How do astrocytes regulate neuroinflammation? Astrocytic TGF beta signaling limits inflammation and neuronal damage in models of stroke and infection [electronic resource]

Reactive astrocytes are increasingly recognized to play a role in controlling the immune response to CNS injury. They form a physical barrier, also known as the astrocytic scar, which sequesters the injury site from neighboring healthier tissue. In addition, reactive astrocytes can regulate neuroinflammation by secreting cytokines and chemokines that inhibit or promote immune cell infiltration, function and distribution in the injury border. These cytokines and chemokines can together be defined as a secretory barrier to inflammation. Reactive astrocytes can have either pro- or anti-inflammatory functions, depending on the type of intracellular signaling that is activated by the environment. Therefore, targeting endogenous signaling pathways in reactive astrocytes after injury could potentially be used for therapeutic purposes, to limit the tissue damage caused by neuroinflammation. This dissertation is focused on investigating the role of endogenous astrocytic signaling by a family of major regulatory cytokines, the transforming growth factor-beta (TGFbeta) family, in controlling these astrocytic barriers to neuroinflammation. TGFbeta is universally upregulated by CNS injury, typically has anti-inflammatory functions, and has previously been shown to increase its signaling in reactive astrocytes after stroke. We inhibited endogenous TGFbeta signaling specifically in GFAP+ astrocytes by constructing a transgenic "DN" mouse model, and studied the outcomes in two models of CNS injury in vivo. The first model was stroke, which primarily elicits the innate immune response, and the second model was a parasitic infection with Toxoplasma gondii, which primarily evokes the adaptive immune response. We show that normal astrocytic TGFbeta signaling is crucial to limit immune cell infiltration and spread in the unaffected tissue during the period of peak inflammation both after stroke and during Toxoplasma infection. Thus, inhibiting astrocytic TGFbeta signaling in DN mice approximately doubles the numbers of immune cells and expands their infiltration and activation beyond the injury site. Inhibiting TGFbeta signaling does not affect the physical barrier formed by astrocytes, but instead alters the astrocyte-generated secretory barrier by changing the cytokine and chemokine environment in the injury border. The specific astrocytic TGFbeta signaling-mediated changes in cytokine and chemokine levels depend on the model of neuroinflammation. Our results suggest that after stroke inhibiting astrocytic TGFbeta signaling prevents the activation and signaling of TGFbeta itself via a positive feedback loop that involves the TGFbeta activator thrombospondin-1. By comparison, during Toxoplasma infection inhibiting astrocytic TGFbeta signaling is associated with an increased astrocytic production of immune cell chemoattractant CCL5 via increased NF-kappaB signaling pathway activation. We further show that the physiological significance of endogenous astrocytic TGFbeta signaling during peak inflammation is to limit neuronal damage in the injury border. Thus, inhibiting astrocytic TGFbeta signaling leads to more neuronal death and dendritic damage during infection, and to larger strokes and worse motor deficit in a stroke model that targets the motor cortex. In summary, we demonstrate that inhibiting endogenous TGFbeta signaling in astrocytes exacerbates neuroinflammation and neuronal damage in stroke and CNS infection. Therefore, astrocytic TGFbeta signaling presents a potential therapeutic target to reduce neuroinflammation and associated neuronal damage that could be investigated in future studies aimed at increasing astrocytic TGFbeta signaling above its endogenous levels.

Advisor: Buckwalter Marion
Huguenard John primary advisor. Thesis advisor
Barres Ben
Giffard Rona Greenberg
Wyss-Coray Anton advisor. Thesis advisor