The maintenance of protein homeostasis is a mammoth task for cells. Due to the elementary importance of transport and degradation processes for the maintenance of cellular survival, even minor disturbances can lead to a restriction of cellular functions or even cell death. In genetic screens designed to detect pathogenic, e.g. mutated DNA sequences, genes involved in protein biosynthesis, their folding, intracellular protein transport or degradation turn up particularly frequently. For example, a number of lysosomal storage diseases are characterized by marked neurodegeneration, and conversely, neurodegenerative diseases regularly show lysosomal dysfunction. These observations underscore the importance of protein homeostasis for neuronal survival in aging and disease.
Misconfigured proteins are secreted from the neuron or recognized by the endoplasmic reticulum (ER) and degraded in the proteasome. The latter mechanism requires special ER proteins, such as membralin (encoded by the TMEM259 gene). These enzymes are grouped together in the ERAD complex, an abbreviation for ER-associated degradation. In aging post-mitotic cells, when the ER is overloaded by accumulation of misfolded proteins, the unfolded protein response (UPR) occurs. With this response, the cell attempts to restore the normal status of the ER. However, if the overload of the ER persists over a longer period of time in the context of so-called ER stress, for example due to aggregation of α-synuclein in neurons, proteins located in the membrane of the ER trigger specific signaling cascades that shut down protein synthesis at ribosomes and lead to DNA strand breaks in the nucleus. In neurons, these changes lead to loss of myelin sheaths, fragmentation of axons, and ultimately apoptosis (cell death).
A cell takes up various substances (yellow) from outside by endocytosis (after invagination of the plasma membrane) in order to pack them into vesicles (endosomes) and distribute them within the cell. Furthermore, protein aggregates (green), but also whole organelles (such as mitochondria, pink) are enclosed by intracellular membranes when needed (which is called autophagy or mitophagy). Autophagosomes fuse with lysosomes, which contain various hydrolases that degrade their contents. This makes individual amino acids, glucose, or lipids available again to cellular metabolism (however, the vast majority of cytoplasmic proteins are degraded in the proteasome). In addition, vesicles are detached inwardly from the endosomal membrane. In this way, multi-vesicular bodies (MVBs) are formed. Various proteins and nucleic acids are also released directly into the extracellular space by exocytosis or are discharged in the form of small vesicles (exosomes). The double-lipid membranes of the cell envelope and the various organelles are not drawn to scale (ER = endoplasmic reticulum; modified Fig. 2.3 from Klimaschewski L.P. , Parkinson's and Alzheimer's diseases today. Springer, 2021).
An example of a lysosomal hydrolase that is mutated in lysosomal storage diseases (Gaucher syndrome) and thus has reduced activity is β-glucocerebrosidase (GCase). This results in a lysosomal increase in lipids. Interestingly, variants of the gene encoding GCase, GBA1, are consistently found in Parkinson's patients. Genetic loss of function of GBA1 is one of the most important risk factors that can trigger this neurodegenerative disease or dementia with Lewy bodies.
PD is associated with an increase and cross-linking (oligomerization) of α-synuclein encoded by the SNCA gene, which normally induces the above-mentioned UPR. In a paper just published in the premium journal of Neuroscience (Neuron), dopaminergic neurons were derived from induced stem cells taken from Parkinson's disease patients (with a threefold amount of the SNCA gene). Surprisingly, no UPR was found in these cells, but the ER partially disintegrated and GCase and other proteins failed to fold and aggregate properly. In this process, the high amounts of synuclein seem to interact with the chaperone proteins necessary for protein folding and inhibit their function. In addition, protein transport at the Golgi apparatus was also disrupted.
The rescue experiments performed in this study showed particularly promising results. Namely, a blocker of ryanodine receptors (RyRs) that inhibits the release of calcium from the ER (diltiazem, an approved antihypertensive drug) prevented the formation of insoluble GCase and led to enhanced maturation of this enzyme in the ER. Genetic knockdown of RyRs (using specific shRNAs) showed similar effects. In addition, farnesyltransferase inhibitors (FTIs) improved lysosomal GCase activity. Together, diltiazem and FTIs acted synergistically, i.e., their combined effects exceeded the summation of the individual effects, thus providing a new therapeutic approach for the treatment of Parkinson's disease (although diltiazem itself may induce Parkinson's syndrome).
References:
Stojkovska I, Wani WY, Zunke F, ..., Mazzulli JR (2022) Rescue of α-synuclein aggregation in Parkinson’s patient neurons by synergistic enhancement of ER proteostasis and protein trafficking. Neuron 110:436-451
Udayar V, Chen Y, Sidransky E, Jagasia R (2022) Lysosomal dysfunction in neurodegeneration: emerging concepts and methods. Trends in Neurosciences 45:184-199
Image credit: istock/Roman Didkivskyi, iStock/Artur Plawgo
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