The tau tubulin kinases TTBK1/2 promote accumulation of pathological TDP-43.

Pathological aggregates of phosphorylated TDP-43 characterize amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD-TDP), two devastating groups of neurodegenerative disease. Kinase hyperactivity may be a consistent feature of ALS and FTLD-TDP, as phosphorylated TDP-43 is not observed in the absence of neurodegeneration. By examining changes in TDP-43 phosphorylation state, we have identified kinases controlling TDP-43 phosphorylation in a C. elegans model of ALS. In this kinome-wide survey, we identified homologs of the tau tubulin kinases 1 and 2 (TTBK1 and TTBK2), which were also identified in a prior screen for kinase modifiers of TDP-43 behavioral phenotypes. Using refined methodology, we demonstrate TTBK1 and TTBK2 directly phosphorylate TDP-43 in vitro and promote TDP-43 phosphorylation in mammalian cultured cells. TTBK1/2 overexpression drives phosphorylation and relocalization of TDP-43 from the nucleus to cytoplasmic inclusions reminiscent of neuropathologic changes in disease states. Furthermore, protein levels of TTBK1 and TTBK2 are increased in frontal cortex of FTLD-TDP patients, and TTBK1 and TTBK2 co-localize with TDP-43 inclusions in ALS spinal cord. These kinases may represent attractive targets for therapeutic intervention for TDP-43 proteinopathies such as ALS and FTLD-TDP.

MSUT2 is a determinant of susceptibility to tau neurotoxicity.

Lesions containing abnormal aggregated tau protein are one of the diagnostic hallmarks of Alzheimer's disease (AD) and related tauopathy disorders. How aggregated tau leads to dementia remains enigmatic, although neuronal dysfunction and loss clearly contribute. We previously identified sut-2 as a gene required for tau neurotoxicity in a transgenic Caenorhabditis elegans model of tauopathy. Here, we further explore the role of sut-2 and show that overexpression of SUT-2 protein enhances tau-induced neuronal dysfunction, neurotoxicity and accumulation of insoluble tau. We also explore the relationship between sut-2 and its human homolog, mammalian SUT-2 (MSUT2) and find both proteins to be predominantly nuclear and localized to SC35-positive nuclear speckles. Using a cell culture model for the accumulation of pathological tau, we find that high tau levels lead to increased expression of MSUT2 protein. We analyzed MSUT2 protein in age-matched post-mortem brain samples from AD patients and observe a marked decrease in overall MSUT2 levels in the temporal lobe of AD patients. Analysis of post-mortem tissue from AD cases shows a clear reduction in neuronal MSUT2 levels in brain regions affected by tau pathology, but little change in regions lacking tau pathology. RNAi knockdown of MSUT2 in cultured human cells overexpressing tau causes a marked decrease in tau aggregation. Both cell culture and post-mortem tissue studies suggest that MSUT2 levels may influence neuronal vulnerability to tau toxicity and aggregation. Thus, neuroprotective strategies targeting MSUT2 may be of therapeutic interest for tauopathy disorders.

Tau isoform regulation is region- and cell-specific in mouse brain.

Tau is a microtubule-associated protein implicated in neurodegenerative tauopathies. Alternative splicing of the tau gene (MAPT) generates six tau isoforms, distinguishable by the exclusion or inclusion of a repeat region of exon 10, which are referred to as 3-repeat (3R) and 4-repeat (4R) tau, respectively. We developed transgenic mouse models that express the entire human MAPT gene in the presence and absence of the mouse Mapt gene and compared the expression and regulation of mouse and human tau isoforms during development and in the young adult. We found differences between mouse and human tau in the regulation of exon 10 inclusion. Despite these differences, the isoform splicing pattern seen in normal human brain is replicated in our mouse models. In addition, we found that all tau, both in the neonate and young adult, is phosphorylated. We also examined the normal anatomic distribution of mouse and human tau isoforms in mouse brain. We observed developmental and species-specific variations in the expression of 3R- and 4R-tau within the frontal cortex and hippocampus. In addition, there were differences in the cellular distribution of the isoforms. Mice transgenic for the human MAPT gene exhibited higher levels of neuronal cell body expression of tau compared to wildtype mice. This neuronal cell body expression of tau was limited to the 3R isoform, whereas expression of 4R-tau was more "synaptic like," with granular staining of neuropil rather than in neuronal cell bodies. These developmental and species-specific differences in the regulation and distribution of tau isoforms may be important to the understanding of normal and pathologic tau isoform expression.

Chronic cortisol exposure promotes the development of a GABAergic phenotype in the primate hippocampus.

Glucocorticoids regulate plasticity and survival of hippocampal neurons. Aberrant exposure to this steroid hormone can result in neurodegeneration, perhaps secondary to disruption of calcium homeostasis. Calbindin, a calcium-binding protein that buffers excess calcium, may protect against neurodegeneration resulting from overabundance of intracellular calcium. In this study, we examined whether chronic treatment (1 year) with cortisol enhances hippocampal calbindin expression in primates. Calbindin is a marker for inhibitory neurons and the dentate gyrus is known to adopt an inhibitory phenotype in response to extreme conditions such as seizures. Thus, we hypothesized that chronic cortisol exposure may also promote a GABAergic phenotype. Therefore, we examined the expression of the GABA-synthesizing enzyme glutamic acid decarboxylase. The expression of brain-derived neurotrophic factor, which is responsive to glucocorticoids, was also examined. Our results demonstrate significant increases in calbindin, glutamic acid decarboxylase and brain-derived neurotrophic factor in several regions of the primate hippocampus, including the dentate gyrus and CA3, in response to chronic cortisol exposure. These results suggest that chronic cortisol exposure may shift the balance towards a GABAergic phenotype, perhaps as part of a compensatory feedback mechanism to dampen the initial excitatory effects of glucocorticoids in the hippocampus.