The PrP-hFUS transgene expresses a hemagglutinin-tagged human wildtype fused in sarcoma cDNA sequence (HA-hFUS) under the direction of the mouse prion protein (PrP) promoter. The exogenous hFUS protein expressed from the PrP-hFUS transgene has a slightly higher molecular weight than endogenous mouse FUS due to eight additional amino acids and the presence of the HA-tag. The phenotype below describes PrP-hFUS mice from founder line WT3 (PrP-hFUS-WT3).
As expected for the PrP promoter, PrP-hFUS-WT3 mice have HA-hFUS expression levels highest in the brain (cortex), spinal cord and testis; with much lower expression in other tissues. Transgenic mice express exogenous HA-hFUS, as well as endogenous mouse FUS. Compared to nontransgenic mice, the total FUS expression levels are ~1.4-fold greater in hemizygous mice, and ~1.7-fold greater in homozygous mice. Transgene expression of HA-hFUS results in significantly decreased expression of mouse FUS in all transgenic lines (endogenous FUS is downregulated ~0.86-fold in hemizygotes, and ~ 0.62-fold in homozygotes). Mice hemizygous for the PrP-hFUS transgene are viable and fertile with no evidence of significant motor phenotype or pathology. FUS overexpression levels in homozygous mice result in an aggressive neurodegenerative phenotype that recapitulates many features of human amyotrophic lateral sclerosis (ALS). Specifically, homozygous mice exhibit early onset tremor after ~4 weeks of age, followed by globular/granular cytoplasmic FUS inclusions and neuroinflammation of the spinal cord (astroglial [GFAP] and microglial [CD68] reactivity), progressive hind-limb paralysis, and eventually death by ~12 weeks of age. This FUS-ALS phenotype is dependent upon the expression level of the FUS protein, and is associated with a significant shift in FUS localization to the cytoplasm without concomitant loss of FUS from the nucleus. Of note, fluorescent labeling of HA-tagged FUS and ubiquitin in the spinal cord anterior horn reveals FUS inclusions with no ubiquitin staining. Fluorescent labeling of HA-tagged FUS and ubiquitin in the motor cortex shows skein-like cytoplasmic FUS deposits surrounded by ubiquitin; but no co-labelling of the two proteins is evident. These PrP-hFUS mice may be useful in studying neurodegenerative disorders such as ALS and frontotemporal lobar degeneration (FTLD).
