Modulating Astrocyte Transition after Stroke to Promote Brain Rescue and Functional Recovery: Emerging Targets Include Rho Kinase

Abstract

Stroke is a common and serious condition, with few therapies. Whilst previous focus has been directed towards biochemical events within neurons, none have successfully prevented the progression of injury that occurs in the acute phase. New targeted treatments that promote recovery after stroke might be a better strategy and are desperately needed for the majority of stroke survivors. Cells comprising the neurovascular unit, including blood vessels and astrocytes, present an alternative target for supporting brain rescue and recovery in the late phase of stroke, since alteration in the unit also occurs in regions outside of the lesion. One of the major changes in the unit involves extensive morphological transition of astrocytes resulting in altered energy metabolism, decreased glutamate reuptake and recycling, and retraction of astrocyte end feed from both blood vessels and neurons. Whilst globally inhibiting transitional change in astrocytes after stroke is reported to result in further damage and functional loss, we discuss the available evidence to suggest that the transitional activation of astrocytes after stroke can be modulated for improved outcomes. In particular, we review the role of Rho-kinase (ROCK) in reactive gliosis and show that inhibiting ROCK after stroke results in reduced scar formation and improved functional recovery.

Keywords: Rho-kinase inhibition; astrogliosis; connectivity; glial scar; neurovascular unit; regeneration.

Figure 1

Schematic diagram of the neurovascular unit. The inter-relationship of microvessels and their dependent neurons via astrocytes and surrounding cells including microglia and oligodendrocytes where injury affects the function of the entire unit. Microvessels consisting of pericytes attached to the abluminal surface of the endothelial cells are surrounded by basement membrane and encompassed by astrocyte end-feet.

Figure 2

Over activation of astrocytes adjacent to the stroke lesion breaks neurovascular coupling in structurally intact nerves: Schematic diagram of healthy astroctyes with end feet coupling to blood vessels and neurons (long black arrow); following stroke (blue arrow) reactive astrocytes retract their end feet connections to break coupling (short black arrow) to form the glial scar. Targeting astrocytes to reduce the glial scar whilst retaining trophic astrocyte support is a new target for brain rescue.

Figure 3

Simple schematic representation of Gliotic scar mediated Rho-kinase (ROCK, blue circle) activation in neurons. Inhibitory molecules released by the glial scar include CSPGs, Nogo (Nogo66 and Amino Nogo), OMgP, RGM, Ephrin and semaphorins. Activation of receptors present on axon membranes (red shapes) signal change in Rho-ROCK activity within neurons resulting in growth cone collapse, neurite retraction and apoptosis. Black arrows reference signaling events from the glial scar that activate receptors on neurons that trigger internal signaling events (Grey arrows) that activate ROCK.

Figure 4

The Rho/ROCK signaling pathway. Rho is kept in the inactive GDP-bound state (yellow/blue complex) by sequestration with Guanine nucleotide Dissociation inhibitors (GDIs; green comples) and the activity of GTPase activating proteins (GAPs). Rho can be activated through Guanine Exchange Factors (GEFs), enabling the exchange of GDP for GTP. Rho-GTP (orange/blue complex) can then activate ROCK (blue/red complex) by binding to the Rho binding domain. Active ROCK can then phosphorylate multiple downstream effectors eliciting changes in actin membrane stabilisation, growth cone collapse and increased cell adhesion, which in astrocytes results in glial scar formation. Arrows indicate activation events, whereas blunted lines indicate inhibitory effects.

Figure 5

Diffuse astroctyes (GFAP; green, DAPI; blue) within the peri-infarct cortex of vehicle rats at 28 days post stroke (A) are attenuated by fasudil (10 mg/kg/i.p./daily) (B); Treatment with fasudil commencing three days after stroke significantly improved contralateral forepaw deficit by 14 days (C). * p < 0.05 vs. 0 h scores; # p < 0.01 vs. vehicle treatment; n = 7/group (unpublished data, [128]).