The role of septin 7 in physiology and pathological disease: A systematic review of current status

Abstract Septins are a conserved family of cytoskeletal GTPases present in different organisms, including yeast, drosophila, Caenorhabditis elegans and humans. In humans, septins are involved in various cellular processes, including exocytosis, apoptosis, leukemogenesis, carcinogenesis and neurodegeneration. Septin 7 is unique out of 13 human septins. Mammalian septin 6, septin 7, septin 2 and septin 9 coisolate together in complexes to form the core unit for the generation of the septin filaments. Physiological septin filaments are hetero‐oligomeric complexes consisting of core septin hexamers and octamers. Furthermore, septin 7 plays a crucial role in cytokinesis and mitosis. Septin 7 is localized to the filopodia and branches of developing hippocampal neurons, and is the most abundant septin in the adult rat forebrain as well as a structural component of the human and mouse sperm annuli. Septin 7 is crucial to the spine morphogenesis and dendrite growth in neurons, and is also a structural constituent of the annulus in human and mouse sperm. It can suppress growth of some tumours such as glioma and papillary thyroid carcinoma. However, the molecular mechanisms of involvement of septin 7 in human disease, especially in the development of cancer, remain unclear. This review focuses on the structure, function and mechanism of septin 7 in vivo, and summarizes the role of septin 7 in cell proliferation, cytokinesis, nervous and reproductive systems, as well as the underlying molecular events linking septin 7 to various diseases, such as Alzheimer's disease, schizophrenia, neuropsychiatric systemic lupus erythematosus, tumour and so on.

