Modeling Human Pacemaker Maturation with SAN-Cardiac Plexus Assembloids

Study Background and Research Question

The sinoatrial node (SAN) is the heart's primary pacemaker, orchestrating rhythmic contraction by generating spontaneous electrical impulses. Its function is intricately modulated by autonomic neural inputs, especially from the cardiac plexus, which fine-tune heart rate and pacemaker dominance. Despite its centrality to cardiac physiology and disease, experimental models capable of recapitulating human neuro-cardiac interactions remain limited. Existing in vitro systems, such as SAN-like organoids from human pluripotent stem cells (hPSCs), often lack the three-dimensional organization and complex innervation observed in vivo. This deficit hampers deeper exploration of pacemaker maturation, conduction disorders, and potential regenerative therapies.

To address these challenges, the reference study (Cell Stem Cell, 2026) set out to engineer a human model system that faithfully recapitulates the spatial, molecular, and electrophysiological features of the SAN and its interactions with intrinsic cardiac neurons. The core research question was: Can a PSC-derived assembloid integrating SAN and cardiac plexus organoids serve as a functional platform for dissecting neuron-pacemaker crosstalk and maturation?

Key Innovation from the Reference Study

The study's principal innovation is the development of SAN-plexus assembloids: composite 3D structures generated by combining hPSC-derived SAN organoids (SANOs) with cardiac ganglionated plexus organoids (CGPOs). In select experiments, atrial-like cardiac organoids were incorporated to model pacemaker-to-atrial conduction. This tri-assembloid system captures the native-like architecture, cellular heterogeneity, and functional interplay between pacemaker cells and their neural regulators.

Crucially, the authors leveraged spatial transcriptomics of human SAN tissue and functional assays in assembloids to identify a neuron-to-pacemaker signaling axis. They demonstrated that CGPO-derived prosaposin (PSAP) activates the SAN-enriched GPR37 receptor, promoting pacemaker maturation—a molecular mechanism previously inaccessible to in vitro studies.

Methods and Experimental Design Insights

The research employed a multi-step differentiation protocol to generate three organoid types from hPSCs: SANOs (recapitulating pacemaker cell heterogeneity), CGPOs (mimicking intrinsic cardiac neurons), and atrial-like cardiac organoids. These were assembled into spatially organized constructs, fostering direct cell-cell interactions.

Electrophysiological analyses, including microelectrode array recordings and optical mapping, assessed spontaneous electrical activity, conduction properties, and response to neural stimulation. Spatial transcriptomics and single-cell RNA sequencing characterized the molecular landscape and identified key signaling pathways activated during maturation. Integration with human SAN spatial data anchored the assembloid findings to native tissue organization.

Core Findings and Why They Matter

The SAN-plexus assembloids demonstrated robust molecular and functional hallmarks of human pacemaker tissue, including:

• Distinct SAN pacemaker cell subtypes (head, tail, and transitional zones) with region-specific marker expression (e.g., SHOX2, ISL1, TBX3, HCN4).
• 3D anatomical coupling between pacemaker and atrial-like regions, enabling assessment of conduction dynamics.
• Intrinsic neural plexus integration, allowing the study of neuro-modulation and disease-associated conduction dysfunction in a human context.

Most notably, functional interrogation of the assembloids revealed that CGPO-derived prosaposin engages GPR37 on SAN cells, triggering transcriptional programs essential for pacemaker maturation and stability. This neuron-to-pacemaker signaling axis was corroborated by parallel spatial transcriptomic analyses of human SAN tissue.

By recapitulating both the structural complexity and neuro-cardiac crosstalk of the human SAN, this platform overcomes key limitations of animal models and simpler in vitro systems, offering new avenues for investigating congenital SAN dysfunction, arrhythmias, and regenerative strategies. The ability to interrogate neural modulation in a physiologically relevant setting is of particular importance for translational cardiac research.

Comparison with Existing Internal Articles

While the reference study targets cardiac pacemaker maturation via neuro-cardiac assembloids, several internal articles—such as "Cucurbitacin I (JSI-124): Protocols for STAT3 Pathway Inhibition"—focus on selective STAT3 inhibition in cancer research. These resources detail workflows for dissecting cell proliferation, invasion, and chemosensitivity using advanced inhibitors like Cucurbitacin I (JSI-124). For example, "Optimizing STAT3 Inhibition Workflows" offers actionable protocol enhancements relevant to tumor biology.

Although the core focus of these internal articles lies in cancer biology, they collectively highlight the value of human organoid models and precise pathway modulation in dissecting complex cellular interactions. The current reference study extends this paradigm to the cardiovascular field, using assembloids to model neural regulation of cardiac function—paralleling the sophistication and translational promise seen in advanced cancer organoid systems.

Limitations and Transferability

Despite its considerable strengths, the SAN-plexus assembloid platform has certain limitations. First, while the system captures major features of human SAN and neural modulation, the complexity of in vivo tissue architecture, long-term maturation, and inter-organ interactions may not be fully recapitulated. Second, the scalability and reproducibility of assembloid generation across multiple hPSC lines and laboratories remain to be systematically validated. Finally, disease modeling is currently limited to intrinsic conduction dysfunction and does not yet encompass the full spectrum of acquired arrhythmias or pharmacological modulation.

Nevertheless, the approach offers a powerful and adaptable toolkit for mechanistic studies of human pacemaker biology, with opportunities for adaptation to personalized medicine and drug discovery applications.

Protocol Parameters

• SAN organoid differentiation: Sequential patterning of hPSCs using stage-specific growth factors to induce TBX18+, ISL1+ progenitors and generate heterogeneous pacemaker subtypes.
• Cardiac plexus organoid (CGPO) generation: Directed differentiation of neural crest-like cells, followed by co-culture with cardiac organoids to promote neural integration.
• Assembloid assembly: Spatially organized fusion of SANO, CGPO, and atrial-like organoids, maintained in 3D culture for at least 7-14 days to permit maturation and cell-cell interactions.
• Electrophysiological assessment: Use of microelectrode array or optical mapping to evaluate pacemaker activity, conduction velocity, and neural modulation.
• Transcriptomic analysis: Integration of single-cell RNA sequencing and spatial transcriptomics to map maturation and signaling pathways.

Research Support Resources

For researchers aiming to dissect signaling pathways in advanced human organoid or assembloid systems—including those investigating proliferation, invasion, or neuro-cardiac modulation—selective pathway inhibitors remain essential workflow tools. Cucurbitacin I (JSI-124, SKU A4512) is widely used for STAT3 pathway inhibition in cancer models, facilitating studies of cell proliferation, migration, and therapy response, as noted in the internal literature. Its robust selectivity and well-characterized protocols support reproducible experimental outcomes. While its primary use is in oncology, the principled application of such inhibitors can inform parallel strategies in organoid-based models of complex tissue interactions, provided their mechanistic relevance is established for the system of interest.