[@dreyfuss2002]
[@han2010]
| HNRNPM |
|---|
| Symbol | HNRNPM |
| Full Name | Heterogeneous Nuclear Ribonucleoprotein M |
| Chromosome | 19p13.3 |
| NCBI Gene ID | [4696](https://www.ncbi.nlm.nih.gov/gene/4696) |
| OMIM | [160993](https://omim.org/entry/160993) |
| Ensembl | [ENSG00000177868](https://www.ensembl.org/Homo_sapiens/ENSG00000177868) |
| UniProt | [P43307](https://www.uniprot.org/uniprot/P43307) |
| Aliases | HNRPM, HNRNPM, NAK |
| Protein Class | RNA-binding protein (RRM family) |
| Tissue Expression | Ubiquitous (brain, heart, muscle) |
HNRNPM encodes Heterogeneous Nuclear Ribonucleoprotein M (hnRNP M), a member of the hnRNP family of RNA-binding proteins. HnRNP M is a multifunctional protein involved in alternative splicing regulation, pre-mRNA processing, mRNA stability, and transcriptional control. It contains multiple RNA recognition motifs (RRMs) that enable it to bind to diverse RNA sequences and regulate post-transcriptional gene expression.
HNRNP M has emerged as an important player in neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). Its role in spliceosome function and RNA processing makes it a critical determinant of neuronal health, and its dysregulation contributes to the widespread spliceopathy observed in these disorders[@tsuiji2015].
HNRNP M is a 730-amino acid protein with several distinct domains[@han2010]:
-
RNA Recognition Motifs (RRMs): Four RRMs (RRM1-4) in the central region enable sequence-specific RNA binding. Each RRM contains the conserved RNP1 and RNP2 motifs.
-
Glycine-rich domain: Located at the N-terminus, involved in protein-protein interactions.
-
C-terminal region: Contains additional binding sites for partner proteins.
HNRNP M participates in multiple RNA-related processes[@dreyfuss2002]:
Alternative Splicing:
- Binds to specific sequence motifs in pre-mRNA (CA-rich elements)
- Recruits spliceosome components or blocks splice site access
- Regulates inclusion/exclusion of specific exons
- Key targets include neuronal transcripts involved in synaptic function
- Also regulates tissue-specific and developmental stage-specific splicing
Pre-mRNA Processing:
- Component of the spliceosome complex (Component of the heterogeneous ribonucleoprotein particle)
- Interacts with PSF (SFPQ) and matrin 3 to form a splicing regulatory complex[@kwon2021]
- Regulates 3' end processing and polyadenylation
- Involved in RNA editing through interaction with ADAR enzymes
mRNA Stability and Transport:
- Binds to 3' UTRs to protect mRNAs from degradation
- Participates in mRNA trafficking to dendritic and axonal compartments in neurons
- Regulates localization of specific transcripts including those involved in synaptic plasticity
- Associates with RNA granules for transport
Transcriptional Regulation:
- Can shuttle between nucleus and cytoplasm
- Associates with chromatin and modulates transcription
- Interacts with transcriptional co-activators and co-repressors
- May regulate expression of genes involved in cell cycle and differentiation
DNA Damage Response:
- Involved in DNA damage response pathways
- Associates with DNA repair complexes
- Regulates expression of DNA repair genes
- Expressed in all tissues examined
- Highest expression in brain, heart, and skeletal muscle
- In the brain, localized to neurons throughout cortex, hippocampus, and cerebellum
- Expressed in both excitatory and inhibitory neurons
- Also expressed in glial cells (astrocytes and microglia)
HNRNP M demonstrates dynamic subcellular distribution:
Nuclear Functions:
- Concentrated in the nucleus, particularly in splicing factor compartments (speckles)
- Associates with chromatin and regulates transcriptional processes
- Part of the spliceosome complex
Cytoplasmic Functions:
- Shuttles between nucleus and cytoplasm
- Involved in mRNA transport to dendritic and axonal compartments
- Regulates mRNA stability in cytoplasm
Dysregulation in Disease:
- In AD, altered nuclear/cytoplasmic distribution observed
- Cytoplasmic accumulation may sequester HNRNPM from nuclear functions
- Contributes to splicing dysregulation
Multiple studies have documented HNRNPM alterations in AD brain[@zhang2019][@kim2023]:
-
Altered Expression: HNRNPM mRNA and protein levels are significantly changed in AD hippocampus. RNA-seq studies show differential expression compared to age-matched controls.
