Mechanisms of tunneling nanotube-based propagation of neurodegenerative disease proteins

Hypothermia promotes tunneling nanotube formation and the transfer of astrocytic mitochondria into oxygen-glucose deprivation/reoxygenation-injured neurons

Mitochondrial transfer occurs between cells, and it is important for damaged cells to receive healthy mitochondria to maintain their normal function and protect against cell death. Accumulating evidence suggests that the functional mitochondria of astrocytes are released and transferred to oxygen-glucose deprivation/reoxygenation (OGD/R)-injured neurons. Mild hypothermia (33 °C) is capable of promoting this process, which partially restores the function of damaged neurons. However, the pathways and mechanisms by which mild hypothermia facilitates mitochondrial transfer remain unclear. We are committed to studying the role of mild hypothermia in neuroprotection to provide reliable evidences and insights for the clinical application of mild hypothermia in brain protection. Tunneling nanotubes (TNTs) are considered to be one of the routes through which mitochondria are transferred between cells. In this study, an OGD/R-injured neuronal model was successfully established, and TNTs, mitochondria, neurons and astrocytes were double labeled using immunofluorescent probes. Our results showed that TNTs were present and involved in the transfer of mitochondria between cells in the mixed-culture system of neurons and astrocytes. When neurons were subjected to OGD/R exposure, TNT formation and mitochondrial transportation from astrocytes to injured neurons were facilitated. Further analysis revealed that mild hypothermia increased the quantity of astrocytic mitochondria transferred into damaged neurons through TNTs, raised the mitochondrial membrane potential (MMP), and decreased the neuronal damage and death during OGD/R. Altogether, our data indicate that TNTs play an important role in the endogenous neuroprotection of astrocytic mitochondrial transfer. Furthermore, mild hypothermia enhances astrocytic mitochondrial transfer into OGD/R-injured neurons via TNTs, thereby promoting neuroprotection and neuronal recovery.

Tunneling nanotubes: The transport highway for astrocyte-neuron communication in the central nervous system

Tunneling nanotubes (TNTs) have emerged as pivotal structures for intercellular communication, enabling the transfer of cellular components across distant cells. Their involvement in neurological disorders has attracted considerable scientific interest. This review delineates the functions of TNTs within the central nervous system, examining their role in the transmission of bioenergetic substrates, and signaling molecules, and their multifaceted impact on both physiological and pathological processes, with an emphasis on neurodegenerative diseases. The review highlights the selectivity and specificity of TNTs as dedicated pathways for intercellular cargo delivery, particularly under stress conditions that provoke increased TNT formation. The potential of TNTs as therapeutic targets is explored in depth. We pay particular attention to the interactions between astrocytes and neurons mediated by TNTs, which are fundamental to brain architecture and function. Dysfunctions in these interactions are implicated in the spread of protein aggregates and mitochondrial anomalies, contributing to the pathogenesis of neurodegenerative diseases. The review culminates with a synthesis of the current understanding of TNT biology and identifies research gaps, advocating for intensified exploration into TNTs as a promising therapeutic frontier.

Casein kinase 1 inhibitor avoids TDP-43 pathology propagation in a patient-derived cellular model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease without a cure to reverse its progression. Its main hallmark is the nuclear protein TDP-43, which undergoes different post-translational modifications leading to a loss of function in the nucleus and an increase in toxicity in the cytoplasm. Previous reports have indicated that pathogenic TDP-43 exhibits prion-like propagation in various contexts. With the aim of advancing therapeutics focused on preventing the propagation of TDP-43 pathology, we studied the potential role of pathogenic TDP-43 in lymphoblasts from sporadic ALS patients. We used lymphoblastoid cell lines from sporadic ALS patients as a source of pathogenic forms of TDP-43, and healthy human cells (lymphoblasts, myoblasts, neuroblastoma SH-SY5Y, or osteosarcoma U2OS) as recipient cells to investigate the seeding and spread of TDP-43 proteinopathy. Furthermore, we evaluated the potential of targeting TDP-43 phosphorylation with a CK-1 inhibitor to prevent the propagation of the pathology. The results presented herein indicate that pathogenic forms of TDP-43 are secreted into the extracellular medium of sporadic ALS lymphoblasts and could be transported by extracellular vesicles, spreading TDP-43 pathology to healthy cells. Moreover, tunneling nanotubes have also been discovered in pathological cells and may be involved in the transport of TDP-43. Interestingly, targeting TDP-43 phosphorylation with an in-house designed CK-1 inhibitor (IGS2.7) was sufficient to halt TDP-43 pathology transmission, in addition to its known effects on restoring the homeostasis of TDP-43 protein in patients-derived cells.

