Human and mouse cortical astrocytes: a comparative view from development to morphological and functional characterization

Impact of perfusion on neuronal development in human derived neuronal networks

Advanced in vitro models of the brain have evolved in recent years from traditional two-dimensional (2D) ones, based on rodent derived cells, to three-dimensional (3D) ones, based on human neurons derived from induced pluripotent stem cells. To address the dynamic changes of the tissue microenvironment, bioreactors are used to control the in vitro microenvironment for viability, repeatability, and standardization. However, in neuronal tissue engineering, bioreactors have primarily been used for cell expansion purposes, while microfluidic systems have mainly been employed for culturing organoids. In this study, we explored the use of a commercial perfusion bioreactor to control the culture microenvironment of neuronal cells in both 2D and 3D cultures. Namely, neurons differentiated from human induced pluripotent stem cells (iNeurons) were cultured in 2D under different constant flow rates for 72 h. The impact of different flow rates on early-stage neuronal development and synaptogenesis was assessed by morphometric characterization and synaptic analysis. Based on these results, two involving variable flow rates were developed and applied again in 2D culture. The most effective protocol, in terms of positive impact on neuronal development, was then used for a preliminary study on the application of dynamic culturing conditions to neuronal cells in 3D. To this purpose, both iNeurons, co-cultured with astrocytes, and the human neuroblastoma cells SH-SY5Y were embedded into a hydrogel and maintained under perfusion for up to 28 days. A qualitative evaluation by immunocytochemistry and confocal microscopy was carried out to assess cell morphology and the formation of a 3D neuronal network.

Reactive astrocytes generated from human iPSC are pro-inflammatory and display altered metabolism

Astrocytes are the most abundant type of glial cell in the central nervous system and they play pivotal roles in both normal health and disease. Their dysfunction is detrimental to many brain related pathologies. Under pathological conditions, such as Alzheimer's disease, astrocytes adopt an activated reactive phenotype which can contribute to disease progression. A prominent risk factor for many neurodegenerative diseases is neuroinflammation which is the purview of glial cells, such as astrocytes and microglia. Human in vitro models have the potential to reveal relevant disease specific mechanisms, through the study of individual cell types such as astrocytes or the addition of specific factors, such as those secreted by microglia. The aim of this study was to generate human cortical astrocytes, in order to assess their protein and gene expression, examine their reactivity profile in response to exposure to the microglial secreted factors IL-1α, TNFα and C1q and assess their functionality in terms of calcium signalling and metabolism. The successfully differentiated and stimulated reactive astrocytes display increased IL-6, RANTES and GM-CSF secretion, and increased expression of genes associated with reactivity including, IL-6, ICAM1, LCN2, C3 and SERPINA3. Functional assessment of these reactive astrocytes showed a delayed and sustained calcium response to ATP and a concomitant decrease in the expression of connexin-43. Furthermore, it was demonstrated these astrocytes had an increased glycolytic capacity with no effect on oxidative phosphorylation. These findings not only increase our understanding of astrocyte reactivity but also provides a functional platform for drug discovery.

Specific cleavage of IGFBP-4 by papp-a in nervous tissue

Astrocytes are subtypes of glial cells involved in metabolic, structural, homeostatic, and neuroprotective processes that help neurons maintain viability. Insulin-like growth factors IGF-1 and IGF-2 are known to have neuroprotective effects on neurons and glial cells through interaction with specific receptors. IGF forms a complex with IGF-binding proteins (IGFBP) in nervous tissue and is released from the complex via IGFBP proteolysis by specific proteases. It has been reported that IGFBP-2, 5 and 6 are cleaved by specific proteases in the central nervous system (CNS), followed by IGF release; however, it was unknown whether IGFBP-4 was exposed to a particular proteolysis in nervous tissue. Using neurons and astrocytes derived from human induced pluripotent stem cell lines (hiPSC), as well as rat brain-sourced primary neuron-glia cultures, we demonstrated that IGFBP-4 is specifically cleaved in nervous tissue by the Pregnancy Associated Plasma Protein A (PAPP-A) protease and that this cleavage is IGF-dependent. Our results indicate that astrocyte rather than neuron PAPP-A cleaves IGFBP-4 in nervous tissue suggesting that this may be one of the fundamental mechanisms for IGF interchange between these two types of cells.

