Neutralization of vascular endothelial growth factor antiangiogenic isoforms is more effective than treatment with proangiogenic isoforms in stimulating vascular development and follicle progression in the perinatal rat ovary

Primordial Follicle Survival and Changes in Ovarian Vasculature May Be Independently Regulated During Chemotherapy

Chemotherapeutic agents induce irreversible gonadotoxic side effects, resulting in endocrine dysfunction and infertility in female cancer survivors. In the current study, we investigated strategies to protect ovarian function from chemotherapy-induced toxicity by evaluating the effects of cisplatin, doxorubicin, or cyclophosphamide on ovarian vasculature and primordial follicle survival. This investigation was conducted using adult CD-1, PDs 5-7 CD-1, and oocyte-specific Trp63 conditional knockout (Trp63 cKO) mice that demonstrated primordial follicles survived following chemotherapy. In control ovaries, vasculature typically surrounds primordial and primary follicles, is in the theca layer as secondary follicles develop, and is distributed among stromal cells. Our findings revealed that the expression pattern of CD31/PECAM-1 (platelet endothelial adhesion molecule-1) was significantly altered in ovaries treated with chemotherapeutic agents compared with controls. The data demonstrate that these agents not only caused the loss of ovarian follicles but also damaged the ovarian vasculature. Using Trp63 cKO mice and CK2II, an inhibitor of checkpoint kinase 2, we demonstrated that vascular damage can occur independently of primordial follicle loss, and VEGFA165 was unable to prevent either outcome. This indicates that the mechanisms governing the death of primordial follicles and vascular damage may not directly affect each other. Long-term ex vivo culture and in vivo experiments demonstrated the ability of ovarian vasculature to recover from cisplatin-induced damage. In conclusion, our study suggests that ovarian follicle survival and vascular integrity may be independently regulated as independent processes, governed by distinct signaling pathway or mechanisms.

The causal association between systemic inflammatory regulators and primary ovarian insufficiency: a bidirectional mendelian randomization study

BACKGROUND

Recent studies have suggested a potential link between systemic inflammatory regulators and primary ovarian insufficiency (POI); however, a causal relationship between them remains unclear. In this study, we explored the causal link between systemic inflammatory regulators and POI risk using a bidirectional, two-sample Mendelian randomization (MR) strategy.

RESULTS

This approach utilized the most extensive genome-wide association study involving 41 systemic inflammatory regulators in a sample of 8,293 Finnish individuals and POI data from the FinnGen consortium (254 cases vs. 118,228 controls). The inverse variance weighting approach served as a primary MR method, and four additional MR techniques (Maximum Likelihood, MR-Egger, Weighted Median, and constrained maximum likelihood and model averaging Bayesian information criterion ) were applied to support and validate results. Cochran's Q statistics were used to assess the heterogeneity of instrumental variables, whereas the MR-Egger and MR Pleiotropy Residual Sum and Outlier tests detected horizontal pleiotropy. The MR Steiger test evaluated the strength of a causal association. Our findings suggest that lower levels of vascular endothelial growth factor (odds ratio [OR] = 0.73, 95% confidence interval [CI]: 0.54-0.99, P = 0.046) and interleukin-10 (OR = 0.54, 95% CI: 0.33-0.85, P = 0.021) are associated with an increased risk of POI. Reverse MR analysis revealed no significant effect of POI on the expression of these 41 systemic inflammatory regulators. No notable heterogeneity or horizontal pleiotropy was observed in the instrumental variables.

CONCLUSIONS

This study revealed a causal association between 41 systemic inflammatory regulators and POI, demonstrating that decreased levels of VEGF and IL-10 are linked to an elevated risk of POI. Further investigations are necessary to assess the potential of these biomarkers as early predictors, preventive strategies, and therapeutic targets for POI.

The vascular endothelial growth factor (VEGF) system as a key regulator of ovarian follicle angiogenesis and growth

The vascular endothelial growth factor-A (VEGFA) system is a complex set of proteins, with multiple isoforms and receptors, including both angiogenic (VEGFxxx, VEGFR2) and antiangiogenic members (VEGFxxxb, VEGFR1 and soluble forms of VEGFR). The members of the VEGF system affect the proliferation, survival, and migration of endothelial and nonendothelial cells and are involved in the regulation of follicular angiogenesis and development. The production of VEGF by secondary follicles stimulates preantral follicular development by directly affecting follicular cells and promoting the acquisition of the follicular vasculature and downstream antrum formation. Additionally, the pattern of expression of the components of the VEGF system may provide a proangiogenic milieu capable of triggering angiogenesis and stimulating follicular cells to promote antral follicle growth, whereas, during atresia, this milieu becomes antiangiogenic and blocks follicular development.

How VEGF-A and its splice variants affect breast cancer development - clinical implications

BACKGROUND

Altered expression levels and structural variations in the vascular endothelial growth factor (VEGF) have been found to play important roles in cancer development and to be associated with the overall survival and therapy response of cancer patients. Particularly VEGF-A and its splice variants have been found to affect physiological and pathological angiogenic processes, including tumor angiogenesis, correlating with tumor progression, mostly caused by overexpression. This review focuses on the expression and impact of VEGF-A splice variants under physiologic conditions and in tumors and, in particular, the distribution and role of isoform VEGF165b in breast cancer.

CONCLUSIONS AND PERSPECTIVES

Many publications already highlighted the importance of VEGF-A and its splice variants in tumor therapy, especially in breast cancer, which are summarized in this review. Furthermore, we were able to demonstrate that cytoplasmatic VEGFA/165b expression is higher in invasive breast cancer tumor cells than in normal tissues or stroma. These examples show that the detection of VEGF splice variants can be performed also on the protein level in formalin fixed tissues. Although no quantitative conclusions can be drawn, these results may be the starting point for further studies at a quantitative level, which can be a major step towards the design of targeted antibody-based (breast) cancer therapies.

