Gliocrine System: Astroglia as Secretory Cells of the CNS

Abstract

Astrocytes are secretory cells, actively participating in cell-to-cell communication in the central nervous system (CNS). They sense signaling molecules in the extracellular space, around the nearby synapses and also those released at much farther locations in the CNS, by their cell surface receptors, get excited to then release their own signaling molecules. This contributes to the brain information processing, based on diffusion within the extracellular space around the synapses and on convection when locales relatively far away from the release sites are involved. These functions resemble secretion from endocrine cells, therefore astrocytes were termed to be a part of the gliocrine system in 2015. An important mechanism, by which astrocytes release signaling molecules is the merger of the vesicle membrane with the plasmalemma, i.e., exocytosis. Signaling molecules stored in astroglial secretory vesicles can be discharged into the extracellular space after the vesicle membrane fuses with the plasma membrane. This leads to a fusion pore formation, a channel that must widen to allow the exit of the Vesiclal cargo. Upon complete vesicle membrane fusion, this process also integrates other proteins, such as receptors, transporters and channels into the plasma membrane, determining astroglial surface signaling landscape. Vesiclal cargo, together with the whole vesicle can also exit astrocytes by the fusion of multivesicular bodies with the plasma membrane (exosomes) or by budding of vesicles (ectosomes) from the plasma membrane into the extracellular space. These astroglia-derived extracellular vesicles can later interact with various target cells. Here, the characteristics of four types of astroglial secretory vesicles: synaptic-like microvesicles, dense-core vesicles, secretory lysosomes, and extracellular vesicles, are discussed. Then machinery for vesicle-based exocytosis, second messenger regulation and the kinetics of exocytotic vesicle content discharge or release of extracellular vesicles are considered. In comparison to rapidly responsive, electrically excitable neurons, the receptor-mediated cytosolic excitability-mediated astroglial exocytotic vesicle-based transmitter release is a relatively slow process.

Keywords: Astrocytes; Exocytosis; Fusion pore; Gliocrine system; Secretory vesicles.

Fig. 4.1

The vesicle network in astrocytes. Lysosomes, first described in 1955, represent a central, prominent intermediate of endo- and exocytotic pathways in all eukaryotic cells, including astroglia. Intracellular secretory organelles (synaptic-like vesicles, dense-core vesicles and primary lysosomes) originate from the endoplasmic reticulum and Golgi complex. Primary lysosomes fuse with endosomes, phagosomes and autophagosomes and convert to secondary lysosomes that undergo exocytosis, thus expelling products of degradation. The multivesicular bodies contain exosomes that may carry various signaling factors. Modified with permission [152]

Fig. 4.2

Secretory vesicles studied by STED and SIM microscopies in acutely isolated rat astrocytes. a Confocal and STED microscopy images of immunostained vesicles d-serine-, V-GLUT1-, ANP- and BDNF-positive vesicles in acutely isolated astrocytes. Histograms display STED-acquired vesicle diameter distributions for 1788 (d-serine), 6787 (V-GLUT1), 1747 (ANP) and 798 (BDNF) vesicles (2 cells per staining). The black curves show Gaussian fits of the diameter distributions; the numbers next to the distribution peaks indicate the average vesicle diameter (expectation value ± SEM). Recalculated values taking into account the microscope’s optical resolution (45 nm) are 80.8 nm for d-serine, 88.4 nm for V-GLUT1, 85.9 nm for ANP and 86.8 nm for BDNF. Scale bar, 500 nm. b Wide-field microscopy and SIM were used to determine the vesicle diameter of immunostained LAMP1 endolysosomes and ATP-loaded vesicles (quinacrine dihydrochloride). Histograms show SIM-acquired vesicle diameter distributions for 557 (LAMP1, 2 cells) and 445 (quinacrine, 2 cells) vesicles in acutely isolated astrocytes (upper two panels) and 338 (LAMP1, 3 cells) and 333 (quinacrine, 6 cells) vesicles in astrocytes isolated from 7- to 8-week-old rats (lower two panels). The black curves show Gaussian fits of the diameter distributions; the average vesicle diameter (expectation value ± SEM) is labeled next to the distribution peaks. Scale bar, 500 nm. Modified with permission [39]

Fig. 4.3

Internalization of exosomes into astrocytes. Internalization of PKH26 nanoparticles and PKH26-positive particles of the exosome-containing samples into subcellular compartments of cultured astrocytes. a, b Representative three-dimensional shaded display of individual live cultured astrocytes that internalized PKH26 nanoparticles (a, con) and PKH26-positive particles present in the PKH26-labeled exosome-containing samples (b, exo) into intracellular compartments, observed as numerous bright fluorescent puncta. Scale bars, 10 μm. Modified with permission [112]

Fig. 4.4

Slowness of astroglial exocytosis. a Neuronal versus astrocytic SNAREs. Neurones and astrocytes alike express SNAREs VAMP2 and syntaxin 1; many astrocytes can also express VAMP3 in lieu of or in addition to VAMP2. Astrocytes express SNAP23, a homologue of neuronal SNAP25. At the plasma membrane, syntaxin 1A can form a binary cis complex with SNAP25B or SNAP23A, which then interacts with vesicular VAMP2 to form a ternary complex. A single ternary complex can tether the vesicle at the plasma membrane for a longer period of time, when it contains SNAP25B rather than SNAP23A, respectively. Of note, truncated syntaxin 1, lacking the N-terminal Habc domain and the linker region to the SNARE domain, is shown for simplicity. Drawings are not to scale. b Comparison of kinetics of neuronal and astroglial exocytosis. Time-dependent changes in membrane capacitance (Cm) recorded in a neuronal cell (trace in red, photoreceptor) and an astrocyte (trace in blue), elicited by a flash photolysis-induced increase in cytosolic Ca²⁺. Note that the blue trace recorded in an astrocyte displays a significant delay between the stimulus (asterisk) and the response (trace components above the dotted line). Modified with permission [139]