Tumor resistance to anti-mesothelin CAR-T cells caused by binding to shed mesothelin is overcome by targeting a juxtamembrane epitope

Despite many clinical trials, CAR-T cells are not yet approved for human solid tumor therapy. One popular target is mesothelin (MSLN) which is highly expressed on the surface of about 30% of cancers including mesothelioma and cancers of the ovary, pancreas, and lung. MSLN is shed by proteases that cleave near the C terminus, leaving a short peptide attached to the cell. Most anti-MSLN antibodies bind to shed MSLN, which can prevent their binding to target cells. To overcome this limitation, we developed an antibody (15B6) that binds next to the membrane at the protease-sensitive region, does not bind to shed MSLN, and makes CAR-T cells that have much higher anti-tumor activity than a CAR-T that binds to shed MSLN. We have now humanized the Fv (h15B6), so the CAR-T can be used to treat patients and show that h15B6 CAR-T produces complete regressions in a hard-to-treat pancreatic cancer patient derived xenograft model, whereas CAR-T targeting a shed epitope (SS1) have no anti-tumor activity. In these pancreatic cancers, the h15B6 CAR-T replicates and replaces the cancer cells, whereas there are no CAR-T cells in the tumors receiving SS1 CAR-T. To determine the mechanism accounting for high activity, we used an OVCAR-8 intraperitoneal model to show that poorly active SS1-CAR-T cells are bound to shed MSLN, whereas highly active h15B6 CAR-T do not contain bound MSLN enabling them to bind to and kill cancer cells.

A novel retinoic acid receptor-γ agonist antagonizes immune checkpoint resistance in lung cancers by altering the tumor immune microenvironment

All-trans-retinoic acid (ATRA), the retinoic acid receptors (RARs) agonist, regulates cell growth, differentiation, immunity, and survival. We report that ATRA-treatment repressed cancer growth in syngeneic immunocompetent, but not immunodeficient mice. The tumor microenvironment was implicated: CD8+ T cell depletion antagonized ATRA's anti-tumorigenic effects in syngeneic mice. ATRA-treatment with checkpoint blockade did not cooperatively inhibit murine lung cancer growth. To augment ATRA's anti-tumorigenicity without promoting its pro-tumorigenic potential, an RARγ agonist (IRX4647) was used since it regulates T cell biology. Treating with IRX4647 in combination with an immune checkpoint (anti-PD-L1) inhibitor resulted in a statistically significant suppression of syngeneic 344SQ lung cancers in mice-a model known for its resistance to checkpoints and characterized by low basal T cell and PD-L1 expression. This combined treatment notably elevated CD4+ T-cell presence within the tumor microenvironment and increased IL-5 and IL-13 tumor levels, while simultaneously decreasing CD38 in the tumor stroma. IL-5 and/or IL-13 treatments increased CD4+ more than CD8+ T-cells in mice. IRX4647-treatment did not appreciably affect in vitro lung cancer growth, despite RARγ expression. Pharmacokinetic analysis found IRX4647 plasma half-life was 6 h in mice. Yet, RARα antagonist (IRX6696)-treatment with anti-PD-L1 did not repress syngeneic lung cancer growth. Together, these findings provide a rationale for a clinical trial investigating an RARγ agonist to augment check point blockade response in cancers.

Hybrid quantum systems with high-T[Formula: see text] superconducting resonators

Superconducting microwave resonators are crucial elements of microwave circuits, offering a wide range of potential applications in modern science and technology. While conventional low-T[Formula: see text] superconductors are mainly employed, high-T[Formula: see text] cuprates could offer enhanced temperature and magnetic field operating ranges. Here, we report the realization of [Formula: see text] superconducting coplanar waveguide resonators, and demonstrate a continuous evolution from a lossy undercoupled regime, to a lossless overcoupled regime by adjusting the device geometry, in good agreement with circuit model theory. A high-quality factor resonator was then used to perform electron spin resonance measurements on a molecular spin ensemble across a temperature range spanning two decades. We observe spin-cavity hybridization indicating coherent coupling between the microwave field and the spins in a highly cooperative regime. The temperature dependence of the Rabi splitting and the spin relaxation time point toward an antiferromagnetic coupling of the spins below 2 K. Our findings indicate that high-Tc superconducting resonators hold great promise for the development of functional circuits. Additionally, they suggest novel approaches for achieving hybrid quantum systems based on high-T[Formula: see text] superconductors and for conducting electron spin resonance measurements over a wide range of magnetic fields and temperatures.

Abstract C058: Remodeling the tumor microenvironment by targeting integrin alpha V beta 3 (αvβ3) expressing cells in pancreatic cancer

ProAgio is a therapeutic cytotoxin that targets and kills integrin αvβ3-expressing cells, resulting in reduced tumor collagen, decreased formation of new blood vessels in the tumor microenvironment (TME), and prolonged survival in mouse models of pancreatic ductal adenocarcinoma (PDAC). ProAgio is currently being tested in a Phase 1 dose-finding study in patients with solid tumors including pancreas cancers. The cell subsets within the PDAC TME that express integrin αvβ3 and could be subject to ProAgio-mediated killing have not been previously studied. Using the KPC genetically engineered mouse model of PDAC, we found that ProAgio (20 mg/kg daily, x7 days) restrained tumor growth by 20% compared to vehicle treatment without changing the cancer cell proliferation rate. We subsequently re-analyzed publicly available scRNA-seq PDAC datasets to identify cells present in human and mouse PDAC tumors that co-express integrins αv and β3 and could be targets of ProAgio. We found that specific subsets of cancer-associated fibroblasts (CAFs), monocytes and macrophages expressed both integrins. Flow cytometry was performed on cells dissociated from orthotopically implanted KPC-derived tumors treated for 15 days with ProAgio or vehicle to identify TME subsets affected by ProAgio treatment. There was no significant difference in the myofibroblast (myCAF) subset, but an increase in the percentage of inflammatory CAFs (iCAFs) was seen. ProAgio treatment also significantly increased the percentage of conventional dendritic cells which play a pivotal role in antigen recognition and T cell priming. T cell abundance was unchanged, but a decrease in B cells was observed. These data demonstrate that ProAgio treatment modifies specific immune and fibroblast subsets within the PDAC TME. Ongoing studies seek to further delineate the cell subtypes affected by ProAgio and the cytokine and chemokine mediators responsible for these changes.

Citation Format: Mayrel Palestino Dominguez, Philip Homan, Xianyu Zhang, Sandra Navas Reyes, Theresa Guerin, Laura Bassel, Zhi-ren Liu, Serguei Kozlov, Christine Alewine. Remodeling the tumor microenvironment by targeting integrin alpha V beta 3 (αvβ3) expressing cells in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr C058.

