Identification of Positionally Distinct Astrocyte Subtypes whose Identities Are Specified by a Homeodomain Code

Astrocytes constitute the most abundant cell type in the central nervous system (CNS) and play diverse functional roles, but the ontogenetic origins of this phenotypic diversity are poorly understood. We have investigated whether positional identity, a fundamental organizing principle governing the generation of neuronal subtype diversity, is also relevant to astrocyte diversification. We identified three positionally distinct subtypes of white-matter astrocytes (WMA) in the spinal cord, which can be distinguished by the combinatorial expression of Reelin and Slit1. These astrocyte subtypes derive from progenitor domains expressing the homeodomain transcription factors Pax6 and Nkx6.1, respectively. Loss- and gain-of-function experiments indicate that the positional identity of these astrocyte subtypes is controlled by Pax6 and Nkx6.1 in a combinatorial manner. Thus, positional identity is an organizing principle underlying astrocyte, as well as neuronal, subtype diversification and is controlled by a homeodomain transcriptional code whose elements are reutilized following the specification of neuronal identity earlier in development

Committed neuronal precursors confer astrocytic potential on residual neural precursor cells

During midgestation, mammalian neural precursor cells (NPCs) differentiate only into neurons. Generation of astrocytes is prevented at this stage, because astrocyte-specific gene promoters are methylated. How the subsequent switch from suppression to expression of astrocytic genes occurs is unknown. We show in this study that Notch ligands are expressed on committed neuronal precursors and young neurons in mid-gestational telencephalon, and that neighboring Notch-activated NPCs acquire the potential to become astrocytes. Activation of the Notch signaling pathway in midgestational NPCs induces expression of the transcription factor nuclear factor I, which binds to astrocytic gene promoters, resulting in demethylation of astrocyte-specific genes. These findings provide a mechanistic explanation for why neurons come first: committed neuronal precursors and young neurons potentiate remaining NPCs to differentiate into the next cell lineage, astrocytes

The Transcription Factor NFIA Controls the Onset of Gliogenesis in the Developing Spinal Cord

The mechanisms controlling the transition from neurogenesis to gliogenesis in the vertebrate CNS are incompletely understood. We identified a family of transcription factors, called NFI genes, which are induced throughout the spinal cord ventricular zone (VZ) concomitantly with the induction of GLAST, an early marker of gliogenesis. NFIA is both necessary and sufficient for GLAST induction in the VZ. Unexpectedly, NFIA is also essential for the continued inhibition of neurogenesis in VZ progenitors. This function is mediated by the requirement of NFIA for the expression of HES5, a Notch effector. However, Notch effectors are unable to promote glial-fate specification in the absence of NFIA. Thus, NFIA links the abrogation of neurogenesis to a generic program of gliogenesis, in both astrocyte and oligodendrocyte VZ progenitors. At later stages, NFIA promotes migration and differentiation of astrocyte precursors, a function that is antagonized in oligodendrocyte precursors by Olig2

Nuclear factor IA is expressed in astrocytomas and is associated with improved survival

Nuclear factor IA (NFIA) is a transcription factor that specifies glial cell identity and promotes astrocyte differentiation during embryonic development. Its expression and function in gliomas are not known. Here, we examined NFIA protein expression in gliomas and its association with clinical outcome in pediatric malignant astrocytomas. We analyzed expression of NFIA by immunohistochemistry in 88 existing glioma specimens from Childrens Hospital Los Angeles and the University of Southern California. Association between NFIA expression and progression-free survival (PFS) was examined in high-grade astrocytomas for which clinical data were available (n = 23, all children). NFIA was highly expressed in astrocytomas of all grades, but only in a minority of cells in oligodendroglial tumors. NFIA was expressed on a higher percentage of tumor cells in low-grade astrocytomas (91 ± 5% and 77 ± 14% in World Health Organization [WHO] I and II, respectively) compared with high-grade astrocytomas (48 ± 18% and 37 ± 16% in WHO III and IV, respectively; P < .001, low- vs high-grade astrocytomas). There was a significant association between NFIA expression and PFS in children with astrocytoma WHO grade III or IV (Cox regression P = .019; logrank trend test for NFIA tertiles P = .0040 and NFIA quartiles P = .014). The association was not consistently significant in this small series of patients after adjustment was made for WHO grade III or IV. This is the first study to demonstrate expression of NFIA protein in astrocytomas and its association with grades of astrocytoma and PFS, suggesting that NFIA may play a role in astrocytoma biology