Vascular remodeling by placenta-derived mesenchymal stem cells restores ovarian function in ovariectomized rat model via the VEGF pathway

Angiogenesis plays an important role in damaged organ or tissue and cell regeneration and ovarian development and function. Primary ovarian insufficiency (POI) is a prevalent pathology in women under 40. Conventional treatment for POI involves hormone therapy. However, due to its side effects, an alternative approach is desirable. Human mesenchymal stem cells (MSCs) from various sources restore ovarian function; however, they have many limitations as stem cell sources. Therefore, it is desirable to study the efficacy of placenta-derived MSCs (PD-MSCs), which possess many advantages over other MSCs, in a rat model of ovarian dysfunction. Here, we investigated the restorative effect of PD-MSCs on injured ovaries in ovariectomized (OVX) rats and the ability of intravenous transplantation (Tx) of PD-MSCs (5 × 105) to enhance ovarian vasculature and follicular development. ELISA analysis of serum revealed that compared to the non-transplantation (NTx) group, the Tx group showed significantly increased levels of anti-Müllerian hormone, follicle stimulating hormone, and estradiol (E2) (*P < 0.05). In addition, histological analysis showed more mature follicles and less atresia and restoration of expanded blood vessels in the ovaries of the OVX PD-MSC Tx group than those of the NTx group (*P < 0.05). Furthermore, folliculogenesis-related gene expression was also significantly increased in the PD-MSC Tx group (*P < 0.05). Vascular endothelial growth factor (VEGF) and VEGF receptor 2 expressions were increased in the ovaries of the OVX PD-MSC Tx group compared to the NTx group through PI3K/AKT/mTOR and GSK3β/β-catenin pathway activation. Interestingly, ex vivo cocultivation of damaged ovaries and PD-MSCs or treatment with recombinant VEGF (50 ng/ml) increased folliculogenic factors and VEGF signaling pathways. Notably, compared to recombinant VEGF, PD-MSCs significantly increased folliculogenesis and angiogenesis (*P < 0.05). These findings suggest that VEGF secreted by PD-MSCs promotes follicular development and ovarian function after OVX through vascular remodeling. Therefore, these results provide fundamental data for understanding the therapeutic effects and mechanism of stem cell therapy based on PD-MSCs and provide a theoretical foundation for their application for obstetrical and gynecological diseases, including infertility and menopause.

The protective effect of fenofibrate, triptorelin, and their combination against premature ovarian failure in rats

Cyclophosphamide (CP) is a chemotherapy alkylating agent that causes a lot of side effects including premature ovarian failure (POF). This study aimed to evaluate the possible protective effect of fenofibrate (FEN) in CP-induced POF. Rats were randomly divided into five groups as follows: negative control, CP, triptorelin (TRI)-treated, FEN (FEN)-treated, and FEN + TRI-treated. Histological study, collagen area fraction, and immunoexpression of proliferating cell nuclear antigen (PCNA) were evaluated. Also, estrogen, anti-mullerian hormone (AMH), follicle-stimulating hormone (FSH), luteinizing hormone (LH) and ovarian malondialdehyde (MDA), nitric oxide (NOx), reduced glutathione (GSH), superoxide dismutase (SOD), interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-α), and vascular endothelial growth factor (VEGF) were measured. CP significantly reduced ovarian follicle count, as compared with the control group (1.00 ± 0.76 versus 7.75 ± 1.83, respectively). Meanwhile, FEN, either solely or in combination with TRI, significantly increased ovarian follicle count, as compared with the CP group (3.88 ± 0.83 and 5.75 ± 1.39, respectively). As compared with the control group, CP increased the levels of MDA, NOx, IL-10, TNF-α, FSH, LH, and collagen area fraction; however, levels of GSH, SOD, VEGF, AMH, estrogen, and PCNA immunoexpression were reduced with CP. Administration of FEN either solely or in combination with TRI showed significant improvement in all the parameters previously mentioned. FEN can protect the ovary from CP-induced side effects possibly through antioxidant and anti-inflammatory actions.

Debunking the Myth of the Endogenous Antiangiogenic Vegfaxxxb Transcripts

In this opinion article we critically assess evidence for the existence of a family of antiangiogenic vascular endothelial growth factor (Vegfaxxxb) transcripts, arising from the use of a phylogenetically conserved alternative distal splice site within exon 8 of the VEGFA gene. We explain that prior evidence for Vegfaxxxb transcripts in tissues rests heavily upon flawed RT-PCR methodologies, with the extensive use of 5'-tailing in primer design being the main issue. Furthermore, our analysis of large RNA-seq data sets (human and ovine) fails to identify a single Vegfaxxxb transcript. Therefore, we challenge the very existence of Vegfaxxxb transcripts, which further questions the physiological relevance of studies based on the use of 'anti-VEGFAxxxb' antibodies. Our analysis has implications for the proposed therapeutic use of isoform-specific anti-VEGFA strategies for treating cancer and retinopathies.

Vascular endothelial growth factor A isoforms modulate follicle development in peripubertal heifers independent of diet through diverse signal transduction pathways

Follicular progression during peripuberty is affected by diet. Vascular endothelial growth factor A (VEGFA) induces follicle progression in many species; however, there are limited studies to determine if diet may alter the effects of angiogenic VEGFA165-stimulated follicle progression or antiangiogenic VEGFA165b follicle arrest. We hypothesized that diet affects the magnitude of angiogenic and antiangiogenic VEGFA isoform actions on follicular development through diverse signal transduction pathways. To test this hypothesis, beef heifers in our first trial received Stair-Step (restricted and refeeding) or control diets from 8 to 13 months of age. Ovaries were collected to determine follicle stages, measure vascular gene expression and conduct ovarian cortical cultures. Ovarian cortical cultures were treated with phosphate-buffered saline (control), 50 ng/ml VEGFA165, VEGFA165b, or VEGFA165 + VEGFA165b. The Stair-Step heifers had more primordial follicles (P &lt; 0.0001), greater messenger RNA abundance of vascular markers VE-cadherin (P &lt; 0.0001) and NRP-1 (P &lt; 0.0051) than controls at 13 months of age prior to culture. After culture, VEGFA isoforms had similar effects, independent of diet, where VEGFA165 stimulated and VEGFA165b inhibited VEGFA165-stimulated follicle progression from early primary to antral follicle stages. In vitro cultures were treated with VEGFA isoforms and signal transduction array plates were evaluated. VEGFA165 stimulated expression of genes related to cell cycle, cell proliferation, and growth while VEGFA165b inhibited expression of those genes. Thus, VEGFA isoforms can act independently of diet to alter follicle progression or arrest. Furthermore, follicle progression can be stimulated by VEGFA165 and inhibited by VEGFA165b through diverse signal transduction pathways.

