α-Actinin-dependent cytoskeletal anchorage is important for ICAM-5-mediated neuritic outgrowth

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Abstract
Intercellular adhesion molecule-5 (ICAM-5, telencephalin) is a dendrite-expressed membrane glycoprotein of telencephalic neurons in the mammalian brain. By deletion of the cytoplasmic and membrane-spanning domains of ICAM-5, we observed that the membrane distribution of ICAM-5 was determined by the cytoplasmic portion. Therefore we have characterized the intracellular associations of ICAM-5 by using a bacterially expressed glutathione S-transferase (GST) fusion protein encompassing the cytoplasmic part of ICAM-5. One of the main proteins in the neuronal cell line Paju that bound to the ICAM-5 cytodomain was α-actinin. ICAM-5 expressed in transfected Paju cells was found in α-actinin immunoprecipitates, and ICAM-5 colocalized with α-actinin both in Paju cells and in dendritic filopodia and spines of primary hippocampal neurons. We were also able to coprecipitate α-actinin from rat brain homogenate. Binding to α-actinin appeared to be mediated mainly through the N-terminal region of the ICAM-5 cytodomain, as the ICAM-5857-861 cytoplasmic peptide (KKGEY) mediated efficient binding to α-actinin. Surface plasmon resonance analysis showed that the turnover of the interaction was rapid. In a mutant cell line, Paju-ICAM-5-KK/AA, the distribution was altered, which implies the importance of the lysines in the interaction. Furthermore, we found that the ICAM-5/α-actinin interaction is involved in neuritic outgrowth and the ICAM-5857-861 cytoplasmic peptide induced morphological changes in Paju-ICAM-5 cells. In summary, these results show that the interaction between ICAM-5 and α-actinin is mediated through binding of positively charged amino acids near the transmembrane domain of ICAM-5, and this interaction may play an important role in neuronal differentiation.