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Aire in medullary thymic epithelial cells (mTECs) plays an important role in the establishment of self-tolerance. Because Aire+ mTECs appear to be a limited subset, they may constitute a unique lineage(s) among mTECs. An alternative possibility is that all mTECs are committed to express Aire in principle, but Aire expression by individual mTECs is conditional. To investigate this issue, we established a novel Aire reporter strain in which endogenous Aire is replaced by the human AIRE-GFP-Flag tag (Aire/hAGF-knockin) fusion gene. The hAGF reporter protein was produced and retained very efficiently within mTECs as authentic Aire nuclear dot protein. Remarkably, snapshot analysis revealed that mTECs expressing hAGF accounted for >95% of mature mTECs, suggesting that Aire expression does not represent a particular mTEC lineage(s). We confirmed this by generating Aire/diphtheria toxin receptor–knockin mice in which long-term ablation of Aire+ mTECs by diphtheria toxin treatment resulted in the loss of most mature mTECs beyond the proportion of those apparently expressing Aire. These results suggest that Aire expression is inherent to all mTECs but may occur at particular stage(s) and/or cellular states during their differentiation, thus accounting for the broad impact of Aire on the promiscuous gene expression of mTECs.

Cortical thymic epithelial cells (cTECs) and medullary thymic epithelial cells (mTECs) play essential roles in the positive and negative selection of developing thymocytes, respectively. Aire in mTECs plays an essential role in the latter process through expression of broad arrays of tissue-restricted Ags. To determine whether the location of Aire within the medulla is absolutely essential or whether Aire could also function within the cortex for establishment of self-tolerance, we used bacterial artificial chromosome technology to establish a semiknockin strain of NOD-background (β5t/Aire-transgenic) mice expressing Aire under control of the promoter of β5t, a thymoproteasome expressed exclusively in the cortex. Although Aire was expressed in cTECs as typical nuclear dot protein in β5t/Aire-Tg mice, cTECs expressing Aire ectopically did not confer transcriptional expression of either Aire-dependent or Aire-independent tissue-restricted Ag genes. We then crossed β5t/Aire-Tg mice with Aire-deficient NOD mice, generating a strain in which Aire expression was confined to cTECs. Despite the presence of Aire+ cTECs, these mice succumbed to autoimmunity, as did Aire-deficient NOD mice. The thymic microenvironment harboring Aire+ cTECs, within which many Aire-activated genes were present, also showed no obvious alteration of positive selection, suggesting that Aire’s unique property of generating a self-tolerant T cell repertoire is functional only in mTECs.

Essential roles of NF-κB–inducing kinase (NIK) for the development of medullary thymic epithelial cells (mTECs) and regulatory T cells have been highlighted by studies using a strain of mouse bearing a natural mutation of the NIK gene (aly mice). However, the exact mechanisms underlying the defect in thymic cross-talk leading to the breakdown of self-tolerance in aly mice remain elusive. In this study, we demonstrated that production of regulatory T cells and the final maturation process of positively selected conventional αβ T cells are impaired in aly mice, partly because of a lack of mature mTECs. Of note, numbers of thymic dendritic cells and their expression of costimulatory molecules were also affected in aly mice in a thymic stroma–dependent manner. The results suggest a pivotal role of NIK in the thymic stroma in establishing self-tolerance by orchestrating cross-talk between mTECs and dendritic cells as well as thymocytes. In addition, we showed that negative selection was impaired in aly mice as a result of the stromal defect, which accounts for the development of organ-specific autoimmunity through a lack of normal NIK.

Understanding the cellular dynamics of Aire-expressing lineage(s) among medullary thymic epithelial cells (AEL-mTECs) is essential for gaining insight into the roles of Aire in establishment of self-tolerance. In this study, we monitored the maturation program of AEL-mTECs by temporal lineage tracing, in which bacterial artificial chromosome transgenic mice expressing tamoxifen-inducible Cre recombinase under control of the Aire regulatory element were crossed with reporter strains. We estimated that the half-life of AEL-mTECs subsequent to Aire expression was ∼7–8 d, which was much longer than that reported previously, owing to the existence of a post-Aire stage. We found that loss of Aire did not alter the overall lifespan of AEL-mTECs, inconsistent with the previous notion that Aire expression in medullary thymic epithelial cells (mTECs) might result in their apoptosis for efficient cross-presentation of self-antigens expressed by AEL-mTECs. In contrast, Aire was required for the full maturation program of AEL-mTECs, as exemplified by the lack of physiological downregulation of CD80 during the post-Aire stage in Aire-deficient mice, thus accounting for the abnormally increased CD80high mTECs seen in such mice. Of interest, increased CD80high mTECs in Aire-deficient mice were not mTEC autonomous and were dependent on cross-talk with thymocytes. These results further support the roles of Aire in the differentiation program of AEL-mTECs.

