Recombinant Mouse Receptor expression-enhancing protein 2 (Reep2)

Basic Research Questions

• What is REEP2 and what are its key structural features?

Receptor expression-enhancing protein 2 (REEP2) belongs to the REEP/DP1/Yop1p family of endoplasmic reticulum (ER)-shaping proteins. It contains two highly conserved hydrophobic domains at its N-terminus that form hairpin structures which insert into membranes and promote homo- or hetero-oligomerization . These hairpin insertions modulate membrane curvature, which is essential for the production and stabilization of highly curved ER tubules and the edges of ER sheets .

REEP2 is approximately 28 kDa in size with 252 amino acids . Unlike its close relative REEP1, REEP2 is not an integral membrane protein but associates with membranes through labile interactions or by interacting with integral membrane proteins such as REEP1, atlastin, or spastin .

Structural domains of REEP2:

• How is REEP2 expressed in different mouse tissues and what techniques are used to detect it?

REEP2 is highly expressed in brain and testis . In the nervous system, it is preferentially expressed in neuronal tissues similar to its ortholog REEP1 . REEP2 is also significantly expressed in taste cells, particularly Type II taste cells that mediate sweet, bitter, and umami taste sensations .

Expression pattern in taste cells:

Several techniques can be used to detect REEP2 expression:

• Immunohistochemistry with anti-REEP2 antibodies (useful for tissue sections)

• Immunofluorescence for co-localization with other markers

• Western blotting for protein detection (observed at approximately 28 kDa)

• RT-PCR and qPCR for mRNA expression analysis

• Flow cytometry for cellular expression analysis

For optimal immunohistochemical detection in mouse brain tissue, antigen retrieval with TE buffer pH 9.0 is recommended .

• What methodologies are used to study the membrane binding properties of REEP2?

Several complementary approaches can be used to characterize REEP2's membrane binding properties:

• Subcellular fractionation: This technique separates cellular components based on their density and size. Studies have shown that both wild-type and p.Val36Glu REEP2 sediment in the membrane fraction .

• Alkaline extraction: This method distinguishes between integral and peripheral membrane proteins. Upon alkaline extraction, both wild-type and mutant REEP2 dissociate from membranes similar to the membrane-associated protein calreticulin, suggesting REEP2 is not an integral membrane protein unlike REEP1 .

• Liposome flotation assay: This technique assesses direct binding to artificial membranes in the absence of other proteins. Wild-type REEP2 purified from E. coli binds to membranes, whereas the p.Val36Glu variant does not. Importantly, when an equimolar mixture of wild-type and p.Val36Glu REEP2 is tested, the interaction with membranes is abolished, demonstrating a dominant-negative effect .

• In vivo biotinylation analysis: This method can determine if REEPs are expressed at the plasma membrane. No REEPs (including REEP2) were precipitated by avidin following biotinylation, demonstrating they are not present at the plasma membrane .

• Microscopy-based approaches: Overexpression of tagged REEP2 in cell lines followed by immunofluorescence microscopy reveals its distribution in a dot-like pattern along both the ER and microtubules .

• How does REEP2 affect endoplasmic reticulum morphology?

REEP2 plays a crucial role in shaping the endoplasmic reticulum through several mechanisms:

• Membrane curvature modulation: The insertion of REEP2's hydrophobic hairpin domains into the ER membrane generates and stabilizes the highly curved ER tubules and the edges of ER sheets .

• ER morphology regulation: siRNA-mediated REEP2 depletion in COS7 cells leads to altered ER morphology, demonstrating its importance in maintaining normal ER structure .

• ER sheet organization: The distribution of ER sheet marker CLIMP-63 is affected by REEP2 expression. In cells overexpressing wild-type REEP2, CLIMP-63 distribution is similar to non-transfected cells, but it becomes more widespread in cells overexpressing the p.Val36Glu REEP2 variant, indicating ER sheet expansion .

• Interaction with other ER-shaping proteins: REEP2 interacts with other proteins involved in ER morphogenesis, including atlastin-1, M1-spastin, and REEP1, as demonstrated by co-immunoprecipitation studies .

These findings highlight REEP2's integral role in the complex network of proteins that maintain the characteristic architecture of the ER, particularly its tubular network and sheet organization.

• How do researchers efficiently produce and purify recombinant REEP2 protein?

While specific protocols for REEP2 production are not detailed in the provided search results, the following methodology has been used for successful purification of functional REEP2:

• Expression system: Recombinant REEP2 has been successfully expressed in E. coli systems . For mammalian expression, vectors directing expression of V5-tagged or GFP-tagged REEP2 in COS7 cells have been used .

• Purification approach: The protein can be purified from E. coli extracts using standard protein purification techniques . His-tagged versions facilitate purification via affinity chromatography.

• Quality control: SDS-PAGE and Western blotting can verify protein integrity and purity. Functional verification through liposome binding assays confirms that the recombinant protein maintains its native properties .

• Storage considerations: Based on protocols for other recombinant proteins, purified REEP2 should be stored at -80°C, preferably in small aliquots to avoid repeated freeze-thaw cycles .

