Dynamics of microglia in the developing rat brain

History of Microglia

The term 'microglia' was first introduced into the scientific literature a century ago. The various eras of microglial research have been defined not only by the number of reports subsequently generated but, more critically, also by the concepts that have shaped our present-day views and understanding of microglia. Key methods, technologies, and models, as well as seminal discoveries made possible through their deployment have enabled breakthroughs, and now pave the way for lines of investigation that could not have been anticipated even a decade ago. Advances in our understanding of the microglial origin, forms, and functions have relied fundamentally on parallel developments in immunology. As the 'neuro-immune' cells of the brain, microglia are now under the spotlight in various disciplines. This chapter surveys the gradual processes and precipitous events that helped form ideas concerning the developmental origin of microglia and their roles in health and disease. It first covers the dawning phase during which the early pioneers of microglial research discovered cellular entities and already assigned functions to them. Following a recess period, the 1960s brought about a renaissance of active interest, with the development of tools and models-and fundamental notions on microglial contributions to central nervous system (CNS) pathologies. These seminal efforts laid the foundation for the awakening of a sweeping research era beginning in the 1980s and spurred on by a blast of immunological discoveries. Finally, this chapter stresses the advancements in molecular, genetic, and imaging approaches to the study of microglia with the turn of the millennium, enabling insights into virtually all facets of microglial physiology. Moving forward, it is clear that the future holds substantial promise for further discoveries. The next epoch in the history of microglial research has just begun.

The inflammatory response to birth requires MyD88 and is driven by both mother and offspring

Birth is an inflammatory event for the newborn, characterized by elevations in interleukin (IL)-6, IL-10, and tumor necrosis factor (TNF)-α peripherally and/or centrally, as well as changes in brain microglia. However, the mechanism(s) underlying these responses is unknown. Toll-like receptors (TLRs) play crucial roles in innate immunity and initiate inflammatory cascades upon recognition of endogenous or exogenous antigens. Most TLR signaling depends on the adaptor molecule myeloid differentiation primary response 88 (MyD88). We independently varied MyD88 gene status in mouse dams and their offspring to determine whether the inflammatory response to birth depends on MyD88 signaling and, if so, whether that signaling occurs in the offspring, the mother, or both. We find that the perinatal surges in plasma IL-6 and brain expression of TNF-α depend solely on MyD88 gene status of the offspring, whereas postnatal increases in plasma IL-10 and TNF-α depend on MyD88 in both the pup and dam. Interestingly, MyD88 genotype of the dam primarily drives differences in offspring brain microglial density and has robust effects on developmental neuronal cell death. Milk cytokines were evaluated as a possible source of postnatal maternal influence; although we found high levels of CXCL1/GROα and several other cytokines in ingested post-partum milk, their presence did not require MyD88. Thus, the inflammatory response previously described in the late-term fetus and newborn depends on MyD88 (and, by extension, TLRs), with signaling in both the dam and offspring contributing. Unexpectedly, naturally-occuring neuronal cell death in the newborn is modulated primarily by maternal MyD88 gene status.

Developmental profile of microglia distribution in nuclei of the superior olivary complex

In the brain, microglia are involved in immune responses and synaptic maturation. During early development, these cells invade the brain, proliferate, and morphologically mature to achieve coverage of the surrounding tissue with their fine processes. Their developmental proliferation overlaps with the postnatal development of neuronal circuits. Within the superior olivary complex (SOC), an auditory brainstem structure, microglia, and their early postnatal development have been documented. A quantification over the full developmental profile of the arrangement and morphological changes in single microglia cells is missing. Here, we used immunofluorescence labeling to quantify their distribution, morphological changes, and coverage during early and late postnatal development in the SOC of Mongolian gerbils. Microglia distributed rather homogenously within each nucleus with a bias to the nucleus borders at postnatal day (P) 5 and more centrally in the nucleus in mature stages. We found a nucleus-specific transient increase in microglia cell number and density reaching its peak at P17 with a subsequent decline to P55 values. Length and branching of microglia protrusions increased especially after P12. The stronger ramification together with the increase in cell density allows coverage of the surrounding tissue from P5 to mature stages, despite the large developmental increase in nucleus size. The transient increase in density during synaptic refinement in SOC nuclei suggests that microglia are important during the pruning period, compensating for developmental increase in tissue volume, and that in mature stages their main function appears tissue surveillance.

Micromotion Derived Fluid Shear Stress Mediates Peri-Electrode Gliosis through Mechanosensitive Ion Channels

The development of bioelectronic neural implant technologies has advanced significantly over the past 5 years, particularly in brain-machine interfaces and electronic medicine. However, neuroelectrode-based therapies require invasive neurosurgery and can subject neural tissues to micromotion-induced mechanical shear, leading to chronic inflammation, the formation of a peri-electrode void and the deposition of reactive glial scar tissue. These structures act as physical barriers, hindering electrical signal propagation and reducing neural implant functionality. Although well documented, the mechanisms behind the initiation and progression of these processes are poorly understood. Herein, in silico analysis of micromotion-induced peri-electrode void progression and gliosis is described. Subsequently, ventral mesencephalic cells exposed to milliscale fluid shear stress in vitro exhibited increased expression of gliosis-associated proteins and overexpression of mechanosensitive ion channels PIEZO1 (piezo-type mechanosensitive ion channel component 1) and TRPA1 (transient receptor potential ankyrin 1), effects further confirmed in vivo in a rat model of peri-electrode gliosis. Furthermore, in vitro analysis indicates that chemical inhibition/activation of PIEZO1 affects fluid shear stress mediated astrocyte reactivity in a mitochondrial-dependent manner. Together, the results suggest that mechanosensitive ion channels play a major role in the development of a peri-electrode void and micromotion-induced glial scarring at the peri-electrode region.

Role of purines in brain development, from neuronal proliferation to synaptic refinement

The purinergic system includes P1 and P2 receptors, which are activated by ATP and its metabolites. They are expressed in adult neuronal and glial cells and are crucial in brain function, including neuromodulation and neuronal signaling. As P1 and P2 receptors are expressed throughout embryogenesis and development, purinergic signaling also has an important role in the development of the peripheral and central nervous system. In this review, we present the expression pattern and activity of purinergic receptors and of their signaling pathways during embryonic and postnatal development of the nervous system. In particular, we review the involvement of the purinergic signaling in all the crucial steps of brain development i.e. in neural stem cell proliferation, neuronal differentiation and migration as well as in astrogliogenesis and oligodendrogenesis. Then, we review data showing a crucial role of the ATP and adenosine signaling pathways in the formation of the peripheral neuromuscular junction and of central GABAergic and glutamatergic synapses. Finally, we examine the consequences of deregulation of the purinergic system during development and discuss the therapeutic potential of targeting it at adult stage in diseases with reactivation of the ATP and adenosine pathway.

Serotonin sensing by microglia conditions the proper development of neuronal circuits and of social and adaptive skills

The proper maturation of emotional and sensory circuits requires fine-tuning of serotonin (5-HT) level during early postnatal development. Consistently, dysfunctions of the serotonergic system have been associated with neurodevelopmental psychiatric diseases, including autism spectrum disorders (ASD). However, the mechanisms underlying the developmental effects of 5-HT remain partially unknown, one obstacle being the action of 5-HT on different cell types. Here, we focused on microglia, which play a role in brain wiring refinement, and we investigated whether the control of these cells by 5-HT is relevant for neurodevelopment and spontaneous behaviors in mice. Since the main 5-HT sensor in microglia is the 5-HT2B receptor subtype, we prevented 5-HT signaling specifically in microglia by conditional invalidation of the Htr2b gene in these cells. We observed that abrogating the serotonergic control of microglia during early postnatal development affects the phagolysosomal compartment of these cells and their proximity to dendritic spines and perturbs neuronal circuits maturation. Furthermore, this early ablation of microglial 5-HT2B receptors leads to adult hyperactivity in a novel environment and behavioral defects in sociability and flexibility. Importantly, we show that these behavioral alterations result from a developmental effect, since they are not observed when microglial Htr2b invalidation is induced later, at P30 onward. Thus, a primary alteration of 5-HT sensing in microglia, during a critical time window between birth and P30, is sufficient to impair social and flexibility skills. This link between 5-HT and microglia may explain the association between serotonergic dysfunctions and behavioral traits like impaired sociability and inadaptability to novelty, which are prominent in psychiatric disorders such as ASD.

