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The work described in the paper cited below communicating the findings of this recently completed project describes the relationship among transforming growth factor-Beta (TGF-β), astrocytes, and pan neurotrophin receptor p75 (p75^NTR) in studies comparing wildtype mice and a special strain of mice created to spontaneously develop various diseases and pathological disfunctions. This study establishes a potential, previously unknown, mechanism by which the brain responds to injury.

The team’s 18 members whose names appear on this paper brought together expertise from UC San Francisco, the University of Freiburg (Germany), UC San Diego and its National Center for Microscopy and Imaging Research, the University of Glasgow (UK), King’s College London, and Stanford University.

Some terms. First we need we need to define various terms.

TGF-β is a secreted protein that controls cell proliferation and differentiation, and is implicated in neuronal functions and neurodegeneration.

Astrocytes are star-shaped cells in the brain and spinal cord. They support the endothelial cells that line blood vessels and form the blood-brain barrier, provide nutrients to the nervous tissue, maintain extracellular ion balance, and play a role in the repair and scarring process of the brain and spinal cord following traumatic injuries.

p75^NTR regulates neural cell death during development and in the adult following injury, making it a target for treatment of neurodegenerative disease.

The nuclear pore complex (NPC) provides a channel to transport substances between the nucleus and the cytoplasm, the jelly-like part of the cell between the envelope that surrounds the nucleus and the outer cell membrane.

Proteolysis is the breakdown of proteins or peptides into amino acids by the action of enzymes.

The research question. Researchers know that TGF-β promotes the transition of astrocytes from quiescence to a reactive state. However, the mechanisms that control this response and the role astrocytes play in communicating with neurons remain elusive.

After brain injury, p75^NTR increases in astrocytes. Knowing this, the team wanted to investigate further the role of p75^NTR in astrocytes and the response to injury. They created a special strain of mice that lacks p75^NTR and spontaneously develops such conditions as astrocytosis (an abnormal increase in the number of astrocytes due to destruction of nearby neurons from central nervous system trauma, infection, etc.), hydrocephaly (massive accumulation of water in the brain), and neuronal disfunction. Surprisingly, the research team observed that the loss of p75^NTR expressed in the mice astrocytes prevented astrocyte activation and rescued the mice’s induced hydrocephalus and neuronal disfunction. But this loss of p75^NTR did not affect their TGF-β levels.

Astrocytes contribute to gamma oscillations, rhythmic or repetitive neural activity in the central nervous system, which control learning, memory, and attention. TGF-β-induced astrocyte activation decreases gamma oscillations and alters locomotor activity. In accordance, after brain trauma, mice lacking p75^NTR had reduced astrocyte activation and deposition of neurocan, a component of the glial scar that inhibits neural regeneration. By contrast, TGF-β treatment of wildtype astrocytes stimulated protein secretion and gene expression of neurocan.

The team further investigated how TGF-β regulates NPC remodeling in astrocytes and found that TGF-β-induced p75^NTR intramembrane cleavage1[1] regulates NPC structure and function. The team’s results suggest that TGF-β activates a key component of the γ-secretase complex localized to the nuclear envelope.

The team worked on understanding the spatiotemporal dynamics of p75^NTR intramembrane cleavage by using a reporter that can differentiate between uncleaved (yellow in the study’s images) and cleaved (green). Time-lapse imaging of astrocytes transfected with the reporter showed that TGF-β induced the release and translocation of the cleaved p75^NTR intracellular domain to the nucleus.

Further results suggest that, in astrocytes, some of the intracellular pool of p75^NTR plays a role in how the NPC regulates the TGF-β signaling cascade. TGF-β binds to its receptor on the surface of the cell. This induces gamma-secretase to cleave p75^NTR, which releases its intracellular domain (ICD) into the cytoplasm. This p75^NTR-ICD then localizes to the nucleus, where it interacts with elements of the nuclear pore complex (i.e., the gates that allow passage of large molecules in and out of the nucleus). It induces a conformational change in the pore gate that allows P-Smad2 to enter the nucleus where it acts as a transcription factor.

In accordance with biophysical models proposed to explain NPC selectivity and transport, it is possible that TGF-β-induced regulated intramembrane cleavage of p75^NTR generates proteolytic fragments that bind along the NPC to modulate P-Smad2 transport. The cleaved p75^NTR is detected in the NPC of astrocytes, but not neurons, in accordance with the differences in NPC composition between cells. Thus, cell-specific regulation of intramembrane proteolysis might contribute to the ability of the NPC to create differences in growth factor signal transduction pathways between neurons and astrocytes. Further, the team found that p75^NTR-mediated TGF-β signaling altered gamma oscillations, suggesting an unanticipated function for p75^NTR in regulating neural information processing and cognition.

The identification of γ-secretase-mediated cleavage of p75^NTR as a molecular link between TGF-β signaling, astrocyte activation, and neuronal functions could provide therapeutic targets for resolving the gliotic scar and promoting neuronal activity.

This study highlights the importance of nuclear structure in normal cell and organism function. The authors found that a piece of a transmembrane receptor on the cell surface localizes to the nucleus and makes contact with nuclear pore complexes, affecting the response to trauma.

This study also establishes a potential mechanism by which astrocytes respond to brain trauma and supports the importance of the nuclear pore complex in normal and pathological brain functions. Further, it points to the need for further research in these areas to identify potential areas for clinical use.

Funding Source: This work was supported in part by the U.S. National Center for Research Resources 5P41RR004050-24 and the U.S. National Institute of General Medical Science 8P41GM103412-24 to M.E. and the BIOSS – Centre for Biological Signaling Studies EXC 294 for the Life Imaging Center to R.N. It was also supported by U.S. National Multiple Sclerosis Society postdoctoral fellowships to J.K.R. and N.L.M., an American Heart Association fellowship to V.R., a German Academic Exchange Service fellowship to K.M., US National Institute on Aging AG047313 to J.J.P., the European Commission FP7 PIRG08-GA-2010-276989 and the German Research Foundation SCHA 1442/3-2 to C.S., and U.S. National Institute for Neurologic Diseases and Stroke R01NS051470, R01NS052189, R01NS066361, and R21NS082976 to K.A.

Relevant Publication: Schachtrup, C., Ryu, J.K., Mammadzada, K., Khan, A.S., Carlton, P.M., Perez, A., Christian, F., Le Moan, N., Vagena, E., Baeza-Raja, B. and Rafalski, V. Nuclear pore complex remodeling by p75^NTR cleavage controls TGF-β signaling and astrocyte functions. Nature N