Tina Pangršič

Fusion-related release of glutamate from astrocytes

Although cell culture studies have implicated the presence of vesicle proteins in mediating the release of glutamate from astrocytes, definitive proof requires the identification of the glutamate release mechanism and the localization of this mechanism in astrocytes at synaptic locales. In cultured murine astrocytes we show an array of vesicle proteins, including SNARE proteins, and vesicular glutamate transporters that are required to fill vesicles with glutamate. Using immunocytochemistry and single-cell multiplex reverse transcription-PCR we demonstrate the presence of these proteins and their transcripts within astrocytes freshly isolated from the hippocampus. Moreover, immunoelectron microscopy demonstrates the presence of VGLUT1 in processes of astrocytes of the hippocampus. To determine whether calcium-dependent glutamate release is mediated by exocytosis, we expressed the SNARE motif of synaptobrevin II to prevent the formation of SNARE complexes, which reduces glutamate release from astrocytes. To further determine whether vesicular exocytosis mediates calcium-dependent glutamate release from astrocytes, we performed whole cell capacitance measurements from individual astrocytes and demonstrate an increase in whole cell capacitance, coincident with glutamate release. Together, these data allow us to conclude that astrocytes in situ express vesicle proteins necessary for filling vesicles with the chemical transmitter glutamate and that astrocytes release glutamate through a vesicle- or fusion-related mechanism.

Exocytotic release of ATP from cultured astrocytes

Astrocytes appear to communicate with each other as well as with neurons via ATP. However, the mechanisms of ATP release are controversial. To explore whether stimuli that increase [Ca2+]i also trigger vesicular ATP release from astrocytes, we labeled ATP-containing vesicles with the fluorescent dye quinacrine, which exhibited a significant co-localization with atrial natriuretic peptide. The confocal microscopy study revealed that quinacrine-loaded vesicles displayed mainly non-directional spontaneous mobility with relatively short track lengths and small maximal displacements, whereas 4% of vesicles exhibited directional mobility. After ionomycin stimulation only non-directional vesicle mobility could be observed, indicating that an increase in [Ca2+]i attenuated vesicle mobility. Total internal reflection fluorescence (TIRF) imaging in combination with epifluorescence showed that a high percentage of fluorescently labeled vesicles underwent fusion with the plasma membrane after stimulation with glutamate or ionomycin and that this event was Ca2+-dependent. This was confirmed by patch-clamp studies on HEK-293T cells transfected with P2X3 receptor, used as sniffers for ATP release from astrocytes. Glutamate stimulation of astrocytes was followed by an increase in the incidence of small transient inward currents in sniffers, reminiscent of postsynaptic quantal events observed at synapses. Their incidence was highly dependent on extracellular Ca2+. Collectively, these findings indicate that glutamate-stimulated ATP release from astrocytes was most likely exocytotic and that after stimulation the fraction of quinacrine-loaded vesicles, spontaneously exhibiting directional mobility, disappeared.

Bassoon and the synaptic ribbon organize Ca2+ channels and vesicles to add release sites and promote refilling

At the presynaptic active zone, Ca²+ influx triggers fusion of synaptic vesicles. It is not well understood how Ca²+ channel clustering and synaptic vesicle docking are organized. Here, we studied structure and function of hair cell ribbon synapses following genetic disruption of the presynaptic scaffold protein Bassoon. Mutant synapses--mostly lacking the ribbon--showed a reduction in membrane-proximal vesicles, with ribbonless synapses affected more than ribbon-occupied synapses. Ca²+ channels were also fewer at mutant synapses and appeared in abnormally shaped clusters. Ribbon absence reduced Ca²+ channel numbers at mutant and wild-type synapses. Fast and sustained exocytosis was reduced, notwithstanding normal coupling of the remaining Ca²+ channels to exocytosis. In vitro recordings revealed a slight impairment of vesicle replenishment. Mechanistic modeling of the in vivo data independently supported morphological and functional in vitro findings. We conclude that Bassoon and the ribbon (1) create a large number of release sites by organizing Ca²+ channels and vesicles, and (2) promote vesicle replenishment.

Properties of Ca2+-dependent exocytosis in cultured astrocytes

Astrocytes, a subtype of glial cells, have numerous characteristics that were previously considered exclusive for neurons. One of these characteristics is a cytosolic [Ca2+] oscillation that controls the release of the chemical transmitter glutamate and atrial natriuretic peptide. These chemical messengers appear to be released from astrocytes via Ca2+‐dependent exocytosis. In the present study, patch‐clamp membrane capacitance measurements were used to monitor changes in the membrane area of a single astrocyte, while the photolysis of caged calcium compounds by a UV flash was used to elicit steps in [Ca2+]i to determine the exocytotic properties of astrocytes. Experiments show that astrocytes exhibit Ca2+‐dependent increases in membrane capacitance, with an apparent Kd value of ∼20 μM [Ca2+]i. The delay between the flash delivery and the peak rate in membrane capacitance increase is in the range of tens to hundreds of milliseconds. The pretreatment of astrocytes by the tetanus neurotoxin, which specifically cleaves the neuronal/neuroendocrine type of SNARE protein synaptobrevin, abolished flash‐induced membrane capacitance increases, suggesting that Ca2+‐dependent membrane capacitance changes involve tetanus neurotoxin‐sensitive SNARE‐mediated vesicular exocytosis. Immunocytochemical experiments show distinct populations of vesicles containing glutamate and atrial natriuretic peptide in astrocytes. We conclude that the recorded Ca2+‐dependent changes in membrane capacitance represent regulated exocytosis from multiple types of vesicles, about 100 times slower than the exocytotic response in neurons.

