Maturation of neural stem cells and integration into hippocampal circuits – a functional study in an in situ model of cerebral ischemia

Abstract

The hippocampus is the region of the brain that is most susceptible to ischemic lesion because it contains pyramidal neurons that are highly vulnerable to ischemic cell death. A restricted brain neurogenesis limits the possibility of reversing massive cell death after stroke and, hence, endorses cell-based therapies for neuronal replacement strategies following cerebral ischemia. Neurons differentiated from neural stem/progenitor cells (NSPCs) can mature and integrate into host circuitry, improving recovery after stroke. However, how the host environment regulates the NSPC behavior in post-ischemic tissue remains unknown. Here, we studied functional maturation of NSPCs in control and post-ischemic hippocampal tissue after modelling cerebral ischemia We traced the maturation of electrophysiological properties and integration of the NSPC-derived neurons into the host circuits, with these cells developing appropriate activity 3 weeks or less after engraftment. In the tissue subjected to ischemia, the NSPC-derived neurons exhibited functional deficits, and differentiation of embryonic NSPCs to glial types - oligodendrocytes and astrocytes - was boosted. Our findings of the delayed neuronal maturation in post-ischemic conditions, while the NSPC differentiation was promoted towards glial cell types, provide new insights that could be applicable to stem cell therapy replacement strategies used after cerebral ischemia.

Monitoring maturation of neural stem cell grafts within a host microenvironment

“…Next, a morphological comparison has been performed with regard to the synapses which NSC-derived neurons constituted with endogenous cells. The visualised structures, either presynaptic terminals containing numerous vesicles or postsynaptic structures, revealed the typical morphology, confirmed by synaptic function ( i.e ., recordings of the postsynaptic currents) detected as early as the first two weeks after engraftment[21]. Extrapolating from the experimental data from this and other studies[22], the maturation of neuronal excitability and synaptogenesis within a host tissue can be envisaged to last up to a few weeks - a time scale much faster than established in dissociated cell cultures across a vast literature (NSC-derived vs iPSC-derived neurons[23]).…”

“…Emerging data from functional studies of the NSC-derived neurons indicate that neuronal differentiation and maturation occur at a much faster rate within a host brain tissue than in in vitro cell cultures. For instance, the maturation of electrophysiological properties of the NSC-derived neurons in organotypic hippocampal tissue has been completed for up to 3 weeks after engraftment[21,22]. By contrast, the maturation of biophysical properties of stem cell-derived neurons in dissociated cell cultures normally requires months to achieve a similar result.…”

“…Functional studies have been carried out in organotypic hippocampal slices, aimed at answering the questions as highlighted earlier. The experimental data from electrophysiological recordings, combined with electron microscopy and immunohistological approaches, have revealed that NSC-derived hippocampal neurons have matured electrophysiological properties, and have functionally integrated into the host circuits within 3 weeks of engraftment[21]. Moreover, the neurophysiological maturation of NSC-derived neurons achieved a similar level of activity as that exhibited by endogenous CA1 pyramidal neurons (varied electrophysiological parameters were quantitatively compared between the groups).…”

“…A substantial bias in the neurogenesis of NSC grafts to glial lineage has been found while monitoring NSC neurogenesis within the ischemic-injured brain tissue[21]. In the post-ischemic environment (organotypic hippocampal slices subjected to ischemic conditions - oxygen-glucose deprivation[40]), NSC grafts have been largely differentiating into glia, with a prompt rise in NSC-derived oligodendrocytes, followed by astrocytes.…”

“…Notably, NSC-derived oligodendrocytes have already been identified at week 1, and astrocytes - by two weeks. In the meantime, NSC-derived neurons matured in terms of their electrophysiological properties with a dramatically slower rate within the post-ischemic milieu than in a physiological environment[21]. Based on the experimental data from a direct comparison between electro-physiological parameters, the promoted glial lineage has been a hallmark of NSC neurogenesis within the post-ischemic tissue (approximately 70% of grafted NSC differentiated into glia), opposing the reduced neuronal lineage (a drop from approximately 70% to approximately 30% in the proportion of NSC-derived neurons; Figure 1).…”