Differential Sensitivity of Brainstem versus Cortical Astrocytes to Changes in pH Reveals Functional Regional Specialization of Astroglia

Abstract

Astrocytes might function as brain interoceptors capable of detecting different (chemo)sensory modalities and transmitting sensory information to the relevant neural networks controlling vital functions. For example, astrocytes that reside near the ventral surface of the brainstem (central respiratory chemosensitive area) respond to physiological decreases in pH with vigorous elevations in intracellular Ca 2ϩ and release of ATP. ATP transmits astroglial excitation to the brainstem respiratory network and contributes to adaptive changes in lung ventilation. Here we show that in terms of pH-sensitivity, ventral brainstem astrocytes are clearly distinct from astrocytes residing in the cerebral cortex. We monitored vesicular fusion in cultured rat brainstem astrocytes using total internal reflection fluorescence microscopy and found that ϳ35% of them respond to acidification with an increased rate of exocytosis of ATP-containing vesicular compartments. These fusion events require intracellular Ca 2ϩ signaling and are independent of autocrine ATP actions. In contrast, the rate of vesicular fusion in cultured cortical astrocytes is not affected by changes in pH. Compared to cortical astrocytes, ventral brainstem astrocytes display higher levels of expression of genes encoding proteins associated with ATP vesicular transport and fusion, including vesicle-associated membrane protein-3 and vesicular nucleotide transporter. These results suggest that astrocytes residing in different parts of the rat brain are functionally specialized. In contrast to cortical astrocytes, astrocytes of the brainstem chemosensitive area(s) possess signaling properties that are functionally relevant-they are able to sense changes in pH and respond to acidification with enhanced vesicular release of ATP.

An astrocyte-dependent mechanism for neuronal rhythmogenesis

“…However, in this case, the mechanism involved is pHdependant and seems specific to the ventral brainstem respiratory area 45 , whereas the mechanism of action reported here may be of a much broader relevance given the widespread distribution of both S100β and I NaP 4,8,9,11,17,18,30 . Regulation of I NaP -mediated bursting by astrocytes may help understand appearance of bursting in normal and pathological conditions such as epileptic seizures where astrocytes are over activated 46 .…”

Vesicular nucleotide transporter (VNUT): appearance of an actress on the stage of purinergic signaling

“…Astrocytes and microglia are other ATPsecreting cells in the brain. In these cells, ATP is stored in dense-cored vesicles [65][66][67], secretory lysosomes [68,69], or non-characterized vesicular compartments [70][71][72]. Consistently, VNUT is reported to be associated with intracellular vesicles in astrocytes and microglia [71][72][73][74][75].…”

“…In these cells, ATP is stored in dense-cored vesicles [65][66][67], secretory lysosomes [68,69], or non-characterized vesicular compartments [70][71][72]. Consistently, VNUT is reported to be associated with intracellular vesicles in astrocytes and microglia [71][72][73][74][75]. Secretory lysosomes have recently been identified as dual functional organelles that serve as both a degradative and as secretory compartments [76].…”

Signal transduction in astrocytes: Localization and release of inorganic polyphosphate

“…An Olympus total internal reflection fluorescence (TIRF) microscope was used to detect vesicular fusion events in astrocytes expressing VNUT–eGFP or CD63‐mKate, as described in detail previously (Angelova et al, 2015; Kasymov et al, 2013). Fluorescence was excited at 488 nm and collected at 500–530 nm for eGFP and excited at 488 nm and collected at 600–660 nm for mKate.…”