Out of the core: the impact of focal ischemia in regions beyond the penumbra

The changes in the necrotic core and the penumbra following induction of focal ischemia have been the focus of attention for some time. However, evidence shows, that ischemic injury is not confined to the primarily affected structures and may influence the remote areas as well. Yet many studies fail to probe into the structures beyond the penumbra, and possibly do not even find any significant results due to their short-term design, as secondary damage occurs later. This slower reaction can be perceived as a therapeutic opportunity, in contrast to the ischemic core defined as irreversibly damaged tissue, where the window for salvation is comparatively short. The pathologies in remote structures occur relatively frequently and are clearly linked to the post-stroke neurological outcome. In order to develop efficient therapies, a deeper understanding of what exactly happens in the exo-focal regions is necessary. The mechanisms of glia contribution to the ischemic damage in core/penumbra are relatively well described and include impaired ion homeostasis, excessive cell swelling, glutamate excitotoxic mechanism, release of pro-inflammatory cytokines and phagocytosis or damage propagation via astrocytic syncytia. However, little is known about glia involvement in post-ischemic processes in remote areas. In this literature review, we discuss the definitions of the terms "ischemic core", "penumbra" and "remote areas." Furthermore, we present evidence showing the array of structural and functional changes in the more remote regions from the primary site of focal ischemia, with a special focus on glia and the extracellular matrix. The collected information is compared with the processes commonly occurring in the ischemic core or in the penumbra. Moreover, the possible causes of this phenomenon and the approaches for investigation are described, and finally, we evaluate the efficacy of therapies, which have been studied for their anti-ischemic effect in remote areas in recent years.

Challenges and Promise for Glioblastoma Treatment through Extracellular Vesicle Inquiry

Glioblastoma (GB) is a rare but extremely aggressive brain tumor that significantly impacts patient outcomes, affecting both duration and quality of life. The protocol established by Stupp and colleagues in 2005, based on radiotherapy and chemotherapy with Temozolomide, following maximum safe surgical resection remains the gold standard for GB treatment; however, it is evident nowadays that the extreme intratumoral and intertumoral heterogeneity, as well as the invasiveness and tendency to recur, of GB are not compatible with a routine and unfortunately ineffective treatment. This review article summarizes the main challenges in the search for new valuable therapies for GB and focuses on the impact that extracellular vesicle (EV) research and exploitation may have in the field. EVs are natural particles delimited by a lipidic bilayer and filled with functional cellular content that are released and uptaken by cells as key means of cell communication. Furthermore, EVs are stable in body fluids and well tolerated by the immune system, and are able to cross physiological, interspecies, and interkingdom barriers and to target specific cells, releasing inherent or externally loaded functionally active molecules. Therefore, EVs have the potential to be ideal allies in the fight against GB and to improve the prognosis for GB patients. The present work describes the main preclinical results obtained so far on the use of EVs for GB treatment, focusing on both the EV sources and molecular cargo used in the various functional studies, primarily in vivo. Finally, a SWOT analysis is performed, highlighting the main advantages and pitfalls of developing EV-based GB therapeutic strategies. The analysis also suggests the main directions to explore to realize the possibility of exploiting EVs for the treatment of GB.

The influence of age and sex on the absolute cell numbers of the human brain cerebral cortex

The human cerebral cortex is one of the most evolved regions of the brain, responsible for most higher-order neural functions. Since nerve cells (together with synapses) are the processing units underlying cortical physiology and morphology, we studied how the human neocortex is composed regarding the number of cells as a function of sex and age. We used the isotropic fractionator for cell quantification of immunocytochemically labeled nuclei from the cerebral cortex donated by 43 cognitively healthy subjects aged 25-87 years old. In addition to previously reported sexual dimorphism in the medial temporal lobe, we found more neurons in the occipital lobe of men, higher neuronal density in women's frontal lobe, but no sex differences in the number and density of cells in the other lobes and the whole neocortex. On average, the neocortex has ~10.2 billion neurons, 34% in the frontal lobe and the remaining 66% uniformly distributed among the other 3 lobes. Along typical aging, there is a loss of non-neuronal cells in the frontal lobe and the preservation of the number of neurons in the cortex. Our study made possible to determine the different degrees of modulation that sex and age evoke on cortical cellularity.

