Rapamycin-Induced Translocation of Meiotic Nuclear Proteins in Saccharomyces cerevisiae.

Conditional depletion of proteins is a potential strategy to elucidate protein function, especially in complex cellular processes like meiosis. Several methods are available to effectively deplete a protein in a conditional manner. Conditional loss of a protein function can be achieved by depleting it from its region of action by degrading it. A conditional loss of protein function can also be achieved by sequestering it to a functionally unavailable compartment inside the cell. This chapter describes anchor away, a conditional depletion tool that can deplete a protein both temporally and spatially by translocation. It utilizes the affinity of FRB to bind FKBP12 in the presence of rapamycin for a quick and efficient translocation of the protein to a designated location. Anchor away is a reliable tool for the study of meiotic proteins, as only small quantities of rapamycin are required to efficiently and rapidly translocate the protein of interest without compromising meiotic progression.

Alkylamine-tethered molecules recruit FBXO22 for targeted protein degradation.

Targeted protein degradation (TPD) relies on small molecules to recruit proteins to E3 ligases to induce their ubiquitylation and degradation by the proteasome. Only a few of the approximately 600 human E3 ligases are currently amenable to this strategy. This limits the actionable target space and clinical opportunities and thus establishes the necessity to expand to additional ligases. Here we identify and characterize SP3N, a specific degrader of the prolyl isomerase FKBP12. SP3N features a minimal design, where a known FKBP12 ligand is appended with a flexible alkylamine tail that conveys degradation properties. We found that SP3N is a precursor and that the alkylamine is metabolized to an active aldehyde species that recruits the SCFFBXO22 ligase for FKBP12 degradation. Target engagement occurs via covalent adduction of Cys326 in the FBXO22 C-terminal domain, which is critical for ternary complex formation, ubiquitylation and degradation. This mechanism is conserved for two recently reported alkylamine-based degraders of NSD2 and XIAP, thus establishing alkylamine tethering and covalent hijacking of FBXO22 as a generalizable TPD strategy.

HMGA1 sensitizes esophageal squamous cell carcinoma to mTOR inhibitors through the ETS1-FKBP12 axis.

Esophageal carcinoma is amongst the prevalent malignancies worldwide, characterized by unclear molecular classifications and varying clinical outcomes. The PI3K/AKT/mTOR signaling, one of the frequently perturbed dysregulated pathways in human malignancies, has instigated the development of various inhibitory agents targeting this pathway, but many ESCC patients exhibit intrinsic or adaptive resistance to these inhibitors. Here, we aim to explore the reasons for the insensitivity of ESCC patients to mTOR inhibitors. We assessed the sensitivity to rapamycin in various ESCC cell lines by determining their respective IC50 values and found that cells with a low level of HMGA1 were more tolerant to rapamycin. Subsequent experiments have supported this finding. Through a transcriptome sequencing, we identified a crucial downstream effector of HMGA1, FKBP12, and found that FKBP12 was necessary for HMGA1-induced cell sensitivity to rapamycin. HMGA1 interacted with ETS1, and facilitated the transcription of FKBP12. Finally, we validated this regulatory axis in in vivo experiments, where HMGA1 deficiency in transplanted tumors rendered them resistance to rapamycin. Therefore, we speculate that mTOR inhibitor therapy for individuals exhibiting a reduced level of HMGA1 or FKBP12 may not work. Conversely, individuals exhibiting an elevated level of HMGA1 or FKBP12 are more suitable candidates for mTOR inhibitor treatment.

A functional interplay between the two BMP-SMAD pathway inhibitors TMPRSS6 and FKBP12 regulates hepcidin expression in vivo.