A general reconstitution protocol would involve:

• Centrifuging the vial before opening

• Reconstituting lyophilized protein in an appropriate buffer (such as PBS)

• Allowing several minutes for complete reconstitution

• For prolonged storage, diluting to working aliquots in a 0.1% BSA solution

Advanced Research Questions

• How do REEP2 mutations lead to hereditary spastic paraplegia (SPG72) and what are the molecular mechanisms involved?

REEP2 mutations cause hereditary spastic paraplegia (SPG72) through distinct molecular mechanisms depending on the inheritance pattern:

Autosomal dominant mutations (e.g., p.Val36Glu, p.Met40Arg):

• These mutations act through a dominant-negative effect

• The mutant protein inhibits wild-type REEP2's normal binding to membranes

• Liposome flotation assays demonstrate that when wild-type and p.Val36Glu REEP2 are mixed, membrane binding is abolished

• This leads to altered ER morphology with expansion of ER sheets and swelling

• The distribution of the ER marker CLIMP-63 becomes more widespread in cells expressing p.Val36Glu REEP2

Autosomal recessive mutations (e.g., p.Phe72Tyr + splice site mutation, p.Met1Thr):

• These mutations cause loss of function through decreased affinity for membranes (p.Phe72Tyr) or destruction of the protein translation initiation site (p.Met1Thr)

• Compound heterozygosity (e.g., p.Phe72Tyr with c.105+3G>T splice site mutation) is expected to cause complete loss of REEP2 function

Mechanistic similarities with other HSPs:
REEP2 interacts with other HSP-associated proteins (atlastin-1/SPG3, spastin/SPG4, and REEP1/SPG31) , suggesting these proteins function in a common pathway involved in ER morphogenesis. Disruption of this pathway appears to be a shared mechanism underlying different forms of HSP, particularly affecting long corticospinal axons that are vulnerable to defects in membrane trafficking and ER morphology .

• What experimental evidence supports the role of REEP2 in G protein-coupled receptor (GPCR) trafficking and function?

REEP2 was initially identified by its ability to enhance cell surface expression of G protein-coupled receptors (GPCRs) , but subsequent research has revealed more nuanced mechanisms:

• Enhanced receptor function without increased surface expression: Studies show that REEP2 specifically enhances responses of heterologously expressed sweet and bitter taste receptors without increasing their cell surface expression .

• Spatial reorganization rather than trafficking: REEP2 does not increase cell surface expression of sweet receptors but alters their spatial organization by recruiting them into lipid raft microdomains .

• Lipid raft recruitment: Discontinuous sucrose density gradient ultracentrifugation experiments reveal that REEP2 significantly increases the amount of T1R2 and T1R3 sweet receptors in lipid rafts without altering their total protein levels .

• Functional impact on endogenous receptors: Downregulation of endogenously expressed REEP2 in the chemosensory enteroendocrine GLUTag cell line dramatically reduced sensitivity of endogenous sweet receptors, confirming its physiological relevance .

• Localization in taste cells: In native taste cells, REEP2 localizes preferentially to the taste bud's apical pore region, suggesting it recruits taste receptors to specific membrane domains for optimal access to tastants .

• What are the differences between autosomal dominant and recessive REEP2 mutations in terms of clinical presentation and disease severity?

The limited clinical cases described in the literature suggest distinct patterns between dominant and recessive REEP2 mutations:

Autosomal Dominant Mutations:

• p.Val36Glu: Found in a French family across three generations with "pure" HSP

• p.Met40Arg: Identified in both a sporadic case and a Nepalese family with "pure" HSP

• Clinical features: Relatively milder phenotype in some cases

• In the Nepalese family, the proband presented with slow and spastic gait at age 2 years with slow progression, while the father and uncle presented even milder symptoms

Autosomal Recessive Mutations:

• Compound heterozygous for c.215T>A (p.Phe72Tyr) and c.105+3G>T (splice site mutation) in a Portuguese family

• Homozygous p.Met1Thr affecting the start codon in two affected individuals

• Clinical features: Potentially earlier onset or more severe disease

• The children with homozygous p.Met1Thr had symptom onset in infancy, although they could still walk unaided at ages 3 and 4 years

• How do the functions of REEP2 compare with other REEP family members, and what techniques can distinguish between them?

The REEP family consists of at least six members (REEP1-6) that can be divided into two structural and functional groups:

Functional comparison:

Techniques to distinguish REEP family members:

• Subcellular fractionation and alkaline extraction: Distinguishes between integral membrane proteins (REEP1) and peripherally associated ones (REEP2)

• Immunostaining with specific antibodies: Can identify specific REEP isoforms in tissues and cells

• Functional assays:

  • Taste receptor enhancement (specific to REEP2)

  • Effect on ER morphology (different patterns among REEP family members)

  • Membrane binding properties (direct binding for REEP2 vs. integral association for REEP1)

• Knockout/knockdown studies: Targeting specific REEP isoforms reveals their unique physiological roles

• Expression analysis: RT-PCR, qPCR, and in situ hybridization can identify tissue-specific expression patterns that distinguish REEP family members

• What is the evidence for REEP2's role in lipid raft recruitment, and how can this be experimentally manipulated?