The multifaceted roles of embryonic microglia in the developing brain

Microglia are the resident immune cells of the central nervous system (CNS). Microglia originate from erythromyeloid progenitors in the yolk sac at the early embryonic stage, and these progenitors then colonize the CNS through extensive migration and proliferation during development. Microglia account for 10% of all cells in the adult brain, whereas the proportion of these cells in the embryonic brain is only 0.5-1.0%. Nevertheless, microglia in the developing brain widely move their cell body within the structure by extending filopodia; thus, they can interact with surrounding cells, such as neural lineage cells and vascular-structure-composing cells. This active microglial motility suggests that embryonic microglia play a pivotal role in brain development. Indeed, recent increasing evidence has revealed diverse microglial functions at the embryonic stage. For example, microglia control differentiation of neural stem cells, regulate the population size of neural progenitors and modulate the positioning and function of neurons. Moreover, microglia exert functions not only on neural lineage cells but also on blood vessels, such as supporting vascular formation and integrity. This review summarizes recent advances in the understanding of microglial cellular dynamics and multifaceted functions in the developing brain, with particular focus on the embryonic stage, and discusses the fundamental molecular mechanisms underlying their behavior.

Origin and Emergence of Microglia in the CNS-An Interesting (Hi)story of an Eccentric Cell

Microglia belong to tissue-resident macrophages of the central nervous system (CNS), representing the primary innate immune cells. This cell type constitutes ~7% of non-neuronal cells in the mammalian brain and has a variety of biological roles integral to homeostasis and pathophysiology from the late embryonic to adult brain. Its unique identity that distinguishes its "glial" features from tissue-resident macrophages resides in the fact that once entering the CNS, it is perennially exposed to a unique environment following the formation of the blood-brain barrier. Additionally, tissue-resident macrophage progenies derive from various peripheral sites that exhibit hematopoietic potential, and this has resulted in interpretation issues surrounding their origin. Intensive research endeavors have intended to track microglial progenitors during development and disease. The current review provides a corpus of recent evidence in an attempt to disentangle the birthplace of microglia from the progenitor state and underlies the molecular elements that drive microgliogenesis. Furthermore, it caters towards tracking the lineage spatiotemporally during embryonic development and outlining microglial repopulation in the mature CNS. This collection of data can potentially shed light on the therapeutic potential of microglia for CNS perturbations across various levels of severity.

Brain effects of gestating germ-free persist in mouse neonates despite acquisition of a microbiota at birth

At birth, mammals experience a massive colonization by microorganisms. We previously reported that newborn mice gestated and born germ-free (GF) have increased microglial labeling and alterations in developmental neuronal cell death in the hippocampus and hypothalamus, as well as greater forebrain volume and body weight when compared to conventionally colonized (CC) mice. To test whether these effects are solely due to differences in postnatal microbial exposure, or instead may be programmed in utero, we cross-fostered GF newborns immediately after birth to CC dams (GF→CC) and compared them to offspring fostered within the same microbiota status (CC→CC, GF→GF). Because key developmental events (including microglial colonization and neuronal cell death) shape the brain during the first postnatal week, we collected brains on postnatal day (P) 7. To track gut bacterial colonization, colonic content was also collected and subjected to 16S rRNA qPCR and Illumina sequencing. In the brains of GF→GF mice, we replicated most of the effects seen previously in GF mice. Interestingly, the GF brain phenotype persisted in GF→CC offspring for almost all measures. In contrast, total bacterial load did not differ between the CC→CC and GF→CC groups on P7, and bacterial community composition was also very similar, with a few exceptions. Thus, GF→CC offspring had altered brain development during at least the first 7 days after birth despite a largely normal microbiota. This suggests that prenatal influences of gestating in an altered microbial environment programs neonatal brain development.

Work hard, play hard: how sexually differentiated microglia work to shape social play and reproductive behavior

Microglia are brain-resident immune cells that play a critical role in synaptic pruning and circuit fine-tuning during development. In the adult brain, microglia actively survey their local environment and mobilize inflammatory responses to signs of damage or infection. Sex differences in microglial gene expression and function across the lifespan have been identified, which play a key role in shaping brain function and behavior. The levels of sex hormones such as androgens, estrogens, and progesterone vary in an age-dependent and sex-dependent manner. Microglia respond both directly and indirectly to changes in hormone levels, altering transcriptional gene expression, morphology, and function. Of particular interest is the microglial function in brain regions that are highly sexually differentiated in development such as the amygdala as well as the pre-optic and ventromedial hypothalamic regions. With a focus on hormone-sensitive developmental windows, this review compares male and female microglia in the embryonic, developing, and adult brain with a particular interest in the influence of sex hormones on microglial wiring of social, reproductive, and disordered behavior circuits in the brain.

Birth triggers an inflammatory response in the neonatal periphery and brain

Birth is preceded by inflammation at the fetal/maternal interface. Additionally, the newborn experiences stimuli that under any other circumstance could elicit an immune response. It is unknown, however, whether birth elicits an inflammatory response in the newborn that extends to the brain. Moreover, it is unknown whether birth mode may alter such a response. To study these questions, we first measured corticosterone and pro- and anti-inflammatory cytokines in plasma of mouse offspring at several timepoints spaced closely before and after a vaginal or Cesarean birth. We found highest levels of IL-6 one day before birth and surges in corticosterone and IL-10 just after birth, regardless of birth mode. We next examined the neuroimmune response by measuring cytokine mRNA expression and microglial number and morphology in the paraventricular nucleus of the hypothalamus and hippocampus around the time of birth. We found a marked increase in TNF-α expression in both brain regions a day after birth, and rapid increases in microglial cell number in the first three days postnatal, with subtle differences by birth mode. To test whether the association between birth and cytokine production or expansion of microglia is causal, we manipulated birth timing. Remarkably, advancing birth by a day advanced the increases in all of the markers tested. Thus, birth triggers an immune response in the body and brain of offspring. Our results may provide a mechanism for effects of birth (e.g., acute changes in cell death and neural activation) previously reported in the newborn brain.

Comparison of Microglial Morphology and Function in Primary Cerebellar Cell Cultures on Collagen and Collagen-Mimetic Hydrogels

Neuronal-glial cell cultures are usually grown attached to or encapsulated in an adhesive environment as evenly distributed networks lacking tissue-like cell density, organization and morphology. In such cultures, microglia have activated amoeboid morphology and do not display extended and intensively branched processes characteristic of the ramified tissue microglia. We have recently described self-assembling functional cerebellar organoids promoted by hydrogels containing collagen-like peptides (CLPs) conjugated to a polyethylene glycol (PEG) core. Spontaneous neuronal activity was accompanied by changes in the microglial morphology and behavior, suggesting the cells might play an essential role in forming the functional neuronal networks in response to the peptide signalling. The present study examines microglial cell morphology and function in cerebellar cell organoid cultures on CLP-PEG hydrogels and compares them to the cultures on crosslinked collagen hydrogels of similar elastomechanical properties. Material characterization suggested more expressed fibril orientation and denser packaging in crosslinked collagen than CLP-PEG. However, CLP-PEG promoted a significantly higher microglial motility (determined by time-lapse imaging) accompanied by highly diverse morphology including the ramified (brightfield and confocal microscopy), more active Ca2+ signalling (intracellular Ca2+ fluorescence recordings), and moderate inflammatory cytokine level (ELISA). On the contrary, on the collagen hydrogels, microglial cells were significantly less active and mostly round-shaped. In addition, the latter hydrogels did not support the neuron synaptic activity. Our findings indicate that the synthetic CLP-PEG hydrogels ensure more tissue-like microglial morphology, motility, and function than the crosslinked collagen substrates.

