Single-molecule imaging of aquaporin-4 array dynamics in astrocytes.

Aquaporin-4 (AQP4) facilitates water transport across astrocytic membranes in the brain, forming highly structured nanometric arrays. AQP4 has a central role in regulating cerebrospinal fluid (CSF) circulation and facilitating the clearance of solutes from the extracellular space of the brain. Adrenergic signaling has been shown to modulate the volume of the extracellular space of the brain via AQP4 localized at the end-feet of astrocytes, but the mechanisms by which AQP4 regulates CSF inflow and outflow in the brain remain elusive. Using advanced imaging techniques, including super-resolution microscopy and single-molecule tracking, we investigated the hypothesis that ß-adrenergic receptor activation induces cellular changes that regulate AQP4 array size and mobility, thus influencing water transport in the brain. We report that the ß-adrenergic agonist, isoproterenol hydrochloride, decreases AQP4 array size and enhances its membrane mobility, while hyperosmotic conditions induce the formation of larger, less mobile arrays. These findings reveal that AQP4 arrays are dynamic structures, responsive to adrenergic signals and osmotic changes, highlighting a novel regulatory mechanism of water transport in the brain. Our results provide insights into the molecular control of CSF circulation and extracellular brain space volume, laying the groundwork for understanding the relationship between astrocyte water transport, sleep physiology, and neurodegeneration.

The Mitochondrial Enzyme 17ßHSD10 Modulates Ischemic and Amyloid-ß-Induced Stress in Primary Mouse Astrocytes.

Severe brain metabolic dysfunction and amyloid-ß accumulation are key hallmarks of Alzheimer's disease (AD). While astrocytes contribute to both pathologic mechanisms, the role of their mitochondria, which is essential for signaling and maintenance of these processes, has been largely understudied. The current work provides the first direct evidence that the mitochondrial metabolic switch 17ß-hydroxysteroid dehydrogenase type 10 (17ßHSD10) is expressed and active in murine astrocytes from different brain regions. While it is known that this protein is overexpressed in the brains of AD patients, we found that 17ßHSD10 is also upregulated in astrocytes exposed to amyloidogenic and ischemic stress. Importantly, such catalytic overexpression of 17ßHSD10 inhibits mitochondrial respiration during increased energy demand. This observation contrasts with what has been found in neuronal and cancer model systems, which suggests astrocyte-specific mechanisms mediated by the protein. Furthermore, the catalytic upregulation of the enzyme exacerbates astrocytic damage, reactive oxygen species (ROS) generation and mitochondrial network alterations during amyloidogenic stress. On the other hand, 17ßHSD10 inhibition through AG18051 counters most of these effects. In conclusion, our data represents novel insights into the role of astrocytic mitochondria in metabolic and amyloidogenic stress with implications of 17ßHSD10 in multiple neurodegenerative mechanisms.