Genetically engineered mice for combinatorial cardiovascular optobiology

Optogenetic effectors and sensors provide a novel real-time window into complex physiological processes, enabling determination of molecular signaling processes within functioning cellular networks. However, the combination of these optical tools in mice is made practical by construction of genetic lines that are optically compatible and genetically tractable. We present a new toolbox of 21 mouse lines with lineage-specific expression of optogenetic effectors and sensors for direct biallelic combination, avoiding the multiallelic requirement of Cre recombinase -mediated DNA recombination, focusing on models relevant for cardiovascular biology. Optogenetic effectors (11 lines) or Ca²⁺ sensors (10 lines) were selectively expressed in cardiac pacemaker cells, cardiomyocytes, vascular endothelial and smooth muscle cells, alveolar epithelial cells, lymphocytes, glia, and other cell types. Optogenetic effector and sensor function was demonstrated in numerous tissues. Arterial/arteriolar tone was modulated by optical activation of the second messengers InsP₃ (optoα1AR) and cAMP (optoß2AR), or Ca²⁺-permeant membrane channels (CatCh2) in smooth muscle (Acta2) and endothelium (Cdh5). Cardiac activation was separately controlled through activation of nodal/conducting cells or cardiac myocytes. We demonstrate combined effector and sensor function in biallelic mouse crosses: optical cardiac pacing and simultaneous cardiomyocyte Ca²⁺ imaging in Hcn4^BAC-CatCh2/Myh6-GCaMP8 crosses. These experiments highlight the potential of these mice to explore cellular signaling in vivo, in complex tissue networks.

Intravital Microscopy of the Beating Murine Heart to Understand Cardiac Leukocyte Dynamics

Cardiovascular disease is the leading cause of worldwide mortality. Intravital microscopy has provided unprecedented insight into leukocyte biology by enabling the visualization of dynamic responses within living organ systems at the cell-scale. The heart presents a uniquely dynamic microenvironment driven by periodic, synchronous electrical conduction leading to rhythmic contractions of cardiomyocytes, and phasic coronary blood flow. In addition to functions shared throughout the body, immune cells have specific functions in the heart including tissue-resident macrophage-facilitated electrical conduction and rapid monocyte infiltration upon injury. Leukocyte responses to cardiac pathologies, including myocardial infarction and heart failure, have been well-studied using standard techniques, however, certain questions related to spatiotemporal relationships remain unanswered. Intravital imaging techniques could greatly benefit our understanding of the complexities of in vivo leukocyte behavior within cardiac tissue, but these techniques have been challenging to apply. Different approaches have been developed including high frame rate imaging of the beating heart, explantation models, micro-endoscopy, and mechanical stabilization coupled with various acquisition schemes to overcome challenges specific to the heart. The field of cardiac science has only begun to benefit from intravital microscopy techniques. The current focused review presents an overview of leukocyte responses in the heart, recent developments in intravital microscopy for the murine heart, and a discussion of future developments and applications for cardiovascular immunology.

Abstract 612: Automated Analysis of Displacement From Intravital Multiphoton Microscopy in Mouse Ventricle

Background: Multiphoton microscopy (MPM) has enabled in vivo time-lapse imaging of the heart that shows motion of cells within the tissue with micrometer resolution. We developed automated analysis techniques to quantify cellular motion from in vivo cardiac MPM images throughout the cardiac cycle.

Methods: Intravital cardiac MPM of the beating mouse heart was performed on 26 week-old, C57Bl6 mice (n=6). Image volumes (100 μm deep) were acquired at 30 frames per second while recording the electrocardiogram and respiratory pressure. An image volume was reconstructed by assembling lines acquired nearest to a specified point in the cardio-respiratory phase space (Fig. a). Motion was calculated as the three-dimensional transformation required to register the reconstructed images to the image at the most stable cardiac phase.

Results: Volumes were reconstructed in 50 intervals across the cardiac phase that show vasculature (intravenous Texas-red dextran, red) and cardiomyocytes (rhodamine 6G, cyan) moving across the field of view (Fig. b). Automated analysis indicated a maximum displacement occurring at 16 % (anterior-posterior), 36 % (base-apex) and 38 % (epi-endocardial) of the cardiac cycle defined by R-wave. Comparison by manual tracking of features across the cardiac cycle at a subset of phases (10 cardiac phases at peak exhalation) validated the automated measurement (Fig. c). Automated motion tracking shows superior performance in the spatial resolution and speed of analysis.

Conclusions: We have shown a novel, fast, and accurate technique for characterizing cardiac motion from in vivo cardiac MPM to study the performance of the contractile cells in health and disease.

Abstract 227: Intravital Multiphoton Microscopy Reveals Increased Capillary Patrolling by Leukocytes and Cardiomyocyte Dysfunction in High Fat Diet Induced Hypertrophy

Background: The study of functional cardiomyocyte adaptation and inflammatory cell behavior at the micro-scale in vivo has been challenging due to limited imaging tools. We recently developed intravital multiphoton microscopy (MPM) methods that enable visualization and quantification of cardiac dynamics at a cell-scale throughout the cardiac cycle. We aimed to determine the dynamic cellular changes that occur due to high fat diet (HFD) induced hypertrophy using intravital cardiac MPM.

Methods: ApoE -/- C57Bl6 mice started a HFD at 6 weeks of age (ApoE -/- -HFD, n=11), while age-matched wild-type mice (WT-ND, n=10) were fed a normal chow diet. At 26-weeks, mice were assessed by cardiac echocardiography and intravital MPM in the intact beating heart. Intravenous injections of rhodamine-6G (R6g) labeled cardiomyocytes and leukocytes, and Texas-Red dextran labeled vasculature. 3D volumes were reconstructed throughout the cardiac cycle to quantify cell motion using automated algorithms.

Results: ApoE -/- -HFD hearts underwent hypertrophy compared to WT-ND with increased heart weight-to-tibial length ratio (10±0.8 vs 13±1.1) and left ventricle wall thickness (1.07±0.03 mm vs 1.13±0.06 mm, respectively, p<0.05 for both) while ejection fraction remained similar (66±3 % vs 59±3 %). In vivo MPM demonstrated that cells move a greater total distance in each cardiac cycle in ApoE -/- -HFD vs WT-ND. Maximum displacement in the apex-base and anterior-posterior directions increased by 46 % in ApoE -/- -HFD compared to WT-ND (30 μm vs 14 μm). R6g+ leukocytes were visible moving in capillaries. The incidence of patrolling behavior (defined as slowing moving cells, visible for longer than one heart beat) increased in capillaries of ApoE -/- -HFD compared to WT-ND (3.4±0.5/min vs 0.12±0.1/min, p<0.01).

Conclusion: These results suggest that hypertrophied cardiomyocytes increase myocardial displacement, and increased leukocyte patrolling behavior is associated with HFD induced cardiac hypertrophy. Intravital cardiac MPM provides a novel perspective to study HFD induced cardiac hypertrophy by capturing the simultaneous contributions of inflammatory cells and myocyte function in the beating heart.