Dysregulation of the Bmi-1/p16(Ink⁴a) pathway provokes an aging-associated decline of submandibular gland function

Involvement of metformin and aging in salivary expression of ACE2 and TMPRSS2

SARS-CoV-2-related proteins, ACE2 and TMPRSS2, are determinants of SARS-CoV-2 infection. Although these proteins are expressed in oral-related tissues, their expression patterns and modulatory mechanisms in the salivary glands remain unknown. We herein showed that full-length ACE2, which has both a fully functional enzyme catalytic site and high-affinity SARS-CoV-2 spike S1-binding sites, was more highly expressed in salivary glands than in oral mucosal epithelial cells and the lungs. Regarding TMPRSS2, zymogen and the cleaved form were both expressed in the salivary glands, whereas only zymogen was expressed in murine lacrimal glands and the lungs. Metformin, an AMPK activator, increased stimulated saliva secretion and full-length ACE2 expression and decreased cleaved TMPRSS2 expression in the salivary glands, and exerted the same effects on soluble ACE2 (sACE2) and sTMPRSS2 in saliva. Moreover, metformin decreased the expression of beta-galactosidase, a senescence marker, and ADAM17, a sheddase of ACE2 to sACE2, in the salivary glands. In aged mice, the expression of ACE2 was decreased in the salivary glands, whereas that of sACE2 was increased in saliva, presumably by the up-regulated expression of ADAM17. The expression of TMPRSS2 in the salivary glands and sTMPRSS2 in saliva were both increased. Collectively, these results suggest that the protein expression patterns of ACE2 and TMPRSS2 in the salivary glands differ from those in other oral-related cells and tissues, and also that metformin and aging affect the salivary expression of ACE2 and TMPRSS2, which have the potential as targets for preventing the transmission of SARS-CoV-2.

B cell senescence promotes age-related changes in oral microbiota

In recent years, there has been increasing attention towards understanding the relationship between age-related alterations in the oral microbiota and age-associated diseases, with reports emphasizing the significance of maintaining a balanced oral microbiota for host health. However, the precise mechanisms underlying age-related changes in the oral microbiota remain elusive. We recently reported that cellular senescence of ileal germinal center (GC) B cells, triggered by the persistent presence of commensal bacteria, results in diminished IgA production with aging and subsequent alterations in the gut microbiota. Consequently, we hypothesize that a similar phenomenon may occur in the oral cavity, potentially contributing to age-related changes in the oral microbiota. Examination of p16-luc mice, wherein the expression of the senescent cell marker p16INK4a can be visualized, raised under specific pathogen-free (SPF) or germ-free (GF) conditions, indicated that, unlike ileal GC B cells, the accumulation of senescent cells in GC B cells of cervical lymph nodes increases with age regardless of the presence of commensal bacteria. Furthermore, longitudinal studies utilizing the same individual mice throughout their lifespan revealed concurrent age-related alterations in the composition of the oral microbiota and a decline in salivary IgA secretion. Further investigation involving Rag1-/- mice transplanted with B cells from wild-type or p16INK4a and p21Waf1/Cip1 -double knockout mice unveiled that B cell senescence leads to reduced IgA secretion and alteration of the oral microbiota. These findings advance our understanding of the mechanism of age-associated changes in the oral microbiota and open up possibilities of their control.

Examination of age-related changes in the submandibular glands of male mice

Salivary gland hypofunction adversely affects the oral environment and daily life by causing dry mouth (xerostomia). Senescence-related atrophy of salivary gland tissues is one cause of xerostomia, and it is particularly common among the elderly. However, the underlying mechanism is poorly understood, and no treatment has been established. Therefore, we examined age-related changes in senescence-associated secretory phenotype (SASP) factors, which regulate stemness and cellular senescence, in mouse submandibular glands. We analyzed the submandibular glands of 6-week-old (young group, n = 6) and 82-week-old mice (aged group, n = 6). We performed salivary flow rate measurements, histological analysis including immunohistochemistry, and quantitative real-time PCR. The salivary flow rate was significantly lower in the aged group than in the young group. In addition, immunostaining and quantitative real-time PCR illustrated that aquaporin-5 and α-amylase expressions were significantly decreased in aged mice, indicating salivary gland hypofunction. c-Kit and cytokeratin 5 expressions were also significantly decreased in this group, suggesting that the regenerative abilities of the submandibular glands were reduced because of decreased stem and progenitor cell counts. Furthermore, the levels of p16INK4a and p21 (the senescence markers) and TGF-β1 and IL-6 (SASP factors) were significantly increased in mice, suggesting that senescence had been promoted. The decreased numbers of stem and progenitor cells and increased levels of SASP factors might be associated with age-related changes in mouse submandibular glands. These results might facilitate the development of treatments for senescence-related submandibular gland hypofunction.