Imatinib mesylate effects on zebrafish reproductive success: Gonadal development, gamete quality, fertility, embryo-larvae viability and development, and related genes

Imatinib (IM) is a tyrosine kinase (TK) inhibitor (TKI) used to treat chronic myeloid leukemia. Clinical case reports and a few laboratory mammal studies provide inconclusive evidence about its deleterious effects on reproduction. The aim of the current study was to evaluate the potential of zebrafish to characterize IM-induced effects on reproduction and clarify IM effects on reproductive success. To this end, we exposed adult zebrafish to four concentrations of IM for 30 days followed by a 30-day depuration period. IM exposure caused a concentration-dependent, irreversible, suppression of folliculogenesis, reversible decrease in sperm density and motility, decreased fecundity and fertility, but no significant change in atretic follicle abundance. We also observed IM-induced premature hatching, but no significant change in embryo-larvae survivability. However, we found significant IM-induced morphometric malformations. IM decreased expression of vegfaa and igf2a (two reproductive-, angiogenic-, and growth-related genes) in testes and ovaries. The results demonstrate IM can induce significant changes in critical reproductive endpoints and zebrafish as a suitable model organism to show effects of IM on reproduction. The findings suggest that TKI effects on reproductive success should be considered.

Expression pattern of new genes regulating female sex differentiation and in vitro maturational status of oocytes in pigs

The processes underlying maturation of mammalian oocytes are considered crucial for the oocytes ability to undergo monospermic fertilization. The same factors of influence are suggested to impact the development of sex associated characteristics, allowing sex differentiation to progress during embryonic growth. The primary aim of the study was to analyze the gene ontology groups involved in regulation of porcine oocytes' response to endogenous stimuli. The results obtained would indicate potential genes influencing sex differentiation. Additionally, they could help to determine new genetic markers, expression profile of which is substantially regulated during porcine oocytes' in vitro maturation. To achieve that, porcine oocytes were collected for analysis before and after in vitro maturation. Pigs were used as they are a readily available model that presents significant similarity to humans in terms of physiology and anatomy. Microarray analysis of oocytes, before and after in vitro maturation was performed and later validated by RT-qPCR. We have particularly detected and analyzed genes belonging to gene ontology groups associated with hormonal stimulation during maturation of the oocytes, that exhibited significant change in expression (fold change ≥ |2|; p < 0.05) namely "Female sex differentiation" (CCND2, MMP14, VEGFA, FST, INHBA, NR5A1), "Response to endogenous stimulus" (INSR, ESR1, CCND2, TXNIP, TACR3, MMP14, FOS, AR, EGR2, IGFBP7, TGFBR3, BTG2, PLD1, PHIP, UBE2B) and "Response to estrogen stimulus" (INSR, ESR1, CCND2, IHH, TXNIP, TACR3, MMP14). Some of them were characteristic for just one of the described ontologies, while some belonged into multiple ontological terms. The genes were analyzed, with their relation to the processes of interest explained. Overall, the study provides us with a range of genes that might serve as molecular markers of in vitro maturation associated processes of the oocytes. This knowledge might serve as a reference for further studies and, after further validation, as a potentially useful knowledge in assessment of the oocytes during assisted reproduction processes.

Anti-angiogenic VEGFAxxxb transcripts are not expressed in the medio-basal hypothalamus of the seasonal sheep

This study investigated Vegfa expression in the pars tuberalis (PT) of the pituitary and medio-basal hypothalamus (MBH) of sheep, across seasons and reproductive states. It has recently been proposed that season impacts alternative splicing of Vegfa mRNA in the PT, which shifts the balance between angiogenic VEGFAxxx and anti-angiogenic VEGFAxxxb isoforms (with xxx the number of amino acids of the mature VEGFA proteins) to modulate seasonal breeding. Here, we used various RT-PCR methodologies and analysis of RNAseq datasets to investigate seasonal variation in expression and splicing of the ovine Vegfa gene. Collectively, we identify 5 different transcripts for Vegfa within the ewe PT/MBH, which correspond to splicing events previously described in mouse and human. All identified transcripts encode angiogenic VEGFAxxx isoforms, with no evidence for alternative splicing within exon 8. These findings led us to investigate in detail how "Vegfaxxxb-like" PCR products could be generated by RT-PCR and misidentified as endogenous transcripts, in sheep and human HEK293 cells. In conclusion, our findings do not support the existence of anti-angiogenic VEGFAxxxb isoforms in the ovine PT/MBH and shed new light on the interpretation of prior studies, which claimed to identify Vegfaxxxb isoforms by RT-PCR.

Kinase inhibitors: the road ahead

Receptor tyrosine kinase signalling pathways have been successfully targeted to inhibit proliferation and angiogenesis for cancer therapy. However, kinase deregulation has been firmly demonstrated to play an essential role in virtually all major disease areas. Kinase inhibitor drug discovery programmes have recently broadened their focus to include an expanded range of kinase targets and therapeutic areas. In this Review, we provide an overview of the novel targets, biological processes and disease areas that kinase-targeting small molecules are being developed against, highlight the associated challenges and assess the strategies and technologies that are enabling efficient generation of highly optimized kinase inhibitors.

Development of Potent, Selective SRPK1 Inhibitors as Potential Topical Therapeutics for Neovascular Eye Disease

Serine/arginine-protein kinase 1 (SRPK1) regulates alternative splicing of VEGF-A to pro-angiogenic isoforms and SRPK1 inhibition can restore the balance of pro/antiangiogenic isoforms to normal physiological levels. The lack of potency and selectivity of available compounds has limited development of SRPK1 inhibitors, with the control of alternative splicing by splicing factor-specific kinases yet to be translated. We present here compounds that occupy a binding pocket created by the unique helical insert of SRPK1, and trigger a backbone flip in the hinge region, that results in potent (<10 nM) and selective inhibition of SRPK1 kinase activity. Treatment with these inhibitors inhibited SRPK1 activity and phosphorylation of serine/arginine splicing factor 1 (SRSF1), resulting in alternative splicing of VEGF-A from pro-angiogenic to antiangiogenic isoforms. This property resulted in potent inhibition of blood vessel growth in models of choroidal angiogenesis in vivo. This work identifies tool compounds for splice isoform selective targeting of pro-angiogenic VEGF, which may lead to new therapeutic strategies for a diversity of diseases where dysfunctional splicing drives disease development.

