Lysosomal reduced thiols are essential for mouse embryonic development

While it has been appreciated for decades that lysosomes can import cysteine, its significance for whole-organism physiology has remained uncertain. Recent work identified MFSD12 as a transmembrane protein required for cysteine import into lysosomes (and melanosomes), enabling genetic interrogation of this pathway. Here, we show that <i>Mfsd12</i> knockout mice die between embryonic days 10.5 and 12.5, indicating that MFSD12 is essential for organogenesis. <i>Mfsd12</i> loss results in the expression of genes involved in cellular stress and thiol metabolism and likely disproportionately affects the erythroid, myeloid, and neuronal lineages. Within lysosomes, imported cysteine is largely oxidized to cystine, which is exported to the cytosol by the cystinosin (CTNS) transporter. However, unlike <i>Mfsd12</i>, loss of <i>Ctns</i> is compatible with viability, suggesting that the essential role of MFSD12 lies not in supplying cystine to the cytosol, but in providing reduced cysteine within the lysosomal lumen. Supporting this model, maternal treatment with cysteamine-a lysosome-penetrant thiol-rescued the development of <i>Mfsd12</i> knockout embryos, yielding viable adult offspring. These findings establish lysosomal thiol import as a critical metabolic pathway and provide genetic tools to further clarify its physiological and biochemical roles.