Recombinant Human Receptor expression-enhancing protein 2 (REEP2)

What is REEP2 and what is its basic molecular structure?

REEP2 (Receptor Expression-Enhancing Protein 2) is a protein encoded by the REEP2 gene located on chromosome 5 in humans. It belongs to the REEP/DP1/Yop1p family of ER-shaping proteins. REEP2 contains two highly conserved hydrophobic domains at its N-terminus, which form hairpin structures that insert into membranes to modulate membrane curvature . These hairpin domains are similar to the Reticulon Homology Domain (RHD) found in reticulon proteins .

REEP2's structure includes cytosolic amino and carboxy termini with the hairpin hydrophobic domains inserting into the cytoplasmic face of membranes, particularly endoplasmic reticulum (ER) membranes . This structural arrangement is crucial for its function in shaping ER tubules and edges of ER sheets.

How does REEP2 expression differ from other REEP family members?

REEP2 exhibits a highly tissue-specific expression pattern that distinguishes it from some other REEP family members:

Unlike REEP5 and REEP6, which are more broadly expressed, REEP2 (along with REEP1) shows a restricted expression pattern primarily in neuronal tissues and tissues with neuronal-like exocytotic properties . This restricted expression pattern is consistent with the neuronal-specific symptoms observed in REEP2-related disorders . DNA microarray analysis has shown that REEP2 mRNA is expressed in murine sympathetic neurons, specifically superior cervical and stellate ganglia, though at lower levels than REEP1 .

What experimental methods are most effective for detecting endogenous REEP2?

For detecting endogenous REEP2, multiple complementary approaches should be considered:

When designing experiments, researchers should be aware that, unlike previous RT-PCR studies suggesting widespread expression, protein-level studies show that REEP2 expression is restricted to neuronal and neuronal-like exocytotic tissues . Therefore, tissue-specific controls are essential for validation.

How does REEP2 contribute to endoplasmic reticulum morphology?

REEP2 shapes the endoplasmic reticulum through its membrane-binding and curvature-inducing properties:

• Hairpin Insertion: The hydrophobic hairpin domains of REEP2 insert into the cytoplasmic face of ER membranes, creating localized curvature that promotes the formation of tubular ER structures .

• Regulation of ER Sheets: REEP2 affects the distribution of sheet proteins such as CLIMP-63. In fibroblasts from patients with REEP2 mutations, CLIMP-63 shows abnormal distribution throughout the peripheral ER, indicating expansion of ER sheets .

• Stabilization of High Curvature: REEP2 helps stabilize the highly curved ER tubules and the edges of ER sheets, which is essential for maintaining the complex architecture of the ER network .

Experimental evidence from transmission electron microscopy shows that mutations in REEP2 lead to abnormal ER structures that are wider (>80 nm) compared to the normal ER tubules (20-60 nm) . This demonstrates REEP2's critical role in maintaining proper ER morphology.

What methodological approaches can researchers use to study REEP2-membrane interactions?

Several complementary approaches can be employed to study REEP2-membrane interactions:

• Liposome Flotation Assay: This in vitro approach can assess the direct binding of purified REEP2 to membranes in the absence of other proteins. Wild-type REEP2 partially binds to membranes in this assay, while disease-causing variants like p.Val36Glu show impaired membrane binding .

• Subcellular Fractionation: This technique can determine the association of REEP2 with membrane fractions in cellular contexts, providing information about its localization within the cell .

• Fluorescence Microscopy with ER Markers: Co-labeling cells with fluorescently tagged REEP2 and ER markers (e.g., GFP-Sec61β for the whole ER, anti-CLIMP-63 for ER sheets) allows visualization of REEP2's distribution relative to different ER domains .

• Transmission Electron Microscopy: This provides high-resolution images of ER morphology, enabling measurement of ER tubule dimensions and assessment of abnormalities caused by REEP2 mutations .

• FRAP (Fluorescence Recovery After Photobleaching): This technique can assess the dynamics of REEP2's association with membranes in living cells.