Nesprin-1 LINC complexes recruit microtubule cytoskeleton proteins and drive pathology in Lmna-mutant striated muscle

Mutations in LMNA, the gene encoding A-type lamins, cause laminopathies-diseases of striated muscle and other tissues. The aetiology of laminopathies has been attributed to perturbation of chromatin organization or structural weakening of the nuclear envelope (NE) such that the nucleus becomes more prone to mechanical damage. The latter model requires a conduit for force transmission to the nucleus. NE-associated Linker of Nucleoskeleton and Cytoskeleton (LINC) complexes are one such pathway. Using clustered regularly interspaced short palindromic repeats to disrupt the Nesprin-1 KASH (Klarsicht, ANC-1, Syne Homology) domain, we identified this LINC complex protein as the predominant NE anchor for microtubule cytoskeleton components, including nucleation activities and motor complexes, in mouse cardiomyocytes. Loss of Nesprin-1 LINC complexes resulted in loss of microtubule cytoskeleton proteins at the nucleus and changes in nuclear morphology and positioning in striated muscle cells, but with no overt physiological defects. Disrupting the KASH domain of Nesprin-1 suppresses Lmna-linked cardiac pathology, likely by reducing microtubule cytoskeleton activities at the nucleus. Nesprin-1 LINC complexes thus represent a potential therapeutic target for striated muscle laminopathies.

T Cell-Mediated Antitumor Immunity Cooperatively Induced By TGFβR1 Antagonism and Gemcitabine Counteracts Reformation of the Stromal Barrier in Pancreatic Cancer

The desmoplastic stroma of pancreatic cancers forms a physical barrier that impedes intratumoral drug delivery. Attempts to modulate the desmoplastic stroma to increase delivery of administered chemotherapy have not shown positive clinical results thus far, and preclinical reports in which chemotherapeutic drugs were coadministered with antistromal therapies did not universally demonstrate increased genotoxicity despite increased intratumoral drug levels. In this study, we tested whether TGFβ antagonism can break the stromal barrier, enhance perfusion and tumoral drug delivery, and interrogated cellular and molecular mechanisms by which the tumor prevents synergism with coadministered gemcitabine. TGFβ inhibition in genetically engineered murine models (GEMM) of pancreas cancer enhanced tumoral perfusion and increased intratumoral gemcitabine levels. However, tumors rapidly adapted to TGFβ-dependent stromal modulation, and intratumoral perfusion returned to pre-treatment levels upon extended TGFβ inhibition. Perfusion was governed by the phenotypic identity and distribution of cancer-associated fibroblasts (CAF) with the myelofibroblastic phenotype (myCAFs), and myCAFs which harbored unique genomic signatures rapidly escaped the restricting effects of TGFβ inhibition. Despite the reformation of the stromal barrier and reversal of initially increased intratumoral exposure levels, TGFβ inhibition in cooperation with gemcitabine effectively suppressed tumor growth via cooperative reprogramming of T regulatory cells and stimulation of CD8 T cell-mediated antitumor activity. The antitumor activity was further improved by the addition of anti-PD-L1 immune checkpoint blockade to offset adaptive PD-L1 upregulation induced by TGFβ inhibition. These findings support the development of combined antistroma anticancer therapies capable of impacting the tumor beyond the disruption of the desmoplastic stroma as a physical barrier to improve drug delivery.

Novel humanized mesothelin-expressing genetically engineered mouse models underscore challenges in delivery of complex therapeutics to pancreatic cancers

Antibody-based therapies designed for human use frequently fail to cross-react with the murine isoform of their target. Due to this problem, pre-clinical studies of antibody-based mesothelin-targeted therapeutics in immune competent systems have been limited by the lack of suitable mouse models. Here, we describe two immune-competent humanized mesothelin transgenic mouse lines that can act as tolerant hosts for C57Bl/6-syngeneic cell lines expressing the human isoform of mesothelin. Thyroid peroxidase (TPO) mice have thyroid-restricted human mesothelin expression. Mesothelin (Msl) mice express human mesothelin in the typical serosal membrane distribution and can additionally be utilized to assess on-target, off-tumor toxicity of human mesothelin-targeted therapeutics. Both transgenic strains shed human mesothelin into the serum like human mesothelioma and ovarian cancer patients and serum human mesothelin can be used as a blood-based surrogate of tumor burden. Using these models, we examined the on-target toxicity and anti-tumor activity of human mesothelin-targeted recombinant immunotoxins. We found that immunotoxin treatment causes acute and chronic histologic changes to serosal membranes in Msl mice while human mesothelin-expressing thyroid follicular cells in TPO mice are resistant to immunotoxin despite excellent drug delivery. Furthermore, poor delivery of immunotoxin to syngeneic orthotopic human mesothelin-expressing pancreatic adenocarcinoma limits anti-tumor activity both alone and in combination with immune checkpoint inhibition. In summary, we have developed two high-fidelity, immunocompetent murine models for human cancer that allow for rigorous pre-clinical evaluation of human mesothelin-targeted therapeutics.

Abstract 2727: Heterodimeric IL-15 (hetIL-15) affects conventional dendritic cells and a distinct novel dendritic cell population in different mouse cancer models of breast and pancreatic cancer

Introduction: IL-15 is a cytokine inducing proliferation and cytotoxic function of lymphocytes, including CD8+ T and NKs cells. We have produced the native heterodimeric form of IL-15 (hetIL-15) which has advanced in clinical trials due to its anticancer activities in many different mouse cancer models. The objective of this study was to explore how hetIL-15 affects lymphoid and myeloid cell populations in the tumor microenvironment in different mouse cancer models of breast and pancreatic cancer.

Study Design and Methods: We studied the therapeutic efficacy of intraperitoneal or locoregional administration of hetIL-15 in three different orthotopic mouse cancer models. We employed 4T1 and EO771 breast cancer models, which mimic human triple negative breast cancer and a KPC derived pancreatic cancer model. hetIL-15 effects on tumor infiltrating immune cells were evaluated by flow cytometry and immunohistochemistry.

Results: hetIL-15 treatment showed a profound effect on the EO771 breast cancer model resulting in complete tumor regression in ~40% of the treated mice. Additionally, hetIL-15 caused tumor delay in 4T1 and KPC derived cancer models. Flow cytometry analysis revealed an increase of cytotoxic effector cells, and especially CD8+ T cells, with enhanced cytotoxic activity and proliferation in all cancer models tested. Immunohistochemistry verified the accumulation of cytotoxic effector cells intratumorally upon hetIL-15 treatment. Furthermore, flow analysis showed an increase of infiltrating conventional type 1 dendritic cells (cDC1s) which was found only in EO771 breast tumors. On the contrary, 4T1 breast and KPC derived pancreatic tumors, but not EO771 tumors, displayed an accumulation of infiltrating conventional type 2 dendritic cells (cDC2s). Importantly, the flow cytometry analysis revealed yet additional novel distinct dendritic cell population characterized by CD103intCD11b+ immunophenotype, which was mostly evident in tumor tissues treated with hetIL-15 in all three models. Phenotypic profiling of this novel DC population identified expression of several cDC1 specific markers, including CD103, IRF8 and XCR1. Both cDC1s and the novel DC population were inversely correlated with the tumor size in EO771 breast cancer model.

