Human PSC-Derived Sinoatrial Node–Cardiac Plexus Assembloids: Advancing In Vitro Models of Pacemaker Maturation

Study Background and Research Question

The sinoatrial node (SAN) is the primary pacemaker of the heart, initiating electrical impulses that regulate cardiac rhythm. Its output is tightly controlled by neuro-cardiac interactions, particularly via autonomic innervation arising from ganglionated plexuses embedded within the atrial myocardium. While animal studies have elucidated several principles of SAN development and neural modulation, significant interspecies differences in electrophysiology and neural control, coupled with limited access to human SAN tissue, have hindered mechanistic understanding in human systems. Existing in vitro models—such as human pluripotent stem cell (hPSC)-derived pacemaker cells and cardiac organoids—have not captured the three-dimensional (3D) complexity or the intrinsic neural interactions characteristic of the human SAN. The central research question addressed by Zhang et al. (reference study) is how to develop a human in vitro model system that recapitulates the maturation and functional modulation of the SAN by its associated neural plexus, enabling detailed analysis of neuro-cardiac signaling crucial for both physiological and pathological pacemaker function.

Key Innovation from the Reference Study

The principal innovation reported in the reference paper is the creation of SAN-plexus “assembloids” by integrating hPSC-derived SAN organoids (SANOs), cardiac ganglionated plexus organoids (CGPOs), and atrial-like cardiac organoids into a tri-component system. This 3D assembloid platform models the anatomical and functional coupling between pacemaker cells and their intrinsic neural circuit, providing a robust environment to study neuro-cardiac crosstalk. Critically, the authors leverage spatial transcriptomics from human SAN tissue to validate the molecular fidelity of their assembloid models and to identify specific neuron-to-pacemaker signaling pathways—most notably, the prosaposin (PSAP)-GPR37 axis—as drivers of SAN maturation. This approach enables the interrogation of both normal and disease-associated conduction phenomena in a human-relevant context.

Methods and Experimental Design Insights

The study employs a multi-step differentiation protocol to generate three key organoid types from hPSCs: (1) SANOs, recapitulating major pacemaker cell subtypes and spatial organization; (2) CGPOs, modeling the intrinsic cardiac neural plexus; and (3) atrial-like cardiac organoids for assessing conduction to working myocardium. The organoids are co-cultured in defined ratios and spatially assembled to mimic the SAN–plexus–atrium axis. Functional maturation and neuro-cardiac coupling are assessed using a combination of techniques, including:

• Electrophysiology: Optical mapping and patch-clamp recordings to measure pacemaker automaticity, action potential characteristics, and conduction velocity.
• Molecular Profiling: Single-cell RNA sequencing and spatial transcriptomics to compare assembloid cell states with native human SAN tissue.
• Functional Perturbation: Pharmacological modulation and genetic manipulation (e.g., knockdown of GPR37) to dissect neuron-to-pacemaker signaling pathways.
• Immunohistochemistry: Analysis of structural organization, marker expression (e.g., SHOX2, ISL1, HCN4 for pacemaker cells; neuronal markers for CGPOs), and tissue architecture.

This integrative approach enables the authors to interrogate how neural signals modulate pacemaker cell maturation, output, and coupling to downstream atrial tissue.

Protocol Parameters

• SAN organoid differentiation: Initiate with hPSCs using mesodermal and cardiac lineage induction factors, followed by exposure to signaling cues (e.g., retinoic acid) guiding SAN lineage specification.
• CGPO generation: Differentiate hPSCs towards neural crest and cardiac autonomic lineages, utilizing stage-specific growth factors and neurotrophic support.
• Assembloid assembly: Co-culture SANOs and CGPOs (typically in a 2:1 ratio) for at least 7–14 days, with subsequent addition of atrial-like organoids for conduction analyses.
• Electrophysiological assessment: Perform optical mapping at baseline and following pharmacological stimulation (e.g., beta-adrenergic agonists) or neural input perturbation.
• Spatial transcriptomics: Integrate assembloid and native SAN tissue data using high-resolution spatial profiling platforms to validate molecular signatures.