REEP2 plays a crucial role in recruiting specific receptors to lipid raft microdomains:

Evidence supporting REEP2's role in lipid raft recruitment:

• Sucrose density gradient analysis: Discontinuous sucrose density gradient ultracentrifugation shows that REEP2—present in the lipid raft fraction—significantly increases the amount of T1R2 and T1R3 sweet taste receptors in lipid rafts without altering their total protein levels .

• Quantification of band intensities: Western blot analysis confirms that REEP2 specifically increases the presence of T1R2 and T1R3 in lipid rafts .

• Localization in taste cells: REEP2 preferentially localizes to the taste bud's apical pore region in all taste papillae, where lipid rafts are present, suggesting it recruits taste receptors to specific domains for optimal tastant access .

• Functional impact: Downregulation of endogenously expressed REEP2 dramatically reduces sensitivity of endogenous sweet receptors, confirming the functional significance of this recruitment .

Experimental manipulation approaches:

• Overexpression studies: Transfection of cells with REEP2 expression constructs increases receptor recruitment to lipid rafts .

• siRNA knockdown: Reduction of endogenous REEP2 expression decreases receptor presence in lipid rafts and reduces receptor function .

• Lipid raft disruption: Treatment with methyl-β-cyclodextrin, which depletes membrane cholesterol and disrupts lipid rafts, could be used to determine if REEP2's effects require intact lipid rafts.

• Mutational analysis: Introduction of mutations in REEP2 (like p.Val36Glu) that affect membrane binding can assess their impact on lipid raft recruitment ability .

• Chimeric proteins: Creating chimeras between REEP2 and other REEP family members could identify domains specifically required for lipid raft recruitment.

• What protein-protein interactions does REEP2 participate in, and how are these affected by disease-causing mutations?

REEP2 participates in multiple protein-protein interactions crucial for its function:

Key REEP2 protein-protein interactions:

• Self-association: REEP2 can form homo-oligomers, as demonstrated by co-immunoprecipitation of differently tagged REEP2 proteins (V5-tagged and GFP-tagged) .

• Interaction with REEP1: REEP2 co-immunoprecipitates with REEP1, suggesting functional interaction between these related proteins .

• Interaction with atlastin-1: REEP2 interacts with atlastin-1, a protein involved in ER fusion and also implicated in HSP (SPG3) .

• Interaction with M1-spastin: REEP2 binds to M1-spastin, another HSP-associated protein (SPG4) involved in microtubule severing .

• Interaction with microtubules: Both wild-type and mutant REEP2 (p.Val36Glu) interact with microtubules in microtubule pull-down assays .

• Association with taste receptors: REEP2 physically associates with both subunits of the T1R2 and T1R3 sweet receptor .

Effects of disease-causing mutations on interactions:

The p.Val36Glu pathogenic variant does not prevent:

• REEP2 self-association

• Interaction with atlastin-1, M1-spastin, or REEP1

• Binding to microtubules

• Direct binding to membranes in liposome flotation assays

• The ability of wild-type REEP2 to bind membranes when both are present (dominant-negative effect)

This suggests that while HSP-causing mutations preserve protein-protein interactions, they critically disrupt REEP2's interaction with membranes, which is essential for its function in ER shaping.

• What are the most effective experimental approaches for studying the loss of function versus dominant-negative mechanisms of REEP2 mutations?

Different REEP2 mutations operate through distinct mechanisms that require specific experimental approaches:

Loss of function mechanisms (e.g., recessive mutations like p.Phe72Tyr):

• Membrane binding assays:

  • Liposome flotation assays with purified recombinant proteins to quantify reduced membrane binding

  • Subcellular fractionation to assess altered membrane association

• Protein expression analysis:

  • Western blotting to detect reduced protein levels

  • Immunofluorescence to assess altered localization

• Splice site mutation analysis:

  • RT-PCR to detect altered splicing patterns

  • Minigene assays to confirm splicing defects

• Complete knockout models:

  • CRISPR/Cas9-mediated REEP2 knockout cells

  • Knockout mouse models to recapitulate recessive disease

Dominant-negative mechanisms (e.g., dominant mutations like p.Val36Glu):

• Co-expression studies:

  • Co-transfection of wild-type and mutant REEP2 to assess interference

  • Titration experiments with different ratios to determine dose effects

• In vitro mixing experiments:

  • Liposome flotation assays with mixtures of wild-type and mutant proteins

  • Analysis of oligomerization in the presence of both proteins

• ER morphology assessment:

  • Immunostaining for ER markers like CLIMP-63 in cells expressing mutant REEP2

  • Live-cell imaging of ER dynamics

• Knock-in models:

  • Creation of heterozygous knock-in mice expressing dominant mutations

  • Analysis of motor function and spinal cord pathology

• Rescue experiments:

  • Overexpression of wild-type REEP2 to determine if it can overcome dominant-negative effects

These approaches provide complementary data to distinguish between mutation mechanisms and may inform potential therapeutic strategies tailored to the specific disease mechanism.