Developmental distribution of astrocytic proteins in the rat cochlear nucleus

Different Flavors of Astrocytes: Revising the Origins of Astrocyte Diversity and Epigenetic Signatures to Understand Heterogeneity after Injury

Astrocytes are a specific type of neuroglial cells that confer metabolic and structural support to neurons. Astrocytes populate all regions of the nervous system and adopt a variety of phenotypes depending on their location and their respective functions, which are also pleiotropic in nature. For example, astrocytes adapt to pathological conditions with a specific cellular response known as reactive astrogliosis, which includes extensive phenotypic and transcriptional changes. Reactive astrocytes may lose some of their homeostatic functions and gain protective or detrimental properties with great impact on damage propagation. Different astrocyte subpopulations seemingly coexist in reactive astrogliosis, however, the source of such heterogeneity is not completely understood. Altered cellular signaling in pathological compared to healthy conditions might be one source fueling astrocyte heterogeneity. Moreover, diversity might also be encoded cell-autonomously, for example as a result of astrocyte subtype specification during development. We hypothesize and propose here that elucidating the epigenetic signature underlying the phenotype of each astrocyte subtype is of high relevance to understand another regulative layer of astrocyte heterogeneity, in general as well as after injury or as a result of other pathological conditions. High resolution methods should allow enlightening diverse cell states and subtypes of astrocyte, their adaptation to pathological conditions and ultimately allow controlling and manipulating astrocyte functions in disease states. Here, we review novel literature reporting on astrocyte diversity from a developmental perspective and we focus on epigenetic signatures that might account for cell type specification.

Neuroglial Involvement in Abnormal Glutamate Transport in the Cochlear Nuclei of the Igf1 -/- Mouse

Insulin-like growth factor 1 (IGF-1) is a powerful regulator of synaptic activity and a deficit in this protein has a profound impact on neurotransmission, mostly on excitatory synapses in both the developing and mature auditory system. Adult Igf1^−/− mice are animal models for the study of human syndromic deafness; they show altered cochlear projection patterns into abnormally developed auditory neurons along with impaired glutamate uptake in the cochlear nuclei, phenomena that probably reflect disruptions in neuronal circuits. To determine the cellular mechanisms that might be involved in regulating excitatory synaptic plasticity in 4-month-old Igf1^−/− mice, modifications to neuroglia, astroglial glutamate transporters (GLTs) and metabotropic glutamate receptors (mGluRs) were assessed in the cochlear nuclei. The Igf1^−/− mice show significant decreases in IBA1 (an ionized calcium-binding adapter) and glial fibrillary acidic protein (GFAP) mRNA expression and protein accumulation, as well as dampened mGluR expression in conjunction with enhanced glutamate transporter 1 (GLT1) expression. By contrast, no differences were observed in the expression of glutamate aspartate transporter (GLAST) between these Igf1^−/− mice and their heterozygous or wildtype littermates. These observations suggest that congenital IGF-1 deficiency may lead to alterations in microglia and astrocytes, an upregulation of GLT1, and the downregulation of groups I, II and III mGluRs. Understanding the molecular, biochemical and morphological mechanisms underlying neuronal plasticity in a mouse model of hearing deficits will give us insight into new therapeutic strategies that could help to maintain or even improve residual hearing when human deafness is related to IGF-1 deficiency.

Methyl-CpG binding proteins are involved in restricting differentiation plasticity in neurons

Neurons and astrocytes are generated from common neural precursors, yet neurogenesis precedes astrocytogenesis, which normally commences at later stages of development. We have previously reported that a particular cytosine residue within a STAT3-binding site in the astrocyte-specific marker glial fibrillary acidic protein (GFAP) gene promoter becomes demethylated in neuroepithelial cells as gestation proceeds. This demethylation correlates tightly with the onset of astrocyte differentiation, suggesting that a change in DNA methylation at cell-type-specific gene promoters controls the switch from neurogenesis to astrocytogenesis in the developing brain. Here, we show that late-gestation neuroepithelial cells, which have already lost the methylation in the STAT3-binding site within the GFAP promoter, can still give rise to neurons and that these neurons do not respond to a STAT3-activating cytokine to express GFAP. Members of a transcriptional repressor family, the methylated-CpG binding proteins (MBDs), including MeCP2, are predominantly expressed in neurons, and ectopic MeCP2 expression inhibited astrocyte differentiation of neuroepithelial cells. Moreover, we found that exon 1 of the GFAP gene remains hypermethylated even in neuroepithelial cells at a late developmental stage and in neurons differentiated from such neuroepithelial cells. We further demonstrate that MeCP2 actually binds to the highly methylated exon 1 of the GFAP gene in neurons. These results suggest that region-specific DNA methylation and MBDs play an important role in the regulation of differentiation plasticity in neurons.

Developmental stage dependent regulation of DNA methylation and chromatin modification in a immature astrocyte specific gene promoter

Astrocytes are generated from neuroepithelial cells after neurons during brain development. However, the mechanism of this sequential generation is not fully understood. Here, we show that a particular cytosine residue in the promoter of the gene encoding the immature astrocyte marker, S100beta, becomes demethylated, correlating with the time when the S100beta expression commences at embryonic day (E) 14. In addition, astrocyte-inducing cytokine, BMP2, increased histone acetylation around the CpG site in neuroepithelial cells at E14 but not E11 when S100beta expressing astrocytes are absent. Furthermore, binding of a methyl DNA binding protein, MeCP2, to the S100beta gene promoter in neuroepithelial cells was reduced at E14 compared to E11. Thus, demethylation of specific CpG site is suggested to be a critical determinant in regulating astrocyte differentiation in the developing brain.