Is cholesterol good or bad for your brain?--NMDARs have a say.

Rapid communication between cells in the nervous systems depends on ion channels that are directly activated by a chemical neurotransmitter. One such ligand-gated ion channel, the N-methyl-d-aspartate receptor (NMDAR), holds a special fascination for neuroscientists and clinicians alike. The NMDAR is involved to some extent in nearly all brain functions including regulating the strength of communication between neurons. Similarly, numerous brain diseases are associated with NMDAR dysfunctions. Hence, NMDAR function is of great interest in both health and disease. In the current issue of The Journal of Physiology, Korinek et al. (2015) present results demonstrating that the membrane lipid cholesterol can alter NMDAR activity. Because cholesterol is present in the central nervous system and can be clinically modified, these results have broad implications to brain function. The impact of NMDARs on cellular communication largely depends on how efficiently the NMDAR ion channel is opened in response to neurotransmitter. Numerous biological and chemical agents, including ions, small molecules and kinases, can modulate this process, referred to as ‘gating’, in NMDARs. Korinek et al. (2015) demonstrate that cholesterol levels in the membrane modulate NMDAR gating and hence how they impact signalling in the brain. Higher levels of membrane cholesterol enhance NMDAR activity whereas decreased levels decrease activity. Defining the effect of membrane lipids on ion channel function is challenging. An elegant aspect of the report by Korinek et al. is the rigour with which they address the question at hand. The authors studied ionotropic glutamate receptors (iGluRs), including NMDA and non-NMDARs, in cultured cerebellar granule cells (CGCs) and used patch-clamp electrophysiology to assay how efficiently iGluRs opened in response to agonists. Initially, they pretreated the CGC cultures with methyl-β-cyclodextrin (MβCD), which removes cholesterol from the lipid bilayer. With increasing duration of the incubation period, NMDARs showed reduced current amplitudes and enhanced desensitization, a process whereby the ion channel closes even in the presence of agonists. Both of these actions – reduced responsiveness to agonist and enhanced desensitization – would diminish the impact of NMDAR on synaptic communication. In contrast, cholesterol levels had no effect on non-NMDAR activity. Thus, however cholesterol acts on iGluRs, it appears specific to the NMDAR. To better characterize their observations, the authors carried out a number of additional experiments. The authors verified that MβCD reduced cholesterol in the membrane. They demonstrated that simvastatin, a clinically used inhibitor of cholesterol biosynthesis, had a similar action on NMDAR activity as MβCD and that NMDAR activity was recovered after adding cholesterol back to MβCD-treated cells. Interestingly, cholesterol enrichment in cultured CGCs enhanced NMDAR current amplitudes. This effect was modest though, suggesting that cholesterol's action was nearly saturated, at least in these cells. Finally, the authors demonstrated experimentally that these alterations in membrane currents did not arise via changes in surface trafficking but rather via an alteration in the channel's open probability. The evidence appears quite strong that NMDAR activity in CGCs depends on membrane cholesterol levels. Still, defining the specific action of membrane lipids on ion channel activity is never an easy task. Membrane lipids may alter ion channel activity either via direct binding (Poveda et al. 2014) or by having broad effects on plasma membrane properties such as thickness, curvature and elasticity (Lundbaek, 2008). Indeed, NMDARs are sensitive to the physical properties of the lipid bilayer (Kloda et al. 2007). Cholesterol, because of its rigid ring structures, dramatically reduces membrane fluidity. Could cholesterol modulate NMDAR by altering the physical properties of the lipid bilayer? To address this challenging issue, the authors estimated membrane fluidity using fluorescence anisotropy. Membrane fluidity was indeed increased following cholesterol removal by MβCD. However, an alternative method using enzymatic degradation of cholesterol did not affect membrane fluidity, yet NMDAR activity was reduced by cholesterol removal regardless of which method was used. Together these results suggest that cholesterol modulates NMDAR activity via a mechanism independent of its effect on membrane fluidity. Given the evidence, cholesterol most likely modulates NMDAR activity via direct binding instead of changes in general membrane properties. However, even though the authors did an outstanding job, direct evidence is still lacking for a specific interaction between cholesterol and NMDAR subunits. Further, the experiments were carried out on CGCs, which express a variety of GluN2 subunits (Lu et al. 2006). Hence, it remains unclear if there is any subunit specificity to cholesterol's action. This consideration is important since defining subunit specificity, if there is any, will help clarify the biological action of cholesterol and will permit more detailed investigations of potential interaction sites. Like any good study, the present results evoke a variety of other physiologically oriented questions. Does cholesterol, which has an uneven membrane distribution, modify the activity of synaptic and/or extrasynaptic NMDARs? Is the level of cholesterol regulated in dendrites? Finally, the significance of cholesterol-induced modification of NMDARs may in certain instances be good and in others bad for the health of the brain. In acute and chronic neurodegenerative disease where NMDAR excitotoxicity comes into play, a reduction of membrane cholesterol may be neuroprotective (Krisanova et al. 2012). On the other hand, NMDAR hypofunction in the prefrontal cortex may be a component of psychiatric disorders such as schizophrenia (Lewis et al. 2012). In these instances, lowered cholesterol might exacerbate symptoms. Hence, cholesterol's modulation of NMDAR activity may be relevant in a number of physiological and pathophysiological contexts. Korinek et al. (2015) provide the first glimpse into this intriguing issue, which will no doubt receive increasing attention in the foreseeable future.