Adrenergic regulation of astroglial aerobic glycolysis and lipid metabolism: Towards a noradrenergic hypothesis of neurodegeneration

Ageing is a key factor in the development of cognitive decline and dementia, an increasing and challenging problem of the modern world. The most commonly diagnosed cognitive decline is related to Alzheimer's disease (AD), the pathophysiology of which is poorly understood. Several hypotheses have been proposed. The cholinergic hypothesis is the oldest, however, recently the noradrenergic system has been considered to have a role as well. The aim of this review is to provide evidence that supports the view that an impaired noradrenergic system is causally linked to AD. Although dementia is associated with neurodegeneration and loss of neurons, this likely develops due to a primary failure of homeostatic cells, astrocytes, abundant and heterogeneous neuroglial cells in the central nervous system (CNS). The many functions that astrocytes provide to maintain the viability of neural networks include the control of ionic balance, neurotransmitter turnover, synaptic connectivity and energy balance. This latter function is regulated by noradrenaline, released from the axon varicosities of neurons arising from the locus coeruleus (LC), the primary site of noradrenaline release in the CNS. The demise of the LC is linked to AD, whereby a hypometabolic CNS state is observed clinically. This is likely due to impaired release of noradrenaline in the AD brain during states of arousal, attention and awareness. These functions controlled by the LC are needed for learning and memory formation and require activation of the energy metabolism. In this review, we address first the process of neurodegeneration and cognitive decline, highlighting the function of astrocytes. Cholinergic and/or noradrenergic deficits lead to impaired astroglial function. Then, we focus on adrenergic control of astroglial aerobic glycolysis and lipid droplet metabolism, which play a protective role but also promote neurodegeneration under some circumstances, supporting the noradrenergic hypothesis of cognitive decline. We conclude that targeting astroglial metabolism, glycolysis and/or mitochondrial processes may lead to important new developments in the future when searching for medicines to prevent or even halt cognitive decline.

Modern Microscopic Approaches to Astrocytes

Microscopy started as the histological analysis based on intrinsic optical properties of tissues such as the refractive index and light absorption, and is expanding to include the visualization of organelles by chemical staining, localization of molecules by immunostaining, physiological measurements such as Ca²⁺ imaging, functional manipulation by optogenetics, and comprehensive analysis of chemical composition by Raman spectra. The microscope is one of the most important tools in neuroscience, which aims to reveal the complex intercellular communications underlying brain function and pathology. Many aspects of astrocytes, including the structures of their fine processes and physiological activities in concert with neurons and blood vessels, were revealed in the course of innovations in modern microscopy. The evolution of modern microscopy is a consequence of breakthroughs in spatiotemporal resolutions and expansions in molecular and physiological targets due to the progress in optics and information technology, as well as the inventions of probes using organic chemistry and molecular biology. This review overviews the modern microscopic approach to astrocytes.

Astrocytes in the pathophysiology of neuroinfection

Key homeostasis providing cells in the central nervous system (CNS) are astrocytes, which belong to the class of cells known as atroglia, a highly heterogeneous type of neuroglia and a prominent element of the brain defence. Diseases evolve due to altered homeostatic state, associated with pathology-induced astroglia remodelling represented by reactive astrocytes, astroglial atrophy and astrodegeneration. These features are hallmarks of most infectious insults, mediated by bacteria, protozoa and viruses; they are also prominent in the systemic infection. The COVID-19 pandemic revived the focus into neurotropic viruses such as SARS-CoV2 (Coronaviridae) but also the Flaviviridae viruses including tick-borne encephalitis (TBEV) and Zika virus (ZIKV) causing the epidemic in South America prior to COVID-19. Astrocytes provide a key response to neurotropic infections in the CNS. Astrocytes form a parenchymal part of the blood-brain barrier, the site of virus entry into the CNS. Astrocytes exhibit aerobic glycolysis, a form of metabolism characteristic of highly morphologically plastic cells, like cancer cells, hence a suitable milieu for multiplication of infectious agent, including viral particles. However, why the protection afforded by astrocytes fails in some circumstances is an open question to be studied in the future.

Neuronal and astrocytic protein connections and associated adhesion molecules

Astrocytes are morphologically complex, with a myriad of processes which allow contact with other astrocytes, blood vessels, and neurons. Adhesion molecules expressed by these cells regulate this connectivity. Adhesion molecules are required to form and maintain functional neural circuits, but their importance and mechanisms of action, particularly in astrocyte-neuron contact, remain unresolved. Several studies of neuron-astrocyte connections have demonstrated the vital functions of adhesion molecules, including neuron-glia cell adhesion molecules, astrotactins, and protocadherins. In this review, we provide an overview and perspective of astrocyte-neuron contacts mediated by adhesion molecules in developing neural circuits and synapse formation, especially in the cerebellum. We also outline a novel mechanism of interaction between neurons and astrocytes in the tripartite synapses that has been recently found by our group.