Age-At-Injury Influences the Glial Response to Traumatic Brain Injury in the Cortex of Male Juvenile Rats

Few translational studies have examined how age-at-injury affects the glial response to traumatic brain injury (TBI). We hypothesized that rats injured at post-natal day (PND) 17 would exhibit a greater glial response, that would persist into early adulthood, compared to rats injured at PND35. PND17 and PND35 rats (n = 75) received a mild to moderate midline fluid percussion injury or sham surgery. In three cortical regions [peri-injury, primary somatosensory barrel field (S1BF), perirhinal], we investigated the glial response relative to age-at-injury (PND17 or PND35), time post-injury (2 hours, 1 day, 7 days, 25 days, or 43 days), and post-natal age, such that rats injured at PND17 or PND35 were compared at the same post-natal-age (e.g., PND17 + 25D post-injury = PND42; PND35 + 7D post-injury = PND42). We measured Iba1 positive microglia cells (area, perimeter) and quantified their activation status using skeletal analysis (branch length/cell, mean processes/cell, cell abundance). GFAP expression was examined using immunohistochemistry and pixel analysis. Data were analyzed using Bayesian multivariate multi-level models. Independent of age-at-injury, TBI activated microglia (shorter branches, fewer processes) in the S1BF and perirhinal cortex with more microglia in all regions compared to uninjured shams. TBI-induced microglial activation (shorter branches) was sustained in the S1BF into early adulthood (PND60). Overall, PND17 injured rats had more microglial activation in the perirhinal cortex than PND35 injured rats. Activation was not confounded by age-dependent cell size changes, and microglial cell body sizes were similar between PND17 and PND35 rats. There were no differences in astrocyte GFAP expression. Increased microglial activation in PND17 brain-injured rats suggests that TBI upregulates the glial response at discrete stages of development. Age-at-injury and aging with an injury are translationally important because experiencing a TBI at an early age may trigger an exaggerated glial response.

Development of microglia in fetal and postnatal neocortex of the pig, the European wild boar (Sus scrofa)

Knowledge on cortical development is based mainly on rodents besides primates and carnivores, all being altricial. Here, we analyzed a precocial animal, the pig, looking at dorsoparietal cortex from E45 to P90. At E45, most ionized calcium-binding adapter molecule 1-positive (Iba1+) cells had a macrophage-like morphology and resided in meninges and choroid plexus. Only a few cells were scattered in the ventricular and subventricular zone (VZ and SVZ). At E60/E70, all laminar compartments displayed microglia cells at a low-to-moderate density, being highest in VZ and SVZ followed by intermediate zone/white matter (IZ/WM). The cortical plate and marginal zone displayed only a few Iba1+ cells. Cells were intensely labeled, but still had poorly arborized somata and many resembled ameboid, macrophage-like microglia. Concurrent with a massive increase in cortical volume, microglia cell density increased until E85, and further until E100/E110 (birth at E114) to densities that resemble those seen postnatally. A fraction of microglia colabeled with Ki67 suggesting proliferation in all laminar compartments. Cell-to-cell distance decreased substantially during this time, and the fraction of microglia to all nuclei and to neurons increases in the laminar compartments. Eventually, of all cortical DAPI+ nuclei 7-12% were Iba1+ microglia. From E70 onwards, more and more cells with ramified processes were present in MZ down to IZ/WM, showing, for instance, a close association with NeuN+, NPY+, and GAD65/67+ somata and axon initial segments. These results suggested that the development of microglia cell density and morphology proceeds rapidly from mid-gestation onwards reaching near-adult status already before birth.

The behavior and functions of embryonic microglia

Microglia are the resident immune cells of the central nervous system. Microglial progenitors are generated in the yolk sac during the early embryonic stage. Once microglia enter the brain primordium, these cells colonize the structure through migration and proliferation during brain development. Microglia account for a minor population among the total cells that constitute the developing cortex, but they can associate with many surrounding neural lineage cells by extending their filopodia and through their broad migration capacity. Of note, microglia change their distribution in a stage-dependent manner in the developing brain: microglia are homogenously distributed in the pallium in the early and late embryonic stages, whereas these cells are transiently absent from the cortical plate (CP) from embryonic day (E) 15 to E16 and colonize the ventricular zone (VZ), subventricular zone (SVZ), and intermediate zone (IZ). Previous studies have reported that microglia positioned in the VZ/SVZ/IZ play multiple roles in neural lineage cells, such as regulating neurogenesis, cell survival and neuronal circuit formation. In addition to microglial functions in the zones in which microglia are replenished, these cells indirectly contribute to the proper maturation of post-migratory neurons by exiting the CP during the mid-embryonic stage. Overall, microglial time-dependent distributional changes are necessary to provide particular functions that are required in specific regions. This review summarizes recent advances in the understanding of microglial colonization and multifaceted functions in the developing brain, especially focusing on the embryonic stage, and discuss the molecular mechanisms underlying microglial behaviors.

Annexin A3 as a Marker Protein for Microglia in the Central Nervous System of Rats

The parenchymal microglia possess different morphological characteristics in cerebral physiological and pathological conditions; thus, visualizing these cells is useful as a means of further investigating parenchymal microglial function. Annexin A3 (ANXA3) is expressed in microglia, but it is unknown whether it can be used as a marker protein for microglia and its physiological function. Here, we compared the distribution and morphology of parenchymal microglia labeled by ANXA3, cluster of differentiation 11b (CD11b), and ionized calcium-binding adaptor molecule 1 (Iba1) and measured the expression of ANXA3 in nonparenchymal macrophages (meningeal and perivascular macrophages). We also investigated the spatiotemporal expression of ANXA3, CD11b, and Iba1 in vivo and in vitro and the cellular function of ANXA3 in microglia. We demonstrated that ANXA3-positive cells were abundant and evenly distributed throughout the whole brain tissue and spinal cord of adult rats. The morphology and distribution of ANXA3-labeled microglia were quite similar to those labeled by the microglial-specific markers CD11b and Iba1 in the central nervous system (CNS). ANXA3 was expressed in the cytoplasm of microglia, and its expression was significantly increased in activated microglia. ANXA3 was almost undetectable in the nonparenchymal macrophages. Meanwhile, the protein and mRNA expression levels of ANXA3 in different regions of the CNS were different from those of CD11b and Iba1. Moreover, knockdown of ANXA3 inhibited the proliferation and migration of microglia, while overexpression of ANXA3 enhanced these activities. This study confirms that ANXA3 may be a novel marker for parenchymal microglia in the CNS of adult rats and enriches our understanding of ANXA3 from expression patterns to physiological function.

The Contribution of Microglia to the Development and Maturation of the Visual System

Microglia, the resident immune cells of the central nervous system (CNS), were once considered quiescent cells that sat in readiness for reacting to disease and injury. Over the last decade, however, it has become clear that microglia play essential roles in maintaining the normal nervous system. The retina is an easily accessible part of the central nervous system and therefore much has been learned about the function of microglia from studies in the retina and visual system. Anatomically, microglia have processes that contact all synapses within the retina, as well as blood vessels in the major vascular plexuses. Microglia contribute to development of the visual system by contributing to neurogenesis, maturation of cone photoreceptors, as well as refining synaptic contacts. They can respond to neural signals and in turn release a range of cytokines and neurotrophic factors that have downstream consequences on neural function. Moreover, in light of their extensive contact with blood vessels, they are also essential for regulation of vascular development and integrity. This review article summarizes what we have learned about the role of microglia in maintaining the normal visual system and how this has helped in understanding their role in the central nervous system more broadly.

