Lysosomal chloride transporter CLH-6 protects lysosome membrane integrity via cathepsin activation.

Lysosomal integrity is vital for cell homeostasis, but the underlying mechanisms are poorly understood. Here, we identify CLH-6, the C. elegans ortholog of the lysosomal Cl-/H+ antiporter ClC-7, as an important factor for protecting lysosomal integrity. Loss of CLH-6 affects lysosomal degradation, causing cargo accumulation and membrane rupture. Reducing cargo delivery or increasing CPL-1/cathepsin L or CPR-2/cathepsin B expression suppresses these lysosomal defects. Inactivation of CPL-1 or CPR-2, like CLH-6 inactivation, affects cargo digestion and causes lysosomal membrane rupture. Thus, loss of CLH-6 impairs cargo degradation, leading to membrane damage of lysosomes. In clh-6(lf) mutants, lysosomes are acidified as in wild type but contain lower chloride levels, and cathepsin B and L activities are significantly reduced. Cl- binds to CPL-1 and CPR-2 in vitro, and Cl- supplementation increases lysosomal cathepsin B and L activities. Altogether, these findings suggest that CLH-6 maintains the luminal chloride levels required for cathepsin activity, thus facilitating substrate digestion to protect lysosomal membrane integrity.

The Metabolite Saccharopine Impairs Neuronal Development by Inhibiting the Neurotrophic Function of Glucose-6-Phosphate Isomerase.

Mutations in the Aminoadipate-Semialdehyde Synthase (AASS) gene encoding α-aminoadipic semialdehyde synthase lead to hyperlysinemia-I, a benign metabolic variant without clinical significance, and hyperlysinemia-II with developmental delay and intellectual disability. Although both forms of hyperlysinemia display biochemical phenotypes of questionable clinical significance, an association between neurologic disorder and a pronounced biochemical abnormality remains a challenging clinical question. Here, we report that Aass mutant male and female mice carrying the R65Q mutation in α-ketoglutarate reductase (LKR) domain have an elevated cerebral lysine level and a normal brain development, whereas the Aass mutant mice carrying the G489E mutation in saccharopine dehydrogenase (SDH) domain exhibit elevations of both cerebral lysine and saccharopine levels and a smaller brain with defective neuronal development. Mechanistically, the accumulated saccharopine, but not lysine, leads to impaired neuronal development by inhibiting the neurotrophic effect of glucose-6-phosphate isomerase (GPI). While extracellular supplementation of GPI restores defective neuronal development caused by G498E mutation in SDH of Aass. Altogether, our findings not only unravel the requirement for saccharopine degradation in neuronal development, but also provide the mechanistic insights for understanding the neurometabolic disorder of hyperlysinemia-II.SIGNIFICANCE STATEMENT The association between neurologic disorder and a pronounced biochemical abnormality in hyperlysinemia remains a challenging clinical question. Here, we report that mice carrying the R65Q mutation in lysine α-ketoglutarate reductase (LKR) domain of aminoadipate-semialdehyde synthase (AASS) have an elevated cerebral lysine levels and a normal brain development, whereas those carrying the G489E mutation in saccharopine dehydrogenase (SDH) domain of AASS exhibit an elevation of both cerebral lysine and saccharopine and a small brain with defective neuronal development. Furthermore, saccharopine impairs neuronal development by inhibiting the neurotrophic effect of glucose-6-phosphate isomerase (GPI). These findings demonstrate saccharopine degradation is essential for neuronal development.