Autologous mesenchymal stem cells offer a new paradigm for salivary gland regeneration

Salivary gland (SG) dysfunction, due to radiotherapy, disease, or aging, is a clinical manifestation that has the potential to cause severe oral and/or systemic diseases and compromise quality of life. Currently, the standard-of-care for this condition remains palliative. A variety of approaches have been employed to restore saliva production, but they have largely failed due to damage to both secretory cells and the extracellular matrix (niche). Transplantation of allogeneic cells from healthy donors has been suggested as a potential solution, but no definitive population of SG stem cells, capable of regenerating the gland, has been identified. Alternatively, mesenchymal stem cells (MSCs) are abundant, well characterized, and during SG development/homeostasis engage in signaling crosstalk with the SG epithelium. Further, the trans-differentiation potential of these cells and their ability to regenerate SG tissues have been demonstrated. However, recent findings suggest that the "immuno-privileged" status of allogeneic adult MSCs may not reflect their status post-transplantation. In contrast, autologous MSCs can be recovered from healthy tissues and do not present a challenge to the recipient's immune system. With recent advances in our ability to expand MSCs in vitro on tissue-specific matrices, autologous MSCs may offer a new therapeutic paradigm for restoration of SG function.

Aging-associated stem/progenitor cell dysfunction in the salivary glands of mice

Although stem cell aging leads to a decline in tissue homeostasis and regenerative capacity, it remains unclear whether salivary gland stem cell function changes during this process. However, the salivary glands are gradually replaced by connective tissue during aging. Here, we show a decline in the stem cell ability of CD133-positive stem/progenitor cells in the salivary glands of aged mice. The CD133-positive cells were isolated from young, adult, and aged mice. The number of CD133-positive cells was significantly decreased in aged mice. They also showed a lower sphere formation capacity compared to young and adult mice. RNA sequencing revealed that CD133-positive cells in aged mice exhibited lower gene expression of several aging-related genes, including FoxO3a, than those in young and adult mice. Salivary gland cells infected with a recombinant lentivirus encoding the FoxO3a gene showed a reduction in oxidative stress induced by hydrogen peroxide compared with those infected with a control virus. Thus, FoxO3a may inhibit stem cell aging via oxidative stress.

Chemokines Up-Regulated in Epithelial Cells Control Senescence-Associated T Cell Accumulation in Salivary Glands of Aged and Sjögren's Syndrome Model Mice

Immunosenescence is characterized by age-associated changes in immunological functions. Although age- and autoimmune-related sialadenitis cause dry mouth (xerostomia), the roles of immunosenescence and cellular senescence in the pathogenesis of sialadenitis remain unknown. We demonstrated that acquired immune cells rather than innate immune cells infiltrated the salivary glands (SG) of aged mice. An analysis of isolated epithelial cells from SG revealed that the expression levels of the chemokine CXCL13 were elevated in aged mice. Senescence-associated T cells (SA-Ts), which secrete large amounts of atypical pro-inflammatory cytokines, are involved in the pathogenesis of metabolic disorders and autoimmune diseases. The present results showed that SA-Ts and B cells, which express the CXCL13 receptor CXCR5, accumulated in the SG of aged mice, particularly females. CD4⁺ T cells derived from aged mice exhibited stronger in vitro migratory activity toward CXCL13 than those from young mice. In a mouse model of Sjögren’s syndrome (SS), SA-Ts also accumulated in SG, presumably via CXCL12-CXCR4 signaling. Collectively, the present results indicate that SA-Ts accumulate in SG, contribute to the pathogenesis of age- and SS-related sialadenitis by up-regulating chemokines in epithelial cells, and have potential as therapeutic targets for the treatment of xerostomia caused by these types of sialadenitis.

Bmi-1 regulates mucin levels and mucin O-glycosylation in the submandibular gland of mice

Mucins, the major components of salivary mucus, are large glycoproteins abundantly modified with O-glycans. Mucins present on the surface of oral tissues contribute greatly to the maintenance of oral hygiene by selectively adhering to the surfaces of microbes via mucin O-glycans. However, due to the complex physicochemical properties of mucins, there have been relatively few detailed analyses of the mechanisms controlling the expression of mucin genes and the glycosyltransferase genes involved in glycosylation. Analysis performed using supported molecular matrix electrophoresis, a methodology developed for mucin analysis, and knockout mice without the polycomb group protein Bmi-1 revealed that Bmi-1 regulates mucin levels in the submandibular gland by suppressing the expression of the mucin Smgc gene, and that Bmi-1 also regulates mucin O-glycosylation via suppression of the glycosyltransferase Gcnt3 gene in the submandibular gland.

