CD40 is expressed and functional on neuronal cells.

We show here that CD40 mRNA and protein are expressed by neuronal cells, and are increased in differentiated versus undifferentiated N2a and PC12 cells as measured by RT-PCR, western blotting and immunofluorescence staining. Additionally, immunohistochemistry reveals that neurons from adult mouse and human brain also express CD40 in situ. CD40 ligation results in a time-dependent increase in p44/42 MAPK activation in neuronal cells. Furthermore, ligation of CD40 opposes JNK phosphorylation and activity induced by NGF-beta removal from differentiated PC12 cells or serum withdrawal from primary cultured neurons. Importantly, CD40 ligation also protects neuronal cells from NGF-beta or serum withdrawal-induced injury and affects neuronal differentiation. Finally, adult mice deficient for the CD40 receptor demonstrate neuronal dysfunction as evidenced by decreased neurofilament isoforms, reduced Bcl-x(L):Bax ratio, neuronal morphological change, increased DNA fragmentation, and gross brain abnormality. These changes occur with age, and are clearly evident at 16 months. Taken together, these data demonstrate a role of CD40 in neuronal development, maintenance and protection in vitro and in vivo.