When investigating dominant negative mutations, researchers should consider comparing cells expressing only wild-type REEP2, only mutant REEP2, or an equimolar mixture of both to characterize how mutations affect the function of the wild-type protein .

How do REEP2 mutations affect its ability to shape membranes?

REEP2 mutations identified in hereditary spastic paraplegia (HSP) patients impair its membrane-shaping functions through several mechanisms:

• Direct Reduction in Membrane Binding: The p.Val36Glu mutation, which causes autosomal dominant HSP, directly impairs REEP2's ability to bind to membranes in liposome flotation assays .

• Dominant Negative Effects: When the p.Val36Glu variant is co-expressed with wild-type REEP2, it prevents the wild-type protein from binding to membranes, explaining the dominant inheritance pattern .

• Loss of Function: The p.Phe72Tyr mutation found in autosomal recessive HSP reduces the affinity of REEP2 for membranes, which, when combined with a splice site mutation (c.105+3G>T) on the other allele, leads to complete loss of REEP2 function .

The functional consequences of these mutations can be observed at the cellular level:

• Fibroblasts from patients with REEP2 mutations show abnormal ER morphology with expanded ER sheets and wider ER structures (>80 nm compared to the normal 20-60 nm) .

• CLIMP-63, an ER sheet protein normally concentrated around the nucleus, shows abnormal distribution throughout the peripheral ER in cells with REEP2 mutations .

• Similar ER phenotypes are observed when REEP2 is downregulated by shRNA or when mutant REEP2 is overexpressed, supporting a loss-of-function or dominant-negative mechanism .

These findings highlight how REEP2 mutations disrupt ER morphology, potentially explaining the pathophysiology of HSP.

What is the mechanism by which REEP2 enhances receptor function?

REEP2 enhances receptor function through a distinct mechanism involving lipid raft recruitment rather than increased cell surface expression:

• Lipid Raft Recruitment: REEP2 recruits receptors, particularly sweet taste receptors (T1R2/T1R3), into lipid raft microdomains localized near the cell's apical region .

• Spatial Organization: Unlike other accessory proteins that primarily promote receptor transit to the cell surface, REEP2 alters the spatial organization of receptors within the membrane .

• Improved G-protein Signaling: The recruitment to lipid rafts improves G-protein-coupled receptor signaling by facilitating interactions with downstream signaling components .

• Tastant Access: REEP2 promotes receptor access to tastants arriving through the apical taste pore, enhancing sensitivity to sweet compounds .

This mechanism is distinct from that of other accessory proteins like RTP1, RTP2, and REEP1, which primarily enhance olfactory receptor function by promoting their transit to the cell surface .

How can researchers experimentally assess REEP2's enhancement of receptor function?

To assess REEP2's enhancement of receptor function, researchers can employ several complementary approaches:

• Calcium Imaging Assays: These can quantify changes in receptor-mediated calcium signaling in cells expressing receptors with or without REEP2 co-expression .

• Electrophysiological Recordings: These can measure receptor-activated currents with high temporal resolution to assess functional enhancement.

• FRET/BRET Assays: These techniques can monitor protein-protein interactions between REEP2 and receptors, as well as interactions with downstream signaling components.

• Lipid Raft Isolation: Detergent-resistant membrane fractions can be isolated to determine whether REEP2 promotes receptor localization to lipid rafts .

• siRNA Knockdown in Native Systems: Downregulation of endogenously expressed REEP2 in chemosensory cells (e.g., GLUTag enteroendocrine cells) can assess the requirement of REEP2 for endogenous receptor sensitivity .

• Dose-Response Curves: These can quantify shifts in receptor sensitivity to ligands when REEP2 is present or absent, providing a measure of functional enhancement .

The study by Vassaux et al. demonstrated that downregulation of endogenously expressed REEP2 in GLUTag cells dramatically reduced sensitivity of endogenous sweet receptors, confirming its functional importance in a physiologically relevant context .

