Mutant Huntingtin Impairs Post-Golgi Trafficking to Lysosomes by Delocalizing Optineurin/Rab8 Complex from the Golgi Apparatus

Abstract

Huntingtin regulates post-Golgi trafficking of secreted proteins. Here, we studied the mechanism by which mutant huntingtin impairs this process. Colocalization studies and Western blot analysis of isolated Golgi membranes showed a reduction of huntingtin in the Golgi apparatus of cells expressing mutant huntingtin. These findings correlated with a decrease in the levels of optineurin and Rab8 in the Golgi apparatus that can be reverted by overexpression of full-length wild-type huntingtin. In addition, immunoprecipitation studies showed reduced interaction between mutant huntingtin and optineurin/Rab8. Cells expressing mutant huntingtin produced both an accumulation of clathrin adaptor complex 1 at the Golgi and an increase of clathrin-coated vesicles in the vicinity of Golgi cisternae as revealed by electron microscopy. Furthermore, inverse fluorescence recovery after photobleaching analysis for lysosomal-associated membrane protein-1 and mannose-6-phosphate receptor showed that the optineurin/Rab8-dependent post-Golgi trafficking to lysosomes was impaired in cells expressing mutant huntingtin or reducing huntingtin levels by small interfering RNA. Accordingly, these cells showed a lower content of cathepsin D in lysosomes, which led to an overall reduction of lysosomal activity. Together, our results indicate that mutant huntingtin perturbs post-Golgi trafficking to lysosomal compartments by delocalizing the optineurin/Rab8 complex, which, in turn, affects the lysosomal function.

Toward an integrative view of Optineurin functions

“…80 Furthermore, mutated Htt that is still able to interact with Optn, delocalized Optn and Rab8 from the Golgi apparatus to the cytosol and impaired post-Golgi trafficking to the plasma membrane and to the lysosomes. 24,80 The ubiquitin-binding function of Optn is also important for antiviral immune response, selective autophagy of cytosolic bacteria and Transferrin receptor (TfR) recycling. 43,55,78 However, the role of Ub-binding activity in some Optn-mediated functions, such as the transport of lysosomal proteins, remains to be determined.…”

“…82 During the G 2 /M phase, disruption of the Optn/ Rab8 complex, considered as an essential component of postGolgi trafficking, may result in a significant reduction of protein transport. 24,26 In addition, the Golgi complex is disassembled in early prophase and is readily reassembled in telophase, suggesting a yet unexplored link between maintenance of the Golgi integrity and regulation of mitosis. 83 Optn may have a general function in macroautophagy by regulating the transport of lysosomal proteins that could impact on the selective autophagy of bacteria.…”

“…[20][21][22][23] Htt plays a crucial role in the regulation of post-Golgi trafficking to the plasma membrane and to the lysosomes, and its depletion affects the spatial organization of late endosomes/lysosomes. 24,25 Interestingly, this function of Htt seems to be dependent on Optn and Rab8, since an altered form of Htt presenting abnormal polyglutamine expansion (mHtt) delocalized Optn and Rab8 from the Golgi apparatus and, in turn, prevents post-Golgi trafficking to lysosomes. 24 However, direct involvement of Optn in this process remains to be demonstrated.…”

“…24,25 Interestingly, this function of Htt seems to be dependent on Optn and Rab8, since an altered form of Htt presenting abnormal polyglutamine expansion (mHtt) delocalized Optn and Rab8 from the Golgi apparatus and, in turn, prevents post-Golgi trafficking to lysosomes. 24 However, direct involvement of Optn in this process remains to be demonstrated. In addition, Optn has been suggested to link Htt to Rab8 and, therefore, to the Golgi, suggesting a possible role for Optn in connecting the Golgi apparatus to the microtubule network to which Htt is associated (Fig.…”

Convergent pathogenic pathways in Alzheimer's and Huntington's diseases: shared targets for drug development

“…The genetic deletion of an endogenous inhibitor of cathepsins, cystatin B, ameliorates the memory deficits and Aβ aggregate load in a mouse model of AD 153 , which indicates that lysosomal proteases are able to clear diseasecausing proteins in AD and that their upregulation should be beneficial (Suppl Table S4). In HD, it was proposed that the expression of mHtt impairs vesicular transport from the Golgi to the lysosomes and thus leads to a reduction in lysosomal cathepsins 154 (Figure 5), which would result in inefficient autophagy in HD. Cystatin B knockdown in models of HD and studies with cathepsin inhibitors in animal models for both AD and HD could help to further confirm and validate these therapeutic targets.…”