Polyazulene is an underrated conductive polymer characterized by its unique building blocks involving a fused-ring structure with an intrinsic dipole moment. It has been studied with a focus on the development of organic electronics, such as photovoltaic cells. However, the biological properties of polyazulene have not been reported to date. This study, the first of its kind, not only describes the properties of electrochemically and chemically synthesised polyazulenes and so-called true polyazulene, but also characterises their cytocompatibility. The physico-chemical properties of polyazulene powders and films were characterized by FTIR, Raman and UV-Vis spectroscopy, profilometry, scanning electron microscopy, voltammetry, van der Pauw conductivity measurement, and thermogravimetric analysis. Their cytocompatibility was explored using NIH/3T3 mouse embryonic fibroblasts, HaCaT human keratinocytes, and ES R1 mouse embryonic stem cells. Our results indicate that the tested polyazulenes exhibit an overall favourable short-term in vitro cytocompatibility with limitations varying according to the synthesis method and conditions. Electrochemically synthesized polyazulene formed continuous films with moderate conductivity under optimized deposition conditions, whereas chemically prepared films were inhomogeneous for reliable conductivity measurements. These differences in conductivity therefore primarily reflect variation in film morphology and deposition conditions rather than intrinsic bulk electronic properties of the respective polyazulene types. Is polyazulene cytocompatible? It depends This study presents the first biological evaluation of polyazulenes synthesized by various methods. It examines their physicochemical characteristics and cytocompatibility with multiple cell lines. The findings demonstrate that synthesis conditions influence both conductivity and biocompatibility. The work highlights polyazulene’s potential in biomedical applications and lays the groundwork for future exploration of its role in bioelectronic materials. • Azulene electrochemical polymerization gives conductive polyazulene films. • Chemical polymerization produces polyazulene types with distinct structures. • Films´ physico-chemical properties strongly depend on synthesis conditions. • Cytocompatibility of polyazulenes varies with synthesis method and processing conditions.

Direct reprogramming (DR) involves inducing cell differentiation by converting somatic cells directly into target cells without bypassing induced pluripotent stem cells. Small molecule-induced DR reduces tumorigenic risk relative to transcription factor-triggered DR. However, identifying small molecules for DR through experimental exploration alone remains challenging. This study presents a computational method, SuperDIRECTEUR, to predict small molecules for DR using single-cell temporal transcriptome data. The cellular conversion processes in DR were simulated by calculating the RNA velocity in each cell. These processes were then classified into multiple stages (i.e., primal, immature, and mature stages), and a variant of simulated annealing was employed to search for the combination of small molecules that mimic stage-specific gene expression patterns during DR. We applied SuperDIRECTEUR to identifying candidate small molecules for DR converting mouse embryonic fibroblasts to induced neurons, and were able to reproduce experimentally verified and functionally related molecules inducing the corresponding conversions in a temporal stage-specific manner. The target proteins of the predicted small molecules in the early and later stages were distinctively involved in biologically relevant pathways toward neuronal differentiation and maturation. The proposed method has potential for practical applications in regenerative medicine.

Stem cell pluripotency is shaped by culture microenvironment. Growth factors, cytokines, and oxygen tension are key regulators of self-renewal and pluripotency state. Here we evaluated the adaptation of bovine embryonic stem cells (bESC) derived on mouse embryonic fibroblasts (MEF) to feeder-free conditions and the effects of low oxygen (5% O2) on their molecular profile. Primed bESC established on MEF and 21% O2 (high oxygen) were adapted to 5% O2 (low oxygen) and to feeder-free conditions in vitronectin to generate four culture conditions: high and low O2 tension in MEF and feeder-free. Adaptation to feeder-free upregulated transcripts related to cell differentiation in bESC cultured in high compared to low oxygen. Transition of bESC on MEF from normoxia to 5% O2 did not alter cell morphology or growth. Interestingly, colony morphology was maintained when transitioning bESC from MEF into feeder-free in low oxygen. Furthermore, there were fewer transcriptomic changes in bESC grown on MEF compared to feeder-free in low compared to high oxygen. While low oxygen tension did not alter the pluripotency profile of bESC, it promoted transcriptional changes related to energetic metabolism and increased mitochondrial membrane potential. These findings emphasize the importance of oxygen tension for derivation, maintenance, and performance of ESC.