Ovarian tissue culture in the presence of VEGF and fetuin stimulates follicle growth and steroidogenesis

Ovarian tissue cryopreservation together with follicle culture provides a promising technique for fertility preservation in cancer patients. The study aimed to evaluate follicle parameters in a culture medium supplemented with VEGFA165 and/or fetuin. Vitrified-warmed ovarian cortical pieces were divided randomly into four culture groups consisting of basic culture medium (control), and the basic culture medium supplemented with VEGFA165, fetuin or both. After six days of culture, we evaluated the following: percentage of resting, primary and secondary growing follicles; survival rate; steroid hormones production; levels of reactive oxygen species, lipid peroxidation and total antioxidant capacity; and developmental and antioxidant gene expression. The addition of VEGFA165 alone or in combination with fetuin to the culture medium caused resting follicle activation and increased the number of growing follicles. In the VEGFA165 group, we found a significant increase in the concentrations of 17β-estradiol at day 6 and progesterone from 4th day of the culture period. In the VEGFA165 + fetuin group, the concentration of 17β-estradiol rose at day 4 of the culture period. The levels of BMP15, GDF9 and INHB mRNAs were increased in all treated groups. In the fetuin and fetuin + VEGFA165 groups, we observed a high level of total antioxidant capacity and expression of SOD1 and CAT genes, low reactive oxygen species and lipid peroxidation levels and increased number of viable follicles. In conclusion, the present study provides useful evidence that supplementation of culture medium with VEGFA165 + fetuin leads to primordial follicle activation and development and increased percentage of healthy secondary growing follicles.

Vascular endothelial growth factor A: just one of multiple mechanisms for sex-specific vascular development within the testis?

Testis development from an indifferent gonad is a critical step in embryogenesis. A hallmark of testis differentiation is sex-specific vascularization that occurs as endothelial cells migrate from the adjacent mesonephros into the testis to surround Sertoli-germ cell aggregates and induce seminiferous cord formation. Many in vitro experiments have demonstrated that vascular endothelial growth factor A (VEGFA) is a critical regulator of this process. Both inhibitors to VEGFA signal transduction and excess VEGFA isoforms in testis organ cultures impaired vascular development and seminiferous cord formation. However, in vivo models using mice which selectively eliminated all VEGFA isoforms: in Sertoli and germ cells (pDmrt1-Cre;Vegfa(-/-)); Sertoli and Leydig cells (Amhr2-Cre;Vegfa(-/-)) or Sertoli cells (Amh-Cre;Vegfa(-/-) and Sry-Cre;Vegfa(-/-)) displayed testes with observably normal cords and vasculature at postnatal day 0 and onwards. Embryonic testis development may be delayed in these mice; however, the postnatal data indicate that VEGFA isoforms secreted from Sertoli, Leydig or germ cells are not required for testis morphogenesis within the mouse. A Vegfa signal transduction array was employed on postnatal testes from Sry-Cre;Vegfa(-/-) versus controls. Ptgs1 (Cox1) was the only upregulated gene (fivefold). COX1 stimulates angiogenesis and upregulates, VEGFA, Prostaglandin E2 (PGE2) and PGD2. Thus, other gene pathways may compensate for VEGFA loss, similar to multiple independent mechanisms to maintain SOX9 expression. Multiple independent mechanism that induce vascular development in the testis may contribute to and safeguard the sex-specific vasculature development responsible for inducing seminiferous cord formation, thus ensuring appropriate testis morphogenesis in the male.

Loss of vascular endothelial growth factor A (VEGFA) isoforms in granulosa cells using pDmrt-1-Cre or Amhr2-Cre reduces fertility by arresting follicular development and by reducing litter size in female mice

Because VEGFA has been implicated in follicle development, the objective of this study was to determine the effects of granulosa- and germ cell-specific VEGFA loss on ovarian morphogenesis, function, and female fertility. pDmrt1-Cre mice were mated to floxed VEGFA mice to develop granulosa-/germ cell-specific knockouts (pDmrt1-Cre;Vegfa^-/-). The time from mating to first parturition was increased when pDmrt1-Cre;Vegfa^-/- females were mated to control males (P = 0.0008) and tended to be longer for heterozygous females (P < 0.07). Litter size was reduced for pDmrt1-Cre;Vegfa^-/- females (P < 0.007). The time between the first and second parturitions was also increased for heterozygous females (P < 0.04) and tended to be increased for pDmrt1-Cre;Vegfa^-/- females (P < 0.07). pDmrt1-Cre;Vegfa^-/- females had smaller ovaries (P < 0.04), reduced plasma estradiol (P < 0.007), fewer developing follicles (P < 0.008) and tended to have fewer corpora lutea (P < 0.08). Expression of Igf1r was reduced (P < 0.05); expression of Foxo3a tended to be increased (P < 0.06); and both Fshr (P < 0.1) and Sirt6 tended to be reduced (P < 0.06) in pDmrt1-Cre;Vegfa^-/- ovaries. To compare VEGFA knockouts, we generated Amhr2-Cre;Vegfa^-/- mice that required more time from mating to first parturition (P < 0.003) with variable ovarian size. Both lines had more apoptotic granulosa cells, and vascular staining did not appear different. Taken together these data indicate that the loss of all VEGFA isoforms in granulosa/germ cells (proangiogenic and antiangiogenic) causes subfertility by arresting follicular development, resulting in reduced ovulation rate and fewer pups per litter.

