Tau tubulin kinases in proteinopathy

Abstract

A number of neurodegenerative diseases are characterized by deposition of abnormally phosphorylated tau or TDP-43 in disease-affected neurons. These diseases include Alzheimer’s disease, frontotemporal lobar degeneration, and amyotrophic lateral sclerosis. No disease-modifying therapeutics is available to treat these disorders, and we have a limited understanding of the cellular and molecular factors integral to disease initiation or progression. Phosphorylated tau and TDP-43 are important markers of pathology in dementia disorders and directly contribute to tau- and TDP- 43-related neurotoxicity and neurodegeneration. Here, we review the scope of tau and TDP-43 phosphorylation in neurodegenerative disease and discuss recent work demonstrating the kinases TTBK1 and TTBK2 phosphorylate both tau and TDP-43, promoting neurodegeneration.

Introduction

Neurodegenerative diseases characterized by accumulation of protein aggregates are termed proteinopathies. In this review, we focus on two aggregating proteins, tau and TDP-43, and their role in neurodegenerative proteinopathies including AD, FTLD, and ALS. Work is ongoing to describe the molecular basis of these diseases; however, we discuss common themes relating pathological tau and TDP-43 in disease as well as new understandings of these proteins from research with model organisms. In particular, we focus on the role that phosphorylation of tau and TDP-43 has in promoting neurotoxicity, with emphasis on the kinases TTBK1 and TTBK2. TTBK 1 and TTBK2 may be targets for therapeutic intervention because they can phosphorylate both tau and TDP-43. This review describes what is known about the biology of these proteins, as well as where knowledge gaps exist, and how TTBK1/2 contribute to disease.

The role of tau in AD and FTLD-tau

Tau protein binds and stabilizes microtubules, regulating tubulin assembly [1,2]. Tau protein is particularly abundant in axons [3] and is important for cargo trafficking along the microtubule network within axons [4]. Phosphorylation of tau decreases its microtubulebinding affinity [5], resulting in destabilization of microtubules. A balance of tau-targeted kinase and phosphatase activity is crucial for microtubule homeostasis and axonal stability. During aging and disease, the balance of tau phosphorylation is disrupted, leading to accumulation of hyperphosphorylated tau that no longer promotes microtubule stabilization. Unbound tau can then associate with other tau species and form soluble oligomers, which may spread between cells resulting in the propagation of aggregation-prone tau conformations. The abnormal assembly of tau into aggregates leads to a cascade of tau fibrillization and neuronal toxicity [6]. Neurodegenerative diseases characterized by the presence of abnormal tau-containing aggregates are termed tauopathies. These disorders include Alzheimer’s disease (AD), frontotemporal lobar degeneration (FTLD-tau), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), Pick’s disease, and chronic traumatic encephalopathy.

AD is the most common tauopathy and the fourth leading cause of death in industrialized nations [7]. The symptoms of AD include memory loss and cognitive impairment presenting with decreased language ability and impaired abstract thinking and judgment. These symptoms are likely the consequence of significant synaptic reduction and neuronal loss, originating in the hippocampus and extending out to the cortices [8,9]. Depletion of synapses in AD can begin 20–40 years prior to diagnosis [10], indicating early stages of pathology may progress slowly. The neuropathological hallmarks of AD are the presence of both neuritic plaques and neurofibrillary tangles. Neuritic plaques form by self-aggregation of amyloid-beta (Aβ), the cytotoxic product of amyloid precursor protein cleavage by the protease BACE-1 [11]. The presence of Aβ peptide precedes and may initiate subsequent tau pathology [12]. Neurofibrillary tangles form from either polymerization of tau monomers into fibrils and hyperphosphorylated tau oligomers or seeding by low level aggregates [13,14]. In AD, soluble tau oligomers can contain both 3R and 4R tau isoforms [15], and are thought to be an important toxic species in AD [16]. Nonetheless, the burden and distribution of pathological filamentous tau is strongly correlated with the severity of cognitive impairment [17,18]. Studies in mouse models of AD show tau reduction in vivo can ameliorate Aβ toxicity, demonstrating tau’s involvement in mediating amyloid- related changes [19–21]. To date, clinical trials to treat AD have all failed to arrest the course of disease or improve cognition in affected individuals [22]. In particular, numerous clinical trials that specifically target Aβ production or accumulation have been halted because the drugs successfully reduced Ab burden but were ineffective at treating cognitive impairment [23–25]. While efforts to target Aβ in disease are ongoing, the combined results from previous therapeutic drug trials, our understanding of human disease, and the biology of AD learned using model organisms all support the idea that Aβ alone does not account for cognitive decline. Taken together, data demonstrate an important role for tau in AD pathogenesis.