The predominance of "astrocytic" intranuclear inclusions in neuronal intranuclear inclusion disease manifesting encephalopathy-like symptoms: A case series with brain biopsy

Neuronal intranuclear inclusion disease (NIID) is a neurodegenerative disorder represented by eosinophilic intranuclear inclusions (EIIs) and GGC/CGG repeat expansion in the NOTCH2NLC gene. We report here two adult cases of NIID, genetically confirmed, with manifestation of encephalopathy-like symptoms and address the histopathologic findings obtained by brain biopsies, with a focus on "astrocytic" intranuclear inclusions (AIIs). Case 1 presented with paroxysmal restlessness, vertigo, or fever and was later involved in severe dementia and tetraparesis. Case 2 presented with forgetfulness and then with paroxysmal fever and headache. In both cases, delimited areas with gadolinium enhancement on magnetic resonance imaging and corresponding hyperperfusion were detected, leading to brain biopsies of the cortex. On histology, Case 1 showed an abnormal lamination, where the thickness of layers was different from usual. Both neurons and astrocytes showed some dysmorphologic features. Notably, astrocytes rather than neurons harbored EIIs. Case 2 showed a cortex, where neurons tended to be arrayed in a columnar fashion. Astrocytes showed some dysmorphologic features. Notably, much more astrocytes than neurons harbored EIIs. By a double-labeling immunofluorescence study for p62/NeuN and p62/glial fibrillary acidic protein, the predominance of AIIs was confirmed in both cases. Considering the physiological functions of astrocytes for the development and maintenance of the cortex, the encephalopathy-like symptoms, dynamic change of cerebral blood flow, and cortical dysmorphology can reasonably be explained by the dysfunction of EII-bearing astrocytes rather than EII-bearing neurons. This study suggests the presence of a subtype of NIID where AIIs rather than "neuronal" intranuclear inclusions are likely a key player in the pathogenesis of NIID, particularly in cases with encephalopathy-like symptoms. The importance of AIIs ("gliopathy") should be more appreciated in future studies of NIID.

Brain stars take the lead during critical periods of early postnatal brain development: relevance of astrocytes in health and mental disorders

In the brain, astrocytes regulate shape and functions of the synaptic and vascular compartments through a variety of released factors and membrane-bound proteins. An imbalanced astrocyte activity can therefore have drastic negative impacts on brain development, leading to the onset of severe pathologies. Clinical and pre-clinical studies show alterations in astrocyte cell number, morphology, molecular makeup and astrocyte-dependent processes in different affected brain regions in neurodevelopmental (ND) and neuropsychiatric (NP) disorders. Astrocytes proliferate, differentiate and mature during the critical period of early postnatal brain development, a time window of elevated glia-dependent regulation of a proper balance between synapse formation/elimination, which is pivotal in refining synaptic connectivity. Therefore, any intrinsic and/or extrinsic factors altering these processes during the critical period may result in an aberrant synaptic remodeling and onset of mental disorders. The peculiar bridging position of astrocytes between synaptic and vascular compartments further allows them to "compute" the brain state and consequently secrete factors in the bloodstream, which may serve as diagnostic biomarkers of distinct healthy or disease conditions. Here, we collect recent advancements regarding astrogenesis and astrocyte-mediated regulation of neuronal network remodeling during early postnatal critical periods of brain development, focusing on synapse elimination. We then propose alternative hypotheses for an involvement of aberrancies in these processes in the onset of ND and NP disorders. In light of the well-known differential prevalence of certain brain disorders between males and females, we also discuss putative sex-dependent influences on these neurodevelopmental events. From a translational perspective, understanding age- and sex-dependent astrocyte-specific molecular and functional changes may help to identify biomarkers of distinct cellular (dys)functions in health and disease, favouring the development of diagnostic tools or the selection of tailored treatment options for male/female patients.

Investigations of Astrocyte Calcium Signaling and Imaging with Classical and Nonclassical Light

The application of light in studying and influencing cellular behavior with improved temporal and spatial resolution remains a key objective in fields such as chemistry, physics, medicine, and engineering. In the brain, nonexcitable cells called astrocytes play essential roles in regulating homeostasis and cognitive function through complex calcium signaling pathways. Understanding these pathways is vital for deciphering brain physiology and neurological disorders like Parkinson's and Alzheimer's. Despite challenges in selectively targeting astrocyte signaling pathways due to shared molecular equipment with neurons, recent advancements in laser technology offer promising avenues. However, the effort to use laser light properties to study astroglial cell function is still limited. This work aims to exploit an in-depth pharmacological analysis of astrocyte calcium channels to determine the physiological mechanism induced by exposure to classical nanosecond-pulsed light. We herein report molecular clues supporting the use of visible-nanosecond laser pulses as a promising approach to excite primary rat neocortical astrocytes and unprecedentedly report on the implementation of entangled two-photon microscopy to image them.

