Disease pathology signatures in a mouse model of Mucopolysaccharidosis type IIIB

Mucopolysaccharidosis type IIIB (MPS IIIB) is a rare and devastating childhood-onset lysosomal storage disease caused by complete loss of function of the lysosomal hydrolase α-N-acetylglucosaminidase. The lack of functional enzyme in MPS IIIB patients leads to the progressive accumulation of heparan sulfate throughout the body and triggers a cascade of neuroinflammatory and other biochemical processes ultimately resulting in severe mental impairment and early death in adolescence or young adulthood. The low prevalence and severity of the disease has necessitated the use of animal models to improve our knowledge of the pathophysiology and for the development of therapeutic treatments. In this study, we took a systematic approach to characterizing a classical mouse model of MPS IIIB. Using a series of histological, biochemical, proteomic and behavioral assays, we tested MPS IIIB mice at two stages: during the pre-symptomatic and early symptomatic phases of disease development, in order to validate previously described phenotypes, explore new mechanisms of disease pathology and uncover biomarkers for MPS IIIB. Along with previous findings, this study helps provide a deeper understanding of the pathology landscape of this rare disease with high unmet medical need and serves as an important resource to the scientific community.

Global chromatin landscapes identify candidate noncoding modifiers of cardiac rhythm

Comprehensive cis-regulatory landscapes are essential for accurate enhancer prediction and disease variant mapping. Although cis-regulatory element (CRE) resources exist for most tissues and organs, many rare - yet functionally important - cell types remain overlooked. Despite representing only a small fraction of the heart's cellular biomass, the cardiac conduction system (CCS) unfailingly coordinates every life-sustaining heartbeat. To globally profile the mouse CCS cis-regulatory landscape, we genetically tagged CCS component-specific nuclei for comprehensive assay for transposase-accessible chromatin-sequencing (ATAC-Seq) analysis. Thus, we established a global CCS-enriched CRE database, referred to as CCS-ATAC, as a key resource for studying CCS-wide and component-specific regulatory functions. Using transcription factor (TF) motifs to construct CCS component-specific gene regulatory networks (GRNs), we identified and independently confirmed several specific TF sub-networks. Highlighting the functional importance of CCS-ATAC, we also validated numerous CCS-enriched enhancer elements and suggested gene targets based on CCS single-cell RNA-Seq data. Furthermore, we leveraged CCS-ATAC to improve annotation of existing human variants related to cardiac rhythm and nominated a potential enhancer-target pair that was dysregulated by a specific SNP. Collectively, our results established a CCS-regulatory compendium, identified novel CCS enhancer elements, and illuminated potential functional associations between human genomic variants and CCS component-specific CREs.

Inducible cardiomyocyte injury within the atrioventricular conduction system uncovers latent regenerative capacity in mice

The cardiac conduction system (CCS) ensures regular contractile function, and injury to any of its components can cause cardiac dysrhythmia. Although all cardiomyocytes (CMs) originate from common progenitors, the CCS is composed of biologically distinct cell types with unique functional and developmental characteristics. In contrast to ventricular cardiomyocytes, which continue to proliferate after birth, most CCS cells terminally exit the cell cycle during fetal development. Although the CCS should thus provide a poor substrate for postnatal injury repair, its regenerative capacity remains untested. Here, we describe a genetic system for ablating CMs that reside within the atrioventricular conduction system (AVCS). Adult mouse AVCS ablation resulted in regenerative failure characterized by persistent atrioventricular conduction defects and contractile dysfunction. In contrast, AVCS injury in neonatal mice led to recovery in a subset of these mice, thus providing evidence for CCS plasticity. Furthermore, CM proliferation did not appear to completely account for the observed functional recovery, suggesting that mechanisms regulating recovery from dysrhythmia are likely to be distinct from cardiac regeneration associated with ventricular injury. Taken together, we anticipate that our results will motivate further mechanistic studies of CCS plasticity and enable the exploration of rhythm restoration as an alternative therapeutic strategy.