Frontotemporal lobar degeneration, the second most prevalent form of presenile dementia after AD, affects 10–30 per 100 000 individuals between the ages of 45–65 years [26]. FTLD clinically presents with personality and behavioral changes and language dysfunction [27], driven by the deterioration of the frontal and temporal lobes of the brain [26]. Roughly 40% of FTLD cases exhibit tau tangles as their primary neuropathology (FTLD-tau, Fig. 1). In FTLD-tau, tau- positive inclusions are found in both neurons and glial cells [28], which differs from the primarily neuronal tau inclusions present in AD [26]. Mutations in MAPT, the gene encoding the tau protein, cause FTDP-17, an inherited form of the disease [29,30], and account for roughly 10% of cases [26]. PSP and CBD are two aging-associated dementia syndromes with Parkinsonian features that are characterized by the presence of abnormal 4R-only tau inclusions in neurons and glia. PSP has a prevalence of 1 in 100 000 [26] and is diagnosed on the basis of supranuclear gaze palsy, Parkinson’s-like movement defects, speech deficits, progressive dementia, and posture instability [26,31–34]. However, roughly three quarters of pathologically confirmed PSP cases do not present with typical clinical indicators of PSP [35], which emphasizes the difficulties in clinical diagnosis. CBD is diagnosed on the basis of focal cortical deficits, such as limb apraxia, aphasia, and sensory loss, and progressive asymmetrical movement disorder, with a prevalence of 3 in 100 000 [26]. Though both PSP and CBD accumulate 4R tau in neurons and glia, the morphology of inclusions is distinct in each disease [36]. CBD manifests filamentous tau-positive inclusions in the hippocampus, basal ganglia, brainstem, and cerebellum. PSP accumulates both filamentous and tangle-like tau deposits in the cortex, basal ganglia, and subthalamic nucleus, as well as tufted astrocytes containing tau inclusions [37]. As in AD, there is evidence for tau oligomers driving neurodegeneration in PSP [38,39]. Furthermore, the burden of tau pathology is positively associated with the degree of cognitive impairment in PSP cases [40]. Pick’s disease is a rare form of dementia originally characterized by Arnold Pick in the late 1800s. Pick’s disease is defined by the presence of eponymous neuronal inclusions (Pick bodies) that are morphologically distinct from neurofibrillary tangles present in the cortex and dentate gyrus [41] and are composed of 3R-only tau [42]. Pick bodies positively label for both ubiquitin and paired helical filamentous tau [43]. Another distinctive pathological feature of Pick’s disease is neuronal swelling [44]. Clinically, Pick’s disease has an earlier onset than AD and has a more variable duration (from 2 to 17 years) [45]. Despite differences in clinical and pathological characteristics of PSP, CBD, and Pick’s disease, modified tau inclusions are a prominent feature of these diseases. This suggests that even if the initiating causes of these diseases differ, the mechanisms leading to neurodegeneration converge on the generation and accumulation of pathological tau.

Fig. 1.

Frontotemporal lobar degeneration-tau and FTLD-TDP-43 exhibit increased TTBK1 and TTBK2. (A) tau neuropathology stained with AT8 (tau pS02/pS205) from frontal cortex of an FTLD case. (B) TDP-43 neuropathology stained with pS409/pS410 TDP-43 antibody from frontal cortex of an FTLD-TDP case. (C, D) TTBK1 kinase domain-specific staining in frontal cortex of FTLD-tau and FTLD-TDP cases respectively. (E, F) TTBK2 kinase domain-specific staining in frontal cortex of FTLD-tau and FTLD-TDP cases respectively. Scale bar = 100 μM.

The molecular features of pathological tau

Although studies are ongoing, current evidence points toward hyper phosphorylated oligomeric and fibrillar tau as important toxic species in tauopathies [16,38,39]. The longest isoform of 4R tau contains 85 potential phosphorylation sites [46], many of which are phosphorylated in disease [47]. Hyperphosphorylated tau can be toxic to cells through several mechanisms. The decreased affinity of phosphorylated tau for microtubules may disrupt cargo trafficking through decreased microtubule stability, leading to axonal impairment and disruption of synapses. Hyperphosphorylation of tau also changes the ability of tau to interact with other proteins that are involved in its native function [48–50]. Furthermore, phosphorylated tau is resistant to turnover, which increases the amount of intracellular tau [51,52]. Lastly, hyperphosphorylation can promote tau aggregation.