VEGF Splicing and the Role of VEGF Splice Variants: From Physiological-Pathological Conditions to Specific Pre-mRNA Splicing

During this past decade, the vascular endothelial growth factor (VEGF) pathway has been extensively studied. VEGF is a paradigm of molecular regulation since its expression is controlled at all possible steps including transcription, mRNA stability, translation, and pre-mRNA splicing. The latter form of molecular regulation is probably the least studied. This field has been neglected; yet different forms of VEGF with different sizes and different physiological properties issued from alternative splicing have been described a long time ago. Recently a new level of complexity was added to the field of splicing of VEGF pre-mRNA. Whereas thousands of publications have described VEGF as a pro-angiogenic factor, an alternative splicing event generates specific anti-angiogenic forms of VEGF that only differ from the others by a modification in the last six amino acids of the protein. According to the scientists who discovered these isoforms, which are indistinguishable from the pro-angiogenic ones with pan VEGF antibodies, some of the literature on VEGF is at least inexact if not completely false. Moreover, the presence of anti-angiogenic forms of VEGF may explain the disappointing efficacy of anti-VEGF therapies on the overall survival of patients with different forms of cancers and with wet age-related macular degeneration. This review focuses on the existence of the different alternative splice variants of VEGF and the molecular mechanisms associated with their expression and function.

Transcriptomic diversification of developing cumulus and mural granulosa cells in mouse ovarian follicles

Cumulus cells and mural granulosa cells (MGCs) have functionally distinct roles in antral follicles, and comparison of their transcriptomes at a global and systems level can propel future studies on mechanisms underlying their functional diversity. These cells were isolated from small and large antral follicles before and after stimulation of immature mice with gonadotropins, respectively. Both cell types underwent dramatic transcriptomic changes, and differences between them increased with follicular growth. Although cumulus cells of both stages of follicular development are competent to undergo expansion in vitro, they were otherwise remarkably dissimilar with transcriptomic changes quantitatively equivalent to those of MGCs. Gene ontology analysis revealed that cumulus cells of small follicles were enriched in transcripts generally associated with catalytic components of metabolic processes, while those from large follicles were involved in regulation of metabolism, cell differentiation, and adhesion. Contrast of cumulus cells versus MGCs revealed that cumulus cells were enriched in transcripts associated with metabolism and cell proliferation while MGCs were enriched for transcripts involved in cell signaling and differentiation. In vitro and in vivo models were used to test the hypothesis that higher levels of transcripts in cumulus cells versus MGCs is the result of stimulation by oocyte-derived paracrine factors (ODPFs). Surprisingly ∼48% of transcripts higher in cumulus cells than MGCs were not stimulated by ODPFs. Those stimulated by ODPFs were mainly associated with cell division, mRNA processing, or the catalytic pathways of metabolism, while those not stimulated by ODPFs were associated with regulatory processes such as signaling, transcription, phosphorylation, or the regulation of metabolism.

The carboxyl terminus of VEGF-A is a potential target for anti-angiogenic therapy

Anti-VEGF-A therapy has become a mainstay of treatment for ocular neovascularisation and in cancer; however, their effectiveness is not universal, in some cases only benefiting a minority of patients. Anti-VEGF-A therapies bind and block both pro-angiogenic VEGF-Axxx and the partial agonist VEGF-Axxxb isoforms, but their anti-angiogenic benefit only comes about from targeting the pro-angiogenic isoforms. Therefore, antibodies that exclusively target the pro-angiogenic isoforms may be more effective. To determine whether C-terminal-targeted antibodies could inhibit angiogenesis, we generated a polyclonal antibody to the last nine amino acids of VEGF-A165 and tested it in vitro and in vivo. The exon8a polyclonal antibody (Exon8apab) did not bind VEGF-A165b even at greater than 100-fold excess concentration, and dose dependently inhibited VEGF-A165 induced endothelial migration in vitro at concentrations similar to the VEGF-A antibody fragment ranibizumab. Exon8apab can inhibit tumour growth of LS174t cells implanted in vivo and blood vessel growth in the eye in models of age-related macular degeneration, with equal efficacy to non-selective anti-VEGF-A antibodies. It also showed that it was the VEGF-Axxx levels specifically that were upregulated in plasma from patients with proliferative diabetic retinopathy. These results suggest that VEGF-A165-specific antibodies can be therapeutically useful.

Loss of vascular endothelial growth factor A (VEGFA) isoforms in the testes of male mice causes subfertility, reduces sperm numbers, and alters expression of genes that regulate undifferentiated spermatogonia

Vascular endothelial growth factor A (VEGFA) isoform treatment has been demonstrated to alter spermatogonial stem cell homeostasis. Therefore, we generated pDmrt1-Cre;Vegfa(-/-) (knockout, KO) mice by crossing pDmrt1-Cre mice to floxed Vegfa mice to test whether loss of all VEGFA isoforms in Sertoli and germ cells would impair spermatogenesis. When first mated, KO males took 14 days longer to get control females pregnant (P < .02) and tended to take longer for all subsequent parturition intervals (9 days; P < .07). Heterozygous males sired fewer pups per litter (P < .03) and after the first litter took 10 days longer (P < .05) to impregnate females, suggesting a more progressive loss of fertility. Reproductive organs were collected from 6-month-old male mice. There were fewer sperm per tubule in the corpus epididymides (P < .001) and fewer ZBTB16-stained undifferentiated spermatogonia (P < .003) in the testes of KO males. Testicular mRNA abundance for Bcl2 (P < .02), Bcl2:Bax (P < .02), Neurog3 (P < .007), and Ret was greater (P = .0005), tended to be greater for Sin3a and tended to be reduced for total Foxo1 (P < .07) in KO males. Immunofluorescence for CD31 and VE-Cadherin showed no differences in testis vasculature; however, CD31-positive staining was evident in undifferentiated spermatogonia only in KO testes. Therefore, loss of VEGFA isoforms in Sertoli and germ cells alters genes necessary for long-term maintenance of undifferentiated spermatogonia, ultimately reducing sperm numbers and resulting in subfertility.

