Recombinant Mouse Cation-dependent mannose-6-phosphate receptor (M6pr)

What is the molecular structure of recombinant mouse M6pr?

Mouse M6pr (also known as CD-MPR) is a type I transmembrane protein with a single transmembrane domain and a C-terminus located on the cytoplasmic side of the lipid membrane. The protein has a molecular mass of approximately 31.2 kDa . Its structure consists of a 28 amino-acid residue N-terminal signal sequence, a 159 amino acid residue luminal domain, a 25 amino acid residue transmembrane domain, and a 67 amino acid residue C-terminal cytoplasmic domain . The receptor typically appears as a homodimer at the membrane and either a dimer or a tetramer in solution .

How does mouse M6pr compare to human M6PR in terms of homology?

Mouse M6pr shows high conservation with human M6PR, with approximately 93% homology between mouse and human sequences . This high degree of conservation suggests evolutionary importance of the receptor's function. Both receptors maintain similar domain organization and transmembrane topology, which facilitates comparative studies between mouse models and human applications.

What are the primary functional roles of M6pr in cellular processes?

M6pr serves as a sorter and deliverer of lysosomal enzymes through receptor-mediated transport. It functions by binding to the mannose-6-phosphate (M6P) residues on lysosomal enzymes . The transport occurs via two main pathways:

• Intracellular pathway: Lysosomal enzyme marker M6P in the Golgi complex combines with M6pr and is transported into the Golgi apparatus.

• Cell surface pathway: M6P-containing lysosomes in the extracellular matrix bind to M6pr on the cell membrane to transport lysosomal enzymes .

In both pathways, a receptor-ligand complex forms when lysosomal enzymes bind to M6pr. This complex is then transported to lysosomes where, in the low pH environment, the receptor and ligand separate, allowing M6pr to recycle back to the Golgi complex or plasma membrane .

What expression systems are optimal for producing functional recombinant mouse M6pr?

For research applications, recombinant mouse M6pr is typically expressed in mammalian cell systems, particularly HEK293T cells, which provide appropriate post-translational modifications . This expression system is preferable because:

• It ensures proper folding and glycosylation patterns critical for M6pr function

• It accommodates the 5 potential Asn-linked glycosylation sites in the N-terminal extracytoplasmic region

• It yields protein with functional binding properties for M6P-tagged ligands

For enhanced purification and detection, recombinant M6pr can be produced with C-terminal tags such as MYC/DDK, which facilitate downstream applications including Western blotting and immunoprecipitation studies .

What are the recommended storage conditions for maintaining recombinant M6pr stability?

For optimal stability, recombinant mouse M6pr should be stored at -80°C immediately after receiving . The recommended storage buffer composition is 25 mM Tris-HCl (pH 7.3), 100 mM glycine, and 10% glycerol . Under proper storage and handling conditions, the protein remains stable for approximately 12 months. Repeated freeze-thaw cycles should be avoided as they can significantly reduce protein activity and integrity .

How can binding affinities between M6pr and its ligands be experimentally determined?

Surface plasmon resonance (SPR) analysis is a primary method for measuring binding affinities between M6pr and its ligands. When studying M6pr interactions, consider the following methodology:

• Immobilize recombinant human soluble M6PR on BIAcore sensor chips (type CM5) to densities of 72-75 fmol/mm²

• Inject samples containing potential ligands in flow buffer

• Analyze binding data using BIAevaluation program to determine dissociation constants (Kd)

For example, SPR analysis revealed that α-Gal A binds to human M6PR with a very high affinity (Kd estimated at 0.2 nM) . Competition assays can also be performed, as demonstrated when 50 μM M6P was shown to markedly inhibit (∼75%) the binding of α-Gal A to M6PR .

What experimental approaches can assess M6pr interactions with other sorting receptors?

