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Adenoviral iPSCs in the Treatment of Deficits in HD Model

Review of “Intrastriatal Transplantation of Adenovirus-Generated Induced Pluripotent Stem Cells for Treating Neuropathological and Functional Deficits in a Rodent Model of Huntington’s Disease” from Stem Cell Translational Medicine by Stuart P. Atkinson.


Huntington’s disease (HD) causes a progressive degeneration of neurons, primarily in the putamen, caudate nucleus, and cerebral cortex [1] and currently has no cure. Clinical studies have assessed have assessed fetal tissues as a therapeutic treatment for HD [2-4], although these present with problems which may be overcome by the use of induced pluripotent stem cells (iPSCs) [5]. Researchers from the group of Gary L. Dunbar at Central Michigan University, Michigan, USA have recently shown the ability of adenovirally generated rat iPSCs to survive and differentiate in the striatum of rats [6], and now in a study in Stem Cells Translational Medicine, they focus on the efficacy of transplantation of these cells into the 3-nitropropionic acid (3-NP) rat model of HD [7], which induces cell death by energy-depleting mechanisms providing an accurate model of cell loss and replicates many features of the disease [8].

Overall, the study utilised rats at 7.5–8.5 weeks of age assigned to 5 different groups; sham control (PBS only), 3-NP control (3-NP) and 3 experimental cohorts – rats treated with 3-NP and injected with iPSCs into the striatum after 7 days (3-NP-iPSC-7), 21 days (3-NP-iPSC-21) and 42 days (3-NP-iPSC-42). The overall trend (See figure 1) was for 3-NP to cause a decrease in the latency times in falling from an accelerod task, a measure of motor coordination, and for iPSCs to partially recover this reduction even after 42 days. Indeed, by the 9 week stage, all iPSC transplanted rats performed at a level not significantly different from sham controls. Histological analyses found that the metabolic activity of striatal tissue, striosome area, and in the area of the lateral ventricles (inner brain areas) was significantly lower than sham rats in 3-NP and 3-NP-iPSC-42 rats, while 3-NP-iPSC-7 and -21 rats had higher levels of metabolism in the striatum than the 3-NP and 3-NP-iPSC-42 rats. 3-NP rats exhibited a significant increase in striosome size, compared to sham, with this being prevented by iPSC treatment at days 7 or 21, but not by cells transplanted at day 42, suggesting that transplantation of iPSCs at day 7 and 21 prevents reorganization of striatal tissue. The lateral ventricles were also larger in 3-NP rats, although iPSC treatment at any time reduced the size, and the 3-NP-iPSC-7 and 3-NP-iPSC-42 rats had ventricles smaller than sham rats.

Further histological analysis assessing immune responses found that compared to sham rats, all 3-NP treated rats had higher numbers of activated microglia (CD11b+), gaining significance over the 3-NP mice in the 3-NP-iPSC-7 and -42 rats, indicating possible local immune responses to the transplanted cells. Astrocyte number (GFAP+) was higher in NP-iPSC-21 and -42 rats than the controls and 3-NP-iPSC-7 rats, although this was not due to iPSCs differentiating into astrocytes as found by colocalization studies, contrary to previous studies [9]. Macrophage response (IBA1+ cells) was significantly higher in the 3-NP control compared to the the sham or iPSC-treated rats suggesting that the iPSCs do not generate a significant host macrophage response. The number of mature neurons (NeuN+) was at their highest in the 3-NP-iPSC-7 rats, which were similar to the sham mice, with 3-NP and NP-iPSC-42 rats having significantly fewer mature neurons than the sham rats. Correlating to this, 3-NP-iPSC-7 rats had higher NeuN+ colocalization with transplanted iPSCs than any of the other iPSC treated rates, suggesting that iPSCs could differentiate into mature neurons post-transplantation. DARPP32-positive cell (marks medium spiny neurons) number decreased significantly in the 3-NP mice compared to sham but only the NP-iPSC-21 rats had significantly higher numbers of cells than 3-NP rats, although there were no differences in colocalization to iPSCs. Day 7 and 42 mice had similar levels of DARPP32-positive cells than the sham suggesting that the iPSCs had a time independent ability to differentiate into these mature neural cells. Further colocalization studies may provide evidence of differentiation of iPSCs into different types of cells in the striatum. Finally, mRNA expression of striatal tissue found no differences in BDNF expression although expression of TNF- in the 3-NP-iPSC-21 and -42 rats had significantly lower levels than sham control and 3-NP-iPSC-7 rats, suggesting that there was not a continuous immune response to iPSCs.

The take home message from this study are that after iPSC-treatment, 3-NP treated mice displayed either a preservation of motor function or behavioral recovery, alongside a preservation of the metabolic activity in the striatum, similar to studies using pluripotent cells from fetal tissue or hESC-derived cells. Additionally, the adenoviral generation of the iPSCs did not lead to any teratomas or other abnormalities suggesting that they are safe for this type of use. However, the data points to early intervention (day 7 and day 21) as generally being more effective than later intervention (day 48) in relation to structural degeneration, as changes may have become irreversible by such a late time point. The authors do point out that perhaps at this point the iPSC may only require a longer time to exert any positive function. Furthermore, assessment of the immune response to transplanted iPSCs was favourable, while cells transplanted at early time points had a better ability to differentiate into mature neural cells, perhaps due to the suppressive effects of the micro-environment at the transplant site at later timepoints. The authors note that their future studies lie in the further assessment of the transplantation of iPSCs into neurodegenerative diseases, and additionally the enhancement of protocols in order to enhance in vivo neuronal lineage differentiation.


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