Trends in Cell Biology
ReviewTherapeutic Dissolution of Aberrant Phases by Nuclear-Import Receptors
Section snippets
NIRs Import Specific Protein Cargo into the Nucleus
The passage of diverse macromolecules between the cytoplasm and nucleus occurs via transport across the nuclear pore complex (NPC) (see Glossary) [1]. The NPC permits passive transit of small molecules, but proteins larger than 20–40 kDa must typically engage NIRs for transport across the NPC [1]. Thus, NIRs bind NLSs of protein cargo in the cytoplasm and transport them across the central channel of the NPC into the nucleus (Figure 1) [1]. Nuclear import of cargo by NIRs is orchestrated by a
Structure of NIRs
Typically, NIRs are members of the karyopherin-β (Kapβ) family of proteins, which drive the majority of nuclear–cytoplasmic transport in eukaryotic cells [1]. For example, Kapβ2 [also known as transportin-1 or importin (Imp)β2] is a NIR that shuttles cargo bearing a proline–tyrosine (PY)-NLS into the nucleus [2]. Similar to other NIRs, Kapβ2 is predominantly comprised of HEAT repeats, which are structural motifs composed of two antiparallel helices A and B [1]. The HEAT repeats coil to form a
Cargo Recognition by NIRs
Kapβ2 cargo generally harbors a PY-NLS (Figure 3), which can be 15–100 amino acids in length 1, 2. Unlike the larger class of lysine-rich cNLSs recognized by Impα/β, PY-NLSs have three important epitopes: (i) an N-terminal hydrophobic or basic epitope, (ii) the arginine of the R-X2–5P-Y/Φ (Φ is a hydrophobic residue) motif, and (iii) the PY or homologous PΦ motif of the C-terminal R-X(2–5)-PY/Φ motif 2, 3, 20, 30. The list of cargo recognized by Kapβ2 is still growing and includes several
Cargo and Stress Granules
During stress, cells must conserve and redirect their limited energy to avoid severe damage. To this end, cells pause translation of most mRNAs except for those encoding various heat-shock proteins, and store transcripts and RBPs in cytoplasmic membraneless organelles called stress granules (SGs) 9, 42, 43, 44. Many NIR cargos and NIRs themselves become sequestered in SGs 10, 45. The formation of SGs is a normal protective process, but it can become deleterious if SGs persist for too long in
Cargo and Disease
Many NIR cargos are RBPs with PrLDs that are connected with ALS and FTD, including TDP-43, FUS, EWSR1, TAF15, hnRNPA1, and hnRNPA2 (Figure 3) 4, 5, 6. These RBPs mislocalize to cytoplasmic inclusions in degenerating neurons 4, 5, 6, 32, 61, 62. For example, TDP-43 is mislocalized to cytoplasmic aggregates in degenerating neurons of ∼97% of ALS cases and ∼45% of FTD cases [4]. By contrast, FUS is mislocalized to cytoplasmic aggregates in degenerating neurons of ∼1% of ALS cases and ∼9% of FTD
Nucleocytoplasmic Transport Dysfunction in Disease
Accumulating evidence suggests that nucleocytoplasmic transport defects are a shared mechanism contributing to the disease initiation and progression of ALS and FTD, even in cases where no apparent mutations in RBP NLSs are observed 76, 77. For example, in FTD, WT FUS mislocalization is seen in ∼9% of cases 78, 79. Additionally, the RBP TDP-43 is cytoplasmically mislocalized in ∼97% of ALS patient and ∼45% of FTD patients [80]. In most cases, WT TDP-43 accumulates in cytoplasmic aggregates, and
New NIR Functions in Preventing and Reversing Aberrant Phase Separation
Several groups have reported new functions for NIRs 10, 11, 12, 13. We reported that NIRs potently inhibit and reverse LLPS, aberrant phase transition, and fibrillization of several RBPs with PrLDs implicated in ALS, FTD, and MSP [10]. While NIRs have been found to function as chaperones before 105, 106, 107, the ability to rapidly disaggregate cargo that has converted to stable cross-β polymers, and to prevent and reverse aberrant phase transitions was unanticipated 10, 11, 12, 13. For
Concluding Remarks
We have established that NIRs inhibit spontaneous and seeded fibrillization of cargo RBPs with PrLDs, which could prevent pathological spreading by prion-like conformers in disease [10]. Moreover, NIRs dissolve fibrils and reverse aberrant phase transitions of cargo RBPs with PrLDs, which we suggest could also be neuroprotective [10]. Indeed, we found that increasing NIR concentrations antagonized neurodegeneration and toxicity caused by cargo RBPs with PrLDs [10]. Moreover, increased
Acknowledgments
We thank Bo Lim (Linda) Lee and Hana Odeh for comments on the manuscript. This work was funded by the Ellison Medical Foundation (L.G.), American Federation for Aging Research (L.G.), Alzheimer’s Association (L.G.), Life Extension Foundation (J.S.), ALS Association (J.S.), Department of Biochemistry and Biophysics Pilot Grant (J.S.), Target ALS (J.S. and L.G.), the Robert Packard Center for ALS Research at Johns Hopkins (J.S.), the Office of the Assistant Secretary of Defense for Health Affairs
Glossary
- Aberrant phase transitions
- transition of the material state from a liquid to a solid-like state. Aberrant phase transition leads to a less dynamic material state and the formation of protein hydrogel, aggregates, and fibrils. Aberrant phase transition is often accelerated by disease mutations in the PrLD and other intrinsically disordered domains.
- Adeno-associated virus (AAV)
- a small virus that infects humans, but elicits only a mild immune response. AAVs have been adapted for gene-therapy
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