Rachel Neve and colleagues at McLean Hospital, Belmont, Massachusetts, together with coworkers in the private sector and at Oregon Health and Science University, Portland, made the connection after they found that mutants of amyloidβ precursor protein (AβPP) known to cause familial AD also lead to apoptosis and DNA synthesis in primary neurons cultured from rats. To understand the molecular basis for these responses, first author Donna McPhie and colleagues used a radiolabeled C-terminal fragment of AβPP to screen rat cDNA libraries for potential binding partners. After several rounds of screening they isolated only one protein that bound tightly to the fragment, PAK3.

To determine if the binding of AβPP mutants with PAK3 is physiologically relevant, McPhie expressed both proteins in primary neurons and then looked for evidence of their interaction. When the authors incubated cell extracts with anti-AβPP antibodies both proteins were precipitated, suggesting that they interact in the cell. Furthermore, using immunohistochemical analysis, McPhie found the two proteins co-localized in neurons, primarily in endosomes.

But what of the role of PAK3? The authors found that in the presence of a dominant though inactive variant of the protein, AβPP mutants failed to induce apoptosis or DNA synthesis, indicating that PAK3 plays a vital role in these responses. However, this response also did not occur in these cells when the authors expressed a dominant active form of PAK3 alone, indicating that the interaction between the kinase and the mutant precursor protein is critical.

PAK family members are known to interact with the G protein family of cell signaling molecules, as is AβPP. To test if G proteins may also be involved in re-entry into the cell cycle or the apoptotic pathway, the authors expressed mutated AβPP in cultured neurons in the presence or absence of pertussis toxin, a well known poisoner of G protein signaling. They found that the toxin does indeed block the action of the AβPP mutants indicating that PAK3, G proteins, and AβPP are all inextricably linked in the neurodegeneration caused by the amyloid precursor.-Tom Fagan.

Reference:
McPhie DL, Coopersmith R, Hines-Peralta A, Chen Y, Ivins KJ, Manly SP, Kozlowski MR, Neve KA, Neve RL. DNA synthesis and neuronal apoptosis caused by familial Alzheimer disease mutants of the amyloid precursor protein are mediated by the p21 activated kinase PAK3. J. Neurosci. 2003 July 30;23:6914-6927. Abstract