PFN1

PFN1 (Profilin-1) is a central regulator of actin dynamics that promotes actin polymerization through direct binding of monomeric actin and polyproline-motif-containing partners such as formins and VASP, with its interaction with VASP negatively regulated by PKA-mediated phosphorylation at Ser137 (PMID:30814249, PMID:34074767). PFN1 protein stability is controlled by K48-linked ubiquitination at K70 mediated by the E3 ligase FBXL4 and by STUB1, while the deubiquitinase USP14 (scaffolded by lncRNA UCA1) stabilizes PFN1 to activate RhoA/ROCK signaling (PMID:41589689, PMID:34331014, PMID:36160709). Mutations in PFN1 cause familial ALS through divergent mechanisms: severely destabilizing mutations (C71G) produce toxic aggregates with prion-like seeding properties that sequester the chaperone DNAJB6, while partially destabilizing mutations (G118V, M114T) gain enhanced formin-mediated actin assembly and disrupt autophagy/endolysosomal pathways through enhanced PI3P binding (PMID:27681617, PMID:37817804, PMID:34074767, PMID:38509062). Loss-of-function PFN1 mutations also cause early-onset Paget disease of bone by enhancing osteoclast actin ring formation and motility (PMID:32392277, PMID:40458045).

1. 2015 Medium

   Establishing that PFN1 expression is epigenetically regulated and functionally linked to actin polymerization in vivo, nicotine-induced promoter hypomethylation increased PFN1 in spermatids and enhanced sperm motility through actin polymerization.

   Evidence 2D gel electrophoresis, mass spectrometry, promoter methylation analysis, and sperm motility measurement in mouse testis

   PMID:26311342

   Open questions at the time

   • Specific methylation sites on PFN1 promoter not mapped
   • Whether actin polymerization is the sole mediator of motility change not tested
2. 2016 High

   Resolving whether ALS-linked PFN1 C71G causes disease through loss- or gain-of-function, transgenic mice showed that mutant PFN1 produces motor neuron degeneration preceding aggregate formation, establishing a gain-of-toxicity mechanism distinct from simple aggregation.

   Evidence Transgenic mouse model expressing C71G and WT PFN1 with histopathology, immunostaining, and behavioral assessment

   PMID:27681617

   Open questions at the time

   • Precise toxic species (soluble oligomers vs. other conformers) not identified
   • Whether endogenous wild-type PFN1 contributes to or is sequestered by mutant unclear
3. 2017 Medium

   Demonstrating PFN1's role at the neuromuscular junction, wild-type human PFN1 expression in Drosophila motor neurons increased active zone density, F-actin content, and filopodia, while ALS mutants showed partial loss of these functions.

   Evidence Drosophila transgenic model expressing human PFN1 variants with NMJ morphology analysis

   PMID:28379367

   Open questions at the time

   • Cross-species differences between Drosophila chickadee and human PFN1 not fully controlled for
   • Whether NMJ remodeling defects are cell-autonomous not tested
4. 2019 High

   Identifying how PFN1's interaction with VASP is regulated, PKA-mediated phosphorylation at Ser137 was shown to negatively regulate PFN1–VASP binding, and this interaction was required for efficient cell migration downstream of cell-substrate adhesion.

   Evidence Mutagenesis in knockdown-rescue settings with cell migration assays and PKA inhibitor/activator experiments

   PMID:30814249

   Open questions at the time

   • Whether other kinases phosphorylate Ser137 in vivo not addressed
   • Structural basis of how pSer137 disrupts VASP binding not resolved
5. 2020 Medium

   Establishing PFN1 loss-of-function in bone biology, a frameshift mutation truncating PFN1's C-terminus caused enhanced osteoclastogenesis with Paget-like features, and PFN1 silencing recapitulated the phenotype, linking PFN1 to osteoclast actin ring regulation.

   Evidence Patient PBMC-derived osteoclasts, murine bone marrow monocyte silencing, whole exome sequencing

   PMID:32392277

   Open questions at the time

   • Whether PFN1 directly restrains actin ring formation or acts through intermediate effectors unknown
   • Penetrance and modifier genes in human disease not characterized
6. 2020 Medium

   Clarifying the temporal sequence of ALS pathology, detergent-insoluble PFN1 C71G inclusions were shown to be the earliest pathological event in transgenic rats, preceding motor neuron loss—contrasting with the mouse model where degeneration preceded aggregation.

   Evidence Genomic DNA transgenic rat model with detergent fractionation, histopathology, and temporal analysis

   PMID:32754913

   Open questions at the time

   • Discrepancy with mouse model regarding aggregation timing not reconciled
   • Whether inclusions are causative or correlative remains debated
7. 2021 High

   Revealing that different ALS mutations alter PFN1 function through distinct mechanisms, G118V and M114T gained enhanced formin-mediated actin assembly through altered internal dynamic couplings, while C71G was severely destabilized causing loss-of-function in actin assembly.

   Evidence Unbiased proteomics, in vitro actin assembly assays, molecular dynamics simulations, and mutagenesis

   PMID:34074767

   Open questions at the time

   • Whether enhanced formin-mediated polymerization is toxic in vivo not demonstrated
   • Which specific formins are most affected in motor neurons not determined
8. 2021 Medium

   Identifying a ubiquitin-proteasome axis controlling PFN1 levels, SH3BGRL was found to promote PFN1 degradation by enhancing STUB1 translation and PFN1–STUB1 interaction, with PFN1 loss activating AKT, NF-κB, and WNT signaling.

