Huntington's disease: lessons from prion disorders

Cytology, architecture, development, and connections of the primate striatum: Hints for human pathology

Degeneration of neurons and circuits across the striatum shows stereotyped time-course and spatial topography patterns that are distinct for Huntington's disease, Parkinson's disease, or the Tauopathies. These patterns of neurodegeneration in humans have not yet been systematically related to developmental, connectional, cellular, and chemical factors studied in human and non-human primates, that may underlie potential differences in selective vulnerability across striatal sectors. Relating primate anatomy to human pathology could provide new venues for identifying molecular, cellular, and connectional factors linked to the degeneration of striatal neurons and circuits. This review describes and summarizes several developmental, cellular, structural, and connectional features of the primate striatum in relation to patterns of neurodegeneration in the striatum of humans and of non-human primate models. We review (1) the types of neurons in the primate striatum, (2) the cyto-, myelo-, and chemoarchitecture of the primate striatum, (3) the developmental origin of the striatum in light of modern patterning studies, (4) the organization of corticostriatal projections in relation to cortical types, and (5) the topography and time-course of neuron loss, glial reaction, and protein aggregation induced by neurodegenerative diseases in humans and in non-human primate models across striatal sectors and their corresponding cortical areas. We summarize current knowledge about key aspects of primate striatal anatomy and human pathology and indicate knowledge gaps that should be addressed in future studies. We aim to identify factors for selective vulnerability to neurodegeneration of striatal neurons and circuits and obtain hints that could help elucidate striatal pathology in humans.

Molecular Mechanisms Mediating the Transfer of Disease-Associated Proteins and Effects on Neuronal Activity

BACKGROUND

Various cellular pathways have been implicated in the transfer of disease-related proteins between cells, contributing to disease progression and neurodegeneration. However, the overall effects of protein transfer are still unclear.

OBJECTIVE

Here, we performed a systematic comparison of basic molecular mechanisms involved in the release of alpha-synuclein, Tau, and huntingtin, and evaluated functional effects upon internalization by receiving cells.

METHODS

Evaluation of protein release to the extracellular space in a free form and in extracellular vesicles using an optimized ultracentrifugation protocol. The extracellular effects of the proteins and extracellular vesicles in primary neuronal cultures were assessed using multi-channel electrophysiological recordings combined with a customized spike sorting framework.

RESULTS

We demonstrate cells differentially release free-forms of each protein to the extracellular space. Importantly, neuronal activity is distinctly modulated upon protein internalization in primary cortical cultures. In addition, these disease-related proteins also occur in extracellular vesicles, and are enriched in ectosomes. Internalization of ectosomes and exosomes by primary microglial or astrocytic cells elicits the production of pro-inflammatory cytokines, and modifies spontaneous electrical activity in neurons.

OBJECTIVE

Overall, our study demonstrates that released proteins can have detrimental effects for surrounding cells, and suggests protein release pathways may be exploited as therapeutic targets in different neurodegenerative diseases.

Prion-like properties of the mutant huntingtin protein in living organisms: the evidence and the relevance

If theories postulating that pathological proteins associated with neurodegenerative disorders behave similarly to prions were initially viewed with reluctance, it is now well-accepted that this occurs in several disease contexts. Notably, it has been reported that protein misfolding and subsequent prion-like properties can actively participate in neurodegenerative disorders. While this has been demonstrated in multiple cellular and animal model systems related to Alzheimer's and Parkinson's diseases, the prion-like properties of the mutant huntingtin protein (mHTT), associated with Huntington's disease (HD), have only recently been considered to play a role in this pathology, a concept our research group has contributed to extensively. In this review, we summarize the last few years of in vivo research in the field and speculate on the relationship between prion-like events and human HD. By interpreting observations primarily collected in in vivo models, our discussion will aim to discriminate which experimental factors contribute to the most efficient types of prion-like activities of mHTT and which routes of propagation may be more relevant to the human condition. A look back at nearly a decade of experimentation will inform future research and whether therapeutic strategies may emerge from this new knowledge.

Unwanted Exacerbation of the Immune Response in Neurodegenerative Disease: A Time to Review the Impact

The COVID-19 pandemic imposed a series of behavioral changes that resulted in increased social isolation and a more sedentary life for many across all age groups, but, above all, for the elderly population who are the most vulnerable to infections and chronic neurodegenerative diseases. Systemic inflammatory responses are known to accelerate neurodegenerative disease progression, which leads to permanent damage, loss of brain function, and the loss of autonomy for many aged people. During the COVID-19 pandemic, a spectrum of inflammatory responses was generated in affected individuals, and it is expected that the elderly patients with chronic neurodegenerative diseases who survived SARSCoV-2 infection, it will be found, sooner or later, that there is a worsening of their neurodegenerative conditions. Using mouse prion disease as a model for chronic neurodegeneration, we review the effects of social isolation, sedentary living, and viral infection on the disease progression with a focus on sickness behavior and on the responses of microglia and astrocytes. Focusing on aging, we discuss the cellular and molecular mechanisms related to immunosenescence in chronic neurodegenerative diseases and how infections may accelerate their progression.

Amyloid-type Protein Aggregation and Prion-like Properties of Amyloids

This review will focus on the process of amyloid-type protein aggregation. Amyloid fibrils are an important hallmark of protein misfolding diseases and therefore have been investigated for decades. Only recently, however, atomic or near-atomic resolution structures have been elucidated from various in vitro and ex vivo obtained fibrils. In parallel, the process of fibril formation has been studied in vitro under highly artificial but comparatively reproducible conditions. The review starts with a summary of what is known and speculated from artificial in vitro amyloid-type protein aggregation experiments. A partially hypothetic fibril selection model will be described that may be suitable to explain why amyloid fibrils look the way they do, in particular, why at least all so far reported high resolution cryo-electron microscopy obtained fibril structures are in register, parallel, cross-β-sheet fibrils that mostly consist of two protofilaments twisted around each other. An intrinsic feature of the model is the prion-like nature of all amyloid assemblies. Transferring the model from the in vitro point of view to the in vivo situation is not straightforward, highly hypothetic, and leaves many open questions that need to be addressed in the future.