Sibling astrocytes share preferential coupling via gap junctions

Spatial organization of astrocyte clones: The role of developmental progenitor timing

Astrocytes represent a diverse and morphologically complex group of glial cells critical for shaping and maintaining nervous system homeostasis, as well as responding to injuries. Understanding the origins of astroglial heterogeneity, originated from a limited number of progenitors, has been the focus of many studies. Most of these investigations have centered on protoplasmic and pial astrocytes, while the clonal relationship of fibrous astrocytes or juxtavascular astrocytes has remained relatively unexplored. In this study, we sought to elucidate the morphological diversity and clonal distribution of astrocytes across adult cortical layers, with particular emphasis on their ontogenetic origins. Using the StarTrack lineage tracing tool, we explored the characteristics of adult astroglial clones derived from single and specific progenitors at various embryonic stages. Our results revealed a heterogeneous spatial distribution of astroglial clones, characterized by variations in location, clonal size, and rostro-caudal dispersion. While a considerable proportion of clones were confined within specific cortical layers, others displayed sibling cells crossing layer boundaries. Notably, we observed a correlation between clone location and developmental stage at earlier embryonic stages, although this relationship diminished in later stages. Fibrous astrocyte clones were exclusively confined to the corpus callosum. In contrast, protoplasmic or juxtavascular clones were located in either the upper or lower cortical layers, with certain clones displayed sibling cells distributed across both regions. Our findings underscore the developmental origins and spatial distribution of astroglial clones within cortical layers, providing new insights into the interplay between their morphology, clonal sizes, and progenitor heterogeneity.

Lineage Relationships Between Subpallial Progenitors and Glial Cells in the Piriform Cortex

The piriform cortex is a paleocortical area, located in the ventrolateral surface of the rodent forebrain, receiving direct input from the olfactory bulb. The three layers of the PC are defined by the diversity of glial and neuronal cells, marker expression, connections, and functions. However, the glial layering, ontogeny, and sibling cell relationship along the PC is an unresolved question in the field. Here, using multi-color genetic lineage tracing approaches with different StarTrack strategies, we performed a rigorous analysis of the derived cell progenies from progenitors located at the subpallium ventricular surface. First, we specifically targeted E12-progenitors with UbC-StarTrack to analyze their adult derived-cell progeny and their location within the piriform cortex layers. The vast majority of the cell progeny derived from targeted progenitors were identified as neurons, but also astrocytes and NG2 cells. Further, to specifically target single Gsx-2 subpallial progenitors and their derived cell-progeny in the piriform cortex, we used the UbC-(Gsx-2-hyPB)-StarTrack to perform an accurate analysis of their clonal relationships. Our results quantitatively delineate the adult clonal cell pattern from single subpallial E12-progenitors, focusing on glial cells. In summary, there is a temporal pattern in the assembly of the glial cell diversity in the piriform cortex, which also reveals spatio-temporal progenitor heterogeneity.

Multicolor strategies for investigating clonal expansion and tissue plasticity

Understanding the generation of complexity in living organisms requires the use of lineage tracing tools at a multicellular scale. In this review, we describe the different multicolor strategies focusing on mouse models expressing several fluorescent reporter proteins, generated by classical (MADM, Brainbow and its multiple derivatives) or acute (StarTrack, CLoNe, MAGIC Markers, iOn, viral vectors) transgenesis. After detailing the multi-reporter genetic strategies that serve as a basis for the establishment of these multicolor mouse models, we briefly mention other animal and cellular models (zebrafish, chicken, drosophila, iPSC) that also rely on these constructs. Then, we highlight practical applications of multicolor mouse models to better understand organogenesis at single progenitor scale (clonal analyses) in the brain and briefly in several other tissues (intestine, skin, vascular, hematopoietic and immune systems). In addition, we detail the critical contribution of multicolor fate mapping strategies in apprehending the fine cellular choreography underlying tissue morphogenesis in several models with a particular focus on brain cytoarchitecture in health and diseases. Finally, we present the latest technological advances in multichannel and in-depth imaging, and automated analyses that enable to better exploit the large amount of data generated from multicolored tissues.

