Faculty Bibliography

Astrocytes play both physiological and pathological roles in maintaining central nervous system (CNS) function. Here, we review the varied functions of astrocytes and how these might change in subsets of reactive astrocytes. We review the current understanding of astrocyte interactions with microglia and the vasculature and protective barriers in the central nervous system as well as highlight recent insights into physiologic and reactive astrocyte sub-states identified by transcriptional profiling. Our goal is to stimulate inquiry into how these molecular identifiers link to specific functional changes in astrocytes and to define the implications of these heterogeneous molecular and functional changes in brain function and pathology. Defining these complex interactions has the potential to yield new therapies in CNS injury, infection, and disease.

Microgliosis is a prominent pathological feature in many neurological diseases including multiple sclerosis (MS), a progressive auto-immune demyelinating disorder. The precise role of microglia, parenchymal central nervous system (CNS) macrophages, during demyelination, and the relative contributions of peripheral macrophages are incompletely understood. Classical markers used to identify microglia do not reliably discriminate between microglia and peripheral macrophages, confounding analyses. Here, we use a genetic fate mapping strategy to identify microglia as predominant responders and key effectors of demyelination in the cuprizone (CUP) model. Colony-stimulating factor 1 (CSF1), also known as macrophage colony-stimulating factor (M-CSF) - a secreted cytokine that regulates microglia development and survival-is upregulated in demyelinated white matter lesions. Depletion of microglia with the CSF1R inhibitor PLX3397 greatly abrogates the demyelination, loss of oligodendrocytes, and reactive astrocytosis that results from CUP treatment. Electron microscopy (EM) and serial block face imaging show myelin sheaths remain intact in CUP treated mice depleted of microglia. However, these CUP-damaged myelin sheaths are lost and robustly phagocytosed upon-repopulation of microglia. Direct injection of CSF1 into CNS white matter induces focal microgliosis and demyelination indicating active CSF1 signaling can promote demyelination. Finally, mice defective in adopting a toxic astrocyte phenotype that is driven by microglia nevertheless demyelinate normally upon CUP treatment implicating microglia rather than astrocytes as the primary drivers of CUP-mediated demyelination. Together, these studies indicate activated microglia are required for and can drive demyelination directly and implicate CSF1 signaling in these events.

Reactive astrocytes are astrocytes undergoing morphological, molecular, and functional remodeling in response to injury, disease, or infection of the CNS. Although this remodeling was first described over a century ago, uncertainties and controversies remain regarding the contribution of reactive astrocytes to CNS diseases, repair, and aging. It is also unclear whether fixed categories of reactive astrocytes exist and, if so, how to identify them. We point out the shortcomings of binary divisions of reactive astrocytes into good-vs-bad, neurotoxic-vs-neuroprotective or A1-vs-A2. We advocate, instead, that research on reactive astrocytes include assessment of multiple molecular and functional parameters-preferably in vivo-plus multivariate statistics and determination of impact on pathological hallmarks in relevant models. These guidelines may spur the discovery of astrocyte-based biomarkers as well as astrocyte-targeting therapies that abrogate detrimental actions of reactive astrocytes, potentiate their neuro- and glioprotective actions, and restore or augment their homeostatic, modulatory, and defensive functions.

Alzheimer's disease (AD) is the most common form of dementia. Despite many years of intense research, there is currently still no effective treatment. Multiple cell types contribute to disease pathogenesis, with an increasing body of data pointing to the active participation of astrocytes. Astrocytes play a pivotal role in the physiology and metabolic functions of neurons and other cells in the central nervous system. Because of their interactions with other cell types, astrocyte functions must be understood in their biologic context, thus many studies have used mouse models, of which there are over 190 available for AD research. However, none appear able to fully recapitulate the many functional changes in astrocytes reported in human AD brains. Our review summarizes the observations of astrocyte biology noted in mouse models of familial and sporadic AD. The limitations of AD mouse models will be discussed and current attempts to overcome these disadvantages will be described. With increasing understanding of the non-neuronal contributions to disease, the development of new methods and models will provide further insights and address important questions regarding the roles of astrocytes and other non-neuronal cells in AD pathophysiology. The next decade will prove to be full of exciting opportunities to address this devastating disease.

