Recombinant Mouse Methyltransferase-like protein 23 (Mettl23)

What is the biochemical function of Mettl23 and how does it differ from other methyltransferases?

Mettl23 is a member of the methyltransferase-like protein family that functions as an S-adenosyl-methionine (SAM)-dependent methyltransferase. Unlike some other methyltransferases that primarily target DNA or RNA, Mettl23 appears to have specialized functions related to protein methylation.

Based on 3D-modeling and structural analyses, Mettl23 contains a characteristic seven-β-strand methyltransferase fold with a binding site for SAM, the universal methyl donor in methylation reactions . This structure is conserved among methyltransferases, but Mettl23's substrate specificity appears distinct, with evidence suggesting it may methylate histone H3R17 and potentially interact with heat shock proteins .

Recent research has shown that Mettl23 plays essential roles in transcriptional regulation through its interaction with GABPA (GA-binding protein transcription factor, alpha subunit), affecting the expression of GABP target genes such as THPO (thrombopoietin) and ATP5B (ATP synthase, H+ transporting, mitochondrial F1 complex, beta polypeptide) . This places Mettl23 in a unique position compared to other methyltransferases that may not have direct transcriptional regulatory functions.

What are the known isoforms of Mettl23 and their expression patterns?

Mouse Mettl23 exists in at least two main isoforms with differing expression patterns and cellular localizations:

Isoform 1 (190 amino acids): This longer isoform contains the complete methyltransferase domain and is encoded by transcript variants 1, 2, and 3. It localizes to both the endoplasmic reticulum and nuclear structures .

Isoform 2 (123 amino acids): This shorter isoform is encoded by transcript variants 4, 5, and 6, and predominantly displays nuclear localization .

The following table summarizes the tissue expression patterns based on research findings:

*RGCs: Retinal ganglion cells; ER: Endoplasmic reticulum

How is Mettl23 protein structure related to its function?

The structure-function relationship of Mettl23 provides important insights for researchers working with recombinant versions:

Mettl23 possesses a typical methyltransferase fold with approximately 25 residues predicted to form the SAM/SAH-binding site, about 50% of which are completely conserved among SAM/SAH-binding proteins . The central β-sheet forms the core of the protein's catalytic domain, which is essential for methyltransferase activity.

3D-modeling studies have revealed that truncating mutations (as seen in intellectual disability cases) disrupt significant portions of this core fold, particularly the central β-sheet, likely preventing proper protein folding and abolishing methyltransferase activity .

Key structural features include:

• SAM-binding pocket formed by conserved residues

• Catalytic domain responsible for methyl transfer

• Protein-protein interaction regions that facilitate binding to substrates and partners like GABPA

Importantly, when producing recombinant Mettl23, researchers should note that the protein tends to be insoluble and form inclusion bodies when expressed in E. coli BL21 host cells. Soluble protein can be generated in the presence of chaperones like GroEL, though the protein remains tightly bound to the chaperone even after purification attempts . This characteristic may reflect Mettl23's natural interaction with heat shock proteins.

What are the optimal methods for producing functional recombinant mouse Mettl23?

Production of functional recombinant Mettl23 presents several challenges that researchers should address:

Expression System Selection:

• E. coli-based systems: When expressed in E. coli BL21, Mettl23 tends to form insoluble inclusion bodies . If pursuing this approach, co-expression with chaperones (particularly GroEL) is essential to obtain soluble protein.

• Mammalian expression systems: HEK293T cells have been successfully used to express Mettl23 for functional studies and are recommended for applications requiring post-translational modifications .

Purification Strategy:

• For bacterial expression:

  • Use His8-tagged constructs for affinity purification

  • Include chaperones during expression

  • Be aware that attempts to release Mettl23 from chaperones (e.g., by ATP addition) have been unsuccessful

• For mammalian expression:

  • C-terminal FLAG-tag constructs have been effective for localization studies

  • Immunoprecipitation with sequence-specific antibodies has been successful for detecting native Mettl23

Protein Solubility Considerations:
The tight binding of Mettl23 to chaperones likely reflects its biological role and may be related to its association with the endoplasmic reticulum membrane. This characteristic should be considered when designing experiments using recombinant Mettl23 .