Astrocytes as essential time-keepers of the central pacemaker

Since the early observations made by Santiago Ramon y Cajal more than a century ago till now, astrocytes have gradually gained protagonism as essential partners of neurons in building brain circuits that regulate complex behavior. In mammals, processes such as sleep-wake cycle, locomotor activity, cognition and memory consolidation, homeostatic and hedonic appetite and stress response (among others), are synchronized in 24-h rhythms by the circadian system. In such a way, physiology efficiently anticipates and adapts to daily recurring changes in the environment. The hypothalamic suprachiasmatic nucleus (SCN) is considered the central pacemaker, it has been traditionally described as a nucleus of around 10,000 neurons nearly all GABAergic able to be entrained by light and to convey time information through multiple neuronal and hormonal pathways. Only recently, this neuro-centered view was challenged by breakthrough discoveries implicating astrocytes as essential time-keepers. In the present review, we will describe the current view on the SCN circuit and discuss whether astrocytic functions described in other brain regions and state-of-the-art experimental approaches, could help explaining better those well- and not so well-known features of the central pacemaker.

Gliogenic Potential of Single Pallial Radial Glial Cells in Lower Cortical Layers

During embryonic development, progenitor cells are progressively restricted in their potential to generate different neural cells. A specific progenitor cell type, the radial glial cells, divides symmetrically and then asymmetrically to produce neurons, astrocytes, oligodendrocytes, and NG2-glia in the cerebral cortex. However, the potential of individual progenitors to form glial lineages remains poorly understood. To further investigate the cell progeny of single pallial GFAP-expressing progenitors, we used the in vivo genetic lineage-tracing method, the UbC-(GFAP-PB)-StarTrack. After targeting those progenitors in embryonic mice brains, we tracked their adult glial progeny in lower cortical layers. Clonal analyses revealed the presence of clones containing sibling cells of either a glial cell type (uniform clones) or two different glial cell types (mixed clones). Further, the clonal size and rostro-caudal cell dispersion of sibling cells differed depending on the cell type. We concluded that pallial E14 neural progenitors are a heterogeneous cell population with respect to which glial cell type they produce, as well as the clonal size of their cell progeny.

Structure and Functions of Gap Junctions and Their Constituent Connexins in the Mammalian CNS

Numerous data obtained in the last 20 years indicate that all parts of the mature central nervous system, from the retina and olfactory bulb to the spinal cord and brain, contain cells connected by gap junctions (GJs). The morphological basis of the GJs is a group of joined membrane hemichannels called connexons, the subunit of each connexon is the protein connexin. In the central nervous system, connexins show specificity and certain types of them are expressed either in neurons or in glial cells. Connexins and GJs of neurons, combining certain types of inhibitory hippocampal and neocortical neuronal ensembles, provide synchronization of local impulse and rhythmic activity, thalamocortical conduction, control of excitatory connections, which reflects their important role in the processes of perception, concentration of attention and consolidation of memory, both on the cellular and at the system level. Connexins of glial cells are ubiquitously expressed in the brain, and the GJs formed by them provide molecular signaling and metabolic cooperation and play a certain role in the processes of neuronal migration during brain development, myelination, tissue homeostasis, and apoptosis. At the same time, mutations in the genes of glial connexins, as well as a deficiency of these proteins, are associated with such diseases as congenital neuropathies, hearing loss, skin diseases, and brain tumors. This review summarizes the existing data of numerous molecular, electrophysiological, pharmacological, and morphological studies aimed at progress in the study of the physiological and pathophysiological significance of glial and neuronal connexins and GJs for the central nervous system.

Approaches to Study Gap Junctional Coupling

Astrocytes and oligodendrocytes are main players in the brain to ensure ion and neurotransmitter homeostasis, metabolic supply, and fast action potential propagation in axons. These functions are fostered by the formation of large syncytia in which mainly astrocytes and oligodendrocytes are directly coupled. Panglial networks constitute on connexin-based gap junctions in the membranes of neighboring cells that allow the passage of ions, metabolites, and currents. However, these networks are not uniform but exhibit a brain region-dependent heterogeneous connectivity influencing electrical communication and intercellular ion spread. Here, we describe different approaches to analyze gap junctional communication in acute tissue slices that can be implemented easily in most electrophysiology and imaging laboratories. These approaches include paired recordings, determination of syncytial isopotentiality, tracer coupling followed by analysis of network topography, and wide field imaging of ion sensitive dyes. These approaches are capable to reveal cellular heterogeneity causing electrical isolation of functional circuits, reduced ion-transfer between different cell types, and anisotropy of tracer coupling. With a selective or combinatory use of these methods, the results will shed light on cellular properties of glial cells and their contribution to neuronal function.