Conclusions: hetIL-15 affects both T cells and conventional dendritic cells in syngeneic murine cancer models of breast and pancreatic cancer. We report that the treatment with hetIL-15 increases a novel distinct dendritic cell population (CD103intCD11b+) in all three models. Since we have previously reported that hetIL-15 induces long term immunological protection from tumor rechallenge, these findings suggest hetIL-15 as a promising therapeutic agent in treatment of triple negative breast and pancreatic cancer.

Citation Format: Vasiliki Stravokefalou, Dimitris Stellas, Sevasti Karaliota, Bethany Nagy, Theresa Guerin, Serguei Kozlov, Barbara K. Felber, George N. Pavlakis. Heterodimeric IL-15 (hetIL-15) affects conventional dendritic cells and a distinct novel dendritic cell population in different mouse cancer models of breast and pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2727.

Mannose receptor (CD206) activation in tumor-associated macrophages enhances adaptive and innate antitumor immune responses

Solid tumors elicit a detectable immune response including the infiltration of tumor-associated macrophages (TAMs). Unfortunately, this immune response is co-opted into contributing toward tumor growth instead of preventing its progression. We seek to reestablish an antitumor immune response by selectively targeting surface receptors and endogenous signaling processes of the macrophage subtypes driving cancer progression. RP-182 is a synthetic 10-mer amphipathic analog of host defense peptides that selectively induces a conformational switch of the mannose receptor CD206 expressed on TAMs displaying an M2-like phenotype. RP-182-mediated activation of this receptor in human and murine M2-like macrophages elicits a program of endocytosis, phagosome-lysosome formation, and autophagy and reprograms M2-like TAMs to an antitumor M1-like phenotype. In syngeneic and autochthonous murine cancer models, RP-182 suppressed tumor growth, extended survival, and was an effective combination partner with chemo- or immune checkpoint therapy. Antitumor activity of RP-182 was also observed in CD206^high patient-derived xenotransplantation models. Mechanistically, via selective reduction of immunosuppressive M2-like TAMs, RP-182 improved adaptive and innate antitumor immune responses, including increased cancer cell phagocytosis by reprogrammed TAMs.

Quantitative Expression of GATA3 Specifies Lineage Fates and Functions of Innate Lymphoid Cells

Innate lymphoid cells (ILCs) are the innate counterparts of the CD4 T helper cells of the adaptive system. While the CD4 T cell development and differentiation have been well studied, the development of ILC subsets is far from clear. Lymphoid tissue inducer (LTi) population is the founding member of ILCs, however, recent study has shown that these cells are not derived from a PLZF-expressing ILC common progenitor that generates other ILCs. The transcription factor(s) determining the fate of non-LTi progenitor versus LTi progenitor are unknown. Here we report that GATA3 is absolutely required for the generation of PLZF-expressing non-LTi progenitors, which express high level of GATA3, but not for the generation of RORgammat-expressing LTi progenitors consistent with low levels of GATA3 expression in these progenitors. Nevertheless, low level of GATA3 expression by LTi progenitors is critical for the generation of functional LTi cells. Thus, quantitative expression of GATA3 functionally determines the fates and functions of distinct ILC progenitors.

LC-MS/MS assay coupled with carboxylic acid magnetic bead affinity capture to quantitatively measure cationic host defense peptides (HDPs) in complex matrices with application to preclinical pharmacokinetic studies

Synthetic host defense peptides (HDP) are a new class of promising therapeutic agents with potential application in a variety of diseases. RP-182 is a 10mer synthetic HDP design, which selectively reduces M2-like tumor associated macrophages via engagement with the cell surface lectin receptor MRC1/CD206 and is currently being developed as an innate immune defense regulator to improve anti-tumor immunity in immunologically cold tumors. Herein, we describe a sensitive and specific liquid chromatography (LC) coupled to quadrupole electron spray tandem mass spectrometry method to measure positively charged HDPs and HDP peptide fragments in complex biological matrices. Carboxylic acid magnetic beads were used as an affinity-capturing agent to extract the positively charged RP-182 from both mouse plasma and tissue homogenates. Beads were eluted with 0.1% (v/v) formic acid and chromatographic separation was achieved on a Waters 2.1 × 100 mm, 3.5 μm XSelect Peptide CSH C18 column with a Vanguard pre-column of the same phase. MS/MS was performed on a Thermo TSQ Quantiva triple quadrupole mass spectrometer operating in Selected Reaction Monitoring (SRM) mode fragmenting the plus three parent ion 458.9⁺³ and monitoring ions 624.0⁺², 550.5⁺², and 597.3⁺¹ for RP-182 and 462.4⁺³ > 629.1⁺², 555.5⁺², and 607.3⁺¹ for isotopic RP-182 standard. The assay had good linearity ranging from 1 ng to 1000 ng in mouse plasma with the lower limit of detection for RP-182 at 1 ng in mouse plasma with good intra- and inter-sample precision and accuracy. Recovery ranged from 66% to 77% with minimum matrix effects. The method was successfully applied to an abbreviated pharmacokinetic study in mice after single IP injection of RP-182. The method was successfully tested on a second HDP, the 17mer D4E1, and the cationic human peptide hormone ghrelin suggesting that it might be a general sensitive method applicable to quantifying HDP peptides that are difficult to extract.

[PRACTICE OF MANAGEMENT OF PATIENTS WITH INFECTIOUS ENDOCARDITIS IN CONDITIONS OF LOW FREQUENCY OF ETIOLOGICALLY SIGNIFICANT PATHOGENS IN THE RUSSIAN FEDERATION]

The article presents the results of a multicenter study of the etiology, antibiotic sensitivity and pharmacoepidemiology of infective endocarditis in the Russian Federation. The purpose of this study is to analyze the current practice of management of patients with infective endocarditis in conditions of low frequency of etiologically significant pathogens in the Russian Federation. The study included patients of both sexes of all age groups with definite and probable infective endocarditis. 406 cases of infectious endocarditis (240 in retrospect and 166 in the prospective part) were analyzed. Etiologically significant pathogen was isolated in 144 cases (35.5%). The structure of pathogens was dominated by gram (+) cocci (90.3%), most often - Staphylococcus aureus (46.5% of all isolated pathogens). Aminoglycosides (22.8%), parenteral cephalosporins of the III generation (22.1%) and glycopeptides (14.5%) were most frequently used in the course of starting antimicrobial therapy. When changing the mode of antimicrobial therapy, glycopeptides (18.6%), aminoglycosides (15.3%), fluoroquinolones (11.2%) and parenteral cephalosporins of generation III (9.5%) were most often prescribed.