The Activin A Receptor type I (ALK2) is a critical component of BMP-SMAD signaling that, in the presence of ligands, phosphorylates cytosolic SMAD1/5/8 and modulates important biological processes, including bone formation and iron metabolism. In hepatocytes, the BMP-SMAD pathway controls the expression of hepcidin, the liver peptide hormone that regulates body iron homeostasis via the BMP receptors ALK2 and ALK3, and the hemochromatosis proteins. The main negative regulator of the pathway in the liver is transmembrane serine protease 6 (TMPRSS6), which downregulates hepcidin by cleaving the BMP coreceptor hemojuvelin. ALK2 function is inhibited also by the immunophilin FKBP12, which maintains the receptor in an inactive conformation. FKBP12 sequestration by tacrolimus or its silencing upregulates hepcidin in primary hepatocytes and in vivo in acute but not chronic settings. Interestingly, gain-of-function mutations in ALK2 that impair FKBP12 binding to the receptor and activate the pathway cause a bone phenotype in patients affected by Fibrodysplasia Ossificans Progressiva but not hepcidin and iron metabolism dysfunction. This observation suggests that additional mechanisms are active in the liver to compensate for the increased BMP-SMAD signaling. Here we demonstrate that Fkbp12 downregulation in hepatocytes by antisense oligonucleotide treatment upregulates the expression of the main hepcidin inhibitor Tmprss6, thus counteracting the ALK2-mediated activation of the pathway. Combined downregulation of both Fkbp12 and Tmprss6 blocks this compensatory mechanism. Our findings reveal a previously unrecognized functional cross talk between FKBP12 and TMPRSS6, the main BMP-SMAD pathway inhibitors, in the control of hepcidin transcription.NEW & NOTEWORTHY This study uncovers a previously unrecognized mechanism of hepcidin and BMP-SMAD pathway regulation in hepatocytes mediated by the immunophilin FKBP12 and the transmembrane serine protease TMPRSS6.

A CUG codon-adapted anchor-away toolkit for functional analysis of genes in Candida albicans.

Promoter shutoff of essential genes in the diploid Candida albicans has often been insufficient to create tight, conditional null alleles due to leaky expression and has been a stumbling block in pathogenesis research. Moreover, homozygous deletion of non-essential genes has often been problematic due to the frequent aneuploidy in the mutant strains. Rapid, conditional depletion of essential genes by the anchor-away strategy has been successfully employed in Saccharomyces cerevisiae and other model organisms. Here, rapamycin mediates the dimerization of human FK506-binding protein (FKBP12) and FKBP12-rapamycin-binding (FRB) domain-containing target protein, resulting in relocalization to altered sub-cellular locations. In this work, we used the ribosomal protein Rpl13 as the anchor and took two nuclear proteins as targets to construct a set of mutants in a proof-of-principle approach. We first constructed a rapamycin-resistant C. albicans strain by introducing a dominant mutation in the CaTOR1 gene and a homozygous deletion of RBP1, the ortholog of FKBP12, a primary target of rapamycin. The FKBP12 and the FRB coding sequences were then CUG codon-adapted for C. albicans by site-directed mutagenesis. Anchor-away strains expressing the essential TBP1 gene or the non-essential SPT8 gene as FRB fusions were constructed. We found that rapamycin caused rapid cessation of growth of the TBP-AA strain within 15 minutes and the SPT8-AA strain phenocopied the constitutive filamentous phenotype of the spt8Δ/spt8Δ mutant. Thus, the anchor-away toolbox for C. albicans developed here can be employed for genome-wide analysis to identify gene function in a rapid and reliable manner, further accelerating anti-fungal drug development in C. albicans. IMPORTANCE: Molecular genetic studies thus far have identified ~27% open-reading frames as being essential for the vegetative growth of Candida albicans in rich medium out of a total 6,198 haploid set of open reading frames. However, a major limitation has been to construct rapid conditional alleles of essential C. albicans genes with near quantitative depletion of encoded proteins. Here, we have developed a toolbox for rapid and conditional depletion of genes that would aid studies of gene function of both essential and non-essential genes.

Generation of Bioactivity-Tailored FK506/FK520 Analogs by CRISPR Editing in Streptomyces tsukubaensis.