Microglia dynamics in adolescent traumatic brain injury

Repetitive, mild traumatic brain injuries (RmTBIs) are increasingly common in adolescents and encompass one of the largest neurological health concerns in the world. Adolescence is a critical period for brain development where RmTBIs can substantially impact neurodevelopmental trajectories and life-long neurological health. Our current understanding of RmTBI pathophysiology suggests key roles for neuroinflammation in negatively regulating neural health and function. Microglia, the brain’s resident immune population, play important roles in brain development by regulating neuronal number, and synapse formation and elimination. In response to injury, microglia activate to inflammatory phenotypes that may detract from these normal homeostatic, physiological, and developmental roles. To date, however, little is known regarding the impact of RmTBIs on microglia function during adolescent brain development. This review details key concepts surrounding RmTBI pathophysiology, adolescent brain development, and microglia dynamics in the developing brain and in response to injury, in an effort to formulate a hypothesis on how the intersection of these processes may modify long-term trajectories.

Cranial irradiation at early postnatal age impairs stroke-induced neural stem/progenitor cell response in the adult brain

Cranial irradiation (IR) is commonly used to treat primary brain tumors and metastatic diseases. However, cranial IR-treated patients often develop vascular abnormalities later in life that increase their risk for cerebral ischemia. Studies in rodents have demonstrated that IR impairs maintenance of the neural stem/precursor cell (NSPC) pool and depletes neurogenesis. We and others have previously shown that stroke triggers NSPC proliferation in the subventricular zone and migration towards the stroke-injured neocortex. Whether this response is sustained in the irradiated brain remains unknown. Here, we demonstrate that cranial IR in mice at an early postnatal age significantly reduced the number to neuronal progenitors responding to cortical stroke in adults. This was accompanied by a reduced number of microglia/macrophages in the peri-infarct cortex; however, the astrocytic response was not altered. Our findings indicate that IR impairs the endogenous repair capacity in the brain in response to stroke, hence pointing to another side effect of cranial radiotherapy which requires further attention.

Cerebellar Cells Self-Assemble into Functional Organoids on Synthetic, Chemically Crosslinked ECM-Mimicking Peptide Hydrogels

Hydrogel-supported neural cell cultures are more in vivo-relevant compared to monolayers formed on glass or plastic substrates. However, there is a lack of synthetic microenvironment available for obtaining standardized and easily reproducible cultures characterized by tissue-mimicking cell composition, cell-cell interactions, and functional networks. Synthetic peptides representing the biological properties of the extracellular matrix (ECM) proteins have been reported to promote the adhesion-driven differentiation and functional maturation of neural cells. Thus, such peptides can serve as building blocks for engineering a standardized, all-synthetic environment. In this study, we have compared the effect of two chemically crosslinked hydrogel compositions on primary cerebellar cells: collagen-like peptide (CLP), and CLP with an integrin-binding motif arginine-glycine-aspartate (CLP-RGD), both conjugated to polyethylene glycol molecular templates (PEG-CLP and PEG-CLP-RGD, respectively) and fabricated as self-supporting membranes. Both compositions promoted a spontaneous organization of primary cerebellar cells into tissue-like clusters with fast-rising Ca2+ signals in soma, reflecting action potential generation. Notably, neurons on PEG-CLP-RGD had more neurites and better synaptic efficiency compared to PEG-CLP. For comparison, poly-L-lysine-coated glass and plastic surfaces did not induce formation of such spontaneously active networks. Additionally, contrary to the hydrogel membranes, glass substrates functionalized with PEG-CLP and PEG-CLP-RGD did not sufficiently support cell attachment and, subsequently, did not promote functional cluster formation. These results indicate that not only chemical composition but also the hydrogel structure and viscoelasticity are essential for bioactive signaling. The synthetic strategy based on ECM-mimicking, multifunctional blocks in registry with chemical crosslinking for obtaining tissue-like mechanical properties is promising for the development of fast and well standardized functional in vitro neural models and new regenerative therapies.

A primary neural cell culture model to study neuron, astrocyte, and microglia interactions in neuroinflammation

Background

Interactions between neurons, astrocytes, and microglia critically influence neuroinflammatory responses to insult in the central nervous system. In vitro astrocyte and microglia cultures are powerful tools to study specific molecular pathways involved in neuroinflammation; however, in order to better understand the influence of cellular crosstalk on neuroinflammation, new multicellular culture models are required.

Methods

Primary cortical cells taken from neonatal rats were cultured in a serum-free “tri-culture” medium formulated to support neurons, astrocytes, and microglia, or a “co-culture” medium formulated to support only neurons and astrocytes. Caspase 3/7 activity and morphological changes were used to quantify the response of the two culture types to different neuroinflammatory stimuli mimicking sterile bacterial infection (lipopolysaccharide (LPS) exposure), mechanical injury (scratch), and seizure activity (glutamate-induced excitotoxicity). The secreted cytokine profile of control and LPS-exposed co- and tri-cultures were also compared.

Results

The tri-culture maintained a physiologically relevant representation of neurons, astrocytes, and microglia for 14 days in vitro, while the co-cultures maintained a similar population of neurons and astrocytes, but lacked microglia. The continuous presence of microglia did not negatively impact the overall health of the neurons in the tri-culture, which showed reduced caspase 3/7 activity and similar neurite outgrowth as the co-cultures, along with an increase in the microglia-secreted neurotrophic factor IGF-1 and a significantly reduced concentration of CX3CL1 in the conditioned media. LPS-exposed tri-cultures showed significant astrocyte hypertrophy, increase in caspase 3/7 activity, and the secretion of a number of pro-inflammatory cytokines (e.g., TNF, IL-1α, IL-1β, and IL-6), none of which were observed in LPS-exposed co-cultures. Following mechanical trauma, the tri-culture showed increased caspase 3/7 activity, as compared to the co-culture, along with increased astrocyte migration towards the source of injury. Finally, the microglia in the tri-culture played a significant neuroprotective role during glutamate-induced excitotoxicity, with significantly reduced neuron loss and astrocyte hypertrophy in the tri-culture.

Conclusions

The tri-culture consisting of neurons, astrocytes, and microglia more faithfully mimics in vivo neuroinflammatory responses than standard mono- and co-cultures. This tri-culture can be a useful tool to study neuroinflammation in vitro with improved accuracy in predicting in vivo neuroinflammatory phenomena.

Location of neonatal microglia drives small extracellular vesicles content and biological functions in vitro

Combining proteomics and systems biology approaches, we demonstrate that neonatal microglial cells derived from two different CNS locations, cortex and spinal cord, and cultured in vitro displayed different phenotypes upon different physiological or pathological conditions. These cells also exhibited greater variability in terms of cellular and small extracellular vesicles (sEVs) protein content and levels. Bioinformatic data analysis showed that cortical microglia exerted anti-inflammatory and neurogenesis/tumorigenesis properties, while the spinal cord microglia were more inflammatory. Interestingly, while both sEVs microglia sources enhanced growth of DRGs processes, only the spinal cord-derived sEVs microglia under LPS stimulation significantly attenuated glioma proliferation. These results were confirmed using the neurite outgrowth assay on DRGs cells and glioma proliferation analysis in 3D spheroid cultures. Results from these in vitro assays suggest that the microglia localized at different CNS regions can ensure different biological functions. Together, this study indicates that neonatal microglia locations regulate their physiological and pathological functional fates and could affect the high prevalence of brain vs spinal cord gliomas in adults.

