Dawn T. Smallwood

Nuclear migration and positioning within cells are critical for many developmental processes and are governed by the cytoskeletal network. Although mechanisms of nuclear-cytoskeletal attachment are unclear, growing evidence links a novel family of nuclear envelope (NE) proteins that share a conserved C-terminal SUN (Sad1/UNC-84 homology) domain. Analysis of Caenorhabditis elegans mutants has implicated UNC-84 in actin-mediated nuclear positioning by regulating NE anchoring of a giant actin-binding protein, ANC-1. Here, we report the identification of SUN1 as a lamin A-binding protein in a yeast two-hybrid screen. We demonstrate that SUN1 is an integral membrane protein located at the inner nuclear membrane. While the N-terminal domain of SUN1 is responsible for detergent-resistant association with the nuclear lamina and lamin A binding, lamin A/C expression is not required for SUN1 NE localization. Furthermore, SUN1 does not interact with type B lamins, suggesting that NE localization is ensured by binding to an additional nuclear component(s), most likely chromatin. Importantly, we find that the luminal C-terminal domain of SUN1 interacts with the mammalian ANC-1 homologs nesprins 1 and 2 via their conserved KASH domain. Our data provide evidence of a physical nuclear-cytoskeletal connection that is likely to be a key mechanism in nuclear-cytoplasmic communication and regulation of nuclear position.

The nuclear envelope (NE) LINC complex, in mammals comprised of SUN domain and nesprin proteins, provides a direct connection between the nuclear lamina and the cytoskeleton, which contributes to nuclear positioning and cellular rigidity. SUN1 and SUN2 interact with lamin A, but lamin A is only required for NE localization of SUN2, and it remains unclear how SUN1 is anchored. Here, we identify emerin and short nesprin-2 isoforms as novel nucleoplasmic binding partners of SUN1/2. These have overlapping binding sites distinct from the lamin A binding site. However, we demonstrate that tight association of SUN1 with the nuclear lamina depends upon a short motif within residues 209-228, a region that does not interact significantly with known SUN1 binding partners. Moreover, SUN1 localizes correctly in cells lacking emerin. Importantly then, the major determinant of SUN1 NE localization has yet to be identified. We further find that a subset of lamin A mutations, associated with laminopathies Emery-Dreifuss muscular dystrophy (EDMD) and Hutchinson-Gilford progeria syndrome (HGPS), disrupt lamin A interaction with SUN1 and SUN2. Despite this, NE localization of SUN1 and SUN2 is not impaired in cell lines from either class of patients. Intriguingly, SUN1 expression at the NE is instead enhanced in a significant proportion of HGPS but not EDMD cells and strongly correlates with pre-lamin A accumulation due to preferential interaction of SUN1 with pre-lamin A. We propose that these different perturbations in lamin A-SUN protein interactions may underlie the opposing effects of EDMD and HGPS mutations on nuclear and cellular mechanics.

Hutchinson–Gilford progeria syndrome (HGPS) is a rare but devastating genetic disorder that mimics premature aging. Most cases are caused by heterozygous mutations in LMNA, encoding lamin A/C. The allelic disorder, mandibuloacral dysplasia (MAD), shares many features with HGPS and can also result from homozygous mutations in ZMPSTE24, which encodes the enzyme responsible for proteolytic processing of prelamin A. Heterozygous mutation of ZMPSTE24 was recently found to be associated with restrictive dermopathy (RD), a lethal neonatal disorder characterised by tight skin and sharing features of MAD/HGPS. N Compound heterozygous ZMPSTE24 mutations, c.1085_1086insT (Leu362PhefsX19) and c.794 ARG (N265S), were identified in a two year old girl with a severe early onset progeroid phenotype displaying features of HGPS, MAD, and RD. N Western blot analysis of skin fibroblasts showed defective lamin A processing, resulting in reduced levels of mature lamin A and increased levels of the prelamin A precursor. N Primary skin fibroblasts showed abnormal nuclear morphology and these abnormalities increased in severity upon culturing. N Our data widen the spectrum of progeroid phenotypes associated with ZMPSTE24 mutation and for the first time provide evidence that these mutations lead to defective prelamin A processing and abnormal nuclear morphology in humans.

The complex interplay between cancer cells, stromal cells, and immune cells in the tumor microenvironment (TME) regulates tumorigenesis and provides emerging targets for immunotherapies. Crosstalk between CD4(+) T cells and proliferating chronic lymphocytic leukemia (CLL) tumor B cells occurs within lymphoid tissue pseudofollicles, and investigating these interactions is essential to understand both disease pathogenesis and the effects of immunotherapy. Tumor-derived extracellular vesicle (EV) shedding is emerging as an important mode of intercellular communication in the TME. In order to characterize tumor EVs released in response to T-cell-derived TME signals, we performed microRNA (miRNA [miR]) profiling of EVs released from CLL cells stimulated with CD40 and interleukin-4 (IL-4). Our results reveal an enrichment of specific cellular miRNAs including miR-363 within EVs derived from CD40/IL-4-stimulated CLL cells compared with parental cell miRNA content and control EVs from unstimulated CLL cells. We demonstrate that autologous patient CD4(+) T cells internalize CLL-EVs containing miR-363 that targets the immunomodulatory molecule CD69. We further reveal that autologous CD4(+) T cells that are exposed to EVs from CD40/IL-4-stimulated CLL cells exhibit enhanced migration, immunological synapse signaling, and interactions with tumor cells. Knockdown of miR-363 in CLL cells prior to CD40/IL-4 stimulation prevented the ability of CLL-EVs to induce increased synapse signaling and confer altered functional properties to CD4(+) T cells. Taken together, these data reveal a novel role for CLL-EVs in modifying T-cell function that highlights unanticipated complexity of intercellular communication that may have implications for bidirectional CD4(+) T-cell:tumor interactions within the TME.

BACKGROUND: Hutchinson-Gilford progeria syndrome (HGPS) is a rare premature ageing disorder that belongs to a group of conditions called laminopathies which affect nuclear lamins. Classical and atypical forms of HGPS have been reported and there are clinical overlaps with mandibulo-acral dysplasia and restrictive dermopathy. To date, mutations in two genes, LMNA and ZMPSTE24, have been found in patients with HGPS. The p.G608G LMNA mutation is the most commonly reported mutation. Correlations between genotype and phenotype in children with progeroid syndromes are beginning to emerge. OBJECTIVES: To establish whether the LMNA p.G608G mutation is associated with a particular phenotype of HGPS. METHODS: We reviewed the clinical features and skin histology of three children with HGPS associated with the p.G608G LMNA mutation, and compared our findings with those reported in the literature. RESULTS: Our patients shared a very similar presentation and clinical course. Skin changes were the earliest finding in all three. Skin histology showed nonspecific changes only. CONCLUSIONS: The LMNA p.G608G mutation results in a uniform phenotype through early to mid-childhood, in keeping with that described in classical HGPS. Skin changes are the earliest distinctive clinical finding and should prompt careful physical and radiological examination for other features of HGPS. Skin biopsy for histology is not a useful investigation when a diagnosis of HGPS is suspected.