We have demonstrated that hTau40 filaments selectively impair anterograde FAT in isolated axoplasm, whereas monomeric hTau40 has no effect at the same concentration. Our results provide a novel link between tau aggregation and neuronal dysfunction and identify a specific gain-of-function mechanism conferred by the aggregation process. Furthermore, our studies suggest that inhibition of kinesin-1-based motility represents an important pathogenic event in AD and other tauopathies.
We have previously described a signaling pathway that regulates conventional kinesin-based motility. In this pathway, increased PP1 activity results in the dephosphorylation and activation of axonal GSK-3 (Morfini et al., 2004
). Activated GSK-3 phosphorylates KLCs, prompting a chaperone-dependent dissociation of kinesin-1 and cargo (Morfini et al., 2002b
). Results presented here suggest that tau filaments inhibit anterograde FAT by triggering this pathway (). It is possible that tau filaments directly activate PP1 in this cascade, because tau reportedly binds to PP1 and stimulates its activity (Liao et al., 1998
). However, more work is necessary to determine whether tau filaments act directly on PP1 or whether additional intermediate components remain undiscovered. Additionally, some feedback from GSK-3 is conceivable, insofar as GSK-3 is a major tau kinase that can also bind directly to tau (Sun et al., 2002
) and is reported to be abnormally activated in AD (Ferrer et al., 2005
Fig. 10 Proposed mechanism underlying tau filament-induced inhibition of anterograde FAT. Pharmacological experiments presented here indicate that FAT inhibition requires the activity of PP1 and GSK-3. We have previously described a pathway in which PP1 dephosphorylates (more ...)
We demonstrated that the inhibitory effect of tau filaments on FAT requires the extreme amino terminus of the protein. A pivotal role for this part of the protein is consistent with studies in other experimental systems. For example, overexpression of tau amino terminus induces cell death in cultured neurons through a mechanism involving abnormal kinase activity (Amadoro et al., 2004
), and expression of the N-terminus of tau induced microtubule disassembly in cells exposed to beta-amyloid (King et al., 2006
). Immunological studies indicate that loss of the N-terminus is an early event in the maturation of tau fibrillar lesions in AD (Horowitz et al., 2004
). Our results suggest that such cleavage events may modulate the toxicity of filamentous tau on FAT. The amino terminus is also the site of AD-related phosphorylation (Lee et al., 2004
) and nitration (Reynolds et al., 2006
) events and of the FTDP-17-associated mutations R5L (Poorkaj et al., 2002
) and R5H (Hayashi et al., 2002
). Analysis of how these modifications influence the effects of tau filaments on FAT may provide further insights into disease progression.
Although hTau40 monomer had no effect on FAT at the concentrations tested, monomers of Tau6P and Tau6D inhibited anterograde FAT as effectively as hTau40 filaments. This result demonstrates that the N-terminus of tau is sufficient to produce the observed effects on FAT, insofar as these isoforms lack the MTBR region and the C-terminal tail of canonical tau. Additionally, this raises the question of why hTau40 monomer failed to show the same inhibitory effect, even though it contains an intact amino terminus. Although monomeric hTau40 was first thought to exist in an extended conformation (Schweers et al., 1994
; Syme et al., 2002
), recent evidence suggests that tau in solution adopts a globally folded conformation in which the N-terminus folds in close proximity to the C-terminus (Horowitz et al., 2006
; Jeganathan et al., 2006
). It is possible that this conformation shields the extreme amino terminus, thus preventing hTau40 monomer from inhibiting FAT. In this scenario, polymerization would freeze hTau40 in a different conformation in which the N-terminus is exposed.
With these results, tau joins a growing list of proteins whose pathogenic forms alter regulatory pathways for FAT. As with tau filaments, AD-associated mutations in presenilin-1 inhibit kinesin-1-based motility through GSK-3 activation (Pigino et al., 2003
), whereas pathogenic forms of androgen receptor and huntingtin inhibit kinesin-1-dependent transport through JNK activation (Morfini et al., 2006
). The current study suggests that AD represents an example of a dysferopathy, in which alterations in FAT lead to a dying back neuropathy (Morfini et al., 2007b
), providing further evidence that alterations in regulatory pathways for FAT represent a common pathogenic event in multiple, otherwise apparently unrelated neurodegenerative diseases (Morfini et al., 2002a