Human Brain Organoids-on-Chip: Advances, Challenges, and Perspectives for Preclinical Applications.

There is an urgent need for predictive in vitro models to improve disease modeling and drug target identification and validation, especially for neurological disorders. Cerebral organoids, as alternative methods to in vivo studies, appear now as powerful tools to decipher complex biological processes thanks to their ability to recapitulate many features of the human brain. Combining these innovative models with microfluidic technologies, referred to as brain organoids-on-chips, allows us to model the microenvironment of several neuronal cell types in 3D. Thus, this platform opens new avenues to create a relevant in vitro approach for preclinical applications in neuroscience. The transfer to the pharmaceutical industry in drug discovery stages and the adoption of this approach by the scientific community requires the proposition of innovative microphysiological systems allowing the generation of reproducible cerebral organoids of high quality in terms of structural and functional maturation, and compatibility with automation processes and high-throughput screening. In this review, we will focus on the promising advantages of cerebral organoids for disease modeling and how their combination with microfluidic systems can enhance the reproducibility and quality of these in vitro models. Then, we will finish by explaining why brain organoids-on-chips could be considered promising platforms for pharmacological applications.

l-Serine links metabolism with neurotransmission.

Brain energy metabolism is often considered as a succession of biochemical steps that metabolize the fuel (glucose and oxygen) for the unique purpose of providing sufficient ATP to maintain the huge information processing power of the brain. However, a significant fraction (10-15 %) of glucose is shunted away from the ATP-producing pathway (oxidative phosphorylation) and may be used to support other functions. Recent studies have pointed to the marked compartmentation of energy metabolic pathways between neurons and glial cells. Here, we focused our attention on the biosynthesis of l-serine, a non-essential amino acid that is formed exclusively in glial cells (mostly astrocytes) by re-routing the metabolic fate of the glycolytic intermediate, 3-phosphoglycerate (3PG). This metabolic pathway is called the phosphorylated pathway and transforms 3PG into l-serine via three enzymatic reactions. We first compiled the available data on the mechanisms that regulate the flux through this metabolic pathway. We then reviewed the current evidence that is beginning to unravel the roles of l-serine both in the healthy and diseased brain, leading to the notion that this specific metabolic pathway connects glial metabolism with synaptic activity and plasticity. We finally suggest that restoring astrocyte-mediated l-serine homeostasis may provide new therapeutic strategies for brain disorders.

Impairment of Glycolysis-Derived l-Serine Production in Astrocytes Contributes to Cognitive Deficits in Alzheimer's Disease.

Alteration of brain aerobic glycolysis is often observed early in the course of Alzheimer's disease (AD). Whether and how such metabolic dysregulation contributes to both synaptic plasticity and behavioral deficits in AD is not known. Here, we show that the astrocytic l-serine biosynthesis pathway, which branches from glycolysis, is impaired in young AD mice and in AD patients. l-serine is the precursor of d-serine, a co-agonist of synaptic NMDA receptors (NMDARs) required for synaptic plasticity. Accordingly, AD mice display a lower occupancy of the NMDAR co-agonist site as well as synaptic and behavioral deficits. Similar deficits are observed following inactivation of the l-serine synthetic pathway in hippocampal astrocytes, supporting the key role of astrocytic l-serine. Supplementation with l-serine in the diet prevents both synaptic and behavioral deficits in AD mice. Our findings reveal that astrocytic glycolysis controls cognitive functions and suggest oral l-serine as a ready-to-use therapy for AD.

Evidence supporting a role for astrocytes in the regulation of cognitive flexibility and neuronal oscillations through the Ca2+ binding protein S100ß.

The medial prefrontal cortex (mPFC) is important for cognitive flexibility, the ability to switch between two task-relevant dimensions. Changes in neuronal oscillations and alterations in the coupling across frequency ranges have been correlated with attention and cognitive flexibility. Here we show that astrocytes in the mPFC of adult male Sprague Dawley rats, participate in cognitive flexibility through the astrocyte-specific Ca2+ binding protein S100ß, which improves cognitive flexibility and increases phase amplitude coupling between theta and gamma oscillations. We further show that reduction of astrocyte number in the mPFC impairs cognitive flexibility and diminishes delta, alpha and gamma power. Conversely, chemogenetic activation of astrocytic intracellular Ca2+ signaling in the mPFC enhances cognitive flexibility, while inactivation of endogenous S100ß among chemogenetically activated astrocytes in the mPFC prevents this improvement. Collectively, our work suggests that astrocytes make important contributions to cognitive flexibility and that they do so by releasing a Ca2+ binding protein which in turn enhances coordinated neuronal oscillations.

Immature Neurons and Radial Glia, But Not Astrocytes or Microglia, Are Altered in Adult Cntnap2 and Shank3 Mice, Models of Autism.

Autism spectrum disorder (ASD) is often associated with cognitive deficits and excessive anxiety. Neuroimaging studies have shown atypical structure and neural connectivity in the hippocampus, medial prefrontal cortex (mPFC), and striatum, regions associated with cognitive function and anxiety regulation. Adult hippocampal neurogenesis is involved in many behaviors that are disrupted in ASD, including cognition, anxiety, and social behaviors. Additionally, glial cells, such as astrocytes and microglia, are important for modulating neural connectivity during development, and glial dysfunction has been hypothesized to be a key contributor to the development of ASD. Cells with astroglial characteristics are known to serve as progenitor cells in the developing and adult brain. Here, we examined adult neurogenesis in the hippocampus, as well as astroglia and microglia in the hippocampus, mPFC, and striatum of two models that display autism-like phenotypes, Cntnap2-/- and Shank3+/ΔC transgenic mice. We found a substantial decrease in the number of immature neurons and radial glial progenitor cells in the ventral hippocampus of both transgenic models compared with wild-type controls. No consistent differences were detected in the number or size of astrocytes or microglia in any other brain region examined. Future work is needed to explore the functional contribution of adult neurogenesis to autism-related behaviors as well as to temporally characterize glial plasticity as it is associated with ASD.