The research, led by Min Wu, Yi-Fei Yang and Xiang-Ping Du of the Department of Orthopedics at Jiangxi Provincial Children’s Hospital in Nanchang, China, began with a simple observation that belied a complex mechanism. When the team exposed BMSCs to hydrogen peroxide, a standard laboratory approach for mimicking oxidative stress, the levels of SPI1, a transcription factor encoded by the Spi-1 proto-oncogene, dropped sharply. The same decline was visible in an animal model: rats whose ovaries had been removed to induce estrogen-deficiency osteoporosis also showed reduced SPI1 in their bone marrow-derived mesenchymal stem cells. The parallel was striking. In both the culture dish and the diseased skeleton, oxidative stress appeared to strip the cells of a factor they apparently needed to stay young.

To test whether that loss mattered, the researchers forced SPI1 expression back up in stressed cells. The effect was dramatic. Overexpressing SPI1 rescued the senescent phenotype that hydrogen peroxide had induced: the cells regained viability, produced fewer reactive oxygen species, and showed reduced staining for senescence-associated beta-galactosidase, a classic enzymatic marker of cellular aging. Just as importantly, SPI1 restoration preserved the cells’ ability to differentiate into bone-forming osteoblasts, which the team quantified using alizarin red S staining of mineralized matrix. In other words, SPI1 was not simply a bystander that declined as cells aged; it appeared to be an active guardian of both youth and bone-building function in the mesenchymal stem cell compartment.

The next question was how SPI1 exerted this protection. Transcription factors work by binding DNA and switching genes on or off, and the researchers had a specific target in mind: NAMPT, the gene encoding nicotinamide phosphoribosyltransferase, the rate-limiting enzyme of the NAD+ salvage pathway. NAD+ has become one of the most intensively studied molecules in aging biology, serving as a coenzyme for hundreds of redox reactions and as the fuel consumed by sirtuins, a family of deacylating enzymes that regulate stress resistance, mitochondrial function and longevity. Declining NAD+ levels are a hallmark of aged tissues, and boosting NAMPT activity has previously been shown to rejuvenate senescent mesenchymal stem cells. Through chromatin immunoprecipitation and luciferase reporter assays, the team demonstrated that SPI1 binds directly to the NAMPT promoter and transcriptionally activates it, raising NAD+ levels inside the stressed cells. This elevation of NAD+ was the mechanistic bridge between SPI1 and senescence resistance, connecting a transcription factor to one of aging biology’s most celebrated metabolic pathways.

But SPI1 is itself a protein, and proteins in the cellular environment are subject to constant quality control. The ubiquitin-proteasome system tags unwanted proteins with chains of ubiquitin molecules, marking them for destruction. For a transcription factor like SPI1, ubiquitination can mean a short half-life and a rapid collapse of expression under stress. This is where USP22 enters the story. USP22 is a deubiquitinating enzyme, a member of a large family of proteases that remove ubiquitin tags and thereby rescue proteins from degradation. The enzyme has attracted growing attention in recent years for its role in stabilizing key regulatory proteins: it has been shown, for example, to deubiquitinate PPARγ in liver cancer cells and FoxM1 in endometrial stromal cells, and to suppress inflammatory signaling by influencing the NLRP3 inflammasome. The Jiangxi team hypothesized that USP22 might perform the same service for SPI1 in BMSCs.

The experimental evidence supported the hypothesis at every step. Co-immunoprecipitation confirmed that USP22 physically interacts with SPI1 inside the cells. Ubiquitination assays then showed that USP22 removes ubiquitin chains from SPI1, and cycloheximide chase experiments, which block new protein synthesis and reveal how fast existing proteins decay, demonstrated that SPI1 protein persists far longer when USP22 is abundant. With SPI1 stabilized, NAMPT transcription stayed high, NAD+ levels were maintained, and the senescence program was held in check. The final piece of the causal puzzle came from a knockdown experiment: when the researchers silenced SPI1 in cells overexpressing USP22, the protective effects of USP22 largely evaporated. Senescence markers rose again, and osteogenic differentiation faltered, showing that SPI1 is the essential downstream mediator of USP22’s anti-aging action in these cells.

Why does this circuit matter beyond the laboratory? Osteoporosis, which affects hundreds of millions of people worldwide, is fundamentally a disease of failed bone regeneration, in which the balance tips from bone formation to bone resorption. Estrogen deficiency accelerates oxidative stress in the bone marrow, and senescent BMSCs not only fail to produce new osteoblasts but actively worsen the inflammatory milieu. The ovariectomized rat model used in this study reproduces key features of postmenopausal bone loss, and the finding that SPI1 declines in this model suggests that the USP22-SPI1-NAMPT axis may be a plausible therapeutic target. If a drug or gene therapy could raise USP22 activity, or stabilize SPI1 by another means, or simply replenish NAD+ through well-known precursors such as nicotinamide mononucleotide, the senescence cascade in transplanted or endogenous stem cells might be blunted before it begins.