Differential Expression of the Transcription Factor GATA3 Specifies Lineage and Functions of Innate Lymphoid Cells

Lymphoid tissue inducer (LTi) cells are regarded as a subset of innate lymphoid cells (ILCs). However, these cells are not derived from the ILC common progenitor, which generates other ILC subsets and is defined by the expression of the transcription factor PLZF. Here, we examined transcription factor(s) determining the fate of LTi progenitors versus non-LTi ILC progenitors. Conditional deletion of Gata3 resulted in the loss of PLZF⁺ non-LTi progenitors but not the LTi progenitors that expressed the transcription factor RORγt. Consistently, PLZF⁺ non-LTi progenitors expressed high amounts of GATA3, whereas GATA3 expression was low in RORγt⁺ LTi progenitors. The generation of both progenitors required the transcriptional regulator Id2, which defines the common helper-like innate lymphoid progenitor (ChILP), but not cytokine signaling. Nevertheless, low GATA3 expression was necessary for the generation of functionally mature LTi cells. Thus, differential expression of GATA3 determines the fates and functions of distinct ILC progenitors.

P721Shear stress induced unfolding of von willebrand factor may be involved in the premature development of myocardial infarction

Background

Normally, von Willebrand factor (vWF) becomes highly reactive with platelets upon unfolding into a fibrillar conformation at critical shear rate (more than 5000 s–1), that may occur in stenotic arteries. At shear rates below critical value (1200–1300 s–1), which occur in intact coronary arteries, normally there is no conformational rearrangement of vWF. Pathologic unfolding of vWF at shear rates below critical value may increase a risk of the development of coronary thrombosis. There is little information on the role of shear stress induced conformational rearrangement of vWF in the development of myocardial infarction in young individuals.

Purpose

To investigate vWF-dependent platelet adhesion of patients with premature myocardial infarction at shear rates below critical value (1200–1300 s–1).

Methods

Using a microfluidic system, we measured platelet adhesion to a fibrinogen-coated optical surface at shear rates of 1200–1300 s–1 during 10 minutes. We assessed platelet-rich plasma of 8 male persons 40–52 years old, who had previous myocardial infarction at the age of 34–39. The control group comprised 6 healthy male volunteers 30–55 years old. We compared the intensity of scattered laser light measured in volts (V) at 10th minute. To study vWF-dependent platelet adhesion, we blocked GPIb receptor with monoclonal antibody to inhibit platelet interaction with vWF. To compare the intensity of vWF-dependent platelet adhesion with normally occurring adhesion to fibrinogen, we blocked GPIIb/IIIa receptor with monoclonal antibody.

Results

The inhibition of GPIb vWF-receptor decreased platelet adhesion to fibrinogen surface at shear rates of 1200–1300 s–1 by 17.8±4.7% in healthy volunteers and by 92±2.8% in persons with premature myocardial infarction (p<0.05). Inhibition of GPIIb/IIIa receptor decreased platelet adhesion by 91.5±3.8% in healthy volunteers and by 97.3±3.2% in persons with premature myocardial infarction.

Conclusion

Pathologic unfolding of vWF at shear rates below critical value may be involved in the development of premature myocardial infarction.

Acknowledgement/Funding

This work was supported by the grant of the Russian Science Foundation (project #16-15-10098)

Metarrestin, a perinucleolar compartment inhibitor, effectively suppresses metastasis

Metastasis remains a leading cause of cancer mortality due to the lack of specific inhibitors against this complex process. To identify compounds selectively targeting the metastatic state, we used the perinucleolar compartment (PNC), a complex nuclear structure associated with metastatic behaviors of cancer cells, as a phenotypic marker for a high-content screen of over 140,000 structurally diverse compounds. Metarrestin, obtained through optimization of a screening hit, disassembles PNCs in multiple cancer cell lines, inhibits invasion in vitro, suppresses metastatic development in three mouse models of human cancer, and extends survival of mice in a metastatic pancreatic cancer xenograft model with no organ toxicity or discernable adverse effects. Metarrestin disrupts the nucleolar structure and inhibits RNA polymerase (Pol) I transcription, at least in part by interacting with the translation elongation factor eEF1A2. Thus, metarrestin represents a potential therapeutic approach for the treatment of metastatic cancer.

Abstract 268: Engineering of transgenic mice expressing human mesothelin for investigation of mesothelin-targeted therapeutics

Most monoclonal antibody-based therapeutics are specific for the human isoform of their targets and do not cross-react with orthologous mouse isoforms. This frequently limits pre-clinical modeling to human-derived tumors growing in immune compromised mice and precludes combination studies with immune modulating agents which rely on an active immune system for activity. Although introduction of human cDNA into mouse tumor cells will make these cells susceptible to targeting, a syngeneic mouse may reject or partially reject cells bearing the foreign transgene, impacting anti-tumor efficacy studies. Mesothelin (MSLN) is the target of many cell- and antibody-based therapies that have reached clinical trials due to its lack of expression in critical normal tissues and robust expression in many solid tumors. We have generated two distinct transgenic C57/Bl6 mouse models with compartment-limited expression of human MSLN (hMSLN) for use in pre-clinical testing of MSLN-targeted therapeutics: 1) TPO/hMSLN mice express full-length hMSLN from an insulated rat thyroid peroxidase gene promoter. Analysis by RT-PCR, immunoblot and IHC demonstrate strong thyroid-specific expression hMSLN. 2) In Nor/hMSLN mice, the hMSLN coding region was inserted as a full-length cDNA into the native mMSLN locus via a homologous recombination knock-in technique such that hMSLN is expressed in place of the mouse isoform under the control of endogenous murine MSLN transcription regulatory elements. IHC studies show expected expression of hMSLN in mouse pleura, pericardium and peritoneum. Shed hMSLN can be readily detected in the serum of both transgenic lines using ELISA, similar to human patients. Treatment of either model with MSLN-targeted immunotoxin LMB-100 at the maximum dose tolerated by non-transgenic C57/Bl6 mice resulted in no gross toxicity. Histologic analysis revealed subclinical pericarditis in Nor/hMSLN mice. No post-treatment thyroid damage was observed in TPO/hMSLN mice even upon detailed histologic examination of thyroid tissue. Syngeneic mouse pancreatic cancer cells expressing hMSLN transgene implanted orthotopically into TPO/hMSLN or Nor/hMSLN grew as well or better as those grown in non-transgenic C57/Bl6 mice. We have established two immunologically proficient transgenic mouse models expressing hMSLN that can be used for evaluating immune effect of hMSLN-targeted therapeutics as well as their on-target/off-tumor toxicities.

Citation Format: Xianyu Zhang, Brendan Hagerty, Theresa Guerin, Norene O'Sullivan, Serguei Kozlov, Christine Alewine. Engineering of transgenic mice expressing human mesothelin for investigation of mesothelin-targeted therapeutics [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 268.

Pharmacokinetic evaluation of the PNC disassembler metarrestin in wild-type and Pdx1-Cre;LSL-KrasG12D/+;Tp53R172H/+ (KPC) mice, a genetically engineered model of pancreatic cancer

PURPOSE

Metarrestin is a first-in-class small molecule clinical candidate capable of disrupting the perinucleolar compartment, a subnuclear structure unique to metastatic cancer cells. This study aims to define the pharmacokinetic (PK) profile of metarrestin and the pharmacokinetic/pharmacodynamic relationship of metarrestin-regulated markers.