Microglia cytoarchitecture in the brain of adenosine A2A receptor knockout mice: Brain region and sex specificities

Microglia cells exert a critical role in brain development, mainly supported by their immune functions, which predicts an impact on the genesis of psychiatric disorders. In fact, microglia stress during gestation is, for instance, associated with chronic anxiety and cognitive deficits accompanied by long-lasting, region- and sex-specific changes in microglia morphology. We recently reported that the pattern of microglia morphologic plasticity, which is sex-determined, impacts on anxious-like behaviour and cognition. We also reported that the pharmacologic blockade of adenosine A_2A receptors (A_{2 A} R) is able to reshape microglia morphology, in a sex-specific manner and with behavioural sequelae. In order to better understand the role of A_{2 A} R in the sex differentiation of microglia, we now compared their morphology in wild-type and A_{2 A} R knockout male and female C57BL/6 mice in two cardinal brain regions implicated in anxiety-like behaviour and cognition, the prefrontal cortex (PFC) and the dorsal hippocampus (dHIP). We report interregional differences between PFC and dHIP in a sex-specific manner: while males presented more complex microglia in the dHIP, microglia from females had a more complex morphology in the PFC. Surprisingly, the genetic deletion of A_{2 A} R did not alter these sex differences, but promoted the exclusive remodelling (increase in complexity) in PFC microglia from females. These findings further support the existence of a heterogeneous microglial network, distinct between sexes and brain regions, and help characterizing the role of A_{2 A} R in the sex- and brain region-specific morphologic differentiation of microglia.

Development and sensory experience dependent regulation of microglia in barrel cortex

The barrel cortex is within the primary somatosensory cortex of the rodent, and processes signals from the vibrissae. Much focus has been devoted to the function of neurons, more recently, the role of glial cells in the processing of sensory input has gained increasing interest. Microglia are the principal immune cells of the nervous system that survey and regulate the cellular constituents of the dynamic nervous system. We investigated the normal and disrupted development of microglia in barrel cortex by chronically depriving sensory signals via whisker trimming for the animals' first postnatal month. Using immunohistochemistry to label microglia, we performed morphological reconstructions as well as densitometry analyses as a function of developmental age and sensory experience. Findings suggest that both developmental age and sensory experience has profound impact on microglia morphology. Following chronic sensory deprivation, microglia undergo a morphological transition from a monitoring or resting state to an altered morphological state, by exhibiting expanded cell body size and retracted processes. Sensory restoration via whisker regrowth returns these morphological alterations back to age-matched control values. Our results indicate that microglia may be recruited to participate in the modulation of neuronal structural remodeling during developmental critical periods and in response to alteration in sensory input.

Microglia: Lifelong patrolling immune cells of the brain

Microglial cells are the predominant parenchymal immune cell of the brain. Recent evidence suggests that like peripheral immune cells, microglia patrol the brain in health and disease. Reviewing these data, we first examine the evidence that microglia invade the brain mesenchyme early in embryonic development, establish residence therein, proliferate and subsequently maintain their numbers throughout life. We, then, summarize established and novel evidence for microglial process surveillance in the healthy and injured brain. Finally, we discuss emerging evidence for microglial cell body dynamics that challenge existing assumptions of their sessile nature. We conclude that microglia are long-lived immune cells that patrol the brain through both cell body and process movements. This recognition has significant implications for neuroimmune interactions throughout the animal lifespan.

Microglia Express Insulin-Like Growth Factor-1 in the Hippocampus of Aged APPswe/PS1ΔE9 Transgenic Mice

Insulin-like growth factor-1 (IGF-1) is a pleiotropic molecule with neurotrophic and immunomodulatory functions. Knowing the capacity of chronically activated microglia to produce IGF-1 may therefore show essential to promote beneficial microglial functions in Alzheimer's disease (AD). Here, we investigated the expression of IGF-1 mRNA and IGF-1 along with the expression of tumor necrosis factor (TNF) mRNA, and the amyloid-β (Aβ) plaque load in the hippocampus of 3- to 24-month-old APPswe/PS1ΔE9 transgenic (Tg) and wild-type (WT) mice. As IGF-1, in particular, is implicated in neurogenesis we also monitored the proliferation of cells in the subgranular zone (sgz) of the dentate gyrus. We found that the Aβ plaque load reached its maximum in aged 21- and 24-month-old APPswe/PS1ΔE9 Tg mice, and that microglial reactivity and hippocampal IGF-1 and TNF mRNA levels were significantly elevated in aged APPswe/PS1ΔE9 Tg mice. The sgz cell proliferation decreased with age, regardless of genotype and increased IGF-1/TNF mRNA levels. Interestingly, IGF-1 mRNA was expressed in subsets of sgz cells, likely neuroblasts, and neurons in both genotypes, regardless of age, as well as in glial-like cells. By double in situ hybridization these were shown to be IGF1 mRNA+ CD11b mRNA+ cells, i.e., IGF-1 mRNA-expressing microglia. Quantification showed a 2-fold increase in the number of microglia and IGF-1 mRNA-expressing microglia in the molecular layer of the dentate gyrus in aged APPswe/PS1ΔE9 Tg mice. Double-immunofluorescence showed that IGF-1 was expressed in a subset of Aβ plaque-associated CD11b+ microglia and in several subsets of neurons. Exposure of primary murine microglia and BV2 cells to Aβ42 did not affect IGF-1 mRNA expression. IGF-1 mRNA levels remained constant in WT mice with aging, unlike TNF mRNA levels which increased with aging. In conclusion, our results suggest that the increased IGF-1 mRNA levels can be ascribed to a larger number of IGF-1 mRNA-expressing microglia in the aged APPswe/PS1ΔE9 Tg mice. The finding that subsets of microglia retain the capacity to express IGF-1 mRNA and IGF-1 in the aged APPswe/PS1ΔE9 Tg mice is encouraging, considering the beneficial therapeutic potential of modulating microglial production of IGF-1 in AD.

Unique microglia expression profile in developing white matter

OBJECTIVE

Recently we demonstrated that amoeboid microglia in white matter regions are essential for proper oligodendrocyte homeostasis and myelinogenesis in the first postnatal week. Amoeboid microglia in the mouse corpus callosum change their activation profile within few days after postnatal day (P)7 with microglia of the cerebellum showing similar features. Here we expanded our previous transcriptional analysis and performed detailed bulk RNA sequencing of microglia from corpus callosum, cortex and cerebellum at P7, P10 and P42. The goal of this study was to identify a specific gene profile for both, white matter and grey matter microglia during development.

RESULTS

Microglia in white matter regions display unique characteristics in the first postnatal week of murine life. In both the corpus callosum and cerebellum microglia show amoeboid morphology and a similar transcription profile during development including high expression of genes related to priming of microglia, phagocytosis and migration at P7; characteristics which are already lost at P10. Together these data verify our previous transcriptional data obtained by microarray analysis and enable a more complete view into white matter and grey matter microglia at different developmental stages.

Microglia: Brain cells on the move

In the last decade, tremendous progress has been made in understanding the biology of microglia - i.e. the fascinating immigrated resident immune cell population of the central nervous system (CNS). Recent literature reviews have largely dealt with the plentiful functions of microglia in CNS homeostasis, development and pathology, and the influences of sex and the microbiome. In this review, the intriguing aspect of their physical plasticity during CNS development will get specific attention. Microglia move around (mobility) and reshape their processes (motility). Microglial migration into and inside the CNS is most prominent throughout development and consequently most of the data described in this review concern mobility and motility in the changing environment of the developing brain. Here, we first define microglia based on their highly specialized age- and region-dependent gene expression signature and associated functional heterogeneity. Next, we describe their origin, the migration route of immature microglial cells towards the CNS, the mechanisms underlying their invasion of the CNS, and their spatiotemporal localization and surveying behaviour inside the developing CNS. These processes are dependent on microglial mobility and motility which are determined by the microenvironment of the CNS. Therefore, we further zoom in on the changing environment during CNS development. We elaborate on the extracellular matrix and the respective integrin receptors on microglia and we discuss the purinergic and molecular signalling in microglial mobility. In the last section, we discuss the physiological and pathological functions of microglia in which mobility and motility are involved to stress the importance of microglial 'movement'.