Many kinases have been identified as tau modifiers, including glycogen synthase kinase-3 (GSK3β) [53,54], cyclin-dependent kinase 5 (Cdk5) [55,56], members of the microtubule affinity-regulating kinase (MARK), cyclic AMP-dependent protein kinase (PKA), calcium/calmodulin-dependent protein kinase (CaMK) families that target serine/threonine phosphorylation sites, and tyrosine kinases such as the SRC and ABL families [46]. More recently, the tau tubulin kinases 1 and 2 (TTBK1/2) were identified as kinases able to phosphorylate tau at residues Thr181, Ser202, Ser208, Ser210, Thr231, Ser396, and Ser404 [57–60]. Increased TTBK1/2 activity exacerbates tau toxicity [60,61], suggesting a role for these kinases in disease initiation or progression.

The role of TDP-43 in ALS and FTLD-TDP

Trans-activating response DNA-binding protein of 43 kDa (TDP-43) is an RNA-binding protein with essential roles in RNA transcription, processing, and export from the nucleus, as well as mRNA transport and translation [62,63]. In normal physiological conditions, TDP-43 is located in the nucleus, but under conditions of stress or neuronal injury, it can be sequestered into cytoplasmic stress granules that potentially act as seeds for the pathological aggregation of TDP-43 observed in disease [64,65]. The TDP-43 protein contains nuclear localization and nuclear export signals, two RNA recognition motifs, and a C-terminal glycine-rich domain. Mutations in TARDBP, which encodes TDP- 43, can cause rare inherited forms of amyotrophic lateral sclerosis (fALS) [66–69]. These fALS mutations cluster in the C terminus of the protein. TDP-43 is the primary protein aggregate found in most cases of ALS, including both sporadic and familial-inherited forms, and is also the major protein aggregate in roughly half of all FTLD cases (FTLD-TDP, Fig. 1) [70]. In addition to ALS and FTLD-TDP, TDP-43 inclusions are a secondary pathology seen in a subset of tauopathy cases including some cases of AD, PSP, CBD, argyrophilic grain disease, and chronic traumatic encephalopathy, as well as in other dementia disorders such as dementia with Lewy bodies [71–76].

Amyotrophic lateral sclerosis, the most common adult onset motor neuron disorder, affects 1–2 per 100 000 individuals worldwide [77]. Symptoms include the progressive loss of motor function, leading to muscle atrophy and death. ALS-causing mutations have been identified most frequently in C9ORF72, SOD1, TARDBP, and FUS, although SOD1 and FUS mutations are not associated with TDP-43 pathology [78]. However, only a small subset of cases have a pattern of familial inheritance (10%); the majority of ALS cases (90%) are sporadic and primarily display TDP-43 neuropathology. Despite a large number of clinical trials in the past decade, only two drugs are available for the treatment of ALS. Daily administration of the FDA-approved drug Riluzole prolongs median survival by several months [79]. Another drug, Edaravone, an antioxidant developed to treat ischemic stroke, modestly slows the rate of ALS progression in a subset of patients also being treated with Riluzole. Edaravone was most effective among patients that had a definite or probable diagnosis of ALS and were symptomatic for less than 2 years [80]. There remains no significant disease-modifying treatment for ALS.

Frontotemporal lobar degeneration-TDP has an age of onset of roughly 60 years and a mean duration of 8 years [28]. About 25–30% of FTLD-TDP cases are familial, and are most often associated with mutations in C9ORF72 and GRN [78]. Clinically, FTLD-TDP may present with behavioral abnormalities, progressive nonfluent aphasia, or semantic dementia [28]. FTLD-TDP is characterized by accumulation of TDP-43-positive lesions in the frontal and temporal lobes leading to reactive gliosis, widespread neuronal loss, and brain atrophy.