METHODS

PK studies included the administration of single or multiple dose of metarrestin at 3, 10, or 25 mg/kg via intravenous (IV) injection, gavage (PO) or with chow to wild-type C57BL/6 mice and KPC mice bearing autochthonous pancreatic tumors. Metarrestin concentrations were analyzed by UPLC-MS/MS. Pharmacodynamic assays included mRNA expression profiling by RNA-seq and qRT-PCR for KPC mice.

RESULTS

Metarrestin had a moderate plasma clearance of 48 mL/min/kg and a large volume of distribution of 17 L/kg at 3 mg/kg IV in C57BL/6 mice. The oral bioavailability after single-dose (SD) treatment was > 80%. In KPC mice treated with SD 25 mg/kg PO, plasma AUC0-∞ of 14400 ng h/mL, Cmax of 810 ng/mL and half-life (t1/2) of 8.5 h were observed. At 24 h after SD of 25 mg/kg PO, the intratumor concentration of metarrestin was high with a mean value of 6.2 µg/g tissue (or 13 µM), well above the cell-based IC50 of 0.4 µM. At multiple dose (MD) 25 mg/kg/day PO in KPC mice, mean tissue/plasma AUC0-24h ratio for tumor, spleen and liver was 37, 30 and 31, respectively. There was a good linear relationship of dosage to AUC0-24h and C24h. AUC0-24h MD to AUC0-24h SD ratios ranged from two for liver to five for tumor indicating additional accumulation in tumors. Dose-dependent normalization of FOXA1 and FOXO6 mRNA expression was observed in KPC tumors.

CONCLUSIONS

Metarrestin is an effective therapeutic candidate with a favorable PK profile achieving excellent intratumor tissue levels in a disease with known poor drug delivery.

Protein Synthesis Inhibition Activity of Mesothelin Targeting Immunotoxin LMB-100 Decreases Concentrations of Oncogenic Signaling Molecules and Secreted Growth Factors

LMB-100 is a mesothelin-targeted recombinant immunotoxin (iTox) that carries a modified Pseuodomonas exotoxin A (PE) payload. PE kills cells by inhibiting synthesis of new proteins. We found that treatment of pancreatic cancer cells with LMB-100 for 24–48 h did not change total protein level despite inducing protein synthesis inhibition (PSI). Further, increased levels of ubiquitinated proteins were detected, indicating that cells may have limited ability to compensate for PSI by reducing protein degradation. Together, these data suggest that PE depletes concentrations of a minority of cellular proteins. We used reverse phase protein array and Luminex assay to characterize this subset. LMB-100 decreased the abundance of 24 of 32 cancer-related proteins (including Bcl-x, Her2, Her3 and MUC16) without compensatory increases in other analytes. Further, cancer cells failed to maintain extracellular concentrations of cancer cell secreted growth factors (CCSGFs), including Vascular Endothelial Growth Factor (VEGF) following treatment with cytostatic LMB-100 doses both in culture and in mouse tumors. Decreased VEGF concentration did not change tumor vasculature density, however, LMB-100 caused tissue-specific changes in concentrations of secreted factors made by non-cancer cells. In summary, our data indicate that PSI caused by cytostatic LMB-100 doses preferentially depletes short-lived proteins such as oncogenic signaling molecules and CCSGFs.

Abstract 104: Identification of the molecular components required for the protumorigenic effect of MSLN in pancreatic cancer

Mesothelin (MSLN) is a GPI-linked protein expressed in &gt;90% of pancreatic adenocarcinomas (PDAC) but not in healthy pancreas nor in the parenchyma of other vital organs. Due to this differential expression, multiple antibody-based therapies are targeting MSLN. The physiologic role of MSLN is yet unknown. The MSLN gene encodes for a precursor protein which is cleaved to produce mature MSLN protein and soluble Megakaryocyte Potentiating Factor (MPF), both of which can be secreted or shed. Previous reports suggested that overexpression of MSLN precursor protein increases tumorigenicity and metastatic potential of PDAC. Here, we have used CRISPR-Cas9 gene editing to delete MSLN from the KLM-1 pancreatic cancer cell line (ΔMSLN) and have examined the growth of these cells compared to mock deleted cells (Mock). The ΔMSLN and Mock cells grew at the same rate both in cell culture and as subcutaneous tumors in nude mice, but ΔMSLN had significantly impaired growth when inoculated into the mouse intraperitoneal (IP) cavity as well as when injected orthotopically into the pancreas. Histological analysis (H&E, Ki67 and Trichrome) showed no significant gross differences between ΔMSLN and Mock IP tumors. RNA deep sequencing analysis comparing ΔMSLN and Mock cell lines showed strong upregulation of MUC-4 in KO cells. Restoration of WT MSLN expression in ΔMSLN cells restored IP growth, however, transduction of a Y318A point mutant could not. Point mutation of Y318 diminished ability of shed MSLN to interact with KLM-1 cells, presumably by impairing association with MUC-16, the only known binding partner of MSLN. To determine the role of mature MSLN and MPF in tumorigenicity, we expressed these proteins separately in ΔMSLN cells and found that neither could restore IP growth, although mature MSLN was appropriately expressed on the cell surface and MPF secreted. This suggests either that both proteins are required for tumorigenicity or that when synthesized individually they are incorrectly processed by the cell. To examine the role of MSLN in tumorigenesis, MSLN KO mouse were bred into the KPC genetically engineered mouse model of PDAC, which spontaneously develops autochthonous tumors. Unlike PDAC patients, KPC typically die from complications of their primary tumors. Loss of MSLN had no effect on the survival of these mice. Effect of MSLN loss on metastasis in this model is currently being evaluated. In summary, we have demonstrated that MSLN enhances the IP growth of PDAC and defined key regions of MSLN important for this activity.

Citation Format: Leela Rani Avula, Michael Rudloff, Danielle Arons, Salma El-Behaedi, Xianyu Zhang, Theresa Guerin, Serguei Kozlov, Christine Alewine. Identification of the molecular components required for the protumorigenic effect of MSLN in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 104.

Synergistic targeted inhibition of MEK and dual PI3K/mTOR diminishes viability and inhibits tumor growth of canine melanoma underscoring its utility as a preclinical model for human mucosal melanoma

Human mucosal melanoma (MM), an uncommon, aggressive and diverse subtype, shares characteristics with spontaneous MM in dogs. Although BRAF and N-RAS mutations are uncommon in MM in both species, the majority of human and canine MM evaluated exhibited RAS/ERK and/or PI3K/mTOR signaling pathway activation. Canine MM cell lines, with varying ERK and AKT/mTOR activation levels reflective of naturally occurring differences in dogs, were sensitive to the MEK inhibitor GSK1120212 and dual PI3K/mTOR inhibitor NVP-BEZ235. The two-drug combination synergistically decreased cell survival in association with caspase 3/7 activation, as well as altered expression of cell cycle regulatory proteins and Bcl-2 family proteins. In combination, the two drugs targeted their respective signaling pathways, potentiating reduction of pathway mediators p-ERK, p-AKT, p-S6, and 4E-BP1 in vitro, and in association with significantly inhibited solid tumor growth in MM xenografts in mice. These findings provide evidence of synergistic therapeutic efficacy when simultaneously targeting multiple mediators in melanoma with Ras/ERK and PI3K/mTOR pathway activation.