How can researchers validate the methyltransferase activity of recombinant Mettl23?

Validating the enzymatic activity of recombinant Mettl23 requires a systematic approach:

In vitro methyltransferase assays:

• SAM-binding assay: Use fluorescently labeled SAM analogs to confirm binding to recombinant Mettl23

• Methylation transfer assays: Incubate recombinant Mettl23 with [³H]-SAM and potential substrates, followed by detection of incorporated methyl groups through:

  • SDS-PAGE separation and fluorography for protein substrates

  • HPLC analysis for smaller substrates

Substrate identification approaches:

• Candidate approach: Based on the research, key candidates to test include:

  • Histone H3 (particularly at R17 position)

  • Heat shock proteins (HSP60/GroEL homologs)

  • GABPA and associated transcription factors

• Unbiased approaches:

  • Protein arrays incubated with Mettl23 and [³H]-SAM

  • Stable isotope labeling with amino acids in cell culture (SILAC) comparing wild-type and Mettl23-knockout cells

Structural validation:
Compare enzymatic activity of wild-type Mettl23 with mutants affecting the SAM-binding pocket and predicted catalytic residues to confirm structure-function relationships.

What phenotypes are observed in Mettl23 knockout mouse models and how can they be assessed?

Mettl23 knockout mice display several phenotypes that provide insights into the protein's function:

Neurodevelopmental phenotypes:

• Intellectual disability-like behaviors can be assessed using:

  • Morris water maze for spatial learning and memory

  • Novel object recognition tests

  • Y-maze for working memory

  • Social interaction tests

Ocular phenotypes:
Mettl23-KO mice develop glaucoma-like phenotypes without elevated intraocular pressure , characterized by:

• Reduction in ganglion cell complex (GCC) thickness in 2-month-old mice that progresses with age

• Normal intraocular pressure (IOP)

• Altered electroretinography (ERG) responses

Assessment methods for ocular phenotypes:

• Optical coherence tomography (OCT) to measure GCC thickness

• Tonometry for IOP measurement

• ERG for functional assessment of retinal responses

• Immunohistochemistry to analyze METTL23 expression in retinal ganglion cells

Research has shown that METTL23 is highly expressed in retinal ganglion cell nuclei and optic nerve fibers in both mice and cynomolgus macaques, with expression markedly reduced in RGCs of Mettl23-KO mice .

What is the relationship between Mettl23 and GABPA, and how can this interaction be studied?

The interaction between Mettl23 and GABPA represents a critical aspect of Mettl23's function in transcriptional regulation:

Experimental evidence for interaction:

• Yeast two-hybrid screens identified Mettl23 as an interacting partner of GABPA

• Co-immunoprecipitation confirmed that endogenous GABPA can be recovered from immunoprecipitates of overexpressed tagged Mettl23

• The interaction has been successfully demonstrated in multiple cell types (HEK293T and N2A) with and without chemical cross-linkers

Functional consequences of interaction:

• Overexpression of Mettl23 significantly increases transcription at the THPO promoter, a known GABP target

• Knockdown of Mettl23 with siRNA results in reduced expression of ATP5B, another GABP target gene

• The positive effects of overexpressed GABPA and Mettl23 appear to be additive, suggesting Mettl23 has a positive modulatory effect on GABP function

Methodological approaches to study the interaction:

• Reporter gene assays: Use luciferase constructs driven by GABP-regulated promoters (THPO, ATP5B) to assess how Mettl23 modulates activity

• ChIP-seq: Determine if Mettl23 is present at GABP binding sites genome-wide

• Domain mapping: Create truncation mutants of Mettl23 and GABPA to identify critical interaction regions

• Methylation analysis: Investigate whether Mettl23 methylates GABPA or associated proteins

This interaction may explain how Mettl23 mutations lead to intellectual disability, as the METTL23/GABP complex regulates genes crucial for neurodevelopment and function .

