Homeodomain-Interacting Protein Kinase 2, a Novel Autoimmune Regulator Interaction Partner, Modulates Promiscuous Gene Expression in Medullary Thymic Epithelial Cells

Abstract

Promiscuous expression of a plethora of tissue-restricted Ags (TRAs) by medullary thymic epithelial cells (mTECs) plays an essential role in T cell tolerance. Although the cellular mechanisms by which promiscuous gene expression (pGE) imposes T cell tolerance have been well characterized, the underlying molecular mechanisms remain poorly understood. The autoimmune regulator (AIRE) is to date the only validated molecule known to regulate pGE. AIRE is part of higher-order multiprotein complexes, which promote transcription, elongation, and splicing of a wide range of target genes. How AIRE and its partners mediate these various effects at the molecular level is still largely unclear. Using a yeast two-hybrid screen, we searched for novel AIRE-interacting proteins and identified the homeodomain-interacting protein kinase 2 (HIPK2) as a novel partner. HIPK2 partially colocalized with AIRE in nuclear bodies upon cotransfection and in human mTECs in situ. Moreover, HIPK2 phosphorylated AIRE in vitro and suppressed the coactivator activity of AIRE in a kinase-dependent manner. To evaluate the role of Hipk2 in modulating the function of AIRE in vivo, we compared whole-genome gene signatures of purified mTEC subsets from TEC-specific Hipk2 knockout mice with control mice and identified a small set of differentially expressed genes. Unexpectedly, most differentially expressed genes were confined to the CD80lo mTEC subset and preferentially included AIRE-independent TRAs. Thus, although it modulates gene expression in mTECs and in addition affects the size of the medullary compartment, TEC-specific HIPK2 deletion only mildly affects AIRE-directed pGE in vivo.

AIRE expands: new roles in immune tolerance and beyond

“…More recent reports have suggested additional partners and/or functions as well as provided confirmation of earlier findings (Figure 3A). For instance, interacting proteins have been described that help Aire to recognize and target TSA genes 25–30 , release stalled RNA polymerase to allow RNA elongation 24, 31–33 , and regulate Aire itself 34–38 . Additionally, inroads have been made into mapping the regions of Aire that bind to interacting partners 25, 38, 39 (Figure 3B).…”

“…Interestingly, Aire interacts with a number of proteins that exert control over Aire’s function through post-translational modifications 34–38 . First, Aire associates with deacetylase and acetyltransferase proteins that act directly upon Aire 34–36 .…”

“…In addition to factors that modulate acetylation, proteins that mediate phosphorylation are also associated with Aire 37, 38 . Aire interacts with DNA dependent protein kinase (DNA PK), which in expressed in mTECs and phosphorylates Aire in vitro 37 .…”

Promiscuous Gene Expression in the Thymus: A Matter of Epigenetics, miRNA, and More?

“…Recent studies documented a role for epigenetic and post-transcriptional mechanisms in regulating pGE in general and the function of Aire in particular (14, 17–19). Indeed, both APECED patients (8) and mouse mutants (20) display variability in the disease severity and the organs affected depending on different genetic backgrounds, indicating that other genetic or epigenetic components define the exact course of the individual disease.…”

Induction de la tolérance centrale dans le thymus par le facteur de transcription Aire

“…Aire interagit également avec le coactivateur transcriptionnel CBP (CREB-binding protein) qui acétyle les histones H3, favorisant ainsi l'accessibilité de la chromatine à la machinerie transcriptionnelle [32]. De manière intéres-sante, un nouveau partenaire de Aire, la protéine kinase HIPK2 (homeodomain interacting protein kinase 2) a été récemment identifiée [33]. Cette kinase régule la fonction de Aire en réprimant par phosphorylation son rôle de coactivateur de la transcription ( Figure 2C).…”