Brain accumulation of lactosylceramide characterizes GALC deficiency in a zebrafish model of Krabbe disease.

Krabbe disease (KD) is an autosomal recessive sphingolipidosis due to mutations of the GALC gene encoding for the lysosomal β-galactosylceramidase (GALC) that removes β-galactose from β-galactosylceramide, β-lactosylceramide (LacCer) and the neurotoxic metabolite β-galactosylsphingosine (psychosine). At present, the accumulation of psychosine is thought to be the main cause of demyelination, neurodegeneration and neuroinflammation that characterize the early infantile KD with a 1.5-2-year median survival. Currently, the standard of care of KD is haematopoietic stem cell transplantation which, however, improves the lifespan of Krabbe patients only when performed before symptoms appear. Thus, a better understanding of the pathogenesis of KD is required for the development of more efficacious therapeutic approaches. This largely depends upon the availability of novel suitable animal models of the disease. Zebrafish (Danio rerio) represents a useful platform for the study of the mechanisms responsible for human hereditary diseases, including sphingolipidoses, and for the identification of new therapeutics. Two co-orthologues of human GALC have been identified in zebrafish, named galca and galcb. Here, we generated a mutant zebrafish line for each of the two co-orthologues by CRISPR/Cas9 genome editing. Galcb knockout (KO), but not galca KO, exerts a dramatic decrease of total GALC activity both in zebrafish embryos and in the brain of adult mutants. At 3-4 months post-fertilization, galcb KO zebrafish showed impaired locomotion and reduced lifespan. Gene expression analysis, immunohistochemistry, spectral confocal reflectance and transmission electron microscopy showed the presence of demyelination, neuroinflammation and neurodegeneration in the brain of galcb KO mutants. Notably, double galca/galcb KO did not cause a further worsening of the disease when compared with galcb KO mutants. Finally, targeted lipidomic analysis demonstrated a dramatic accumulation of the bioactive sphingolipid LacCer in the brain of both galcb KO and double galca/galcb KO mutants with a modest increase of psychosine levels. Accordingly, activation of LacCer-related signalling occurs in the brain of galcb KO animals. Furthermore, intraventricular injection of LacCer upregulates the expression of various proinflammatory markers and increase mpeg1-positive macrophage infiltration in the brain of 5 dpf zebrafish embryos. In conclusion, galcb KO zebrafish recapitulates several pathological features of KD and is characterized by the accumulation of the bioactive LacCer. This model sheds new light on a possible role of LacCer as a neuroinflammatory/neurodegenerative metabolite in KD with implications for the development of novel therapeutic strategies.