Neural Guidance Cues Research Articles

Emerging Three‐Dimensional Integrated Systems for Biomimetic Neural In Vitro Cultures

Abstract3D in vitro neural cultures have gained interest in recent years as promising biomimetic micro‐environments which can regulate cell response to neural guidance cues. Several bioengineered technologies have been developed in combination with biomaterials to produce 3D models, such as scaffold‐based neural constructs, neurospheres, cerebral organoids, and brain‐on‐chip devices. While the strategic selection of bioengineering approaches and associated biomaterials has opened a multitude of opportunities in building increasingly advanced neural tissues, technical limitations remain a challenge in modeling the complexity of in vivo cell–cell and cell–microenvironment interactions. In this context, potential synergies between different model systems can provide solutions for long‐standing in vitro challenges in neural tissue engineering. Here, the authors provide a critical and comprehensive overview of biomimetic in vitro brain modeling strategies, and explore how 3D integrated brain systems can be implemented in multidisciplinary neural tissue engineering, regenerative and personalized medicine, disease modeling, and drug discovery applications.

Revisiting and refining roles of neural guidance cues in circuit assembly.

Neural guidance mechanisms ensure the precise targeting and synaptogenesis events essential for normal circuit function. Neuronal growth cones encounter numerous attractive and repulsive cues as they navigate toward their intermediate and final targets; temporal and spatial regulation of these responses are critical for circuit assembly. Recent work highlights the complexity of these events throughout neural development and the multifaceted functions of a wide range of guidance cues. Here, we discuss recent studies that leverage advances in genetics, single cell tracing, transcriptomics and proteomics to further our understanding of the molecular mechanisms underlying neural guidance and overall circuit organization.

Serum semaphorin 7A is associated with the risk of acute atherothrombotic stroke.

Semaphorin 7A (Sema7A), a neural guidance cue, was recently identified to regulate atherosclerosis in mice. However, the clinical relevance of Sema7A with atherosclerotic diseases remains unknown. The aim of this study was to investigate the association between serum Sema7A and the risk of acute atherothrombotic stroke (AAS). We measured serum concentrations of Sema7A in 105 newly onset AAS cases and 105 age‐ and sex‐matched controls, showing that median Sema7A level in AAS cases was over three times of that in controls (5.86 vs 1.66 ng/mL). Adjusted for hypertension, body mass index, fasting blood glucose, total cholesterol, triglyceride, high‐density lipoprotein (HDL)‐cholesterol, low‐density lipoprotein (LDL)‐cholesterol, current smoking and alcohol consumption, multivariate logistic regression showed that higher Sema7A was independently associated with the odds of AAS (OR = 6.40, 95% CI: 2.88‐14.25). Each 1‐standard deviation increase in Sema7A was associated with a threefold higher odds of AAS (OR = 3.42, 95% CI: 1.84‐6.35). Importantly, adding Sema7A to a multivariate logistic model containing conventional cardiovascular risk factors improved the area under receiver operating characteristic curves from 0.831 to 0.891 for the association with AAS. In conclusion, elevated serum Sema7A is independently associated with the risk of AAS, suggesting that it may play a potential role in AAS.

Dual actions of Netrin-1 on islet insulin secretion and immune modulation.

Netrin-1 is typically known as a neural guidance cue, which has been implicated in pancreas development. Since regenerative, angiogenic and anti-inflammatory properties of Netrin-1 have been reported in multiple tissues, we have investigated the potential role of Netrin-1in the endocrine islet and its implication in mice with high-fat diet (HFD)/streptozotocin (STZ)-induced diabetes. Effects of exogenous Netrin-1 on β-cell [Ca(2+)]i, cyclic AMP (cAMP) and insulin production were assessed invitro The long-term impact of Netrin-1 treatment was then evaluated in HFD/STZ-induced diabetic mice by subcutaneous implantation of osmotic minipumps which release Netrin-1in a sustained manner for 4weeks. Immunostaining of pancreases of Netrin-1-treated and control animals were employed to examine islet morphology, vascularization and macrophage infiltration. Plasma insulin, glucagon and pro-inflammatory cytokine concentrations were quantified by ELISA. Expression of endogenous Netrin-1 was also assessed by PCR and immunohistochemistry. We observed a stimulatory effect of Netrin-1 on invitro insulin secretion by promoting β-cell Ca(2+) influx and cAMP production. After 4-week continuous exposure, a hypoglycaemic property of Netrin-1 was demonstrated, which is probably attributable to improved β-cell function, shown as increased insulin content and preproinsulin mRNA expression. Enhanced islet vascularization, reduced islet macrophage infiltration and ameliorated systemic inflammation were detected from HFD/STZ-induced diabetic mice after Netrin-1 administration. We propose a dual action of Netrin-1in islets during pathophysiological hyperglycaemia: by maintaining insulin secretion while attenuating inflammation.