The findings also connect to a much broader scientific conversation about the biology of aging. Cellular senescence was once viewed as a passive consequence of damage accumulation; it is now understood as a tightly regulated program, orchestrated by p53, p21, p16 and chromatin changes, and driven upstream by metabolic deficits such as NAD+ depletion. Sirtuins sit at the junction of metabolism and chromatin, consuming NAD+ as they modify histones and regulatory proteins, which is why the NAD+-sirtuin axis has become a focal point for anti-aging interventions. What this study adds is an upstream control point: a deubiquitinase-transcription factor module that feeds into NAMPT expression and thereby into the entire NAD+ economy of the cell. It suggests that protein stability, not merely gene expression or enzyme activity, may be a decisive lever for keeping stem cells youthful under stress.

There are, of course, important caveats and open questions. USP22 has a complicated double life in the literature: in some contexts it promotes tumor growth and inflammation, and its substrates are numerous and tissue-dependent. Any clinical strategy that elevates USP22 would need to weigh potential oncogenic risks, particularly since SPI1 itself is best known as an oncogene in hematopoietic malignancies, even as the new work assigns it a protective role in mesenchymal stem cells. The current study also relies on cell culture and a rat model; whether the same circuit operates in human bone marrow, and whether it can be safely modulated in patients, remains to be demonstrated. Still, the elegance of the pathway is hard to ignore: oxidative stress degrades SPI1, loss of SPI1 silences NAMPT, silencing of NAMPT drains NAD+, and drained NAD+ leaves stem cells old and bone barren. By inserting USP22 at the top of that cascade, evolution appears to have built a safeguard, and researchers have now found the switch. For an aging global population facing a rising tide of fractures, that switch is worth pulling.

Subject of Research: How the deubiquitinase USP22 protects bone marrow mesenchymal stem cells from oxidative stress-induced senescence

Article Title: USP22 alleviates oxidative stress-induced BMSCs senescence by stabilizing SPI1 protein

Article References: Wu, M., Yang, Y.-F., & Du, X.-P. (2026). USP22 alleviates oxidative stress-induced BMSCs senescence by stabilizing SPI1 protein. Molecular Genetics and Genomics, 301(1), Article 194. https://doi.org/10.1007/s00438-026-02519-8

Image Credits: AI Generated

DOI: 10.1007/s00438-026-02519-8

Keywords: USP22, SPI1, NAMPT, NAD+, bone marrow mesenchymal stem cells, cellular senescence, oxidative stress, deubiquitination, osteoporosis, osteogenic differentiation, sirtuins, protein stability

Postmenopausal osteoporosis arises when declining estrogen levels tip the balance of bone remodeling toward resorption, producing reduced bone mass, deteriorated trabecular microstructure, and elevated fracture risk. As populations age worldwide, the disease imposes a growing public health burden on elderly women. Current first-line treatments, including bisphosphonates and hormone replacement therapy, effectively slow bone loss, but long-term use carries well-documented concerns ranging from gastrointestinal damage to cardiovascular risks and potential carcinogenic hazards. That trade-off has driven researchers to search for natural compounds with genuine anti-osteoporotic activity and favorable safety profiles.

The research team, led by Zhaoguo Wang of Southern Medical University, turned to Auricularia auricula, an edible medicinal fungus whose polysaccharides have previously shown antioxidant, anti-inflammatory, and gut-regulating properties. To maximize yield, the group developed an ultrasound-assisted enzymatic extraction method, first screening cellulase, pectinase, and a combination of the two. The enzyme cocktail proved decisively superior, lifting the extraction yield to 27.72 percent compared with 24.02 percent for cellulase alone and 15.23 percent for pectinase alone, a synergy attributed to the enzymes attacking different structural components of the fungal cell wall simultaneously.

With the enzyme system selected, the researchers systematically optimized each extraction variable. Single-factor experiments identified a cellulase-to-pectinase ratio of 1.5:1, an enzyme concentration of 3000 units per 100 milliliters, a four-hour enzymolysis, a 1:40 solid-to-liquid ratio, twenty minutes of sonication, and an ultrasonic power of 2 watts per milliliter as favorable conditions. A Box-Behnken response surface design then refined four key parameters into a predictive model with strong explanatory power, achieving a coefficient of determination of 0.9156 and a non-significant lack-of-fit test. Validation runs under practical near-optimal conditions delivered an experimental yield of 30.15 percent, within 2.87 percent of the model prediction and inside the 95 percent prediction interval, confirming the reliability of the optimized process.

Characterization revealed that the resulting Auricularia auricula polysaccharide, or AAP, is an acidic heteropolysaccharide rich in glucose, mannose, and glucuronic acid, with smaller contributions from fucose and xylose. Total sugar content reached 74.07 percent, uronic acid 10.37 percent, and protein only 2.63 percent, while endotoxin levels remained low across the tested concentration range. Infrared spectroscopy identified hallmark polysaccharide absorptions, including hydroxyl stretching near 3000 to 3500 inverse centimeters, a carbonyl band consistent with glucuronic acid carboxyl groups, and a signal at 848 inverse centimeters indicating beta-configured sugar units. Molecular weight analysis showed a heterogeneous mixture spanning roughly one thousand to one and a half million Daltons, and thermal analysis demonstrated stability up to approximately 270 degrees Celsius, well above physiological temperatures.