The three-dimensional (3D) organization of the genome is strongly influenced by interactions between chromatin and lamin proteins at the nuclear envelope. Here, we investigate the role of lamina-associated domains (LADs) in shaping genome architecture using coarse-grained polymer models of mouse embryonic fibroblasts and embryonic stem cells. By integrating genome-wide LAD maps from DamID assays, we simulate chromatin conformations with and without LAD attachment. Incorporating LAD-lamina interactions reproduces the experimentally observed radial chromatin distribution and reveals that LADs induce extensive long-range (70-120 Mbp) chromatin contacts beyond typical loops and topologically associating domains (TADs). We describe these contacts in terms of two limiting geometric scenarios: LAD crowding, in which peripheral tethering increases the proximity of nearby non-LAD regions to LADs, and LAD anchoring, in which lamina-bound LADs constrain neighboring chromatin positions. LAD-induced interactions were especially prominent in chromatin regions lacking architectural proteins such as CTCF, and were associated with lower gene density and reduced transcriptional activity. Together, these results suggest that LAD-lamina tethering reshapes long-range chromatin contact probabilities through boundary-driven effects and is associated with gene-poor, less transcriptionally active chromatin regions.

Altered lipid profile in mice lacking the DNA repair protein ERCC1.

Apigenin and chrysin are chemically close and related flavonoids. Our previous works have demonstrated both apigenin and chrysin's ability to reduce cholesterol and uric acid biosynthesis along with upregulation of ketone bodies and endogenous anti-inflammatory lipids. Current work based on transcriptomics approach has shed light into their ability to influence endogenous metabolites or xenobiotic metabolism by regulating CYP isoforms. This work is based on analyzing raw data for transcriptomics, which we previously performed on mouse embryonic fibroblasts (MEFs). Following transcriptomics, we have added our raw data to NIH - Gene Expression Omnibus (GEO) with Accession Number - GSE155852. We have selected top 5 altered targets which are CYP450 family members and provided our perspective on how apigenin or chrysin administration could influence our metabolic status or drug metabolizing ability. In addition, we have also analyzed the impact of apigenin and chrysin treatment on glutathione S-transferase and ATP-binding cassette family members. This work will really shed light upon metabolizing and drug interaction capabilities of these flavonoids through the regulation of major cellular drug metabolizers and transporters.

Mitochondria orchestrate energy conversion and cell fate, yet label-free approaches that report both functional and physical states at the single-organelle level are nonexistent. Here, we combine atomic force microscopy (AFM) imaging with single-mitochondrion phenotyping by quantifying stiffness, height, and spontaneous low-frequency height fluctuations at the nanoscale. Across respiratory activators, inhibitors, and uncouplers, the integrated 0- to 20-Hz fluctuation power correlates with mitochondrial membrane potential (ΔΨm) and does not covary with changes in mitochondrial height (a proxy for swelling). In liver mitochondria lacking mitochondrial carrier homolog 2 (MTCH2), a regulator of mitochondrial metabolism, dynamics, and apoptosis, AFM reveals a compact, mechanically stiff, high-fluctuation state consistent with hyperpolarization and distinct from inhibited/uncoupled signatures. Extending the assay to mitochondria isolated from mouse embryonic fibroblasts, AFM data can distinguish between genotypes: loss of the mitochondrial pro-fusion proteins mitofusin 1 or 2 (MFN1 or MFN2) yields stiff, low-fluctuation mitochondria with reduced ΔΨm, whereas MTCH2 loss produces stiff, high-fluctuation, high-ΔΨm mitochondria. These three label-free features provide reproducible single-organelle "fingerprints" that resolve bioenergetic states and molecular defects and complement fluorescence and respirometry.

This study evaluates cinnamoyl-diphenylalanine dipeptide (Cin-FF), a newly introduced gelator, for its antimicrobial efficacy, biocompatibility, and potential anticancer properties in solution and gel forms. We show that Cin-FF exhibits antibacterial efficacy in the gel phase (0.2%, w/v), particularly against Escherichia coli, Acinetobacter baumannii, Staphylococcus aureus, and Bacillus subtilis, attributed to its sustained release and high concentration. In contrast, the solution phase has limited antimicrobial effects, particularly at lower concentrations (0.01% and 0.001%, w/v). The cytotoxicity of Cin-FF was evaluated across five mammalian cell lines: BJ human fibroblasts, SH-SY5Y neuroblastoma cells, mouse embryonic fibroblasts (MEF), HeLa cervical cancer cells, and Vero monkey kidney epithelial cells. Higher concentrations in the solution (0.1%, w/v) and in the gel phase (0.2%, w/v) are notably cytotoxic, particularly in the gel form. Lower concentrations in the solution form (0.01% and 0.001%, w/v) initially supported cell viability but induced delayed cytotoxicity by Day 4. Importantly, Cin-FF demonstrated potent cytotoxic effects against cancerous cell lines (HeLa and SH-SY5Y), suggesting its potential as a localized anticancer therapy. While Cin-FF in gel form shows considerable potential for both antimicrobial and anticancer applications, its cytotoxicity toward the noncancerous cells underscores the need for careful dosage control, formulation strategies, and targeted delivery to optimize therapeutic efficacy while minimizing harm to healthy tissues. This paves the way for further development as a multifunctional therapeutic agent.

