In my conversation with neuro-drug discover researchers, I am frequently being asked about the potential of using our STEMEZ(TM) hNP1 Human Neural Progenitors Expansion Kits for studying ion channels. How effective are these cells as a source for studying neurodegenerative diseases and for drug screening assays? There is good news.
When differentiated, these neural progenitors express subunits of glutamatergic, GABAergic, nicotinic, purinergic and transient receptor potential receptors. In addition, sodium and calcium channel subunits were also expressed. Functionally, virtually all the NP cells exhibited delayed rectifier potassium channel currents and some differentiated cells exhibited tetrodotoxin sensitive, voltage-dependent sodium channel current under whole-cell voltage clamp and action potentials could be elicited by current injection under whole-cell current clamp. These results indicate that removing basic fibroblast growth factor from the neural progenitor cell cultures leads to a post-mitotic state, and also results in the capability to produce excitable cells that can generate action potentials. This is the first data demonstrating capabilitiesof these cells for ionotrophic receptor assays and ultimately for electrically active human neural cell assays for drug discovery.
Images: Glutamate receptor expression in hNP cells and differentiated hNP cells The expression of ionotropic glutamate receptors might also be an indicator of neuronal maturation. These receptors are composed of three distinct families: NMDA, kainate and AMPA receptors. The hNP cells and differentiated hNP cells cultured in the absence of bFGF for 2 weeks were analyzed for mRNA expression of subunits of each glutamate receptor subtype relative to hESCs. Significant increases (p<0.05) in Grin2b were seen in hNP cells (20 fold) and differentiated hNP cells (25 fold) relative to hESCs (Figure 3A). Additionally, Grin1 and Grin2d were significantly increased (p<0.05) only in differentiated hNP cells relative to hESCs, but not in undifferentiated hNP cells (Figure 3A). Of the kainate receptors, Grik4 and Grik5 were significantly (p<0.05) increased only in undifferentiated hNP cells relative to hESCs (Figure 3B); whereas, Grik2 was significantly (p<0.05) increased only in hNP cells where bFGF had been removed (Figure 3B). AMPA receptor subunits were also examined. Gria1 and Gria4 were up regulated in hNP cells relative to hESCs (Figure 3C). Two week differentiated hNP cells showed significant (p<0.05) up regulation of Gria2 and Gira4 relative to hESCs (Figure 3C). To determine if functional glutamate channels exist in differentiated hNP cells, calcium influx in response to AMPA, kainic acid or NMDA application was measured on hNP cells, 14 days after the removal of bFGF. Figure 3G indicates that NMDA could not depolarize differentiated or undifferentiated hNP cells enough to cause significant calcium influx above background. In contrast, AMPA and kainic acid can cause calcium influx which can be potentiated by AMPA receptor specific modulator, cyclothiazide (50 μM, Figure 3G).Calcium influx was detected in the presence of cyclothiazide in calcium activity as measured (Figure 3H).
Images: Sodium channel activity in differentiated hNP cells was measured using whole cell voltage clamp. 81 total hNP cells cultured in the absence of bFGF from 4 to 27 days were analyzed. Of these, 34 exhibited no fast inward currents in response to a step depolarization indicating the 348 absence of functional voltage gated sodium channels (Figure 4G). The remaining cells yielded between 0.04 - 1.5 nA of inward current in response to the step depolarization (Figures 4B and 4G). These currents inactivated rapidly in all cases (Figures 4B and 4C) and could be abolished with the addition of 1 μM TTX (n = 3 cells; Figure 4C). Voltage-dependent steady state inactivation (n = 11 cells; Figure 4D) and recovery from fast inactivation (n = 5 cells; Figure 4E) were also observed on several positive cells. A subset of these cells was subjected to current clamp and action potentials were elicited by current injection (n = 8 cells, Figure 4F). In support of this, increasing concentrations of a sodium channel activator veratridine in a FLIPR assay on differentiated hNP cells show an increasing calcium response (Figure 4H). This probably resulted from voltage-gated sodium channel depolarization of cells that subsequently allowed calcium influx through calcium channels. These data indicate that differentiation of hNP cells by removal of bFGF can lead to a neuronal cell that can generate action potentials and depolarize the cell. The 58% hit rate for voltage-gated sodium channel function (Figure 4G), does not reflect the true proportion of sodium channel positive cells in our differentiated hNP cells, but rather our ability to morphologically distinguish these cells from negative cells by eye. An example of the morphology of a sodium channel positive cell is shown in Figure 4A. The positive cells were phase bright with a few long processes.