-
Spliceopathy: Genome-wide splicing analyses reveal widespread abnormal splicing in AD brain, with many changes affecting predicted HNRNPM target genes.
-
Subcellular Localization: HNRNPM shows altered nuclear/cytoplasmic distribution in AD neurons, potentially affecting its splicing function.
Tau Pathology: HNRNPM is directly affected by tau pathology in AD[@wang2024]:
- Hyperphosphorylated tau sequesters HNRNPM in the cytoplasm
- This reduces HNRNPM availability for nuclear splicing functions
- Abnormal splicing of tau-related transcripts ensues
Amyloid-beta Effects:
- Aβ oligomers alter HNRNPM expression in cellular models
- Affected transcripts include those involved in synaptic function
- Leads to impaired synaptic protein expression
Neuroinflammation: Chronic neuroinflammation in AD affects HNRNPM[@liu2022]:
- Pro-inflammatory cytokines alter HNRNPM expression in astrocytes
- Contributes to astrocyte reactivity
- May affect RNA processing of inflammatory genes
HNRNP M regulates splicing of numerous synaptic proteins[@park2020]:
- Pre-synaptic: Synaptophysin, synaptotagmin, SV2
- Post-synaptic: Glutamate receptors, PSD95, SHANK family
- Loss of HNRNPM function contributes to synaptic dysfunction
While less studied than in AD, HNRNPM is implicated in PD:
- Altered expression in substantia nigra
- May interact with α-synuclein pathology
- Contributes to RNA processing defects in dopaminergic neurons
HNRNPM has been directly implicated in ALS pathogenesis[@tsuiji2015][@yang2022]:
Spliceosome Defects:
- Mutations in RNA-binding proteins (including HNRNPM) cause spliceosome dysfunction
- Loss of HNRNPM leads to abnormal splicing of survival motor neuron (SMN) complex transcripts
- Contributes to splicing deficits characteristic of ALS
C9orf72 Connection:
- The hexanucleotide repeat expansion in C9orf72 is the most common cause of familial ALS/FTD
- HNRNPM may be sequestered by repeat RNA foci (similar to other hnRNPs)
- Contributes to RNA processing defects
FTD Links:
- HNRNPM alterations found in frontotemporal dementia
- Shared molecular mechanisms with ALS (TDP-43 pathology)
HNRNP M plays a role in mitochondrial biology through its splicing functions[@choi2021]:
Splicing Targets:
- Mitochondrial electron transport chain subunits
- mtDNA maintenance genes
- Mitochondrial translation factors
Implications for Neurodegeneration:
- Loss of HNRNPM leads to mitochondrial dysfunction
- Impaired OXPHOS in neurons
- Increased susceptibility to oxidative stress
Modulating HNRNPM function could have therapeutic benefits:
| Strategy |
Approach |
Stage |
| Splicing modulation |
Small molecules to restore normal splicing |
Preclinical |
| ASO therapy |
Anti-sense oligonucleotides targeting aberrant splicing |
Early research |
| Protein stabilization |
Enhance HNRNPM nuclear localization |
Research |
- Biomarkers: HNRNPM splicing patterns as biomarkers for neurodegenerative disease
- Drug targets: Developing small molecules that modulate HNRNPM function
- Gene therapy: Restoring normal HNRNPM expression patterns
| Partner |
Function |
Reference |
| PSF (SFPQ) |
Splicing regulation, transcriptional repression |
[@kwon2021] |
| Matrin 3 |
Splicing complex, DNA damage response |
[@kwon2021] |
| TDP-43 (TARDBP) |
ALS/FTD pathology, RNA processing |
[@tsuiji2015] |
| FUS |
ALS pathology, RNA processing |
[@tsuiji2015] |
- Dreyfuss et al., mRNA-binding proteins (2002)[@dreyfuss2002]
- Han et al., Functional diversity of hnRNPs (2010)[@han2010]
- Tsuiji et al., Spliceosome integrity in ALS (2015)[@tsuiji2015]
- Zhang et al., HNRNPM in AD (2019)[@zhang2019]
- Wang et al., HNRNPM modulates tau pathology (2024)[@wang2024]
¶ Domain Organization
HNRNP M contains several distinct structural domains that enable its diverse functions[@dreyfuss2002]:
RNA Recognition Motifs (RRMs):
- RRM1 (positions 92-171): Primary RNA binding
- RRM2 (positions 180-258): Sequence specificity
- RRM3 (positions 291-370): Auxiliary binding
- RRM4 (positions 414-492): C-terminal binding
Each RRM contains the conserved RNP1 (8 residues) and RNP2 (6 residues) motifs that directly contact RNA. The four RRMs work cooperatively to recognize diverse RNA targets.