Vascular contributions to early ovarian development: potential roles of VEGFA isoforms

Vascularisation is an essential component of ovarian morphogenesis; however, little is known regarding factors regulating the establishment of vasculature in the ovary. Angiogenesis involving extensive endothelial cell migration is a critical component of vessel formation in the embryonic testis but vasculogenic mechanisms appear to play a prominent role in ovarian vascularisation. Vasculature has a strong influence on the formation of ovarian structures, and the early developmental processes of ovigerous cord formation, primordial follicle assembly and follicle activation are all initiated in regions of the ovary that are in close association with the highly vascular medulla. The principal angiogenic factor, vascular endothelial growth factor A (VEGFA), has an important role in both endothelial cell differentiation and vascular pattern development. Expression of VEGFA has been localised to ovigerous cords and follicles in developing ovaries and an increased expression of pro-angiogenic Vegfa isoform mRNA in relation to anti-angiogenic isoform mRNA occurs at the same time-point as the peak of primordial follicle assembly in perinatal rats. Elucidation of specific genes that affect vascular development within the ovary may be critical for determining not only the normal mechanisms of ovarian morphogenesis, but also for understanding certain ovarian reproductive disorders.

The role of VEGF 165b in pathophysiology

Anti-angiogenic vascular endothelial growth factor A (VEGF) 165b and pro-angiogenic VEGF 165 are generated from the same transcript, and their relative amounts are dependent on alternative splicing. The role of VEGF 165b has not been investigated in as much detail as VEGF 165, although it appears to be highly expressed in non-angiogenic tissues and, in contrast with VEGF 165, is downregulated in tumors and other pathologies associated with abnormal neovascularization such as diabetic retinopathy or Denys Drash syndrome. VEGF 165b inhibits VEGFR2 signaling by inducing differential phosphorylation, and it can be used to block angiogenesis in in vivo models of tumorigenesis and angiogenesis-related eye disease. Recent reports have identified three serine/arginine-rich proteins, SRSF1, SRSF2 and SRSF6, and studied their role in regulating terminal splice-site selection. Since the balance of VEGF isoforms is lost in cancer and angiogenesis-related conditions, control of VEGF splicing could also be used as a basis for therapy in these diseases.

Do anti-angiogenic VEGF (VEGFxxxb) isoforms exist? A cautionary tale

Splicing of the human vascular endothelial growth factor-A (VEGF-A) gene has been reported to generate angiogenic (VEGFxxx) and anti-angiogenic (VEGFxxxb) isoforms. Corresponding VEGFxxxb isoforms have also been reported in rat and mouse. We examined VEGFxxxb expression in mouse fibrosarcoma cell lines expressing all or individual VEGF isoforms (VEGF120, 164 or 188), grown in vitro and in vivo, and compared results with those from normal mouse and human tissues. Importantly, genetic construction of VEGF164 and VEGF188 expressing fibrosarcomas, in which exon 7 is fused to the conventional exon 8, precludes VEGFxxxb splicing from occurring. Thus, these two fibrosarcoma cell lines provided endogenous negative controls. Using RT-PCR we show that primers designed to simultaneously amplify VEGFxxx and VEGFxxxb isoforms amplified only VEGFxxx variants in both species. Moreover, only VEGFxxx species were generated when mouse podocytes were treated with TGFβ-1, a reported activator of VEGFxxxb splice selection in human podocytes. A VEGF164/120 heteroduplex species was identified as a PCR artefact, specifically in mouse. VEGFxxxb isoform-specific PCR did amplify putative VEGFxxxb species in mouse and human tissues, but unexpectedly also in VEGF188 and VEGF164 fibrosarcoma cells and tumours, where splicing to produce true VEGFxxxb isoforms cannot occur. Moreover, these products were only consistently generated using reverse primers spanning more than 5 bases across the 8b/7 or 8b/5 splice junctions. Primer annealing to VEGFxxx transcripts and amplification of exon 8b primer ‘tails’ explained the artefactual generation of VEGFxxxb products, since the same products were generated when the PCR reactions were performed with cDNA from VEGF164/VEGF188 ‘knock-in’ vectors used in the generation of single VEGF isoform-expressing transgenic mice from which the fibrosarcoma lines were developed. Collectively, our results highlight important pitfalls in data interpretation associated with detecting VEGFxxxb isoforms using current methods, and demonstrate that anti-angiogenic isoforms are not commonly expressed in mouse or human tissues.

Ovarian VEGF(165)b expression regulates follicular development, corpus luteum function and fertility

Angiogenesis and vascular regression are critical for the female ovulatory cycle. They enable progression and regression of follicular development, and corpora lutea formation and regression. Angiogenesis in the ovary occurs under the control of the vascular endothelial growth factor-A (VEGFA) family of proteins, which are generated as both pro-(VEGF(165)) and anti(VEGF(165)b)-angiogenic isoforms by alternative splicing. To determine the role of the VEGF(165)b isoforms in the ovulatory cycle, we measured VEGF(165)b expression in marmoset ovaries by immunohistochemistry and ELISA, and used transgenic mice over-expressing VEGF(165)b in the ovary. VEGF(165)b was expressed in the marmoset ovaries in granulosa cells and theca, and the balance of VEGF(165)b:VEGF(165) was regulated during luteogenesis. Mice over-expressing VEGF(165)b in the ovary were less fertile than wild-type littermates, had reduced secondary and tertiary follicles after mating, increased atretic follicles, fewer corpora lutea and generated fewer embryos in the oviduct after mating, and these were more likely not to retain the corona radiata. These results indicate that the balance of VEGFA isoforms controls follicle progression and luteogenesis, and that control of isoform expression may regulate fertility in mammals, including in primates.

The balance of proangiogenic and antiangiogenic VEGFA isoforms regulate follicle development

Vascular endothelial growth factor A (VEGFA) has been extensively studied because of its role in follicular development and is a principal angiogenic factor essential for angiogenesis. Since vascularization of the theca layer increases as follicles progress in size through preantral and antral stages, VEGFA might influence follicle growth via the regulation of angiogenesis. However, VEGFA might also influence follicular development through nonangiogenic mechanisms, since its expression has been localized in nonvascular follicles and cells. Alternative mRNA splicing of eight exons from the VEGFA gene results in the formation of various VEGFA isoforms. Each isoform has unique properties and is identified by the number of amino acids within the mature protein. Proangiogenic isoforms (VEGFA_XXX) are encoded by exon 8a, whereas a sister set of isoforms (VEGFA_XXXB) with antiangiogenic properties is encoded by exon 8b. The antiangiogenic VEGFA_XXXB isoforms comprise the majority of VEGFA expressed in most tissues, whereas expression of the proangiogenic VEGFA isoforms is upregulated in tissues undergoing active angiogenesis. Although proangiogenic and antiangiogenic isoforms can now be distinguished from one another, many studies evaluating VEGFA in ovarian and follicular development up to now have not differentiated proangiogenic VEGFA from antiangiogenic VEGFA. Experiments from our laboratory indicate that proangiogenic VEGFA promotes follicle recruitment and early follicular development and antiangiogenic VEGFA inhibits these processes. The balance of proangiogenic versus antiangiognic VEGFA isoforms is thus of importance during follicle development. Further studies are warranted to elucidate the way that this balance regulates follicular formation and progression.

