BioLinks Journal Club 5 - Parkinson’s Disease and Cell Differentiation

Paper: “Reversing a model of Parkinson’s Disease with in situ converted nigral neurons”

Please read the introduction to this week’s research paper below, which includes the most crucial concepts and background involved. Don’t worry if you don’t understand everything in this summary - we will break it down over the course of the week and dive into the techniques and exact experiments included in the paper . Please remember questions that come up from reading this or anything of particular interest you would like to discuss more on Monday!

How can we compensate for the lack of dopamine neurons in individuals with Parkinson’s Disease (PD)? The release of dopamine by these neurons is crucial for body movement. Studies have demonstrated that the impaired motor function observed in Parkinson’s patients may be attributed to the loss of dopaminergic neurons in the substantia nigra region of the midbrain. While substantial research has been performed on prevention of this phenomenon, this paper proposes a method where other existing brain cells can be converted into the needed dopamine neurons by suppressing the activity of a single protein. How? By manipulating the same principle that is responsible for differentiating our muscle cells from skin cells or blood cells.

How do different parts of our bodies perform different functions? While we possess over 200 highly specialized cell types, all of our body cells contain the same genetic code within their nucleus and arise from generic embryonic stem cells with the capability to become almost any type of specialized cell. Several factors as early as fertilization are responsible for the differentiation of our cells. This includes environmental cues directed at select cells that can trigger secretion of signaling molecules, and ​transcription factors (TFs)​, which are proteins that heavily influence whether a particular gene will be transcribed and translated into protein or not. Since genes can only perform the function they encode for when in protein form, the control of cells with different transcription factors means that different genes are being made into proteins at different times in each type of cell.

Within the nervous system, not all of our cells are neurons. In fact, over 90 percent of brain cells are non-neuronal (glial) cells, classified by the fact that they do not send and receive electrical impulses. This paper focuses on astrocytes, a subtype of glial cells that are responsible for energy metabolism and blood flow regulation in the brain, as well as mediation of synapses and nutrition. While both neurons and glial cells arise from neural stem cells, the process of neural differentiation is key to the structure and function of our nervous system. ​RE1-Silencing Transcription factor (​REST​) is a key protein in this differentiation process. Acting as a transcriptional repressor to several neuron specific genes, REST is active in astrocytes but

suppressed in neurons. As observed successfully in fibroblasts (another non-neuronal cell type), this group hypothesizes that the abnormal inactivation of REST in astrocytes could cause the conversion of these non-neuronal cells into functional neurons, specifically dopamine neurons when targeting the substantia nigra region. But how can the activity of the REST complex be manipulated?

Polypyrimidine Tract Binding Protein ​(PTB) ​belongs to a class of proteins called RNA binding proteins, which bind to and process our cells’ RNA strands. RNA binding proteins like PTB are particularly important in mRNA post-transcriptional regulation and processing. This makes PTB another significant factor in neuronal differentiation because it can influence whether mRNAs go on to be translated into neuron or non-neuron specific proteins. In the case of this paper, PTB presence in astrocytes is transitively responsible for activated REST protein. In neurons, the PTB equivalent nPTB does not perform this same function and the REST complex remains inactive, allowing aspects of neuron function and morphology to develop. As the first component of this particular differentiation pathway, this group targets PTB levels in order to convert astrocytes into dopamine neurons in the substantia nigra.

Through a series of tests in mouse brains, it was determined that down-regulating PTB protein levels in midbrain astrocytes is a sufficient catalyst to the efficient conversion of these non-neuronal cells into neurons. By targeting the substantia nigra, this team was able to limit the class of the majority of new converted neurons to the desired dopaminergic neurons lacking in PD, which may be important for limiting therapeutic side effects in the future. Furthermore, these new neurons were determined to be highly functional by the standard that they responded to an array of neuron specific markers, displayed typical neuron morphology, responses to stimuli and dopamine release. Externally, the elevated levels of dopaminergic neurons were able to mostly rescue the motion impairment phenotype observed in their PD models.

Overall, this paper is a demonstration of the powerful potential of manipulation of gene expression for reversing disease conditions. This study has the potential to revolutionize Parkinson’s treatment because of the discovery that a one protein manipulation can cause efficient conversion of astrocytes to neurons and highly effective rescue of PD relevant motion impairment. The success with astrocytes and fibroblasts suggests that perhaps other cell types can also contribute to neuron levels through this conversion, especially glial cells that undergo a similar differentiation process to astrocytes.