The structural profile matters because polysaccharide bioactivity is thought to depend heavily on molecular weight and monosaccharide composition. Acidic, uronic-acid-rich polysaccharides cannot cross cell membranes, but their negatively charged carboxyl groups can engage positively charged receptor domains through electrostatic attraction. Notably, the extracellular domain of the insulin-like growth factor 1 receptor is enriched in arginine and lysine residues, making it a plausible anchoring target for anionic polysaccharides. This structural logic shaped the team’s central hypothesis: that AAP might stimulate osteoblasts not through the canonical BMP/Smad or Wnt/beta-catenin routes used by neutral fungal glucans, but through the IGF-1/IGF-1R signaling axis.

To test the hypothesis in living animals, the researchers used an ovariectomized rat model, the standard experimental surrogate for postmenopausal bone loss. Female rats underwent bilateral ovariectomy and, after a week of recovery, received daily oral AAP at 100, 300, or 500 milligrams per kilogram for twelve weeks, alongside sham, ovariectomy-only, and estradiol-treated control groups. Micro-computed tomography of the proximal tibia showed that only the high dose significantly increased bone mineral density and improved trabecular microarchitecture relative to untreated ovariectomized animals. Histological staining corroborated the imaging: high-dose AAP increased the bone area fraction and reduced the osteoclast-covered bone surface, mirroring the effects of estradiol without stimulating uterine tissue, a key safety distinction for any non-hormonal therapy.

Serum biomarkers told a consistent story. Ovariectomized rats displayed elevated CTX-1 and RANKL, markers of rampant bone resorption, alongside depressed P1NP, OPG, and the protective OPG-to-RANKL ratio. High-dose AAP reversed all of these shifts and restored serum IGF-1, which had fallen sharply after ovariectomy. Because IGF-1 is the most abundant growth factor stored in bone matrix and a well-established driver of osteoblast differentiation, proliferation, and survival, its recovery pointed directly toward the signaling axis the team suspected. Importantly, no abnormalities appeared in major organ indices or histology across the treatment groups, and AAP did not raise the uterine index at any dose tested.

In vitro experiments then dissected the mechanism at cellular resolution. AAP did not alter the proliferation of bone marrow stromal cells or MC3T3-E1 pre-osteoblasts, but the high concentration markedly enhanced osteogenic differentiation, boosting alkaline phosphatase activity at day seven and matrix mineralization at day twenty-one. Western blotting revealed increased IGF-1 and IGF-1R protein levels, elevated phosphorylation of IGF-1R, IRS-1, AKT, and ERK, and upregulation of core osteogenic genes including RUNX2, ALP, COL-1, OSX, and OPG. Total protein levels of Smad2, Smad3, Wnt4, and beta-catenin were unchanged, though the authors caution that this alone cannot exclude activation of those pathways. The decisive experiment came with CRISPR/Cas9-generated IGF-1R knockout cells: in these, the AAP-driven gains in alkaline phosphatase activity, mineralization, and osteogenic gene expression were largely abolished, providing functional proof that the receptor is required for the full effect.

The team also addressed a question critical to any functional food: does the compound survive digestion? Simulated oral, gastric, and intestinal digestion phases left AAP’s osteogenic activity essentially intact, with all digested fractions sustaining elevated alkaline phosphatase activity in human osteoblasts indistinguishable from the undigested compound. The authors acknowledge limitations, including the imperfect fidelity of the ovariectomy model to human disease and the absence of formal pharmacokinetic and long-term toxicity studies, though pharmacokinetic data from a structurally analogous Auricularia polysaccharide suggest efficient oral absorption. Further work to purify individual molecular weight fractions, map upstream receptor interactions, and complete safety profiling will determine whether this kitchen-staple fungus can move from the wok to the clinic as a bone-protective functional food.

Subject of Research: Osteoprotective effects and IGF-1/IGF-1R mechanism of Auricularia auricula polysaccharides in postmenopausal osteoporosis

Article Title: Preparation and physicochemical characterization of polysaccharides from Auricularia auricula and their osteogenic potential in vitro and in vivo

Article References: Wang, Z., Liu, G., Lai, F., Xiao, X., & Xu, S. (2026). Preparation and physicochemical characterization of polysaccharides from Auricularia auricula and their osteogenic potential in vitro and in vivo. Journal of Agriculture and Food Research, 31, Article 103263. https://doi.org/10.1016/j.jafr.2026.103263

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103263

Keywords: Auricularia auricula, wood ear mushroom, polysaccharides, postmenopausal osteoporosis, osteogenic differentiation, IGF-1/IGF-1R signaling, bone mineral density, ovariectomized rats, ultrasound-assisted enzymatic extraction, functional food, bone metabolism, natural products