Emerging Roles of Neural Guidance Molecules in Atherosclerosis

Neural guidance cues are attracting attention in atherosclerosis. In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology , Wanschel et al1 add yet another neural guidance molecule, Semaphorin 3E, to the growing list of neural guidance cues that may contribute to atherosclerosis. Indeed, this team of scientists, led by Kathryn Moore at New York University, has in the last year implicated 3 other neural guidance cues in atherosclerosis, including netrin-1,2,3 semaphorin 3A,3 and ephrin B3 in the regulation of inflammatory atherosclerosis. See accompanying article on page 886 In the first of a series of papers on these molecules, this group showed that netrin-1–deficient bone marrow transplanted into LDLR–/− mice led to markedly reduced atherosclerotic plaques.2 Using a method to track monocyte entry and persistence in a quantitative manner,4,5 the authors argued that netrin-1 was essential for retaining monocyte-derived foamy macrophages in plaques. In the absence of netrin-1, monocytes could …

Navigation rules for vessels and neurons: cooperative signaling between VEGF and neural guidance cues.

Many organs, such as lungs, nerves, blood and lymphatic vessels, consist of complex networks that carry flows of information, gases, and nutrients within the body. The morphogenetic patterning that generates these organs involves the coordinated action of developmental signaling cues that guide migration of specialized cells. Precision guidance of endothelial tip cells by vascular endothelial growth factors (VEGFs) is well established, and several families of neural guidance molecules have been identified to exert guidance function in both the nervous and the vascular systems. This review discusses recent advances in VEGF research, focusing on the emerging role of neural guidance molecules as key regulators of VEGF function during vascular development and on the novel role of VEGFs in neural cell migration and nerve wiring.

Transmembrane semaphorin5B is proteolytically processed into a repulsive neural guidance cue

Developing neuronal growth cones respond to a number of post-transcriptionally modified guidance cues to establish functional neural networks. The Semaphorin family has well-established roles as both secreted and transmembrane guidance cues. Here, we describe the first evidence that a transmembrane Semaphorin, Semaphorin 5B (Sema5B), is proteolytically processed from its transmembrane form and can function as a soluble growth cone collapsing guidance cue. Over-expression of A Disintegrin and Metalloprotease (ADAM)-17, results in an enhanced release of the Sema5B ectodomain, while removal of a predicted ADAM-17 cleavage site prevents its release. In contrast, knockdown of ADAM-17 does not significantly reduce Sema5B release, indicating there are additional unknown compensating proteases. This modulation of the transmembrane Sema5B to a diffusible cue represents a sophisticated method to regulate neuronal guidance in vivo.

Neuropilin Signalling in Vascular Development and Pathology

The creation of functional blood vessel circuits is a critical step during embryonic development to support organ growth and differentiation. In the adult, angiogenesis occurs naturally in the female reproductive system, during exercise- induced muscle growth and in wound healing. However, adult angiogenesis also promotes tumour growth and contributes to many other diseases with tissue ischemia. The transmembrane protein neuropilin 1 (NRP1) is an isoformselective receptor for the most potent vascular growth and permeability factor, VEGF-A. NRP1 also functions as a receptor for an archetypical neural guidance cue called SEMA3A that inhibits tumour angiogenesis. In the adult vasculature, both VEGF-A and SEMA3A increase vascular permeability and thereby promote the formation of tissue oedema. Here, we review current knowledge about NRP1 function during blood vessel growth and vascular pathology. Keywords: Angiogenesis, VEGF, VEGF-A, SEMA3A, neuropilin, VEGFR2, endothelial cells, Neuropilin Signalling, Vascular Development, Pathology

Semaphorin 3A Contributes to Distal Pulmonary Epithelial Cell Differentiation and Lung Morphogenesis

