Introduction
Role | Experimental paradigm | Adverse effects | References |
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Microtubules Regulation of microtubule dynamics | Tau knockdown | ↓ In labile microtubule mass, ↑ in the stable domain | Qiang et al. [213] |
Tau knockdown | ↓ Neuronal outgrowth | Liu et al. [150] | |
Tau knockdown | Impaired repulsive response of the growth cone | ||
Tau knockdown | Disruption to axonal extension | Caceres and Kosik [36] | |
Tau knockdown/ knockout | Delayed neuronal maturation | ||
Tau knockout | ↓ Microtubule density in small caliber axons | Harada et al. [96] | |
No tau added to microtubules in vitro (compared to tau presence) | ↑ EB1 binding to microtubule ends, ↑ catastrophe frequency | Ramirez-Rios et al. [217] | |
Regulation of axonal transport | 4R tau knockdown | ↑ Velocity of mitochondrial axonal transport | Beevers et al. [15] |
Protection of microtubules from cleavage | Tau knockdown | Katanin-mediated cleavage, loss of microtubules and ↓ axon length | Qiang et al. [214] |
Tau knockdown | ↑ Neuronal branching | Yu et al. [287] | |
Synaptic Activity LTP, LTD and memory | Tau knockout | Age-dependent cognitive deficits in contextual fear conditioning, Y-maze, Morris Water Maze and reversal learning tests | |
Tau knockout | Severe LTP deficit | Ahmed et al. [3] | |
Tau knockout | LTD deficits | ||
Acute tau knockdown using shRNA | ↓ Dendritic spine density, loss of synaptic proteins and significant spatial memory impairments (no compensatory MAP upregulation) | Velazquez et al. [266] | |
Regulation of neuronal hyperexcitability | Tau knockout | Hyperpolarised neuronal membrane potential | Pallas‐Bazarra et al. [197] |
Tau knockout | Impaired basal neurotransmission when crossed with APP transgenic mouse | Puzzo et al. [209] | |
Neurogenesis and synaptogenesis | Acute tau knockdown using shRNA | ↓ In baseline spine numbers, pro-synaptic response to BDNF blocked | Chen et al. [44] |
Acute tau knockdown using shRNA | ↓ Apical and basal dendrite density | Velazquez et al. [266] | |
Tau knockout | Failed normal migration of new-born granule neurons in the dentate gyrus | ||
Tau knockout | ↓ Dendritic length, disrupted PSD and mossy fiber terminal formation | Pallas‐Bazarra et al. [197] | |
Tau knockout | Impaired neurogenesis | Hong et al. [107] | |
Tau knockout | Delayed neuronal maturation | Dawson et al. [54] | |
Tau knockout | Transcriptional repression of neuronal genes | de Barreda et al. [11] | |
Behaviour Hyperactivity | Tau knockout | Hyperactivity | |
Anxiety | Tau knockout | ↑ Rearing behaviour | Lei et al. [146] |
Tau knockout | ↑ Anxiety in open field arenas | Gonçalves et al. [86] | |
Sleep | Tau knockout | ↑ Wakefulness and disruption to normal circadian activities | |
Motor function | Tau knockout | FTD-P17-like motor dysfunction | Lei et al. [145] |
Tau knockout | Changes in gait, ↓ locomotion and muscle weakness | ||
Tau knockout | Loss of dopaminergic neurons | ||
Tau knockout, tau 4R knockout, acute tau knockdown using shRNA | Significant impairment in balance beam or rotarod performance | ||
Myelination Regulation of myelination | Tau knockdown using siRNA | ↓ Oligodendrocyte process outgrowth, ↓ myelin basic protein expression, ↓contact with axons | Seiberlich et al. [230] |
Tau knockdown using siRNA | ↓ Recovery after sciatic nerve damage, defective myelin debris clearance, impaired Schwann cell migration and differentiation | Yi et al. [285] | |
Tau knockout | Age-dependent degeneration of myelinated fibers, ↓ nerve conduction and progressive hypomyelination, resulting in motor and nociceptive impairments | ||
Tau knockout | Worse clinical outcome after experimental autoimmune encephalomyelitis (EAE) | Weinger et al. [277] | |
Expression of an inducible, truncated tau | Demyelination and development of gait abnormalities | LoPresti [160] | |
Response to injury Promotion of recovery | Tau knockout | ↓ Recovery after sciatic nerve damage | Yi et al. [285][] |
Tau knockout | Worse outcome after EAE | Weinger et al. [277] | |
Mitochondrial activity Mitochondrial mobility and health | Tau knockdown | ↓ Mitochondrial mobility and ↑ number of abnormal mitochondria | Sapir et al. [227] |
Iron Regulation of iron homeostasis | Tau knockout | Age-dependent iron accumulation associated with neurodegeneration, cognitive deficits and parkinsonian-like motor deficits, deficits rescued by treatment with the iron chelator clioquinol | |