Physiological Implications of Microglia-Synapse Interactions

Microglia are the sole immune responding cells in the central nervous system. Their role as neuroimmune cells in the pathogenesis of various neurodegenerative and infectious diseases of the brain have been extensively studied. Upon brain disease and infection, they adopt an activated phenotype associated with the release of cytokines and neurotrophic factors and resulting in neuroprotective or neurotoxic outcomes. However, microglia are resident also in the healthy or physiological brain, but much less is known about their role(s) in the healthy brain, partly due to technical limitations regarding investigation of these highly reactive cells in the intact brain. Recent developments in molecular probes and in vivo optical imaging techniques has now helped to characterize microglia in the physiological or healthy brain. In vivo two-photon imaging of fluorescently labeled microglia have revealed that they are highly motile cells in the healthy brain, extending and retracting their processes that extend from a largely stationary cell soma. In this chapter, we briefly summarize some of the physiological functions of microglia in the uninjured brain, with a focus on interactions they have with synapses.

Neuronal Adenosine A2A Receptors Are Critical Mediators of Neurodegeneration Triggered by Convulsions

Neurodegeneration is a process transversal to neuropsychiatric diseases and the understanding of its mechanisms should allow devising strategies to prevent this irreversible step in brain diseases. Neurodegeneration caused by seizures is a critical step in the aggravation of temporal lobe epilepsy, but its mechanisms remain undetermined. Convulsions trigger an elevation of extracellular adenosine and upregulate adenosine A_2A receptors (A_2AR), which have been associated with the control of neurodegenerative diseases. Using the rat and mouse kainate model of temporal lobe epilepsy, we now tested whether A_2AR control convulsions-induced hippocampal neurodegeneration. The pharmacological or genetic blockade of A_2AR did not affect kainate-induced convulsions but dampened the subsequent neurotoxicity. This neurotoxicity began with a rapid A_2AR upregulation within glutamatergic synapses (within 2 h), through local translation of synaptic A_2AR mRNA. This bolstered A_2AR-mediated facilitation of glutamate release and of long-term potentiation (LTP) in CA1 synapses (4 h), triggered a subsequent synaptotoxicity, heralded by decreased synaptic plasticity and loss of synaptic markers coupled to calpain activation (12 h), that predated overt neuronal loss (24 h). All modifications were prevented by the deletion of A_2AR selectively in forebrain neurons. This shows that synaptic A_2AR critically control synaptic excitotoxicity, which underlies the development of convulsions-induced neurodegeneration.

Bidirectional Microglia-Neuron Communication in Health and Disease

Microglia are ramified cells that exhibit highly motile processes, which continuously survey the brain parenchyma and react to any insult to the CNS homeostasis. Although microglia have long been recognized as a crucial player in generating and maintaining inflammatory responses in the CNS, now it has become clear, that their function are much more diverse, particularly in the healthy brain. The innate immune response and phagocytosis represent only a little segment of microglia functional repertoire that also includes maintenance of biochemical homeostasis, neuronal circuit maturation during development and experience-dependent remodeling of neuronal circuits in the adult brain. Being equipped by numerous receptors and cell surface molecules microglia can perform bidirectional interactions with other cell types in the CNS. There is accumulating evidence showing that neurons inform microglia about their status and thus are capable of controlling microglial activation and motility while microglia also modulate neuronal activities. This review addresses the topic: how microglia communicate with other cell types in the brain, including fractalkine signaling, secreted soluble factors and extracellular vesicles. We summarize the current state of knowledge of physiological role and function of microglia during brain development and in the mature brain and further highlight microglial contribution to brain pathologies such as Alzheimer’s and Parkinson’s disease, brain ischemia, traumatic brain injury, brain tumor as well as neuropsychiatric diseases (depression, bipolar disorder, and schizophrenia).

Immunology of West Nile Virus Infection and the Role of Alpha-Synuclein as a Viral Restriction Factor

West Nile virus (WNV) is a single-stranded RNA flavivirus and is a major cause of viral encephalitis worldwide. Experimental models of WNV infection in mice are commonly used to define acute neuroinflammatory responses in the brain. Alpha-synuclein (Asyn) is a protein of primarily neuronal origin and is a major cause of Parkinson's disease (PD), a disorder characterized by loss of dopaminergic neurons. Both WNV and PD pathologies are largely mediated by inflammation of the central nervous system (neuroinflammation) and have overlapping inflammatory pathways. In this review, we highlight the roles of the immune system in both diseases while comparing and contrasting both protective and pathogenic roles of immune cells and their effector proteins. Additionally, we review the current literature showing that Asyn is an important mediator of the immune response with diverging roles in PD (pathogenic) and WNV disease (neuroprotective).

Carbamylated Erythropoietin Decreased Proliferation and Neurogenesis in the Subventricular Zone, but Not the Dentate Gyrus, After Irradiation to the Developing Rat Brain

Cranial radiotherapy for pediatric brain tumors causes progressive, debilitating late effects, including cognitive decline. Erythropoietin (EPO) has been shown to be neuroprotective and to promote neuroregeneration. Carbamylated erythropoietin (CEPO) retains the protective properties of EPO but is not erythrogenic. To study the effects of CEPO on the developing brain exposed to radiotherapy, a single irradiation (IR) dose of 6 Gy was administered to the brains of postnatal day 9 (P9) rats, and CEPO (40 μg/kg s.c.) was injected on P8, P9, P11, P13, and P15. To examine proliferation, 5-Bromo-2-deoxyuridine (BrdU) was injected on P15, P16, and P17. CEPO administration did not affect BrdU incorporation in the granule cell layer (GCL) of the hippocampus or in the subventricular zone (SVZ) as quantified 7 days after the last BrdU injection, whereas IR decreased BrdU incorporation in the GCL and SVZ by 63% and 18%, respectively. CEPO did not affect BrdU incorporation in the GCL of irradiated brains, although it was reduced even further (to 31%) in the SVZ. To evaluate the effect of CEPO on neurogenesis, BrdU/doublecortin double-positive cells were quantified. CEPO did not affect neurogenesis in non-irradiated brains, whereas IR decreased neurogenesis by 58% in the dentate gyrus (DG) but did not affect it in the SVZ. In the DG, CEPO did not affect the rate of neurogenesis following IR, whereas in the SVZ, the rate decreased by 30% following IR compared with the rate in vehicle-treated rats. Neither CEPO nor IR changed the number of microglia. In summary, CEPO did not promote neurogenesis in non-irradiated or irradiated rat brains and even aggravated the decreased neurogenesis in the SVZ. This raises concerns regarding the use of EPO-related compounds following radiotherapy.

Microglia heterogeneity along a spatio-temporal axis: More questions than answers

Microglia are resident macrophages of the central nervous system; they arise during early embryonic development and persist throughout adulthood. These unique cells provide developmental support, contribute to adult brain homeostasis and impart immune protection during infection. Dysregulated microglia are implicated in the pathophysiology of several neurological disorders, including Alzheimer disease, and as such, a better understanding of their regulation and function is required for rational therapeutic design. Recent studies have highlighted the various heterogeneous aspects of microglia, such as their wide differentiation spectrum from early embryogenesis to adulthood, their location in different brain regions and their responses to ageing, infection and inflammation. In this review, we discuss microglial heterogeneity in time and space and highlight the remaining questions arising from such heterogeneity.

Proliferative cells in the rat developing neocortical grey matter: new insights into gliogenesis

The postnatal brain development is characterized by a substantial gain in weight and size, ascribed to increasing neuronal size and branching, and to massive addition of glial cells. This occurs concomitantly to the shrinkage of VZ and SVZ, considered to be the main germinal zones, thus suggesting the existence of other germinative niches. The aim of this study is to characterize the cortical grey matter proliferating cells during postnatal development, providing their stereological quantification and identifying the nature of their cell lineage. We performed double immunolabeling for the proliferation marker Ki67 and three proteins which identify either astrocytes (S100β) or oligodendrocytes (Olig2 and NG2), in addition to a wider panel of markers apt to validate the former markers or to investigate other cell lineages. We found that proliferating cells increase in number during the first postnatal week until P10 and subsequently decreased until P21. Cell lineage characterization revealed that grey matter proliferating cells are prevalently oligodendrocytes and astrocytes along with endothelial and microglial cells, while no neurons have been detected. Our data showed that astrogliogenesis occurs prevalently during the first 10 days of postnatal development, whereas contrary to the expected peak of oligodendrogenesis at the second postnatal week, we found a permanent pool of proliferating oligodendrocytes enduring from birth until P21. These data support the relevance of glial proliferation within the grey matter and could be a point of departure for further investigations of this complex process.

