Human PSC-Derived SAN-Plexus Assembloids: A Platform for Investigating Neuro-Cardiac Pacemaker Maturation

Study Background and Research Question

The sinoatrial node (SAN) serves as the heart’s primary pacemaker, orchestrating the rhythmic initiation of cardiac contractions. While animal studies have elucidated principles of SAN formation and neural regulation, direct study of human SAN development is constrained by limited tissue accessibility and interspecies differences in electrophysiology and autonomic control. Moreover, traditional in vitro systems, such as 2D cultures of SAN-like cells or cardiac organoids, generally lack the three-dimensional (3D) spatial organization and intrinsic neural inputs essential for recapitulating the complex environment of the human SAN. This study addresses a fundamental gap: the absence of a robust human model to dissect neuron-pacemaker interactions and the mechanisms underlying pacemaker maturation and disease (see internal overview).

Key Innovation from the Reference Study

The authors developed a tri-organoid "assembloid" platform by integrating human pluripotent stem cell (hPSC)-derived SAN organoids (SANOs), cardiac ganglionated plexus organoids (CGPOs), and atrial-like cardiac organoids. This system, termed the SAN-plexus assembloid, models the spatial, molecular, and electrophysiological features of the native human SAN and its innervation. Critically, the assembloid recapitulates neuro-cardiac crosstalk, enabling functional interrogation of how cardiac neuronal inputs modulate SAN pacemaker automaticity, maturation, and conduction to atrial tissue (reference study).

Methods and Experimental Design Insights

The experimental design leveraged established hPSC differentiation protocols to generate three distinct organoid types:

• SANOs: Enriched for pacemaker cell subtypes (including head, tail, and transitional SANPCs), expressing canonical transcription factors (SHOX2, ISL1, TBX3) and ion channels (notably HCN4).
• CGPOs: Recapitulating the cardiac ganglionated plexus, providing intrinsic neuronal inputs for autonomic regulation.
• Atrial-like organoids: Modeled working atrial myocardium to facilitate analysis of pacemaker-to-atrial conduction.

These organoids were assembled into a 3D tri-assembloid configuration. The system’s structural fidelity was validated using spatial transcriptomics mapped against native human SAN tissue, confirming the presence and spatial organization of SAN-specific cell populations and neural elements. Functional interrogation employed multielectrode array (MEA) electrophysiology to resolve pacemaker activity, conduction velocity, and the effects of neural modulation. Additionally, the authors used genetic and pharmacological perturbations to dissect signaling pathways within the assembloid context.

Protocol Parameters

• Tri-organoid assembly: Combine hPSC-derived SAN, CGPO, and atrial organoids in defined ratios on low-attachment plates to promote 3D integration and spatial organization.
• Spatial transcriptomics: Apply single-cell or spatial RNA-seq to validate cell-type composition and anatomical mapping relative to native SAN tissue.
• Electrophysiology: Use MEA recordings or patch-clamp to assess spontaneous pacemaker activity, conduction, and response to autonomic agonists/antagonists.
• Signaling pathway interrogation: Manipulate neuron-derived factors (e.g., PSAP) or receptor expression (e.g., GPR37) through genetic editing or targeted inhibitors to probe neuron-to-pacemaker signaling.

Core Findings and Why They Matter

Recapitulation of Pacemaker Maturation and Neuro-Cardiac Modulation: The SAN-plexus assembloid successfully mimicked key features of human SAN physiology, including heterogeneous SANPC subtypes, robust diastolic depolarization, and dynamic modulation of pacemaker output by intrinsic cardiac neurons. This system enabled the authors to model disease-associated conduction dysfunction and investigate how neural inputs shape pacemaker activity in a human context.

Discovery of a Prosaposin-GPR37 Signaling Axis: By integrating spatial transcriptomics with functional assays, the study identified a neuron-to-pacemaker signaling program involving CGPO-derived prosaposin (PSAP) and its receptor GPR37, which is enriched in SAN tissue. Targeted activation of this pathway promoted pacemaker maturation, highlighting a previously unappreciated mechanism of neuro-cardiac development (reference study).

Modeling Conduction and Disease: The tri-assembloid system allowed detailed analysis of pacemaker-to-atrial conduction, offering a tractable model to study congenital and acquired SAN dysfunction, including arrhythmias and conduction blocks, in a fully human context.

Comparison with Existing Internal Articles

While the present study centers on cardiac development and neuro-cardiac interactions, there are instructive methodological parallels with advanced cancer research workflows. For example, internal resources such as "Cucurbitacin I (JSI-124): Advanced Protocols for STAT3 Inhibition" and "STAT3 Inhibition and Advanced Assay Design" emphasize the value of organoid and assembloid systems for dissecting cell signaling and tissue context—principles mirrored in the SAN-plexus assembloid work. Both domains benefit from 3D modeling and high-content functional assays (e.g., STAT3 DNA binding inhibition assay in oncology, neuro-cardiac conduction analysis in cardiology). However, the cardiac assembloid system is specifically tailored to capture the spatial and neural complexity of the human SAN, whereas cancer-focused organoid platforms typically prioritize tumor microenvironment and signaling pathway modulation, such as JAK2/STAT3 inhibition using Cucurbitacin I (see related workflow).

Limitations and Transferability

This assembloid system represents a significant advance in modeling human neuro-cardiac biology in vitro, but several caveats remain. The differentiation protocols, while robust, may not fully recapitulate the complete cellular diversity and maturation states found in adult human SAN and ganglionated plexus. Electrophysiological properties, though closely matching human SAN, could be influenced by culture conditions and the absence of additional systemic factors present in vivo. Finally, while spatial transcriptomics enables detailed mapping, it relies on available reference datasets and may miss rare or transient cell states. As with all stem cell-derived platforms, transferability to disease modeling and drug discovery should be interpreted with these constraints in mind.

Research Support Resources

Researchers seeking to implement similar assembloid platforms or dissect complex cell signaling pathways can leverage advanced 3D culture methods and high-content functional assays described in this and related literature. For cancer research applications, including STAT3 pathway interrogation and functional assays such as cancer cell invasion or proliferation inhibition, Cucurbitacin I (JSI-124, SKU A4512) from APExBIO offers a well-characterized JAK2/STAT3 inhibitor with demonstrated utility in both in vitro and in vivo models. Experimental details, including solubility, dosing, and cell culture protocols, are available via the product information. Integrating such targeted compounds into assembloid or organoid workflows can facilitate mechanistic studies of signaling modulation, apoptosis, and tissue-specific responses.