complexes. 7,8 Septins have a unique ability to assemble into heteropolymers and form a variety of high-order structures, including filaments, loops and cages. 9 These unique structures can control cellular processes and localize at various cellular locations, 10 including the plasma membrane, 11 the annulus of spermatozoa, 12 the bases of cilia 13 and dendrites, 14 as well as surrounding invasive bacteria. 15,16 Septins have been identified as the "cell-division cycle" proteins, 17 and they play a critical role during cytokinesis. 18 It has been reported that septins are indispensable in co-ordinating myosin motor proteins and bind with non-muscle myosin II to activate myosin II in interphase and dividing cells 19,20 and reorganizingx membrane during cytokinesis, 21 and anchoring the midbody ring structure in the membrane 22 when a daughter cell separates from its mother cell. Septins can assemble into hetero-oligomeric protein complexes which can further form filaments and microscopic bundles or ring structures in vitro and in vivo to control cellular processes. Septin filaments and intermediate filaments are non-polar, distinguishing them from actin filaments and microtubules. 17 Septins have been suggested to be cytoskeletal components owing to these structural features and their association with the membrane, F-actin and microtubules. 8,23 Recent data indicate that they also serve as scaffolds which recruit factors to particular sites in a cell and/or act as blocks to separate different membrane areas into discrete domains to promote changes in cytoskeletal and membrane organization. 24 The common septin structure consists of a highly variable N-terminal domain, a central GTP-binding domain and a C-terminal domain, which normally includes sequences compatible with a coiled coil structure. 25 Septin filaments are usually 7-9 nm in width and vary in length, with unit length of 25-32 nm observed under high salt concentration. [26][27][28] G-domains can form linear filaments using either the guanine nucleotide binding site (G interface) or N-and C-terminal extensions (NC interface). 17 Furthermore, exploring the physiological significance of GTP/GDP binding and/or GTP hydrolytic activity of septins may contribute to further understanding of their structural organization and functions. 29 2 | THE STRUCTURE OF THE SEPTIN 7  30 cDNA sequence of septin 7 in humans is homologous to Cdc10 in yeast, and septin 7 in humans was even named hCdc10.
Three-dimensional X-ray structures of individual septins have shown that septin 7 shares with other septins a canonical Ras-like G-domain consisting of 6 ß-strands and 5 a-helixes. 31 29,38 . Human septins are divided into the septin 2, septin 6, septin 7 and septin 9 groups. The septin 7 group seems to be unique compared to other groups, as it contains only one member in all organisms. The absence of septin 7 will lead to loss of other septin proteins in homo-and heterooligomeric complexes, and this protein appears essential to the generation of filaments. 8  | 3299 septin 2/6/7, 8,24 septin 7/9b/11 29 and septin 5/7/11. 14,40-42 Septin 2/6/7 hetero-polymer is the only one of septin 7-associated complexes for which a crystal structure is currently available. 31 Septin 2/6/7 is the most abundant septin complex out of those affinity purified from brain tissues or HeLa cells. 8  F I G U R E 1 A, Organization chart of the septin2/6/7 complex. Septin 7 is a core component of septin 2/6/7. Septin 2/6/7 is the most abundant septin complex and the structure of the complex shows a universal bipolar polymer, composed of an extended G domain and/or the N-and C-terminal extensions. 24 Kinoshita et al identified that septin 2/6/7 is a non-polar hexamer and 2 copies of each septin symmetrically arranged (septin 7/septin 6/septin 2/septin 2/septin 6/septin 7). B, Organization chart of the septin7/9b/11 complex. Septin 9b binds to C termini of both septin 7 and septin 11 through its long N-terminal extension and septin 7/9b/11 forms a filamentous pattern along actin stress fibres in the actin filament-dependent manner. Structure of the filaments containing septins 7/9b/11 depends on the integrity of actin filaments in REF52 cells. C, Organization chart of the septin5/7/11 complex. Septin 5 and septin 11 can colocalize and coimmunoprecipitate with septin 7. The existence of septin 5/7/11 complexes does not contradict the existence of previously reported septin 2/6/7 or septin 7/9b/11 complexes. D, Schematic Organization chart of the role of septin 7 in different diseases and the related regulation mechanism. The common structure of the septin 7 in human and other organisms and its molecular mechanism of action in physiology and disease pathology was summarized in various diseases including Alzheimer's disease (AD), schizophrenia, neuropsychiatric systemic lupus erythematosus (NPSLE), glioma, papillary thyroid carcinoma (PTC) and hepatocellular carcinoma (HCC) The in vitro septin 7-septin 6-septin 2-septin 2-septin 6-septin 7 hexamer is an incomplete mammalian septin complex. Mammalian septin complexes can form octamers that are arranged as setpin 9septin 7-septin 6-septin 2-septin 2-septin 6-septin 7-septin 9. Septin 9 occupies the ends of an octameric mammalian septin complex. 44 Septin 9b binds to C termini of both septin 7 and septin 11 through its long N-terminal extension, which lacks a predicted coiled-coil region and does not contain any predicted domain structure. 29 Nagata et al argued that septin 7/9b/11 forms a filamentous pattern along actin stress fibres which is distributed in REF52 cells (rat embryonic fibroblast cells) in the actin filament-dependent manner ( Figure 1B). The 3 septins interact in vitro and in vivo. Structure of the filaments containing septins 7/9b/11 depends on the integrity of actin filaments in REF52 cells. 29 Moreover, septin 5 and septin 11 can colocalize and coimmunoprecipitate with septin 7, and expression levels of both septin 5 and septin 11 decreased in septin 7-deficient neurons. 14,40 These data suggest the existence of a septin 5/7/11 complex in neuronal dendrites, a result consistent with an earlier finding that septin 7 level is significantly decreased in homozygotic septin 5 null mice. 41 Interestingly, several human septins might be exchangeable in septin complexes. 14,40 It has been suggested that septin 2 can be replaced by septin 5 (or septin 1/septin 4) and septin 6 by septin 11 (or possibly by septin 8/septin 10) in a septin 2/6/7 complex. 25 Therefore, the existence of septin 5/7/11 complexes does not contradict the existence of previously reported septin 2/6/7 or septin 7/9b/11 complexes, and is consistent with earlier findings that down-regulation of septin 7 decreased the expression of other septin complex members ( Figure 1C). 8,42 Septin 7 occupies terminal positions in above mentioned hexamers, 31 which is further associated with forming non-polar linear septin filaments. 14,17 Septin complexes have been purified from human tissues, and some components have been identified, but mutual influences of the septins in the complexes have not been studied.