Astrocytes: The Housekeepers and Guardians of the CNS

Astroglia are a diverse group of cells in the central nervous system. They are of the ectodermal, neuroepithelial origin and vary in morphology and function, yet, they can be collectively defined as cells having principle function to maintain homeostasis of the central nervous system at all levels of organisation, including homeostasis of ions, pH and neurotransmitters; supplying neurones with metabolic substrates; supporting oligodendrocytes and axons; regulating synaptogenesis, neurogenesis, and formation and maintenance of the blood-brain barrier; contributing to operation of the glymphatic system; and regulation of systemic homeostasis being central chemosensors for oxygen, CO₂ and Na⁺. Their basic physiological features show a lack of electrical excitability (inapt to produce action potentials), but display instead a rather active excitability based on variations in cytosolic concentrations of Ca²⁺ and Na⁺. It is expression of neurotransmitter receptors, pumps and transporters at their plasmalemma, along with transports on the endoplasmic reticulum and mitochondria that exquisitely regulate the cytosolic levels of these ions, the fluctuation of which underlies most, if not all, astroglial homeostatic functions.

Coming full circle: In vivo Veritas, or expanding the neuroscience frontier

The 25-century long history of brain science can be divided into four distinct 'Eras': Era 1 (∼2000 years) started around 500 BC with reductionist studies of human and animal brains using invasive in vivo and ex vivo methods; Era 2 (∼200 years) started in the 17^th century and introduced the first invasive methods to study the function of living tissues; Era 3 started around 1838 and is the ongoing era of cellular neurophysiology; finally, Era 4 (in statu nascendi) is the era of non-invasive, holistic yet mechanistic, studies of the brain. Animal experimentation is becoming increasingly more holistic as multimodal imaging and recording techniques are combined in a single experiment on the brain of awake behaving animal. This newly emerged approach can be called in vigilo (from Latin 'in awake' or 'in vigilant state'), by analogy to the earlier introduced terms in vivo or in vitro. We introduce the Special Issue "In Vigilo Veritas: New Frontiers of Optical Imaging and Electrical Recording in the Brain of Awake Behaving Mice", which features original research articles and reviews that represent some of the finest examples of a truly multimodal studies, where behavioural readouts and tasks are combined in the same longitudinal experiment and on the same mouse with the two-photon imaging, optogenetics and/or electrophysiological recordings. This exciting multi-methodological approach creates a fertile ground for breakthrough discoveries in neurophysiology and neuropsychology of an awake behaving mammalian brain.

Microglia and its Genetics in Alzheimer's Disease

Alzheimer's Disease (AD) is the most prevalent form of dementia across the world. While its discovery and pathological manifestations are centered on protein aggregations of amyloid- beta (Aβ) and hyperphosphorylated tau protein, neuroinflammation has emerged in the last decade as a main component of the disease in terms of both pathogenesis and progression. As the main innate immune cell type in the central nervous system (CNS), microglia play a very important role in regulating neuroinflammation, which occurs commonly in neurodegenerative conditions, including AD. Under inflammatory response, microglia undergo morphological changes and status transition from homeostatic to activated forms. Different microglia subtypes displaying distinct genetic profiles have been identified in AD, and these signatures often link to AD risk genes identified from the genome-wide association studies (GWAS), such as APOE and TREM2. Furthermore, many AD risk genes are highly enriched in microglia and specifically influence the functions of microglia in pathogenesis, e.g. releasing inflammatory cytokines and clearing Aβ. Therefore, building up a landscape of these risk genes in microglia, based on current preclinical studies and in the context of their pathogenic or protective effects, would largely help us to understand the complex etiology of AD and provide new insight into the unmet need for effective treatment.