Dynamin-related protein 1 (Drp1), encoded by DNM1L, is essential for mitochondrial fission, but its functional roles remain unclear due to isoform-specific effects from alternative splicing. Short-read RNA sequencing fails to resolve full-length isoforms involving distant exons, limiting our understanding. Here, we applied targeted long-read sequencing to profile full-length DNM1L transcripts in human left ventricle and induced pluripotent stem cell-derived cardiomyocytes, recovering all annotated isoforms with conserved expression patterns and isoforms 1-4 being the most abundant. Functional assays revealed that isoform abundance does not predict enzymatic activity. Extending this to six different mouse tissues, we identified distinct, tissue-enriched expression profiles. Functional rescue in Drp1-knockout mouse embryonic fibroblasts showed isoform-dependent differences in mitochondrial fission. Isoforms lacking the A-insert robustly rescued mitochondrial fission, with maximal activity observed for variants also lacking the B-insert (e.g. isoform b), consistent with a modulatory role of exon 3 in Drp1 activity. Our cross-species atlas integrates long-read transcriptomics with functional validation, revealing how isoform diversity underpins tissue-specific mitochondrial dynamics and physiological roles of Drp1.

L-arginine, a semi-essential amino acid, is metabolised in the cell to generate nitric oxide (NO) and L-citrulline via the enzyme nitric oxide synthase (NOS) or urea and L-ornithine via arginase activity. L-citrulline and L-ornithine are the products of L-arginine degradation. Mouse liver epithelial (BNL CL2) and mouse embryonic fibroblast (3T3 L1) insulin-sensitive cell lines were used as model systems and cultured with 0, 400 or 800 µM L-Arg. This study focuses on the analysis of the residual concentrations of amino acids (L-Arg, L-Cit and L-Orn) in cell culture medium samples using high performance liquid chromatography that involves precolumn derivatization with o-phthaldialdehyde. In BNL CL2 cells, most of the culture supernatant has increased amount of L-Arg in comparison to the control complete DMEM addition. L-ornithine levels showed an overall increase over time, with higher concentrations observed at 72 h compared with 24 h across all samples. In 3T3 L1 cells, residual L-Arg concentration decreased in most of the cell supernatant in comparison to the control at 72 h. Noticeably, L-Arg at 0 µM and the control complete DMEM had highest amount of L-Orn among all samples. Interestingly, L-Cit was very much high in culture medium of both untreated BNL CL2 (85.96 µM) and 3T3 L1 (37.49 µM) cells at T = 0 compared to the control. Collectively, the results show that excess L-Arg is sensed by the cell which then regulates the residual amount of amino acids concentration. The spectroscopy technique used here is highly sensitive, specific and accurate, can be readily automated and serves as a valuable tool for investigating the modulation of the arginine-nitric oxide pathway.