| SEPTIN 7 AND INTRACE LLULAR CALCIUM
Septin 7 can be regarded as a novel regulator of neuronal Ca 2+ homoeostasis based on physiological and behavioural phenotypes. 43 Drosophila has been identified to have 5 septin-encoding genes [45][46][47] including dseptin 7, a homologue of human septin 7. The septin 7 group is unique because it consists of a single protein both in drosophila and in humans. 17,47 Overexpression of dseptin 7 in neurons of wild-type drosophila causes significant flight defects. 43 Furthermore, knockdown or partial genetic depletion of dseptin 7 rescues the flight defects of animals when the reduction of inositol-1,4,5-trisphosphate receptor (IP3R), a protein which contributes to the release of intracellular Ca 2+ , 48,49 and the septin7 in drosophlia (dseptin7)-deficient can compensate for the lessened function of IP3R. 43

NERVOUS SYSTEM
Septin 7 is widely distributed in the brain 39  Septin 7 interacting with HDAC6 decreases the microtubule stability during the formation of the collateral in cortical neurons. 61 In addition, septin 7 can maintain the shape of the dorsal root ganglia (DRG) neuron and their bipolar processes, meanwhile, septin 7 is required for regulating the polarity of cortical neuron rather than the DRG neuron. 60 Overall, down-regulation of septin 7 alters the morphology of dendritic protrusions in mature neurons. 40 Interestingly, dendrites can be altered to form elongated protuberances, similar to the elongated buds of the septin-deficient yeast cells. 62 These similarities indicate that septin 7 plays a conserved function in neurons. Similar to that in yeast bud neck, it may be a key component of the dendrite diffusion barrier. This function will be a long sought after molecular association to explain the fact that dendrites are unique compartments, and that their motility is crucial for synaptic plasticity.
Alzheimer's disease (AD) is a chronic neurodegenerative disease that usually starts slowly and worsens over time. 63 The pathological change in the AD is cerebral cortex atrophy, loss of cortical neurons, accumulation and progressive deposition of b-amyloid in brain parenchyma and vessel wall, glial proliferation and formation of neurofibrillary tangles accompanied by the appearance of hyperphosphorylated tau. 64 The cyclin-dependent kinase 5 (Cdk5) is a tau kinase up-regulated in AD, 65 and the Cdk5 activator p35 can be cleaved to p25 to increase the Cdk5 activity. 66 Interestingly, expression of p25 varies depending on AD stage. Reduction of p25 levels contributes to memory formation in early AD, 67,68 while the ongoing overexpression of p25 leads to neurodegeneration in later AD. 69,70 Septin 7 has been suggested to be a p25-regulated protein localized in spine necks, where it can control the formation and shape of spines. 12,38,71 Low expression of septin 7 induces lower spine density and increased size of spines in vivo. 14,40 Interestingly, septin 7 expression was specifically up-regulated in female, but not male mice. Septin 7 can increase the spine density and reduce spine size in female p25 transgenic mice. 72 Meanwhile, in water maze experiments septin 7 level was increased in trained mice but not in control mice, which indicates that septin 7 expression is up-regulated during spatial memory formation. 66 As the synapse has been identified to be affected in early AD 73  Cdk5 signalling can alter expression of various candidate genes related to schizophrenia. [74][75][76] Reduction of the level of p35, as Cdk5 activator, 66 is sufficient to reduce septin 7 expression. 72 Cdk5 activity can be regulated by the glutamatergic and dopaminergic cell signalling. [77][78][79][80] In addition,the dysfunction of the TAOK2 and septins are found in the neurofibrillary tangles in AD, 81 which may offer a novel pathway to treat AD. Schizophrenia is a common and chronic psychiatric disorder with imprecise etiopathogenesis 82 and has various symptoms, including disorders in sensory perception, cognition, emotions and behaviour, that could be induced by the glutamatergic dysfunction and dopaminergic disbalance. 83 Septin 7 expression is changed in schizophrenia post-mortem tissue and down-regulated in the prefrontal cortex, but not the hippocampus, and expression of p35 and septin 7 are not altered by clozapine treatment. Clozapine is a kind of antipsychotics by regulating dopamine receptor D2 and other neurotransmitter receptors. 72 Low expression of septin 7 leads to the abnormal spine density in the prefrontal cortex of schizophrenia patients. 84,85 Interestingly, septin 7 levels are reduced in prefrontal cortex of male mice, but not in female mice 86 88 This difference may reflect the deletion of pathogenic antibodies associated with alteration of brain tissue or the lack of regulatory antibodies required for maintaining neuroprotection, 88 and it has been previously identified in normal brain tissue by control sera. 71 Septin 7 is involved in neuronal microtubule stability, suggesting a role of microtubules in the pathophysiology of NPSLE. 88 Taken together, septin 7 can be used to investigate the mechanism of the NPSLE.

