Endoglin Wild Type and Variants Associated With Hereditary Hemorrhagic Telangiectasia Type 1 Undergo Distinct Cellular Degradation Pathways

Nesrin Gariballa,Bassam R Ali,Anne John,Praseetha Kizhakkedath,Nadia Akawi

doi:10.3389/fmolb.2022.828199

Abstract

Endoglin, also known as cluster of differentiation 105 (CD105), is an auxiliary receptor in the TGFβ signaling pathway. It is predominantly expressed in endothelial cells as a component of the heterotetrameric receptor dimers comprising type I, type II receptors and the binding ligands. Mutations in the gene encoding Endoglin (ENG) have been associated with hereditary hemorrhagic telangiectasia type 1 (HHT1), an autosomal dominant inherited disease that is generally characterized by vascular malformation. Secretory and many endomembrane proteins synthesized in the Endoplasmic reticulum (ER) are subjected to stringent quality control mechanisms to ensure that only properly folded and assembled proteins are trafficked forward through the secretory pathway to their sites of action. We have previously demonstrated that some Endoglin variants causing HHT1 are trapped in the ER and fail to traffic to their normal localization in plasma membrane, which suggested the possible involvement of ER associated protein degradation (ERAD) in their molecular pathology. In this study, we have investigated, for the first time, the degradation routes of Endoglin wild type and two mutant variants, P165L and V105D, and previously shown to be retained in the ER. Stably transfected HEK293 cells were treated with proteasomal and lysosomal inhibitors in order to elucidate the exact molecular mechanisms underlying the loss of function phenotype associated with these variants. Our results have shown that wild type Endoglin has a relatively short half-life of less than 2 hours and degrades through both the lysosomal and proteasomal pathways, whereas the two mutant disease-causing variants show high stability and predominantly degrades through the proteasomal pathway. Furthermore, we have demonstrated that Endoglin variants P165L and V105D are significantly accumulated in HEK293 cells deficient in HRD1 E3 ubiquitin ligase; a major ERAD component. These results implicate the ERAD mechanism in the pathology of HHT1 caused by the two variants. It is expected that these results will pave the way for more in-depth research studies that could provide new windows for future therapeutic interventions.

Highlights

Hereditary hemorrhagic telangiectasia type 1 (HHT1; OMIM 187300), known as Rendu-Osler-Weber syndrome, is an autosomal dominant inherited disease that is generally characterized by vascular malformation which can range from small cutaneous and mucous membrane telangiectases to large arteriovenous malformation (AVM) in the lungs, liver, brain, and gastrointestinal tracts (McAllister et al, 1994; Richards-Yutz et al, 2010)
HHT1 has been associated with mutations in the transforming growth factor Beta (TGFβ) coreceptor Endoglin that is encoded by ENG (McAllister et al, 1994)
Due to the stringent quality control mechanisms dedicated to protein folding, it was estimated that 12–15% of newly synthesized protein in the Endoplasmic reticulum (ER) are eliminated co-translationally through the proteasomal system (Sun and Brodsky 2019)

Summary

Introduction

Hereditary hemorrhagic telangiectasia type 1 (HHT1; OMIM 187300), known as Rendu-Osler-Weber syndrome, is an autosomal dominant inherited disease that is generally characterized by vascular malformation which can range from small cutaneous and mucous membrane telangiectases to large arteriovenous malformation (AVM) in the lungs, liver, brain, and gastrointestinal tracts (McAllister et al, 1994; Richards-Yutz et al, 2010). The disease affects 1 in 10,000 individuals, the age of onset of the disease and phenotype penetrance may vary considerably amongst affected individuals (McDonald et al, 2015). AVMs usually manifest as congenital lesions that vary significantly in terms of lesion sites, numbers, and severity of symptoms (McDonald, 2011). Large AVMs often account for serious consequences such as stroke and fatal hemorrhages that can lead to death (Karabegovic et al, 2004). Haploinsufficiency due to loss of function is widely accepted as the underlying functional mechanism for HHT1 (Galaris et al, 2021)

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