Abstract A17: p53 missense mutants R172H and R270H exhibit differential effects on tumorigenesis

p53 is a well-characterized transcription factor that is mutated in about 50% of human cancers. The majority of p53 genomic alterations are missense mutations which result in expression of p53 protein isoforms with deficient functionality. Such missense mutations cluster in several hotspots, with the most frequent alterations located at amino acids 175, 248 and 273 of human p53 (mouse amino acids 172, 245 and 270). p53 missense mutants disrupt the function of p53 tetramers, thereby behaving similarly to loss of p53. However, there is evidence that p53 missense mutants have additional tumor-promoting functions that differ from p53 deletions: p53 missense mutants can transform p53 null cells in vitro and in mouse models, and the p53 missense mutants induce a different tumor spectrum from that induced by p53 loss or other p53 missense mutants.

The information obtained from the engineered murine models featuring p53 missense mutations has greatly elucidated the role of p53 mutagenesis in cancer. However, the currently-available p53 models bear several drawbacks that make them sub-optimal for preclinical studies. These models contain a global p53 haploinsufficiency, potentially impacting the biology of tumor-modifying structures like stroma, the immune system, and the vasculature. Furthermore, activation of the p53 missense mutant alleles results in a transition from a single functional wild-type allele to a combination of a wild-type and a mutant allele. These features substantially deviate from those of observed in human tumorigenesis, where somatic p53 mutagenesis in cells undergoing transformation results in a transition from two wild-type alleles to a combination of a wild-type and a mutant alleles, while tumor-modifying structures retain an unaltered p53 biallelic configuration. Here we describe the molecular and phenotypic characterization of a new allelic series of conditional p53 missense mutant mouse lines in which Cre-mediated recombination converts p53 from a wild-type to a missense mutant (R172H, R270H or R270C) configuration.

To characterize the gradual molecular changes induced by expression of R172H and R270H mutants, we derived MEF lines harboring the alleles in a heterozygous conditional (p53-R172H fl/+ or p53-R270H fl/+) configuration. We observed that the wild-type p53 mRNA was lost but p53 protein abundance was increased after Cre-mediated activation of the mutant allele. Interestingly, we found that the R270H mutant allele provided a greater proliferative advantage, a distinct growth pattern and a greater ability to grow in vitro under starvation conditions than a similarly engineered R172H mutant allele. Consistent with published evidence, these observations suggest that the acquired functional changes are dissimilar between different p53 missense mutants. We further characterized the transcriptome profiles in MEF lines harboring R172H and R270H alleles and identified a number of unique and common transcriptional changes that could be causal for gain-of-function phenotypes and informative for uncovering additional mechanisms for carcinogenesis driven by p53 missense mutants.

We used the newly-derived p53 mutant alleles to establish improved autochthonous and orthotopic pancreatic cancer models suitable for preclinical efficacy studies and we are currently developing lung cancer, ovarian cancer, and medullary thyroid carcinoma models.

Citation Format: Tomas Vilimas, Keith Collins, Theresa Guerin, Roackie Awasthi, Lionel Feigenbaum, Guillermina Lozano, Terry Van Dyke, Serguei Kozlov. p53 missense mutants R172H and R270H exhibit differential effects on tumorigenesis. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A17.

Vitamin D receptor-mediated stromal reprogramming suppresses pancreatitis and enhances pancreatic cancer therapy

The poor clinical outcome in pancreatic ductal adenocarcinoma (PDA) is attributed to intrinsic chemoresistance and a growth-permissive tumor microenvironment. Conversion of quiescent to activated pancreatic stellate cells (PSCs) drives the severe stromal reaction that characterizes PDA. Here, we reveal that the vitamin D receptor (VDR) is expressed in stroma from human pancreatic tumors and that treatment with the VDR ligand calcipotriol markedly reduced markers of inflammation and fibrosis in pancreatitis and human tumor stroma. We show that VDR acts as a master transcriptional regulator of PSCs to reprise the quiescent state, resulting in induced stromal remodeling, increased intratumoral gemcitabine, reduced tumor volume, and a 57% increase in survival compared to chemotherapy alone. This work describes a molecular strategy through which transcriptional reprogramming of tumor stroma enables chemotherapeutic response and suggests vitamin D priming as an adjunct in PDA therapy. PAPERFLICK:

Selective inhibition of the T cell p38 alternative activation pathway and pancreatic cancer (TUM3P.1046)

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive neoplasm with a five year survival rate of ~5%, that is characterized by a marked fibro-inflammatory microenvironment, the presence of which can promote cancer induction and growth. Therefore, selective manipulation of local cytokines is an attractive if unrealized therapeutic approach. T cells possess a unique mechanism for activation of p38 MAPK downstream of T cell receptor (TCR) engagement in which it is phosphorylated on Tyr-323 (pY323), which is required for pro-inflammatory cytokine production. Analysis of 192 human pancreatic carcinomas revealed that having a high percentage of infiltrating pY323+ T cells was accompanied by large numbers of TNFa and IL-17-producing CD4+ TIL and very aggressive disease. Cancer progression was inhibited in two different murine pancreatic tumors models by genetic ablation of the alternative p38 activation pathway. Strikingly, a plasma membrane-permeable peptide derived from Gadd45α, the naturally-occurring inhibitor of p38 pY323+, reduced infiltrating CD4+ T cell production of TNFa, IL-17A, IL-10, and induction of secondary cytokines, slowed the growth of implanted tumor cells and inhibited progression of spontaneous K-ras-driven adenocarcinoma. Thus, TCR-mediated alternative p38 activation in tumor-infiltrating CD4+ T cells, which promotes the production of pro-tumorigenic inflammatory factors, can be targeted for therapeutic benefit.

The LINC complex is essential for hearing

Hereditary hearing loss is the most common sensory deficit. We determined that progressive high-frequency hearing loss in 2 families of Iraqi Jewish ancestry was due to homozygosity for the protein truncating mutation SYNE4 c.228delAT. SYNE4, a gene not previously associated with hearing loss, encodes nesprin-4 (NESP4), an outer nuclear membrane (ONM) protein expressed in the hair cells of the inner ear. The truncated NESP4 encoded by the families' mutation did not localize to the ONM. NESP4 and SUN domain-containing protein 1 (SUN1), which localizes to the inner nuclear membrane (INM), are part of the linker of nucleoskeleton and cytoskeleton (LINC) complex in the nuclear envelope. Mice lacking either Nesp4 or Sun1 were evaluated for hair cell defects and hearing loss. In both Nesp4-/- and Sun1-/- mice, OHCs formed normally, but degenerated as hearing matured, leading to progressive hearing loss. The nuclei of OHCs from mutant mice failed to maintain their basal localization, potentially affecting cell motility and hence the response to sound. These results demonstrate that the LINC complex is essential for viability and normal morphology of OHCs and suggest that the position of the nucleus in sensory epithelial cells is critical for maintenance of normal hearing.

