Volker Busskamp Group
Engineering synthetic human neuronal circuits
Previously, we studied the human and mouse retina in health and disease. In a blinding disease called Retinitis pigmentosa, we repaired non-functional cone photoreceptors using optogenetics, specifically by expressing microbial Halorhodopsins, to restore visual function in mice. We further translated this approach to post mortem human retinas (Busskamp et al. 2010), which forms the basis for on-going clinical trials (www.gensight-biologics.com). Furthermore, we were interested in basic functions of microRNAs in photoreceptors. We discovered that some of these non-coding RNAs had a high turnover in an activity depended manner (Krol et al. 2010). Upon cell type specific knockout of the microRNA processing machinery, it turned out that a sensory-specific microRNA cluster maintains the structure of cone outer segments and the genetic identity of this cell type. Overexpression of this microRNA cluster in embryonic stem cell-derived retinas resulted in the formation of outer segments and light sensitivity of photoreceptors (Busskamp et al. 2014).
Translating these approaches from mice to humans has been hindered by the lack of functional human neuronal tissues. This has prompted us to find novel routes to generate neurons from human induced pluripotent stem (iPS) cells with a focus on retinal neurons. For this purpose, we decellularized adult mouse retinas such that only the extra cellular matrix (ECM) remained – the idea is that the ECM may contain signals that coax stem cells to differentiate into neurons. Thus, we used this ECM as a scaffold to grow and differentiate human iPS cells to retinal neurons. Although the preliminary results looked promising, the time line of differentiation was still long - in the range of several months. To overcome these limitations, we explored an alternative technique via forced expression of transcription factors in iPS cells. The overexpression of two transcription factors resulted in the differentiation of neurons within four days. By capturing the coding and non-coding transcriptome, we aimed to identify the molecular routes for this rapid neurogenesis at the systems-level and to use knowledge to rationally engineer diverse neurons (Busskamp et al. 2014). Having succeeded with the generation of sets of different neurons, the next step is to assemble them into synthetic functional human neuronal circuits in vitro.
Future Projects and Goals
In order to understand how parts of the human brain function in health and disease, our aim is to reverse engineer functional human neuronal circuits from scratch combining neuroscience with stem cell research and bioengineering. The goal is to generate the basic parts, namely electrically active neurons, from adult human stem cells and to connect these cells in a controlled and reproducible way into defined functional neuronal circuits in vitro. Disease-causing mutations will be introduced to some circuit members trying to model brain diseases in a human setting to explore novel therapeutic interventions. Furthermore, from an engineering point of view, it is aimed to create biological computers using living cells to compute signals as our brain does with extreme efficiency.
Methodological and Technical Expertise
- Stem cell cultures
- Transcription factor-based neuronal differentiation of human stem cells
- Viral gene transfer
- Multi electrode array recordings