Migraine therapeutics differentially modulate the CGRP pathway

Background

The clinical efficacy of migraine therapeutic agents directed towards the calcitonin-gene related peptide (CGRP) pathway has confirmed the key role of this axis in migraine pathogenesis. Three antibodies against CGRP – fremanezumab, galcanezumab and eptinezumab – and one antibody against the CGRP receptor, erenumab, are clinically approved therapeutics for the prevention of migraine. In addition, two small molecule CGRP receptor antagonists, ubrogepant and rimegepant, are approved for acute migraine treatment. Targeting either the CGRP ligand or receptor is efficacious for migraine treatment; however, a comparison of the mechanism of action of these therapeutic agents is lacking in the literature.

Methods

To gain insights into the potential differences between these CGRP pathway therapeutics, we compared the effect of a CGRP ligand antibody (fremanezumab), a CGRP receptor antibody (erenumab) and a CGRP receptor small molecule antagonist (telcagepant) using a combination of binding, functional and imaging assays.

Results

Erenumab and telcagepant antagonized CGRP, adrenomedullin and intermedin cAMP signaling at the canonical human CGRP receptor. In contrast, fremanezumab only antagonized CGRP-induced cAMP signaling at the human CGRP receptor. In addition, erenumab, but not fremanezumab, bound and internalized at the canonical human CGRP receptor. Interestingly, erenumab also bound and internalized at the human AMY₁ receptor, a CGRP receptor family member. Both erenumab and telcagepant antagonized amylin-induced cAMP signaling at the AMY₁ receptor while fremanezumab did not affect amylin responses.

Conclusion

The therapeutic effect of agents targeting the CGRP ligand versus receptor for migraine prevention (antibodies) or acute treatment (gepants) may involve distinct mechanisms of action. These findings suggest that differing mechanisms could affect efficacy, safety, and/or tolerability in migraine patients.

Rational Reprogramming of Cellular States by Combinatorial Perturbation

Ectopic expression of transcription factors (TFs) can reprogram cell state. However, because of the large combinatorial space of possible TF cocktails, it remains difficult to identify TFs that reprogram specific cell types. Here, we develop Reprogram-Seq to experimentally screen thousands of TF cocktails for reprogramming performance. Reprogram-Seq leverages organ-specific cell-atlas data with single-cell perturbation and computational analysis to predict, evaluate, and optimize TF combinations that reprogram a cell type of interest. Focusing on the cardiac system, we perform Reprogram-Seq on MEFs using an undirected library of 48 cardiac factors and, separately, a directed library of 10 epicardial-related TFs. We identify a combination of three TFs, which efficiently reprogram MEFs to epicardial-like cells that are transcriptionally, molecularly, morphologically, and functionally similar to primary epicardial cells. Reprogram-Seq holds promise to accelerate the generation of specific cell types for regenerative medicine.

PAN-INTACT enables direct isolation of lineage-specific nuclei from fibrous tissues

Recent studies have highlighted the extraordinary cell type diversity that exists within mammalian organs, yet the molecular drivers of such heterogeneity remain elusive. To address this issue, much attention has been focused on profiling the transcriptome and epigenome of individual cell types. However, standard cell type isolation methods based on surface or fluorescent markers remain problematic for cells residing within organs with significant connective tissue. Since the nucleus contains both genomic and transcriptomic information, the isolation of nuclei tagged in specific cell types (INTACT) method provides an attractive solution. Although INTACT has been successfully applied to plants, flies, zebrafish, frogs, and mouse brain and adipose tissue, broad use across mammalian organs remains challenging. Here we describe the PAN-INTACT method, which can be used to isolate cell type specific nuclei from fibrous mouse organs, which are particularly problematic. As a proof-of-concept, we demonstrate successful isolation of cell type-specific nuclei from the mouse heart, which contains substantial connective tissue and harbors multiple cell types, including cardiomyocytes, fibroblasts, endothelial cells, and epicardial cells. Compared to established techniques, PAN-INTACT allows more rapid isolation of cardiac nuclei to facilitate downstream applications. We show cell type-specific isolation of nuclei from the hearts of Nkx2-5^Cre/+; R26^{Sun1-2xsf-GFP-6xmyc/+} mice, which we confirm by expression of lineage markers. Furthermore, we perform Assay for Transposase Accessible Chromatin (ATAC)-Seq to provide high-fidelity chromatin accessibility maps of Nkx2-5⁺ nuclei. To extend the applicability of PAN-INTACT, we also demonstrate successful isolation of Wt1⁺ podocytes from adult kidney. Taken together, our data suggest that PAN-INTACT is broadly applicable for profiling the transcriptional and epigenetic landscape of specific cell types. Thus, we envision that our method can be used to systematically probe mechanistic details of cell type-specific functions within individual organs of intact mice.

