The list of enzymes that can regulate tau phosphorylation in the test tube is quite long—in addition to MARK/PAR-1, there are GSK-3β, MAP kinase, CDK2 and 5 (see ARF related news story), PKA, CaMKII, and protein phosphatases 1, 2A, 2B, and 2C. GSK-3 and CDK5 are most likely involved in vivo, too. But which one is the ringleader of the neurodegenerative process?

In the current study, first author Isao Nishimura of the Rockefeller University in New York City, with Lu and Yufeng Yang, who are now at Stanford University in Palo Alto, California, take advantage of the ease with which researchers can manipulate Drosophila genetics. They studied the role the fly’s MARK homologue PAR-1 (see ARF related news story) plays in tau-related degeneration of photoreceptor cells. The researchers first established that overexpression of PAR-1 in photoreceptor neurons led to degeneration of these cells in a dose-dependent manner. This effect was mediated by an endogenous fly tau homologue. PAR-1 overexpression also increased neurodegeneration in flies transfected with either wild-type or mutant (4 repeat with R406W mutation) human tau.

These experiments did not produce obvious neurofibrillary tangles (NFTs). Conversely, George Jackson and colleagues have found that coexpression of GSK-3 with human tau did lead to evidence of NFTs in a similar photoreceptor model (see ARF related news story).

The researchers next sought to answer the question of what happens to tau if there is no PAR-1 present. Since embryos lacking PAR-1 don't survive, the researchers created tissue clones that lacked PAR-1 but contained mutant human tau. TUNEL assays revealed a threefold drop in apoptosis in cells lacking PAR-1, accompanied by a drop in tau phosphorylation at the S262 and S356 residues. These two sites are frequented by MARK and other tau phosphorylators, and the researchers confirmed their importance by showing that point mutations in S262 and S356 abolished the toxicity of mutant h-tau in photoreceptors.

Yet more interesting was the finding that these mutations in the PAR-1/MARK binding site also abolished phosphorylation at other sites implicated in AD, including those recognized by the antibodies AT100 (pT212 and pS214) and AT8 (pS202 and pT205), but not the AT270 site (S199 and S202). The suggestion that phosphorylation at S262 and/or S356 is necessary for phosphorylation at these other sites was confirmed in neurons without PAR-1.

What does this mean for other phosphorylation suspects? The authors found that elevation of GSK-3 and CDK5, which have affinity for these inhibited sites, raised phosphorylation at these residues, an effect that was compounded by overexpression of PAR-1. Thus, PAR-1 phosphorylation seems to be a prerequisite for GSK-3- and CDK5-mediated phosphorylation.

In an editorial in the same issue of Cell, Mark Fortini of the National Cancer Institute in Frederick, Maryland, suggests an emerging picture "in which PAR-1 initiates a temporally ordered series of tau phosphorylations, with the early PAR-1 phosphorylation step generating soluble non-aggregating tau forms that are converted into hyperphosphorylated, aggregation-prone tau through downstream phosphorylations performed by CDK5, GSK-3β and perhaps additional kinases."

Speculating on tau's role in AD and other tauopathies, the authors make special mention of the possibility that hyperphosphorylation of tau could be involved in the early synaptic dysfunction postulated in AD (see also Mandelkow portion of ARF related news story). They suggest several candidate mechanisms for such an effect, including a direct effect of hyperphosphorylated tau at the synapse, or an effect via disrupted microtubule dynamics.—Hakon Heimer.

References:
Nishimura I, Yang Y, Lu B. PAR-1 kinase plays an initiator role in a temporally ordered phosphorylation process that confers tau toxicity in Drosophila. Cell. 2004 Mar 5;116:671-82. Abstract

Fortini ME. PAR-1 for the course of neurodegeneration. Cell. 2004 Mar 5;116(5):631-2. Abstract