Core Findings and Why They Matter

The SAN-plexus assembloid system recapitulates several key features of human pacemaker biology:

• Molecular fidelity: The assembloids reproduce the regional transcriptional heterogeneity observed in native SAN, including canonical pacemaker transcription factors (SHOX2, ISL1, TBX3) and ion channel expression (HCN4).
• Functional neuro-cardiac coupling: Integration of CGPOs into the assembloid enhances spontaneous pacemaker activity, increases conduction velocity to atrial-like organoids, and enables dynamic modulation of leading pacemaker sites in response to neural signals.
• Neuron-to-pacemaker signaling: Combining spatial transcriptomics and functional assays, the study identifies a key role for CGPO-derived prosaposin acting through the GPR37 receptor, which is enriched in SAN cells and promotes electrophysiological maturation. Genetic or pharmacological disruption of this pathway impairs pacemaker function and conduction.
• Modeling disease: The platform supports interrogation of conduction disorders by introducing mutations or perturbing neural input, enabling mechanistic studies of congenital SAN dysfunctions that are otherwise challenging in human tissue.

These findings provide a flexible, human-relevant system for dissecting the cellular and molecular basis of SAN development, neural regulation, and disease, with implications for both basic cardiovascular research and translational modeling of arrhythmias.

Comparison with Existing Internal Articles

Several recent internal studies have contributed to the development and application of human PSC-derived SAN-plexus assembloid models. The article "Modeling Human SAN-Neural Maturation via PSC-Derived Assembloids" outlines the foundational strategy for integrating pacemaker and neural organoids, emphasizing the platform's utility for mapping neuron-to-pacemaker communication. Similarly, "Modeling Human SAN-Plexus Interactions via PSC-Derived Assembloids" details protocols for establishing functional connections and highlights applications in modeling conduction disorders. The present reference study extends these frameworks by leveraging spatial transcriptomic validation and delineating the PSAP-GPR37 signaling axis, thereby providing a higher-resolution mechanistic view. Compared to earlier work, the new assembloid model offers improved structural fidelity, more precise molecular characterization, and expanded functional readouts, positioning it as a reference standard for future neuro-cardiac research.

Limitations and Transferability

Despite significant advances, the assembloid system described in the reference study has certain limitations:

• Cellular complexity: While the model incorporates major pacemaker and neural cell types, it does not yet fully recapitulate the diversity of supporting cells (e.g., fibroblasts, endothelial cells) or immune components present in vivo.
• Maturation state: Although the assembloids display enhanced maturation compared to 2D cultures, the electrophysiological and structural properties still fall short of fully adult SAN tissue.
• Long-term stability: Functional and molecular features are typically assessed over weeks, but longer-term studies are needed to capture chronic adaptation or degeneration relevant to disease modeling.
• Transferability: Protocols are robust but require optimization for different hPSC lines, and batch-to-batch variability can affect reproducibility.

Nonetheless, the platform represents a substantial step forward in modeling human SAN biology and neuro-cardiac signaling, with clear potential for adaptation to study diverse cardiovascular and electrophysiological phenomena.

Research Support Resources

For laboratories seeking to replicate or extend these workflows, reliable reagents are critical for modeling beta-adrenergic receptor signaling and neuro-cardiac modulation. Isoproterenol sulfate dihydrate (SKU C6402) is a high-purity, water-soluble, non-selective beta-adrenergic agonist widely used to probe GPCR/cAMP/PKA pathways in cardiovascular research. According to the product information, it is supplied as a hemisulfate salt dihydrate, suitable for rapid preparation and acute stimulation protocols, and is routinely applied in advanced assembloid models to analyze beta-adrenergic signaling dynamics. For further optimization of neuro-cardiac research, APExBIO offers relevant technical details on stability and storage conditions for this reagent.