A point mutation in Semaphorin 4A associates with defective endosomal sorting and causes retinal degeneration

The role of semaphorins in allergic diseases

Semaphorins were originally identified as guidance molecules in neural development. However, accumulating evidence indicates that 'immune semaphorins' are critically involved in regulating immune cell activation, differentiation, mobility and migration. Semaphorins are also intimately associated with the pathogenesis of allergic diseases including asthma, allergic rhinitis, atopic dermatitis, allergic conjunctivitis, and eosinophilic chronic rhinosinusitis. Interestingly, reflecting their function in positive or negative regulation of immune cells, levels of some semaphorins are increased while others are decreased in patients with allergic diseases. This review presents the pathogenic functions of immune semaphorins in allergic inflammation and discusses the potential use of these molecules as therapeutic targets for allergic diseases.

Class IV semaphorins in disease pathogenesis

Semaphorins are a large family of secreted and/or transmembrane proteins, originally identified as proteins that function in axon guidance during neuronal development. However, semaphorins play crucial roles in other physiological and pathological processes, including immune responses, angiogenesis, maintenance of tissue homeostasis, and cancer progression. Class IV semaphorins may be present as transmembrane and soluble forms and are implicated in the pathogenesis of various diseases. This review discusses recent progress on the roles of class IV semaphorins determined by clinical and experimental pathology studies.

Body-fat sensor triggers ribosome maturation in the steroidogenic gland to initiate sexual maturation in Drosophila

Fat stores are critical for reproductive success and may govern maturation initiation. Here, we report that signaling and sensing fat sufficiency for sexual maturation commitment requires the lipid carrier apolipophorin in fat cells and Sema1a in the neuroendocrine prothoracic gland (PG). Larvae lacking apolpp or Sema1a fail to initiate maturation despite accruing sufficient fat stores, and they continue gaining weight until death. Mechanistically, sensing peripheral body-fat levels via the apolipophorin/Sema1a axis regulates endocytosis, endoplasmic reticulum remodeling, and ribosomal maturation for the acquisition of the PG cells' high biosynthetic and secretory capacity. Downstream of apolipophorin/Sema1a, leptin-like upd2 triggers the cessation of feeding and initiates sexual maturation. Human Leptin in the insect PG substitutes for upd2, preventing obesity and triggering maturation downstream of Sema1a. These data show how peripheral fat levels regulate the control of the maturation decision-making process via remodeling of endomembranes and ribosomal biogenesis in gland cells.

Secretory proteostasis of the retinal pigmented epithelium: Impairment links to age-related macular degeneration

Secretory proteostasis integrates protein synthesis, processing, folding and trafficking pathways that are essential for efficient cellular secretion. For the retinal pigment epithelium (RPE), secretory proteostasis is of vital importance for the maintenance of the structural and functional integrity of apical (photoreceptors) and basal (Bruch's membrane/choroidal blood supply) sides of the environment it resides in. This integrity is achieved through functions governed by RPE secreted proteins, which include extracellular matrix modelling/remodelling, angiogenesis and immune response modulation. Impaired RPE secretory proteostasis affects not only the extracellular environment, but leads to intracellular protein aggregation and ER-stress with subsequent cell death. Ample recent evidence implicates dysregulated proteostasis as a key factor in the development of age-related macular degeneration (AMD), the leading cause of blindness in the developed world, and research aiming to characterise the roles of various proteins implicated in AMD-associated dysregulated proteostasis unveiled unexpected facets of the mechanisms involved in degenerative pathogenesis. This review analyses cellular processes unveiled by the study of the top 200 transcripts most abundantly expressed by the RPE/choroid in the light of the specialised secretory nature of the RPE. Functional roles of these proteins and the mechanisms of their impaired secretion, due to age and genetic-related causes, are analysed in relation to AMD development. Understanding the importance of RPE secretory proteostasis in relation to maintaining retinal health and how it becomes impaired in disease is of paramount importance for the development and assessment of future therapeutic advancements involving gene and cell therapies.

The cell biology of the retinal pigment epithelium

The retinal pigment epithelium (RPE), a monolayer of post-mitotic polarized epithelial cells, strategically situated between the photoreceptors and the choroid, is the primary caretaker of photoreceptor health and function. Dysfunction of the RPE underlies many inherited and acquired diseases that cause permanent blindness. Decades of research have yielded valuable insight into the cell biology of the RPE. In recent years, new technologies such as live-cell imaging have resulted in major advancement in our understanding of areas such as the daily phagocytosis and clearance of photoreceptor outer segment tips, autophagy, endolysosome function, and the metabolic interplay between the RPE and photoreceptors. In this review, we aim to integrate these studies with an emphasis on appropriate models and techniques to investigate RPE cell biology and metabolism, and discuss how RPE cell biology informs our understanding of retinal disease.

