The Tau Protein
Tau is one of several types of microtubule-associated proteins (MAPs) which regulate the assembly and stability of microtubule networks. Although microtubule networks exist in all kinds of animal and plant cells, Tau is present only in neurons and predominantly localized in axons. This unique feature suggests that Tau may have neuron-specific functions.
Recently, Tau has received great attention because mounting evidence has indicated that hyperphosphorylation of Tau is the origin of Alzheimer's disease (AD) (see another page). Phosphorylation is a process that adds a phosphate group to a protein, particularly on the amino acid serine, threonine or tyrosine. In an AD brain, too many residues in the Tau protein are phosphorylated. This page will discuss both normal Tau phosphorylation and pathological consequences of hyperphosphorylation. It is organized as: (1) functional domains of the Tau protein, (2) normal functions of Tau phosphorylation, (3) regulation of excitability by Tau, (4) effects of aberrant phosphorylation, and (5) lipid rafts and beta amyloid.
Functional Domains
Tau has six isoforms produced from a single gene through alternative RNA splicing (more info). The above figure shows the longest isoform which has 441 amino acid residues. The shortest one has 352 residues. They differ in the number of repeats at the C-terminal half and the number of inserts at the N-terminal half. The number of repeats may be either 3 or 4 and the number of inserts may be 0,1 or 2.
Tau binds to the microtubule through its C-terminal repeats. Upon binding, the N-terminal half protrudes from the microtubule surface. For this reason, the N-terminal half is called the projection domain, which may bind to other cell components such as the plasma membrane (review).
In the middle of the Tau protein, there is a proline-rich region which contains several PXXP motifs. This motif is known to interact with proteins containing Src homology 3 domains (SH3). An important example is Fyn, which is a tyrosine kinase - the enzyme that catalyzes phosphorylation specifically on tyrosine. Fyn works with Tau during the production of the myelin sheath - a layer wrapping around the axon (reference). Experiments have also demonstrated that, without Fyn, beta amyloid cannot induce neurotoxicity (reference).
Normal Functions of Tau Phosphorylation
Protein phosphorylation is widely used to regulate cellular processes, because it can affect binding between two proteins. For the Tau protein, its association with microtubules is inhibited if certain residues in its microtubule binding domain are phosphorylated (review). Tau's association with the plasma membrane also decreases with increasing level of phosphorylation (reference). Mutation of serine or threonine to negatively charged amino acids (which mimics phosphorylation) can reduce binding between Tau and the membrane (reference).
Tyrosine phosphorylation is important in Tau's normal function because it contains the PXXP motif which can interact with tyrosine kinase. All Tau isoforms have five tyrosine residues, located at 18, 29, 197, 310 and 394 (numbering is based on the longest isoform). The tyrosine kinase Fyn preferentially targets tyosine-18 (Y18), which is phosphorylated in the AD brain and at early developmental stages in mice, but not in healthy adult. Phosphorylation of Y18 did not significantly alter the microtubule binding (reference), consistent with its location far from the microtubule binding domain.
Another tyrosine residue, Y394, is also phosphorylated in both AD brain and fetal brain. The phosphorylation has little effect on microtubule binding. Unlike Y18, Y394 is targeted by tyrosine kinase c-Abl (reference). The finding that these residues are phosphorylated in fetus indicates that their phosphorylation has normal functions during development. In fact, it has been known for many years that Tau is highly phosphorylated in the fetal brain, but minimally phosphorylated in the healthy adult brain (reference).
As mentioned above, Tau's association with the membrane decreases with increasing level of phosphorylation. This result, together with the finding that Tau phosphorylation is high in fetus but low in adult, suggest that Tau's functions in fetus and in adult are entirely different. In fetus, Tau's function requires high level of phosphorylation, while in adult it requires low level of phosphorylation so that Tau can associate with the plasma membrane. The question is: what does Tau do at the membrane when it is not phosphorylated? Knowing Tau's membrane function may help us understand why Tau hyperphosphorylation is normal in fetus, but pathological in adult.
Regulation of Excitability
It has been well documented that Alzheimer's disease is linked to overexcitation of neurons, resulting in excessive Ca2+ influx into the neuron (reference). A direct manifestation of overexcitation is seizures which often occur in AD (reference). Since Tau plays a central role in AD, it may be involved in neuronal excitability.
A possible membrane function for Tau proteins. The membrane-associated Tau proteins may bind to a microtubule, which is highly negatively charged. Upon binding, the microtubule can exert a hyperpolarizing field on voltage-gated sodium and potassium channels in the plasma membrane of AIS, thereby reducing neuronal excitability. Dissociation of Tau proteins from the membrane may cause overexcitation.
The above figure illustrates a possible membrane function for Tau proteins: regulation of neuronal excitability. It is assumed that the membrane-associated Tau proteins may also bind to "gating microtubules" - the short and free microtubules NOT embedded in a network. This hypothesis was originally proposed to explain the mind-brain interaction, independent of Alzheimer's disease. At that time, the hypothesis seemed too speculative because in axons microtubules are typically quite long and tightly connected with each other to form a network. However, the short and free microtubules were assumed to exist only in the axon initial segment (AIS), located between the axon hillock (where the axon starts) and the beginning of the myelin sheath. AIS is the initiation site of action potentials (reference). It is a specialized region and could have different microtubular organization.
