A biochemical view on the septins, a less known component of the cytoskeleton

Adipocyte Septin-7 attenuates obesogenic adipogenesis and promotes lipolysis to prevent obesity

OBJECTIVES

The white adipose tissue (WAT) expansion plays a significant role in the development of obesity. Cytoskeletal remodeling directly impacts adipogenic program, however, the precise mechanism remains poorly understood. Here, we identified a crucial role of Septin-7 (SEPT7), a cytoskeleton component, in the regulation of diet-induced processes of adipogenesis, lipogenesis, and lipolysis in WAT.

METHODS

A high-fat diet (HFD)-induced obesity model was constructed using mice with inducible adipocyte-specific SEPT7 deficiency. The impact of SEPT7 on adipocyte morphology, cell number and metabolism capacity were evaluated with immunofluorescence, isoproterenol induced lipolysis assay, glucose tolerance test and insulin tolerance test. Adipocyte mTmG reporter line was established to trace in vivo adipogenesis. The preadipocyte 3T3-L1 cell was induced for exploring role of SEPT7 in adipocyte differentiation. qRT-PCR and Western-blot were used to investigate the expression of PPARγ, C/EBPα, and HSL in 3T3-L1 cell with siRNA-mediated SEPT7 knockdown.

RESULTS

SEPT7 expression was greatly induced in obesogenic human and murine adipocytes. Mice lacking SEPT7 in mature white adipocytes demonstrated defective differentiation of preadipocyte into mature adipocytes when fed HFD resulting in larger adipocytes, increased WAT inflammation and reduced lipolysis, which leading to increased WAT mass, liver fat accumulation and impaired glucose tolerance. Mechanistically, we identified SEPT7 restrains store-operated Ca2+ entry (SOCE) and regulates adipocyte adipogenesis and lipolysis by targeting PPARγ, C/EBPα and HSL.

CONCLUSIONS

We demonstrated that SEPT7 negatively regulates adipogenesis while promotes lipolysis and its repression drives WAT expansion and impaired metabolic health.

Interface integrity in septin protofilaments is maintained by an arginine residue conserved from yeast to man

The septins are conserved, filament-forming, guanine nucleotide binding cytoskeletal proteins. They assemble into palindromic protofilaments which polymerize further into higher-ordered structures that participate in essential intracellular processes such as cytokinesis or polarity establishment. Septins belong structurally to the P-Loop NTPases but, unlike their relatives Ras or Rho, do not mediate signals to effectors through GTP binding and hydrolysis. Biochemical approaches addressing how and why septins utilize nucleotides are hampered by the lack of nucleotide-free complexes. Using molecular dynamics simulations, we determined structural alterations and intersubunit binding free energies in human and yeast septin dimer structures and in their in silico generated apo forms. An interchain salt bridge network around the septin unique β-meander, conserved across all kingdoms of septin containing species, is destabilized upon nucleotide removal, concomitant with disruption of the entire G-interface. Within this network, we confirmed a conserved arginine residue, which coordinates the guanine base of the nucleotide, as the central interaction hub. The essential role of this arginine for interface integrity was experimentally confirmed to be conserved in septins from yeast to human.

Dysregulation of septin cytoskeletal organization in the trabecular meshwork contributes to ocular hypertension

Ocular hypertension, believed to result partly from increased contractile activity, cell adhesive interactions, and stiffness within the trabecular meshwork (TM), is a major risk factor for glaucoma, a leading cause of blindness. However, the identity of molecular mechanisms governing organization of actomyosin and cell adhesive interactions in the TM remains limited. Based on our previous findings, in which proteomics analyses revealed elevated levels of septins, including septin-9 in human TM cells treated with the ocular hypertensive agent dexamethasone, here, we evaluated the effects of septin-9 overexpression, deficiency, and pharmacological targeting in TM cells. These studies demonstrated a profound impact on actomyosin organization, cell adhesion, contraction, and phagocytosis. Overexpression raised intraocular pressure (IOP) in mice, while inhibition increased cell permeability. In addition, we replicated a significant association between a common variant (rs9038) in SEPT9 with IOP in the Genetic Epidemiology Research on Adult Healthy and Aging (GERA) cohort. Collectively, these data reveal a link between dysregulated septin cytoskeletal organization in the TM and increased IOP, likely due to enhanced cell contraction, adhesive interactions, and fibrotic activity. This suggests that targeting the septin cytoskeleton could offer a novel approach for lowering IOP in patients with glaucoma.