For a potential application of FK506 in the treatment of acute kidney failure only the FKBP12 binding capability of the compound is required, while the immunosuppressive activity via calcineurin binding is considered as a likely risk to the patients. The methoxy groups at C13 and C15 are thought to have significant influence on the immunosuppressive activity of the molecule. Consequently, FK506 analogs with different functionalities at C13 and C15 were generated by targeted CRISPR editing of the AT domains in module 7 and 8 of the biosynthetic assembly line in Streptomyces tsukubaensis. In addition, the corresponding FK520 (C21 ethyl derivative of FK506) analogs could be obtained by media adjustments. The compounds were tested for their bioactivity in regards to FKBP12 binding, BMP potentiation and calcineurin sparing. 15-desmethoxy FK506 was superior to the other tested analogs as it did not inhibit calcineurin but retained high potency towards FKBP12 binding and BMP potentiation.

SPI1 activates TGF-ß1/PI3K/Akt signaling through transcriptional upregulation of FKBP12 to support the mesenchymal phenotype of glioma stem cells.

Glioma stem cells (GSCs) exhibit diverse molecular subtypes with the mesenchymal (MES) population representing the most malignant variant. The oncogenic potential of Salmonella pathogenicity island 1 (SPI1), an oncogenic transcription factor, has been established across various human malignancies. In this study, we explored the association between the SPI1 pathway and the MES GSC phenotype. Through comprehensive analysis of the Cancer Genome Atlas and Chinese Glioma Genome Atlas glioma databases, along with patient-derived GSC cultures, we analyzed SPI1 expression. Using genetic knockdown and overexpression techniques, we assessed the functional impact of SPI1 on GSC MES marker expression, invasion, proliferation, self-renewal, and sensitivity to radiation in vitro, as well as its influence on tumor formation in vivo. Additionally, we investigated the downstream signaling cascades activated by SPI1. Our findings revealed a positive correlation between elevated SPI1 expression and the MES phenotype, which in turn, correlated with poor survival. SPI1 enhanced GSC MES differentiation, self-renewal, and radioresistance in vitro, promoting tumorigenicity in vivo. Mechanistically, SPI1 augmented the transcriptional activity of both TGF-ß1 and FKBP12 while activating the non-canonical PI3K/Akt pathway. Notably, inhibition of TGF-ß1/PI3K/Akt signaling partially attenuated SPI1-induced GSC MES differentiation and its associated malignant phenotype. Collectively, our results underscore SPI1's role in activating TGF-ß1/PI3K/Akt signaling through transcriptional upregulation of FKBP12, thereby supporting the aggressive MES phenotype of GSCs. Therefore, SPI1 emerges as a potential therapeutic target in glioma treatment.

Propagation of conformational instability in FK506-binding protein FKBP12.

FKBP12 is the archetype of the FK506 binding domains that define the family of FKBP proteins which participate in the regulation of various distinct physiological signaling processes. As the drugs FK506 and rapamycin inhibit many of these FKBP proteins, there is need to develop therapeutics which exhibit selectivity within this family. The long ß4-ß5 loop of the FKBP domain is known to regulate transcriptional activity for the steroid hormone receptors and appears to participate in regulating calcium channel activity for the cardiac and skeletal muscle ryanodine receptors. The ß4-ß5 loop of FKBP12 has been shown to undergo extensive conformational dynamics, and here we report hydrogen exchange measurements for a series of mutational variants in that loop which indicate deviations from a two-state kinetics for those dynamics. In addition to a previously characterized local transition near the tip of this loop, evidence is presented for a second site of conformational dynamics in the stem of this loop. These mutation-dependent hydrogen exchange effects extend beyond the ß4-ß5 loop, primarily by disrupting the hydrogen bond between the Gly 58 amide and the Tyr 80 carbonyl oxygen which links the two halves of the structural rim that surrounds the active site cleft. Mutationally-induced opening of the cleft between Gly 58 and Tyr 80 not only modulates the global stability of the protein, it promotes a conformational transition in the distant ß2-ß3a hairpin that modulates the binding affinity for a FKBP51-selective inhibitor previously designed to exploit a localized conformational transition at the homologous site.

FK506 bypasses the effect of erythroferrone in cancer cachexia skeletal muscle atrophy.

