Stroke and brain trauma are leading causes of death and disability worldwide. The most common type of stroke, ischaemic stroke, is caused by blood clots which cut off the supply of blood to the brain. Although people who have had an ischaemic stroke can be treated with ‘clot-busting’ medicines, the window of opportunity for effective thrombolysis is only about 3 hours from the onset of stroke. Following a stroke, levels of glutamate increase rapidly, leading to over-activation of N-methyl-D-aspartic acid receptors (NMDARs) and neuronal excitotoxicity. Although this pathway is believed to be a major contributor to neuronal injury after stroke and brain trauma, NMDAR antagonists have not proved effective in reducing brain injury after stroke. Some of the limitations of NMDAR antagonists, including interference with the normal physiological function of NMDARs and a narrow therapeutic window, might be overcome by targeting downstream signalling pathways and scientists at the Brain Research Centre, University of British Columbia and the China Medical University Hospital, Taiwan have identified one such pathway.
Over-activation of NR2B subunit–containing NMDARs was found to activate SREBP-1, a transcription factor that regulates genes involved in lipid biosynthesis. Activation of SREBP-1 requires cleavage of an inactive membrane-bound precursor protein by two dedicated proteases in the Golgi apparatus, releasing an N-terminal fragment which translocates to the nucleus. Under basal, unstimulated conditions, immature SREBPs form a stable complex with SREBP cleavage–activating protein (SCAP) and are retained in the endoplasmic reticulum (ER) by the interaction between SCAP and the ER membrane–resident protein, Insig-1. Treatment of neuronal cultures with Insig-1–specific siRNA resulted in a 40% reduction in Insig-1 and a corresponding increase in active SREBP-1. Challenging these neurons with excitotoxic NMDA stimuli resulted in increased cell death compared to controls, supporting a crucial role for the degradation of Insig-1 in mediating NMDAR-dependent SREBP-1 activation and excitotoxicity, and also suggesting that inhibition of Insig-1 degradation may be neuroprotective. Insig-1 is degraded by the proteasome after ubiquitination, and an Insig-1–derived interference peptide (Indip) that competes for ubiquitination was found to block NMDA-induced degradation of Insig-1 and reduce cell death.
In animal studies, prophylactic Indip treatment 45 minutes before 90-min transient middle cerebral artery occlusion (MCAo) reduced the size of the infarct compared to saline-injected animals and also markedly improved the combined behavioral outcome 24 h post-ischemia. In an experiment more closely resembling clinical intervention, Indip administered 2 h after 90-min transient MCAo also produced significantly reduced brain infarct volume 7 days later. Although the details of exactly how SREBP-1 activation leads to neuronal damage have not been fully established, the study suggests that agents that reduce activation may be neuroprotective after stroke. Since excitotoxicity is also believed to be associated with chronic neurodegenerative disorders such as Huntington’s disease and Parkinson’s disease, reducing SREBP-1 activation may also be of benefit in these conditions.
The study is published in Nature Medicine.