Cullin 4b-RING ubiquitin ligase targets IRGM1 to regulate Wnt signaling and intestinal homeostasis.

Hierarchical organization of intestine relies on the self-renewal and tightly regulated differentiation of intestinal stem cells (ISCs). Although signals like Wnt are known to sustain the continued intestinal renewal by maintaining ISCs activity and lineage commitment, molecular mechanisms underlying ISCs 'stemness' and supportive niche have not been well understood. Here, we found that CUL4B-RING ubiquitin ligase (CRL4B) regulates intestinal homeostasis by targeting immunity-related GTPase family M member 1 (IRGM1) for proteasomal degradation. CUL4B was mainly expressed at ISCs zone. Deletion of Cul4b led to reduced self-renewal of ISCs and a decreased lineage differentiation towards secretory progenitors through downregulated Wnt signals. Besides, Cul4b-null mice exhibited impaired Paneth cells number and structure. Mechanistically, CRL4B complex were associated with WD40 proteins and targeted IRGM1 at K270 for ubiquitination and proteosomal degradation. Impaired intestinal function caused by CUL4B deletion was rescued by down-regulation of its substrate IRGM1. Our results identified CUL4B as a novel regulator of ISCs and revealed a new 26 S proteasome degradation mechanism in intestine self-renewal and lineage commitment.