1679.6 Induction of Metanephric Nephron Progenitors
from Pluripotent Stem Cells
In contrast to conventionally accepted Nodal−/activin- and Bmp-driven patterning
within the PS, we found that a high concentration of Wnt agonist combined with an
optimal concentration of Bmp4 induced the T+/Cdx2+ posterior nascent mesoderm
(or tail bud mesenchyme, late primitive streak). This caudal immature population
should be further maintained by high concentrations of Wnt to posteriorize the pop-
ulation (see 1.4.5). Indeed, prolonged Wnt activation followed by exposure to the
differentiation cocktail efficiently induced metanephric nephron progenitors from
both mouse embryonic stem cells and human-induced pluripotent stem cells
(Fig. 9.7). Importantly, the nephron progenitors derived from mouse ES cells
express representative key transcription factors at similar levels to those observed in
the embryonic mouse metanephric nephron progenitors.
In response, several groups investigating kidney induction have recently revised
their original protocols and now employ a prolonged Wnt-driven strategy, although
the duration of Wnt treatment slightly varies among protocols, from 4 (Takasato
et al. 2015 ; Morizane et al. 2015 ) to 6 days (Taguchi et al. 2014 ). The subsequent
conditions used to induce PIM and mature nephron progenitor development are also
somewhat different among the protocols. Nevertheless, the shared stream of differ-
entiation—exposure to a prolonged strong Wnt signal followed by attenuation of the
Wnt signal and administration of Fgf9—could presumably facilitate programming
into nephron progenitors to a certain extent. Importantly, however, the endogenous
activity of each growth factor signal varies among cell lines and maintenance
conditions. Therefore, the fine tuning of each growth factor signaling is indispens-
able to optimize the process in each laboratory.
Intriguingly, a recent study that aimed to induce caudal paraxial mesoderm
reported the efficacy of initial Wnt treatment, concluding that the effect of Wnt
signaling was the induction of the late primitive streak (Mendjan et al. 2014 ). Other
studies that aimed to induce neuromesodermal progenitors—the bipotent precursors
of caudal PAM and the neural tube—have shown the efficacy of initial prolonged
Wnt and FGF treatment (Gouti et al. 2014 ; Turner et al. 2014 ; Henrique et al. 2015 ).
Taken together, in vitro directed differentiation experiments may indicate the slope,
or local accumulation of Wnt signals within the primitive streak confers the caudal
identity in addition to the Nodal/BMP gradient (Fig. 9.3a).
9.7 Reconstitution of a Three-Dimensional Nephron
by Self- Organizing Morphogenesis
Pluripotent stem cell-derived metanephric nephron progenitors can differentiate
into three-dimensional nephron structures in a self-organizing manner under the
influence of Wnt (see 1.2.4). Indeed, the conventional coculture method that
9 Early Kidney Specification and Its Recapitulation by Pluripotent Stem Cells