Biology subjects

Borges-Merjane, C.

Publications and source records attributed to Borges-Merjane, C..

3 recordsLinked to original sources

bioRxiv2022

A two-pool mechanism of vesicle release in medial habenula terminals underlies GABAB receptor-mediated potentiation

GABAB receptor (GBR) activation inhibits neurotransmitter release in axon terminals in the brain, except in medial habenula (MHb) terminals, which show robust potentiation. However, mechanisms underlying this enigmatic potentiation remain elusive. Here, we report that GBR activation induces a transition from tonic to phasic release accompanied by a 4-fold increase in readily releasable pool (RRP) size in MHb terminals, mirrored by a similar increase in the docked vesicle number at the presynaptic active zone (AZ). The tonic and phasic release vesicles have distinct coupling distances. We identified two vesicle-associated molecules, synaptoporin and CAPS2, selectively involved in tonic and phasic release, respectively. Synaptoporin mediates augmentation of tonic release and CAPS2 stabilizes readily releasable vesicles during phasic release. A newly developed "Flash and Freeze-fracture" method revealed selective recruitment of CAPS2 to the AZ during phasic release. Thus, we propose a novel two-pool mechanism underlying the GBR-mediated potentiation of release from MHb terminals.

neuroscience↗

bioRxiv2022

Validation of a stereological method for estimating particle size and density from 2D projections with high accuracy

AO_SCPLOWBSTRACTC_SCPLOWStereological methods for estimating the 3D particle size and density from 2D projections are essential to many research fields. These methods are, however, prone to errors arising from undetected particle profiles due to sectioning and limited resolution, known as lost caps. A potential solution by Keiding et al. (1972) accounts for lost caps by quantifying the smallest detectable profiles in terms of their limiting section angle ({phi}). However, this simple solution has not been widely adopted nor validated. Rather, model-independent design-based stereological methods, which do not explicitly account for lost caps, have come to the fore. Here, we provide the first experimental validation of the Keiding model by quantifying {phi} of synaptic vesicles using electron-tomography 3D reconstructions. This analysis reveals a Gaussian distribution for {phi} rather than a single value. Nevertheless, curve fits of the Keiding model to the 2D diameter distribution accurately estimate the mean {phi} and 3D diameter distribution. While systematic testing using Monte Carlo simulations revealed an upper limit to determining {phi}, our analysis shows that mean {phi} can be used to estimate the 3D particle density from the 2D density under a wide range of conditions, and this method is potentially more accurate than minimum-size-based lost-cap corrections and disector methods. We applied the Keiding model to estimate the size and density of somata, nuclei and vesicles in rodent cerebella, where high packing density can be problematic for design-based methods.

neuroscience↗

bioRxiv2020

Incomplete removal of extracellular glutamate controls synaptic transmission and integration at a cerebellar synapse

Synapses of glutamatergic mossy fiber onto cerebellar unipolar brush cells (UBCs) generate slow excitatory (ON) or inhibitory (OFF) postsynaptic responses dependent on the complement of glutamate receptors expressed on the UBCs large dendritic brush. Using brain slice recording and computational modeling of synaptic transmission, we found that substantial glutamate is maintained in the UBC synaptic cleft, sufficient to modify spontaneous firing in OFF UBCs and tonically desensitize AMPARs of ON UBC. The source of ambient glutamate was spontaneous, spike-independent exocytosis from the mossy fiber terminal, and its level was dependent on activity of glutamate transporters EAAT1-2. Changing levels of ambient glutamate shifted the polarity of evoked synaptic responses in ON UBCs and altered the phase of responses to in vivo-like synaptic activity. Unlike classical fast synapses, receptors at the UBC synapse are virtually always exposed to a significant level of glutamate, which varies in a graded manner during transmission.

neuroscience↗