This study aimed to investigate the role of lactate in the progression of cardiac dysfunction after myocardial infarction (MI) and clarify the effect of aerobic exercise (AE) on improving post-infarction cardiac function by regulating lactate metabolism, so as to provide experimental evidence for the clinical improvement of cardiac function after MI. NIH 3 T3 mouse embryonic fibroblasts and C57BL/6 J mice were used as research subjects. In vitro, fibroblasts were treated with different concentrations of lactate, and the activation state of fibroblasts was evaluated by detecting the expression levels of Collagen I (COL I) and α-Smooth Muscle Actin (α-SMA). In vivo, four mouse models (SED-SHAM, AE-SHAM, SED-MI, and AE-MI) were established. Cardiac function was assessed by echocardiography for left ventricular ejection fraction (LVEF) and left ventricular fractional shortening (LVFS). Lactate levels in cardiac tissue and serum were detected using a lactate assay kit, and serum metabolite changes mediated by lactate were analyzed via metabolomics technology. In vitro experiments confirmed that lactate could significantly induce fibroblast activation. Metabolomics results showed that elevated lactate levels after MI led to abnormal accumulation of arachidonoyl carnitine. In vivo experiments revealed that lactate levels in cardiac tissue and serum were significantly increased, while LVEF and LVFS were decreased in the SED-MI group. In contrast, lactate levels in cardiac tissue and serum were reduced, and LVEF and LVFS were elevated in the AE-SHAM group. Lactate levels in cardiac tissue and serum of the AE-MI group were lower than those of the SED-MI group but higher than those of the AE-SHAM group, and the LVEF and LVFS of the AE-MI group were significantly higher than those of the SED-MI group. This study confirmed that lactate accumulation is involved in the pathological process of cardiac dysfunction after myocardial infarction, and AE can improve cardiac function after MI by reducing lactate levels in cardiac tissue and serum. These findings provide new experimental evidence for the prevention and treatment of post-infarction cardiac dysfunction by targeting lactate metabolism, and also offer theoretical support for the application of AE in cardiac rehabilitation.

Expression of concern: "PTEN deletion leads to deregulation of antioxidants and increased oxidative damage in mouse embryonic fibroblasts" by Yan-Ying Huo [Free Radic. Biol. Med. 44 (2008) 1578-1591, https://doi.org/10.1016/j.freeradbiomed.2008.01.013

Naive human pluripotent stem cells (hPSCs) represent a pre-implantation epiblast state able to efficiently differentiate into embryonic and extraembryonic pre-implantation lineages and to self-organise in vitro into blastocyst-like structures called blastoids. Naive hPSC maintenance routinely relies on co-culture with mouse embryonic fibroblast (MEFs) as feeder cells, a method prone to variability and analytical confounders. Here, we describe a feeder-free culture system based on serum coating that supports long-term maintenance of naive hPSCs. Across five laboratories, 30 serum batches were evaluated for the expansion of eight naive hPSCs lines for up to 25 passages. Mass spectrometry analysis identified fibronectin and collagens as extracellular matrix proteins consistently present in serum coating. Cells cultured on serum coating displayed growth kinetics, clonogenic capacity, mutation rates, and global gene expression profiles comparable to MEF-based cultures. Importantly, serum-cultured naive hPSCs efficiently underwent germ layer specification, retained trophectoderm competence, and generated blastoids with efficiency similar to MEF-based cultures. Collectively, serum coating provides a scalable, cost-effective, and robust alternative to feeder-based systems, preserving genomic stability and developmental potential while eliminating MEF-associated disadvantages and variability. This platform facilitates large-scale applications of naive hPSCs and enables more reproducible mechanistic studies.

Fused filament fabrication (FFF) three-dimensional (3D) printing technologies offer new opportunities for fabricating customizable, low-cost platforms for tissue engineering applications. Here, we developed and characterized 3D-printed scaffolds using conductive thermoplastic polyurethane (cTPU) filaments and evaluated their mechanical, electrical, and biological performance in vitro. Dynamic mechanical analysis (DMA) across a range of temperatures and frequencies revealed that both TPU and cTPU exhibit temperature- and rate-dependent elastic moduli, with cTPU showing enhanced mechanical stiffness due to the incorporation of conductive fillers. Electrical testing confirmed that cTPU exhibited a stable conductivity (∼1-2mS/cm) resembling physiological conditions. Surface characterization showed that cTPU was significantly more hydrophilic and exhibited higher nanoscale roughness, both of which are favorable for cell-material interactions. Mouse embryonic fibroblasts (MEFs) cultured on both scaffolds showed high viability (>85%) and significant proliferation. Notably, immunofluorescence analysis of cultured hippocampal neurons revealed significantly higher density of neuronal networks represented by higher microtubule-associated protein 2 (MAP-2)-positive cell density, greater MAP-2 area coverage, larger average MAP-2 cell area, and enhanced postsynaptic density protein 95 (PSD-95) expression on cTPU scaffolds. Together, these results demonstrate that FFF 3D-printed cTPU platforms can support long-term neuronal growth and synaptic maturation, offering promising applications in neural tissue modeling and bioelectronic interfaces. Practical Application: Characterizing soft viscoelastic materials whose properties strongly depend on temperature and strain rate is challenging and typically requires extensive testing across multiple conditions. Using a single-specimen Dynamic Mechanical Analysis-based mechanical testing method and a viscoelastic-elastic transformation that converts frequency-domain viscoelastic measurements into elastic constants over a broad range of test conditions, validated by tensile tests, we efficiently generated reliable modulus data across a range of conditions, enhancing testing throughput without sacrificing accuracy. As a case study, we demonstrate the successful fabrication and comprehensive characterization of FDM 3D-printed conductive TPU (cTPU) scaffolds for potential applications in neural tissue modeling and bioelectronic interfaces, with the results positioning cTPU composites as cost-effective, tunable, cytocompatible, and electrically active platforms capable of supporting neuronal growth and function.