Using Gjd3-CreEGFP mice to examine atrioventricular node morphology and composition

The atrioventricular node (AVN) coordinates the timing of atrial and ventricular contraction to optimize cardiac performance. To study this critical function using mouse genetics, however, new reagents are needed that allow AVN-specific manipulation. Here we describe a novel Gjd3-CreEGFP mouse line that successfully recombines floxed alleles within the AVN beginning at E12.5. These mice have been engineered to express CreEGFP under the control of endogenous Gjd3 regulatory elements without perturbing native protein expression. Detailed histological analysis of Gjd3-CreEGFP mice reveals specific labeling of AVN cardiomyocytes and a subset of cardiac endothelial cells. Importantly, we show that Gjd3-CreEGFP mice have preserved cardiac mechanical and electrical function. In one application of our newly described mouse line, we provide a three-dimensional (3D) view of the AVN using tissue clearing combined with confocal microscopy. With this 3D model as a reference, we identify specific AVN sub-structures based on marker staining characteristics. In addition, we use our Gjd3-CreEGFP mice to guide microdissection of the AVN and construction of a single-cell atlas. Thus, our results establish a new transgenic tool for AVN-specific recombination, provide an updated model of AVN morphology, and describe a roadmap for exploring AVN cellular heterogeneity.

pouC Regulates Expression of bmp4 During Atrioventricular Canal Formation in Zebrafish

BACKGROUND

Many human gene mutations have been linked to congenital heart disease (CHD), yet CHD remains a major health issue worldwide due in part to an incomplete understanding of the molecular basis for cardiac malformation.

RESULTS

Here we identify the orthologous mouse Pou6f1 and zebrafish pouC as POU homeodomain transcription factors enriched in the developing heart. We find that pouC is a multi-functional transcriptional regulator containing separable activation, repression, protein-protein interaction, and DNA binding domains. Using zebrafish heart development as a model system, we demonstrate that pouC knockdown impairs cardiac morphogenesis and affects cardiovascular function. We also find that levels of pouC expression must be fine-tuned to enable proper heart formation. At the cellular level, we demonstrate that pouC knockdown disrupts atrioventricular canal (AVC) cardiomyocyte maintenance, although chamber myocyte specification remains intact. Mechanistically, we show that pouC binds a bmp4 intronic regulatory element to mediate transcriptional activation.

CONCLUSIONS

Taken together, our study establishes pouC as a novel transcriptional input into the regulatory hierarchy that drives AVC morphogenesis in zebrafish. We anticipate that these findings will inform future efforts to explore functional conservation in mammals and potential association with atrioventricular septal defects in humans. Developmental Dynamics 248:173-188, 2019. © 2018 Wiley Periodicals, Inc.

Abstract 106: Identifying Cardiac Conduction System-specific Enhancers That Link Cell-type Specification With Human Conduction Defects

The Cardiac Conduction System (CCS) is composed of a small set of specialized cardiomyocytes (CMs) that generate and propagate the electrical impulse required for contraction of the cardiac chambers. Genome Wide Association Studies (GWAS) have identified nearly fifty cardiac rhythm associated Single Nucleotide Polymorphisms (SNPs), and recent reports have characterized how selected SNPs impact CCS gene regulatory networks to influence cardiac conduction. However, a global view of CCS regulatory networks remains to be elucidated, and the vast majority of cardiac conduction associated SNPs remain uncharacterized. Such in-depth analyses have been hampered by our inability to specifically interrogate genome accessibility and RNA expression in the CCS. In order to identify genome-wide cis regulatory elements that correlate with CCS cell-type specification, we generated and rigorously characterized novel knock-in (KI) Cre driver lines that label the Atrio-Ventricular Node (AVN) and ventricular conduction system (VCS) to add to the limited CCS-specific genetic toolkit. In our study, we also used a previously characterized Shox2 KI-Cre driver to label the sinoatrial node (SAN), the primary pacemaker of the heart. Given a variety of issues associated with FACS and Laser Capture Microdissection, we optimized the INTACT (Isolation of Nuclei TAgged in specific Cell Types) method for mammalian heart tissue. To isolate cell-type-specific nuclei, we used Rosa26-Sun1-myc-sfGFP-Tag mice in combination with CCS-specific Cre driver lines. Following isolation and enrichment of CCS-specific nuclei, we performed Omni-ATAC-seq to identify open regions of chromatin (e.g. enhancer elements) and differentially enriched transcription factor motifs for individual CCS cellular sub-types without prior knowledge of cell-type specific regulators. Most cardiac transcriptomes in the literature do not distinguish between individual subtypes. Therefore, using bulk nuclear RNA-Seq, we also generated CCS-subtype specific transcriptomes. We anticipate that our datasets will provide an important starting point for establishing mechanistic links between CCS-specific enhancer elements and disease-associated SNPs identified by GWAS.