Tunneling Nanotube: An Enticing Cell-Cell Communication in the Nervous System

The field of neuroscience is rapidly progressing, continuously uncovering new insights and discoveries. Among the areas that have shown immense potential in research, tunneling nanotubes (TNTs) have emerged as a promising subject of study. These minute structures act as conduits for the transfer of cellular materials between cells, representing a mechanism of communication that holds great significance. In particular, the interplay facilitated by TNTs among various cell types within the brain, including neurons, astrocytes, oligodendrocytes, glial cells, and microglia, can be essential for the normal development and optimal functioning of this complex organ. The involvement of TNTs in neurodegenerative disorders, such as Alzheimer's disease, Huntington's disease, and Parkinson's disease, has attracted significant attention. These disorders are characterized by the progressive degeneration of neurons and the subsequent decline in brain function. Studies have predicted that TNTs likely play critical roles in the propagation and spread of pathological factors, contributing to the advancement of these diseases. Thus, there is a growing interest in understanding the precise functions and mechanisms of TNTs within the nervous system. This review article, based on our recent work on Rhes-mediated TNTs, aims to explore the functions of TNTs within the brain and investigate their implications for neurodegenerative diseases. Using the knowledge gained from studying TNTs could offer novel opportunities for designing targeted treatments that can stop the progression of neurodegenerative disorders.

The Synucleins and the Astrocyte

Synucleins consist of three proteins exclusively expressed in vertebrates. α-Synuclein (αS) has been identified as the main proteinaceous aggregate in Lewy bodies, a pathological hallmark of many neurodegenerative diseases. Less is understood about β-synuclein (βS) and γ-synuclein (γS), although it is known βS can interact with αS in vivo to inhibit aggregation. Likewise, both γS and βS can inhibit αS's propensity to aggregate in vitro. In the central nervous system, βS and αS, and to a lesser extent γS, are highly expressed in the neural presynaptic terminal, although they are not strictly located there, and emerging data have shown a more complex expression profile. Synapse loss and astrocyte atrophy are early aspects of degenerative diseases of the brain and correlate with disease progression. Synucleins appear to be involved in synaptic transmission, and astrocytes coordinate and organize synaptic function, with excess αS degraded by astrocytes and microglia adjacent to the synapse. βS and γS have also been observed in the astrocyte and may provide beneficial roles. The astrocytic responsibility for degradation of αS as well as emerging evidence on possible astrocytic functions of βS and γS, warrant closer inspection on astrocyte-synuclein interactions at the synapse.

The Hidden Cell-to-Cell Trail of α-Synuclein Aggregates

The progressive accumulation of insoluble aggregates of the presynaptic protein alpha-synuclein (α-Syn) is a hallmark of neurodegenerative disorders including Parkinson's disease (PD), Multiple System Atrophy, and Dementia with Lewy Bodies, commonly referred to as synucleinopathies. Despite considerable progress on the structural biology of these aggregates, the molecular mechanisms mediating their cell-to-cell transmission, propagation, and neurotoxicity remain only partially understood. Numerous studies have highlighted the stereotypical spatiotemporal spreading of pathological α-Syn aggregates across different tissues and anatomically connected brain regions over time. Experimental evidence from various cellular and animal models indicate that α-Syn transfer occurs in two defined steps: the release of pathogenic α-Syn species from infected cells, and their uptake via passive or active endocytic pathways. Once α-Syn aggregates have been internalized, little is known about what drives their toxicity or how they interact with the endogenous protein to promote its misfolding and subsequent aggregation. Similarly, unknown genetic factors modulate different cellular responses to the aggregation and accumulation of pathogenic α-Syn species. Here we discuss the current understanding of the molecular phenomena associated with the intercellular spreading of pathogenic α-Syn seeds and summarize the evidence supporting the transmission hypothesis. Understanding the molecular mechanisms involved in α-Syn aggregates transmission is essential to develop novel targeted therapeutics against PD and related synucleinopathies.