**9. Conclusion**

specification. In addition, we developed a specific profile of marker genes, which was

Our results demonstrated that neural differentiation took place in mES cell-derived cultures resulting in the generation of neural progenitors and neurons in a time-frame which mirrors telencephalic neurogenesis *in vivo*. The expression levels of telencephalic markers were com‐ parable between *in vivo* and *in vitro* differentiated populations. We demonstrated that the neural differentiation in human cells can be temporally aligned with mouse cells in the pro‐ posed neurogenic time-windows. Thus, our temporally aligned, comparative cell culture model offered a novel platform for analyzing the effect of signaling molecules on the gener‐

**Figure 3.** Hh signaling for modulation of dorsal ventral patterning in mouse and human telencephalon in vivo and in

To exemplify the value of this approach we analyzed in greater detail a single process, the step of D/V telencephalic patterning. Thus, we monitored the effect of pharmacological modulators of the Hh signaling pathway, purmorphamine—an agonist and cyclopamine an antagonist acting on the Smoothened receptor (Smo), regarding the expression of region-

Purmorphamine strongly up-regulated the expression of telencephalic ventral markers Nkx2.1, Nkx6.2, Lhx6, and Lhx8 in mouse and human cells, thus reflecting the *in vivo* process of the MGE patterning and specification. Cyclopamine up-regulated the expression of telencephalic dorsal markers, but at lower levels in human compared with mouse cells. Interestingly, the modula‐ tion of Smo *in vitro* differentially affected the expression of molecules of the Hh pathway, espe‐

We additionally examined how the SHH expression itself was modulated by Smo agonist or antagonist treatment. We reported that SHH expression is regulated in a very dynamic way

specific TFs and signaling molecules relevant for telencephalic development *in vivo*.

cially the Gli1 and Gli3 effectors and Ptch receptors, in mouse *vs* human cells.

vitro, in ES stem cell-derived cell cultures (modified from Nat et al 2012 [3]).

ation of specific telencephalic populations in mouse and human cell cultures.

derived from *in vivo* studies.

228 Trends in Cell Signaling Pathways in Neuronal Fate Decision

Recent studies have shown that both mES cells and hES cells differentiate into region specif‐ ic progenitors, following the same developmental principles that have been identified by studying mouse CNS development. Together with previous findings, our own data support the model in which the human neural progenitors in culture develop a reverse default D/V phenotype compared with mouse. However, early human NE cells can be efficiently differ‐ entiated into dorsal and ventral telencephalic progenitors *via* modulating similar molecular pathways as described in rodents.

Therefore, mES and hES cell-derived models, directly compared in parallel experiments and temporally aligned to *in vivo* telencephalic development, offer a platform for testing the ef‐ fect of morphogens, growth factors, and pharmacological substances for the generation of specific neuronal subtypes.

Additionally, telencephalic progenitors and neurons generated *in vitro* from human pluripo‐ tent cells provide a unique paradigm to study the human telencephalic development.

Even more importantly, the telencephalic differentiation of human induced PS (IPS) cells has recently been reported [119;120].

The application of optimized telencephalic differentiation protocols to IPS cell cultures de‐ rived from patients with neurodegenerative or neurogenetic diseases will provide unique new opportunities to develop *in vitro* models of human diseases such as Alzheimer's dis‐ ease, Huntington's disease, epilepsy, and neuropsychiatric disorders. These models, based on human neurons in culture, will critically complement existing animal models, which do not fully reflect important features specific for the normal and pathological human brain.

### **Acknowledgements**

This work was supported by SPIN FWF W1206-B05, Austria.
