Microglial cells protect cerebellar granule neurons from apoptosis: Evidence for reciprocal signaling

The microglia are the immune cell population of the nervous system and play important roles both in normal function and in disease. Reciprocal neuron- microglia interactions are not well understood, in particular those concern ing the crosstalk between the two cell populations when neuronal damage doe s occur. We have used a well-established model of apoptosis in cerebellar g ranule neurons to test the effect of co-culturing microglial cells with the m or of exposing them to microglia-conditioned medium. Microglial cells, de rived from cortical or cerebellar mixed glial cultures and plated over cere bellar granule neurons, protected these neurons from apoptosis induced by s hifting them, at 7 days in vitro, for 24 h from a depolarizing (high-potass ium) to a nondepolarizing (low-potassium) medium. The same result was achie ved when microglial cells obtained from mixed glial cortical cultures were plated over a membrane well insert in the culture chamber, permitting mediu m exchange without physical contact with granule neurons. A similar result was obtained when the low-potassium, apoptosis-inducing medium was conditio ned by 48-h exposure to microglial cells; 24-h exposure to microglial cells was not enough to confer neuroprotective capability to the conditioned med ium. However in double-conditioned medium experiments, in which the medium was first exposed to apoptotic neurons and then to microglial cells, unknow n signal(s) released by apoptotic neurons, conferred to the 24-h conditione d medium a strong neuroprotective action, similar to that observed in the c o-cultures experiments. This finding, together with the results from co-cul ture experiments, is explained by admitting that molecules released in the medium by apoptotic neurons potentiate the anti-apoptotic activity of micro glia. Our results, therefore, demonstrate not only that normally microglial cells release in the medium molecule(s) able to rescue neurons from apopto tic death, but that unknown diffusible signal(s) from apoptotic neurons enh ance(s) microglial neuroprotective properties as well. (C) 2001 Wiley-Liss, Inc.