To investigate interactions between M6pr and other sorting receptors like sortilin:

• Immobilization assays: Immobilize sortilin and test binding with M6PR (41-1365aa) containing the two distinct M6P-binding sites

• Competition binding assays: Determine if M6PR binding to sortilin affects binding of other ligands (e.g., α-Gal A) to sortilin

• Co-localization studies: Use fluorescently labeled proteins to visualize potential co-localization in cellular compartments

Research has demonstrated that M6PR (41-1365aa) binds to immobilized sortilin, and this binding prevents the binding of α-Gal A to sortilin, indicating that binding of M6PR to sortilin does not enhance α-Gal A binding to the M6PR-sortilin receptor complex .

What methods can track M6pr trafficking between cellular compartments?

Several approaches can effectively track M6pr trafficking:

• Immunofluorescence microscopy: Using antibodies against M6pr to visualize its localization in fixed cells. Counterstaining with organelle markers (e.g., GFP-Rab7 for late endosomes) enables co-localization analysis .

• Live-cell imaging: Expressing GFP-tagged M6pr to track movement in real-time, as demonstrated in studies showing co-movement of antisense oligonucleotides (ASOs) with GFP-M6PR-CD in live cells .

• Quantitative co-localization analysis: Measuring overlap between M6pr and compartment markers before and after stimulus (e.g., PS-ASO treatment) .

In SVGA cells, M6PR-CI primarily localizes to the perinuclear trans-Golgi network (TGN) region with some scattered dot-like structures co-localizing with GFP-Rab7 (late endosomes) . After PS-ASO treatment, the number of cytoplasmic foci containing M6PR-CI increases, with these foci also containing PS-ASO and Rab7, indicating increased late endosome localization .

How does M6pr shuttling between cellular compartments impact experimental design?

When designing experiments to study M6pr function, researchers should consider its dynamic shuttling between late endosomes and the trans-Golgi network. This shuttling behavior has several implications:

• Temporal considerations: Experimental time points should account for recycling kinetics between compartments

• Compartment isolation: When isolating cellular fractions, cross-contamination between endosomal and Golgi fractions must be minimized

• Trafficking modulators: GCC2 (a Golgi-localized protein) affects M6pr trafficking and should be considered when designing experiments

Research has shown that both GCC2 and M6pr act in the same pathway to influence PS-ASO activity, as simultaneous reduction of GCC2 and M6PR-CD did not cause additive effects on PS-ASO activity .

How can knockdown/knockout approaches be optimized for studying M6pr function?

For effective knockdown/knockout studies:

• siRNA approach: Target specific regions of M6pr mRNA to achieve selective knockdown. In cell culture systems, both M6PR-CI and M6PR-CD can be targeted separately or simultaneously .

• In vivo knockdown: For mouse studies, antisense oligonucleotides (ASOs) can be administered subcutaneously (e.g., 15 mg/kg GalNAc-conjugated PS-ASOs) to target M6pr expression . This approach has been successfully used to reduce M6PR levels in mouse liver.

• Validation: Confirm knockdown efficiency at both mRNA (qRT-PCR) and protein (Western blot) levels .

• Functional assessment: After confirming knockdown, evaluate phenotypic changes related to lysosomal enzyme trafficking or other M6pr-dependent processes.

Research has demonstrated different effects when targeting M6PR-CD versus M6PR-CI in mice, with reduction of M6PR-CI decreasing PS-ASO activity while depletion of M6PR-CD had minimal effect .

What are the species-specific differences in M6pr function between mouse and human models?

Important species-specific differences must be considered when translating findings between mouse and human models:

• While the protein structure shows high conservation (93% homology), functional differences exist .

• In mouse cells and in vivo studies, reduction of M6PR-CI significantly decreases PS-ASO activity, whereas reduction of M6PR-CD shows minimal effect .

• In human cells (e.g., A431 and HeLa), both M6PR-CI and M6PR-CD affect PS-ASO activity, with simultaneous reduction causing additive effects .

How does M6pr impact antisense oligonucleotide (ASO) delivery and activity?

M6pr plays a significant role in antisense oligonucleotide (ASO) trafficking and activity:

• Endosomal release mechanism: M6pr facilitates PS-ASO release from late endosomes through its shuttling between late endosomes and the trans-Golgi network .

