The Sel1L-Hrd1 Endoplasmic Reticulum-Associated Degradation Complex Manages a Key Checkpoint in B Cell Development

Abstract

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a principal mechanism that targets ER-associated proteins for cytosolic proteasomal degradation. Here our data demonstrate a critical role for the Sel1L-Hrd1 complex, the most conserved branch of ERAD, in early B cell development. Loss of Sel1L-Hrd1 ERAD in B cell precursors leads to a severe developmental block at the transition from the large to small pre-B cells. Mechanistically, we show that Sel1L-Hrd1 ERAD selectively recognizes and targets the pre-B cell receptor (pre-BCR) for proteasomal degradation in a BiP-dependent manner. The pre-BCR complex accumulates both intracellularly and at the cell surface in Sel1L-deficient pre-B cells, leading to persistent pre-BCR signaling and pre-B cell proliferation. This study thus implicates ERAD mediated by Sel1L-Hrd1 as a key regulator of B cell development and reveals the molecular mechanism underpinning the transient nature of pre-BCR signaling.

ER-associated degradation is required for vasopressin prohormone processing and systemic water homeostasis

“…Our data show that mammalian SEL1L-HRD1 ERAD is responsible for the clearance of misfolded proAVP in the ER and thereby ERAD deficiency is not necessarily associated with elevated cell death is further supported by recent reports of ERAD-deficient adipocytes (25,26), B cells (27), and colonic epithelium (21). We speculate that cells with compromised ERAD function may adapt in a number of ways including autophagy, reprogramming of BiP or other ER chaperone expression, and/or sequestering of misfolded proteins into "nontoxic" amyloid aggregates (51) in order to reset the parameters of ER homeostasis and thereby allow cells to survive for an extended period of time.…”

“…We next explored how Sel1L deficiency in the ER of AVP neurons leads to systemic water imbalance. As in other cell types (20,21,27), Hrd1 protein levels were markedly reduced in Sel1L-deficient AVP neurons ( Figure 4A the axons toward synaptic terminals. We costained brain tissue sections with 2 different proAVP antibodies specifically recognizing AVP and NPII domains ( Figure 4E) (39,40) and found that at 1 week of age, there was already a notable reduction of AVP signal in the axons of Sel1L AVP neurons ( Figure 4F and Supplemental Figure 4).…”

“…Thus far, studies of ERAD in a cell type-specific context lag behind those of the UPR, but evidence published to date suggests that the physiological role of Sel1L-Hrd1 ERAD can be UPR independent, depending on the specific substrate(s) found in specific cell types (20,21,(25)(26)(27)(46)(47)(48). In this study, we showed that ER retention of proAVP occurs early and coincides with the initiation of diabetes insipidus in Sel1L-deficient mice.…”

“…For example, IRE1α, which is a Sel1L-Hrd1 ERAD substrate in many cell types (20), accumulates in the ER but remains functional to sense misfolded ER proteins in ERAD-deficient cells. The pre-B cell receptor, a Sel1L-Hrd1 ERAD substrate in developing B cells, accumulates in the ER and leads to an increased level of functional proteins at the cell surface of ERAD-deficient cells (27). In both cases, ERAD deficiency leads to a gain-of-function phenotype mimicking the effect of substrate overexpression.…”

“…Germline Sel1L or Hrd1 deficiency is embryonically lethal in mice (23,24), and acute global loss of Sel1L or Hrd1 in adult mice leads to premature death within 2 to 3 weeks (20,23), suggesting that Sel1L and Hrd1 are indispensable for both development and postnatal survival (11). Recent studies of cell type-specific Sel1L-Hrd1 deficiency in adipocytes (25,26), B cells (27), and colonic epithelium (21) have delineated the physiological importance of ERAD in these various tissues and identified several endogenous substrates (11), including IRE1α of the unfolded protein response (UPR) in many cell types (20); lipoprotein lipase and PGC1β in adipocytes (25,26); and pre-B cell receptor in developing B cells (27). However, the role of ERAD in neuroendocrine cells has not been explored.…”