Septins as key players in spermatogenesis, fertilisation and pre-implantation embryogenic cytoplasmic dynamics

Septins are a family of cytokinesis-related proteins involved in regulating cytoskeletal design, cell morphology, and tissue morphogenesis. Apart from cytokinesis, as a fourth component of cytoskeleton, septins aid in forming scaffolds, vesicle sorting and membrane stability. They are also known to be involved in the regulation of intracellular calcium (Ca2+) via the STIM/Orai complex. Infertility affects ~ 15% of couples globally, while male infertility affects ~ 7% of men. Global pregnancy and live birth rates following fertility treatment remain relatively low, while there has been an observable decline in male fertility parameters over the past 60 years. Low fertility treatment success can be attributed to poor embryonic development, poor sperm parameters and fertilisation defects. While studies from the past few years have provided evidence for the role of septins in fertility related processes, the functional role of septins and its related complexes in cellular processes such as oocyte activation, fertilization, and sperm maturation are not completely understood. This review summarizes the available knowledge on the role of septins in spermatogenesis and oocyte activation via Ca2+ regulation, and cytoskeletal dynamics throughout pre-implantation embryonic development. We aim to identify the currently less known mechanisms by which septins regulate these immensely important mechanisms with a view of identifying areas of investigation that would benefit our understanding of cell and reproductive biology, but also provide potential avenues to improve current methods of fertility treatment.

The evolutionary origins and ancestral features of septins

Septins are a family of membrane-associated cytoskeletal guanine-nucleotide binding proteins that play crucial roles in various cellular processes, such as cell division, phagocytosis, and organelle fission. Despite their importance, the evolutionary origins and ancestral function of septins remain unclear. In opisthokonts, septins form five distinct groups of orthologs, with subunits from multiple groups assembling into heteropolymers, thus supporting their diverse molecular functions. Recent studies have revealed that septins are also conserved in algae and protists, indicating an ancient origin from the last eukaryotic common ancestor. However, the phylogenetic relationships among septins across eukaryotes remained unclear. Here, we expanded the list of non-opisthokont septins, including previously unrecognized septins from glaucophyte algae. Constructing a rooted phylogenetic tree of 254 total septins, we observed a bifurcation between the major non-opisthokont and opisthokont septin clades. Within the non-opisthokont septins, we identified three major subclades: Group 6 representing chlorophyte green algae (6A mostly for species with single septins, 6B for species with multiple septins), Group 7 representing algae in chlorophytes, heterokonts, haptophytes, chrysophytes, and rhodophytes, and Group 8 representing ciliates. Glaucophyte and some ciliate septins formed orphan lineages in-between all other septins and the outgroup. Combining ancestral-sequence reconstruction and AlphaFold predictions, we tracked the structural evolution of septins across eukaryotes. In the GTPase domain, we identified a conserved GAP-like arginine finger within the G-interface of at least one septin in most algal and ciliate species. This residue is required for homodimerization of the single Chlamydomonas septin, and its loss coincided with septin duplication events in various lineages. The loss of the arginine finger is often accompanied by the emergence of the α0 helix, a known NC-interface interaction motif, potentially signifying the diversification of septin-septin interaction mechanisms from homo-dimerization to hetero-oligomerization. Lastly, we found amphipathic helices in all septin groups, suggesting that membrane binding is an ancestral trait. Coiled-coil domains were also broadly distributed, while transmembrane domains were found in some septins in Group 6A and 7. In summary, this study advances our understanding of septin distribution and phylogenetic groupings, shedding light on their ancestral features, potential function, and early evolution.

The Evolutionary Origins and Ancestral Features of Septins

Septins are a family of membrane-associated cytoskeletal GTPases that play crucial roles in various cellular processes, such as cell division, phagocytosis, and organelle fission. Despite their importance, the evolutionary origins and ancestral function of septins remain unclear. In opisthokonts, septins form five distinct groups of orthologs, with subunits from multiple groups assembling into heteropolymers, thus supporting their diverse molecular functions. Recent studies have revealed that septins are also conserved in algae and protists, indicating an ancient origin from the last eukaryotic common ancestor. However, the phylogenetic relationships among septins across eukaryotes remained unclear. Here, we expanded the list of non-opisthokont septins, including previously unrecognized septins from rhodophyte red algae and glaucophyte algae. Constructing a rooted phylogenetic tree of 254 total septins, we observed a bifurcation between the major non-opisthokont and opisthokont septin clades. Within the non-opisthokont septins, we identified three major subclades: Group 6 representing chlorophyte green algae (6A mostly for species with single septins, 6B for species with multiple septins), Group 7 representing algae in chlorophytes, heterokonts, haptophytes, chrysophytes, and rhodophytes, and Group 8 representing ciliates. Glaucophyte and some ciliate septins formed orphan lineages in-between all other septins and the outgroup. Combining ancestral-sequence reconstruction and AlphaFold predictions, we tracked the structural evolution of septins across eukaryotes. In the GTPase domain, we identified a conserved GAP-like arginine finger within the G-interface of at least one septin in most algal and ciliate species. This residue is required for homodimerization of the single Chlamydomonas septin, and its loss coincided with septin duplication events in various lineages. The loss of the arginine finger is often accompanied by the emergence of the α0 helix, a known NC-interface interaction motif, potentially signifying the diversification of septin-septin interaction mechanisms from homo-dimerization to hetero-oligomerization. Lastly, we found amphipathic helices in all septin groups, suggesting that curvature-sensing is an ancestral trait of septin proteins. Coiled-coil domains were also broadly distributed, while transmembrane domains were found in some septins in Group 6A and 7. In summary, this study advances our understanding of septin distribution and phylogenetic groupings, shedding light on their ancestral features, potential function, and early evolution.