RationaleSemaphorin 3A (Sema3A) is a neural guidance cue that also mediates cell migration, proliferation and apoptosis, and inhibits branching morphogenesis. Because we have shown that genetic deletion of neuropilin-1, which encodes an obligatory Sema3A co-receptor, influences airspace remodeling in the smoke-exposed adult lung, we sought to determine whether genetic deletion of Sema3A altered distal lung structure.MethodsTo determine whether loss of Sema3A signaling influenced distal lung morphology, we compared pulmonary histology, distal epithelial cell morphology and maturation, and the balance between lung cell proliferation and death, in lungs from mice with a targeted genetic deletion of Sema3A (Sema3A-/-) and wild-type (Sema3A+/+) littermate controls.ResultsGenetic deletion of Sema3A resulted in significant perinatal lethality. At E17.5, lungs from Sema3A-/- mice had thickened septae and reduced airspace size. Distal lung epithelial cells had increased intracellular glycogen pools and small multivesicular and lamellar bodies with atypical ultrastructure, as well as reduced expression of type I alveolar epithelial cell markers. Alveolarization was markedly attenuated in lungs from the rare Sema3A-/- mice that survived the immediate perinatal period. Furthermore, Sema3A deletion was linked with enhanced postnatal alveolar septal cell death.ConclusionsThese data suggest that Sema3A modulates distal pulmonary epithelial cell development and alveolar septation. Defining how Sema3A influences structural plasticity of the developing lung is a critical first step for determining if this pathway can be exploited to develop innovative strategies for repair after acute or chronic lung injury.

Transplantation of MSCs in Combination with Netrin-1 Improves Neoangiogenesis in a Rat Model of Hind Limb Ischemia

Similar to the neural network, the vascular network is formed from central axial structures that send sprouts along predetermined trajectories to their distal destinations. Indeed, recent evidence indicates that neuronal guidance factors and their receptors function as angiogenic regulators. As neural guidance cues, netrin-1 is the most extensively studied gene in the field of angiogenesis. Despite achieving some advances in mesenchymal stem cell (MSC) therapy in angiogenesis, there are still a certain number of patients who fail to respond to cell therapy. Thus, a novel therapeutic strategy to enhance the angiogenic property of transplanted cells is desirable. This study examined the impact of combined netrin-1 protein and MSC implantation on therapeutic angiogenesis in a rat model of hind limb ischemia. Hind limb ischemic rats (n = 24) were divided randomly into four groups (six rats per group): control group (0.05 mL saline); netrin-1 group (1 μg netrin-1 dissolved in 0.05 mL saline); MSC group (1 × 10(6) MSCs); and netrin-1/MSCs group (1 μg netrin-1 combined with MSCs). Netrin-1 and/or MSCs were injected directly into the muscle of the ischemic limb. Gross appearance of ischemic limb, collateral vessel formation, and vascular endothelial growth factor (VEGF) level were assessed 28 d after treatment. The results showed that pretreatment of MSCs with a recombinant netrin-1 protein markedly augmented the angiographic score and capillary density, improved function of the ischemic limb, and increased levels of VEGF in the plasma and damaged tissues. Further studies assaying the cell migration and network formation suggested that netrin-1 promoted MSC migration and enhanced its ability to participate in tube formation. These results demonstrated that transplantation of MSCs pretreated with netrin-1 protein significantly improved the therapeutic effect of MSCs and, therefore, may provide a novel therapeutic approach for ischemic disease.

Emerging mechanisms of vascular stabilization.

Neural guidance cues are essential for a growing axon to correctly course through the body and innervate target tissues. Interestingly, the vascular network follows a parallel trajectory along nerves, suggesting that guidance cues important for neural patterning may also be required for proper vascular patterning. However, while an axon arises from one cell, a blood vessel is composed of many endothelial cells. Recent evidence suggests that neural repulsive cues are usurped by multi-cellular blood vessels to ensure vascular stabilization cues. Additional clues into the signaling mechanisms that promote vascular stabilization are emerging from cerebral cavernous malformations, a disease characterized by headache, epilepsy, and stroke. Thus, neurobiology and neurology are providing insights into the concepts of vascular stability.

