Differentiation of pluripotent stem cells into striatal projection neurons: a pure MSN fate may not be sufficient

Abstract

Huntington’s disease (HD) is an autosomal dominant inherited disorder leading to the loss inter alia of DARPP-32 positive medium spiny projection neurons (“MSNs”) in the striatum. There is no known cure for HD but the relative specificity of cell loss early in the disease has made cell replacement by neural transplantation an attractive therapeutic possibility. Transplantation of human fetal striatal precursor cells has shown “proof-of-principle” in clinical trials; however, the practical and ethical difficulties associated with sourcing fetal tissues have stimulated the need to identify alternative source(s) of donor cells that are more readily available and more suitable for standardization. We now have available the first generation of protocols to generate DARPP-32 positive MSN-like neurons from pluripotent stem cells and these have been successfully grafted into animal models of HD. However, whether these grafts can provide stable functional recovery to the level that can regularly be achieved with primary fetal striatal grafts remains to be demonstrated. Of particular concern, primary fetal striatal grafts are not homogenous; they contain not only the MSN subpopulation of striatal projection neurons but also include all the different cell types that make up the mature striatum, such as the multiple populations of striatal interneurons and striatal glia, and which certainly contribute to normal striatal function. By contrast, present protocols for pluripotent stem cell differentiation are almost entirely targeted at specifying just neurons of an MSN lineage. So far, evidence for the functionality and integration of stem-cell derived grafts is correspondingly limited. Indeed, consideration of the features of full striatal reconstruction that is achieved with primary fetal striatal grafts suggests that optimal success of the next generations of stem cell-derived replacement therapy in HD will require that graft protocols be developed to allow inclusion of multiple striatal cell types, such as interneurons and/or glia. Almost certainly, therefore, more sophisticated differentiation protocols will be necessary, over and above replacement of a specific population of MSNs. A rational solution to this technical challenge requires that we re-address the underlying question—what constitutes a functional striatal graft?

Temporal establishment of neural cell identityin vivoandin vitro

“…The knowledge gained from these analyses would enable one to devise better strategies for generating specific neurons that recapitulate the process by which reprogrammed cells develop and integrate into the nervous system and function properly, without perturbing the pre‐existing neural connections. For example, in replacing missing medium‐spiny neurons in Huntington's disease, it appears that the performance achieved by grafting a population of DARPP‐32‐positive, medium‐spiny neuron‐like neurons is not equivalent to the performance achieved by primary fetal striatal grafts, which contain a variety of cell types that constitute the mature striatum (Reddington et al , ). Thus, additional studies are needed to understand the relationship between neurons and non‐neuronal cells within the graft, and the exact mechanisms that allow transplanted primary cells to integrate into the neural circuit.…”

Neuronal Replacement as a Tool for Basal Ganglia Circuitry Repair: 40 Years in Perspective

“…Graft-induced functional recovery has been observed in behavioral tasks at different levels of complexity: locomotor activity, skilled paw use, habit learning, and conditioned motivational behaviors (for review see Dunnett et al, 2000;Reddington et al, 2014). The ability of the GE grafts to restore function across this range of unconditioned and conditioned motor behaviors constitute the very best example, so far, of functional circuitry repair.…”

Axonal mRNA in human embryonic stem cell derived neurons

“…Direct evaluation of primary human neurons is severely restricted due to the limited ability to obtain tissue or culture isolated neurons. Human stem cell derived neurons are a biologically relevant and tractable in vitro system and protocols have been developed to differentiate stem cells into specific neuronal subtypes, such as motor, striatal, dopaminergic and glutamatergic neurons [15][16][17][18][19][20] . Here we sought to identify the axonal transcriptome of human embryonic stem cells differentiated into neurons enriched for glutamatergic lineage 20,21 and compare this transcriptome to axonal transcriptomes from rodent model systems.…”