The structure of a tetrameric septin complex reveals a hydrophobic element essential for NC-interface integrity

The septins of the yeast Saccharomyces cerevisiae assemble into hetero-octameric rods by alternating interactions between neighboring G-domains or N- and C-termini, respectively. These rods polymerize end to end into apolar filaments, forming a ring beneath the prospective new bud that expands during the cell cycle into an hourglass structure. The hourglass finally splits during cytokinesis into a double ring. Understanding these transitions as well as the plasticity of the higher order assemblies requires a detailed knowledge of the underlying structures. Here we present the first X-ray crystal structure of a tetrameric Shs1-Cdc12-Cdc3-Cdc10 complex at a resolution of 3.2 Å. Close inspection of the NC-interfaces of this and other septin structures reveals a conserved contact motif that is essential for NC-interface integrity of yeast and human septins in vivo and in vitro. Using the tetrameric structure in combination with AlphaFold-Multimer allowed us to propose a model of the octameric septin rod.

An in silico approach to determine inter-subunit affinities in human septin complexes

The septins are a conserved family of filament-forming guanine nucleotide binding proteins, often named the fourth component of the cytoskeleton. Correctly assembled septin structures are required for essential intracellular processes such as cytokinesis, vesicular transport, polarity establishment, and cellular adhesion. Structurally, septins belong to the P-Loop NTPases but they do not mediate signals to effectors through GTP binding and hydrolysis. GTP binding and hydrolysis are believed to contribute to septin complex integrity, but biochemical approaches addressing this topic are hampered by the stability of septin complexes after recombinant expression and the lack of nucleotide-depleted complexes. To overcome this limitation, we used a molecular dynamics-based approach to determine inter-subunit binding free energies in available human septin dimer structures and in their apo forms, which we generated in silico. The nucleotide in the GTPase active subunits SEPT2 and SEPT7, but not in SEPT6, was identified as a stabilizing element in the G interface. Removal of GDP from SEPT2 and SEPT7 results in flipping of a conserved Arg residue and disruption of an extensive hydrogen bond network in the septin unique element, concomitant with a decreased inter-subunit affinity. Based on these findings we propose a singular "lock-hydrolysis" mechanism stabilizing human septin filaments.

Effect of 11-Deoxycorticosterone in the Transcriptomic Response to Stress in Rainbow Trout Skeletal Muscle

In aquaculture, many stressors can negatively affect growth in teleosts. It is believed that cortisol performs glucocorticoid and mineralocorticoid functions because teleosts do not synthesize aldosterone. However, recent data suggest that 11-deoxycorticosterone (DOC) released during stress events may be relevant to modulate the compensatory response. To understand how DOC modifies the skeletal muscle molecular response, we carried out a transcriptomic analysis. Rainbow trout (Oncorhynchus mykiss) were intraperitoneally treated with physiological doses of DOC in individuals pretreated with mifepristone (glucocorticoid receptor antagonist) or eplerenone (mineralocorticoid receptor antagonist). RNA was extracted from the skeletal muscles, and cDNA libraries were constructed from vehicle, DOC, mifepristone, mifepristone plus DOC, eplerenone, and eplerenone plus DOC groups. The RNA-seq analysis revealed 131 differentially expressed transcripts (DETs) induced by DOC with respect to the vehicle group, mainly associated with muscle contraction, sarcomere organization, and cell adhesion. In addition, a DOC versus mifepristone plus DOC analysis revealed 122 DETs related to muscle contraction, sarcomere organization, and skeletal muscle cell differentiation. In a DOC versus eplerenone plus DOC analysis, 133 DETs were associated with autophagosome assembly, circadian regulation of gene expression, and regulation of transcription from RNA pol II promoter. These analyses indicate that DOC has a relevant function in the stress response of skeletal muscles, whose action is differentially modulated by GR and MR and is complementary to cortisol.