Pure ALS and FTLD-TDP appear to exist on opposite ends of a disease spectrum. Both ALS and FTLD- TDP share clinical, neuropathological, and genetic features that regularly occur concurrently [63,81]. In roughly half of ALS cases, there is evidence of cognitive impairment and ultimately 15% of ALS patients also get diagnosed with FTLD [82]. Similarly, about 40% of FTLD cases display motor dysfunction and 15% result in a diagnosis of motor neuron disease [83]. Both ALS and FTLD accumulate aggregated, phosphorylated TDP-43 in disease-affected neurons. Finally, ALS and FTLD-TDP are linked genetically. Expansions of an intronic hexanucleotide repeat (GGGGCC) in C9ORF72 are the most common genetic cause of ALS and FTLD-TDP [84,85]. Within C9ORF72 families, affected members can present with ALS, FTLD, or a mixed disease course with both motor and cognitive dysfunction. C9ORF72 mutation carriers accumulate TDP-43-positive inclusions as well as repeat-expansion specific features including RNA foci and dipeptide repeat inclusions. The mechanisms of disease initiation and progression in both sporadic and familial ALS and FTLD-TDP remain poorly understood. However, the common underlying pathology is the aggregation of TDP-43, and elucidating the causes and consequences of dysfunctional TDP-43 remains an important goal.

The molecular features of pathological TDP-43

Much work has been done to elucidate the mechanisms of TDP-43 toxicity. TDP-43 is an essential protein [86–88], and loss of TDP-43 function or toxic gain-of-function are both possible avenues to neurodegeneration [89]. Abundance of TDP-43 protein correlates with its neurotoxicity [90], and higher amounts of TDP-43 pathology are associated with more rapid cognitive decline [91]. TDP-43 protein is aberrantly post-translationally modified in disease states. These modifications include ubiquitination, acetylation, SUMOylation, and phosphorylation [92–95]. Of these modifications, abnormal phosphorylation of TDP-43 most consistently marks lesions in TDP-43 proteinopathies, and is used diagnostically to identify TDP-43-positive protein inclusions in brain and spinal cord [70]. TDP-43 has 64 potential sites of phosphorylation (including serine, threonine, and tyrosine residues). Only five sites have evidence for phosphorylation in disease: serines 409 and 410 (S409/410) are consistently phosphorylated in cases of TDP-43 proteinopathy, while serines 379, 403, and 404 are phosphorylated in a subset of cases [70,94–96]. Notably, all five phosphorylation sites reside in the TDP-43 C terminus. Phosphorylation at S409/410 potentiates a number of toxic disruptions in normal TDP-43 metabolism, including decreased TDP-43 protein turnover, increased TDP-43 stabilization, cellular mislocalization of TDP-43, protein aggregation, and neurodegeneration [96–101]. Four kinases have been shown to phosphorylate TDP-43: CK1, CDC7, TTBK1, and TTBK2 [96,97,102]. We do not yet know the relative contribution of each of these kinases to TDP-43 phosphorylation in disease. However, TTBK1 and TTBK2 are both upregulated in TDP-43 proteinopathy (Fig. 1) [60,102].

The tau tubulin kinase family

Tau tubulin kinase 1 and tau tubulin kinase 2 (TTBK1/2) belong to the casein kinase superfamily along with CK1. TTBK1/2 were originally characterized as kinases that phosphorylate microtubule-bound tau and are upregulated in AD cases [56,58]. In Drosophila melanogaster, the TTBK1/2 homolog Asator is an essential gene that localizes to the mitotic spindle during mitosis and directly interacts with the interchromosomal protein Magator [103]. TTBK1/2 play an important role in a variety of tauopathy disorders [57,60,104–107]. In addition to their tau-targeted activities, TTBK1/2 can directly phosphorylate TDP-43 in vitro and promote TDP-43 phosphorylation in vivo [102]. Furthermore, a reduction in TTBK1/2 levels protects against TDP-43 phosphorylation in simple models of TDP-43 proteinopathy. Finally, TTBK1/2 colocalize with phosphorylated TDP-43 in ALS and FTLD cases [102]. Taken together, these data suggest that TTBK1/2 have the potential to initiate or contribute to both tau and TDP-43 proteinopathy disorders (Figs 1–3).

Fig. 3.

Tau tubulin kinase 2 phosphorylates tau but not TDP-43 in disease-state neurons. In healthy neurons (top), TTBK2 regulates Na+ -coupled transporters, ciliogenesis, and neuronal maturation. Dysregulated TTBK2 kinase activities (bottom) can promote pathological phosphorylation of tau and subsequent disease.