• Co-localization evidence: Upon PS-ASO treatment, M6PR-CI increasingly localizes to late endosomes (Rab7-positive compartments) that also contain PS-ASOs .

• Isoform-specific effects: In mice, M6PR-CI, but not M6PR-CD, is necessary for optimal PS-ASO activity, while in human cells, both isoforms contribute .

This understanding has potential applications for enhancing ASO delivery strategies that leverage endogenous trafficking pathways.

What experimental designs can assess M6pr-mediated lysosomal enzyme targeting?

To study M6pr-mediated lysosomal enzyme targeting:

• Enzyme uptake assays: Measure cellular uptake of recombinant lysosomal enzymes in the presence/absence of M6pr expression

• Competition assays: Determine if free M6P inhibits enzyme uptake (e.g., 50 μM M6P markedly inhibits α-Gal A binding to M6PR by ~75%)

• Domain-specific binding: Test binding to specific M6pr domains (e.g., M6PR (41-1365aa) containing the two M6P-binding sites vs. M6PR (1510-2108aa) containing the IGF-II-binding site)

• In vivo enzyme distribution: Track distribution of administered enzymes in M6pr knockdown animal models versus controls

These approaches help delineate the specific role of M6pr in enzyme targeting and can inform therapeutic strategies for lysosomal storage disorders.

How do cation-dependent (CD) and cation-independent (CI) M6P receptors differ functionally?

The following table summarizes key differences between CD-M6PR (M6pr) and CI-M6PR:

While both receptors participate in lysosomal enzyme trafficking, their functions do not entirely overlap . Simultaneous reduction of both M6PR-CD and M6PR-CI causes additive effects on PS-ASO activity in human cells, indicating distinct functional roles .

What methodological approaches can distinguish between the roles of CD-M6PR and CI-M6PR?

To differentiate between CD-M6PR and CI-M6PR functions:

• Selective knockdown: Target each receptor individually and in combination to assess specific and overlapping functions

• Domain-specific binding assays: Test ligand binding to specific domains (e.g., M6PR (41-1365aa) for M6P binding vs. M6PR (1510-2108aa) for IGF-II binding)

• Overexpression studies: Express each receptor type independently to determine if they can compensate for each other's functions (e.g., overexpression of M6PR-CD enhances ASO activity in HeLa cells)

• Species comparative analysis: Compare effects in mouse vs. human systems, as M6PR-CD shows different impacts on PS-ASO activity between species

These approaches help delineate the unique contributions of each receptor type to cellular processes.

What are the optimal antibody selection criteria for M6pr detection in various applications?

When selecting antibodies for M6pr research:

• Epitope specificity: Choose antibodies that target conserved regions for cross-species applications or species-specific epitopes for selective detection

• Application compatibility: Verify antibody performance in specific applications (Western blot, immunofluorescence, immunoprecipitation)

• Isoform selectivity: Select antibodies that can distinguish between CD-M6PR and CI-M6PR when needed (the anti-M6PR-CI antibody has been noted to perform better than anti-M6PR-CD antibody for certain applications)

• Validation in knockout/knockdown systems: Confirm specificity by testing in systems where M6pr expression is reduced or eliminated

Research has employed M6PR antibodies to investigate evolutionary implications of different sized M6PR tags and to evaluate chimeric expression constructs for studying receptor extracellular domains .

What purification approaches yield high-quality recombinant M6pr for structural studies?

For structural and functional studies requiring high-purity M6pr:

• Expression optimization: Use mammalian expression systems (HEK293T cells) to ensure proper folding and post-translational modifications

• Affinity purification: Employ tag-based purification (e.g., using C-terminal MYC/DDK tags) for initial capture

• Quality control: Verify purity via SDS-PAGE and Coomassie blue staining (target >80% purity)

• Functional validation: Confirm binding activity using SPR or other binding assays

• Storage optimization: Maintain in appropriate buffer (25 mM Tris.HCl, pH 7.3, 100 mM glycine, 10% glycerol) and store at -80°C to preserve stability

These approaches help ensure that the purified protein retains its native conformation and functional properties essential for meaningful structural and interaction studies.