Temporal molecular and biological assessment of an erlotinib-resistant lung adenocarcinoma model reveals markers of tumor progression and treatment response

Patients with lung cancer with activating mutations in the EGF receptor (EGFR) kinase, who are treated long-term with tyrosine kinase inhibitors (TKI), often develop secondary mutations in EGFR associated with resistance. Mice engineered to develop lung adenocarcinomas driven by the human EGFR T790M resistance mutation are similarly resistant to the EGFR TKI erlotinib. By tumor volume endpoint analysis, these mouse tumors respond to BIBW 2992 (an irreversible EGFR/HER2 TKI) and rapamycin combination therapy. To correlate EGFR-driven changes in the lung with response to drug treatment, we conducted an integrative analysis of global transcriptome and metabolite profiling compared with quantitative imaging and histopathology at several time points during tumor progression and treatment. Responses to single-drug treatments were temporary, whereas combination therapy elicited a sustained response. During tumor development, metabolomic signatures indicated a shift to high anabolic activity and suppression of antitumor programs with 11 metabolites consistently present in both lung tissue and blood. Combination drug treatment reversed many of the molecular changes found in tumored lung. Data integration linking cancer signaling networks with metabolic activity identified key pathways such as glutamine and glutathione metabolism that signified response to single or dual treatments. Results from combination drug treatment suggest that metabolic transcriptional control through C-MYC and SREBP, as well as ELK1, NRF1, and NRF2, depends on both EGFR and mTORC1 signaling. Our findings establish the importance of kinetic therapeutic studies in preclinical assessment and provide in vivo evidence that TKI-mediated antiproliferative effects also manifest in specific metabolic regulation.

Behavioral and molecular exploration of the AR-CMT2A mouse model Lmna (R298C/R298C)

In 2002, we identified LMNA as the first gene responsible for an autosomal recessive axonal form of Charcot-Marie-Tooth disease, AR-CMT2A. All patients were found to be homozygous for the same mutation in the LMNA gene, p.Arg298Cys. In order to investigate the physiopathological mechanisms underlying AR-CMT2A, we have generated a knock-in mouse model for the Lmna p.Arg298Cys mutation. We have explored these mice through an exhaustive series of behavioral tests and histopathological analyses, but were not able to find any peripheral nerve phenotype, even at 18 months of age. Interestingly at the molecular level, however, we detect a downregulation of the Lmna gene in all tissues tested from the homozygous knock-in mouse Lmna (R298C/R298C) (skeletal muscle, heart, peripheral nerve, spinal cord and cerebral trunk). Importantly, we further reveal a significant upregulation of Pmp22, specifically in the sciatic nerves of Lmna (R298C/R298C) mice. These results indicate that, despite the absence of a perceptible phenotype, abnormalities exist in the peripheral nerves of Lmna (R298C/R298C) mice that are absent from other tissues. Although the mechanisms leading to deregulation of Pmp22 in Lmna (R298C/R298C) mice are still unclear, our results support a relation between Lmna and Pmp22 and constitute a first step toward understanding AR-CMT2A physiopathology.

Functional coupling between the extracellular matrix and nuclear lamina by Wnt signaling in progeria

The segmental premature aging disease Hutchinson-Gilford Progeria (HGPS) is caused by a truncated and farnesylated form of Lamin A. In a mouse model for HGPS, a similar Lamin A variant causes the proliferative arrest and death of postnatal, but not embryonic, fibroblasts. Arrest is due to an inability to produce a functional extracellular matrix (ECM), because growth on normal ECM rescues proliferation. The defects are associated with inhibition of canonical Wnt signaling, due to reduced nuclear localization and transcriptional activity of Lef1, but not Tcf4, in both mouse and human progeric cells. Defective Wnt signaling, affecting ECM synthesis, may be critical to the etiology of HGPS because mice exhibit skeletal defects and apoptosis in major blood vessels proximal to the heart. These results establish a functional link between the nuclear envelope/lamina and the cell surface/ECM and may provide insights into the role of Wnt signaling and the ECM in aging.

Structure of the lamin A/C R482W mutant responsible for dominant familial partial lipodystrophy (FPLD)

Proteins of the A-type lamin family, which consists of two members, lamin A and lamin C, are the major components of a thin proteinaceous filamentous meshwork, the lamina, that underlies the inner nuclear membrane. A-type lamins have recently become the focus of extensive functional studies as a consequence of the linking of at least eight congenital diseases to mutations in the lamin A/C gene (LMNA). This spectrum of pathologies, which mostly manifest themselves as dominant traits, includes muscle dystrophies, dilated cardiomyopathies, the premature aging syndrome Hutchinson-Guilford progeria and familial partial lipodystrophy (FPLD). The crystal structure of the lamin A/C mutant R482W, a variant that causes FPLD, has been determined at 1.5 A resolution. A completely novel aggregation state of the C-terminal globular domain and the position of the mutated amino-acid residue suggest means by which the mutation may affect lamin A/C-protein and protein-DNA interactions.

Loss of nucleoplasmic LAP2alpha-lamin A complexes causes erythroid and epidermal progenitor hyperproliferation

Lamina-associated polypeptide (LAP) 2alpha is a chromatin-associated protein that binds A-type lamins. Mutations in both LAP2alpha and A-type lamins are linked to human diseases called laminopathies, but the molecular mechanisms are poorly understood. The A-type lamin-LAP2alpha complex interacts with and regulates retinoblastoma protein (pRb), but the significance of this interaction in vivo is unknown. Here we address the function of the A-type lamin-LAP2alpha complex with the use of LAP2alpha-deficient mice. We show that LAP2alpha loss causes relocalization of nucleoplasmic A-type lamins to the nuclear envelope and impairs pRb function. This causes inefficient cell-cycle arrest in dense fibroblast cultures and hyperproliferation of epidermal and erythroid progenitor cells in vivo, leading to tissue hyperplasia. Our results support a disease-relevant model in which LAP2alpha defines A-type lamin localization in the nucleoplasm, which in turn affects pRb-mediated regulation of progenitor cell proliferation and differentiation in highly regenerative tissues.

[Forms of LonB protease from Archaeoglobus fulgidus devoid of the transmembrane domain: the contribution of the quaternary structure to the regulation of enzyme proteolytic activity]

Deletion of the transmembrane domain (TM-domain) of Archaeoglobus flggidus LonB protease (AfLon) was shown to result in uncontrollable activation of the enzyme proteolytic site and in vivo autolysis yielding a stable and functionally inactive fragment consisting of both alpha-helical and proteolytic domains (alphaP). The deltaTM-AfLonTM-S590A enzyme form, obtained by site-directed mutagenesis of the catalytic Ser residue, is capable of recombination with the alphaP fragment. The mixed oligomers were shown to be proteolytically active, which indicates a crucial role of subunit interactions in the activation of the AfLon proteolytic site. The thermophilic nature of AfLon protease was found to be due to the special features of the enzyme activity regulation, the structure of ATPase domain, and the quaternary structure.

