SLC38A9 and Alzheimer disease: The impact of arginine levels and arginine signaling warrants further consideration in Alzheimer’s disease (AD) because of its high demand for multiple metabolic pathways and recent discoveries of mTORC1 activation associated bona fide and putative arginine sensors that locate at the lysosome [SLC38A9 (1), TM4SF5 (2)], cytoplasm [CASTORs (3)], and plasma membrane [GPRC6A (4)].