Abstract Body (Do not enter title and authors here): Background: Tachycardia can drive heart failure (HF). Traditionally, this is experimentally modeled using an implantable pacemaker, an invasive, technically demanding, and low-throughput methodology. Genetically-expressed light-activated ion channels (‘optogenetics’) provide an alternative approach for in vivo cardiac stimulation, which if applied in transparent zebrafish embryos, may be used for non-invasive, cost-effective, high-throughput HF and drug screening studies.

Objective: Establish a non-invasive in vivo experimental model of HF using optogenetic tachypacing in larval zebrafish.

Methods: Larval zebrafish expressing cation-nonspecific channelrhodopsin-2 (ChR2), chloride-specific anion channelrhodopsin-1 (ACR1), or green fluorescent protein (GFP, as a light-exposed, non-paced control) in cardiomyocytes were exposed to intermittent tachypacing (15s on, 15s off) by programmed light pulses from 2-7 days post fertilization (dpf). At 7 dpf: (i) in vivo atrial and ventricular mechanical function was assessed by end-diastolic area (EDA), end-systolic area (ESA), and ejection fraction (EF) with video microscopy; (ii) cardiac structure was measured by atrial and ventricular area in fixed larvae with fluorescent imaging; and (iii) gene expression of cardiac-specific HF biomarkers (atrial and brain natriuretic peptide [anp, bnp], atrial- and ventricular-specific myosin heavy chain-a and -b [myh6, myh7], and a-smooth muscle actin [acta2]) was quantified by qPCR. Values across groups were compared by one-way ANOVA, with Šídák post hoc tests.

Results: In vivo, atrial and ventricular EDA were greater in ChR2 (+191%, p<0.0001; +66%, p<0.0001) and ACR1 (+58%, p=0.005; +40%, p=0.0004) hearts compared to control, as was ESA (ChR2: +245%, p<0.0001; +67%, p<0.0001 and ACR1: +89%, p<0.0001; +53%, p<0.0001). Atrial EF was lower in ChR2 (-26%, p=0.0006) and ACR1 (-26%, p=0.0032) hearts, while ventricular EF was unchanged. Atrial and ventricular area were also greater in fixed ChR2 hearts (+191%, p<0.0001; +35%, p=0.0005), while only atrial area was greater in ACR1 hearts (+55%, p=0.0236). Expression of anp, bnp, and myh6 were greater in both ChR2 (11x, 22x, 58x; p<0.05 for each) and ACR1 (6x, 4x, 15x; p<0.05 for each) hearts.

Conclusion: Intermittent optogenetic-tachypacing with either ChR2 or ACR1 induces HF in larval zebrafish, which may represent novel experimental models for insight into HF mechanisms and as pre-clinical tools for the development of new HF therapies.