Skeletal muscle atrophy is a hallmark of cachexia, a wasting condition typical of chronic pathologies, that still represents an unmet medical need. Bone morphogenetic protein (BMP)-Smad1/5/8 signaling alterations are emerging drivers of muscle catabolism, hence, characterizing these perturbations is pivotal to develop therapeutic approaches. We identified two promoters of "BMP resistance" in cancer cachexia, specifically the BMP scavenger erythroferrone (ERFE) and the intracellular inhibitor FKBP12. ERFE is upregulated in cachectic cancer patients' muscle biopsies and in murine cachexia models, where its expression is driven by STAT3. Moreover, the knock down of Erfe or Fkbp12 reduces muscle wasting in cachectic mice. To bypass the BMP resistance mediated by ERFE and release the brake on the signaling, we targeted FKBP12 with low-dose FK506. FK506 restores BMP-Smad1/5/8 signaling, rescuing myotube atrophy by inducing protein synthesis. In cachectic tumor-bearing mice, FK506 prevents muscle and body weight loss and protects from neuromuscular junction alteration, suggesting therapeutic potential for targeting the ERFE-FKBP12 axis.

FKBP12 inhibits hepcidin expression by modulating BMP receptors interaction and ligand responsiveness in hepatocytes.

The expression of the iron regulatory hormone hepcidin in hepatocytes is regulated by the BMP-SMAD pathway through the type I receptors ALK2 and ALK3, the type II receptors ACVR2A and BMPR2, and the ligands BMP2 and BMP6. We previously identified the immunophilin FKBP12 as a new hepcidin inhibitor that acts by blocking ALK2. Both the physiologic ALK2 ligand BMP6 and the immunosuppressive drug Tacrolimus (TAC) displace FKBP12 from ALK2 and activate the signaling. However, the molecular mechanism whereby FKBP12 regulates BMP-SMAD pathway activity and thus hepcidin expression remains unclear. Here, we show that FKBP12 acts by modulating BMP receptor interactions and ligand responsiveness. We first demonstrate that in primary murine hepatocytes TAC regulates hepcidin expression exclusively via FKBP12. Downregulation of the BMP receptors reveals that ALK2, to a lesser extent ALK3, and ACVR2A are required for hepcidin upregulation in response to both BMP6 and TAC. Mechanistically, TAC and BMP6 increase ALK2 homo-oligomerization and ALK2-ALK3 hetero-oligomerization and the interaction between ALK2 and the type II receptors. By acting on the same receptors, TAC and BMP6 cooperate in BMP pathway activation and hepcidin expression both in vitro and in vivo. Interestingly, the activation state of ALK3 modulates its interaction with FKBP12, which may explain the cell-specific activity of FKBP12. Overall, our results identify the mechanism whereby FKBP12 regulates the BMP-SMAD pathway and hepcidin expression in hepatocytes, and suggest that FKBP12-ALK2 interaction is a potential pharmacologic target in disorders caused by defective BMP-SMAD signaling and characterized by low hepcidin and high BMP6 expression.

The blue light receptor CRY1 interacts with FIP37 to promote N6 -methyladenosine RNA modification and photomorphogenesis in Arabidopsis.

Light is a particularly important environmental cue that regulates a variety of diverse plant developmental processes, such as photomorphogenesis. Blue light promotes photomorphogenesis mainly through the activation of the photoreceptor cryptochrome 1 (CRY1). However, the mechanism underlying the CRY1-mediated regulation of growth is not fully understood. Here, we found that blue light induced N6 -methyladenosine (m6 A) RNA modification during photomorphogenesis partially via CRY1. Cryptochrome 1 mediates blue light-induced expression of FKBP12-interacting protein 37 (FIP37), which is a component of m6 A writer. Moreover, we showed that CRY1 physically interacted with FIP37 in vitro and in vivo, and mediated blue light activation of FIP37 binding to RNA. Furthermore, CRY1 and FIP37 modulated m6 A on photomorphogenesis-related genes PIF3, PIF4, and PIF5, thereby accelerating the decay of their transcripts. Genetically, FIP37 repressed hypocotyl elongation under blue light, and fip37 mutation could partially rescue the short-hypocotyl phenotype of CRY1-overexpressing plants. Together, our results provide a new insight into CRY1 signal in modulating m6 A methylation and stability of PIFs, and establish an essential molecular link between m6 A modification and determination of photomorphogenesis in plants.