Phenotypically silent Cre recombination within the postnatal ventricular conduction system

The cardiac conduction system (CCS) is composed of specialized cardiomyocytes that initiate and maintain cardiac rhythm. Any perturbation to the normal sequence of electrical events within the heart can result in cardiac arrhythmias. To understand how cardiac rhythm is established at the molecular level, several genetically modified mouse lines expressing Cre recombinase within specific CCS compartments have been created. In general, Cre driver lines have been generated either by homologous recombination of Cre into an endogenous locus or Cre expression driven by a randomly inserted transgene. However, haploinsufficiency of the endogenous gene compromises the former approach, while position effects negatively impact the latter. To address these limitations, we generated a Cre driver line for the ventricular conduction system (VCS) that preserves endogenous gene expression by targeting the Contactin2 (Cntn2) 3’ untranslated region (3’UTR). Here we show that Cntn2^{3’UTR-IRES-Cre-EGFP/+} mice recombine floxed alleles within the VCS and that Cre expression faithfully recapitulates the spatial distribution of Cntn2 within the heart. We further demonstrate that Cre expression initiates after birth with preservation of native Cntn2 protein. Finally, we show that Cntn2^{3’UTR-IRES-Cre-EGFP/+} mice maintain normal cardiac mechanical and electrical function. Taken together, our results establish a novel VCS-specific Cre driver line without the adverse consequences of haploinsufficiency or position effects. We expect that our new mouse line will add to the accumulating toolkit of CCS-specific mouse reagents and aid characterization of the cell-autonomous molecular circuitry that drives VCS maintenance and function.

Induction of diverse cardiac cell types by reprogramming fibroblasts with cardiac transcription factors

Various combinations of cardiogenic transcription factors, including Gata4 (G), Hand2 (H), Mef2c (M) and Tbx5 (T), can reprogram fibroblasts into induced cardiac-like myocytes (iCLMs) in vitro and in vivo. Given that optimal cardiac function relies on distinct yet functionally interconnected atrial, ventricular and pacemaker (PM) cardiomyocytes (CMs), it remains to be seen which subtypes are generated by direct reprogramming and whether this process can be harnessed to produce a specific CM of interest. Here, we employ a PM-specific Hcn4-GFP reporter mouse and a spectrum of CM subtype-specific markers to investigate the range of cellular phenotypes generated by reprogramming of primary fibroblasts. Unexpectedly, we find that a combination of four transcription factors (4F) optimized for Hcn4-GFP expression does not generate beating PM cells due to inadequate sarcomeric protein expression and organization. However, applying strict single-cell criteria to GHMT-reprogrammed cells, we observe induction of diverse cellular phenotypes, including those resembling immature forms of all three major cardiac subtypes (i.e. atrial, ventricular and pacemaker). In addition, we demonstrate that cells induced by GHMT are directly reprogrammed and do not arise from an Nxk2.5(+) progenitor cell intermediate. Taken together, our results suggest a remarkable degree of plasticity inherent to GHMT reprogramming and provide a starting point for optimization of CM subtype-specific reprogramming protocols.

