A yeast two-hybrid screen reveals that osteopontin associates with MAP1A and MAP1B in addition to other proteins linked to microtubule stability, apoptosis and protein degradation in the human brain.

Osteopontin (OPN) expression is reduced in surviving dopaminergic neurones in the substantia nigra (SN) in Parkinson's disease (PD), and protects against MPP(+) -induced cell death in primary mesencephalic cultures and 6-OHDA-induced cell loss in the rat, while inactivation of OPN aggravates cell death. OPN is thought to act through interactions with integrin receptors or CD44. However, the specific protein interactions involved in OPN-mediated neuroprotection are unknown and are the focus of this study. The yeast two-hybrid (YTH) technique was utilised to investigate OPN-protein interactions, using full-length human OPN to screen a human foetal brain cDNA library. Proteins involved in apoptosis, protein degradation and microtubule stability were identified as OPN binding partners. These included: MAP1A and MAP1B, which regulate microtubule stability; RNF138, an E3 ubiquitin-ligase; proteasome β1 subunit, a subunit of the 20S proteasome involved in the ubiquitin-dependent cleavage of peptides; BAG6, SGTΑ and EF1A, proteins implicated in control of apoptosis; DnaJB1, a co-chaperone of Hsp70s; and pleiotrophin, a growth factor. The use of site-directed mutagenesis to modify known OPN protein binding sites outside the RGD integrin binding domain, specifically Y165A and D139E, inhibited some of these interactions. Further investigation using affinity pull-down assays, co-immunoprecipitation and immunohistochemistry confirmed that OPN associates with MAP1A and MAP1B in rat SN and striatum. These findings indicate a role for OPN in the regulation of microtubule dynamics, apoptosis and proteolysis in the brain, suggesting that OPN may act as an endogenous multifunctional protective protein in PD.