Figure 5. Model depicting differential requirements for mTORC1 regulation by nutrient signaling through RagA in homeostatic versus stress hematopoiesis.

(A) Under homeostatic conditions, HSC do not depend on mTORC1. Under hematopoietic stress, mTORC1 is critical for HSC function, as previously shown by Raptor deletion (Raptor^–/–). However, nutrient sensing by RagA is not required for mTORC1 activity in HSC and Rraga^–/– HSC are indifferent to aa levels. (B) In contrast, progenitors (STRC) depend on RagA-regulated mTORC1 activation to maintain normal production of mature blood cells. Distinct effects are noted on monocyte production (increased) compared with other blood cells (decreased) with either Raptor or Rraga deletion.