Modeling Human Pacemaker Maturation with SAN-Plexus Assembloids

Study Background and Research Question

The sinoatrial node (SAN) is the intrinsic pacemaker of the human heart, initiating electrical impulses that regulate cardiac rhythm and drive atrial contraction. While animal models have provided insight into SAN development and function, species-specific differences in electrophysiology and neural regulation limit their translational relevance. Human tissue studies are further constrained by the anatomical complexity and limited accessibility of the SAN. Therefore, there has been a critical need for human in vitro systems that recapitulate the essential features of SAN structure, cellular diversity, and—most importantly—innervation by intrinsic cardiac neurons.

The reference study—Zhang et al., Cell Stem Cell, 2026—addresses this gap by developing human pluripotent stem cell (PSC)-derived assembloids that integrate SAN organoids (SANOs), cardiac ganglionated plexus organoids (CGPOs), and atrial-like cardiac organoids. The central research question is how intrinsic neural inputs modulate pacemaker maturation and function in a human context, and whether such interactions can be modeled in vitro for mechanistic study and disease modeling.

Key Innovation from the Reference Study

This work introduces a tri-assembloid system that functionally reconstitutes the neuro-cardiac interface of the human SAN. By integrating PSC-derived SANOs with CGPOs and atrial organoids, the authors model both the molecular and electrophysiological characteristics of the human pacemaker complex, as well as its modulation by intrinsic cardiac neurons. This approach enables, for the first time, systematic investigation of neuron-to-pacemaker signaling and neurogenic control of SAN automaticity in a human-specific, three-dimensional context. A pivotal discovery is the identification of CGPO-derived prosaposin (PSAP) engaging the SAN-enriched GPR37 receptor, a signaling axis that promotes pacemaker maturation.

Methods and Experimental Design Insights

The experimental strategy employed several advanced methodologies:

• PSC Differentiation: Human PSCs were directed to form SAN-like pacemaker cells (SAN-LPCs), CGPOs containing neural crest-derived cardiac neurons, and atrial-like cardiomyocytes. Each organoid type was validated by marker expression and functional assays.
• 3D Assembloid Construction: The three organoid types were spatially integrated to form tri-assembloids recapitulating SAN-plexus-atrial interactions. This arrangement enabled modeling of pacemaker-to-atrial conduction and neurogenic modulation.
• Electrophysiological Characterization: The assembloids exhibited spontaneous action potentials with diastolic depolarization and slow upstroke—characteristics of SAN pacemaker cells—along with evidence of neural modulation of firing rate and conduction.
• Spatial Transcriptomics: Integration with spatial transcriptomic profiling of native human SAN tissue allowed comparison of cellular heterogeneity, gene expression patterns, and identification of region-specific markers within the assembloids.
• Functional Interrogation of Signaling Pathways: PSAP-GPR37 signaling was investigated using loss- and gain-of-function approaches, confirming its role in promoting pacemaker phenotype maturation.

Core Findings and Why They Matter

The study demonstrates several advances:

• The tri-assembloid platform recapitulates key aspects of SAN anatomy and function, including distinct head, tail, and transitional pacemaker cell populations, and robust conduction to atrial-like tissue.
• Integration of CGPOs provides functional innervation, allowing the assembloid SAN to respond to neural inputs. This is evidenced by modulation of pacemaker activity upon neural stimulation and pharmacological manipulation.
• Spatial transcriptomics confirms that the assembloid platform faithfully models the cellular heterogeneity and gene expression architecture observed in native human SAN tissue.
• Mechanistically, CGPO-derived prosaposin activates the GPR37 receptor, enriched in SAN cells, to promote pacemaker cell maturation. Disruption of this axis impairs SAN phenotypes and conduction properties. This neuron-to-cardiac signaling program represents a novel regulatory pathway for human pacemaker development (Zhang et al., 2026).
• The model enables interrogation of disease-associated conduction defects, paving the way for mechanistic and pharmacological studies relevant to congenital SAN dysfunction.

This study thus fills a major gap in human cardiology research: enabling direct study of neuro-cardiac crosstalk and SAN maturation in a controlled, human-relevant system. Such platforms are essential for understanding arrhythmia mechanisms, modeling congenital defects, and screening candidate therapeutics—domains previously limited by lack of appropriate models.

Comparison with Existing Internal Articles

As of this writing, there are no directly related internal articles available for cross-reference. However, the technology and workflow developed here align with current efforts in organoid biology and neuro-cardiac interface modeling. Future internal resources may build on this work to refine disease-specific protocols or integrate additional signaling pathway analyses, such as STAT3 pathway interrogation using selective inhibitors.

Limitations and Transferability

Despite its significant advances, several limitations should be noted:

• Model Complexity: While the tri-assembloid system captures many features of the SAN neuro-cardiac axis, it may not fully recapitulate the in vivo microenvironment, including vascularization and long-range extrinsic innervation.
• Developmental Maturity: PSC-derived organoids often reflect fetal or neonatal states; the extent to which SAN-plexus assembloids achieve adult-like maturity remains to be fully established.
• Transferability: The platform is optimized for mechanistic and pharmacological studies in vitro. Translation to in vivo contexts, such as regenerative medicine or transplantation, will require further validation.
• Domain Specificity: The current model is focused on neuro-cardiac interactions in the SAN; extrapolation to other cardiac regions or non-cardiac systems should be approached cautiously unless supported by additional evidence.

Protocol Parameters

• SAN organoid differentiation: Direct hPSCs toward TBX18⁺ and ISL1⁺ lineages using stage-specific growth factors (as detailed in the reference methods).
• Cardiac plexus organoid specification: Induce neural crest differentiation before 3D aggregation with SANO and atrial organoids.
• Tri-assembloid assembly: Combine SANOs, CGPOs, and atrial-like organoids in Matrigel or hydrogel scaffolds; co-culture time frames range from 7 to 21 days for maturation and phenotypic analyses.
• Electrophysiological assessment: Use patch clamp and calcium imaging to monitor pacemaker activity and neural modulation.
• Signaling pathway interrogation: Apply small-molecule inhibitors or recombinant proteins to dissect neuron-to-cardiac communication, following published dosing and exposure protocols.

Research Support Resources

For researchers aiming to extend these findings or interrogate related pathways such as JAK2/STAT3 signaling in cardiac or cancer models, Cucurbitacin I (JSI-124) (SKU A4512) is a potent and selective STAT3 pathway inhibitor suitable for in vitro and in vivo studies. It can be used to assess the impact of STAT3 signaling on cell proliferation, apoptosis, and invasion in various experimental systems, including those involving organoid-based assays. For further details, including solubility and storage, consult the product dossier provided by APExBIO.