α-Synuclein is a major constituent of pathological intracellular inclusion bodies, a common feature of several neurodegenerative diseases. Two missense mutations in the α-synuclein gene have been identified in confirmed autosomal-dominant familial Parkinson's disease, which segregate with the illness. However, the physiological function of α-synuclein remains unknown. After biochemical investigations we have revealed tubulin to be an α-synuclein associated/binding protein. Here, we show that α-synuclein induces polymerization of purified tubulin into microtubules. Mutant forms of α-synuclein lose this potential. The binding site of α-synuclein to tubulin is identified, and co-localization of α-synuclein with microtubules is shown in cultured cells. To our knowledge, this is the first demonstration of microtubule-polymerizing activity of α-synuclein. Now we can see a striking resemblance between α-synuclein and tau: both have the same physiological function and pathological features, making abnormal structures in diseased brains known as synucleinopathies and tauopathies. The discovery of a physiological role for α-synuclein may provide a new dimension in researches into the mechanisms of α-synuclein-associated neurodegenerative diseases.

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A pathological hallmark of Alzheimer's disease is the deposition of amyloid fibrils in the brain. The principal component of amyloid fibrils is beta/A4 amyloid protein, which can be generated by the aberrant processing of a large membrane-bound glycoprotein, the beta/A4 amyloid protein precursor (APP)3. To test whether overexpression of APP generates abnormally processed derivatives that affect the viability of neurons, we stably transfected full-length human APP complementary DNA into murine embryonal carcinoma P19 cells. These cells differentiate into post-mitotic neurons and astrocytes after exposure to retinoic acid. When differentiation of the APP cDNA-transfected P19 cells was induced, all neurons showed severe degenerative changes and disappeared within a few days. The degenerating neurons contained large amounts of APP derivatives that were truncated at the amino terminus and encompassed the entire beta/A4 domain. These results suggest that post-mitotic neurons are vulnerable to overexpressed APP, which undergoes aberrant processing to generate potentially amyloidogenic fragments.

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