Nucleocytoplasmic transport at the crossroads of proteostasis, neurodegeneration and neuroprotection

Abstract

Nucleocytoplasmic transport comprises the multistep assembly, transport and disassembly of protein and RNA cargoes entering and exiting nuclear pores. Accruing evidence supports that impairments to nucleocytoplasmic transport are a hallmark of neurodegenerative diseases. These impairments cause dysregulations in nucleocytoplasmic partitioning and proteostasis of nuclear transport receptors and client substrates that promote intracellular deposits – another hallmark of neurodegeneration. Disturbances in liquid–liquid phase separation (LLPS) between dense and dilute phases of biomolecules implicated in nucleocytoplasmic transport promote micrometer‐scale coacervates, leading to proteinaceous aggregates. This Review provides historical and emerging principles of LLPS at the interface of nucleocytoplasmic transport, proteostasis, aging and noxious insults, whose dysregulations promote intracellular aggregates. E3 SUMO‐protein ligase Ranbp2 constitutes the cytoplasmic filaments of nuclear pores, where it acts as a molecular hub for rate‐limiting steps of nucleocytoplasmic transport. A vignette is provided on the roles of Ranbp2 in nucleocytoplasmic transport and at the intersection of proteostasis in the survival of photoreceptor and motor neurons under homeostatic and pathophysiological environments. Current unmet clinical needs are highlighted, including therapeutics aiming to manipulate aggregation‐dissolution models of purported neurotoxicity in neurodegeneration.

Proteostatic Remodeling of Small Heat Shock Chaperones─Crystallins by Ran-Binding Protein 2─and the Peptidyl-Prolyl cis–trans Isomerase and Chaperone Activities of Its Cyclophilin Domain

“…1,9,10 Notably, dysregulations of the moonlighting activity of CY of Ranbp2 cause proteostatic imbalances of a restricted set of client substrates, which are implicated in neuroprotection, and some of these substrates are also affected in mice with haploinsufficiency of Ranbp2. 1,10,114 Third, Ranbp2 plays a selective role in chorioretinal proteostasis by mechanisms that are not well understood. For example, the CLD of Ranbp2 specifically associates to a subset of ATPase and non-ATPase subunits of the 19S cap of the proteasome selectively in retinal extracts but not in other tissue extracts.…”

“…We focused on proteostatic impairments affecting the RPE and the retina for several reasons. First, these tissues are chronically exposed to and vulnerable to environmental insults (e.g., photo-oxidative stress) and thus predisposed to proteotoxicity and AMD. , AMD is a multifactorial disease characterized by the induction of heat shock proteins, the formation of large laminar heterogeneous aggregates (drusen) with aging, and by the irreversible loss of photoreceptor neurons and RPE cells. ,− Second, we have previously shown that haploinsufficiency of Ranbp2 preordains these tissues to neuroprotection against proteostatic stressors, such as photo-oxidative stress. ,, Notably, dysregulations of the moonlighting activity of CY of Ranbp2 cause proteostatic imbalances of a restricted set of client substrates, which are implicated in neuroprotection, and some of these substrates are also affected in mice with haploinsufficiency of Ranbp2 . ,, Third, Ranbp2 plays a selective role in chorioretinal proteostasis by mechanisms that are not well understood. For example, the CLD of Ranbp2 specifically associates to a subset of ATPase and non-ATPase subunits of the 19S cap of the proteasome selectively in retinal extracts but not in other tissue extracts. − Loss of small ubiquitin-like modifier-1 (SUMO-1)-binding activity of CLD caused by mutations in the consensus SUMO-1-interacting motif (SIM), which overlaps with the internal repeat 1 (IR1) of Ranbp2 (Figure a), protects photoreceptor neurons from death against phototoxicity, but unlike Ranbp2 haploinsufficient mice, photoreceptors are not protected from photodamage. , Further, loss of CY PPIase activity of Ranbp2 diminishes the levels of polyubiquitylated substrates in the retina and RPE, whereas neither the selective loss of Ranbp2 in motoneurons nor the constitutive loss of its CY PPIase activity affect, respectively, the overall levels of polyubiquitylated substrates in sciatic nerves and liver. , Hence, the Ranbp2 scaffold appears to regulate proteostasis and neuroprotection to stressors by coupling modular activities of CY and CLD (Figure a).…”

“…9−20 Impairments of these biophysical processes are a cardinal hallmark of neurodegenerative diseases. 1,8,21 These disturbances culminate in the formation of insoluble protein aggregates and membraneless bodies that become resistant or inaccessible to degradation by the ubiquitin−proteasome system (UPS) in neurodegenerative diseases. 22,23 Molecular chaperones have critical but complementary roles in proteostasis under homeostatic and stress environments, 24,25 whereas aging impairs the induction of chaperones.…”

“…The Ran-binding protein 2 (Ranbp2; aka, Nup358) comprises the cytoplasmic filaments emanating from nuclear pores, where it acts as a molecular hub for rate-limiting steps of nuclear export. − Ranbp2 mediates the disassembly of Ran-GTP-bound protein ensembles upon exiting the cytosolic face of nuclear pores. These processes involve phase transitions of Ran-GTP-bound and liberated cargoes. , Ranbp2 regulates the nucleocytoplasmic partition and phase transitions of nuclear transport receptors and client substrates elicited by genetic or environmental insults and by aging. − Impairments of these biophysical processes are a cardinal hallmark of neurodegenerative diseases. ,, These disturbances culminate in the formation of insoluble protein aggregates and membraneless bodies that become resistant or inaccessible to degradation by the ubiquitin–proteasome system (UPS) in neurodegenerative diseases. , …”

“…Our prior findings have shown that haploinsufficiency of Ranbp2 promotes age-dependent neuroprotection of the chorioretina against phototoxicity by suppressing the damage and apoptosis of photoreceptor neurons and the accumulation of intracellular deposits in the RPE. 1,9,10 Among other effects, this neuroprotection is reflected by the suppression of the light-elicited accumulation of polyubiquitylated substrates and by the proteostatic regulation of neuroprotective substrates of Ranbp2. 1,10 Our subsequent studies found that loss of PPIase activity of the cyclophilin domain (CY) of Ranbp2 in mice also causes a decrease of polyubiquitylated substrates in the retina absent of stressors.…”