REPRODUCTIVE SYSTEM
Septin 7 is a structural constituent of the annulus of mouse and human sperm. 89  | 3303 compared to control tumours, and proliferation of the septin 7-transfected U251 cells was significantly lower than that of control cells. 104 These studies suggest that septin 7 can inhibit the growth and proliferation and induce apoptosis in glioma cells acting as a tumour-suppressor protein. In xenograft tumours in mice treated with septin 7, proliferating cell nuclear antigen (PCNA) is down-regulated while glial fibrillary acidic protein (GFAP) is up-regulated. 104 In addition, down-regulation of Bcl-2 and up-regulation of caspase-3 may indicate that septin 7 functions as a tumour suppressor in glioma. 104 Hence, inhibition of glioma cell proliferation or promotion of apoptosis by septin 7 may be regulated by the positive or negative cell-cycle regulators. 104 Furthermore, up-regulation of GFAP in TJ905 and U251 xenograft tumours treated with septin 7 indicates that septin 7 can reverse the glioma phenotypes in differentiation. 110 Down-regulation of MMP2/9, MTI-MMP, 99  MiR-30a-5p is a small non-coding RNA (microRNA) that may facilitate the formation of glioma since its expression is up-regulated in glioma cell lines and specimens. 105 Septin 7 gene contains the highly conserved putative binding sites to miR-30a-5p which regulate the post-transcriptional expression of septin 7. 105 Septin 7 expression in control glioma cells is much lower than in glioma cells treated with miR-30a-5p antisense oligonucleotide. Septin 7 can be negatively regulated by miR-30a-5p during its translation. 105 Furthermore, adenovirus-mediated overexpression of septin 7 can partly reverse the increased glioma cells growth because of the down-regulation of miR-30a-5p. 105 Hence, there is an inverse correlation between septin 7 and miR-30a-5p, and miR-30a-5p decreases septin 7 expression at the translational level in glioma cells.  111 Expression of septin 7 and its subcellular location have been shown to be associated with specific subtypes of PTC. 111 Nuclear, cytoplasmic and overall septin 7 expression were much lower in FVPTC tissues in contrast with benign hyperfunctioning thyroid nodules. In CVPTC group, the septin 7 expression was only decreased in the nucleus while its overall and cytoplasmic expressions were stable. 106 The difference in septin 7 expression patterns between FVPTC and CVPTC may be associated with different molecular regulatory mechanisms and signalling pathways. 99

| CONCLUSION
As a highly evolutionarily conserved GTPase, septin 7 is a member of septin family which includes 13 human septins involving in exocytosis, apoptosis, leukemogenesis, carcinogenesis and neurodegeneration. 105 Septin 7 can combine with other septins to form heteropolymers and is a core component of these multimeric septin complexes. 39 These heteropolymers can form a diverse array of higher order structures which include filaments, gauzes and rings. 9 However, the function and molecular mechanism of action of these heteropolymers have not received enough attention. In this review, we described the common structure of the septin 7 in human and other organisms and its molecular mechanism of action in physiology and disease pathology, summarized recent studies of the function of septin 7 in nervous and reproductive systems and showed its diverse functions in various diseases including AD, schizophrenia, NPSLE, glioma, PTC and HCC ( Figure 1D). The role of septin 7 in physiology and disease pathology may provide us novel ideas for exploration of the therapeutic targets in human disease.

ACKNOWLEDG EMENTS
This work was supported in part by grants from the National Science Foundation of China (#81472729 and #81672426), the Foundation of Tianjin Health Bureau (15KG112) and the Foundation of Committee on Science and Technology of Tianjin (17YFZCSY00700 and 17ZXMFSY00120).

CONFLI CT OF INTEREST
The authors declare that there is no conflict of interest.