Regulating gene expression at the protein level is becoming increasingly important for answering basic questions in neurobiology. Several techniques using destabilizing domains (DD) on transgenes, which can be activated or deactivated by specific drugs, have been developed to achieve this goal. A DD from bacterial dihydrofolate reductase bound and stabilized by trimethoprim (TMP) represents such a tool. To control transgenic protein levels in the brain, the DD-regulating drugs need to have sufficient penetration into the central nervous system (CNS). Yet, very limited information is available on TMP pharmacokinetics in the CNS following systemic injection. Here, we performed a pharmacokinetic study on the penetration of TMP into different CNS compartments in the rat. We used mass spectrometry to measure TMP concentrations in serum, cerebrospinal fluid (CSF) and tissue samples of different CNS regions upon intraperitoneal TMP injection. We show that TMP quickly (within 10 min) penetrates from serum to CSF through the blood-CSF barrier. TMP also shows quick penetration into brain tissue but concentrations were an order of magnitude lower compared to serum or CSF. TMP concentration in spinal cord was lower than in any other analyzed CNS area. Nevertheless, effective levels of TMP to stabilize DDs can be reached in the CNS with half-lives around 2 h. These data show that TMP has good and fast penetration properties into the CNS and is therefore a valuable ligand for precisely controlling protein expression in the CNS in rodents.

Aberrant aggregation of RNA binding protein TDP-43 in neurons is a hallmark of frontotemporal lobar degeneration caused by progranulin haploinsufficiency^1,2. However, the mechanism leading to TDP-43 proteinopathy remains unclear. Here we use single-nucleus RNA-sequencing (snRNA-seq) to show that progranulin deficiency promotes microglial transition from a homeostatic to disease-specific state that causes endolysosomal dysfunction and neurodegeneration. These defects persist even when Grn^-/- microglia are cultured ex vivo. In addition, snRNA-seq reveals selective loss of excitatory neurons at disease end-stage, characterized by prominent nuclear and cytoplasmic TDP-43 granules and nuclear pore defects. Remarkably, conditioned media from Grn^-/- microglia is sufficient to promote TDP-43 granule formation, nuclear pore defects and cell death in excitatory neurons via the complement activation pathway. Consistent with these results, deleting C1qa and C3 mitigates microglial toxicity, and rescues TDP-43 proteinopathy and neurodegeneration. These results uncover previously unappreciated contributions of chronic microglial toxicity to TDP-43 proteinopathy during neurodegeneration.

Microglia-astrocyte interactions represent a delicate balance affecting neural cell functions in health and disease. Tightly controlled to maintain homeostasis during physiological conditions, rapid and prolonged departures during disease, infection, and following trauma drive multiple outcomes: both beneficial and detrimental. Recent sequencing studies at the bulk and single-cell level in humans and rodents provide new insight into microglia-astrocyte communication in homeostasis and disease. However, the complex changing ways these two cell types functionally interact has been a barrier to understanding disease initiation, progression, and disease mechanisms. Single cell sequencing is providing new insights; however, many questions remain. Here, we discuss how to bridge transcriptional states to specific functions so we can develop therapies to mediate negative effects of altered microglia-astrocyte interactions.

Reactive astrocytes have been implicated in the pathogenesis of neurodegenerative diseases, including a non-cell autonomous effect on motor neuron survival in ALS. We previously defined a mechanism by which microglia release three factors, IL-1α, TNFα, and C1q, to induce neurotoxic astrocytes. Here we report that knocking out these three factors markedly extends survival in the SOD1^G93A ALS mouse model, providing evidence for gliosis as a potential ALS therapeutic target.

Glaucoma is a neurodegenerative disease that features the death of retinal ganglion cells (RGCs) in the retina, often as a result of prolonged increases in intraocular pressure. We show that preventing the formation of neuroinflammatory reactive astrocytes prevents the death of RGCs normally seen in a mouse model of glaucoma. Furthermore, we show that these spared RGCs are electrophysiologically functional and thus still have potential value for the function and regeneration of the retina. Finally, we demonstrate that the death of RGCs depends on a combination of both an injury to the neurons and the presence of reactive astrocytes, suggesting a model that may explain why reactive astrocytes are toxic only in some circumstances. Altogether, these findings highlight reactive astrocytes as drivers of RGC death in a chronic neurodegenerative disease of the eye.

The COVID19 crisis has magnified the issues plaguing academic science, but it has also provided the scientific establishment with an unprecedented opportunity to reset. Shoring up the foundation of academic science will require a concerted effort between funding agencies, universities, and the public to rethink how we support scientists, with a special emphasis on early career researchers.