TTBK1/2 have a highly conserved homologous kinase domain (88% identity, 96% similarity between human TTBK1 and TTBK2), which is present in C. elegans, D. melanogaster, and mammalian species. The TTBK1/2 kinase domain contains dual-kinase properties enabling it to phosphorylate serine, threonine, and tyrosine residues. However, TTBK1 and TTBK2 possess mostly distinct regulatory domain sequences [107]. However, TTBK1 and TTBK2 do share one region of homology in the C terminus of the regulatory region, spanning amino acids 1053–1117 in TTBK1 and 942–1006 in TTBK2 with 43% identity and 58% similarity [107]. The N-terminal kinase domain structure for TTBK1 has been solved [108,109], and predictive modeling of TTBK2 suggests its kinase domain is functionally identical to that of TTBK1 [110]. A basic local alignment search (BLAST) targeted against the TTBK2 regulatory domains produced no significant similarities to other known proteins, pointing toward a unique regulatory mechanism for TTBK2 that is distinct from TTBK1 regulation [110]. The complete protein structures for TTBK1/2 have yet to be solved. Both TTBK1/2 undergo proteolytic processing into numerous small fragments ranging in size from 25 to 100 kDa [106] suggesting a complex native protein regulation mechanism may exist. However, the nature of TTBK1/2 regulation remains largely unexplored.

Tau tubulin kinase 1

The TTBK1 gene on chromosome 6p21.1 encodes a 1321 amino acid protein localized to the cytoplasm of the cell [57]. TTBK1 mRNA is expressed as multiple alternatively spliced isoforms predominantly in the neurons of the central nervous system and at low levels in testes [107]. TTBK1 protein includes an N-terminal kinase domain, accounting for the first 297 amino acids of the protein sequence, and a regulatory region with a unique 39-amino acid polyglutamate stretch (Fig. 4A) [107]. No other protein has been identified with a polyglutamate region this extensive, and the role it plays in TTBK1 regulation and activity remains unknown. Likewise, the function of TTBK1 is largely unknown, although recent evidence suggests a role in synaptic vesicle formation through phosphorylation of synaptic vesicle protein 2A (SV2A) (Fig. 2) [111]. The biological function, binding partners, and overall regulation of TTBK1 activity remain mostly unknown.

Fig. 4.

Protein domains of TTBK1 and TTBK2. (A) The longest isoform of TTBK1 protein has an N-terminal kinase domain and two regulatory domains (Domain A and Domain B) separated by 39 amino acid polyglutamine stretch. (B) The longest isoform of TTBK2 has an N-terminal kinase domain and a C terminus with no homology to known protein domains. The sites of familial-inherited SCA11-causing frameshift mutations are labeled.

Fig. 2.

Tau tubulin kinase 1 phosphorylates tau and TDP-43 in disease-state neurons. In healthy neurons (top), TTBK1 promotes the formation of synapses and neuronal development. TTBK1 likely has additional unknown roles in the central nervous system. Dysregulated TTBK1 kinase activities (bottom) can promote pathological phosphorylation of tau or TDP-43 and subsequent disease.

TTBK1 is upregulated in Alzheimer’s disease (AD) brain [106] and colocalizes to phospho-tau in pretangles [112]. Two independent GWAS studies in Han Chinese and Spanish populations identified single nucleotide polymorphisms of TTBK1 associated with reduced risk of AD [113,114]. Another study identified a de novo variant of TTBK1 as a potential cause of childhood onset schizophrenia [115]. These findings suggest that TTBK1 plays an important role in the brain both during development and postdevelopmental aging. Transgenic mouse lines expressing full-length human TTBK1 display age-dependent cognitive and molecular changes similar to those seen in AD mouse models. These transgenic mice have learning impairments, neurofilament aggregation, microgliosis, altered CDK5/p35 activity, and decreased expression of NMDA receptors [106]. A double-transgenic mouse model expressing both mutant FTLD-causing tau and human TTBK1 had increased accumulation of oligomeric tau, enhanced motor deficits, and loss of spinal cord motor neurons [104]. Interestingly, these changes are associated with increased neuroinflammation through TTBK1-dependent activation of mononuclear phagocytes, indicating TTBK1 may promote tau toxicity through both direct phosphorylation and pro-inflammatory pathways [116]. In concert, these data suggest a key role of TTBK1 in accelerating tau-related neurodegeneration.

Tau tubulin kinase 2

The TTBK2 gene located on chromosome 15q15.2 encodes multiple alternatively spliced mRNAs that translate into a protein up to 1244 amino acids in length. TTBK2 exhibits widespread tissue distribution including expression in liver, skeletal muscle, pancreas, heart, and brain, with the highest expression levels occurring in the testes [58,107]. TTBK2 was first isolated from bovine and mouse brain samples and characterized as a kinase phosphorylating tau residues Ser208 and Ser210 [58,59]. Recent publications have demonstrated roles for TTBK2 in the initiation of ciliogenesis through the regulation of microtubule dynamics, as well as in controlling cilia length, ciliary trafficking, and stability (Fig. 3) [117,118]. TTBK2 is known to interact with EB1, EB3, and Cep164 during cilia formation [119,120]. Specifically, TTBK2 phosphorylates Cep164 when it is recruited to the basal body of a nascent cilium and regulates the removal of CP110, a suppressor of ciliogenesis [119,121]. Additionally, TTBK2 has been implicated in sodium-coupled transporter regulation [122,123]. However, characterization of TTBK2 function in neurons remains incomplete.