Mouth-Watering Results: Clinical Need, Current Approaches, and Future Directions for Salivary Gland Regeneration

Permanent damage to the salivary glands and resulting hyposalivation and xerostomia have a substantial impact on patient health, quality of life, and healthcare costs. Currently, patients rely on lifelong treatments that alleviate the symptoms, but no long-term restorative solutions exist. Recent advances in adult stem cell enrichment and transplantation, bioengineering, and gene transfer have proved successful in rescuing salivary gland function in a number of animal models that reflect human diseases and that result in hyposalivation and xerostomia. By overcoming the limitations of stem cell transplants and better understanding the mechanisms of cellular plasticity in the adult salivary gland, such studies provide encouraging evidence that a regenerative strategy for patients will be available in the near future.

p16 deficiency attenuates intervertebral disc degeneration by adjusting oxidative stress and nucleus pulposus cell cycle

The cell cycle regulator p16 is known as a biomarker and an effector of aging. However, its function in intervertebral disc degeneration (IVDD) is unclear. In this study, p16 expression levels were found to be positively correlated with the severity of human IVDD. In a mouse tail suspension (TS)-induced IVDD model, lumbar intervertebral disc height index and matrix protein expression levels were reduced significantly were largely rescued by p16 deletion. In TS mouse discs, reactive oxygen species levels, proportions of senescent cells, and the senescence-associated secretory phenotype (SASP) were all increased, cell cycling was delayed, and expression was downregulated for Sirt1, superoxide dismutase 1/2, cyclin-dependent kinases 4/6, phosphorylated retinoblastoma protein, and transcription factor E2F1/2. However, these effects were rescued by p16 deletion. Our results demonstrate that p16 plays an important role in IVDD pathogenesis and that its deletion attenuates IVDD by promoting cell cycle and inhibiting SASP, cell senescence, and oxidative stress.

Neck and shoulder pain, lower back pain and leg numbness are conditions that many people will encounter as years go by. This is because intervertebral discs, the padding structures that fit between the bones in the spine, degenerate with age: their cells enter a ‘senescent’, inactive state, and stop multiplying.

A protein known as p16, an important regulator of cell growth and division, is known to accumulate in senescent cells. In fact, in mouse fat tissue, muscles or eyes, removing the cells that contain high levels of p16 delays aging-associated disorders. However, it was still unknown whether deactivating the gene that codes p16 in senescent cells could delay disc degeneration.

Here, Che, Li et al. discovered that p16 is highly present in the senescent cells of severely degenerated human intervertebral discs. The cells in the nucleus pulposus, the jelly-like and most critical tissue in the intervertebral discs, were extracted and grown in the lab under conditions that replicate the early stages of damage to the spine. Drugs and genetic manipulations were then used to decrease the amount of p16 in these cells.

The experiments showed that reducing the levels of p16 results in the senescent cells multiplying more and showing fewer signs of damage and aging. In addition, the discs of mice in which the gene that codes for p16 had been deleted were less prone to degeneration compared to ‘normal’ mice in similar conditions.

Overall, the work by Che, Li et al. shows that inhibiting p16 in disc cells delays the aging process and reduces the degeneration of intervertebral discs. These findings may one day be applicable to people with intervertebral disc diseases who, for example, could potentially benefit from a gene therapy targeting the cells which produce p16.

Sirt1 Promotes Osteogenic Differentiation and Increases Alveolar Bone Mass via Bmi1 Activation in Mice

Sirtuin 1 (Sirt1), a protein deacetylase, is a novel target for bone metabolism. To investigate whether overexpression of Sirt1 in mandibular mesenchymal stem cells (M-MSCs) increased alveolar bone mass in vivo, we generated Sirt1 transgenic mice (Sirt1^TG ), with Sirt1 gene expression driven by the Prx1 gene, which represents the mesenchymal lineage. Our results demonstrated that overexpression of Sirt1 in M-MSCs increased the alveolar bone volume in 1-month-old, 9-month-old, and 18-month-old Sirt1^TG mice compared with age-matched wild-type (WT) mice, and in ovariectomized Sirt1^TG mice compared with ovariectomized WT mice by stimulating M-MSC differentiation into osteoblasts. Treatment with resveratrol, a Sirt1 activator, increased Sirt1 binding with Bmi1 and reduced Bmi1 acetylation in a dose-dependent manner demonstrated in M-MSC cultures. Both treatment with resveratrol in M-MSC cultures and overexpressed Sirt1 in M-MSCs ex vivo cultures increased nuclear translocation of Bmi1. Furthermore, we demonstrated that deletion of Bmi1 blocked the increased alveolar bone volume in Sirt1^TG mice. The Sirt1 activator resveratrol inhibited human MSC senescence and promoted their differentiation into osteoblasts, which were associated with upregulating the expression levels of Sirt1 and nuclear translocation of Bmi1. The present results suggested that Sirt1 promotes MSC proliferation and osteogenic differentiation, inhibits MSC senescence to increase alveolar bone volume by promoting the deacetylation and nuclear translocation of Bmi1. Thus, our study elucidated the mechanism by which Sirt1 increases alveolar bone mass, and these findings are important for the clinical application of the Sirt1 activator resveratrol for the promotion of alveolar bone formation and prevention of alveolar bone loss. © 2019 American Society for Bone and Mineral Research.