Current models suggest that MIRO GTPases anchor cytoskeletal motors to the mitochondrial outer membrane (MOM). However, our previous findings indicate that the unconventional myosin, MYO19, interacts with MIRO weakly and that a MIRO-independent MOM-localizing domain interacts more tightly with the MOM. To test the hypothesis that other MIRO interactors may also have MIRO-independent MOM binding, we examined interactions between TRAK proteins (microtubule motor-mitochondria adaptor proteins) and the MOM via quantitative fluorescence microscopy and steady-state kinetic approaches. Using GFP-TRAK truncations expressed in MIRO1-2 double knockout mouse embryonic fibroblasts, we identified a MIRO-independent mitochondrial-binding domain in the C-terminus of TRAK1 and TRAK2, with a MOM localization pattern similar to what we observed for full-length GFP-TRAK proteins. The MIRO-binding domains (MBD) of the TRAK proteins were only able to localize to mitochondria when MIRO is expressed. Importantly, fluorescence recovery after photobleaching (FRAP) demonstrated that the steady-state kinetics of TRAKMBD/MIRO interactions were faster exchanging than for either full-length TRAK or the TRAK C-terminal MOM-binding domain expressed alone. These data support a model where TRAK/MIRO associations may be serving functions beyond anchoring cytoskeletal motors and their adapters to the MOM.

Late-stage metastatic cutaneous melanoma (MCM) and neuroblastoma (NB) are the most aggressive skin and childhood cancers with survival rates of <50%, mainly due to the emergence of resistance to available drugs, thus requiring an urgent solution. Quaternary phosphonium salts (QPSs) can exhibit strong anticancer effects, regardless of the developed resistance. Triphenyl (1) and diphenyl (3 and 4) phosphonium salts were synthesized, treating commercial triphenyl phosphine and synthesizing 11-diphenylphosphanyl-undecan-1-ol (2), respectively, with benzyl bromide. Upon full characterization, they were tested, for the first time, on MeTRAV (BRAFV600D) and MeOV (BRAFV600E) vemurafenib (PLX)-resistant MCM cells, etoposide (ETO)-sensitive (HTLA 230) and multidrug resistant (MDR) (HTLA ER) NB cells, non-tumorigenic human keratinocytes (HaCaT), and mouse embryonic fibroblasts (3T3), as well as red blood cells (RBCs). Viability of MeTRAV cells was decreased to 44.8% by administration of 1 (100 µM), in intermediate-time (48 h) treatments, while short-time exposure (24 h) to 3 (≥75 µM) and 4 (≥50 µM) was sufficient to reduce their viability to 33.6 and 32.2%. Viability of MeOV was decreased under 50% with 5 µM concentrations of 1 and 25 µM of 3 and 4, While cells were exterminated (26.9, 20.6, and 21.8%) with higher concentrations after 48 h exposure. Collectively, 1 was the better performing compound (IC50 = 6.4 µM, 48 h). Viability of HTLA ER cells was decreased under 50% upon 72 h administrations of 1 at concentrations ≥ 50 µM, 48 h (≥75 µM) and 72 h (≥50 µM) of 3, and after 72 h (≥75 µM) of 4, but 72 h exposure and high concentrations of all compounds were necessary for their extermination (31.2, 28.7, and 29.7%). Viability of HTLA 230 cells was not <50% when 1 and 4 were administered for only 24 h, while their viability was <50% after administration of 3 at all times of exposure. At high concentrations, all compounds exterminated cells (33.6, 25.3%, 1, 48-72 h; 38.6, 30.2, and 24.7%, 3, 24-72 h; 33.2%, 4, 72 h). The best-performing compounds were 1 (IC50 = 4.0 µM, HTLA 230) and 3 (IC50 = 27.8 µM, HTLA ER) at 72 h exposure. The cytotoxic effects of compound 4 on MeTRAV cells, when exposed to 24/48 h treatments, were comparable to those of PLX on the same cells in 72 h treatments. Compound 1, in shorter 48 h treatments of PLX-R MeOV, was 2.5-fold more cytotoxic than PLX in 72 h ones. All compounds were not cytotoxic to 3T3 cells at all times of exposure; they had low cytotoxicity to HaCaT cells in 24 and 48 h treatments and were slightly cytotoxic to RBCs in 24 h ones. Compound 1 could be a promising platform to develop new intermediate-time therapies for PLX-R MeOV cells, while 4 could be used to develop 24 and 48 h treatments for PLX-R MeTRAV cells. Also, all compounds could be developed as new treatment options for both ETO-sensitive and MDR late-stage HR-NB cells, being even more effective than ETO by 1.2, 2.0, and 1.3 times (HTLA 230) and 3.2, 4.7, and 3.2 times (HTLA ER). All compounds have the potential to be developed as adjuvants in already existing anticancer cocktails to treat MCM and/or NB.