WT1 mutants reveal SRPK1 to be a downstream angiogenesis target by altering VEGF splicing

Angiogenesis is regulated by the balance of proangiogenic VEGF(165) and antiangiogenic VEGF(165)b splice isoforms. Mutations in WT1, the Wilms' tumor suppressor gene, suppress VEGF(165)b and cause abnormal gonadogenesis, renal failure, and Wilms' tumors. In WT1 mutant cells, reduced VEGF(165)b was due to lack of WT1-mediated transcriptional repression of the splicing-factor kinase SRPK1. WT1 bound to the SRPK1 promoter, and repressed expression through a specific WT1 binding site. In WT1 mutant cells SRPK1-mediated hyperphosphorylation of the oncogenic RNA binding protein SRSF1 regulated splicing of VEGF and rendered WT1 mutant cells proangiogenic. Altered VEGF splicing was reversed by wild-type WT1, knockdown of SRSF1, or SRPK1 and inhibition of SRPK1, which prevented in vitro and in vivo angiogenesis and associated tumor growth.

Expression of pro- and anti-angiogenic isoforms of VEGF in the mouse model of oxygen-induced retinopathy

Retinopathy of prematurity (ROP) has become one of the leading causes of blindness and visual loss in children over the last half century. Vascular endothelial growth factor (VEGF-A) is the principal stimulator of angiogenesis. Recently, it has been identified that VEGF was differentially spliced from exons 8 to exons 8a and 8b to form two families: the pro-angiogenic VEGFxxx family and the anti-angiogenic VEGFxxxb family. This alternate splicing produced VEGFxxxb proteins of the same length as VEGFxxx family, but with different C terminal amino acid sequences. VEGFxxxb appeared to be able to inhibit VEGFxxx-dependent angiogenesis. In our study, we investigated the protein expression course of VEGFxxx and VEGFxxxb by Western-blot in a mouse model of Oxygen-induced Retinopathy (OIR) from postnatal day 1 (P1) to postnatal day 21 (P21). We also analyzed the relative protein expression level of VEGF(165)b isoform in the OIR mouse model. We found that both VEGFxxx and VEGFxxxb were present in the mouse retina, among which, VEGF(164) and VEGF(165)b appeared to be predominant VEGFxxx and VEGFxxxb isoforms respectively in the mouse retina. We also found that the two family had different expression pattern correlated with neovascularization development and that the relative expression level of VEGF(165)b isoform switched during the neovascularization development in the OIR mouse model. In OIR group, the protein level of total VEGF isoforms (a mix of VEGF(164) and VEGF(165)b, detected by pan-VEGF antibody) continuously increased and peaked at P17 while VEGF(165)b continuously decreased from P9 which was well related with the vessel obliteration and neovascularization development in the mouse model of OIR. The neovascularization development correlates with an increase of total VEGF isoforms and the decrease of VEGF(165)b, indicating that there is a pro-angiogenic VEGF shift. Therefore, anti-angiogenic therapy that could alter the ratio of VEGFxxxb/VEGFxxx may be more effective.

Progress in understanding the relationship between vascular endothelial growth factor and digestive tumors

Digestive tumors account for a large portion of human malignancies, and their incidence and mortality are on the rise. Neovascularization plays a critical role in the metastasis of tumors. Vascular endothelial growth factor (VEGF) is one of the best characterized angiogenic regulators. There is close relationship between VEGF and tumor growth, invasion and metastasis. VEGF has become a research hot for diagnosis, targeted therapy and prognosis of tumors. The purpose of this review is to review the recent progress in understanding the relationship between VEGF and digestive tumors.

The evolution of alternative splicing exons in vascular endothelial growth factor A

The C-terminus alternative splicing in VEGFA (vascular endothelial growth factor A) is known for its impact on physiological and pathological angiogenesis. Based on our prediction and RT-PCR verification, we identified anti-angiogenic VEGFA165b isoforms in mouse and rabbit for the first time. We also found that the relative expression level of VEGFA165b isoform had been increasing from rodents to human, and exon8b may have experienced a minor-to-major form exon conversion, possibly correlated with its gain-of-function. It is suggested that introduction of alternative splicing exons (esp. exon6 and exon8b) made important contributions to the transcriptional diversity of VEGFA and played a crucial role in the evolution of its regulatory mechanism.

Neutralization of vascular endothelial growth factor antiangiogenic isoforms or administration of proangiogenic isoforms stimulates vascular development in the rat testis

Vascular endothelial growth factor A (VEGFA) plays a role in both angiogenesis and seminiferous cord formation, and alternative splicing of the Vegfa gene produces both proangiogenic isoforms and antiangiogenic isoforms (B-isoforms). The objectives of this study were to evaluate the expression of pro- and antiangiogenic isoforms during testis development and to determine the role of VEGFA isoforms in testis morphogenesis. Quantitative RT-PCR determined that Vegfa_165b mRNA was most abundant between embryonic days 13.5 and 16 (E13.5 and 16; P<0.05). Compared with ovarian mRNA levels, Vegfa_120 was more abundant at E13-14 (P<0.05), Vegfa_164 was less abundant at E13 (P<0.05), and Vegfa_165b tended to be less abundant at E13 (P<0.09) in testes. Immunohistochemical staining localized antiangiogenic isoforms to subsets of germ cells at E14-16, and western blot analysis revealed similar protein levels for VEGFA_165B, VEGFA_189B, and VEGFA_206B at this time point. Treatment of E13 organ culture testes with VEGFA_120, VEGFA_164, and an antibody to antiangiogenic isoforms (anti-VEGFAxxxB) resulted in less organized and defined seminiferous cords compared with paired controls. In addition, 50 ng/ml VEGFA_120 and VEGFA_164 treatments increased vascular density in cultured testes by 60 and 48% respectively, and treatment with VEGFAxxxB antibody increased vascular density by 76% in testes (0.5 ng/ml) and 81% in ovaries (5 ng/ml) compared with controls (P<0.05). In conclusion, both pro- and antiangiogenic VEGFA isoforms are involved in the development of vasculature and seminiferous cords in rat testes, and differential expression of these isoforms may be important for normal gonadal development.

