The BRISC de-ubiquitinating enzyme complex limits hematopoietic stem cell expansion by regulating JAK2 K63-ubiquitination.

Hematopoietic stem cell (HSC) homeostasis is controlled by cytokine receptor-mediated JAK2 signaling. We previously found that JAK2 is promptly ubiquitinated upon cytokine stimulation. Whether a competing JAK2 de-ubiquitination activity exists is unknown. LNK is an essential adaptor protein that constrains HSC expansion through dampening thrombopoietin (TPO)- induced JAK2 signaling. We show here that a LNK-associated lysine63 - (K63-) deubiquitinating (DUB) complex, BRISC attenuates HSC expansion through control of JAK2 signaling. We pinpoint a direct interaction between the LNK SH2 domain and a phosphorylated tyrosine residue in KIAA0157 (Abraxas2), a unique and defining BRISC component. Kiaa0157 deficiency in mice led to an expansion of phenotypic and functional HSCs. Endogenous JAK2 and phospho-JAK2 were rapidly K63-ubiquitinated upon TPO stimulation, and this was augmented in cells depleted of BRISC core components KIAA0157, MERIT40, or BRCC36. This increase in JAK2 ubiquitinationn following BRISC knockdown was associated with increased TPO-mediated JAK2 activation and protein levels, and increased MPL receptor presence at the cell surface. Additionally, BRISC depletion promoted membrane-proximal association between the MPL receptor and pJAK2/JAK2, thus enhancing activated JAK2/MPL at the cell membrane. These findings define a novel pathway by which K63-ubiquitination promotes JAK2 stability and activation in a proteasome independent manner. Moreover. mutations in BRCC36 are found in clonal hematopoiesis in humans. This work may shed light into the mechanistic understanding of a potential role of BRCC36 in human HSCs.