Tissue-restricted inhibition of mTOR using chemical genetics.

Mammalian target of rapamycin (mTOR) is a highly conserved eukaryotic protein kinase that coordinates cell growth and metabolism, and plays a critical role in cancer, immunity, and aging. It remains unclear how mTOR signaling in individual tissues contributes to whole-organism processes because mTOR inhibitors, like the natural product rapamycin, are administered systemically and target multiple tissues simultaneously. We developed a chemical-genetic system, termed selecTOR, that restricts the activity of a rapamycin analog to specific cell populations through targeted expression of a mutant FKBP12 protein. This analog has reduced affinity for its obligate binding partner FKBP12, which reduces its ability to inhibit mTOR in wild-type cells and tissues. Expression of the mutant FKBP12, which contains an expanded binding pocket, rescues the activity of this rapamycin analog. Using this system, we show that selective mTOR inhibition can be achieved in Saccharomyces cerevisiae and human cells, and we validate the utility of our system in an intact metazoan model organism by identifying the tissues responsible for a rapamycin-induced developmental delay in Drosophila.

Development of NanoLuc-targeting protein degraders and a universal reporter system to benchmark tag-targeted degradation platforms.

Modulation of protein abundance using tag-Targeted Protein Degrader (tTPD) systems targeting FKBP12F36V (dTAGs) or HaloTag7 (HaloPROTACs) are powerful approaches for preclinical target validation. Interchanging tags and tag-targeting degraders is important to achieve efficient substrate degradation, yet limited degrader/tag pairs are available and side-by-side comparisons have not been performed. To expand the tTPD repertoire we developed catalytic NanoLuc-targeting PROTACs (NanoTACs) to hijack the CRL4CRBN complex and degrade NanoLuc tagged substrates, enabling rapid luminescence-based degradation screening. To benchmark NanoTACs against existing tTPD systems we use an interchangeable reporter system to comparatively test optimal degrader/tag pairs. Overall, we find the dTAG system exhibits superior degradation. To align tag-induced degradation with physiology we demonstrate that NanoTACs limit MLKL-driven necroptosis. In this work we extend the tTPD platform to include NanoTACs adding flexibility to tTPD studies, and benchmark each tTPD system to highlight the importance of comparing each system against each substrate.

Leveraging Fungal and Human Calcineurin-Inhibitor Structures, Biophysical Data, and Dynamics To Design Selective and Nonimmunosuppressive FK506 Analogs.

Calcineurin is a critical enzyme in fungal pathogenesis and antifungal drug tolerance and, therefore, an attractive antifungal target. Current clinically accessible calcineurin inhibitors, such as FK506, are immunosuppressive to humans, so exploiting calcineurin inhibition as an antifungal strategy necessitates fungal specificity in order to avoid inhibiting the human pathway. Harnessing fungal calcineurin-inhibitor crystal structures, we recently developed a less immunosuppressive FK506 analog, APX879, with broad-spectrum antifungal activity and demonstrable efficacy in a murine model of invasive fungal infection. Our overarching goal is to better understand, at a molecular level, the interaction determinants of the human and fungal FK506-binding proteins (FKBP12) required for calcineurin inhibition in order to guide the design of fungus-selective, nonimmunosuppressive FK506 analogs. To this end, we characterized high-resolution structures of the Mucor circinelloides FKBP12 bound to FK506 and of the Aspergillus fumigatus, M. circinelloides, and human FKBP12 proteins bound to the FK506 analog APX879, which exhibits enhanced selectivity for fungal pathogens. Combining structural, genetic, and biophysical methodologies with molecular dynamics simulations, we identify critical variations in these structurally similar FKBP12-ligand complexes. The work presented here, aimed at the rational design of more effective calcineurin inhibitors, indeed suggests that modifications to the APX879 scaffold centered around the C15, C16, C18, C36, and C37 positions provide the potential to significantly enhance fungal selectivity. IMPORTANCE Invasive fungal infections are a leading cause of death in the immunocompromised patient population. The rise in drug resistance to current antifungals highlights the urgent need to develop more efficacious and highly selective agents. Numerous investigations of major fungal pathogens have confirmed the critical role of the calcineurin pathway for fungal virulence, making it an attractive target for antifungal development. Although FK506 inhibits calcineurin, it is immunosuppressive in humans and cannot be used as an antifungal. By combining structural, genetic, biophysical, and in silico methodologies, we pinpoint regions of the FK506 scaffold and a less immunosuppressive analog, APX879, centered around the C15 to C18 and C36 to C37 positions that could be altered with selective extensions and/or deletions to enhance fungal selectivity. This work represents a significant advancement toward realizing calcineurin as a viable target for antifungal drug discovery.