Lithium-mediated tau reduction | ↑ Iron accumulation in the brain, ↓ cellular efflux of iron | Lei et al. [143] | |
Nuclear activity Protection of DNA from damage | Tau knockout | Extensive heat shock damage (DNA breaks) in neurons | Sultan et al. [246] |
Tau knockout | ↑ DNA fragmentation under physiological conditions and high susceptibility to DNA breakage after hyperthermic stress | Violet et al. [267] | |
Tau knockout | Delayed repair of double-strand breaks after heat shock | Violet et al. [267] | |
Maintenance of chromosomal stability | Knockout of one or both copies of tau | Marked ↑ in aneuploidy | |
Tau knockout | Disrupted pericentromeric heterochromatin | ||
Regulation of transcription | Tau knockdown using shRNA | ↓ mRNA and protein levels of VGLUT1 | Siano et al. [234] |
Tau knockout | Upregulation of proteins such as BAF-57 (involved in neuron-specific gene repression) | de Barreda et al. [11] | |
Tau knockdown | rDNA transcription altered | ||
Tumour suppression | Tau knockdown | Enhanced cell growth and invasion in clear cell renal cell carcinoma | Han et al. [95] |
Glucose metabolism | Tau knockout | Insulin resistance in the hippocampus | Marciniak et al. [170] |
Tau knockout | Pancreatic β cell dysfunction and glucose intolerance | Wijesekara et al. [279] |
Role | Experimental paradigm | Adverse effects | References |
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Synaptic activity LTP | APP or BACE1 knockout | Cognitive deficits induced and impaired LTP | |
Treatment with anti-Aβ antibody 4G8 | LTP formation prevented | ||
Infusion of anti-Aβ antibody 4G8 or siRNA to APP | Short-term memory abolished in contextual fear conditioning or the Morris Water Maze | ||
BACE1 inhibitor treatment (wild-type mice) | Suppression of LTP, impaired cognitive performance | Filser et al. [74] | |
Regulation of neuronal hyperexcitability | APP or BACE1 knockout | Hypersensitivity to spontaneous and induced seizures | |
Neurogenesis and synaptogenesis | APP knockout | ↓ Neuronal branching and synapse formation | Southam et al. [239] |
APP knockout | Loss of synaptic proteins | ||
BACE1 knockout | Hearing impairment linked to aberrant synaptic organisation in the cochlea | Dierich et al. [60] | |
BACE1 inhibitor treatment (wild-type mice) | ↓ Spine density, ↓ spine formation | Filser et al. [74] | |
Myelination Regulation of myelination | BACE1 knockout | Delayed myelination, ↓ myelin thickness | |
BACE1 knockout | Impaired remyelination of peripheral nerves after injury | ||
Role in blood vessels Promotion of angiogenesis | BACE1 knockout | ↓ In retinal vascular density | Cai et al. [38] |
APP-deficiency or BACE1 inhibitor treatment | Shorter hindbrain vessels, fewer cerebrovascular branches | Luna et al. [163] | |
γ-secretase inhibitor treatment | ↑ Angiogenesis and vascularisation | Cameron et al. [40] | |
“Vascular plug” | Aβ-targeting drugs (active or passive Aβ immunisation) in human clinical trials | Microhaemorrhages and brain oedema (“Amyloid-Related Imaging Abnormalities” (ARIA)) | |
Aβ immunisation (animal models) | ARIA-like cerebral microbleeds | ||
APP or BACE1 knockout | ↑ Mortality after ischaemic injury, deficits in reactive blood flow | Koike et al. [133] | |
Response to injury Promotion of recovery | BACE1 knockout | Impaired remyelination after sciatic nerve lesion | |
BACE1 knockout | Worse functional outcome after spinal cord injury | Pajoohesh-Ganji et al. [195] | |
BACE1 knockout | Worse outcome after controlled cortical impact (rescued by Aβ application) | ||
BACE1 or APP knockout | ↑ Risk of mortality following cerebral ischaemia | Koike et al. [133] | |
Antimicrobial activity | APP knockout | ↑ Mortality after infection | Kumar et al. [138] |
Aβ-targeting therapies | ↑ Incidence of infections | Gosztyla et al. [87] | |
Iron homeostasis Regulation of iron homeostasis | APP knockout | ↑ Neuronal iron retention in vitro, ↑ vulnerability to oxidative damage from dietary iron in vivo | Duce et al. [63] |
APP knockout | Age-dependent iron accumulation in the brain and liver | Belaidi et al. [16] | |
Glucose metabolism | BACE1 knockout | ↓ Insulin expression in the pancreas | Hoffmeister et al. [104] |
BACE1 knockdown (siRNA) | ↓ Insulin mRNA and protein in insulinoma cells | Hoffmeister et al. [104] |
The origin of Aβ and tau
Aβ biogenesis
Tau production
Aβ and tau are expressed throughout the body
Aβ and tau are evolutionarily conserved
Regulation of microtubules: the primary physiological role of tau?
