Lysosomal reduced thiols are essential for mouse embryonic development.

While it has been appreciated for decades that lysosomes can import cysteine, its for organismal physiology is unclear. Recently, the MFSD12 transmembrane protein was shown to be necessary to import cysteine into lysosomes (and melanosomes), enabling the study of these processes using genetic tools. Here, we find that mice lacking die between embryonic days 10.5-12.5, suggesting that MFSD12 is essential for organogenesis. Within lysosomes, it is well known that a significant fraction of the cysteine is converted into cystine (oxidized cysteine), which accumulates and can be released into the cytosol via the CTNS (cystinosin) transporter. However, in contrast to , loss of results in live animals, indicating that it is not an essential gene. This suggests that the essential function of MFSD12 is not to enable cystine storage for use in the cytosol but rather to supply reduced cysteine to the lysosomal lumen itself. Consistent with this idea, the treatment of breeding heterozygous mice with cysteamine, a lysosome-penetrant thiol, rescued the development of knockout mice and resulted in live births. Our work implicates lysosomal thiol import as an essential metabolic pathway and provides tools for deciphering its complex genetic and metabolic interactions.