Background: Tau inhibits kinesin on GDP-microtubules in vitro, but the physiological significance in neurons is unclear. Results: On GTP-microtubules, Tau loses its inhibitory effect, and kinesin becomes less processive. Conclusion:The nucleotide-binding state of the microtubule influences the behavior of both kinesin and Tau. Significance: Tau has different functions, both inhibitory and non-inhibitory, in regulating axonal transport.

The neck-linker is a structurally conserved region among most members of the kinesin superfamily of molecular motor proteins that is critical for kinesin's processive transport of intracellular cargo along the microtubule surface. Variation in the neck-linker length has been shown to directly modulate processivity in different kinesin families; for example, kinesin-1, with a shorter neck-linker, is more processive than kinesin-2. Although small differences in processivity are likely obscured in vivo by the coupling of most cargo to multiple motors, longer and more flexible neck-linkers may allow different kinesins to navigate more efficiently around the many obstacles, including microtubule-associated proteins (MAPs), that are found on the microtubule surface within cells. We hypothesize that, due to its longer neck-linker, kinesin-2 can more easily navigate obstacles (e.g., MAPs) on the microtubule surface than kinesin-1. We used total internal reflection fluorescence microscopy to observe single-molecule motility from different kinesin-1 and kinesin-2 neck-linker chimeras stepping along microtubules in the absence or presence of two Tau isoforms, 3RS-Tau and 4RL-Tau, both of which are MAPs that are known to differentially affect kinesin-1 motility. Our results demonstrate that unlike kinesin-1, kinesin-2 is insensitive to the presence of either Tau isoform, and appears to have the ability to switch protofilaments while stepping along the microtubule when challenged by an obstacle, such as Tau. Thus, although kinesin-1 may be more processive, the longer neck-linker length of kinesin-2 allows it to be better optimized to navigate the complex microtubule landscape. These results provide new insight, to our knowledge, into how kinesin-1 and kinesin-2 may work together for the efficient delivery of cargo in cells.

It has been demonstrated that Tau exists on the microtubule lattice in both diffusing and static populations, but how this may relate to Tau function is currently unclear. Tau isoforms are developmentally regulated and have been shown to have disparate effects on microtubule polymerization, the ability to bind microtubules, and the ability to inhibit kinesin. It has also been shown that Tau is sensitive to microtubule stabilizing agents and the ability to affect the persistence length of microtubules and to inhibit kinesin can be altered by stabilizing microtubules with various nucleotide analogs. Given these observations, it is likely the behavior of Tau is dictated by both the isoform of Tau and by structural changes in the microtubule lattice. In the present study, we use single molecule imaging to examine the behavior of the three-repeat short (3RS) isoform and the four-repeat long (4RL) isoform on different microtubule tracks stabilized with either paclitaxel or guanylyl-(α, β)-methylene-diphosphate (GMPCPP). On paclitaxel-stabilized microtubules, we find 3RS-Tau favors the static conformation and forms complexes consisting of 2–3 molecules, while 4RL-Tau predominantly exists as a single molecule equally distributed between the static and diffusing populations. However, on GMPCPP-stabilized microtubules both isoforms favor the diffusing conformation and do not form static complexes composed of more than one Tau molecule. We find both isoforms of Tau interconvert between static and diffusing populations on the microtubule surface, and the equilibrium between these two states depends on both the isoform of Tau and the structure of the underlying microtubule lattice.

Synaptic plasticity often involves changes in the structure and composition of dendritic spines. Vesicular cargos and organelles enter spines either by exocytosing in the dendrite shaft and diffusing into spines or through a kinesin to myosin hand-off at the base of spines. Here we present evidence for microtubule (MT)-based targeting of a specific motor/cargo pair directly into hippocampal dendritic spines. During transient MT polymerization into spines, the kinesin KIF1A and an associated cargo, synaptotagmin-IV (syt-IV), are trafficked in unison along MTs into spines. This trafficking into selected spines is activity-dependent and results in exocytosis of syt-IV-containing vesicles in the spine head. Surprisingly, knockdown of KIF1A causes frequent fusion of syt-IV-containing vesicles throughout the dendritic shaft and diffusion into spines. Taken together, these findings suggest a mechanism for targeting dendritic cargo directly into spines during synaptic plasticity and indicate that MT-bound kinesins prevent unregulated fusion by sequestering vesicular cargo to MTs.

Synaptic plasticity is a hallmark of the nervous system and is thought to be integral to higher brain functions such as learning and memory. Calcium, acting as a second messenger, and the calcium/calmodulin dependent kinase CaMKII are key regulators of neuronal plasticity. Given the importance of the actin and microtubule (MT) cytoskeleton in dendritic spine morphology, composition and plasticity, it is not surprising that many regulators of these cytoskeletal elements are downstream of the CaMKII pathway. In this review we discuss the emerging role of calcium and CaMKII in the regulation of MTs and cargo unloading during synaptic plasticity.