Better Utilization of Mouse Models of Neurodegenerative Diseases in Preclinical Studies: From the Bench to the Clinic.

The major symptom of Alzheimer's disease is dementia progressing with age. Its clinical diagnosis is preceded by a long prodromal period of brain pathology that encompasses both formation of extracellular amyloid and intraneuronal tau deposits in the brain and widespread neuronal death. At present, familial cases of dementia provide the most promising foundation for modeling neurodegenerative tauopathies, a group of heterogeneous disorders characterized by prominent intracellular accumulation of hyperphosphorylated tau protein. In this chapter, we describe major behavioral hallmarks of tauopathies, briefly outline the genetics underlying familial cases, and discuss the arising implications for modeling the disease in transgenic mouse systems. The selection of tests performed to evaluate the phenotype of a model should be guided by the key behavioral hallmarks that characterize human disorder and their homology to mouse cognitive systems. We attempt to provide general guidelines and establish criteria for modeling dementia in a mouse; however, interpretations of obtained results should avoid a reductionist "one gene, one disease" explanation of model characteristics. Rather, the focus should be directed to the question of how the mouse genome can cope with the over-expression of the protein coded by transgene(s). While each model is valuable within its own constraints and the experiments performed are guided by specific hypotheses, we seek to expand upon their methodology by offering guidance spanning from issues of mouse husbandry to choices of behavioral tests and routes of drug administration that might increase the external validity of studies and consequently optimize the translational aspect of preclinical research.

Kinase inhibitors arrest neurodegeneration in cell and C. elegans models of LRRK2 toxicity.

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most frequent known cause of late-onset Parkinson's disease (PD). To explore the therapeutic potential of small molecules targeting the LRRK2 kinase domain, we characterized two LRRK2 kinase inhibitors, TTT-3002 and LRRK2-IN1, for their effects against LRRK2 activity in vitro and in Caenorhabditis elegans models of LRRK2-linked neurodegeneration. TTT-3002 and LRRK2-IN1 potently inhibited in vitro kinase activity of LRRK2 wild-type and mutant proteins, attenuated phosphorylation of cellular LRRK2 and rescued neurotoxicity of mutant LRRK2 in transfected cells. To establish whether LRRK2 kinase inhibitors can mitigate pathogenesis caused by different mutations including G2019S and R1441C located within and outside of the LRRK2 kinase domain, respectively, we evaluated effects of TTT-3002 and LRRK2-IN1 against R1441C- and G2019S-induced neurodegeneration in C. elegans models. TTT-3002 and LRRK2-IN1 rescued the behavioral deficit characteristic of dopaminergic impairment in transgenic C. elegans expressing human R1441C- and G2019S-LRRK2. The inhibitors displayed nanomolar to low micromolar rescue potency when administered either pre-symptomatically or post-symptomatically, indicating both prevention and reversal of the dopaminergic deficit. The same treatments also led to long-lasting prevention and rescue of neurodegeneration. In contrast, TTT-3002 and LRRK2-IN1 were ineffective against the neurodegenerative phenotype in transgenic worms carrying the inhibitor-resistant A2016T mutation of LRRK2, suggesting that they elicit neuroprotective effects in vivo by targeting LRRK2 specifically. Our findings indicate that the LRRK2 kinase activity is critical for neurodegeneration caused by R1441C and G2019S mutations, suggesting that kinase inhibition of LRRK2 may represent a promising therapeutic strategy for PD.

Sex difference in pathology and memory decline in rTg4510 mouse model of tauopathy.

Abnormal phosphorylation of tau protein is a common event in many neurodegenerative disorders, including Alzheimer's disease and other tauopathies. We investigated the relationship between hyperphosphorylated tau in brain extracts and mnemonic functions in rTg4510 mouse model of tauopathy. We report that rTg4510 mice showed rapid deterioration in spatial learning and memory, which paralleled a significant increase of hyperphosphorylated tau in the brain between 3 and 5.5 months of age. At 5.5 months, rTg4510 females showed significantly higher levels of hyperphosphorylated tau than males, with no evidence of differential tau transgene expression between the sexes. The increased levels of hyperphosphorylated tau in females were associated with more severe impairment in spatial learning and memory as compared to transgenic males. We also showed that within studied age range, the decrease in memory performance was accompanied by other behavioral disturbances in the water maze related to search strategy, like thigmotaxic swim and cue response. These findings suggest that the onset of abnormal tau biochemistry and coincident cognitive deficits in the rTg4510 mouse model is sex-dependent with females being affected earlier and more aggressively than males.

Inhibition of PHF-like tau hyperphosphorylation in SH-SY5Y cells and rat brain slices by K252a.