Deciphering neural heterogeneity through cell lineage tracing

Understanding how an adult brain reaches an appropriate size and cell composition from a pool of progenitors that proliferates and differentiates is a key question in Developmental Neurobiology. Not only the control of final size but also, the proper arrangement of cells of different embryonic origins is fundamental in this process. Each neural progenitor has to produce a precise number of sibling cells that establish clones, and all these clones will come together to form the functional adult nervous system. Lineage cell tracing is a complex and challenging process that aims to reconstruct the offspring that arise from a single progenitor cell. This tracing can be achieved through strategies based on genetically modified organisms, using either genetic tracers, transfected viral vectors or DNA constructs, and even single-cell sequencing. Combining different reporter proteins and the use of transgenic mice revolutionized clonal analysis more than a decade ago and now, the availability of novel genome editing tools and single-cell sequencing techniques has vastly improved the capacity of lineage tracing to decipher progenitor potential. This review brings together the strategies used to study cell lineages in the brain and the role they have played in our understanding of the functional clonal relationships among neural cells. In addition, future perspectives regarding the study of cell heterogeneity and the ontogeny of different cell lineages will also be addressed.

Clonal Analysis of Gliogenesis in the Cerebral Cortex Reveals Stochastic Expansion of Glia and Cell Autonomous Responses to Egfr Dosage

Development of the nervous system undergoes important transitions, including one from neurogenesis to gliogenesis which occurs late during embryonic gestation. Here we report on clonal analysis of gliogenesis in mice using Mosaic Analysis with Double Markers (MADM) with quantitative and computational methods. Results reveal that developmental gliogenesis in the cerebral cortex occurs in a fraction of earlier neurogenic clones, accelerating around E16.5, and giving rise to both astrocytes and oligodendrocytes. Moreover, MADM-based genetic deletion of the epidermal growth factor receptor (Egfr) in gliogenic clones revealed that Egfr is cell autonomously required for gliogenesis in the mouse dorsolateral cortices. A broad range in the proliferation capacity, symmetry of clones, and competitive advantage of MADM cells was evident in clones that contained one cellular lineage with double dosage of Egfr relative to their environment, while their sibling Egfr-null cells failed to generate glia. Remarkably, the total numbers of glia in MADM clones balance out regardless of significant alterations in clonal symmetries. The variability in glial clones shows stochastic patterns that we define mathematically, which are different from the deterministic patterns in neuronal clones. This study sets a foundation for studying the biological significance of stochastic and deterministic clonal principles underlying tissue development, and identifying mechanisms that differentiate between neurogenesis and gliogenesis.

Unraveling the adult cell progeny of early postnatal progenitor cells

NG2-glia, also referred to as oligodendrocyte precursor cells or polydendrocytes, represent a large pool of proliferative neural cells in the adult brain that lie outside of the two major adult neurogenic niches. Although their roles are not fully understood, we previously reported significant clonal expansion of adult NG2-cells from embryonic pallial progenitors using the StarTrack lineage-tracing tool. To define the contribution of early postnatal progenitors to the specific NG2-glia lineage, we used NG2-StarTrack. A temporal clonal analysis of single postnatal progenitor cells revealed the production of different glial cell types in distinct areas of the dorsal cortex but not neurons. Moreover, the dispersion and size of the different NG2 derived clonal cell clusters increased with age. Indeed, clonally-related NG2-glia were located throughout the corpus callosum and the deeper layers of the cortex. In summary, our data reveal that postnatally derived NG2-glia are proliferative cells that give rise to NG2-cells and astrocytes but not neurons. These progenitors undergo clonal cell expansion and dispersion throughout the adult dorsal cortex in a manner that was related to aging and cell identity, adding new information about the ontogeny of these cells. Thus, identification of clonally-related cells from specific progenitors is important to reveal the NG2-glia heterogeneity.

