlined in (A)]. Lower panel shows a single vesicle as nine bright pixels. Upper panel presents a schematic view of the two-dimensional Gaussian curve fitted to the pixel intensity distribution representing a vesicle. The pixel intensities are gray and the surface of the two-dimensional Gaussian curve is white; (C) Vesicles analyzed with ParticleTR software; (C) Examples of two modes of vesicle mobility. Vesicles 2 and 3 show directional mobility, whereas vesicles 1 and 4 have nondirectional mobility. The line on vesicle 3 depicts the maximal displacement that the vesicle attained in the observation time of 15 s. Reproduced with permission from [19].

Figure 1

Distinct types of mobility of pro-ANP-Emd labeled vesicles. (A) Magnification of part of a rat astrocyte with fluorescently labeled vesicles. Bar: 2.5 μm; (B) An example of a two-dimensional Gaussian curve fitted to a single vesicle [see area outlined in (A)]. Lower panel shows a single vesicle as nine bright pixels. Upper panel presents a schematic view of the two-dimensional Gaussian curve fitted to the pixel intensity distribution representing a vesicle. The pixel intensities are gray and the surface of the two-dimensional Gaussian curve is white; (C) Vesicles analyzed with ParticleTR software; (C) Examples of two modes of vesicle mobility. Vesicles 2 and 3 show directional mobility, whereas vesicles 1 and 4 have nondirectional mobility. The line on vesicle 3 depicts the maximal displacement that the vesicle attained in the observation time of 15 s. Reproduced with permission from [19].

Figure 2

Single vesicle mobility tracks. (A) Displacement from the origin versus time for a directional vesicle. The first quarter of the tracking vesicle’s mobility from the origin consists of almost constant displacements (a to b) and during this time the vesicle remains close to the origin of tracking [see inset (A)]. In the next third of the tracking time (b–d1), vesicle displacements increase rapidly in a preferential direction with a brief pause (c). After a short period with equal displacements (d1–d2), the vesicle seems to move backwards, however, apparently not on the same track (d2–e1 and inset); (B) The displacement from the origin for a nondirectional vesicle. Minor mobility was observed and the vesicle did not translocate far from the origin of tracking (see inset). The mean square displacement (MSD) shown in (C,D) was calculated according to the equation MSD = [d(t) − d(t + Δt)]²; (C) The MSD of directional vesicles. The dashed line represents a linear function fitted to the data using an equation with the form MSD (μm²) = (0.4702 ± 0.0099) × time (s). The upwardly curving line represents a quadratic function fitted to the data following the equation MSD (μm²) = (0.2189 ± 0.0148) × time (s) + (0.0221 ± 0.0013) × time² (s²); (D) The MSD of nondirectional vesicles. The linear function was fitted to the data following the equation MSD (μm²) = (0.0038 ± 0.0001) × time (s). Bar: 2.5 μm. Reproduced with permission from [19].

Figure 3

The IFN-γ-induced increase in the mobility of MHC-II compartments in astrocytes is IF dependent. (A) Alexa Fluor 546-dextran labels MHC-II–positive compartments in IFN-γ-treated WT and GFAP^−/−Vim^−/− (IF-deficient) primary mouse astrocytes. Fluorescence images of astrocytes labeled with dextran, fixed, and immunostained with antibodies against MHC-II molecules. White pixels (Mask) represent the colocalization mask of green (MHC-II) and red fluorescence pixels (Dextran). Scale bars: 10 μm; (B) Histogram of average vesicle track lengths in control (Ctrl.) and IFN-γ-treated (+IFN-γ) WT and GFAP^−/−Vim^−/− cells; (C) Histogram of the mean maximal displacements of vesicles in control (Ctrl.) and IFN-γ-treated (+IFN-γ) WT and GFAP^−/−Vim^−/− cells. Numbers on the bars are the numbers of vesicles analyzed. Values are mean ± SEM. ^*p < 0.05. Adapted with permission from [21].