On variants and disease-causing mutations: Case studies of a SEMA4A variant identified in inherited blindness

The p.R713Q variant of the semaphorin-4a-encoding gene, SEMA4a, has been reported to cause autosomal dominant retinitis pigmentosa. Here we show three families with retinal degeneration in which unaffected family members are either homozygous or heterozygous for the variant. The p.R713Q variant in SEMA4A is insufficient to cause either autosomal recessive or autosomal dominant retinitis pigmentosa and is unlikely to be pathogenic.

The role of Sema4A in angiogenesis, immune responses, carcinogenesis, and retinal systems

Semaphorins were originally identified as axon guidance cues that regulate the functional activity of axons in the nervous system. In addition, accumulating evidence indicates that semaphorins have multiple functions in physiological and pathogenic processes, including vascular development, tumor progression, and immune responses. Sema4A is a semaphorin expressed in immune cells, and is thus termed an “immune semaphorin.” Sema4A has 4 types of receptors: Plexin D family, Plexin B family, Tim-2, and Nrp-1. Recent studies suggest that Sema4A plays critical roles in many processes including cell–cell interactions, immune-cell activation, differentiation, and migration. In other studies, Sema4A is also associated with carcinogenesis and retinal systems. In this review, we summarize current knowledge regarding the biology of Sema4A in relation to angiogenesis, immune responses, colorectal cancer, and the retina.

A role of Heat Shock Protein 70 in Photoreceptor Cell Death: Potential as a Novel Therapeutic Target in Retinal Degeneration

Retinal degenerative diseases (RDs) such as retinitis pigmentosa (RP) are a genetically heterogeneous group of disorders characterized by night blindness and peripheral vision loss, which caused by the dysfunction and death of photoreceptor cells. Although many causative gene mutations have been reported, the final common end stage is photoreceptor cell death. Unfortunately, no effective treatments or therapeutic agents have been discovered. Heat shock protein 70 (HSP70) is highly conserved and has antiapoptotic activities. A few reports have shown that HSP70 plays a role in RDs. Thus, we focused on the role of HSP70 in photoreceptor cell death. Using the N-methyl-N-nitrosourea (MNU)-induced photoreceptor cell death model in mice, we could examine two stages of the novel cell death mechanism; the early stage, including HSP70 cleavage through protein carbonylation by production of reactive oxygen species, lipid peroxidation and Ca(2+) influx/calpain activation, and the late stage of cathepsin and/or caspase activation. The upregulation of intact HSP70 expression by its inducer is likely to protect photoreceptor cells. In this review, we focus on the role of HSP70 and the novel cell death signaling process in RDs. We also describe candidate therapeutic agents for RDs.

Congenital diseases and semaphorin signaling: overview to date of the evidence linking them

Semaphorins and their receptors, neuropilins and plexins, were initially characterized as a modulator of axonal guidance during development, but are now recognized as a regulator of a wide range of developmental events including morphogenesis and angiogenesis, and activities of the immune system. Owing to the development of next-generation sequencing technologies together with other useful DNA assays, it has also become clear that semaphorin signaling plays a crucial role in many congenital diseases such as retinal degeneration and congenital heart defects. This review summarizes the recent knowledge about the relationship between a variety of congenital diseases and semaphorin signaling.

Up-regulation of semaphorin 4A expression in human retinal pigment epithelial cells by PACAP released from cocultured neural cells

Development and homeostasis of multicellular organisms require interactions between neighbouring cells. We recently established an in vitro model of cell-cell interaction based on a collagen vitrigel membrane. We have now examined the role of neural cells in retinal homeostasis by coculture of human retinal pigment epithelial (RPE) cells and neural cells on opposite sides of such a membrane. The neural cells (differentiated PC12 cells) induced up-regulation of semaphorin 4A (Sema4A), a member of the semaphorin family of neural guidance proteins, in RPE (ARPE19) cells. This effect of the neural cells was mimicked by the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) and was abolished by the PACAP antagonist PACAP(6-38). Coculture with neural cells or stimulation with PACAP also induced the phosphorylation of extracellular-signal-regulated kinase in ARPE19 cells, and this effect of the neural cells was inhibited by PACAP(6-38). Finally, among various cytokines examined, only the amount of interleukin-6 released by cocultures of ARPE19 and neural cells differed from that released by ARPE19 cells cultured alone. Interleukin-6 was not detected in culture supernatants of neural cells, and the reduction in the amount of interleukin-6 released by the cocultures compared with that released by ARPE19 cells alone was prevented by PACAP(6-38). Our findings suggest that PACAP released from retinal neural cells (photoreceptors or optic nerve cells) may regulate Sema4A expression in RPE cells and thereby contribute to the maintenance of retinal structure and function. Development and homeostasis of multicellular organisms require interactions between neighbouring cells. With the use of a coculture system based on a collagen vitrigel membrane, we have now shown that neural cells induce up-regulation of the neural guidance protein Sema4A in RPE cells. This effect of neural cells appears to be mediated by the neuropeptide PACAP. PACAP released from retinal neural cells (photoreceptors or optic nerve cells) may thus regulate Sema4A expression in RPE cells and thereby contribute to the maintenance of retinal structure and function.