Contraception targets in Mammalian ovarian development

In the human ovary, early in pre-natal life, oocytes are surrounded by pre-granulosa follicular cells to form primordial follicles. These primordial oocytes remain dormant, often for decades, until recruited into the growing pool throughout a woman's adult reproductive years. Activation of follicle growth and subsequent development of growing oocytes in pre-antral follicles are major biological checkpoints that determine an individual females reproductive potential. In the past decade, great strides have been made in the elucidation of the molecular and cellular mechanisms underpinning maintenance of the quiescent primordial follicle pool and initiation and development of follicle growth. Gaining an in-depth knowledge of the intracellular signalling systems that control oocyte preservation and follicle activation has significant implications for improving female reproductive productivity and alleviating infertility. It also has application in domestic animal husbandry, feral animal population control and contraception in women.

The anti-angiogenic isoforms of VEGF in health and disease

Anti-angiogenic VEGF (vascular endothelial growth factor) isoforms, generated from differential splicing of exon 8, are widely expressed in normal human tissues but down-regulated in cancers and other pathologies associated with abnormal angiogenesis (cancer, diabetic retinopathy, retinal vein occlusion, the Denys-Drash syndrome and pre-eclampsia). Administration of recombinant VEGF(165)b inhibits ocular angiogenesis in mouse models of retinopathy and age-related macular degeneration, and colorectal carcinoma and metastatic melanoma. Splicing factors and their regulatory molecules alter splice site selection, such that cells can switch from the anti-angiogenic VEGF(xxx)b isoforms to the pro-angiogenic VEGF(xxx) isoforms, including SRp55 (serine/arginine protein 55), ASF/SF2 (alternative splicing factor/splicing factor 2) and SRPK (serine arginine domain protein kinase), and inhibitors of these molecules can inhibit angiogenesis in the eye, and splice site selection in cancer cells, opening up the possibility of using splicing factor inhibitors as novel anti-angiogenic therapeutics. Endogenous anti-angiogenic VEGF(xxx)b isoforms are cytoprotective for endothelial, epithelial and neuronal cells in vitro and in vivo, suggesting both an improved safety profile and an explanation for unpredicted anti-VEGF side effects. In summary, C-terminal distal splicing is a key component of VEGF biology, overlooked by the vast majority of publications in the field, and these findings require a radical revision of our understanding of VEGF biology in normal human physiology.

Molecular diversity of VEGF-A as a regulator of its biological activity

The vascular endothelial growth factor (VEGF) family of proteins regulates blood flow, growth, and function in both normal physiology and disease processes. VEGF-A is alternatively spliced to form multiple isoforms, in two subfamilies, that have specific, novel functions. Alternative splicing of exons 5-7 of the VEGF gene generates forms with differing bioavailability and activities, whereas alternative splice-site selection in exon 8 generates proangiogenic, termed VEGF(xxx), or antiangiogenic proteins, termed VEGF(xxx)b. Despite its name, emerging roles for VEGF isoforms on cell types other than endothelium have now been identified. Although VEGF-A has conventionally been considered to be a family of proangiogenic, propermeability vasodilators, the identification of effects on nonendothelial cells, and the discovery of the antiangiogenic subfamily of splice isoforms, has added further complexity to their regulation of microvascular function. The distally spliced antiangiogenic isoforms are expressed in normal human tissue, but downregulated in angiogenic diseases, such as cancer and proliferative retinopathy, and in developmental pathologies, such as Denys Drash syndrome and preeclampsia. Here, we examine the molecular diversity of VEGF-A as a regulator of its biological activity and compare the role of the pro- and antiangiogenic VEGF-A splice isoforms in both normal and pathophysiological processes.

VEGF(121)b, a new member of the VEGF(xxx)b family of VEGF-A splice isoforms, inhibits neovascularisation and tumour growth in vivo

Background:

The key mediator of new vessel formation in cancer and other diseases is VEGF-A. VEGF-A exists as alternatively spliced isoforms - the pro-angiogenic VEGF_xxx family generated by exon 8 proximal splicing, and a sister family, termed VEGF_xxxb, exemplified by VEGF₁₆₅b, generated by distal splicing of exon 8. However, it is unknown whether this anti-angiogenic property of VEGF₁₆₅b is a general property of the VEGF_xxxb family of isoforms.

Methods:

The mRNA and protein expression of VEGF₁₂₁b was studied in human tissue. The effect of VEGF₁₂₁b was analysed by saturation binding to VEGF receptors, endothelial migration, apoptosis, xenograft tumour growth, pre-retinal neovascularisation and imaging of biodistribution in tumour-bearing mice with radioactive VEGF₁₂₁b.

Results:

The existence of VEGF₁₂₁b was confirmed in normal human tissues. VEGF₁₂₁b binds both VEGF receptors with similar affinity as other VEGF isoforms, but inhibits endothelial cell migration and is cytoprotective to endothelial cells through VEGFR-2 activation. Administration of VEGF₁₂₁b normalised retinal vasculature by reducing both angiogenesis and ischaemia. VEGF₁₂₁b reduced the growth of xenografted human colon tumours in association with reduced microvascular density, and an intravenous bolus of VEGF₁₂₁b is taken up into colon tumour xenografts.

Conclusion:

Here we identify a second member of the family, VEGF₁₂₁b, with similar properties to those of VEGF₁₆₅b, and underline the importance of the six amino acids of exon 8b in the anti-angiogenic activity of the VEGF_xxxb isoforms.