Modulation of recombinant, α2*, α3* or α4*-nicotinic acetylcholine receptor (nAChR) function by nAChR β3 subunits

The nicotinic acetylcholine receptor (nAChR) β3 subunit is thought to serve an accessory role in nAChR subtypes expressed in dopaminergic regions implicated in drug dependence and reward. When β3 subunits are expressed in excess, they have a dominant-negative effect on function of selected nAChR subtypes. In this study, we show, in Xenopus oocytes expressing α2, α3 or α4 plus either β2 or β4 subunits, that in the presumed presence of similar amounts of each nAChR subunit, co-expression with wild-type β3 subunits generally (except for α3*-nAChR) lowers amplitudes of agonist-evoked, inward peak currents by 20-50% without having dramatic effects (≤ 2-fold) on agonist potencies. By contrast, co-expression with mutant β3(V9'S) subunits generally (except for α4β2*-nAChR) increases agonist potencies, consistent with an expected gain-of-function effect. This most dramatically demonstrates formation of complexes containing three kinds of subunit. Moreover, for oocytes expressing nAChR containing any α subunit plus β4 and β3(V9'S) subunits, there is spontaneous channel opening sensitive to blockade by the open channel blocker, atropine. Collectively, the results indicate that β3 subunits integrate into all of the studied receptor assemblies and suggest that natural co-expression with β3 subunits can influence levels of expression and agonist sensitivities of several nAChR subtypes.

Stimulation of murine intestinal secretion by daily genistein injections: gender-dependent differences

BACKGROUND/AIMS

The effect of daily injections with genistein (naturally occurring phytoestrogen) on intestinal chloride (Cl(-)) secretion was measured with Ussing chamber short circuit current (I(sc), μA/cm(2)), in C57BL/6J male and female mice, using 600 mg/kg genistein/day (600G), 300 mg/kg genistein/day (300G), 150 mg/kg genistein/day (150G) or genistein-free vehicle control (0G) for 1- or 2-weeks.

METHODS AND RESULTS

Injecting with 600G elicited significant increases in basal I(sc) in females after 1-week (ñ70 μA/cm(2), n=15, p < 0.05) and in males after 2-weeks (ñ80 μA/cm(2), n=5, p < 0.05) compared to their 0G counterparts. Chloride-free ringer significantly reduced basal I(sc) by 65% in 600G males and 72% in 600G females, suggesting that Cl(-) was the major anion comprising the genistein-stimulated secretion. The forskolin-stimulated (10 μM) I(sc) was significantly inhibited by the CFTR chloride channel inhibitors, glibenclamide (500 μM) and CFTR(inh)-172 (100 μM) in 600G males and females, suggesting some contribution by genistein-dependent CFTR-mediated Cl(-) secretion. We found no associated changes in intestinal morphology, nor change in total CFTR protein with 600G. There was a 5% increase in apical/subapical ratio in 600G males compared to controls (no change in females).

CONCLUSION

These data suggest that male and female mice both exhibit increased Cl- secretion with 600G, however, the mechanisms mediating this are gender-dependent.

Identification of N-terminal extracellular domain determinants in nicotinic acetylcholine receptor (nAChR) α6 subunits that influence effects of wild-type or mutant β3 subunits on function of α6β2*- or α6β4*-nAChR

Despite the apparent function of naturally expressed mammalian α6*-nicotinic acetylcholine receptors (α6*-nAChR; where * indicates the known or possible presence of additional subunits), their functional and heterologous expression has been difficult. Here, we report that coexpression with wild-type β3 subunits abolishes the small amount of function typically seen for all-human or all-mouse α6β4*-nAChR expressed in Xenopus oocytes. However, levels of function and agonist potencies are markedly increased, and there is atropine-sensitive blockade of spontaneous channel opening upon coexpression of α6 and β4 subunits with mutant β3 subunits harboring valine-to-serine mutations at 9'- or 13'-positions. There is no function when α6 and β2 subunits are expressed alone or in the presence of wild-type or mutant β3 subunits. Interestingly, hybrid nAChR containing mouse α6 and human (h) β4 subunits have function potentiated rather than suppressed by coexpression with wild-type hβ3 subunits and potentiated further upon coexpression with hβ3(V9'S) subunits. Studies using nAChR chimeric mouse/human α6 subunits indicated that residues involved in effects seen with hybrid nAChR are located in the α6 subunit N-terminal domain. More specifically, nAChR hα6 subunit residues Asn-143 and Met-145 are important for dominant-negative effects of nAChR hβ3 subunits on hα6hβ4-nAChR function. Asn-143 and additional residues in the N-terminal domain of nAChR hα6 subunits are involved in the gain-of-function effects of nAChR hβ3(V9'S) subunits on α6β2*-nAChR function. These studies illuminate the structural bases for effects of β3 subunits on α6*-nAChR function and suggest that unique subunit interfaces involving the complementary rather than the primary face of α6 subunits are involved.