Stimulator of interferon genes (STING) signaling, as a pivotal DNA-sensing mechanism, orchestrates antiviral and antitumor immunity through the induction of type I interferon response. Precise modulation of STING signaling is critical for maintaining immune homeostasis, yet its regulatory landscape has not been fully elucidated. Here, we identify the Deltex E3 ubiquitin ligase 2 (DTX2) as a positive regulator of STING-type I interferon response. Loss of Dtx2 in mouse macrophages and embryonic fibroblasts (MEFs) markedly impairs the type I interferon production upon double-stranded DNA (dsDNA) or cyclic guanosine monophosphate (GMP)-adenosine monophosphate (AMP) (cGAMP) stimulation. Correspondingly, Dtx2-/- mice exhibit more susceptibility to DNA viral infection compared to its counterparts. Mechanistically, DTX2 interacts with STING to promote K63-linked ubiquitination at residue K236 and K370, which facilitates the translocation of STING from the endoplasmic reticulum (ER) to the Golgi apparatus and activates downstream signaling cascades. Furthermore, we demonstrate that DTX2 potentiates STING-mediated type I interferon response in multiple tumor cell lines, and enhances anti-tumor immunity in murine head and neck cancer models. Collectively, our work uncovers DTX2 as a previously unrecognized regulator of STING, revealing a ubiquitin-dependent mechanism for fine-tuning innate immune response with implications for combating infections and cancer.

Vanadyl Complexes (VOp-Dmada) Promote Healthy Aging by Regulating Electron Transport Mediated by Mitochondrial Complex II.

Spontaneous whole genome duplication renders mouse embryonic fibroblasts resistant to reprogramming.

Through alternative splicing, the TP53 gene can generate multiple protein isoforms with distinct biochemical properties. The p53psi isoform has been identified as a shorter variant than full-length p53 as it lacks nuclear localization, oligomerization, and part of the DNA binding domains due to the use of an alternative 3' splice site in intron 6. Several TP53-truncating mutations, including those producing p53psi, have been detected in a significant proportion of human tumors. However, the mechanistic roles of these truncated p53 proteins remain poorly understood. Here, we describe the generation and analysis of a genetically engineered mouse model that expresses the p53psi protein in place of the full-length p53 protein. In the C57/BL6J genetic background, mice heterozygous for the targeted p53psi allele (p53KI/+) appear phenotypically normal, survive to adulthood, and reproduce. However, heterozygote matings fail to yield viable p53psi homozygote knock-in (p53KI/KI) pups, indicating that forced p53psi expression disrupts embryogenesis. Timed matings revealed that homozygous p53psi expression is embryonically lethal on day E16.5. E14.5-16.5 embryos were pale, reduced in size, and exhibited exencephaly, a defect typically associated with neural tube closure failure. Mouse embryonic fibroblasts (MEFs) derived from p53psi embryos and transformed with the E1A and H-RasV12 oncogenes formed tumors with a decreased growth rate compared to their p53null counterparts, suggesting that p53psi retains at least some tumor-suppressive functions. Our mechanistic studies suggest that p53psi modulates tumorigenesis by triggering senescence. These findings provide insights into the role of the p53psi variant, paving the way for a better interpretation of TP53 mutational patterns in human cancers.