Compromised astrocyte function and survival negatively impact neurons in infantile neuronal ceroid lipofuscinosis

The neuronal ceroid lipofuscinoses (NCLs) are the most common cause of childhood dementia and are invariably fatal. Early localized glial activation occurs in these disorders, and accurately predicts where neuronal loss is most pronounced. Recent evidence suggests that glial dysfunction may contribute to neuron loss, and we have now explored this possibility in infantile NCL (INCL, CLN1 disease). We grew primary cultures of astrocytes, microglia, and neurons derived from Ppt1 deficient mice (Ppt1^-/-) and assessed their properties compared to wildtype (WT) cultures, before co-culturing them in different combinations (astrocytes with microglia, astrocytes or microglia with neurons, all three cell types together). These studies revealed that both Ppt1^-/- astrocytes and microglia exhibit a more activated phenotype under basal unstimulated conditions, as well as alterations to their protein expression profile following pharmacological stimulation. Ppt1^{- /-} astrocytes also displayed abnormal calcium signalling and an elevated cytoplasmic Ca²⁺ level, and a profound defect in their survival. Ppt1^-/- neurons displayed decreased neurite outgrowth, altered complexity, a reduction in cell body size, and impaired neuron survival with prolonged time in culture. In co-cultures, the presence of both astrocytes and microglia from Ppt1^-/- mice further impaired the morphology of both wild type and Ppt1^-/- neurons. This negative influence was more pronounced for Ppt1^-/- microglia, which appeared to trigger increased Ppt1^-/- neuronal death. In contrast, wild type glial cells, especially astrocytes, ameliorated some of the morphological defects observed in Ppt1^-/- neurons. These findings suggest that both Ppt1^-/- microglia and astrocytes are dysfunctional and may contribute to the neurodegeneration observed in CLN1 disease. However, the dysfunctional phenotypes of Ppt1^-/- glia are different from those present in CLN3 disease, suggesting that the pathogenic role of glia may differ between NCLs.