Unexpected phenotypic and molecular changes of combined glucocerebrosidase and acid sphingomyelinase deficiency

Abstract

Heterozygous variants in GBA1 encoding glucocerebrosidase (GCase) are the most common genetic risk factor for Parkinson's disease (PD). Moreover, sporadic PD patients also have a substantial reduction of GCase activity. Genetic variants in SMPD1 are also overrepresented in PD cohorts, whilst a reduction of its encoded enzyme (ASM) activity is linked to an earlier age of PD onset. Despite both converging on the ceramide pathway, how combined deficiencies of both enzymes may interact to modulate PD has yet to be explored. Therefore, we created a double knock out (DKO) zebrafish line for both gba1 and smpd1 to test for an interaction in vivo, hypothesising an exacerbation of phenotypes in the DKO compared to single mutants. Unexpectedly, DKOs maintained conventional swimming behaviour and had normalised neuronal gene expression signatures when compared to single mutants. We further identified rescue of mitochondrial Complexes I and IV in DKOs. Despite having an unexpected rescue effect, our results confirm ASM as a modifier of GBA1 deficiency in vivo. Our study highlights the need for validating how genetic variants and enzymatic deficiencies may interact in vivo.

A zebrafish model of combined saposin deficiency identifies acid sphingomyelinase as a potential therapeutic target

“…The early rise in NFκB and Jak-Stat signaling in the psap KO brain supports pharmacological modulation of these pathways as one strategy toward symptom alleviation. Additionally, given recent evidence of successful Farber and Gaucher disease rescue via acid sphingomyelinase knockout ( Keatinge et al, 2023 ; Beckmann et al, 2019 ), we also examined this genetic rescue strategy for the psap KO model.…”

“…In addition to small-molecule-based treatment, a genetic rescue was also explored based on previous reports of successful Farber and Gaucher disease mitigation by crossing the respective mouse and zebrafish models with an acid sphingomyelinase (SMPD1) deficiency model ( Keatinge et al, 2020 preprint; Beckmann et al, 2019 ). In the context of the sphingolipid metabolic pathway, it is possible that SMPD1 knockout-induced ceramide depletion may counter ceramide accumulation in Farber disease and enhance lifespan in the Gaucher model by shifting complex sphingolipid catabolism toward ceramide instead of psychosine.…”

“…Although water tank MMF administration did not alter the phenotype, a modest increase in lifespan was observed upon crossing the psap KO line into a zebrafish model of smpd1 deficiency. This study was inspired by previous reports of increased survival in Farber ( ASAH1 ) ( Beckmann et al, 2019 ) and Gaucher ( gba ) ( Keatinge et al, 2023 ) disease animal models following SMPD1 knockout. In ASAH1 −/− mice, SMPD1 knockout ameliorated multiple disease pathologies, including ceramide accumulation, peripheral organ histopathology and inflammation ( Beckmann et al, 2019 ).…”

“…In ASAH1 −/− mice, SMPD1 knockout ameliorated multiple disease pathologies, including ceramide accumulation, peripheral organ histopathology and inflammation ( Beckmann et al, 2019 ). In gba −/− zebrafish, smpd1 knockout further raised sphingolipid levels, but rescued mitochondrial respiratory chain function, leading to improved motor behavior and survival ( Keatinge et al, 2023 ). Morphologically, the progressive cachexia and impaired swimming associated with Psap loss more closely resembles the disease course of gba −/− zebrafish ( Keatinge et al, 2015 ; Lelieveld et al, 2019 ), rather than the overall reduced size (with no visible cachexia or behavioral change) previously reported in a Farber disease zebrafish model ( Zhang et al, 2019 ).…”

C9ORF72 deficiency results in degeneration of the zebrafish retinain vivo

“…The zebrafish has shown to be a promising complementary in vivo vertebrate model of neurodegeneration, demonstrating the key markers of pathology, including neuronal loss and gliosis, even during early larval stages, in models of various neurodegenerative conditions 22,[25][26][27][28][29][30][31][32] . Previous studies characterising c9orf72 deficient larval zebrafish utilised a knock down approach with Morpholinos 33 , and indicated that knockdown of c9orf72 in zebrafish resulted in axon outgrowth deficits in motor neurons and reduction in spontaneous movement 33 .…”