The Polycomb Group Protein Pcgf1 Is Dispensable in Zebrafish but Involved in Early Growth and Aging

Polycomb Repressive Complex (PRC) 1 regulates the control of gene expression programs via chromatin structure reorganization. Through mutual exclusion, different PCGF members generate a variety of PRC1 complexes with potentially distinct cellular functions. In this context, the molecular function of each of the PCGF family members remains elusive. The study of PCGF family member expression in zebrafish development and during caudal fin regeneration reveals that the zebrafish pcgf genes are subjected to different regulations and that all PRC1 complexes in terms of Pcgf subunit composition are not always present in the same tissues. To unveil the function of Pcgf1 in zebrafish, a mutant line was generated using the TALEN technology. Mutant pcgf1^-/- fish are viable and fertile, but the growth rate at early developmental stages is reduced in absence of pcgf1 gene function and a significant number of pcgf1^-/- fish show signs of premature aging. This first vertebrate model lacking Pcgf1 function shows that this Polycomb Group protein is involved in cell proliferation during early embryogenesis and establishes a link between epigenetics and aging.

Elevated p16ink4a Expression in Human Labial Salivary Glands as a Potential Correlate of Cognitive Aging in Late Midlife

BACKGROUND

The cell-cycle inhibitor and tumor suppressor cyclin dependent kinase inhibitor, p16ink4a, is one of the two gene products of the ink4a/ARF (cdkn2a) locus on chromosome 9q21. Up-regulation of p16ink4a has been linked to cellular senescence, and findings from studies on different mammalian tissues suggest that p16ink4a may be a biomarker of organismal versus chronological age.

OBJECTIVE

The aim of this study was to examine the immunolocalization pattern of p16ink4a in human labial salivary gland (LSG) tissue, and to analyze whether its expression level in LSGs is a peripheral correlate of cognitive decline in late midlife.

METHODS

The present study was a part of a study of causes and predictors of cognitive decline in middle-aged men in a Danish birth cohort. It is based on data from 181 male participants from the Danish Metropolit birth cohort, born in 1953, who were examined for age-associated alterations in cognition, dental health, and morphological and autonomic innervation characteristics of the LSGs. The participants were allocated to two groups based on the relative change in cognitive performance from young adulthood to late midlife. LSG biopsies were analyzed by qRT-PCR for the expression level of p16ink4a. Immunohistochemistry was performed on formalin-fixed, paraffin-embedded sections of LSGs.

RESULTS

p16ink4a immunoreactivity was observed in LSG ductal, myoepithelial, and stromal cells, but not in acinar cells. The mean relative expression of p16ink4a in LSGs was higher in the group of participants with decline in cognitive performance. A logistic regression analysis revealed that the relative p16 expression was predictive of the participant's group assignment. A negative correlation was found between relative p16ink4a expression and the participant's standardized regression residuals from early adulthood to late midlife cognitive performance scores.

CONCLUSIONS

p16ink4a expression in human LSGs may constitute a potential peripheral correlate of cognitive decline. Human labial salivary glands seem suitable for studies on organismal as opposed to chronological age.

Similar ex vivo expansion and post-irradiation regenerative potential of juvenile and aged salivary gland stem cells

BACKGROUND AND PURPOSE

Salivary gland dysfunction is a major side effect of radiotherapy for head and neck cancer patients, which in the future might be salvaged by autologous adult salivary gland stem cell (SGSC) therapy. Since frail elderly patients may have decreased activity of SGSCs, we aimed to characterize the potential of aged SGSC-population in a murine model.

MATERIALS AND METHODS

Salivary glands and salisphere-derived cells from young and old mice were tested for CD24 and CD29 stem cell marker expression using FACS. Moreover, in vitro expansion capability and in vivo regeneration potential upon post-irradiation transplantation of young and aged SGSCs were measured.

RESULTS

An increase in CD24(hi)/CD29(hi) putative stem cells was detected in aged salivary glands albeit with a decrease in functional ability to form salispheres. However, the salispheres formed from aged mice salivary glands expressed CD24(hi)/CD29(hi) to the same extent as the ones from young mice. Moreover, following exposure to adequate growth conditions old and young SGSCs exhibited similar in vitro expansion- and in vivo regeneration potential.

CONCLUSIONS

Aged SGSCs although reduced in number are in vitro indistinguishable from young SGSCs and could potentially be used to ameliorate age- or treatment related salivary gland dysfunction.