Vasodilation by in vivo activation of astrocyte endfeet via two-photon calcium uncaging as a strategy to prevent brain ischemia

Therapeutic Potential of Astrocyte Transplantation

Cell transplantation is an attractive treatment strategy for a variety of brain disorders, as it promises to replenish lost functions and rejuvenate the brain. In particular, transplantation of astrocytes has come into light recently as a therapy for amyotrophic lateral sclerosis (ALS); moreover, grafting of astrocytes also showed positive results in models of other conditions ranging from neurodegenerative diseases of older age to traumatic injury and stroke. Despite clear differences in etiology, disorders such as ALS, Parkinson's, Alzheimer's, and Huntington's diseases, as well as traumatic injury and stroke, converge on a number of underlying astrocytic abnormalities, which include inflammatory changes, mitochondrial damage, calcium signaling disturbance, hemichannel opening, and loss of glutamate transporters. In this review, we examine these convergent pathways leading to astrocyte dysfunction, and explore the existing evidence for a therapeutic potential of transplantation of healthy astrocytes in various models. Existing literature presents a wide variety of methods to generate astrocytes, or relevant precursor cells, for subsequent transplantation, while described outcomes of this type of treatment also differ between studies. We take technical differences between methodologies into account to understand the variability of therapeutic benefits, or lack thereof, at a deeper level. We conclude by discussing some key requirements of an astrocyte graft that would be most suitable for clinical applications.

Unraveling ChR2-driven stochastic Ca2+ dynamics in astrocytes: A call for new interventional paradigms

Optogenetic targeting of astrocytes provides a robust experimental model to differentially induce Ca2+ signals in astrocytes in vivo. However, a systematic study quantifying the response of optogenetically modified astrocytes to light is yet to be performed. Here, we propose a novel stochastic model of Ca2+ dynamics in astrocytes that incorporates a light sensitive component-channelrhodopsin 2 (ChR2). Utilizing this model, we investigated the effect of different light stimulation paradigms on cells expressing select variants of ChR2 (wild type, ChETA, and ChRET/TC). Results predict that depending on paradigm specification, astrocytes might undergo drastic changes in their basal Ca2+ level and spiking probability. Furthermore, we performed a global sensitivity analysis to assess the effect of variation in parameters pertinent to the shape of the ChR2 photocurrent on astrocytic Ca2+ dynamics. Results suggest that directing variants towards the first open state of the ChR2 photocycle (o1) enhances spiking activity in astrocytes during optical stimulation. Evaluation of the effect of Ca2+ buffering and coupling coefficient in a network of ChR2-expressing astrocytes demonstrated basal level elevations in the stimulated region and propagation of calcium activity to unstimulated cells. Buffering reduced the diffusion range of Ca2+ within the network, thereby limiting propagation and influencing the activity of astrocytes. Collectively, the framework presented in this study provides valuable information for the selection of light stimulation paradigms that elicit desired astrocytic activity using existing ChR2 constructs, as well as aids in the engineering of future application-oriented optogenetic variants.

Temporal assessment of vascular reactivity and functionality using MRI during postischemic proangiogenenic vascular remodeling

Postischemic angiogenesis is an important recovery mechanism. Both arteries and veins are upregulated during angiogenesis, but eventually there are more angiogenic veins than arteries in terms of number and length. It is critical to understand how the veins are modulated after ischemia and then transitioned into angiogenic vessels during the proangiogenic stage to finally serve as a restorative strength to the injured area. Using a rat model of transient focal cerebral ischemia, the hypercapnic blood oxygen level-dependent (BOLD) response was used to evaluate vascular reactivity, while the hyperoxic BOLD and tissue oxygen level-dependent (TOLD) responses were used to evaluate the vascular functionality at 1, 3, and 7days after ischemia. Vessel-like venous signals appeared on R2* maps on days 3 and 7, but not on day 1. The large hypercapnic BOLD responses on days 3 and 7 indicated that these areas have high vascular reactivity. The temporal correlation between vascular reactivity and the immunoreactivity to desmin and VEGF further indicates that the integrity of vascular reactivity is associated with the pericyte coverage as regulated by the VEGF level. Vascular functionality remained low on days 1, 3, and 7, as reflected by the small hyperoxic BOLD and large hyperoxic TOLD responses, indicating the low oxygen consumption of the ischemic tissues. These functional changes in proangiogenic veins may be critical for angiogenesis.

Olfactory plays a key role in spatiotemporal pathogenesis of cerebral malaria

Cerebral malaria is a complication of Plasmodium falciparum infection characterized by sudden coma, death, or neurodisability. Studies using a mouse model of experimental cerebral malaria (ECM) have indicated that blood-brain barrier disruption and CD8 T cell recruitment contribute to disease, but the spatiotemporal mechanisms are poorly understood. We show by ultra-high-field MRI and multiphoton microscopy that the olfactory bulb is physically and functionally damaged (loss of smell) by Plasmodium parasites during ECM. The trabecular small capillaries comprising the olfactory bulb show parasite accumulation and cell occlusion followed by microbleeding, events associated with high fever and cytokine storm. Specifically, the olfactory upregulates chemokine CCL21, and loss or functional blockade of its receptors CCR7 and CXCR3 results in decreased CD8 T cell activation and recruitment, respectively, as well as prolonged survival. Thus, early detection of olfaction loss and blockade of pathological cell recruitment may offer potential therapeutic strategies for ECM.