Glial fibrillary acidic protein (GFAP) is the characteristic protein in the intermediate filament (IF) network of astrocytes. At least 10 different GFAP splice isoforms are known to be expressed, and the main isoforms are GFAPα and GFAPδ. The canonical GFAPα isoform is able to assemble into filaments by itself. GFAPδ, however, is assembly compromised, but low amounts of GFAPδ ( ~ 10% of total GFAP) are tolerated into an IF network composed of vimentin or GFAPα. An increasing amount of GFAPδ will lead to an imbalance with the other IFs present in the cells, and will cause a collapse of the IF network. An IF network is a dynamic structure and IF proteins continuously assemble and disassemble into the network. To study potential differences in the dynamic properties of GFAPα and GFAPδ, both in a physiological and a collapsed IF network, we performed a fluorescence recovery after photobleaching (FRAP) analysis on astrocytoma cells transfected with fluorescently-tagged GFAPs. Here we show for the first time that exchange of GFP-GFAPδ (t1/2 = 2.3 min) into the IF network of astrocytoma cells was significantly slower than the exchange of GFP-GFAPα (t1/2 = 1.1 min). Furthermore, we provide preliminary data that a collapsed IF network, induced by a high GFAPδ expression, led to an even more pronounced decrease in fluorescent recovery of both GFP-GFAPα (t1/2 = 4.5 min) and GFP-GFAPδ (t1/2 = 5.5 min). In the collapsed condition, however, the difference in dynamic properties between GFPGFAPα and GFP-GFAPδ was not present anymore. Our study provides a first insight into biochemical properties of two GFAP isoforms that leads to differences in IF network dynamics. As the IF cytoskeleton has been implicated in cell signaling, a change in the dynamic properties of the network could play a role in regulating cellular processes. More importantly, a GFAP collapse mimicks GFAP aggregates present in Rosenthal fibers in Alexander Disease and some glioma subyptes, therefore our data might help to understand the functional consequences of these aggregates in astrocytes.
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