Glycine-Rich Domain (GRD):
- Positions 1-60: Low complexity region
- Mediates protein-protein interactions
- Involved in phase separation and granule formation
C-terminal Acidic Region:
- Positions 600-730: Highly acidic tail
- Important for interactions with partner proteins
- Regulates subcellular localization
HNRNP M undergoes various PTMs:
Phosphorylation:
- Multiple serine/threonine phosphorylation sites
- Regulates RNA binding affinity
- Affects subcellular localization
- CDK-mediated phosphorylation during cell cycle
Methylation:
- Arginine methylation by PRMTs
- Modulates protein-RNA interactions
- Influences splicing regulation
Ubiquitination:
- Controls protein stability
- Regulates stress response
- Degradation signals
HNRNP M is an integral component of the spliceosome:
Core Complexes:
- HNRNP M associates with U2 and U5 snRNPs
- Component of the prespliceosome
- Remodels during spliceosome assembly
Splicing Cycle:
- E complex: Recognition of 5' splice site and branch point
- A complex: U2 snRNP recruitment
- B complex: U1/U4/U6 tri-snRNP entry
- C complex: Catalytic steps (two transesterifications)
- Post-spliceosome: Disassembly and recycling
Exon Definition:
- HNRNP M regulates exon recognition
- Influences splice site selection
- Modulates exon inclusion/skipping
Alternative Splicing Patterns:
- Mutually exclusive exons
- Alternative 5' splice sites
- Alternative 3' splice sites
- Retained introns
Neuron-Specific Splicing:
- HNRNP M regulates neuronal-specific exons
- Critical for synaptic protein isoforms
- Activity-dependent splicing regulation
¶ RNA Granules and Transport
HNRNP M localizes to stress granules under cellular stress:
Granule Composition:
- Translation initiation factors (eIF4E, eIF4G)
- 40S ribosomal subunits
- Other hnRNPs (HNRNPA1, HNRNPC)
- TIA-1, G3BP1
Formation Mechanism:
- Translation arrest triggers granule assembly
- HNRNP M sequestered during stress
- Reversible process upon stress resolution
Functional Implications:
- mRNA storage and protection
- Stress response modulation
- Translation regulation
In neurons, HNRNP M participates in transport:
Dendritic RNA Localization:
- Transport of synaptic protein mRNAs
- Activity-dependent delivery
- Local translation at synapses
Axonal RNA Transport:
- Growth cone guidance mRNAs
- Injury-responsive transcripts
- Long-range delivery
Granule Components:
- Motor proteins (KIF5, dynein)
- Adaptor proteins (ZIP3, STAU2)
- Other RNA-binding proteins
HNRNP M regulates splicing of mitochondrial proteins:
Electron Transport Chain:
- Complex I: NDUFA, NDUFB subunits
- Complex III: UQCRB, UQCRH
- Complex IV: COX subunits
- Complex V: ATP synthase subunits
mtDNA Maintenance:
- Replication proteins
- Transcription factors
- Translation machinery
HNRNP M affects:
Fission/Fusion:
- Regulates Drp1 splicing
- Influences Mfn and OPA1 isoforms
- Affects mitochondrial morphology
Biogenesis:
- PGC-1α processing
- TFAM expression
- Mitochondrial DNA replication
Quality Control:
- Mitophagy receptor splicing
- Apoptotic protein isoforms
- DNA repair factors
Oxidative Phosphorylation:
