MARKs, or microtubule-associated protein (MAP)/microtubule affinity regulatory kinases, are involved in controlling MAP binding to microtubules and are conserved among a wide range of organisms from yeast to mammals. While MARKs play a key role in many processes, including cell division, the recognition that they can phosphorylate tau has sparked interest in their role in AD. Of the four MARK paralogs in the human genome, only three have been characterized, so joint first authors Bernhard Trinczek and Miro Brajenovic decided to probe the nature and activity of MARK 4. Other kinases implicated in tau phosphorylation relevant to AD include GSK-3β, CDK5, and MAP kinase.

The authors used PCR to clone the complete human gene. Sequence analysis showed that the overall identity between MARK 4 and the other three family members is about 70 percent. However, one area, called the spacer domain because it falls between the ubiquitin-associated and the C-terminal kinase domains, is much less conserved in MARK4, setting this protein apart from the others in the family. Subjecting it to a battery of biochemical and molecular tests, the authors found that, like MARKs 1, 2, and 3, MARK4 readily phosphorylates tau, but it also has some unique properties.

Immunohistochemistry to localize MARKs overexpressed in CHO cells showed that only MARK4 seems to co-localize with microtubules, particularly in centrosomes. This localization appears to be dependent on the protein’s kinase activity, because an inactive mutant did not segregate in the same fashion. When the scientists studied the distribution of a green fluorescent protein-tagged MARK4 in neurons, they found it is associated with microtubules not only in the cell bodies, but also in the tips of neurite-like processes. Similarly tagged MARKs 1-3 were not visible in these structures.

In neurites, the microtubule matrix is predicted to be most dynamic, yet previous observations have shown that the tau/tubulin ratio is high there. As tau stabilizes microtubules, this seems contradictory. Drewes and colleagues suggest that MARK4 in these processes may help to counterbalance this stabilizing effect of tau, because MARK-catalyzed phosphorylation of tau reduces its affinity for the tubules. The authors also suggest that a "gain or loss of function of MARK4 in Alzheimer's disease may result in the loss of axonal-dendritic polarity that is a hallmark of neurodegeneration."—Tom Fagan.

Reference:
Trinczek B, Brajenovic M, Ebneth A, Drewes G. MARK4 is a novel MAP/microtubule affinity-regulating kinase that binds to the cellular microtubule network and to centrosomes. J Biol Chem. 2003 Nov 1 [Epub ahead of print]. Abstract