A team of researchers led by Ken Kwok-Keung Chan at the A*STAR Bioprocessing Technology Institute have identified the signaling pathway that causes human embryonic stem cells (hESCs) to differentiate into neurons, providing new insights into the development of stem cell therapies for neurological disorders1.
Sonic Hedgehog (SHH) is one of three proteins in a family of signaling pathways called hedgehog, which is known to control patterning of the developing brain and spinal cord by inducing differentiation of a number of neuronal subtypes in specified locations. Exactly how SHH regulates these diverse cellular responses and the target genes involved is unknown.
“We recently showed that SHH signaling influences early differentiated hESCs toward the neuroectoderm lineage2,” says Chan. “This prompted us to study the role of SHH in hESC neural differentiation.”
To do so, Chan and his co-workers generated a line of hESCs that expresses high levels of SHH and found that the cells can be maintained in an undifferentiated state for long periods of time. When the cells were induced to differentiate, however, they produced greater numbers of neural progenitors than those expressing normal levels of SHH, and this led to a subsequent increase in the numbers of mature neurons that synthesize the neurotransmitter dopamine.
The researchers then used sophisticated microarray technology combined with bioinformatics to determine how increased SHH levels affect the expression of other genes. Their analysis revealed a total of 182 genes whose activity was increased by SHH overexpression. Many of these are well established SHH target genes, all of which play a role in the SHH signaling pathway.
The study also identified four new SHH target genes, all of which contained sites for binding the transcription factor GLI—a component of the Notch signaling pathway, which is activated by SHH. This subset of genes is involved in inducing neural ectoderm during the earliest stages of embryonic development, neural stem cell proliferation and the development and function of dopamine-producing neurons.
A better understanding of these mechanisms could aid the development of stem cell-based therapies for Parkinson’s disease, a neurodegenerative condition in which dopamine-producing neurons in the midbrain die. “Understanding the molecular mechanisms of hESC with respect to their maintenance and cell fate determination is critical to harnessing their true potential for future regenerative medicine,” says Chan.
“Our data provide the first evidence of cross-talk between the Notch and SHH pathways in hESC-derived neuroprogenitors,” adds Chan. “We will continue to investigate how these two pathways interact.”
The A*STAR-affiliated researchers contributing to this research are from the Bioprocessing Technology Institute.