The role of Fused in Sarcoma (FUS) in the alternative splicing of TAU

Neurodegenerative disease patients suffer from cognitive decline
and/or motor dysfunctions, depending on the different regions
affected by the neuron loss. With aging being the major risk factor
and a society with increased life expectancy, there is an urgent
need to develop new effective treatments to alleviate the situation
faced by patients, their families and society. Although
neurodegenerative diseases including Alzheimer’s disease (AD),
amyotrophic lateral sclerosis (ALS) or frontotemporal dementia
(FTD) lead to different clinical symptoms, they share common
pathomechanisms, such as protein aggregation and altered RNA
metabolism. A subset of ALS and FTD cases, for instance, is
pathologically characterized by neuronal cytoplasmic inclusions
containing aggregated Fused in Sarcoma (FUS) protein. There is also
a genetic link, since FUS mutations cause ALS with FUS pathology.
FUS is a DNA/RNA-binding protein known to regulate different steps
of RNA metabolism, however, its exact function and target genes in
neurons were unknown. In this study, I evaluated the neuronal role
of FUS in alternative splicing using a candidate approach focusing
on the microtubule-associated protein TAU. TAU is one of the most
widely studied proteins in neurodegeneration research due to its
aggregation in different tauopathies, most notably AD. Mutations in
the TAU gene MAPT, that affect alternative splicing of exon 10, are
known to cause another subtype of FTD. Here, I demonstrate that FUS
depleted rat neurons, although having normal viability, show
aberrant alternative splicing of TAU, with increased inclusion of
exon 3 and exon 10, resulting in higher expression of the 2N and 4R
TAU isoforms. Importantly, reintroduction of human FUS rescues
aberrant splicing of TAU in FUS depleted neurons. Accordingly,
overexpression of FUS decreases expression of 2N and 4R TAU
isoforms. In mouse brain lysates, I detected direct FUS binding to
TAU pre-mRNA, with strong binding around the regulated exon 10,
often at AUU-rich RNA stretches. Since TAU splicing is regulated
differently in humans and rodents, I also confirmed the role of
human FUS in TAU exon 10 splicing using a TAU minigene and a human
neuronal cell line. In addition, I analyzed the morphology and
development of axons to evaluate the functional consequences of FUS
depletion in neurons. Although FUS depleted neurons develop
neurites normally, their axons are significantly shorter than in
the control cells. Similar to observations in TAU/MAP1B knockout
neurons, axons of FUS depleted neurons develop significantly larger
growth cones with abnormal cytoskeletal organization. The
development of growth cones in vivo is an essential step in axonal
maintenance and repair. Altogether, this study identified TAU as
the first physiological splice target of FUS in neurons. The newly
discovered role of FUS in regulating the axonal cytoskeleton
indicates that aberrant axonal function could contribute to the
neuron loss seen in ALS/FTD cases with FUS aggregates.