Germline variants in the SEMA4A gene predispose to familial colorectal cancer type X

Familial colorectal cancer type X (FCCTX) is characterized by clinical features of hereditary non-polyposis colorectal cancer with a yet undefined genetic background. Here we identify the SEMA4A p.Val78Met germline mutation in an Austrian kindred with FCCTX, using an integrative genomics strategy. Compared with wild-type protein, SEMA4A(V78M) demonstrates significantly increased MAPK/Erk and PI3K/Akt signalling as well as cell cycle progression of SEMA4A-deficient HCT-116 colorectal cancer cells. In a cohort of 53 patients with FCCTX, we depict two further SEMA4A mutations, p.Gly484Ala and p.Ser326Phe and the single-nucleotide polymorphism (SNP) p.Pro682Ser. This SNP is highly associated with the FCCTX phenotype exhibiting increased risk for colorectal cancer (OR 6.79, 95% CI 2.63 to 17.52). Our study shows previously unidentified germline variants in SEMA4A predisposing to FCCTX, which has implications for surveillance strategies of patients and their families.

[The role of semaphorin family in immune systems]

Semaphorins are soluble and membrane-bound proteins originally identified as axonal growth cone-collapsing guidance molecules which are involved in the development of the neuronal system. Recently, cumulative evidences indicate that several semaphorins also participate in various phases of immune responses, both physiological and pathological. They are so-called 'immune semaphorins' such as sema3A, 3E, 4A, 4D, 6D, and 7A. Some semaphorins regulate immune cell activation or differentiation, whereas others navigate the trafficking of immune cells. Moreover, Plexin family members and neuropilins are the most representative receptors for semaphorins, which have cell type-specific patterns of expression and are involved in multiple signaling responses. At present, semaphorins and their receptors are considered to be potential diagnostic and therapeutic targets for many kinds of diseases. Here, we review the current knowledge of the function of semaphorins and their corresponding receptors in immune systems, which is especially focused on class3 and class4 semaphorins.

Immunological functions of the neuropilins and plexins as receptors for semaphorins

Semaphorins were originally identified as axon-guidance molecules that function during neuronal development. However, cumulative evidence indicates that semaphorins also participate in immune responses, both physiological and pathological, and they are now considered to be potential diagnostic and/or therapeutic targets for a range of diseases. The primary receptors for semaphorins are neuropilins and plexins, which have cell type-specific patterns of expression and are involved in multiple signalling responses. In this Review, we focus on the roles of neuropilin 1 (NRP1) and plexins in the regulation of the immune system, and we summarize recent advances in our understanding of their pathological implications.

RD3: a challenge and a promise

Photoreceptor (PR) cells are highly specialized cells that convert light into electrical signals. Ten percent of their outer segment (OS) membranes (approximately 77 cm² of membrane) are renewed every day. Therefore, PR cells must possess an extraordinary trafficking system to provide all of the needed material to build up the OS discs through a 0.3 µm diameter connecting cilium. The mechanism of trafficking of membrane proteins in the retina and corresponding degenerative diseases is still elusive. The retinal degeneration(rd3) is the gene responsible for a murine autosomal recessive hereditary retinal degeneration, which is known as Leber Congenital Amaurosis 12 (LCA12). Degeneration starts at about two weeks of age and is completed between 2-4 months. We generated the first antibody against this protein and by a protein-protein interaction analysis discovered that RD3 protein directly interacts with guanylate cyclase 1 (GC1) and partially expresses in the OS. We also detected the major binding site between these two proteins and realized that RD3 is directly involved in trafficking of this crucial protein. In a separate study, we reported that RD3 negatively regulates GC1, which is crucial for efficient trafficking of GC1 during the trafficking path, and RD3 prevents unnecessary production of cGMP. It is possible that RD3 is still involved in regulating GC1 even after targeting. Several mutations that cause visual difficulties have been reported for the mouse and human ortholog of RD3. The symptoms these mutations cause are very similar to those reported for a more severe form of blindness referred to as LCA1. Therefore, RD3 might cause a broader range of retinal diseases. Gene replacement of RD3 has shown to restore the GC1 across the retina. This makes RD3 a novel therapeutic target for retinal targeting impaired degenerative diseases.