Astrocytes in Alzheimer's Disease: Pathological Significance and Molecular Pathways

Astrocytes perform a wide variety of essential functions defining normal operation of the nervous system and are active contributors to the pathogenesis of neurodegenerative disorders such as Alzheimer's among others. Recent data provide compelling evidence that distinct astrocyte states are associated with specific stages of Alzheimer´s disease. The advent of transcriptomics technologies enables rapid progress in the characterisation of such pathological astrocyte states. In this review, we provide an overview of the origin, main functions, molecular and morphological features of astrocytes in physiological as well as pathological conditions related to Alzheimer´s disease. We will also explore the main roles of astrocytes in the pathogenesis of Alzheimer´s disease and summarize main transcriptional changes and altered molecular pathways observed in astrocytes during the course of the disease.

Connexins as therapeutic targets in neurological and neuropsychiatric disorders

Astrocytes represent the reticular part of the central nervous system; gap junctions formed by connexins Cx43, Cx30- and Cx26 provide for homocellular astrocyte-astrocyte coupling, whereas connexins Cx30, Cx32, Cx43, and Cx47 connect astrocytes and oligodendrocytes. Astroglial networks are anatomically and functionally segregated being homologous to neuronal ensembles. Connexons, gap junctions and hemichannels (unpaired connexons) are affected in various neuropathologies from neuropsychiatric to neurodegenerative diseases. Manipulation of astrocytic connexins modulates the size and outreach of astroglial syncytia thus affecting astroglial homeostatic support. Modulation of astrocytic connexin significantly modifies pharmacological profile of many CNS drugs, which represents an innovative therapeutic approach for CNS disorders; this approach is now actively tested in pre-clinical and clinical studies. Wide combination of connexin modulators with CNS drugs open new promising perspectives for fundamental studies and therapeutic strategies.

Calcium signals in astrocytes of the fly brain promote sleep

Astrocytes are morphologically and functionally linked to neuronal synapses, and can regulate the activity of neural circuits, brain function, and behavior. However, the molecular mechanisms by which astrocytes regulate fundamental biological processes such as sleep are not completely understood. Wu and colleagues show that an increase in calcium signals within the processes of astrocytes of the fruit fly brain can promote sleep by upregulating the expression of a monoamine receptor, TyrRII, which in turn activates sleep promoting neurons via the astrocytic release of an interleukin-1 analog, spatzle. This study provides compelling evidence for a novel molecular mechanism by which increases in astrocytic calcium signals can induce sleep by activating sleep promoting neurons in the fly brain.

Physiology of Astroglial Excitability

Classic physiology divides all neural cells into excitable neurons and nonexcitable neuroglia. Neuroglial cells, chiefly responsible for homeostasis and defense of the nervous tissue, coordinate their complex homeostatic responses with neuronal activity. This coordination reflects a specific form of glial excitability mediated by complex changes in intracellular concentration of ions and second messengers organized in both space and time. Astrocytes are equipped with multiple molecular cascades, which are central for regulating homeostasis of neurotransmitters, ionostasis, synaptic connectivity, and metabolic support of the central nervous system. Astrocytes are further provisioned with multiple receptors for neurotransmitters and neurohormones, which upon activation trigger intracellular signals mediated by Ca²⁺, Na⁺, and cyclic AMP. Calcium signals have distinct organization and underlying mechanisms in different astrocytic compartments thus allowing complex spatiotemporal signaling. Signals mediated by fluctuations in cytosolic Na⁺ are instrumental for coordination of Na⁺ dependent astrocytic transporters with tissue state and homeostatic demands. Astroglial ionic excitability may also involve K⁺, H^+, and Cl^-. The cyclic AMP signalling system is, in comparison to ions, much slower in targeting astroglial effector mechanisms. This evidence review summarizes the concept of astroglial intracellular excitability.

The Concept of Neuroglia

Neuroglia represent a diverse population of non-neuronal cells in the nervous systems, be that peripheral, central, enteric or autonomic nervous system. Arguably, these cells represent about half of the volume of the human brain. This volumetric ratio, and by extension glia to neurone ratio, not only widely differ depending on the size of the animal species brain and its positioning on the phylogenetic tree, but also vary between the regions of an individual brain. Neuroglia derived from a dual origin (ectoderm and mesodermal) and in an assorted morphology, yet their functional traits can be mainly classified into being keepers of homeostasis (water, ions, neurotransmitters, metabolites, fuels, etc.) and defenders (e.g., against microbial organisms, etc.) of the nervous system. As these capabilities go awry, neuroglia ultimately define their fundamental role in most, if not, all neuropathologies. This concept presented in this chapter serves as a general introduction into the world of neuroglia and subsequent topics covered by this book.