The distribution of β-tubulin isotypes in cultured neurons from embryonic, newborn, and adult mouse brains.

Tubulin, the subunit protein of microtubules, is an α/β heterodimer. Both α- and β-tubulin exist as numerous isotypes, differing in their amino acid sequences and encoded by different genes. The differences are highly conserved in evolution, suggesting that they are functionally significant. Neurons are a potentially very useful system for elucidating this significance, because they are highly differentiated cells and rich in tubulin isotypes. We have examined the distribution of β-tubulin isotypes in mouse primary cultured cortical neurons from embryonic fetus, newborn pups and adults. Neurons from both embryonic and adult mouse brains express the βI, βII, and βIII isotypes, but apparently not βIV or βV. βI, βII, and βIII are found in both cell bodies and neurites. However, the situation is different in newborn mice. Although βI and βIII are present in these neurons in both cell bodies and neurites and βIV is absent, just like in embryonic and adult mice, two striking differences were noted in the neurons from newborn mice. First, βV is apparently present evanescently in the neurons in both cell bodies and neurites. Interestingly, the βV was expressed strongly in newborn neurons after one day of culture; expression became much weaker after 3days, and almost disappeared after 5days. Second, the distribution of βII is different from other isotypes. After newborn mouse neurons were cultured for 3days, βII started to disappear partly from the cell bodies; this was much more pronounced after five days in culture. Our findings suggest that βII's major function may involve the neurites and not the cell body. They also raise the possibility that βV has a unique role in the neurons of newborn mice.