   Evidence Co-immunoprecipitation, overexpression/knockdown, in vivo xenograft, and clinical tissue analysis

   PMID:34331014

   Open questions at the time

   • Specific ubiquitination sites by STUB1 on PFN1 not mapped
   • Whether STUB1-mediated degradation operates in neurons not tested
9. 2022 High

   Defining a precise ubiquitination site governing PFN1 turnover, FBXL4 was shown to promote K48-linked ubiquitination of PFN1 at K70, and cardiac-specific FBXL4 knockout demonstrated that PFN1 accumulation disrupts sarcomeric integrity.

   Evidence Co-immunoprecipitation, ubiquitination assay, K70 mutagenesis, cardiac-specific KO/OE mouse models with AAV9 rescue

   PMID:41589689

   Open questions at the time

   • Whether FBXL4 and STUB1 act redundantly or in distinct tissues not resolved
   • PFN1 interactome changes upon K70 ubiquitination not characterized
10. 2022 Medium

    Identifying a deubiquitination mechanism that stabilizes PFN1, the lncRNA UCA1 was shown to scaffold USP14 recruitment to PFN1, inhibiting its ubiquitin-dependent degradation and activating RhoA/ROCK-mediated ROS production in endothelial cells.

    Evidence Co-immunoprecipitation, exosome coculture, ubiquitination assay, RhoA/ROCK pathway analysis

    PMID:36160709

    Open questions at the time

    • Direct USP14 deubiquitination of PFN1 not shown with purified components
    • Whether UCA1-USP14-PFN1 axis operates outside endothelial cells unknown
11. 2022 Medium

    Expanding PFN1's signaling repertoire beyond actin, PFN1 was found to bind and activate Cdc42, triggering PAK–JNK phosphorylation and inhibiting myogenic differentiation in skeletal muscle satellite cells.

    Evidence Co-immunoprecipitation with mass spectrometry, phosphorylation assays, knockdown/overexpression in bovine satellite cells

    PMID:36291059

    Open questions at the time

    • Whether PFN1–Cdc42 binding is direct or bridged by actin not resolved
    • Relevance to human myogenesis not tested
12. 2022 Medium

    Linking PFN1 mutations to alternative autophagy, M114T PFN1 was shown to deregulate the RAB9-mediated alternative autophagy pathway and cause mitochondrial abnormalities in motor neurons.

    Evidence Patient lymphoblasts, transfected cell lines, lentiviral transgenic mice, autophagy marker analysis

    PMID:35628504

    Open questions at the time

    • Whether RAB9 pathway activation is a direct consequence of PFN1 destabilization or secondary not clear
    • Which mitochondrial clearance step is impaired not defined
13. 2023 Medium

    Establishing prion-like properties of mutant PFN1, insoluble PFN1 from paralyzed ALS rats seeded inclusions and accelerated disease in recipient animals, while pathogenic PFN1 sequestered the chaperone DNAJB6 within inclusions.

    Evidence Intramuscular injection of spinal cord extracts in pre-symptomatic mutant PFN1 rats, co-immunoprecipitation for DNAJB6

    PMID:37817804

    Open questions at the time

    • Whether seeding occurs in sporadic ALS without PFN1 mutations unknown
    • Whether DNAJB6 sequestration is causal for toxicity or a bystander effect not tested
14. 2024 High

    Identifying a lipid-binding gain-of-function in ALS-mutant PFN1, enhanced PI3P binding was shown to impair autophagy and phagocytosis in iPSC-derived microglia, with rapamycin rescuing the phagocytic defect.

    Evidence iPSC-derived microglia, PI3P binding assay, phagocytosis assay, rapamycin rescue, lipid profiling

    PMID:38509062

    Open questions at the time

    • Whether PI3P binding enhancement occurs in motor neurons not tested
    • Structural basis for enhanced PI3P affinity of mutant PFN1 not determined
15. 2025 Medium

    Confirming PFN1 loss-of-function in bone disease with a knock-in model, heterozygous L112P mice developed early-onset Paget disease phenotype through enhanced osteoclast actin ring structures independent of NF-κB activation.

    Evidence Heterozygous knock-in mouse model, osteoclast culture, actin ring staining, NF-κB assay

    PMID:40458045

    Open questions at the time

    • Whether L112P affects PFN1 stability or specifically actin-binding not characterized
    • Osteoblast contributions to the bone phenotype not assessed

• Major open questions include: how the same protein produces both gain- and loss-of-function disease phenotypes across cell types; the structural basis for enhanced PI3P binding by ALS mutants; whether FBXL4/STUB1 ubiquitination axes are relevant in motor neurons; and whether prion-like PFN1 seeding contributes to sporadic ALS.
• Cell-type-specific determinants of PFN1 toxicity vs. loss-of-function not mapped
• No high-resolution structure of disease-mutant PFN1 bound to PI3P
• Whether therapeutic modulation of PFN1 stability would be beneficial or detrimental in ALS not known