- Loss of HNRNPM reduces OXPHOS capacity
- Increased glycolytic dependency
- ATP production deficits
ROS Production:
- Enhanced mitochondrial ROS
- Oxidative stress susceptibility
- DNA damage accumulation
Calcium Handling:
- Altered calcium buffering
- Mitochondrial calcium dynamics
- Excitotoxicity susceptibility
HNRNP M modulates astrocyte responses:
Inflammatory Signaling:
- Cytokine-mediated HNRNPM changes
- Alters splicing of inflammatory genes
- Creates feed-forward loop
Reactive Astrogliosis:
- GFAP splicing regulation
- Proliferation control
- Scar formation genes
RNA Processing in Microglia:
- HNRNP M in microglial activation
- Regulates inflammatory transcripts
- Implications for neurodegeneration
Phagocytosis:
- Complement protein splicing
- Scavenger receptor isoforms
- Clearance efficiency
Small Molecule Modulators:
- spliceosome modulators in development
- Specificity challenges
- Future directions
Antisense Oligonucleotides (ASOs):
- Target-specific splice correction
- Delivery challenges to CNS
- Clinical trial progress
RNA-Binding Protein Targeting:
- Small molecules that modulate HNRNPM
- Protein-protein interaction inhibitors
- Selective approaches needed
Stabilization Strategies:
- Enhance nuclear localization
- Protect from mislocalization
- Prevent pathogenic sequestration
Function Restoration:
- Restore tau-mediated mislocalization
- Enhance nuclear import
- Boost spliceosome function
With Other RNA-Binding Proteins:
- TDP-43 targeting
- FUS modulation
- Coordinated approaches
With Neuroprotective Strategies:
- Antioxidants
- Neurotrophic factors
- Metabolic support
Cell Lines:
- SH-SY5Y (neuroblastoma)
- HEK293 (human embryonic kidney)
- Primary cortical neurons
- iPSC-derived neurons
Disease Models:
- Aβ-treated neurons
- Tau-expressing cells
- α-Synuclein models
- Oxidative stress models
Mouse Models:
- Hnrnpm knockout
- Conditional deletions
- Disease model crosses
- Reporter lines
Behavior Testing:
- Memory and learning tests
- Motor function assays
- Social behavior
- Electrophysiology
iPSC Lines:
- From AD/PD/ALS patients
- Isogenic controls
- Gene-corrected lines
Brain Organoids:
- Patient-derived organoids
- Differentiation protocols
- Disease modeling
Diagnostic Signatures:
- Specific exon inclusion patterns
- Predict disease state
- Distinguish disease subtypes
Progression Markers:
- Longitudinal changes
- Correlate with severity
- Treatment response
CNS Biomarkers:
- CSF HNRNPM measurement
- Brain imaging correlation
- Peripheral correlates
Therapeutic Monitoring:
- Target engagement markers
- Pathway activity readouts
- Safety monitoring
Core Partners:
- PSF (SFPQ): Splicing regulation
- Matrin 3: Nuclear matrix
- TDP-43: ALS/FTD pathology
- FUS: ALS pathology
Extended Network:
- Other hnRNPs (A1, A2, C)
- Splicing factors (SF3B1, U2AF1)
- Chromatin regulators
- Transcription factors
Major Targets:
- Synaptic protein transcripts
- Mitochondrial transcripts
- Cell cycle regulators
- DNA repair genes
Disease-Relevant Targets:
- APP processing variants
- Tau isoforms
- α-Synuclein transcripts
- Apoptotic proteins