Netrin-1 increases proliferation and migration of renal proximal tubular epithelial cells via the UNC5B receptor

The cellular hallmark of kidney repair is a rapid proliferation of renal tubular epithelial cells ultimately leading to the restoration of nephron structure and function. Netrin-1 was discovered as a neural guidance cue and found to be expressed outside the nervous system, including in kidney. Previous work showed that netrin-1 is upregulated in response to ischemic injury and ameliorates ischemic injury. The objectives of this study were to determine the role of netrin-1 in renal tubular epithelial cell proliferation and migration in vitro. Real-time RT-PCR analysis showed that netrin-1 and its receptors UNC5B and neogenin are highly expressed in cultured mouse renal epithelial cells (TKPTS), whereas the expression of the Deleted in Colon Cancer (DCC), UNC5A, UNC5C, and UNC5D receptors is negligible or undetectable. Netrin-1 protein was induced in the edges of mechanical wounds in vitro. Netrin-1 increased TKPTS cell proliferation in a dose-dependent manner. The netrin-1-induced increase in TKPTS cell proliferation was completely prevented by small interfering RNA (siRNA) inhibition of UNC5B receptor but not UNC5C receptor expression. Netrin-1 also increased TKPTS cell migration in vitro, and this was also mediated through the UNC5B receptor. Netrin-1 increased the phosphorylation of Akt and ERK. Inhibition of phosphatidylinositol 3-kinase and MEK1/2 completely inhibited netrin-1-induced cell proliferation but not migration. These results indicate that netrin-1 increases renal tubular epithelial cell proliferation and migration through the UNC5B receptor. Moreover, the increase in cell proliferation, but not migration, was mediated via activation of Akt and ERK pathways.

Semaphoring Vascular Morphogenesis

In vertebrate embryos, development of an architecturally optimized blood vessel network allows the efficient transport of oxygen and nutrients to all other tissues. The final shape of the vascular system results from vasculogenesis and angiogenesis, during which motile endothelial cells (ECs) modify their integrin-mediated interactions with the extracellular matrix (ECM) in response to pro- and anti-angiogenic factors. There is mounting evidence that different members of the semaphorin (SEMA) family of neural guidance cues participate in developmental and postnatal vessel formation and patterning as well. It turns out that paracrine secretion of class 3 SEMA (SEMA3) by nonendothelial tissues cooperates with vascular endothelial growth factor in regulating EC precursor migration and assembly during vasculogenesis and funnels navigating blood vessel through tissue boundaries during sprouting angiogenesis. Autocrine loops of endothelial SEMA3 instead appears to regulate vascular remodeling, which occurs through blood vessel intussusception and fusion. SEMA3 activity both on the vascular and nervous systems relies upon their ability to hamper the affinity of integrin receptors towards ECM ligands. Indeed, signaling from SEMA-activated plexin receptors negatively regulates cell-ECM adhesive interactions by inhibiting two key integrin activators, such as the small GTPase R-Ras and the focal adhesion protein talin.

The axonal attractant Netrin-1 is an angiogenic factor

Blood vessels and nerves often follow parallel trajectories, suggesting that distal targets use common cues that induce vascularization and innervation. Netrins are secreted by the floor plate and attract commissural axons toward the midline of the neural tube. Here, we show that Netrin-1 is also a potent vascular mitogen. Netrin-1 stimulates proliferation, induces migration, and promotes adhesion of endothelial cells and vascular smooth muscle cells with a specific activity comparable to vascular endothelial growth factor and platelet-derived growth factor. Our evidence indicates that the netrin receptor, Neogenin, mediates netrin signaling in vascular smooth muscle cells, but suggests that an unidentified receptor mediates the proangiogenic effects of Netrin-1 on endothelial cells. Netrin-1 also stimulates angiogenesis in vivo and augments the response to vascular endothelial growth factor. Thus, we demonstrate that Netrin-1 is a secreted neural guidance cue with the unique ability to attract both blood vessels and axons, and suggest that other cues may also function as vascular endothelial growth factors.

Semaphorin 7A promotes axon outgrowth through integrins and MAPKs.

Striking parallels exist between immune and nervous system cellular signalling mechanisms. Molecules originally shown to be critical for immune responses also serve neuronal functions, and similarly neural guidance cues can modulate immune function. We show here that semaphorin 7A (Sema7A), a membrane-anchored member of the semaphorin family of guidance proteins previously known for its immunomodulatory effects, can also mediate neuronal functions. Unlike many other semaphorins, which act as repulsive guidance cues, Sema7A enhances central and peripheral axon growth and is required for proper axon tract formation during embryonic development. Unexpectedly, Sema7A enhancement of axon outgrowth requires integrin receptors and activation of MAPK signalling pathways. These findings define a previously unknown biological function for semaphorins, identify an unexpected role for integrins and integrin-dependent intracellular signalling in mediating semaphorin responses, and provide a framework for understanding and interfering with Sema7A function in both immune and nervous systems.