HD iPSC-derived neural progenitors accumulate in culture and are susceptible to BDNF withdrawal due to glutamate toxicity

Abstract

Huntington's disease (HD) is a fatal neurodegenerative disease, caused by expansion of polyglutamine repeats in the Huntingtin gene, with longer expansions leading to earlier ages of onset. The HD iPSC Consortium has recently reported a new in vitro model of HD based on the generation of induced pluripotent stem cells (iPSCs) from HD patients and controls. The current study has furthered the disease in a dish model of HD by generating new non-integrating HD and control iPSC lines. Both HD and control iPSC lines can be efficiently differentiated into neurons/glia; however, the HD-derived cells maintained a significantly greater number of nestin-expressing neural progenitor cells compared with control cells. This cell population showed enhanced vulnerability to brain-derived neurotrophic factor (BDNF) withdrawal in the juvenile-onset HD (JHD) lines, which appeared to be CAG repeat-dependent and mediated by the loss of signaling from the TrkB receptor. It was postulated that this increased death following BDNF withdrawal may be due to glutamate toxicity, as the N-methyl-d-aspartate (NMDA) receptor subunit NR2B was up-regulated in the cultures. Indeed, blocking glutamate signaling, not just through the NMDA but also mGlu and AMPA/Kainate receptors, completely reversed the cell death phenotype. This study suggests that the pathogenesis of JHD may involve in part a population of 'persistent' neural progenitors that are selectively vulnerable to BDNF withdrawal. Similar results were seen in adult hippocampal-derived neural progenitors isolated from the BACHD model mouse. Together, these results provide important insight into HD mechanisms at early developmental time points, which may suggest novel approaches to HD therapeutics.

iPSC-based drug screening for Huntington׳s disease

“…One of these phenotypes is BDNF expression, which is suppressed in HD NSCs compared to isogenic corrected NSCs with a normal CAG repeat length (30, 66). This effect is probably due to transcriptional dysregulation caused by mHTT (25) and has been confirmed in other human HD derived neurons from iPSCs (36). In HD NSCs, when growth factors in normal medium (full medium) are taken away, cells show phenotypes relevant to the disease.…”

“…Following this development, we demonstrated that HD-iPSCs when differentiated into NSCs had a number of features relevant to HD pathology, including susceptibility to cell death upon growth factor withdrawal as measured by caspase activity (30). In the years following, many iPSC-based human HD cell models have been generated and utilized (23–30, 36). These models together with human ESC based HD models are summarized in Table 1.…”

Induced Pluripotent Stem Cells in Huntington’s Disease: Disease Modeling and the Potential for Cell-Based Therapy

“…Furthermore, CAG-repeat-associated alterations in actin cytoskeleton, cell-cell adhesion, ATP/ADP levels, and electrophysiological properties have been observed in derived neural cells. Moreover, HD-derived lines exhibited increased susceptibility to cell stressors such as BDNF withdrawal and glutamate, which was verified by a more recent study in the juvenile-onset HD lines [62, 63]. …”

Pluripotent Stem Cells to Model and Treat Huntington’s Disease

“…After removing BDNF from the cell culture medium, neurons derived from HD-iPSCs (109 and 180 CAG repeats) have a robust increase in 3/7 caspase activity and die [107]. This and many other studies indicate that BDNF is a critical factor in the pathology of HD and is a putative candidate for HD treatment [108][109][110].…”