Neurons and astrocytes in an infantile neuroaxonal dystrophy (INAD) mouse model show characteristic alterations in glutamate-induced Ca signaling.

INAD (infantile neuroaxonal dystrophy, OMIM#256600), an autosomal recessive inherited degenerative disease, is associated with PLA2G6 mutations. PLA2G6 encodes Ca-independent phospholipase A (VIA iPLA). However, it is unclear how the PLA2G6-mutations lead to disease. Non-canonical functions, which were suggested for VIA iPLA, such as regulation of cellular and mitochondrial Ca are promising candidates. Therefore, we investigate glutamate (Glu)-evoked Ca signals in neurons and astrocytes in co-culture obtained from three INAD mouse model strains with Pla2g6 mutations, (i) hypomorphic Pla2g6 allele with reduced transcript levels, (ii) knocked-out Pla2g6, and (iii) (G373R)-point mutation with inactive VIA iPLA enzyme. Homozygous offspring from these strains develop pathology similar to that observed in INAD patients. We found that in mouse neurons the Pla2g6 mutation disrupted the dependency of Glu-induced extracellular Ca influx on mitochondrial Ca uptake. Thus, in neurons with Pla2g6 mutation we did not detect the characteristic reduction in Glu-induced Ca influx upon treatment with Ru360, a blocker of mitochondrial Ca uniporter, or with rotenone. In contrast to neurons, in astrocytes, both with Pla2g6 mutation or wild-type cells, the treatment with Ru360 or rotenone reduced the rate of Glu-induced Ca influx ∼2-fold. This Ca influx in astrocytes represents capacitative Ca entry. In astrocytes with Pla2g6 mutation, the Glu-induced Ca influx was ∼2-fold lower than in wild-type controls. We suggest that this is the mechanism for strongly decreased durations of Glu-induced Ca responses in astrocytes with Pla2g6 mutation. We could mimic the mutation by pharmacological inhibition of iPLA using S-BEL. Thus, lack of VIA iPLA activity caused effects in astrocytes. In summary, three INAD mouse models show comparable changes in Glu-induced Ca signaling, but specific for neurons or astrocytes. This finding helps to identify pathways altered during INAD and highlights non-canonical VIA iPLA functions, like regulation of cellular Ca fluxes by mitochondria or capacitative Ca-entry.