Abnormal hyperphosphorylation of tau is believed to constitute a critical biochemical event in the process of neurofibrillary degeneration of Alzheimer's disease. We have developed a cellular model where apparently authentic PHF-like tau hyperphosphorylation is induced by okadaic acid. To gain deeper insight into the complex mechanisms of this pathological process we tested a variety of kinase inhibitors in this model. We found that K252a is differentiated from staurosporine by its inhibition of ERK2: both compounds are structurally related microbial metabolites generally believed to have only moderate kinase selectivity. However, since ERK2 inhibitors are exceedingly rare, we used this differential inhibitory property of K252a to demonstrate the involvement of ERK2 in PHF-type tau hyperphosphorylation. K252a was uniquely able to completely suppress the okadaic acid-induced tau hyperphosphorylation in SH-SY5Y cells and rat brain slices by way of including ERK2 in its inhibitory spectrum, and to conserve the normal binding of tau to tubulin. GSK3 inhibitors partially affected the normal state of tau phosphorylation in SH-SY5Y cells, but had no impact on okadaic acid-induced tau hyperhosphorylation. As K252a is the first molecule identified capable of preventing the spectrum of PHF-like tau hyperphosphorylation markers, it may represent a conceptual starting point for therapeutic development of suitable spectrum kinase inhibitors.

Accumulation of pathological tau species and memory loss in a conditional model of tauopathy.

Neurofibrillary tangles (NFTs) are a pathological hallmark of Alzheimer's disease and other tauopathies, but recent studies in a conditional mouse model of tauopathy (rTg4510) have suggested that NFT formation can be dissociated from memory loss and neurodegeneration. This suggests that NFTs are not the major neurotoxic tau species, at least during the early stages of pathogenesis. To identify other neurotoxic tau protein species, we performed biochemical analyses on brain tissues from the rTg4510 mouse model and then correlated the levels of these tau proteins with memory loss. We describe the identification and characterization of two forms of tau multimers (140 and 170 kDa), whose molecular weight suggests an oligomeric aggregate, that accumulate early in the pathogenic cascade in this mouse model. Similar tau multimers were detected in a second mouse model of tauopathy (JNPL3) and in tissue from patients with Alzheimer's disease and FTDP-17 (frontotemporal dementia and parkinsonism linked to chromosome 17). Moreover, levels of the tau multimers correlated consistently with memory loss at various ages in the rTg4510 mouse model. Our findings suggest that accumulation of early-stage aggregated tau species, before the formation of NFT, is associated with the development of functional deficits during the pathogenic progression of tauopathy.

Microtubule-associated protein tau as a therapeutic target in neurodegenerative disease.

Interest in the biology of the microtubule-associated protein tau, not only as a pathologic marker, but as a therapeutic target has surged considerably over the last few years. This is due, in part, to the discovery of mutations in tau causing a group of aggressively degenerative neurologic disorders characterized by abnormalities of tau very similar to what is seen in Alzheimer's disease where mutations in tau are absent. As these same mutations also precipitate authentic forms of neurofibrillary degeneration in tau transgenic mice, the gateways to testing therapeutic ideas preclinically have opened. Other Alzheimer's disease animal models have been notoriously bare of this feature, limiting their predictive power for clinical success. In this review, the authors discuss some of the main therapeutic ideas presently advanced in the field and their molecular rationales.

An inhibitor of tau hyperphosphorylation prevents severe motor impairments in tau transgenic mice.

An orally bioavailable and blood-brain barrier penetrating analog of the kinase inhibitor K252a was able to prevent the typical motor deficits in the tau (P301L) transgenic mouse model (JNPL3) and markedly reduce soluble aggregated hyperphosphorylated tau. However, neurofibrillary tangle counts were not reduced in the successfully treated cohort, suggesting that the main cytotoxic effects of tau are not exerted by neurofibrillary tangles but by lower molecular mass aggregates of tau. Our findings strongly suggest that abnormal tau hyperphosphorylation plays a critical role in the development of tauopathy and suggest a previously undescribed treatment strategy for neurodegenerative diseases involving tau pathology.

Prospect of therapeutic approaches to tauopathies.

Alois Alzheimer described the concurrence of two conspicuous proteinacious aggregates in 1906. Today it is clear that the two types of protein aggregates are fundamentally different. One consists of a short fragment (Abeta peptide) of a membrane protein APP and is found mainly outside of cells and the other is formed from a biochemically modified cytoskeleton-associated protein known as tau. The latter is found exclusively inside cells. Aggregated tau in all tauopathies including AD is abnormally modified by the excess incorporation of chemical groups called phosphates (hyperphosphorylation), and the appearance of this biochemical change coincides with the onset all tauopathies, suggesting that it is both a necessary and sufficient cause for such diseases. Data indicate that the basis for tau hyperphosphorylation is the dysregulation of key enzymes known as kinases. These enzymes are therapeutic targets for AD and other neurodegenerative diseases, which feature pathological tau structures in brain