Physiological function of FKBP12, a primary target of rapamycin/FK506: a newly identified role in transcription of ribosomal protein genes in yeast.

In this review, we have summarized the information from a study on FKBP12 (FK506 binding protein 12 kDa) with a view to understand its drug-free, physiological roles in transcription of ribosomal protein gene in Saccharomyces cerevisiae. FKBP12 with peptidyl-prolylisomerase (PPIase) activity is widely conserved among many eukaryotes. FKBP12 is a primary target for the two structurally related drugs, FK506 and rapamycin. FKBP12 bound with FK506 or rapamycin inhibits calcineurin and target of rapamycin complex 1 (TORC1), respectively. The molecular mechanisms of the effect of FKBP12 in the presence of these drugs have been elucidated. Conversely, the physiological role of FKBP12 has been unclear, especially in yeast. Our study revealed that the deletion of FPR1 (FK506-sensitive prolinerotamase 1 gene), which encodes yeast FKBP12, induced severe growth defect synthetically with deletion of HMO1 (high mobility group family 1). HMO1 encodes an HMGB family protein involved in transcription of ribosomal component genes. Fpr1 was shown to bind specifically to the promoters of ribosomal protein genes (RPGs) dependent on Rap1 (repressor/activator binding protein 1). Importantly, Fpr1 and Hmo1 promote the binding of Fhl1/Ifh1 (forkhead-like 1/interacts with forkhead 1), key regulators of RPG transcription, to certain RPG promoters independently and/or cooperatively with each other. Taken together, we conclude that Fpr1 physiologically functions as transcription factor of RPGs in S. cerevisiae. To our knowledge, this is the first study to demonstrate that FKBP12 participates in ribosome synthesis independently of drugs, and it may also provide a clue to the unidentified function of other PPIase proteins.

A Split Methyl Halide Transferase AND Gate That Reports by Synthesizing an Indicator Gas.

Monitoring microbial reactions in highly opaque or autofluorescent environments like soils, seawater, and wastewater remains challenging. To develop a simple approach for observing post-translational reactions within microbes situated in environmental matrices, we designed a methyl halide transferase (MHT) fragment complementation assay that reports by synthesizing an indicator gas. We show that backbone fission within regions of high sequence variability in the Rossmann domain yields split MHT (sMHT) AND gates whose fragments cooperatively associate to synthesize CH3Br. Additionally, we identify a sMHT whose fragments require fusion to pairs of interacting partner proteins for maximal activity. We also show that sMHT fragments fused to FKBP12 and the FKBP-rapamycin binding domain of mTOR display significantly enhanced CH3Br production in the presence of rapamycin. This gas production is reversed in the presence of the competitive inhibitor of FKBP12/FKPB dimerization, indicating that sMHT is a reversible reporter of post-translational reactions. This sMHT represents the first genetic AND gate that reports on protein-protein interactions via an indicator gas. Because indicator gases can be measured in the headspaces of complex environmental samples, this assay should be useful for monitoring the dynamics of diverse molecular interactions within microbes situated in hard-to-image marine and terrestrial matrices.

Rapid and direct control of target protein levels with VHL-recruiting dTAG molecules.

Chemical biology strategies for directly perturbing protein homeostasis including the degradation tag (dTAG) system provide temporal advantages over genetic approaches and improved selectivity over small molecule inhibitors. We describe dTAGV-1, an exclusively selective VHL-recruiting dTAG molecule, to rapidly degrade FKBP12F36V-tagged proteins. dTAGV-1 overcomes a limitation of previously reported CRBN-recruiting dTAG molecules to degrade recalcitrant oncogenes, supports combination degrader studies and facilitates investigations of protein function in cells and mice.