Mouse models of the laminopathies

The A and B type lamins are nuclear intermediate filament proteins that comprise the bulk of the nuclear lamina, a thin proteinaceous structure underlying the inner nuclear membrane. The A type lamins are encoded by the lamin A gene (LMNA). Mutations in this gene have been linked to at least nine diseases, including the progeroid diseases Hutchinson-Gilford progeria and atypical Werner's syndromes, striated muscle diseases including muscular dystrophies and dilated cardiomyopathies, lipodystrophies affecting adipose tissue deposition, diseases affecting skeletal development, and a peripheral neuropathy. To understand how different diseases arise from different mutations in the same gene, mouse lines carrying some of the same mutations found in the human diseases have been established. We, and others have generated mice with different mutations that result in progeria, muscular dystrophy, and dilated cardiomyopathy. To further our understanding of the functions of the lamins, we also created mice lacking lamin B1, as well as mice expressing only one of the A type lamins. These mouse lines are providing insights into the functions of the lamina and how changes to the lamina affect the mechanical integrity of the nucleus as well as signaling pathways that, when disrupted, may contribute to the disease.

Loss of emerin at the nuclear envelope disrupts the Rb1/E2F and MyoD pathways during muscle regeneration

Emery-Dreifuss muscular dystrophy (EDMD1) is caused by mutations in either the X-linked gene emerin (EMD) or the autosomal lamin A/C (LMNA) gene. Here, we describe the derivation of mice lacking emerin in an attempt to derive a mouse model for EDMD1. Although mice lacking emerin show no overt pathology, muscle regeneration in these mice revealed defects. A bioinformatic array analysis of regenerating Emd null muscle revealed abnormalities in cell-cycle parameters and delayed myogenic differentiation, which were associated with perturbations to transcriptional pathways regulated by the retinoblastoma (Rb1) and MyoD genes. Temporal activation of MyoD transcriptional targets was significantly delayed, whereas targets of the Rb1/E2F transcriptional repressor complex remained inappropriately active. The inappropriate modulation of Rb1/MyoD transcriptional targets was associated with up-regulation of Rb1, MyoD and their co-activators/repressors transcripts, suggesting a compensatory effort to overcome a molecular block to differentiation at the myoblast/myotube transition during regeneration. This compensation appeared to be effective for MyoD transcriptional targets, although was less effective for Rb1 targets. Analysis of Rb1 phosphorylation states showed prolonged hyper-phosphorylation at key developmental stages in Emd null myogenic cells, both in vivo and in vitro. We also analyzed the same pathways in Lmna null muscle, which shows extensive dystrophy. Surprisingly, Lmna null muscle did not show the same perturbations to Rb- and MyoD-dependent pathways. We did observe increased transcriptional expression of Lap2alpha and delayed expression of Rb1, which may regulate alternative transcriptional pathways in the Lmna null myoblasts. We suggest that the dominant LMNA mutations seen in many clinically disparate laminopathies may similarly alter Rb function, with regard to either the timing of exit from the cell cycle or terminal differentiation programs or both.

Atomic-resolution crystal structure of the proteolytic domain of Archaeoglobus fulgidus lon reveals the conformational variability in the active sites of lon proteases

The atomic-resolution crystal structure of the proteolytic domain (P-domain, residues 415-621) of Archaeoglobus fulgidus B-type Lon protease (wtAfLonB) and the structures of several mutants have revealed significant differences in the conformation of the active-site residues when compared to other known Lon P-domains, despite the conservation of the overall fold. The catalytic Ser509 is facing the solvent and is distant from Lys552, the other member of the catalytic dyad. Instead, the adjacent Asp508 forms an ion pair with the catalytic lysine residue. Glu506, an analog of the putative third catalytic residue from a related Methanococcus jannaschii LonB, also faces the solvent and does not interact with the catalytic dyad. We have established that full-length wtAfLonB is proteolytically active in an ATP-dependent manner. The loss of enzymatic activity of the S509A mutant confirms the functional significance of this residue, while retention of considerable level of activity by the D508A and E506A mutants rules out their critical involvement in catalysis. In contrast to the full-length enzymes, all individually purified P-domains (wild-type and mutants) were inactive, and the mutations had no influence on the active-site structure. These findings raise the possibility that, although isolated proteolytic domains of both AfLonB and E.coli LonA are able to assemble into expected functional hexamers, the presence of the other domains, as well as substrate binding, may be needed to stabilize the productive conformation of their active sites. Thus, the observed conformational variability may reflect the differences in the stability of active-site structures for the proteolytic counterparts of single-chain Lon versus independently folded proteolytic subunits of two-chain AAA+ proteases.

Abnormal nuclear shape and impaired mechanotransduction in emerin-deficient cells

Emery-Dreifuss muscular dystrophy can be caused by mutations in the nuclear envelope proteins lamin A/C and emerin. We recently demonstrated that A-type lamin-deficient cells have impaired nuclear mechanics and altered mechanotransduction, suggesting two potential disease mechanisms (Lammerding, J., P.C. Schulze, T. Takahashi, S. Kozlov, T. Sullivan, R.D. Kamm, C.L. Stewart, and R.T. Lee. 2004. J. Clin. Invest. 113:370–378). Here, we examined the function of emerin on nuclear mechanics and strain-induced signaling. Emerin-deficient mouse embryo fibroblasts have abnormal nuclear shape, but in contrast to A-type lamin-deficient cells, exhibit nuclear deformations comparable to wild-type cells in cellular strain experiments, and the integrity of emerin-deficient nuclear envelopes appeared normal in a nuclear microinjection assay. Interestingly, expression of mechanosensitive genes in response to mechanical strain was impaired in emerin-deficient cells, and prolonged mechanical stimulation increased apoptosis in emerin-deficient cells. Thus, emerin-deficient mouse embryo fibroblasts have apparently normal nuclear mechanics but impaired expression of mechanosensitive genes in response to strain, suggesting that emerin mutations may act through altered transcriptional regulation and not by increasing nuclear fragility.

Conditional alleles for activation and inactivation of the mouse Rx homeobox gene

The Rx homeobox gene is a transcriptional regulator indispensable for development of the eye and ventral forebrain. Rx-null homozygotes lack optic pits, which are the earliest ocular structures. To study the roles Rx may play at various stages of eye and brain development, we generated an allelic series at the Rx locus. The targeted allele, Rx(neo), is a severely hypomorphic or null allele. This Rx(neo) allele is converted via FLP-mediated recombination to the Rx(flox) allele, which is phenotypically identical to the wildtype allele. Cre-mediated conversion of Rx(flox) generates the RxDelta2 allele, which, when homozygous, results in an Rx-null phenotype that includes perinatal lethality, anophthalmia, and anterior neural and craniofacial defects. Mice carrying these alleles allow both Cre-mediated inactivation and FLP-mediated activation of Rx gene activity on a conditional basis and will be useful in examining Rx function at different developmental stages and in distinct tissue environments.