Astrocyte function and role in motor neuron disease: a future therapeutic target?

Astrocytes are the most numerous cell type within the central nervous system (CNS) and perform a variety of tasks, from axon guidance and synaptic support, to the control of the blood brain barrier and blood flow. To perform these roles, there is a great variety of astrocytes. In this review, we summarize the function of astrocytes, in particular, their role in maintaining homeostasis at the synapse, regulating neuronal signaling, protecting neurons from oxidative damage, and determining the fate of endogenous neural precursors. The review also highlights recent developments indicating the role of astrocytes in motor neuron disease (MND), emphasizing their potential as therapeutic targets and agents in cell replacement therapy. The Cu-Zn superoxide dismutase (SOD1) gene that has been implicated in 20% of cases of familial MND must be expressed in the glial cells as well as motor neurons to produce the disease state in murine models of disease. Selectively reducing mutant SOD1 (mSOD1) in astrocytes does not affect disease onset but slows disease progression, whereas reducing mSOD1 in motor neurons delays disease onset and slows early disease but has less effect on life span. This suggests that glial cells represent potential therapeutic targets in MND. However, the lack of specific markers for astrocytes, their precursors, and sub-types means that our knowledge of astrocyte development/differentiation and response to injury lags far behind our understanding of function. Only by filling this knowledge gap can astrocytes be effectively targeted or replaced to successfully treat chronic CNS disorders such as MND.