What types of REEP2 mutations have been identified in hereditary spastic paraplegia?

Several types of REEP2 mutations have been identified in hereditary spastic paraplegia (HSP), specifically SPG72, with both autosomal dominant and recessive inheritance patterns:

These mutations affect different aspects of REEP2 function:

• Dominant Mutations: The p.Val36Glu and p.Met40Arg mutations have dominant negative effects, inhibiting the normal binding of wild-type REEP2 to membranes .

• Recessive Mutations: The compound heterozygous mutations (p.Phe72Tyr with c.105+3G>T) or homozygous p.Met1Thr mutation cause complete loss of REEP2 function .

This spectrum of mutations illustrates how different genetic mechanisms can lead to similar clinical phenotypes and explains the different inheritance patterns observed in SPG72 families.

How do REEP2-related neurological disorders manifest clinically?

REEP2 mutations cause a form of hereditary spastic paraplegia (HSP) known as SPG72, which typically presents as a "pure" form of HSP:

• Clinical Presentation:

  • Progressive weakness and spasticity in the lower limbs

  • Increased deep tendon reflexes

  • Positive Babinski signs

  • Often includes pes cavus (high-arched feet)

• Age of Onset:

  • Variable, ranging from infancy to adulthood

  • Earlier onset typically seen in recessive cases (infancy)

  • Later onset in dominant cases (childhood to adulthood)

• Disease Progression:

  • Generally slow progression

  • Patients with dominant mutations often have milder symptoms

  • Complete REEP2 loss of function (recessive mutations) may lead to more severe phenotypes

• Neuroimaging: Brain and spine MRI are typically unremarkable, suggesting the pathology primarily affects axonal function rather than causing visible structural changes .

In a reported Nepalese family with the p.Met40Arg mutation, the proband presented with slow and spastic gait at age 2 years, while affected family members (father and uncle) showed milder symptoms, highlighting the variable expressivity of dominant REEP2 mutations .

What cellular models are most effective for studying REEP2 mutations in disease contexts?

Several cellular models have proven effective for studying REEP2 mutations in disease contexts:

• Patient-Derived Fibroblasts: Fibroblasts from affected individuals have been successfully used to study the effects of REEP2 mutations on ER morphology. These cells show abnormal ER structure with expanded ER sheets and wider ER tubules .

• COS7 Cell Overexpression: COS7 cells overexpressing wild-type or mutant REEP2 provide a system to study subcellular localization and effects on ER morphology. This model has demonstrated that the p.Val36Glu variant affects ER organization similar to what is observed in patient fibroblasts .

• shRNA Knockdown Models: Downregulation of REEP2 using shRNA vectors in COS7 cells replicates phenotypes observed in patient cells, supporting loss-of-function mechanisms .

• GLUTag Enteroendocrine Cells: This chemosensory cell line expresses endogenous REEP2 and can be used to study its role in sweet receptor function through siRNA knockdown approaches .

For biochemical studies of REEP2-membrane interactions, in vitro approaches using purified proteins have been effective:

• Liposome Flotation Assays: These have been used to directly assess the membrane-binding properties of wild-type and mutant REEP2 proteins .

• Microtubule Pull-Down Assays: These can assess interactions between REEP2 and microtubules, which may be relevant to its function .

When selecting a model system, researchers should consider the specific aspect of REEP2 function they wish to study and whether endogenous expression of interaction partners is required.

How does REEP2 interact with other proteins involved in ER morphology and function?

REEP2 participates in a network of protein interactions that collectively shape the ER:

• Self-Association: REEP2 can form homo-oligomers, which may be important for its membrane-shaping functions. The p.Val36Glu disease-causing variant does not prevent this self-interaction .

• Interaction with REEP Family Members: REEP2 can potentially interact with other REEP proteins, though this interaction appears distinct from REEP5, as they localize to different compartments when mutated .