Dissecting reactive astrocyte responses: lineage tracing and morphology-based clustering

Brain damage triggers diverse cellular and molecular events, with astrocytes playing a crucial role in activating local neuroprotective and reparative signaling within damaged neuronal circuits. Here, we investigated reactive astrocytes using a multidimensional approach to categorize their responses into different subtypes based on morphology. This approach utilized the StarTrack lineage tracer, single-cell imaging reconstruction and multivariate data analysis. Our findings identified three profiles of reactive astrocyte responses, categorized by their effects on cell size- and shape- related morphological parameters: "moderate", "strong," and "very strong". We also examined the heterogeneity of astrocyte reactivity, focusing on spatial and clonal distribution. Our research revealed a notable enrichment of protoplasmic and fibrous astrocytes within the "strong" and "very strong" response subtypes. Overall, our study contributes to a better understanding of astrocyte heterogeneity in response to an injury. By characterizing the diverse reactive responses among astrocyte subpopulations, we provide insights that could guide future research aimed at identifying novel therapeutic targets to mitigate brain damage and promote neural repair.

Astrocytes adopt a progenitor-like migratory strategy for regeneration in adult brain

Mature astrocytes become activated upon non-specific tissue damage and contribute to glial scar formation. Proliferation and migration of adult reactive astrocytes after injury is considered very limited. However, the regenerative behavior of individual astrocytes following selective astroglial loss, as seen in astrocytopathies, such as neuromyelitis optica spectrum disorder, remains unexplored. Here, we performed longitudinal in vivo imaging of cortical astrocytes after focal astrocyte ablation in mice. We discovered that perilesional astrocytes develop a remarkable plasticity for efficient lesion repopulation. A subset of mature astrocytes transforms into reactive progenitor-like (REPL) astrocytes that not only undergo multiple asymmetric divisions but also remain in a multinucleated interstage. This regenerative response facilitates efficient migration of newly formed daughter cell nuclei towards unoccupied astrocyte territories. Our findings define the cellular principles of astrocyte plasticity upon focal lesion, unravelling the REPL phenotype as a fundamental regenerative strategy of mature astrocytes to restore astrocytic networks in the adult mammalian brain. Promoting this regenerative phenotype bears therapeutic potential for neurological conditions involving glial dysfunction.

Identification of the abnormalities in astrocytic functions as potential drug targets for neurodegenerative disease

INTRODUCTION

Historically, astrocytes were seen primarily as a supportive cell population within the brain; with neurodegenerative disease research focusing exclusively on malfunctioning neurons. However, astrocytes perform numerous tasks that are essential for maintenance of the central nervous system`s complex processes. Disruption of these functions can have negative consequences; hence, it is unsurprising to observe a growing amount of evidence for the essential role of astrocytes in the development and progression of neurodegenerative diseases. Targeting astrocytic functions may serve as a potential disease-modifying drug therapy in the future.

AREAS COVERED

The present review emphasizes the key astrocytic functions associated with neurodegenerative diseases and explores the possibility of pharmaceutical interventions to modify these processes. In addition, the authors provide an overview of current advancement in this field by including studies of possible drug candidates.

EXPERT OPINION

Glial research has experienced a significant renaissance in the last quarter-century. Understanding how disease pathologies modify or are caused by astrocyte functions is crucial when developing treatments for brain diseases. Future research will focus on building advanced models that can more precisely correlate to the state in the human brain, with the goal of routinely testing therapies in these models.

Human stem cell transplantation models of Alzheimer's disease

Alzheimer's disease (AD) is the most frequent form of dementia. It is characterized by pronounced neuronal degeneration with formation of neurofibrillary tangles and deposition of amyloid β throughout the central nervous system. Animal models have provided important insights into the pathogenesis of AD and they have shown that different brain cell types including neurons, astrocytes and microglia have important functions in the pathogenesis of AD. However, there are difficulties in translating promising therapeutic observations in mice into clinical application in patients. Alternative models using human cells such as human induced pluripotent stem cells (iPSCs) may provide significant advantages, since they have successfully been used to model disease mechanisms in neurons and in glial cells in neurodegenerative diseases in vitro and in vivo. In this review, we summarize recent studies that describe the transplantation of human iPSC-derived neurons, astrocytes and microglial cells into the forebrain of mice to generate chimeric transplantation models of AD. We also discuss opportunities, challenges and limitations in using differentiated human iPSCs for in vivo disease modeling and their application for biomedical research.

Neuron-Astrocyte Interactions: A Human Perspective

This chapter explores the intricate interactions between neurons and astrocytes within the nervous system with a particular emphasis on studies conducted in human tissue or with human cells. We specifically explore how neuron-astrocyte interactions relate to processes of cellular development, morphology, migration, synapse formation, and metabolism. These findings enrich our understanding of basic neurobiology and how disruptions in these processes are relevant to human diseases.The study of human neuron-astrocyte interactions is made possible because of transformative in vitro advancements that have facilitated the generation and sustained culture of human neural cells. In addition, the rise of techniques like sequencing at single-cell resolution has enabled the exploration of numerous human cell atlases and their comparisons to other animal model systems. Thus, the innovations outlined in this chapter illuminate the convergence and divergence of neuron-astrocyte interactions across species. As technologies progress, continually more sophisticated in vitro systems will increasingly reflect in vivo environments and deepen our command of neuron-glial interactions in human biology.

