Caspase-6 inhibitors(Elgia)

Tau post-translational modifications (PTMs) are associated with progressive tau accumulation and neuronal loss in tauopathies, including forms of frontotemporal lobar degeneration (FTLD) and Alzheimer's disease (AD). Proteolytic cleavage of tau by active caspases, including caspase-6, represents an underexplored tau PTM implicated in tau pathology by generating tau species with an enhanced propensity for self-aggregation and accumulation into neuronal cytoplasmic inclusions.

To elucidate the presence and temporal course of caspase activation, tau cleavage, and neuronal death, we generated two neoepitope monoclonal antibodies (mAbs) against caspase-6 tau proteolytic sites (D13 and D402) and cortical neurons (iNs) from induced pluripotent stem cells (iPSCs) with the frontotemporal dementia (FTD)-causing V337M MAPT mutation (tau^V337M ).

FTLD tau^V337M  and AD postmortem brains showed positivity for both cleaved tau mAbs as well as active caspase-6 (Fig. 1). Three month tau^V337M  iNs showed a three-fold increase in caspase-cleaved D402 and D13 tau protein levels and increased p-tau and oligomeric tau species compared to controls (Fig. 2). Three month tau^V337M  iNs also showed a significantly higher rate of neuronal death than the tau^WT  counterpart. Treatment with staurosporine (STS), a stress inducer, led to a 5-fold increase of cytotoxicity levels in STS-treated tau^V337M  iNs compared to vehicle-treated tau^V337M  iNs. Although STS-induced cytotoxicity was anticipated, the increase was significantly higher in STS-treated tau^V337M  iNs than in STS-treated control iNs. Similarly, we observed a 4-fold increase of caspase-6 activity in STS-treated tau^V337M  iNs compared to vehicle-treated tau^V337M  iNs that was reversed following co-treatment with the caspase-6 inhibitor z-VEID-fmk (Fig. 2).

We propose a model in which time-dependent accumulation of caspase-cleaved tau in V337M MAPT neurons promotes neurotoxicity that is reversed by caspase-6 inhibition. Caspase-cleaved tau may be a biomarker of tauopathy, and caspases could be viable targets for therapeutic intervention against tau pathogenesis in FTLD and other tauopathies.

Current antidepressant therapies meet with variable therapeutic success and there is increasing interest in therapeutic approaches not based on monoamine signaling. Histone deacetylase 6 (HDAC6), which also deacetylates α-tubulin shows altered expression in mood disorders and HDAC6 knockout mice mimic traditional antidepressant treatments. Nonetheless, a mechanistic understanding for HDAC6 inhibitors in the treatment of depression remains elusive. Previously, we have shown that sustained treatment of rats or glioma cells with several antidepressants translocates Gα_s from lipid rafts toward increased association with adenylyl cyclase (AC). Concomitant with this is a sustained increase in cAMP production. While Gα_s modifies microtubule dynamics, tubulin also acts as an anchor for Gα_s in lipid-rafts. Since HDAC-6 inhibitors potentiate α-tubulin acetylation, we hypothesize that acetylation of α-tubulin disrupts tubulin-Gα_s raft-anchoring, rendering Gα_s free to activate AC. To test this, C6 Glioma (C6) cells were treated with the HDAC-6 inhibitor, tubastatin-A. Chronic treatment with tubastatin-A not only increased α-tubulin acetylation but also translocated Gα_s from lipid-rafts, without changing total Gα_s. Reciprocally, depletion of α-tubulin acetyl-transferase-1 ablated this phenomenon. While escitalopram and imipramine also disrupt Gα_s/tubulin complexes and translocate Gα_s from rafts, they evoke no change in tubulin acetylation. Finally, two indicators of downstream cAMP signaling, cAMP response element binding protein phosphorylation (pCREB) and expression of brain-derived-neurotrophic-factor (BDNF) were both elevated by tubastatin-A. These findings suggest HDAC6 inhibitors show a cellular profile resembling traditional antidepressants, but have a distinct mode of action. They also reinforce the validity of antidepressant-induced Gα_s translocation from lipid-rafts as a biosignature for antidepressant response that may be useful in the development of new antidepressant compounds.