Oligodendrocytes are a lifelong source of nuclear and ribosomal material for neurons in the mouse brain

Nuclear and ribosomal components define cell identity and function by regulating chromatin dynamics, gene expression, and protein turnover. Here we report that in the mouse central nervous system (CNS) under normal conditions, neurons accumulate nuclear and ribosomal material of oligodendrocyte (OL) origin. We show that neuronal accumulation of OL-derived nuclear and ribosomal material is brain area-specific, and in the cortex and hippocampal dentate gyrus gradually propagates during postnatal brain maturation. We further demonstrate that OL-to-neuron material transfer persists throughout adulthood and responds to neuroinflammation. We found that satellite OL of the gray matter form internuclear contacts with receiving neurons in the mouse brain. Similar close internuclear associations between satellite OL and neurons are present in the adult human cortex. Our findings provide the first evidence of wide-spread dynamic and selective OL-to-neuron nuclear and ribosomal material transfer in the mouse CNS and indicate that satellite OL serve as powerful mediators of neuronal function. Equivalent processes may occur in the human CNS and cause neurological disorders when dysregulated.

Expression of immunoglobulin constant domain genes in neurons of the mouse central nervous system

General consensus states that immunoglobulins are exclusively expressed by B lymphocytes to form the first line of defense against common pathogens. Here, we provide compelling evidence for the expression of two heavy chain immunoglobulin genes in subpopulations of neurons in the mouse brain and spinal cord. RNA isolated from excitatory and inhibitory neurons through ribosome affinity purification revealed Ighg3 and Ighm transcripts encoding for the constant (Fc), but not the variable regions of IgG3 and IgM. Because, in the absence of the variable immunoglobulin regions, these transcripts lack the canonical transcription initiation site used in lymphocytes, we screened for alternative 5′ transcription start sites and identified a novel 5′ exon adjacent to a proposed promoter element. Immunohistochemical, Western blot, and in silico analyses strongly support that these neuronal transcripts are translated into proteins containing four Immunoglobulin domains. Our data thus demonstrate the expression of two Fc-encoding genes Ighg3 and Ighm in spinal and supraspinal neurons of the murine CNS and suggest a hitherto unknown function of the encoded proteins.

Inactivation of Hedgehog signal transduction in adult astrocytes results in region-specific blood–brain barrier defects

In this study, we use molecular genetic approaches to clarify the role of the Hedgehog (Hh) pathway in regulating the blood–brain/spinal cord barrier (BBB) in the adult mouse central nervous system (CNS). Our work confirms and extends prior studies to demonstrate that astrocytes are the predominant cell type in the adult CNS that transduce Hh signaling, revealed by the expression of Gli1, a target gene of the canonical pathway that is activated in cells receiving Hh, and other key pathway transduction components. Gli1+ (Hh-responsive) astrocytes are distributed in specific regions of the CNS parenchyma, including layers 4/5/6 of the neocortex, hypothalamus, thalamus, and spinal cord, among others. Notably, although BBB properties in endothelial cells are normally regulated by both paracellular and transcellular mechanisms, conditional inactivation of Hh signaling in astrocytes results in transient, region-specific BBB defects that affect transcytosis but not paracellular diffusion. These findings stand in contrast to prior studies that implicated astrocytes as a source of Sonic hedgehog that limited extravasation via both mechanisms [J. I. Alvarez et al., Science 334, 1727–1731 (2011)]. Furthermore, using three distinct Cre driver lines as well as pharmacological approaches to inactivate Hh-pathway transduction globally in CNS astrocytes, we find that these specific BBB defects are only detected in the rostral hypothalamus and spinal cord but not the cortex or other regions where Gli1+ astrocytes are found. Together, our data show that Gli1+ Hh-responsive astrocytes have regionally distinct molecular and functional properties and that the pathway is required to maintain BBB properties in specific regions of the adult mammalian CNS.

Influenza causes alterations to oligodendrocyte-specific transcripts and the myelin lipidome in the adult mouse central nervous system.

