New GFAP splice isoform (GFAPµ) differentially expressed in glioma translates into 21 kDa N‐terminal GFAP protein

Abstract

Glial fibrillary acidic protein (GFAP) is a type III intermediate filament (IF) protein, which forms together with vimentin, synemin, and nestin the cytoskeletal IF-network in astrocytes. GFAP is often used as a marker of astrocytes, and increased expression of GFAP as an indicator of reactive gliosis in the injured and diseased brain. 1,2 Moreover, GFAP is a classical diagnostic marker for the most malignant tumors of the central nervous system (CNS); gliomas. 3-5 Gliomas are

Elevated GFAP isoform expression promotes protein aggregation and compromises astrocyte function

“…While the ~30 kDa GFAP fragment generated by active caspase 3 44 may correlate with alterations in astrocyte function through cytoskeletal reorganization, 51 the 26 kDa GFAP fragments generated by caspase 6 may act as an aggravating factor that promotes further GFAP aggregation 43,45 . A recent study on a novel isoform GFAP‐μ, 52 resulting from exon 2 skipping that translates into a 21 kDa GFAP N‐terminal fragment, provides additional evidence in support of the role of a truncated form of GFAP in promoting IF aggregation. Although GFAP‐μ mRNA is expressed at a very low level compared to GFAP‐α in glioma cells and healthy brain tissues, the translated GFAP protein can be detected in the IF‐enriched fraction isolated from the human spinal cord.…”

GFAP Alternative Splicing and the Relevance for Disease – A Focus on Diffuse Gliomas

“…The consensus now is that GFAPδ and GFAPϵ are the same variant (Middeldorp and Hol, 2011;Roelofs et al, 2005). In following years, more GFAP splice variants were discovered: GFAPΔ135, GFAPΔexon 6, and GFAPΔ164 in human and mouse brain (Hol et al, 2003;Kamphuis et al, 2012), GFAPΔexon 7 in mouse brain (Kamphuis et al, 2012), GFAPκ in pig brain (Blechingberg et al, 2007a), GFAPλ in human brain (Helman et al, 2020), and GFAPμ in human brain and glioma (van Bodegraven et al, 2021). Note that in the original GFAPδ paper the authors described 2 transcripts: (i) with an intron 7 retention (exon 7a) and alternative polyadenylation site ( = GFAPδ/ϵ) and (ii) with an intron 7 retention and including exon 8 and 9 ( = GFAPλ) (Condorelli et al, 1999a).…”

“…References: (1) (Lewis et al, 1984), (2) (Brenner et al, 1990), (3) (Kamphuis et al, 2012), (4) (Feinstein et al, 1992), (5) (Galea et al, 1995), (6) (Condorelli et al, 1999b), ( 7) (Kamphuis et al, 2014), (8) (Roelofs et al, 2005), (9) (Mamber et al, 2012), (10) (Middeldorp et al, 2010), (11) (Middeldorp et al, 2009), ( 12) (Stassen et al, 2017), (13) (Blechingberg et al, 2007a), ( 14) (Zelenika et al, 1995), (15) (Boer et al, 2010), ( 16) (Hol et al, 2003), ( 17) (Helman et al, 2020), ( 18) (van Bodegraven et al, 2021).…”

“…Unique to GFAP, in comparison to the other type III IF proteins, is the regulation of the GFAP pre-RNA transcript by alternative splicing and alternative polyadenylation. Since the discovery of the canonical isoform GFAPα, seven murine and twelve human splice-isoforms have been described (Middeldorp and Hol, 2011), of which the isoforms GFAPλ and GFAPμ were only discovered recently (Helman et al, 2020;van Bodegraven et al, 2021). Although the difference in functionality between the isoforms is not entirely known, neurological disorders are associated with alterations in isoform expression levels (Bugiani et al, 2011;Clairembault et al, 2014;Flint et al, 2012;Helman et al, 2020;Kamphuis et al, 2014;Stassen et al, 2017).…”

GFAP-directed Inactivation of Men1 Exploits Glial Cell Plasticity in Favor of Neuroendocrine Reprogramming

“…To test whether this interaction also occurs in glial cells we performed co-immunoprecipitation using extracts from a rat enteric glial cell line (EGC) and confirmed interactions between GFAP and menin in whole cell lysates as well as nuclear and cytoplasmic fractions (Figure 4F). Although intermediate filaments including GFAP are elaborate cytoskeletal structures residing in the cytoplasm, their expression also occurs in the nucleus and peri-nuclear regions of various cell types [25][26][27] . Similarly, the subcellular localization of menin is dynamic with its movement in and out of the nucleus regulated by multiple factors [14,28,29] .…”