IMPACT OF CLN8 DEFICIENCY IN THE AUTOPHAGY/LYSOSOMAL DEGRADATION PATHWAY

The CLN8 protein is an ER-resident transmembrane protein belonging to the NCL family proteins involved in the neuronal ceroid lipofuscinoses (NCLs), a genetically heterogeneous group of lysosomal diseases that is manifest mainly in childhood and shows accumulation of lipopigment, progressive cognitive and motor decline, epilepsy and vision loss. Function of CLN8 and pathogenesis of CLN8-associated disease are largely unknown. We recently identified several protein partners of CLN8, including VAPA, c14orf1/hERG28, STX8, GATE16/GABARAPL2, BNIP3 and BNIP3L/NIX proteins that are associated with synthesis and transport of lipids, vesicular/membrane trafficking and directly or indirectly influence the autophagic/mitophagic processes. Here, using the CLN8mnd (motor neuron degeneration) mouse model of NCL, we investigated whether a compromised CLN8 function may affect the autophagy/lysosomal degradation pathway. We found that in the cerebellum of CLN8mnd mice at 2 weeks of age, there were increased beclin1 and decreased phosphorylation of mTOR-p70S6K, and accumulation of LC3/GABARAP/GATE16 and p62 that did not localize with the lysosomal LAMP1, suggesting lack of fusion of autophagosomal and lysosomal compartments. The mitophagic marker BNIP3 and BNIP3L/NIX also increased but no mitochondrial translocation of parkin was detected. Furthermore, isolated p62 aggregates together with an enhancement of the chaperone Hsp70 and HDAC6 revealed presence of aggresome-like structures. These results and the link of CLN8 to sphyngolipid synthesis we also reported, indicate a role of CLN8 in autophagic network that likely accounts for the observed dysfunction in the mouse model. However, CLN8 deficiency may set in motion adaptive responses to delay accumulation of toxic substrates during disease progression.