• Interaction with Atlastin-1: REEP2 co-immunoprecipitates with atlastin-1, a GTPase involved in ER membrane fusion that is mutated in another form of HSP (SPG3). This interaction suggests coordination in ER remodeling processes .

• Interaction with M1-Spastin: REEP2 interacts with M1-spastin, a microtubule-severing protein mutated in the most common form of HSP (SPG4). This interaction connects ER morphology regulation with microtubule dynamics .

• Interaction with REEP1: REEP2 can interact with REEP1, which is mutated in SPG31, another form of HSP. This suggests potential functional redundancy or cooperation .

These interactions place REEP2 within a network of proteins implicated in HSP, explaining the similar clinical presentations of different genetic forms of the disease. The interactions are maintained even in the presence of the p.Val36Glu REEP2 variant, suggesting that the pathogenic mechanism involves disruption of membrane binding rather than protein-protein interactions .

What is the relationship between REEP2 and the formation of a specialized vesicular compartment?

Recent research has revealed that REEP2, along with other members of the REEP1-4 subfamily, resides in a unique vesicular compartment distinct from the bulk ER:

• Novel Vesicular Localization: REEP1-4 proteins localize to specific vesicular structures rather than throughout the ER network .

• Curvature-Dependent Localization: This vesicular localization depends on the proteins' membrane curvature-generating capabilities. Mutations that compromise curvature generation, including disease-causing mutations, relocalize the proteins to the bulk ER .

• Atlastin-1 Content: These REEP1-4 vesicles contain the membrane fusogen atlastin-1 but lack general ER proteins, suggesting a specialized function .

• Vesicle Generation Mechanism: It is proposed that REEP1-4 proteins generate these vesicles themselves by budding directly from the ER, and that the vesicles cycle back to the ER through atlastin-mediated fusion .

• Potential Function: These vesicles may serve to regulate ER tubule dynamics, potentially explaining why mutations in REEP1-4 and atlastin-1 cause similar neurodegenerative disorders .

This model represents a significant advancement in understanding REEP1-4 function, suggesting that they are not simply static ER-shaping proteins but are involved in dynamic vesicular processes that may be particularly important in neurons with long axons.

How can CRISPR/Cas9 technology be applied to study REEP2 function?

CRISPR/Cas9 technology offers versatile approaches for investigating REEP2 function:

• Gene Knockout Studies: Complete deletion of REEP2 can reveal its essential functions and potential compensatory mechanisms by other REEP family members.

• Knock-in of Disease Mutations: CRISPR can be used to introduce specific disease-associated mutations (e.g., p.Val36Glu, p.Met40Arg) into cell lines or animal models to study their effects on ER morphology and neuronal function.

• Tagging Endogenous REEP2: Adding fluorescent or affinity tags to endogenous REEP2 allows for visualization of its localization and dynamics without overexpression artifacts.

• Conditional Knockout Systems: For studying REEP2 function in specific cell types or developmental stages, conditional knockout approaches using Cre-loxP systems can be combined with CRISPR/Cas9.

• High-throughput Screening: CRISPR screens targeting genes that interact with REEP2 can identify new functional relationships and pathways.

Recent advances in prime editing techniques, which allow for precise genomic modifications without double-strand breaks, could be particularly valuable for introducing specific point mutations in REEP2 .

When designing CRISPR experiments for REEP2, researchers should consider:

• The tissue-specific expression pattern of REEP2, primarily in neuronal tissues

• Potential functional redundancy with other REEP family members

• The importance of examining ER morphology and receptor function as readouts

What expression systems are optimal for producing recombinant human REEP2?

For producing recombinant human REEP2, researchers should consider several expression systems with their respective advantages:

• Mammalian Expression Systems:

  • HEK293/HEK293A cells: Successfully used for REEP2 expression with various tags (V5, GFP, FLAG) .

  • Advantages: Proper folding, post-translational modifications, and native-like membrane insertion.

  • Applications: Cellular localization studies, protein-protein interaction analyses, and dominant-negative experiments.