The notion that myelin remains static during adulthood has been challenged in recent years. Myelin is not only crucial for proper cognitive function and behavior, but it is vulnerable to alterations from external factors outside the window of development. Here, in the adult mouse CNS, RNA analysis revealed global downregulation and subsequent recovery of oligodendrocyte-specific transcripts in response to peripheral influenza viral infection. Furthermore, shot-gun lipidomic analysis revealed that infection alters the lipid profile in the prefrontal cortex as well as in purified brain myelin. Finally, treatment with the colony stimulating factor receptor (CSFR)1 antagonist GW2580 during infection suppressed glial activation and partially restored oligodendrocyte-specific myelin transcripts to baseline levels. Together, these findings reveal a yet unforeseen consequence of peripheral infection on oligodendrocyte homeostasis in the adult mouse.

Pathogenic tau accelerates aging-associated activation of transposable elements in the mouse central nervous system

ABSTRACTTransposable elements comprise almost half of the mammalian genome. A growing body of evidence suggests that transposable element dysregulation accompanies brain aging and neurodegenerative disorders, and that transposable element activation is neurotoxic. Recent studies have identified links between pathogenic forms of tau, a protein that accumulates in Alzheimer disease and related tauopathies, and transposable element-induced neurotoxicity. Starting with transcriptomic analyses, we find that age- and tau-induced transposable element activation occurs in the mouse brain. Among transposable elements that are activated at the RNA level in the context of brain aging and tauopathy, we find that the endogenous retrovirus (ERV) class of retrotransposons is particularly enriched. We show that protein encoded by Intracisternal A-particle, a highly active mouse ERV, is elevated in brains of tau transgenic mice. We further demonstrate that brains of tau transgenic mice contain increased DNA copy number of transposable elements, raising the possibility that these elements actively retrotranspose in the context of tauopathy. Taken together, our study lays the groundwork for future mechanistic studies focused on transposable element regulation in the aging mouse brain and in mouse models of tauopathy, while providing support for therapeutic approaches targeting transposable element activation for Alzheimer disease and related tauopathies.

Proliferative and Nonproliferative Lesions of the Rat and Mouse Central and Peripheral Nervous Systems: New and Revised INHAND Terms

Harmonization of diagnostic terminology used during the histopathologic analysis of rodent tissue sections from nonclinical toxicity studies will improve the consistency of data sets produced by laboratories located around the world. The INHAND Project ( International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a cooperative enterprise of 4 major societies of toxicologic pathology to develop a globally accepted standard vocabulary for proliferative and nonproliferative lesions in rodents. A prior manuscript ( Toxicol Pathol 2012;40[4 Suppl]:87S-157S) defined multiple diagnostic terms for toxicant-induced lesions, common spontaneous and age-related changes, and principal confounding artifacts in the rat and mouse central nervous system (CNS) and peripheral nervous system (PNS). The current article defines 9 new diagnostic terms and updates 2 previous terms for findings in the rodent CNS and PNS, the need for which has become evident in the years since the publication of the initial INHAND nomenclature for findings in rodent neural tissues. The nomenclature presented in this document is also available electronically on the Internet at the goRENI website ( http://www.goreni.org/ ).

Single-cell profiling of histone modifications in the mouse brain

The development of the mouse central nervous system (CNS) involves coordinated execution of transcriptional and epigenetic programs. These programs have been extensively studied through single-cell technologies in a pursuit to characterize the underlying cell heterogeneity. However, histone modifications pose additional layers of both positive and negative regulation that defines cellular identity. Here we show that the Cut&Tag technology can be coupled with a droplet-based single cell library preparation platform to produce high quality chromatin modifications data at a single cell resolution in tens of thousands of cells. We apply single-cell Cut&Tag (scC&T) to probe histone modifications characteristic of active promoters (H3K4me3), active promoters and enhancers (H3K27ac), active gene bodies (H3K36me3) and inactive regions (H3K27me3) and generate scC&T profiles for almost 50,000 cells. scC&T profiles of each of these histone modifications were sufficient to determine cell identity and deconvolute at single cell level regulatory principles such as promoter bivalency, spreading of H3K4me3 and promoter-enhancer connectivity. Moreover, we used scC&T to investigate the single-cell chromatin occupancy of transcription factor Olig2 and the cohesin complex component Rad21. Our results indicate that analysis of histone modifications and transcription factor occupancy at a single cell resolution can provide unique insights of epigenomic landscapes in the CNS. We also provide an online resource that can be used to interactively explore the data at https://castelobranco.shinyapps.io/BrainCutAndTag2020/.