Hearing requires otoferlin-dependent efficient replenishment of synaptic vesicles in hair cells.

Inner hair cell ribbon synapses indefatigably transmit acoustic information. The proteins mediating their fast vesicle replenishment (hundreds of vesicles per s) are unknown. We found that an aspartate to glycine substitution in the C2F domain of the synaptic vesicle protein otoferlin impaired hearing by reducing vesicle replenishment in the pachanga mouse model of human deafness DFNB9. In vitro estimates of vesicle docking, the readily releasable vesicle pool (RRP), Ca2+ signaling and vesicle fusion were normal. Moreover, we observed postsynaptic excitatory currents of variable size and spike generation. However, mutant active zones replenished vesicles at lower rates than wild-type ones and sound-evoked spiking in auditory neurons was sparse and only partially improved during longer interstimulus intervals. We conclude that replenishment does not match the release of vesicles at mutant active zones in vivo and a sufficient standing RRP therefore cannot be maintained. We propose that otoferlin is involved in replenishing synaptic vesicles.

Cytoskeleton and Vesicle Mobility in Astrocytes

Exocytotic vesicles in astrocytes are increasingly viewed as essential in astrocyte‐to‐neuron communication in the brain. In neurons and excitable secretory cells, delivery of vesicles to the plasma membrane for exocytosis involves an interaction with the cytoskeleton, in particular microtubules and actin filaments. Whether cytoskeletal elements affect vesicle mobility in astrocytes is unknown. We labeled single vesicles with fluorescent atrial natriuretic peptide and monitored their mobility in rat astrocytes with depolymerized microtubules, actin, and intermediate filaments and in mouse astrocytes deficient in the intermediate filament proteins glial fibrillary acidic protein and vimentin. In astrocytes, as in neurons, microtubules participated in directional vesicle mobility, and actin filaments played an important role in this process. Depolymerization of intermediate filaments strongly affected vesicle trafficking and in their absence the fraction of vesicles with directional mobility was reduced.

Exocytosis at the hair cell ribbon synapse apparently operates without neuronal SNARE proteins

SNARE proteins mediate membrane fusion. Neurosecretion depends on neuronal soluble NSF attachment protein receptors (SNAREs; SNAP-25, syntaxin-1, and synaptobrevin-1 or synaptobrevin-2) and is blocked by neurotoxin-mediated cleavage or genetic ablation. We found that exocytosis in mouse inner hair cells (IHCs) was insensitive to neurotoxins and genetic ablation of neuronal SNAREs. mRNA, but no synaptically localized protein, of neuronal SNAREs was present in IHCs. Thus, IHC exocytosis is unconventional and may operate independently of neuronal SNAREs.

Otoferlin: a multi-C2 domain protein essential for hearing

Sound is encoded at synapses between cochlear inner hair cells and the auditory nerve. These synapses are anatomically and functionally specialized to transmit acoustic information with high fidelity over a lifetime. The molecular mechanisms of hair-cell transmitter release have recently attracted substantial interest. Here we review progress toward understanding otoferlin, a multi-C2 domain protein identified a decade ago by genetic analysis of human deafness. Otoferlin functions in hair-cell exocytosis. Several otoferlin C2 domains bind to Ca2+, phospholipids, and proteins. Current research reveals requirements for otoferlin in priming and fusion of synaptic vesicles during sound encoding. Understanding the molecular mechanisms through which otoferlin functions also has important implications for understanding the disease mechanisms that lead to deafness.

Developmental refinement of hair cell synapses tightens the coupling of Ca2+ influx to exocytosis.

Cochlear inner hair cells (IHCs) develop from pre‐sensory pacemaker to sound transducer. Here, we report that this involves changes in structure and function of the ribbon synapses between IHCs and spiral ganglion neurons (SGNs) around hearing onset in mice. As synapses matured they changed from holding several small presynaptic active zones (AZs) and apposed postsynaptic densities (PSDs) to one large AZ/PSD complex per SGN bouton. After the onset of hearing (i) IHCs had fewer and larger ribbons; (ii) CaV1.3 channels formed stripe‐like clusters rather than the smaller and round clusters at immature AZs; (iii) extrasynaptic CaV1.3‐channels were selectively reduced, (iv) the intrinsic Ca2+ dependence of fast exocytosis probed by Ca2+ uncaging remained unchanged but (v) the apparent Ca2+ dependence of exocytosis linearized, when assessed by progressive dihydropyridine block of Ca2+ influx. Biophysical modeling of exocytosis at mature and immature AZ topographies suggests that Ca2+ influx through an individual channel dominates the [Ca2+] driving exocytosis at each mature release site. We conclude that IHC synapses undergo major developmental refinements, resulting in tighter spatial coupling between Ca2+ influx and exocytosis.

Harmonin inhibits presynaptic Cav1.3 Ca2+ channels in mouse inner hair cells

Harmonin is a scaffolding protein that is required for normal mechanosensory function in hair cells. We found a presynaptic association of harmonin and Cav1.3 Ca2+ channels at the mouse inner hair cell synapse, which limits channel availability through a ubiquitin-dependent pathway.