REEP5

REEP5 is an endoplasmic/sarcoplasmic reticulum membrane-shaping protein that organizes tubular SR/ER architecture to support cardiomyocyte function, including Ca2+ cycling and excitation-contraction coupling (PMID:32075961, PMID:29431104). Loss of REEP5 destabilizes SR/ER membranes and produces luminal vacuolization, reduced contractility, and disrupted Ca2+ handling in cardiomyocytes, and causes dilated cardiomyopathy-like dysfunction in vivo across zebrafish and mouse models (PMID:32075961); ultrastructurally, REEP5 deficiency specifically deforms SR membrane geometry while sparing transverse tubules and Ca2+-handling protein levels, yielding depressed SR Ca2+ release and reduced EC-coupling gain (PMID:29431104). Through this membrane-shaping role REEP5 also coordinates inter-organelle contacts: it binds Mitofusins 1 and 2 to enable mitochondrial 'hitchhiking' along tubular ER on microtubules, controlling cytosolic mitochondrial distribution and ER-mitochondria tethering, with REEP5 loss causing perinuclear mitochondrial clustering and altered mitochondrial ROS (PMID:39133213), and in cardiac tissue its loss fragments mitochondrial networks, raises ROS, and remodels both the SR/ER membrane-shaping proteome and mitochondrial/microsomal proteomes (PMID:41672147). Beyond its structural role, REEP5 overexpression suppresses PERK/IRE1α/ATF6 ER-stress signaling and apoptosis after myocardial infarction and binds CLEC5A to counter CLEC5A-induced ER stress (PMID:39044150). REEP5 additionally acts as an accessory protein promoting ligand-stimulated endocytosis of CXCR1 (PMID:27966653), and forms a complex with TRAM1 that engages SARS-CoV-2 NSP3 at replication organelles to promote viral replication (PMID:37768083).

1. 2016 Medium

   Established a non-structural signaling role for REEP5 by asking whether it modulates GPCR trafficking, showing it is required for ligand-stimulated CXCR1 internalization and downstream signaling.

   Evidence Co-IP, siRNA depletion, receptor internalization and β-arrestin2 clustering imaging, ERK/invasion assays, xenograft model

   PMID:27966653

   Open questions at the time

   • Mechanism by which REEP5 promotes CXCR1 endocytosis (vesicle scission vs membrane shaping) not defined
   • Receptor selectivity beyond CXCR1 vs CXCR2 untested
   • Relationship to its ER membrane-shaping role unclear
2. 2018 High

   Resolved whether REEP5 shapes cardiac SR membranes and whether this geometry is functionally coupled to Ca2+ release, distinguishing structural deformation from altered Ca2+-handling protein expression.

   Evidence REEP5 knockout rats with FIB-SEM 3D SR reconstruction and simultaneous patch-clamp of Ca2+ currents and transients

   PMID:29431104

   Open questions at the time

   • How REEP5 physically curves/stabilizes SR membrane at molecular level not shown
   • Whether the same mechanism operates in non-cardiac ER untested here
3. 2020 High

   Confirmed REEP5 as essential for SR/ER structural integrity and cardiac function across orthogonal in vitro and in vivo systems, linking membrane destabilization to contractile and Ca2+-cycling failure.

   Evidence shRNA/siRNA in mouse cardiomyocytes, CRISPR/Cas9 zebrafish, AAV9 knockdown in mice, Ca2+ imaging and contractility measurements

   PMID:32075961

   Open questions at the time

   • Whether observed dilated cardiomyopathy phenotype reflects a primary membrane defect or secondary remodeling not fully separated
   • Human disease relevance not established
4. 2023 Medium

   Extended REEP5 function to host-pathogen biology by asking whether it participates in viral replication organelle formation, identifying a REEP5-TRAM1 complex that engages SARS-CoV-2 NSP3.

   Evidence Host-viral interactome mapping with co-expression of NSP3/NSP4/NSP6 and co-IP/MS

   PMID:37768083

   Open questions at the time

   • Direct vs indirect REEP5-NSP3 interaction not resolved
   • Whether membrane-shaping activity is required for RO formation untested
   • Limited mechanistic detail on replication promotion
5. 2024 High

   Defined a mechanism for inter-organelle organization by asking how tubular ER positions mitochondria, showing REEP5 binds MFN1/2 to mediate mitochondrial hitchhiking and tethering.

   Evidence Co-IP, rapamycin-induced forced dimerization, live-cell imaging, ROS measurement, siRNA and overexpression

   PMID:39133213

   Open questions at the time

   • Stoichiometry and structural basis of the REEP5-MFN interface unknown
   • How tethering dynamics regulate ROS mechanistically not defined
6. 2024 Medium

   Addressed whether REEP5 protects cardiomyocytes from ER-stress injury, showing its overexpression suppresses PERK/IRE1α/ATF6 signaling and CLEC5A-driven apoptosis after myocardial infarction.

   Evidence Co-IP (REEP5-CLEC5A), gain-of-function overexpression in MI mouse and hypoxia cell models, western blot for ER-stress markers, apoptosis assays

   PMID:39044150

   Open questions at the time

   • Whether ER-stress suppression is a direct consequence of CLEC5A binding or of membrane stabilization unclear
   • Loss-of-function effect on ER stress not tested
   • Single-lab gain-of-function design
7. 2026 Medium

   Connected REEP5 SR/ER shaping to mitochondrial health at the proteome level, showing cardiac REEP5 loss fragments mitochondria, raises ROS, and remodels membrane-shaping and mitochondrial proteomes.

   Evidence AAV9-shRNA knockdown in mouse heart, subcellular fractionation with DIA mass spectrometry, live imaging of mitochondrial networks, ROS measurement

   PMID:41672147

   Open questions at the time

   • Causal chain from SR/ER membrane defect to mitochondrial import/antioxidant proteome changes not established
   • Whether RTN4/ATL3/CKAP4 upregulation is true functional compensation untested

• How REEP5's intrinsic membrane-curving biochemistry generates SR/ER tubule geometry, and how this single activity is shared across its cardiac, inter-organelle, GPCR-trafficking, and viral roles, remains unresolved.
• No structural model of REEP5 membrane-shaping mechanism in the corpus
• No unifying biochemical assay linking the disparate functional contexts
• No human Mendelian disease link established in the corpus