How does Mettl23 contribute to histone methylation and what are the appropriate experimental approaches?

Recent research suggests Mettl23 functions as a histone arginine methyltransferase, specifically targeting H3R17:

Evidence for histone methylation activity:

• Mettl23 mutation alters histone H3R17 methylation in normal eyes, contributing to glaucoma phenotypes

• Dimethylation of H3R17 in the retina requires Mettl23 and affects transcription

Experimental approaches to study histone methylation:

• In vitro histone methyltransferase assays:

  • Incubate recombinant Mettl23 with histone H3 peptides and [³H]-SAM

  • Use antibodies specific for H3R17me2 to detect methylation by western blot

  • Mass spectrometry to identify specific methylated residues

• Cellular approaches:

  • Compare H3R17 methylation levels in wild-type versus Mettl23-knockout or knockdown cells

  • ChIP-seq using anti-H3R17me2 antibodies to identify genomic regions affected by Mettl23 loss

  • RNA-seq to correlate changes in H3R17 methylation with gene expression

• Tissue-specific analysis:

  • Immunohistochemistry for H3R17me2 in tissues from wild-type and Mettl23-mutant mice

  • Focus on retinal ganglion cells where Mettl23 is highly expressed

A comprehensive experimental design should include both biochemical and cellular approaches, with particular attention to tissue-specific effects given Mettl23's diverse roles across different cell types.

What are the effects of disease-associated Mettl23 mutations on protein function and how can they be modeled?

Several Mettl23 mutations have been identified in patients with intellectual disability and other phenotypes:

Types of pathogenic mutations:

• Frameshift deletions (e.g., c.281_285delAAGAT)

• Nonsense mutations leading to truncated protein

• Splice site mutations (e.g., c.A83G, c.322+1del)

• Missense mutations affecting protein structure

Functional consequences:

• Truncating mutations disrupt the predicted catalytic domain

• Splice variants alter subcellular localization (shifting from ER/cytoplasmic to nuclear localization)

• Mutations can affect interaction with binding partners like GABPA

• Some mutations result in complete loss of expression in affected tissues

Experimental approaches to model mutations:

• In vitro expression systems:

  • Express wild-type and mutant Mettl23 in cell lines to assess protein stability, localization, and interactions

  • Use fluorescent tags to visualize subcellular localization (mutants often show altered patterns)

• Animal models:

  • CRISPR/Cas9-generated mice carrying specific mutations:

    • Mettl23-KI mice with c.A83G mutation show exon 2 skipping

    • Mettl23-KO mice with c.221_224del (A74fs) show absence of METTL23 protein

• Patient-derived cells:

  • iPSCs from patients carrying Mettl23 mutations reveal complex aberrant RNA splicing

  • Lymphoblastoid cell lines from affected individuals show loss of C-terminal METTL23 protein sequence

The table below summarizes key mutations and their functional effects:

What methods are most effective for detecting and measuring endogenous Mettl23 expression?

Detecting endogenous Mettl23 presents challenges due to its relatively low expression levels in most tissues:

RNA detection methods:

• Quantitative RT-PCR:

  • Distinguish between isoform-specific transcripts using primers targeting alternative exons

  • Use reference genes (B2M, HPRT1) shown to be stable relative to Mettl23

  • Design assays that can detect aberrant splicing events common in mutants

• RNA-seq:

  • Provides comprehensive view of all transcript variants

  • Can reveal tissue-specific expression patterns

  • Important for detecting complex splicing variants in mutants

Protein detection methods:

• Immunoblotting:

  • Use polyclonal antibodies recognizing full-length Mettl23

  • For detecting truncated variants, use sequence-specific antibodies targeting the C-terminal portion (positions 137-166)

