ATP and adenosine induce ramification of microglia in vitro

Microglial cells in the healthy adult brain possess a characteristic ramifi ed morphology with multiple branched processes, small somata and down-regul ated inflammatory properties. In contrast, microglial cells isolated from n ew-born rat brain inevitably show a non-ramified amoeboid phenotype, which is observed in vivo after pathologic activation or during development. To i dentify factors that control microglial morphology we investigated the effe cts of purines alone or in combination with astrocyte-conditioned medium (A CM). Under optimized culture conditions postnatal rat microglial cells deve loped an amoeboid to ovoid phenotype. Addition of 0.6-1 mM ATP or adenosine induced the outgrowth of numerous processes after 2-3 days that could be o bserved also in the presence of ACM as previously reported. Culture in ACM plus ATP or adenosine yielded an optimized ramified phenotype. ATP or adeno sine, but not ACM alone, also prevented the formation of a flat, amoeboid m orphology induced by lipopolysaccharide (LPS); however, at 0.6-1 mM they di d not reduce the initial LPS-induced activation of the transcription factor NF-kappaB. By using specific agonists or antagonists the morphological tra nsformations could not be confined to a distinct purinoreceptor subtype, bu t appeared to be mediated by long-term presence of adenosine in the medium to which phosphorylated purines were rapidly hydrolyzed by microglial cells . Since ACM did not contain sufficient concentrations of ATP or adenosine, purines are not the only ramification-inducing factors present in ACM; howe ver, they are a valuable tool to induce microglial ramification in vitro. ( C) 2001 Elsevier Science B.V. All rights reserved.