Tau binds to and regulates the structure of microtubules
Beyond microtubule stability: tau as a regulator of microtubule dynamics
Tau regulates axonal transport
Tau protects microtubules from cleavage
The future of therapeutics targeting microtubule (dys)function
Physiological roles at the synapse
Physiological concentrations of Aβ enhance LTP
Tau in LTP, LTD, and memory
Regulating neuronal hyperexcitability: could tau and Aβ act at opposite ends of the spectrum?
Neurogenesis, synaptogenesis, and structural plasticity
Regulating behaviour
Tau, hyperactivity, anxiety, and sleep
Tau and the motor system
Regulation of myelination
Tau is important for normal myelination
Aβ and its cleavage enzymes may regulate myelination
Aβ regulates vasculature
Angiogenesis and Aβ: a question of balance?
Aβ as a “vascular plug”
Response to injury
Tau and Aβ increase after injury: a protective or pathological response?
Aβ may promote recovery after brain injury
The effect of tau depends on the type of injury induced
Mitochondria and oxidative stress
Mitochondrial dysfunction is a key component of AD pathology
Aβ and tau deposition: cause of, or response to, oxidative damage?
Tau as a regulator of mitochondrial mobility and health?
Aβ may function as an antimicrobial peptide
The antimicrobial protection/infection hypotheses of AD
Aβ rises after infection and is associated with pathogens in the CNS
Aβ has antimicrobial activity
Regulation of iron homeostasis
Iron homeostasis may be disrupted in neurodegenerative disorders
Tau and APP play a role in physiological iron transport
The role of tau and Aβ in the nucleus
Tau is located at the nucleus and interacts with oligonucleotides
Tau protects DNA from damage
Tau maintains chromosomal stability
Tau as a regulator of transcription
Aβ at the nucleus - a caution for tau-lowering therapeutics?
Aβ and tau may function as tumour suppressors
Do individuals with AD have reduced incidence of cancer?
Aβ can act as a tumour suppressor
Loss of tau function may increase tumour incidence
Glucose metabolism
Dysregulation of insulin signalling in AD
Tau regulates normal glucose metabolism
Aβ-processing enzymes regulate insulin signalling
Conclusion
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The functions of tau and Aβ are influenced by their location. Shifts within the cell, expression in different cells or locations within the body, can alter the roles that these proteins play, whether they are beneficial or harmful, and how therapeutic treatments will influence function.
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Different isoforms, aggregation status, and post-translational modifications can dramatically alter the function of tau and Aβ.
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The concentrations of tau and Aβ are crucial for regulating physiological versus pathological function. This “hormetic nature”, where too much or too little protein causes functional deficits, raises the likelihood of a “therapeutic sweet spot” where the physiologically optimal concentrations lie, outside of which may result in damage (Fig. 4).
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AD is likely a combination of both gain- and loss-of-function phenotypes. Physiological responses can become hijacked to become toxic, detrimental functions can appear, or normal functions can be lost during disease-related changes.
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Contradictory findings are common in the literature. The potential for developmental compensation in constitutive knockouts, as well as experimental differences in factors such as age, sex, and environment, will all impact outcome. Future studies testing multiple timepoints, acute versus constitutive knockdown, different genetic backgrounds, mixed-sex cohorts, and increasing comparisons between animal and human tissue will greatly clarify genuine phenotypes from experimental artefacts.