  • Caution: commercial antibodies may have variable specificity

• Immunohistochemistry/Immunofluorescence:

  • Most effective in tissues with higher expression (retinal ganglion cells)

  • Can reveal subcellular localization patterns

  • Use co-staining with ER and nuclear markers to distinguish isoforms

• Mass spectrometry:

  • For unbiased detection and quantification

  • Can detect post-translational modifications

  • May require enrichment strategies due to low abundance

The following tissues show most reliable detection of endogenous Mettl23:

• Retina (particularly RGCs)

• Brain tissue (low-to-moderate levels)

• Lymphoblastoid cells (for patient studies)

What are the critical quality control parameters for recombinant Mettl23 preparations?

Researchers working with recombinant Mettl23 should implement rigorous quality control:

Purity assessment:

• SDS-PAGE analysis under reducing and non-reducing conditions

• Size exclusion chromatography to verify monomeric state

• Mass spectrometry to confirm intact protein mass

Functional validation:

• SAM binding assays

• Methyltransferase activity toward known substrates (H3R17)

• Interaction with known binding partners (GABPA)

Structural integrity:

• Circular dichroism to assess secondary structure

• Thermal shift assays to determine stability

• Limited proteolysis to evaluate protein folding

Special considerations:

• Always check for co-purifying chaperones as Mettl23 tends to remain bound to GroEL/HSP60

• Verify that recombinant constructs maintain appropriate subcellular localization when expressed in mammalian cells

• For truncated variants, confirm they match the expected molecular weight (e.g., 21.5 kDa for wild-type vs. absence in patient samples)

How can Mettl23's interaction with heat shock proteins be leveraged for research applications?

The tight association between Mettl23 and heat shock proteins offers unique research opportunities:

Experimental approaches to study the interaction:

• Co-immunoprecipitation:

  • Pull down Mettl23 and identify associated chaperones

  • Reciprocal experiments pulling down chaperones to detect Mettl23

• In vitro methylation assays:

  • Test whether Mettl23 can methylate heat shock proteins (particularly HSP60, the human homolog of GroEL)

  • Map methylation sites using mass spectrometry

• Functional consequences:

  • Investigate how methylation affects chaperone activity

  • Determine if Mettl23-mediated methylation alters substrate specificity of chaperones

Research applications:

• Use Mettl23 as a tool to study chaperone function in neurodevelopment

• Investigate whether disruption of this interaction contributes to intellectual disability

• Explore therapeutic approaches targeting this interaction for neurodevelopmental disorders

Evidence suggests that several human methyltransferases interact with molecular chaperones and regulate their activity through methylation of conserved lysine residues . This may represent a broader regulatory mechanism with Mettl23 serving as a model system.

What are the most promising applications for recombinant Mettl23 in neurodevelopmental research?

Recombinant Mettl23 has several promising applications in neurodevelopmental research:

Therapeutic development:

• Screen for compounds that enhance Mettl23 activity as potential treatments for intellectual disability

• Develop gene therapy approaches to restore Mettl23 function in affected individuals

Mechanistic studies:

• Map the transcriptional networks regulated by the Mettl23-GABPA interaction

• Identify all substrates methylated by Mettl23 in neuronal cells

• Investigate how Mettl23-mediated methylation affects chromatin structure and gene expression

Biomarker development:

• Develop assays to measure Mettl23 activity in patient samples

• Investigate correlations between Mettl23 function and cognitive outcomes

Model systems:

• Use Mettl23-knockout or knock-in mice as models for intellectual disability

• Create patient-specific iPSC-derived neurons to study neurodevelopmental consequences of Mettl23 mutations

The growing body of evidence linking methyltransferases to intellectual disability highlights the importance of methylation processes in neuronal function and brain development . Recombinant Mettl23 provides a valuable tool to further explore these connections and potentially develop novel therapeutic approaches.