Molecular mechanism of neuritogenesis driven by prion protein

The cellular form of prion protein (PrPC) is a ubiquitous component of both the central and peripheral nervous systems from early stages of development to adulthood. Its misfolded isoform PrPSc is the pathological agent of prion diseases, a group of fatal neurodegenerative diseases. PrPC has been suggested to play different roles in neuroprotection, synaptic activities, neuritogenesis and metal homeostasis. Particularly, we were interested in its neurotrophic function and molecular mechanism involved the prion protein (PrP) with the process. By combining genomic approaches, cellular assays and focal stimulation technique, we explored PrP could act as a guidance cue, attracting the growth cone (GC) protrusion forward and eventually neurite outgrowth. In the study, we made different forms of the recombinant prion proteins (recPrP) from mouse without GPI anchoring residues mimicking secreted forms of PrPC. Our data suggest that full-length and wild-type recPrP(23-231) protein, not its truncated forms at N or C-terminal (23-90, 23-120, 89-231), could attract GC turning toward the protein source and enhance neurite growth in a dosedependent manner. recPrP may act through homophilic interaction with the GPI-anchor PrPC and form trans-signaling complex with neural cell adhesion molecule (NCAM) on the target cells to induce multiple intracellular signaling cascades known for cell growth including the Src-family kinase Fyn, extracellular regulated kinases MEK-ERK and phosphatidylinositol 3-kinase (PI3K). In addition, we discovered the functional sites for PrP function as a signaling molecule in neuritogenesis lying directly on N-terminal copper binding sites by mutating these residues to partially or completely prevent copper binding. In detail, minimal change in the copper binding site could lead to changes in the protein structure preventing PrP from functioning correctly and disrupting all the copper-binding sites at the N-terminus could turn the protein to be toxic to neurons. Especially, copper coordination at non-octarepeat (non-OR) region was shown to be essential for PrP to activate the proper growth signaling. GSS-linked mutation P102L (P101L in mouse numbering) that impacts indirectly to non-OR copper coordination could also abolish the function of PrP on neuritogenesis. Altogether, our findings indicate the crucial role of copper binding sites in maintaining functional structure for PrP interaction in neuritogenesis and suggest a potential link between loss-of-function of the protein and prion disease initiation.