Human Astrocytes Exhibit Tumor Microenvironment-, Age-, and Sex-Related Transcriptomic Signatures

: Astrocytes are critical for the development and function of synapses. There are notable species differences between human astrocytes and commonly used animal models. Yet, it is unclear whether astrocytic genes involved in synaptic function are stable or exhibit dynamic changes associated with disease states and age in humans, which is a barrier in understanding human astrocyte biology and its potential involvement in neurological diseases. To better understand the properties of human astrocytes, we acutely purified astrocytes from the cerebral cortices of over 40 humans across various ages, sexes, and disease states. We performed RNA sequencing to generate transcriptomic profiles of these astrocytes and identified genes associated with these biological variables. We found that human astrocytes in tumor-surrounding regions downregulate genes involved in synaptic function and sensing of signals in the microenvironment, suggesting involvement of peri-tumor astrocytes in tumor-associated neural circuit dysfunction. In aging, we also found downregulation of synaptic regulators and upregulation of markers of cytokine signaling, while in maturation we identified changes in ionic transport with implications for calcium signaling. In addition, we identified subtle sexual dimorphism in human cortical astrocytes, which has implications for observed sex differences across many neurological disorders. Overall, genes involved in synaptic function exhibit dynamic changes in the peritumor microenvironment and aging. This data provides powerful new insights into human astrocyte biology in several biologically relevant states, that will aid in generating novel testable hypotheses about homeostatic and reactive astrocytes in humans.SIGNIFICANCE STATEMENTAstrocytes are an abundant class of cells playing integral roles at synapses. Astrocyte dysfunction is implicated in a variety of human neurological diseases. Yet our knowledge of astrocytes is largely based on mouse studies. Direct knowledge of human astrocyte biology remains limited. Here, we present transcriptomic profiles of human cortical astrocytes, and we identified molecular differences associated with age, sex, and disease state. We found that peritumor and aging astrocytes downregulate genes involved in astrocyte-synapse interactions. These data provide necessary insight into human astrocyte biology that will improve our understanding of human disease

Glial development: The crossroads of regeneration and repair in the CNS

Given the complexities of the mammalian CNS, its regeneration is viewed as the holy grail of regenerative medicine. Extraordinary efforts have been made to understand developmental neurogenesis, with the hopes of clinically applying this knowledge. CNS regeneration also involves glia, which comprises at least 50% of the cellular constituency of the brain and is involved in all forms of injury and disease response, recovery, and regeneration. Recent developmental studies have given us unprecedented insight into the processes that regulate the generation of CNS glia. Because restorative processes often parallel those found in development, we will peer through the lens of developmental gliogenesis to gain a clearer understanding of the processes that underlie glial regeneration under pathological conditions. Specifically, this review will focus on key signaling pathways that regulate astrocyte and oligodendrocyte development and describe how these mechanisms are reutilized in these populations during regeneration and repair after CNS injury

The Role of Hyperexcitability in Gliomagenesis

Glioblastoma is the most common malignant primary brain tumor. Recent studies have demonstrated that excitatory or activity-dependent signaling—both synaptic and non-synaptic—contribute to the progression of glioblastoma. Glutamatergic receptors may be stimulated via neuron–tumor synapses or release of glutamate by the tumor itself. Ion currents generated by these receptors directly alter the structure of membrane adhesion molecules and cytoskeletal proteins to promote migratory behavior. Additionally, the hyperexcitable milieu surrounding glioma increases the rate at which tumor cells proliferate and drive recurrent disease. Inhibition of excitatory signaling has shown to effectively reduce its pro-migratory and -proliferative effects