Expanding the Chemogenetic Toolbox by Circular Permutation.

To expand the repertoire of chemogenetic tools tailored for molecular and cellular engineering, we describe herein the design of cpRAPID as a circularly permuted rapamycin-inducible dimerization system composed of the canonical FK506-binding protein (FKBP) and circular permutants of FKBP12-rapamycin binding domain (cpFRB). By permuting the topology of the four helices within FRB, we have created cpFRB-FKBP pairs that respond to ligand with varying activation kinetics and dynamics. The cpRAPID system enables chemical-controllable subcellular redistribution of proteins, as well as inducible transcriptional activation when coupled with the CRISPR activation (CRISPRa) technology to induce a GFP reporter and endogenous gene expression. We have further demonstrated the use of cpRAPID to generate chemically switchable split nanobody (designated Chessbody) for ligand-gated antigen recognition in living cells. Collectively, the circular permutation approach offers a powerful means for diversifying the chemogenetics toolbox to benefit the burgeoning synthetic biology field.

FKBP12 dimerization mutations effect FK506 binding and differentially alter calcineurin inhibition in the human pathogen Aspergillus fumigatus.

The 12-kDa FK506-binding protein (FKBP12) is the target of the commonly used immunosuppressive drug FK506. The FKBP12-FK506 complex binds to calcineurin and inhibits its activity, leading to immunosuppression and preventing organ transplant rejection. Our recent characterization of crystal structures of FKBP12 proteins in pathogenic fungi revealed the involvement of the 80's loop residue (Pro90) in the active site pocket in self-substrate interaction providing novel evidence on FKBP12 dimerization in vivo. The 40's loop residues have also been shown to be involved in reversible dimerization of FKBP12 in the mammalian and yeast systems. To understand how FKBP12 dimerization affects FK506 binding and influences calcineurin function, we generated Aspergillus fumigatus FKBP12 mutations in the 40's and 50's loop (F37 M/L; W60V). Interestingly, the mutants exhibited variable FK506 susceptibility in vivo indicating differing dimer strengths. In comparison to the 80's loop P90G and V91C mutants, the F37 M/L and W60V mutants exhibited greater FK506 resistance, with the F37M mutation showing complete loss in calcineurin binding in vivo. Molecular dynamics and pulling simulations for each dimeric FKBP12 protein revealed a two-fold increase in dimer strength and significantly higher number of contacts for the F37M, F37L, and W60V mutations, further confirming their varying degree of impact on FK506 binding and calcineurin inhibition in vivo.

Tacrolimus-induced hypomagnesemia and hypercalciuria requires FKBP12 suggesting a role for calcineurin.

Calcineurin inhibitors (CNIs) are immunosuppressive drugs used to prevent graft rejection after organ transplant. Common side effects include renal magnesium wasting and hypomagnesemia, which may contribute to new-onset diabetes mellitus, and hypercalciuria, which may contribute to post-transplant osteoporosis. Previous work suggested that CNIs reduce the abundance of key divalent cation transport proteins, expressed along the distal convoluted tubule, causing renal magnesium and calcium wasting. It has not been clear, however, whether these effects are specific for the distal convoluted tubule, and whether these represent off-target toxic drug effects, or result from inhibition of calcineurin. The CNI tacrolimus can inhibit calcineurin only when it binds with the immunophilin, FKBP12; we previously generated mice in which FKBP12 could be deleted along the nephron, to test whether calcineurin inhibition is involved, these mice are normal at baseline. Here, we confirmed that tacrolimus-treated control mice developed hypomagnesemia and urinary calcium wasting, with decreased protein and mRNA abundance of key magnesium and calcium transport proteins (NCX-1 and Calbindin-D28k ). However, qPCR also showed decreased mRNA expression of NCX-1 and Calbindin-D28k , and TRPM6. In contrast, KS-FKBP12-/- mice treated with tacrolimus were completely protected from these effects. These results indicate that tacrolimus affects calcium and magnesium transport along the distal convoluted tubule and strongly suggests that inhibition of the phosphatase, calcineurin, is directly involved.