Two distinct ontogenies confer heterogeneity to mouse brain microglia

Hoxb8 mutant mice show compulsive behavior similar to trichotillomania, a human obsessive-compulsive-spectrum disorder. The only Hoxb8 lineage-labeled cells in the brains of mice are microglia, suggesting that defective Hoxb8 microglia caused the disorder. What is the source of the Hoxb8 microglia? It has been posited that all microglia progenitors arise at embryonic day (E) 7.5 during yolk sac hematopoiesis, and colonize the brain at E9.5. In contrast, we show the presence of two microglia subpopulations: canonical, non-Hoxb8 microglia and Hoxb8 microglia. Unlike non-Hoxb8 microglia, Hoxb8 microglia progenitors appear to be generated during the second wave of yolk sac hematopoiesis, then detected in the aorto-gonad-mesonephros (AGM) and fetal liver, where they are greatly expanded, prior to infiltrating the E12.5 brain. Further, we demonstrate that Hoxb8 hematopoietic progenitor cells taken from fetal liver are competent to give rise to microglia in vivo Although the two microglial subpopulations are very similar molecularly, and in their response to brain injury and participation in synaptic pruning, they show distinct brain distributions which might contribute to pathological specificity. Non-Hoxb8 microglia significantly outnumber Hoxb8 microglia, but they cannot compensate for the loss of Hoxb8 function in Hoxb8 microglia, suggesting further crucial differences between the two subpopulations.

HIV-1 Tat alters neuronal intrinsic excitability

Objective

In HIV+ individuals, the virus enters the central nervous system and invades innate immune cells, producing important changes that result in neurological deficits. We aimed to determine whether HIV plays a direct role in neuronal excitability. Of the HIV peptides, Tat is secreted and acts in other cells. In order to examine whether the HIV Tat can modify neuronal excitability, we exposed primary murine hippocampal neurons to that peptide, and tested its effects on the intrinsic membrane properties, 4 and 24 h after exposure.

Results

The exposure of hippocampal pyramidal neurons to Tat for 4 h did not alter intrinsic membrane properties. However, we found a strong increase in intrinsic excitability, characterized by increase of the slope (Gain) of the input–output function, in cells treated with Tat for 24 h. Nevertheless, Tat treatment for 24 h did not alter the resting membrane potential, input resistance, rheobase and action potential threshold. Thus, neuronal adaptability to Tat exposure for 24 h is not applicable to basic neuronal properties. A restricted but significant effect on coupling the inputs to the outputs may have implications to our knowledge of Tat biophysical firing capability, and its involvement in neuronal hyperexcitability in neuroHIV.

Interleukin 4 modulates microglia homeostasis and attenuates the early slowly progressive phase of amyotrophic lateral sclerosis

Microglia activation is a commonly pathological hallmark of neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), a devastating disorder characterized by a selective motor neurons degeneration. Whether such activation might represent a causal event rather than a secondary epiphenomenon remains elusive. Here, we show that CNS-delivery of IL-4—via a lentiviral-mediated gene therapy strategy—skews microglia to proliferate, inducing these cells to adopt the phenotype of slowly proliferating cells. Transcriptome analysis revealed that IL-4-treated microglia express a broad number of genes normally encoded by embryonic microglia. Since embryonic microglia sustain CNS development, we then hypothesized that turning adult microglia to acquire such phenotype via IL-4 might be an efficient in vivo strategy to sustain motor neuron survival in ALS. IL-4 gene therapy in SOD1^G93A mice resulted in a general amelioration of clinical outcomes during the early slowly progressive phase of the disease. However, such approach did not revert neurodegenerative processes occurring in the late and fast progressing phase of the disease.

Recent advances in the understanding of microglial development and homeostasis

Microglia are the resident macrophages of the central nervous system (CNS). These pivotal cells arise early during embryonic development and provide both developmental support and immune protection to the brain. In adults, microglia contribute to brain homeostasis and mediate an intriguing interplay between the CNS and the gut microbiota. When dysregulated, microglia are also implicated in numerous neurological disorders, and thus fully understanding their regulation and functions will facilitate rational design of therapies to alleviate these conditions; however it remains unclear how the multiple factors modulating microglial activity are integrated at the organism and cellular levels. In this review, we will discuss recent advances in the understanding of microglial regulation and highlight the key questions that remain to be answered around microglial development, homeostasis and functions.

The microbiota influences cell death and microglial colonization in the perinatal mouse brain

The mammalian fetus develops in a largely sterile environment, and direct exposure to a complex microbiota does not occur until birth. We took advantage of this to examine the effect of the microbiota on brain development during the first few days of life. The expression of anti- and pro-inflammatory cytokines, developmental cell death, and microglial colonization in the brain were compared between newborn conventionally colonized mice and mice born in sterile, germ-free (GF) conditions. Expression of the pro-inflammatory cytokines interleukin 1β and tumor necrosis factor α was markedly suppressed in GF newborns. GF mice also had altered cell death, with some regions exhibiting higher rates (paraventricular nucleus of the hypothalamus and the CA1 oriens layer of the hippocampus) and other regions exhibiting no change or lower rates (arcuate nucleus of the hypothalamus) of cell death. Microglial labeling was elevated in GF mice, due to an increase in both microglial cell size and number. The changes in cytokine expression, cell death and microglial labeling were evident on the day of birth, but were absent on embryonic day 18.5, approximately one-half day prior to expected delivery. Taken together, our results suggest that direct exposure to the microbiota at birth influences key neurodevelopmental events and does so within hours. These findings may help to explain some of the behavioral and neurochemical alterations previously seen in adult GF mice.

Histology Atlas of the Developing Prenatal and Postnatal Mouse Central Nervous System, with Emphasis on Prenatal Days E7.5 to E18.5

Evaluation of the central nervous system (CNS) in the developing mouse presents unique challenges, given the complexity of ontogenesis, marked structural reorganization over very short distances in 3 dimensions each hour, and numerous developmental events susceptible to genetic and environmental influences. Developmental defects affecting the brain and spinal cord arise frequently both in utero and perinatally as spontaneous events, following teratogen exposure, and as sequelae to induced mutations and thus are a common factor in embryonic and perinatal lethality in many mouse models. Knowledge of normal organ and cellular architecture and differentiation throughout the mouse's life span is crucial to identify and characterize neurodevelopmental lesions. By providing a well-illustrated overview summarizing major events of normal in utero and perinatal mouse CNS development with examples of common developmental abnormalities, this annotated, color atlas can be used to identify normal structure and histology when phenotyping genetically engineered mice and will enhance efforts to describe and interpret brain and spinal cord malformations as causes of mouse embryonic and perinatal lethal phenotypes. The schematics and images in this atlas illustrate major developmental events during gestation from embryonic day (E)7.5 to E18.5 and after birth from postnatal day (P)1 to P21.

