Functional Human Retinohypothalamic Tract Assembloid Model for Circadian Rhythm Research

The retinohypothalamic tract (RHT) is the primary pathway for circadian photoentrainment. Rodent models exhibit a significant translational gap for human physiology due to their nocturnal nature. To overcome this, we developed a functional human RHT assembloid by fusing human pluripotent stem cell (hPSC) derived retinal and hypothalamus organoids. Characterization revealed mature retinal brush borders and the preservation of melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) integrated via excitatory glutamatergic synapses. Multielectrode array (MEA) analysis confirmed synchronized network activity across the interface. The development of the human RHT assembloid represents a significant leap forward in chronobiology. The "gold standard" for circadian models--self-sustained gene expression oscillations--was demonstrated using a PER2::Luciferase reporter, showing robust 20- 30 hour rhythms. This validates the hypothalamic component as a functional "clock in a dish". This platform provides a readout to screen drugs or test light-pulse effects on circadian phase, directly modeling jet lag or phase-shifting. Overall, this model offers a high-fidelity system for investigating human-specific chronobiological mechanisms in vitro. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=75 SRC="FIGDIR/small/700761v1_ufig1.gif" ALT="Figure 1"> View larger version (27K): org.highwire.dtl.DTLVardef@1327027org.highwire.dtl.DTLVardef@61183corg.highwire.dtl.DTLVardef@7e280eorg.highwire.dtl.DTLVardef@77e7ce_HPS_FORMAT_FIGEXP M_FIG C_FIG