Faculty Bibliography

Melanoma brain metastasis is the largest cause of melanoma morbidity and mortality, and melanoma has the highest rate of brain metastasis of any cancer. The mechanisms that mediate melanoma brain metastasis remain poorly understood. We characterized patient-derived Short-Term Cultures (STCs) as a novel model system for the study of melanoma brain metastasis. Unbiased proteomics analysis of STCs revealed striking alterations in brain metastasis vs non-brain metastasis derived STCs in proteins related to neurodegeneration. Through in-vivo assays, we show that loss of Amyloid Precursor Protein (APP) in melanoma cells dramatically inhibits melanoma brain metastasis formation without affecting metastasis to other organs and that amyloid beta is the form of APP critically required for melanoma brain metastasis. Additionally, we demonstrate that APP is required for late growth and survival of melanoma cells in the brain parenchyma. Furthermore, we demonstrate that melanoma-derived amyloid beta polarizes astrocytes to an anti-inflammatory secretory phenotype that inhibits microglial phagocytosis of melanoma cells. Finally, we show that treatment of mice with a beta secretase inhibitor (LY2886721), which prevents amyloid beta production, decreases brain metastatic burden. Our results demonstrate a critical role for amyloid beta in melanoma brain metastasis, establish a novel connection between brain metastasis and neurodegenerative pathologies, and show that amyloid beta is a promising therapeutic target for brain metastasis treatment. Studies to further characterize how amyloid beta acts in the melanoma brain metastasis microenvironment are currently underway

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.

In neurodegenerative diseases, debris of dead neurons are thought to trigger glia-mediated neuroinflammation, thus increasing neuronal death. Here we show that the expression of neurotoxic proteins associated with these diseases in microglia alone is sufficient to directly trigger death of naive neurons and to propagate neuronal death through activation of naive astrocytes to the A1 state. Injury propagation is mediated, in great part, by the release of fragmented and dysfunctional microglial mitochondria into the neuronal milieu. The amount of damaged mitochondria released from microglia relative to functional mitochondria and the consequent neuronal injury are determined by Fis1-mediated mitochondrial fragmentation within the glial cells. The propagation of the inflammatory response and neuronal cell death by extracellular dysfunctional mitochondria suggests a potential new intervention for neurodegeneration-one that inhibits mitochondrial fragmentation in microglia, thus inhibiting the release of dysfunctional mitochondria into the extracellular milieu of the brain, without affecting the release of healthy neuroprotective mitochondria.

Altered metabolic function is common in stressed immune cells, but alteration in brain microglia during neurodegeneration is not understood. In this issue, Baik et al. (2019) provide insight into microglial metabolism. They demonstrate a switch from oxidative phosphorylation to glycolysis following interaction with amyloid beta acutely, and breakdown in both pathways chronically.

Astrocytes are essential for central nervous system health, regulating homeostasis, metabolism, and synaptic transmission. In addition to these and many other physiological roles, the pathological impact of astrocytes ('reactive astrocytes') in acute trauma and chronic disease like Alzheimerêž‹s disease (AD) is well established. Growing evidence supports a fundamental and active role of astrocytes in multiple neurodegenerative diseases. With a growing interest in normal astrocyte biology, and countless studies on changes in astrocyte function in the context of disease, it may be a surprise that no therapies exist incorporating astrocytes as key targets. Here, we examine unintentional effects of current AD therapies on astrocyte function and theorise how astrocytes may be intentionally targeted for more efficacious therapeutic outcomes. Given their integral role in normal neuronal functioning, incorporating astrocytes as key criteria for AD drug development can only lead to more effective therapies for the millions of AD sufferers worldwide.

Reactive astrocytes generated following CNS injury and neurodegenerative disease have poorly understood functions. We have reported two types of reactive astrocyte dependent on the type of inducing injury. We named these "A1" and "A2" and based on gene profiles hypothesized they were harmful and helpful respectively. We have shown that A1 astrocytes are induced by microglia-secreted IL1alpha, TNFalpha, and C1q, which are necessary and sufficient to induce A1s both in vitro and in vivo. A1s have little ability to promote neuron survival, synaptogenesis or phagocytosis and instead are powerfully neurotoxic. We also showed that A1s are present in human neurodegenerative disease, and that death of axotomized CNS neurons is prevented when A1s are blocked. We now show the role of A1s in the context of neurodegeneration in acute (optic nerve crush/axotomy) and chronic (glaucoma) mouse models. Together, our findings suggest that A1s drive death of neurons, and point the way to develop new treatments for disease

Reactive astrocytes are strongly induced by central nervous system injury and disease, but their role is poorly understood. We previously found that a subtype of reactive astrocytes is induced by Il-1a, TNFa, and C1q secreted from neuroinflammatory microglia. Further, we showed that these neuroinflammatory reactive astrocytes secrete a factor(s) that kills neurons and oligodendrocytes and saw upregulation of these reactive astrocytes in tissue from patients with neurodegenerative diseases. We are now using our serum-free system for astrocyte culture to determine the identity of the astrocyte-secreted toxin(s) and using genetic mouse lines that inhibit reactive astrocyte formation to determine if reactive astrocytes contribute to neurodegenerative disease

Astrogliosis and activation of microglia are hallmarks of prion diseases in humans and animals. Both were viewed to be rather independent events in disease pathophysiology, with proinflammatory microglia considered to be the potential neurotoxic species at late disease stages. Recent investigations have provided substantial evidence that a proinflammatory microglial cytokine cocktail containing TNF-α, IL-1α and C1qa reprograms a subset of astrocytes to change their expression profile and phenotype, thus becoming neurotoxic (designated as A1-astrocytes). Knockout or antibody blockage of the three cytokines abolish formation of A1-astrocytes, therefore, this pathway is of high therapeutic interest in neurodegenerative diseases. Since astrocyte polarization profiles have never been investigated in prion diseases, we performed several analyses and could show that C3⁺-PrP^Sc-reactive-astrocytes, which may represent a subtype of A1-astrocytes, are highly abundant in prion disease mouse models and human prion diseases. To investigate their impact on prion disease pathophysiology and to evaluate their potential therapeutic targeting, we infected TNF-α, IL-1α, and C1qa Triple-KO mice (TKO-mice), which do not transit astrocytes into A1, with prions. Although formation of C3⁺-astrocytes was significantly reduced in prion infected Triple-KO-mice, this did not affect the amount of PrP^Sc deposition or titers of infectious prions. Detailed characterization of the astrocyte activation signature in thalamus tissue showed that astrocytes in prion diseases are highly activated, showing a mixed phenotype that is distinct from other neurodegenerative diseases and were therefore termed C3⁺-PrP^Sc-reactive-astrocytes. Unexpectedly, Triple-KO led to a significant acceleration of prion disease course. While pan-astrocyte and -microglia marker upregulation was unchanged compared to WT-brains, microglial homeostatic markers were lost early in disease in TKO-mice, pointing towards important functions of different glia cell types in prion diseases.