Modeling Human SAN-Plexus Interactions via PSC-Derived Assembloids

Study Background and Research Question

The sinoatrial node (SAN) is the heart's primary pacemaker, responsible for initiating and regulating each heartbeat through rhythmic electrical impulses. This system’s output is dynamically modulated by intrinsic cardiac neural inputs, especially those from the right atrial ganglionated plexus (RAGP), which influence both heart rate and the anatomical site of pacemaker activity. While animal studies have provided important insights into SAN formation and function, significant interspecies differences in electrophysiology and autonomic regulation limit the translation of these findings to human biology. Moreover, direct studies of the human SAN are hampered by limited tissue availability and the node’s complex three-dimensional organization. As a result, in vitro models that accurately recapitulate the intricate neuro-cardiac interactions in human tissue are needed to advance our understanding of pacemaker development, disease mechanisms, and autonomic regulation.

Key Innovation from the Reference Study

The reference study by Zhang et al. (Cell Stem Cell, 2026) presents a significant methodological advance by generating human SAN-plexus assembloids. These assembloids are constructed by integrating human pluripotent stem cell (hPSC)-derived SAN organoids (SANOs) with cardiac ganglionated plexus organoids (CGPOs) and, in an extended tri-assembloid configuration, with atrial-like cardiac organoids. This platform captures spatial, molecular, and electrophysiological characteristics of the human SAN, enabling the study of neuron-to-pacemaker signaling and functional neural modulation of pacemaker automaticity.

Methods and Experimental Design Insights

The researchers used a multi-organoid fusion approach to assemble SANOs and CGPOs, generating a two-component SAN-plexus assembloid. For in-depth evaluation of pacemaker-to-atrial conduction, a third component—atrial-like cardiac organoids—was incorporated, resulting in a tri-assembloid system. Each organoid was differentiated from hPSCs using established protocols that recapitulate developmental signaling pathways. The SANOs were enriched for pacemaker cell subtypes characterized by transcriptional markers (SHOX2, ISL1, TBX3) and pacemaker-associated ion channels (HCN4), while CGPOs modeled intrinsic cardiac neurons. Spatial transcriptomics and single-cell RNA sequencing were employed to compare the assembloid components to human SAN tissue, validating the fidelity of the model at the molecular level. Electrophysiological recordings—including optical mapping—enabled assessment of pacemaker activity, conduction properties, and neural modulation.

A particularly novel aspect involved functional experiments to dissect neuron-to-pacemaker signaling. The study identified prosaposin (PSAP) secretion from CGPOs and its interaction with the GPR37 receptor, which is enriched in SAN-like pacemaker cells, as a key pathway promoting pacemaker maturation. This was confirmed using loss-of-function and gain-of-function perturbations in the assembloid system, linking molecular signaling to electrophysiological outcomes.

Core Findings and Why They Matter

The SAN-plexus assembloid model successfully recapitulated several critical features of human SAN biology:

• Spatial and molecular organization of pacemaker subtypes, including head, tail, and transitional regions, mirroring the heterogeneity observed in human tissue.
• Functional coupling between SAN and atrial components, enabling the study of pacemaker-to-atrial conduction and arrhythmic phenotypes.
• Demonstration of intrinsic neural modulation: CGPO-derived neurons modulated SAN automaticity and conduction, revealing mechanisms of autonomic control and potential disease susceptibility.
• Identification of the PSAP-GPR37 signaling axis as a critical neuro-cardiac pathway for promoting pacemaker maturation, supported by integrated spatial transcriptomics and functional perturbation data (reference study).

These findings are important because they establish a scalable, human-based platform for mechanistic studies of neuro-cardiac crosstalk, SAN development, and conduction disorders—areas where existing animal models and simple monolayer cultures fall short. The model also enables exploration of genetic or pharmacologic interventions in a context that closely matches human physiology, offering potential for disease modeling and preclinical drug testing.

Comparison with Existing Internal Articles

For researchers interested in beta-adrenergic receptor signaling or GPCR pathways within cardiovascular models, the internal article "Isoproterenol sulfate dihydrate: Reliable Beta-Adrenergic Signaling in Human Cell Models" provides practical insights into optimizing cell-based assays with high-fidelity agonists. While Zhang et al. focus on neuro-cardiac interactions and intrinsic signaling, the internal article addresses experimental challenges such as solubility, reproducibility, and data quality when using compounds like Isoproterenol hemisulfate in cardiovascular research. Together, these resources illustrate how advanced human cell models and well-characterized beta-adrenergic agonists synergize to dissect signaling pathways that control heart rhythm and contractility.

Limitations and Transferability

Despite its robust design, the SAN-plexus assembloid system has limitations. As with all organoid-based models, there are constraints in recapitulating the full tissue complexity and long-term maturation seen in vivo. While spatial transcriptomics and single-cell profiling confirm high fidelity to human SAN tissue, the model may not fully capture all aspects of autonomic regulation or chronic disease progression. Furthermore, the reliance on hPSC differentiation protocols introduces variability between batches and cell lines. Transferability to other cardiac or neurological disease contexts should be approached with caution, as the system is optimized for studying SAN-specific neuro-cardiac interactions and may require further adaptation for broader applications.

Protocol Parameters

• SAN organoid differentiation: Initiate from hPSCs using sequential modulation of WNT, BMP, and retinoic acid signaling; duration and concentrations should align with published protocols for pacemaker cell enrichment.
• CGPO generation: Differentiate neural crest-derived progenitors and aggregate with SANOs to form functional plexus innervation; validate neuronal markers before fusion.
• Tri-assembloid assembly: Fuse SANO, CGPO, and atrial-like organoids in low-adhesion plates; co-culture for at least 7–14 days to support organization and functional maturation.
• Electrophysiology assays: Employ optical mapping or patch clamp to assess spontaneous depolarization, conduction velocity, and response to neural modulation.
• Pharmacological modulation: For studying beta-adrenergic responses, apply Isoproterenol sulfate dihydrate at literature-supported concentrations (typically 0.1–10 μM) to evaluate cAMP/PKA pathway activation and rate changes; always confirm batch solubility and stability prior to use.
• Transcriptomics validation: Perform single-cell RNA sequencing or spatial transcriptomics to compare assembloid cell states to primary human SAN tissue.

Research Support Resources

To enable precise interrogation of beta-adrenergic receptor signaling in human SAN or organoid models, researchers can incorporate Isoproterenol sulfate dihydrate (SKU C6402), a high-purity non-selective beta-adrenergic agonist validated for GPCR research and cAMP/PKA pathway modulation. The product’s solubility and stability profiles, as described in the product information, make it suitable for acute assays in sensitive cardiovascular systems. For additional guidance on optimizing protocols and troubleshooting experimental challenges with Isoproterenol hemisulfate, consult the internal scenario-based article linked above. Integrating well-characterized reagents with advanced assembloid systems will support rigorous, reproducible research in human cardiac signaling.