To my greatest delight, while I was writing this page, I found information about the microtubular organization in AIS (reference). Short and free microtubules DO exist in AIS !
Under electron microscope, it can be seen that microtubules in AIS assemble into bundles called fascicles. In the pyramidal neuron of the rat cerebral cortex, most fascicles contain 6 to 12 microtubules, but some fascicles contain only 3 or 4. Single microtubules are also present, but less visible under the microscope. The sizes of both the individual and fasciculated microtubules appear to be the same. The total number of microtubules in AIS varies between 22 and 50. These microtubules run parallel to the long axis of the axon (reference).
The microtubules in AIS, either single or in fascicles, are called gating microtubules because they may affect the opening probability of ion channels when they bind to the membrane-associated Tau proteins. AIS contains high density of voltage-gated sodium and potassium channels. During information transmission, dendrites receive inputs from other neurons, resulting in graded membrane potential changes which converge at the AIS. If the membrane potential at AIS exceeds a threshold, the action potential will be generated. Tau proteins and gating microtubules may regulate neuronal excitability at this strategic location.
A microtubule molecule is highly negatively charged, about 50 electron charges per tubulin dimer (the building block of microtubules) (reference). Upon binding to the membrane-associated Tau proteins, the gating microtubule can exert a hyperpolarizing field on voltage-gated sodium and potassium channels, reducing their opening probability. Thus, binding between the gating microtubule and membrane-associated Tau proteins has inhibitory effects on neuronal firing. Dissociation of the gating microtubule from membrane-associated Tau has excitatory effects.
In experiments, knockout of the Tau gene from mouse DNA did not have dramatic effects on cognitive functions, suggesting that other MAPs may compensate for the role of Tau, such as MAP1B (reference). However, Tau is the smallest MAP. The microtubule can exert greater hyperpolarizing field when it is bridged by Tau than other MAPs. This may explain why the Tau deficient mice showed hyperactivity in a novel environment. Tau knockout also caused muscle weakness and impairment in fear conditioning (reference).
In humans, there is a genetic disorder resulting from 17q21.31 microdeletion. The deleted region contains several genes, including the Tau gene. This deletion results in hypotonia (muscle weakness), mental retardation and other symptoms (reference).
Effects of Aberrant Phosphorylation
In adult, Tau phosphorylation should be kept to a minimum. If its microtubule binding domain is phosphorylated, the gating microtubules would not be able to bind the membrane-associated Tau proteins, which may cause overexcitation. If Tau's projection domain is phosphorylated, the Tau protein would dissociate from the membrane, which can lead to overexcitation and other damages resulting from neurofibrillary tangles. For this reason, the axon contains high level of phosphatase to dephosphorylate aberrant phosphorylation. However, in the soma and dendrites where Tau is normally absent, the phosphatase activity is low (reference). Therefore, if free phosphorylated Tau proteins migrate to the somato-dendritic compartment, they will have sufficient time to aggregate into neurofibrillary tangles, resulting in severe damages to the neuron.
The main phosphatase for Tau is protein phosphatase 2A (PP2A) which is sensitive to temperature: lower temperature reduces its activity. General anesthesia often induces prolonged hypothermia (lower body temperature) which reduces the activity of PP2A, thereby increasing Tau phosphorylation. This may impair cognitive functions and increase the risk of AD (reference).
The free phosphorylated Tau proteins before forming a tangle represent an early modification in the progression of AD. They were often found in the proximal dendrites (near soma) and axon hillock (reference), consistent with the view that free phosphorylated Tau proteins may originate from the axon initial segment.
In fetus, phosphorylation is not a problem because the factors that can cause problems have not been developed yet. For instance, AIS may not contain sufficient density of ion channels to cause overexcitation. Another important factor is beta amyloid whose neurotoxicity is mainly mediated by lipid rafts.
Lipid Rafts and Beta Amyloid
A lipid raft is a membrane microdomain, enriched in cholesterol and sphingolipids. One of its functions is to confine proteins within the microdomain to facilitate their interaction. For instance, the cleavage of amyloid precursor protein (APP) by the γ-secretase takes place in lipid rafts (reference). At the postsynaptic membrane in dendrites, lipid rafts facilitate Fyn to regulate the normal function of NMDA receptors (reference).
Beta amyloid (Aβ) may be generated either inside or outside the neuron. The intracellularly generated Aβ is normally not sufficient to cause neuronal damages. However, the extracellularly generated Aβ may enter the neuron through lipid rafts (reference). It has been demonstrated that the oligomer of Aβ peptides can activate glycogen synthase kinase-3β (GSK3β) - a major enzyme that catalyzes the phosphorylation of Tau proteins (reference). Therefore, accumulation of Aβ inside the neuron is very toxic.
Aβ can also exert its toxicity at the lipid raft which contains Fyn. The Tau protein can bind to Fyn through its SH3-recognizing motif. When free Tau proteins move to the dendrites, they may interact with Fyn at the lipid raft. The participation of Aβ increases Tau phosphorylation, resulting in Tau aggregation, and eventually leads to synaptic loss (reference).
Author: Frank Lee
Last updated: March 1, 2011