Antiglioma effects of cytarabine on leptomeningeal metastasis of high-grade glioma by targeting the PI3K/Akt/mTOR pathway

Leptomeningeal metastasis (LM) of high-grade glioma is a highly lethal disease requiring new effective therapeutic measures. For both de novo or relapsed glioma with LM, intrathecal cytarabine chemotherapy is not frequently used for first-line and relapse protocols. We encountered a clinical case demonstrating effective application of cytarabine in high-grade glioma with LM, prompting us to explore the effects of cytarabine on malignant glioma and molecular mechanisms of such effects through in vivo and in vitro experiments. The U87 cell line was selected to represent human glioma for studies. Cell viability was measured by MTT assay, plate colony formation assay, and trypan-blue dye exclusion test. Apoptosis was assessed by flow cytometry. Protein expression levels were detected by Western blot assay and immunohistochemistry. mRNA expression was examined by quantitative real-time reverse transcription polymerase chain reaction. Cytarabine inhibited tumor growth during the in vivo experiment. The present study confirmed that cytarabine inhibits proliferation and promotes apoptosis of U87 cells, and molecular analysis of this effect showed that cytarabine significantly reduces expression of phosphatidylinositol 3-kinase/serine/threonine kinase also known as the protein kinase B/mechanistic target of rapamycin (PI3K/Akt/mTOR) pathway, Ki-67, BCL2, and 4-1BB, and upregulates Bax and cleaved caspase-3. Our findings indicated that intrathecal administration of cytarabine manifests potential in prophylaxis and treatment of malignant glioma with LM. Effective medications for high-grade glioma with LM should contain cytarabine.

Cranial irradiation induces transient microglia accumulation, followed by long-lasting inflammation and loss of microglia

The relative contribution of resident microglia and peripheral monocyte-derived macrophages in neuroinflammation after cranial irradiation is not known. A single dose of 8 Gy was administered to postnatal day 10 (juvenile) or 90 (adult) CX3CR1GFP/+ CCR2RFP/+ mouse brains. Microglia accumulated in the subgranular zone of the hippocampal granule cell layer, where progenitor cell death was prominent. The peak was earlier (6 h vs. 24 h) but less pronounced in adult brains. The increase in juvenile, but not adult, brains was partly attributed to proliferation. Microglia numbers then decreased over time to 39% (juvenile) and 58% (adult) of controls 30 days after irradiation, largely as a result of cell death. CD68 was expressed in 90% of amoeboid microglia in juvenile hippocampi but only in 9% of adult ones. Isolated hippocampal microglia revealed reduced CD206 and increased IL1-beta expression after irradiation, more pronounced in juvenile brains. CCL2 and IL-1 beta increased after irradiation, more in juvenile hippocampi, and remained elevated at all time points. In summary, microglia activation after irradiation was more pronounced, protracted and pro-inflammatory by nature in juvenile than in adult hippocampi. Common to both ages was long-lasting inflammation and the absence of monocyte-derived macrophages.

Axonal Cleaved Caspase-3 Regulates Axon Targeting and Morphogenesis in the Developing Auditory Brainstem

Caspase-3 is a cysteine protease that is most commonly associated with cell death. Recent studies have shown additional roles in mediating cell differentiation, cell proliferation and development of cell morphology. We investigated the role of caspase-3 in the development of chick auditory brainstem nuclei during embryogenesis. Immunofluorescence from embryonic days E6–13 revealed that the temporal expression of cleaved caspase-3 follows the ascending anatomical pathway. The expression is first seen in the auditory portion of VIIIth nerve including central axonal regions projecting to nucleus magnocellularis (NM), then later in NM axons projecting to nucleus laminaris (NL), and subsequently in NL dendrites. To examine the function of cleaved caspase-3 in chick auditory brainstem development, we blocked caspase-3 cleavage in developing chick embryos with the caspase-3 inhibitor Z-DEVD-FMK from E6 to E9, then examined NM and NL morphology and NM axonal targeting on E10. NL lamination in treated embryos was disorganized and the neuropil around NL contained a significant number of glial cells normally excluded from this region. Additionally, NM axons projected into inappropriate portions of NL in Z-DEVD-FMK treated embyros. We found that the presence of misrouted axons was associated with more severe NL disorganization. The effects of axonal caspase-3 inhibition on both NL morphogenesis and NM axon targeting suggest that these developmental processes are coordinated, likely through communication between axons and their targets.

Minocycline causes widespread cell death and increases microglial labeling in the neonatal mouse brain

Minocycline, an antibiotic of the tetracycline family, inhibits microglia in many paradigms and is among the most commonly used tools for examining the role of microglia in physiological processes. Microglia may play an active role in triggering developmental neuronal cell death, although findings have been contradictory. To determine whether microglia influence developmental cell death, we treated perinatal mice with minocycline (45 mg/kg) and quantified effects on dying cells and microglial labeling using immunohistochemistry for activated caspase-3 (AC3) and ionized calcium-binding adapter molecule 1 (Iba1), respectively. Contrary to our expectations, minocycline treatment from embryonic day 18 to postnatal day (P)1 caused a > tenfold increase in cell death 8 h after the last injection in all brain regions examined, including the primary sensory cortex, septum, hippocampus and hypothalamus. Iba1 labeling was also increased in most regions. Similar effects, although of smaller magnitude, were seen when treatment was delayed to P3-P5. Minocycline treatment from P3 to P5 also decreased overall cell number in the septum at weaning, suggesting lasting effects of the neonatal exposure. When administered at lower doses (4.5 or 22.5 mg/kg), or at the same dose 1 week later (P10-P12), minocycline no longer increased microglial markers or cell death. Taken together, the most commonly used microglial "inhibitor" increases cell death and Iba1 labeling in the neonatal mouse brain. Minocycline is used clinically in infant and pediatric populations; caution is warrented when using minocycline in developing animals, or extrapolating the effects of this drug across ages. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 753-766, 2017.

Microglia contact induces synapse formation in developing somatosensory cortex

Microglia are the immune cells of the central nervous system that play important roles in brain pathologies. Microglia also help shape neuronal circuits during development, via phagocytosing weak synapses and regulating neurogenesis. Using in vivo multiphoton imaging of layer 2/3 pyramidal neurons in the developing somatosensory cortex, we demonstrate here that microglial contact with dendrites directly induces filopodia formation. This filopodia formation occurs only around postnatal day 8-10, a period of intense synaptogenesis and when microglia have an activated phenotype. Filopodia formation is preceded by contact-induced Ca(2+) transients and actin accumulation. Inhibition of microglia by genetic ablation decreases subsequent spine density, functional excitatory synapses and reduces the relative connectivity from layer 4 neurons. Our data provide the direct demonstration of microglial-induced spine formation and provide further insights into immune system regulation of neuronal circuit development, with potential implications for developmental disorders of immune and brain dysfunction.

Developmental changes in microglial mobilization are independent of apoptosis in the neonatal mouse hippocampus

During CNS development, microglia transform from highly mobile amoeboid-like cells to primitive ramified forms and, finally, to highly branched but relatively stationary cells in maturity. The factors that control developmental changes in microglia are largely unknown. Because microglia detect and clear apoptotic cells, developmental changes in microglia may be controlled by neuronal apoptosis. Here, we assessed the extent to which microglial cell density, morphology, motility, and migration are regulated by developmental apoptosis, focusing on the first postnatal week in the mouse hippocampus when the density of apoptotic bodies peaks at postnatal day 4 and declines sharply thereafter. Analysis of microglial form and distribution in situ over the first postnatal week showed that, although there was little change in the number of primary microglial branches, microglial cell density increased significantly, and microglia were often seen near or engulfing apoptotic bodies. Time-lapse imaging in hippocampal slices harvested at different times over the first postnatal week showed differences in microglial motility and migration that correlated with the density of apoptotic bodies. The extent to which these changes in microglia are driven by developmental neuronal apoptosis was assessed in tissues from BAX null mice lacking apoptosis. We found that apoptosis can lead to local microglial accumulation near apoptotic neurons in the pyramidal cell body layer but, unexpectedly, loss of apoptosis did not alter overall microglial cell density in vivo or microglial motility and migration in ex vivo tissue slices. These results demonstrate that developmental changes in microglial form, distribution, motility, and migration occur essentially normally in the absence of developmental apoptosis, indicating that factors other than neuronal apoptosis regulate these features of microglial development.