• Bacterial Expression Systems:

  • E. coli: Can be used for producing segments of REEP2 or full-length protein for biochemical studies.

  • Advantages: High yield, cost-effectiveness, and simplicity.

  • Considerations: May require optimizing codons, using solubility tags, or refolding from inclusion bodies.

• Cell-Free Systems:

  • Allow for direct incorporation into liposomes during translation.

  • Useful for membrane protein studies without cell lysis steps.

For purification of functional REEP2:

• Detergent Selection: Critical for maintaining structure and function of membrane-associated regions.

• Affinity Tags: His, GST, or FLAG tags have been successfully used with REEP2 .

• Buffer Conditions: Should maintain protein stability while preserving membrane-binding capacity.

When designing expression constructs, researchers should consider:

• Including appropriate tags for detection and purification

• Using strong but controllable promoters (e.g., CMV for mammalian cells)

• The impact of tags on protein function, particularly membrane binding

For biochemical studies of REEP2-membrane interactions, in vitro translation has been successfully used, allowing the protein to be synthesized and used directly in liposome flotation assays .

What are the critical considerations for preserving REEP2 function in recombinant systems?

Preserving REEP2 function in recombinant systems requires careful attention to several factors:

• Membrane Environment:

  • REEP2 is a membrane-shaping protein that forms hairpin structures inserted into membranes.

  • Maintaining appropriate lipid composition is crucial for preserving its function.

  • Consider using microsomes or liposomes when studying purified protein .

• Hydrophobic Domains Integrity:

  • The N-terminal hydrophobic hairpin domains are essential for REEP2's membrane-shaping functions.

  • Mutations or tags that disrupt these domains can alter protein function.

  • Position tags away from the hydrophobic domains (preferably at the C-terminus) .

• Oligomerization Capability:

  • REEP2 forms homo-oligomers and interacts with other proteins.

  • Expression conditions should preserve these interaction capabilities.

  • Avoid strong detergents that might disrupt protein-protein interactions .

• Expression Level Control:

  • Overexpression may lead to aggregation or mislocalization.

  • Use inducible promoters to control expression levels.

  • For localization studies, compare with endogenous expression patterns .

• Validation of Functional Properties:

  • Verify membrane binding using liposome flotation assays.

  • Assess subcellular localization using fluorescent tags or immunostaining.

  • Confirm protein-protein interactions through co-immunoprecipitation .

When studying disease-causing mutations, it's particularly important to compare wild-type and mutant proteins under identical conditions to accurately assess functional differences. The p.Val36Glu variant has been successfully expressed in COS7 cells and used for both localization studies and biochemical assays of membrane binding .

What are the emerging areas of investigation for REEP2 function beyond ER shaping?

Several emerging research directions are expanding our understanding of REEP2 function beyond its established role in ER shaping:

• Specialized Vesicular Compartment: Recent evidence suggests REEP1-4 proteins reside in a unique vesicular compartment and may be involved in vesicle budding and fusion cycles with the ER, potentially regulating ER tubule dynamics .

• Receptor Signaling Enhancement: REEP2 enhances sweet receptor function through recruitment to lipid rafts, suggesting broader roles in sensory signaling and potentially other receptor systems .

• Neuron-Specific Functions: Given its restricted expression in neuronal tissues, REEP2 likely has specialized functions in neurons that may relate to their unique morphology and needs, particularly in long axons where ER architecture is critical .

• Interplay with Microtubule Dynamics: REEP2 interactions with spastin suggest potential roles in coordinating ER morphology with microtubule dynamics, which could be particularly important in axonal transport and maintenance .

• Potential Roles in Organelle Contacts: Other REEP family members modulate ER-mitochondria contacts, suggesting REEP2 might also function at organelle contact sites in neurons .

