2009 – THE ENDOSOMAL ADAPTOR PROTEIN MYCT1 CONTROLS HUMAN HEMATOPOIETIC STEM CELL FUNCTION

• Published in: Experimental hematology 2021-08, Vol.100, p.S31-S32
• Main Authors: Aguade-Gorgorio, Julia, Kardouh, Maya, Jami-alahmadi, Yasaman, Fares, Iman, Calvanese, Vincenzo, Magnusson, Mattias, Shin, Juliana, Goodridge, Helen, Wohlschlegel, James, Mikkola, Hanna
• Format: Article
• Language: English
• ISSN: 0301-472X; 1873-2399
• DOI: 10.1016/j.exphem.2021.12.374

Hematopoietic stem cells (HSC) need to integrate multiple environmental cues to maintain the balance between quiescence, self-renewal and differentiation through life. During culture however, HSC self-renewal and engraftment ability become severely compromised, hindering our ability to manipulate and expand functional HSCs for therapeutic use. To understand the molecular programs governing HSC function that become dysregulated in culture, we analyzed multiple gene expression data sets of cultured and uncultured human HSPCs. We identified MYCT1 (Myc target 1) as an endothelial and HSC gene that becomes drastically downregulated and epigenetically silenced in culture, concomitantly with loss of engraftment ability. Knockdown (KD) experiments revealed that MYCT1 is critical for human HSC expansion and engraftment. Using subcellular fractionation, immunofluorescence, and immunoprecipitation coupled with high-sensitivity mass spectrometry we discovered that MYCT1 is a membrane-associated protein localized in endosomes, where it interacts with vesicle trafficking components and multiple signaling receptors, such as TGFBR1 and 2. MYCT1 KD in human HSPCs led to hyperactivation of endocytosis, a crucial regulatory step determining the responsiveness to extracellular cues and governing cell fate decisions. Moreover, MYCT1 KD dysregulated cell division and self-renewal genes in human HSPCs, while MYCT1 overexpression had the opposite effect. These data suggest that MYCT1 governs HSC stemness and balance between fate options by fine-tuning multiple signaling pathways with critical functions in HSC biology through the control of endocytosis. As MYCT1 expression is downregulated in cultured HSPC, this work also elucidates mechanisms underlying culture-associated HSC dysfunction that will need to be overcome to restore compromised transplantability of ex vivo expanded human HSCs.