Heterogeneity of astrocytes: Electrophysiological properties of juxtavascular astrocytes before and after brain injury

Astrocyte heterogeneity is increasingly recognized, but still little is known about juxtavascular astrocytes with their somata directly adjacent to blood vessels, despite their importance after brain injury. As juxtavascular astrocytes originate from common progenitor cells, that is, have a clonal origin, they may intrinsically differ from other, non-juxtavascular astrocytes. To explore this, we examined the electrophysiological properties of these groups of astrocytes and the underlying ion channels. Using brain slices of BAC Aldh1l1-eGFP transgenic mice with astrocytes labeled by GFP expression, we compared juxtavascular and non-juxtavascular astrocytes in the somatosensory cortex by means of whole-cell patch-clamp recordings and immunohistochemical staining. Prior to injury, juxta- and non-juxtavascular astrocytes exhibit comparable electrophysiological properties with characteristic mostly passive conductance and a typical negative resting membrane potential. Immunohistochemical analysis of K⁺ channels showed that all astrocytes were K_ir 4.1⁺ , but revealed an intriguing difference for K_v 4.3. The expression of K_v 4.3 in sibling astrocytes (non-juxtavascular, juxtavascular and pial) was dependent on their ontogenetic origin with lowest levels in juxtavascular astrocytes located in upper cortical layers. After traumatic brain injury (TBI), we found profound changes in the electrophysiological type of astrocytes with a predominance of non-passive properties and this pattern was significantly enriched in juxtavascular astrocytes. This was accompanied by pronounced down-regulation of K_ir 4.1 in proliferating astrocytes, which was significantly more in juxtavascular compared to non-juxtavascular astrocytes. Taken together, TBI induces profound differences in electrophysiological properties between juxtavascular and non-juxtavascular astrocytes that might be related to the preponderance of juxtavascular astrocyte proliferation.

Cell Progeny in the Olfactory Bulb After Targeting Specific Progenitors with Different UbC-StarTrack Approaches

The large phenotypic variation in the olfactory bulb may be related to heterogeneity in the progenitor cells. Accordingly, the progeny of subventricular zone (SVZ) progenitor cells that are destined for the olfactory bulb is of particular interest, specifically as there are many facets of these progenitors and their molecular profiles remain unknown. Using modified StarTrack genetic tracing strategies, specific SVZ progenitor cells were targeted in E12 mice embryos, and the cell fate of these neural progenitors was determined in the adult olfactory bulb. This study defined the distribution and the phenotypic diversity of olfactory bulb interneurons from specific SVZ-progenitor cells, focusing on their spatial pallial origin, heterogeneity, and genetic profile.

Lineage Tracing and Cell Potential of Postnatal Single Progenitor Cells In Vivo

Understanding the contribution of adult neural progenitor cells (NPCs) and their lineage potential is a great challenge in neuroscience. To reveal progenitor diversity and cell-lineage relationships of postnatal NPCs in the subventricular zone (SVZ), we performed in vivo lineage-tracing genetic analysis using the UbC-StarTrack. We determined the progeny of single SVZ-NPCs, the number of cells per clone, the dispersion of sibling cells, and the cell types within clones. Long-term analysis revealed that both the cell-dispersion pattern and number of cells comprising clones varied depending on the glial/neuronal nature of sibling cells. Sibling-olfactory interneurons were primarily located within the same layer, while sibling-glial cells populated SVZ-adjacent areas. Sibling astrocytes and interneurons did not form big clones, whereas oligodendroglial-lineage clones comprised the largest clones originated in adult brains. These results demonstrate the existence of SVZ postnatal bipotential progenitors that give rise to clones widely dispersed across the olfactory bulb and SVZ-adjacent areas.

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Bipotent postnatal progenitors produce clones of olfactory neurons and glial cells

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Different clonal cell patterns in astroglial, oligodendroglial, and neuronal lineages

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Sibling neuroblasts migrating to the olfactory bulb widespread along the RMS axis

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Sibling astrocytes and interneurons form discrete cell clones