Mutations in TTBK2 cause an autosomal dominant form of spinocerebellar ataxia, type 11 (SCA11), a very rare progressive degenerative disease characterized by changes in gait, speech, and eye movements. The SCA11 causative TTBK2 mutations result in premature termination of the TTBK2 mRNA and depleted TTBK2 levels (Fig. 4B) [105,124,125]. TTBK2 depletion in SCA11 significantly reduces kinase activity levels and stimulates localization of TTBK2 to the nucleus, acting as a dominant negative allele that interferes with normal function of full-length TTBK2 [118,126]. Homozygous transgenic mice carrying the SCA11 mutation in TTBK2 exhibit neurodevelopmental abnormalities and are embryonic lethal [126]. These data suggest that TTBK2 is essential for proper neuronal development and may have a role in neuronal maturation. The symptoms of SCA11 are distinct from more typical symptoms of primary ciliopathies, which range from retinal degeneration and renal complications to cerebral malformation [127]. SCA11 patients accumulate phosphorylated tau inclusions in disease-affected neurons, similar to what is seen in AD [105]. This may be due to upregulation of TTBK1 that compensates for the loss of TTBK2 activity. Altered TTBK2 expression has also been associated with cancer [128], highlighting other possible functions for TTBK2 beyond cilia formation.

The therapeutic potential of the TTBKs

As described above, many age-related neurodegenerative diseases including AD, FTLD, and ALS are characterized by the presence of either phospho-tau or phospho-TDP-43. While these phosphorylated protein aggregates are diagnostic hallmarks of disease, their contributions as toxic species in disease remain a topic of debate in the field. Both tau and TDP-43 are capable of propagating disease-associated protein conformations between neurons in a prion-like manner [129]; however, the contribution of phosphorylation to this process is unknown. Similarly, whether TTBK1/2 phosphorylation of tau and TDP-43 occurs as a primary or secondary event in the initiation of proteinopathy is unknown. However, a body of literature strongly supports targeting tau or TDP-43 phosphorylation as a neuroprotective strategy [16,101,130,131].

Despite numerous clinical trials aimed at treating AD and ALS, therapies remain elusive. Targeting neurotoxicity directly through an upstream molecular pathway, which drives neurodegeneration, is a favored strategy. However, target selection is critical. Ideally, the drug target should have limited off-pathway roles. An ideal drug target would therefore be one that is expressed only in disease-relevant tissues and has no other essential cellular functions. Because TTBK1/2 phosphorylate both tau and TDP-43, they make attractive candidates for inhibition since targeting them could potentially ameliorate both tauopathies and TDP-43 proteinopathies. TTBK1 in particular is a good candidate for targeted therapeutics since its expression is largely limited to the central nervous system. TTBK2 has less favorable drug target characteristics since it is expressed in more tissue types, is essential for viability in mice, and haploinsufficiency is directly linked to disease. Additionally, TTBK1 phosphorylates both tau and TDP-43 in model systems, unlike TTBK2, which has a strong specificity for only tau [60]. This supports the idea that TTBK1 could be a viable therapeutic for both tau and TDP-43 proteinopathies, which would be particularly impactful for pathologically uncharacterized FTLD patients. While it remains an open question whether TTBK1 depletion will have neurological consequences, developing selective inhibitors of TTBK1 may be a viable approach for TTBK-focused therapeutics. However, utilizing either TTBK1 or TTBK2 as a therapeutic target will first require a better understanding of TTBK1/2 activities, regulation, and functions within the nervous system.

Abbreviations

Aβ

Amyloid-beta

AD

Alzheimer’s disease

ALS

Amyotrophic lateral sclerosis

CBD

Corticobasal degeneration

fALS

Familial amyotrophic lateral sclerosis

FTLD

frontotemporal lobar degeneration

PSP

progressive supranuclear palsy

SCA11

Spinocerebellar ataxia 11

TDP-43

TAR DNA-binding protein 43 kilodaltons

TTBK1

Tau tubulin kinase 1

TTBK2

Tau tubulin kinase 2