Testis and somatic Xrcc-1 DNA repair gene expression

The human XRCC1 gene has been shown to be involved in DNA strand-break repair using the Chinese hamster ovary cell mutant EM9. The purpose of this study was to characterize the expression of Xrcc-1 to determine if there is tissue-specific expression and to provide a baseline of information for future studies that may involve altering Xrcc-1 expression in mice. Normal young adult male testis and enriched populations of pachytene spermatocytes and round spermatids displayed significantly higher levels of Xrcc-1 expression than other mouse tissues, although Xrcc-1 transcripts were found in low abundance in all tested tissues. Cultured mouse cell lines displayed levels of expression similar to male germ cells, which is a striking contrast to the levels of expression obtained in somatic tissues from the mouse. The relatively high levels of expression identified in male germ cells indicate Xrcc-1 may have an important role in male germ cell physiology.

Brain and liver targeted overexpression of O6-methylguanine DNA methyltransferase in transgenic mice

O6-Methylguanine DNA methyltransferase (MGMT; EC 2.1.1.63) is an unusual DNA repair protein in that it directly and specifically repairs a premutagenic DNA lesion without involving other proteins. MGMT removes the alkyl group from O6-alkylguanine in DNA in a unique stoichiometric reaction by accepting the alkyl group on a cysteine residue. The intracellular level of MGMT varies among tissues and appears to be inversely correlated to tissue-specific tumorigenesis induced by monofunctional alkylating agents. Because MGMT acts in solo, genetic manipulation of its expression may provide valuable insight into its contribution to cellular resistance to alkylation toxicity and to tumor induction. The human MGMT full length cDNA has been fused with a portion of the human transferrin (TF) 5'-flanking region (TF/MGMT). Transgenic founder mice were produced carrying the TF/MGMT transgene and then bred to establish stable transgenic lines. Human MGMT transcripts were specifically expressed in abundance in transgenic brain and liver tissues. In vitro MGMT assays revealed approximately 150-fold and approximately 25-fold increases in MGMT activity in transgenic brain and liver extracts respectively. Western blot analysis confirmed that human MGMT protein is specifically synthesized in transgenic brain and liver tissues.

Demonstration of two forms of vitellogenin in serum of estradiol-17 beta-treated Indian major carp, Labeo rohita

Vitellogenin (Vg) synthesis was induced in the male and non-vitellogenic female Rohu, the Indian major carp, by estradiol-17 beta(E2) where effect was more in female. A crude preparation of Vg was isolated in the second peak after gel filtration on Ultrogel AcA 34 from the sera of vitellogenic female Rohu and E2-treated male and female Rohu. Estimation of alkali-labile phosphorus was shown to be used as an index of Vg. Native-PAGE analysis has revealed the presence of two forms of Vg (Vg1: 430,000 dalton and Vg2:240,000 dalton) in Vg fraction obtained after gel filtration as well as in the sera of E2-treated male and female Rohu. Immunological cross-reaction studies between antiserum to yolk protein and Vg fractions as well as the sera from E2-treated male and female Rohu further indicates the presence of two precipitin lines (not clearly visible as the two lines fused to form a thick line) suggesting the occurrence of two forms of Vg in the Rohu.

Clonogenic assay for Wilms' tumor: improved technique for obtaining single-cell suspensions and evidence for tumor cell specificity

Wilms' tumors from seven patients were dissociated by mechanical and enzymatic means; this technique resulted in single-cell suspensions for five specimens and a few aggregates for two. By dye exclusion, cell viability ranged from 56 to 100% (median, 92%). All seven preparations produced more than five colonies/2 x 10(5) cells plated. Forty-three colonies grown from cells of a glucose-6-phosphate dehydrogenase heterozygote were of the same glucose-6-phosphate dehydrogenase isoenzyme type as the original tumor, indicating that the assay is specific for tumor cells. We attribute the high rate of colony formation to an improved method of cell preparation (combined mechanical and enzymatic dissociation of tumors) which may be applicable to other primary human tumors assayed in the soft agar system.