Autophagy controls differentiation of Drosophila blood cells by regulating Notch levels in response to nutrient availability

Introduction

Drosophila larval hematopoiesis takes place at the lymph gland, where blood cell progenitors localized centrally at the so-called medullary zone undergo differentiation to give rise to myeloid-like lineage cells, which include plasmatocytes and crystal cells (CCs), while a third cell-type, the lamellocytes, differentiates only upon certain specific immune challenges, such as parasitic wasp egg invasion^1,2. Plasmatocytes are macrophage-like cells that represent around 95% of total circulating blood cells, while CCs account for the remaining 5%³. CCs display large cytoplasmic prophenoloxidase crystalline inclusions, which breakdown after the immune insult, releasing phenoloxidases to the blood stream, thus contributing to melanization and destruction of the pathogen^4,5.

The balance between maintenance and differentiation of blood cell progenitors of the lymph gland is tightly controlled by a wide array of factors ranging from signaling proteins^{6, 7, 8, 9–10} and metabolites^9,11, to chemical and mechanical cues^12,13, which ultimately determine the progenitor fate. Particularly, differentiation of progenitors into CCs depends largely on the Notch pathway, activated, at least in part, by its ligand Serrate, which is expressed in specialized scattered cells localized at the outer-most region of the medullary zone^{13, 14, 15, 16–17}. Two populations of blood cell progenitors occur at the medullary zone of the lymph gland: Internally localized core progenitors, and externally localized distal progenitors^18,19. Distal progenitors, but not core progenitors, are competent to undergo terminal differentiation, acquiring either a plasmatocyte fate in response to MAP kinase signaling mediated by the transcription factor Pointed², or a crystal cell fate in response to Notch activation¹⁸. Notch pathway inhibition in blood cell progenitors provokes reduction or complete loss of CCs, while genetic overactivation of the pathway in progenitors leads to excess of CCs^18,20.

Macroautophagy (referred hereafter as autophagy) is an evolutionary conserved self-degradative process, essential for housekeeping functions, such as recycling of macromolecules and organelles, as well as for removing protein aggregates. During autophagy, the cytoplasmic components to be degraded are sequestered in a double membrane organelle called autophagosome, which then fuses with lysosomes, where hydrolytic enzymes execute degradation of the cargo^{21, 22, 23–24}. Alternatively, an intermediate step may take place, as autophagosomes can fuse with late endosomes/multivesicular...