Expression of the p16Ink4a tumor suppressor gene, a sensor of oncogenic stress, is up-regulated by a variety of potentially oncogenic stimuli in cultured primary cells. However, because p16Ink4a expression is also induced by tissue culture stress, physiological mechanisms regulating p16Ink4a expression remain unclear. To eliminate any potential problems arising from tissue culture–imposed stress, we used bioluminescence imaging for noninvasive and real-time analysis of p16Ink4a expression under various physiological conditions in living mice. In this study, we show that oncogenic insults such as ras activation provoke epigenetic derepression of p16Ink4a expression through reduction of DNMT1 (DNA methyl transferase 1) levels as a DNA damage response in vivo. This pathway is accelerated in the absence of p53, indicating that p53 normally holds the p16Ink4a response in check. These results unveil a backup tumor suppressor role for p16Ink4a in the event of p53 inactivation, expanding our understanding of how p16Ink4a expression is regulated in vivo.

Controlled proteolytic degradation of specialized junctional structures, corneodesmosomes, by epidermal proteases is an essential process for physiological desquamation of the skin. Corneodesmosin (CDSN) is an extracellular component of corneodesmosomes and, although considerable debate still exists, genetic studies have suggested that the CDSN gene in the major psoriasis-susceptibility locus ( PSORS1 ) may be responsible for susceptibility to psoriasis, a human skin disorder characterized by excessive growth and aberrant differentiation of keratinocytes. CDSN is also expressed in the inner root sheath of hair follicles, and a heterozygous nonsense mutation of the CDSN gene in humans is associated with scalp-specific hair loss of poorly defined etiology. Here, we have investigated the pathogenetic roles of CDSN loss of function in the development of skin diseases by generating a mouse strain with targeted deletion of the Cdsn gene. Cdsn -deficient mouse skin showed detachment of the stratum corneum from the underlying granular layer and/or detachment within the upper granular layers due to the disrupted integrity of the corneodesmosomes. When grafted onto immunodeficient mice, Cdsn -deficient skin showed rapid hair loss together with epidermal abnormalities resembling psoriasis. These results underscore the essential roles of CDSN in hair physiology and suggest functional relevance of CDSN gene polymorphisms to psoriasis susceptibility.

Although the role of p21 Waf1/Cip1 gene expression is well documented in various cell culture studies, its in vivo roles are poorly understood. To gain further insight into the role of p21 Waf1/Cip1 gene expression in vivo , we attempted to visualize the dynamics of p21 Waf1/Cip1 gene expression in living animals. In this study, we established a transgenic mice line (p21- p -luc) expressing the firefly luciferase under the control of the p21 Waf1/Cip1 gene promoter. In conjunction with a noninvasive bioluminescent imaging technique, p21- p -luc mice enabled us to monitor the endogenous p21 Waf1/Cip1 gene expression in vivo . By monitoring and quantifying the p21 Waf1/Cip1 gene expression repeatedly in the same mouse throughout its entire lifespan, we were able to unveil the dynamics of p21 Waf1/Cip1 gene expression in the aging process. We also applied this system to chemically induced skin carcinogenesis and found that the levels of p21 Waf1/Cip1 gene expression rise dramatically in benign skin papillomas, suggesting that p21 Waf1/Cip1 plays a preventative role(s) in skin tumor formation. Surprisingly, moreover, we found that the level of p21 Waf1/Cip1 expression strikingly increased in the hair bulb and oscillated with a 3-week period correlating with hair follicle cycle progression. Notably, this was accompanied by the expression of p63 but not p53. This approach, together with the analysis of p21 Waf1/Cip1 knockout mice, has uncovered a novel role for the p21 Waf1/Cip1 gene in hair development. These data illustrate the unique utility of bioluminescence imaging in advancing our understanding of the timing and, hence, likely roles of specific gene expression in higher eukaryotes.