Blockade of mGluR5 in astrocytes derived from human iPSCs modulates astrocytic function and increases phagocytosis

TNF-α is essential for induction and maintenance of inflammatory responses and its dysregulation is associated with susceptibility to various pathogens that infect the central nervous system. Activation of both microglia and astrocytes leads to TNF-α production, which in turn triggers further activation of these cells. Astrocytes have been implicated in the pathophysiology of a wide range of neurodegenerative diseases with either harmful or protective roles, as these cells are capable of secreting several inflammatory factors and also promote synapse elimination and remodeling. These responses are possible because they sense their surroundings via several receptors, including the metabotropic glutamate receptor 5 (mGluR5). Under neuroinflammatory conditions, mGluR5 activation in astrocytes can be neuroprotective or have the opposite effect. In the current study, we investigated the role of mGluR5 in hiPSC-derived astrocytes subjected to pro-inflammatory stimulation by recombinant TNF-α (rTNF-α). Our results show that mGluR5 blockade by CTEP decreases the secreted levels of pro-inflammatory cytokines (IL-6 and IL-8) following short rTNF-α stimulation, although this effect subsides with time. Additionally, CTEP enhances synaptoneurosome phagocytosis by astrocytes in both non-stimulated and rTNF-α-stimulated conditions, indicating that mGluR5 blockade alone is enough to drive synaptic material engulfment. Finally, mGluR5 antagonism as well as rTNF-α stimulation augment the expression of the reactivity marker SERPINA3 and reduces the expression of synaptogenic molecules. Altogether, these data suggest a complex role for mGluR5 in human astrocytes, since its blockade may have beneficial and detrimental effects under inflammatory conditions.

Distinctive biophysical features of human cell-types: insights from studies of neurosurgically resected brain tissue

Electrophysiological characterization of live human tissue from epilepsy patients has been performed for many decades. Although initially these studies sought to understand the biophysical and synaptic changes associated with human epilepsy, recently, it has become the mainstay for exploring the distinctive biophysical and synaptic features of human cell-types. Both epochs of these human cellular electrophysiological explorations have faced criticism. Early studies revealed that cortical pyramidal neurons obtained from individuals with epilepsy appeared to function "normally" in comparison to neurons from non-epilepsy controls or neurons from other species and thus there was little to gain from the study of human neurons from epilepsy patients. On the other hand, contemporary studies are often questioned for the "normalcy" of the recorded neurons since they are derived from epilepsy patients. In this review, we discuss our current understanding of the distinct biophysical features of human cortical neurons and glia obtained from tissue removed from patients with epilepsy and tumors. We then explore the concept of within cell-type diversity and its loss (i.e., "neural homogenization"). We introduce neural homogenization to help reconcile the epileptogenicity of seemingly "normal" human cortical cells and circuits. We propose that there should be continued efforts to study cortical tissue from epilepsy patients in the quest to understand what makes human cell-types "human".

Deepening the role of excitation/inhibition balance in human iPSCs-derived neuronal networks coupled to MEAs during long-term development

Objective.The purpose of this study is to investigate whether and how the balance between excitation and inhibition ('E/I balance') influences the spontaneous development of human-derived neuronal networksin vitro. To achieve that goal, we performed a long-term (98 d) characterization of both homogeneous (only excitatory or inhibitory neurons) and heterogeneous (mixed neuronal types) cultures with controlled E/I ratios (i.e. E:I 0:100, 25:75, 50:50, 75:25, 100:0) by recording their electrophysiological activity using micro-electrode arrays.Approach.Excitatory and inhibitory neurons were derived from human induced pluripotent stem cells (hiPSCs). We realized five different configurations by systematically varying the glutamatergic and GABAergic percentages.Main results.We successfully built both homogeneous and heterogeneous neuronal cultures from hiPSCs finely controlling the E/I ratios; we were able to maintain them for up to 3 months. Homogeneity differentially impacted purely inhibitory (no bursts) and purely excitatory (few bursts) networks, deviating from the typical traits of heterogeneous cultures (burst dominated). Increased inhibition in heterogeneous cultures strongly affected the duration and organization of bursting and network bursting activity. Spike-based functional connectivity and image-based deep learning analysis further confirmed all the above.Significance.Healthy neuronal activity is controlled by a well-defined E/I balance whose alteration could lead to the onset of neurodevelopmental disorders like schizophrenia or epilepsy. Most of the commonly usedin vitromodels are animal-derived or too simplified and thus far from thein vivohuman condition. In this work, by performing a long-term study of hiPSCs-derived neuronal networks obtained from healthy human subjects, we demonstrated the feasibility of a robustin vitromodel which can be further exploited for investigating pathological conditions where the E/I balance is impaired.