Calpains in the Molecular Pathogenesis of Polyglutamine Disorders and Their Potential as a Therapeutic Target

Together with cancer and cardiovascular disorders, neurodegenerative diseases such as Alzheimer and Parkinson disease are an increasingly important medical issue for the aging society of the 21st century. Multiple factors such as environmental influences and individual living conditions act as important modulators of neuropathology. Despite various forms of neurodegenerative diseases, many similarities exist in the underlying molecular pathomechanisms. These are thus promising targets for medical interventions. Therefore, research on genetically determined forms featuring a singular trigger allows to understand these common processes and to transfer acquired knowledge to other neurological diseases. In view of this fact, the current work investigates the validity of the toxic fragment hypothesis using the example of two polyglutamine diseases, Huntington disease and spinocerebellar ataxia type 3. This widely-described theory assumes that molecularly processed disease proteins in the form of toxic and aggregation-prone fragments enhance neurodegenerative effects. Amongst others, endogenous enzymes such as caspases and calcium-dependent calpains have been associated with the proteolytic cleavage of the disease-causing proteins, a source for toxic fragments. The present study focused on the analysis of the calpain-mediated fragmentation of the mutant proteins huntingtin and ataxin-3, as well as on the detection of the enzymatic overactivation of calpains in cell and animal models of Huntington disease and spinocerebellar ataxia type 3. The investigations carried out here confirmed the influence of calpains on neurodegenerative processes in both polyglutamine diseases. Moreover, huntingtin and ataxin-3 were found to be cleaved at specific amino acid positions within the protein, and the resulting fragments exhibited both an increased toxicity and aggregation propensity. In addition, direct as well as indirect pharmacological inhibition of calpains and genetic modification of cleavage sites within the disease protein attenuated these processes, yielding positive effects on the respective molecular pathology. Future studies including the extension to other neurodegenerative diseases are necessary to further investigate the general validity of this pathomechanism. In this way, therapeutically applicable and effective strategies against neurodegeneration may be developed

From Pathways to Targets: Understanding the Mechanisms behind Polyglutamine Disease

The history of polyglutamine diseases dates back approximately 20 years to the discovery of a polyglutamine repeat in the androgen receptor of SBMA followed by the identification of similar expansion mutations in Huntington’s disease, SCA1, DRPLA, and the other spinocerebellar ataxias. This common molecular feature of polyglutamine diseases suggests shared mechanisms in disease pathology and neurodegeneration of disease specific brain regions. In this review, we discuss the main pathogenic pathways including proteolytic processing, nuclear shuttling and aggregation, mitochondrial dysfunction, and clearance of misfolded polyglutamine proteins and point out possible targets for treatment

Killing Two Angry Birds with One Stone: Autophagy Activation by Inhibiting Calpains in Neurodegenerative Diseases and Beyond

Proteolytic machineries execute vital cellular functions and their disturbances are implicated in diverse medical conditions, including neurodegenerative diseases. Interestingly, calpains, a class of Ca2+-dependent regulatory proteases, can modulate the degradational system of autophagy by cleaving proteins involved in this pathway. Moreover, both machineries are common players in many molecular pathomechanisms and have been targeted individually or together, as a therapeutic strategy in experimental setups. In this review, we briefly introduce calpains and autophagy, with their roles in health and disease, and focus on their direct pathologically relevant interplay in neurodegeneration and beyond. The modulation of calpain activity may comprise a promising treatment approach to attenuate the deregulation of these two essential mechanisms

Implications of specific lysine residues within ataxin-3 for the molecular pathogenesis of Machado-Joseph disease

Lysine residues are one of the main sites for posttranslational modifications of proteins, and lysine ubiquitination of the Machado-Joseph disease protein ataxin-3 is implicated in its cellular function and polyglutamine expansion-dependent toxicity. Despite previously undertaken efforts, the individual roles of specific lysine residues of the ataxin-3 sequence are not entirely understood and demand further analysis. By retaining single lysine residues of otherwise lysine-free wild-type and polyglutamine-expanded ataxin-3, we assessed the effects of a site-limited modifiability on ataxin-3 protein levels, aggregation propensity, localization, and stability. We confirmed earlier findings that levels of lysine-free ataxin-3 are reduced due to its decreased stability, which led to a diminished load of SDS-insoluble species of its polyglutamine-expanded form. The isolated presence of several single lysine residues within the N-terminus of polyglutamine-expanded ataxin-3 significantly restored its aggregate levels, with highest fold changes induced by the presence of lysine 8 or lysine 85, respectively. Ataxin-3 lacking all lysine residues presented a slightly increased nuclear localization, which was counteracted by the reintroduction of lysine 85, whereas presence of either lysine 8 or lysine 85 led to a significantly higher ataxin-3 stability. Moreover, lysine-free ataxin-3 showed increased toxicity and binding to K48-linked polyubiquitin chains, whereas the reintroduction of lysine 85, located between the ubiquitin-binding sites 1 and 2 of ataxin-3, normalized its binding affinity. Overall, our data highlight the relevance of lysine residues 8 and 85 of ataxin-3 and encourage further analyses, to evaluate the potential of modulating posttranslational modifications of these sites for influencing pathophysiological characteristics of the Machado-Joseph disease protein