Factors that determine the spectrum of target organs involved in autoimmune destruction are poorly understood. Although loss of function of autoimmune regulator (AIRE) in thymic epithelial cells is responsible for autoimmunity, the pathogenic roles of AIRE in regulating target-organ specificity remain elusive. In order to gain insight into this issue, we have established NOD mice, an animal model of type 1 diabetes caused by autoimmune attack against beta cell islets, in which Aire has been abrogated. Remarkably, acinar cells rather than beta cell islets were the major targets of autoimmune destruction in Aire-deficient NOD mice, and this alteration of intra-pancreatic target-organ specificity was associated with production of autoantibody against pancreas-specific protein disulfide isomerase (PDIp), an antigen expressed predominantly by acinar cells. Consistent with this pathological change, the animals were resistant to the development of diabetes. The results suggest that Aire not only is critical for the control of self-tolerance but is also a strong modifier of target-organ specificity through regulation of T cell repertoire diversification. We also demonstrated that transcriptional expression of PDIp was retained in the Aire-deficient NOD thymus, further supporting the concept that Aire may regulate the survival of autoreactive T cells beyond transcriptional control of self-protein expression in the thymus.

IκB kinase (IKK) α exhibits diverse biological activities through protein kinase-dependent and -independent functions, the former mediated predominantly through a noncanonical NF-κB activation pathway. The in vivo function of IKKα, however, still remains elusive. Because a natural strain of mice with mutant NF-κB-inducing kinase (NIK) manifests autoimmunity as a result of disorganized thymic structure with abnormal expression of Rel proteins in the thymic stroma, we speculated that the NIK-IKKα axis might constitute an essential step in the thymic organogenesis that is required for the establishment of self-tolerance. An autoimmune disease phenotype was induced in athymic nude mice by grafting embryonic thymus from IKKα-deficient mice. The thymic microenvironment that caused autoimmunity in an IKKα-dependent manner was associated with defective processing of NF-κB2, resulting in the impaired development of thymic epithelial cells. Thus, our results demonstrate a novel function for IKKα in thymic organogenesis for the establishment of central tolerance that depends on its protein kinase activity in cooperation with NIK.

It has recently become clear that signals mediated by members of the TNFR superfamily, including lymphotoxin-β receptor (LTβR), receptor activator for NF-κB (RANK), and CD40, play essential roles in organizing the integrity of medullary thymic epithelial cells (mTECs) required for the establishment of self-tolerance. However, details of the mechanism responsible for the unique and cooperative action of individual and multiple TNFR superfamily members during mTEC differentiation still remain enigmatic. In this study, we show that the LTβR signal upregulates expression of RANK in the thymic stroma, thereby promoting accessibility to the RANK ligand necessary for mTEC differentiation. Cooperation between the LTβR and RANK signals for optimal mTEC differentiation was underscored by the exaggerated defect of thymic organogenesis observed in mice doubly deficient for these signals. In contrast, we observed little cooperation between the LTβR and CD40 signals. Thus, the LTβR signal exhibits a novel and unique function in promoting RANK activity for mTEC organization, indicating a link between thymic organogenesis mediated by multiple cytokine signals and the control of autoimmunity.

OBJECTIVE—Insulin binds to the α-subunit of the insulin receptor (IRα) and subsequently exerts its effects in the cells. The soluble ectodomains of several receptors have been found to circulate in the plasma. Therefore, we hypothesized that soluble human insulin receptor (hIR) ectodomain (α-subunit and a part of β-subunit) may exist in the plasma of diabetic patients. RESEARCH DESIGN AND METHODS—We identified soluble hIR ectodomain in human plasma by a two-step purification followed by immunoblotting and gel-filtration chromatography. Furthermore, we established an hIRα-specific enzyme-linked immunosorbent assay and measured the plasma IRα levels in patients with diabetes. We also investigated this phenomenon in streptozotocin-induced diabetic hIR transgenic mice. RESULTS—Soluble hIRα, but not intact hIRβ or whole hIR, exists in human plasma. The plasma IRα levels were significantly higher in type 1 (2.00 ± 0.60 ng/ml; n = 53) and type 2 (2.26 ± 0.80; n = 473) diabetic patients than in control subjects (1.59 ± 0.40 ng/ml; n = 123 (P < 0.001 vs. control). Plasma IRα level was positively correlated with blood glucose level, and 10–20% of the insulin in plasma bound to hIRα. In the in vivo experiments using diabetic hIR transgenic mice, hyperglycemia was confirmed to increase the plasma hIRα level and the half-life estimated to be ∼6 h. CONCLUSIONS—We propose that the increased soluble IR ectodomain level appears to be a more rapid glycemic marker than A1C or glycoalbumin.