TY  - GEN
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/33701/
Y1  - 2023///
ID  - heidok33701
AV  - public
TI  - Organoid assembloids modelling the role of serotonin during human cortical development
CY  - Heidelberg
A1  - Siekmann, Marco Torben
N2  - The development of the human cortex involves the coordinated interplay between cell proliferation,
migration and differentiation. Serotonergic projections from the raphe nuclei were shown to reach the
developing cortex at early stages, prior to the main expansion period. Subsequently, serotonin has
been linked to several aspects of corticogenesis, including proliferation. However, the lack of
appropriate models impedes deepening our understanding of the role of serotonin in human
corticogenesis. In this context, human induced stem cell (hiPSC)-derived culture systems became of
interest as regionalized brain organoids have been shown to faithfully recapitulate certain aspects of
human brain development.
In this thesis, hiPSC-derived 2D and 3D model systems were applied to decipher the effect of serotonin
on distinct cortical progenitor pools. Thus, apical progenitor cells (aPCs) were identified as recipients
of serotonin. Specifically, activation of the serotonergic receptor 5-HT2C was required and sufficient to
induce proliferation. Secondly, a mitogenic effect of serotonin was also observed in basal progenitor
cells (bPCs), a precursor type fundamental for human brain development, e.g. human-specific cortical
expansion. The increased proliferation was here mediated by the receptor 5-HT2A. Interestingly, no
change in proliferation behavior was observed in intermediate progenitors (IPs).
To closer resemble the interplay between serotonergic neurons and the developing cortex, cortical-
raphe assembloids were established. Whereby, a new organoid model was developed mimicking the
raphe nuclei. At early time points, raphe organoids (RO) were composed of progenitors exhibiting
expression of characteristic markers of serotonergic precursors including NKX2.2 and FOXA2. RO
ultimately generated serotonergic neurons marked by TPH2, VMAT2 and serotonin itself. Moreover,
by applying the serotonin sensor sDarken the endogenous release of serotonin could be detected.
By fusing raphe with cortical organoids (CO) to assembloids, a model system was created representing
the influence of serotonergic innervation on human corticogenesis. Specifically, RO-derived
serotonergic projections were observed in cortical structures and the utilization of sDarken in CO
demonstrated serotonergic innervation. Finally, ventricular zones showed elevated proliferation in the
presence of serotonergic projections.
Taken together, hiPSC-derived organoids displayed distinct mitogenic effects of exogenous serotonin
on different progenitor populations, namely aPCs and bPCs but not IPs. Moreover, the generated
assembloid proved valuable by displaying endogenous serotonergic innervation and recapitulating the
mitogenic effect of serotonin. Thus, RO-CO assembloids enable further decipherment of the
serotonergic role during human corticogenesis and open the possibility to study associated diseases,
including autism spectrum disorder.
ER  -