• Temporal Regulation in Neuronal Development: Evidence from cultured sympathetic ganglion neurons shows that REEP1 expression is temporally regulated, appearing between Day 4 and Day 8 of culture. Similar temporal regulation might exist for REEP2, potentially coordinating with developmental stages of neuronal maturation .

These emerging areas provide exciting opportunities for researchers to explore REEP2's multifaceted roles in cellular function, particularly in neuronal contexts where its dysfunction leads to disease.

How might therapeutic approaches targeting REEP2 be developed for hereditary spastic paraplegia?

Developing therapeutic approaches for REEP2-related hereditary spastic paraplegia (HSP) requires addressing different mechanisms based on mutation type:

• For Dominant Negative Mutations (e.g., p.Val36Glu, p.Met40Arg):

  • RNA Interference Approaches: Allele-specific siRNAs or antisense oligonucleotides could selectively silence the mutant allele while preserving wild-type expression.

  • Protein Quality Control Modulation: Enhancing degradation of mutant REEP2 might prevent its interference with wild-type protein.

  • Small Molecule Stabilizers: Compounds that stabilize wild-type REEP2-membrane interactions even in the presence of mutant protein.

• For Loss-of-Function Mutations (recessive cases):

  • Gene Replacement Therapy: Delivering functional REEP2 using viral vectors (AAV) targeted to affected neural tissues.

  • Upregulation of Compensatory Proteins: Enhancing expression of other REEP family members (particularly REEP1) that might partially compensate for REEP2 loss.

  • Read-through Approaches: For nonsense mutations, compounds promoting read-through of premature stop codons.

• For All Mutation Types:

  • ER Stress Modulators: Compounds reducing ER stress that might develop from altered ER morphology.

  • Lipid Composition Modification: Approaches altering membrane lipid composition to compensate for defective membrane shaping.

  • Downstream Pathway Targeting: Identifying and modulating pathways downstream of ER morphology disruption that lead to axonal degeneration.

The development of these therapeutic approaches requires:

• Better understanding of REEP2's exact function in neurons

• Identification of key interaction partners and pathways

• Development of relevant disease models (cellular and animal)

• High-throughput screening methods to identify potential therapeutic compounds

Research into related HSP proteins (atlastin-1, spastin, REEP1) may also inform therapeutic approaches for REEP2-related disease, given their functional interconnections .

What animal models would be most appropriate for studying REEP2 function in vivo?

Selecting appropriate animal models for studying REEP2 function requires consideration of several factors:

• Mouse Models:

  • Conventional Knockout: Complete deletion of REEP2 to study loss-of-function effects.

  • Knock-in Models: Introduction of specific patient mutations (e.g., p.Val36Glu, p.Met40Arg) to study dominant negative effects.

  • Conditional Knockouts: Tissue-specific or temporally controlled deletion to assess REEP2 requirements in different contexts.

  • Double Knockouts: Combined deletion of REEP2 with related proteins (e.g., REEP1) to address functional redundancy.

  • Advantages: Mammalian nervous system, established genetic tools, ability to assess complex behaviors and pathology.

• Zebrafish Models:

  • Morpholino Knockdown: Rapid assessment of developmental requirements.

  • CRISPR/Cas9 Mutants: Generation of stable genetic models.

  • Advantages: Transparent embryos for in vivo imaging, rapid development, ability to perform high-throughput drug screening.

• Drosophila Models:

  • REEP2 has orthologs conserved to Drosophila, making it a viable model organism .

  • Advantages: Rapid generation time, powerful genetic tools, simpler nervous system for functional studies.

• C. elegans Models:

  • The conserved nature of REEP proteins extends to C. elegans .

  • Advantages: Transparent body, fully mapped neural circuits, ease of genetic manipulation.

When developing these models, researchers should consider:

• The tissue-specific expression pattern of REEP2, primarily in neuronal and neuronal-like exocytotic tissues

• The potential compensatory roles of other REEP family members

• The need to examine both structural (ER morphology) and functional (neuronal activity, motor function) outcomes

• The differences in disease severity between dominant and recessive mutations