Neurovascular crosstalk coordinates the central nervous system development

Purpose of the review: The synchronic development of vascular and nervous systems is orchestrated by common molecules that regulate the communication between both systems. The identification of these common guiding cues and the developmental processes regulated by neurovascular communication are slowly emerging. In this review, we describe the molecules modulating the neurovascular development and their impact in processes such as angiogenesis, neurogenesis, neuronal migration, and brain homeostasis. Recent findings: Blood vessels not only are involved in nutrient and oxygen supply of the central nervous system (CNS) but also exert instrumental functions controlling developmental neurogenesis, CNS cytoarchitecture, and neuronal plasticity. Conversely, neurons modulate CNS vascularization and brain endothelial properties such as blood–brain barrier and vascular hyperemia. Summary: The integration of the active role of endothelial cells in the development and maintenance of neuronal function is important to obtain a more holistic view of the CNS complexity and also to understand how the vasculature is involved in neuropathological conditions.     

The Yin and Yang of Wnt/Ryk axon guidance in development and regeneration

In the developing embryo,nascent axons navigate towards their specific targets to establish the intricate network of axonal connections linking neurons within the mature nervous system.Molecular navigational systems comprising repulsive and attractive guidance cues form chemotactic gradients along the pathway of the exploring growth cone.Axon-bound receptors detect these gradients and determine the trajectory of the migrating growth cone.In contrast to their benevolent role in the developing nervous system,repulsive guidance receptors are detrimental to the axon’s ability to regenerate after injury in the adult.In this review we explore the essential and beneficial role played by the chemorepulsive Wnt receptor,Ryk/Derailed in axon navigation in the embryonic nervous system(the Yin function).Specifically,we focus on the role of Wnt5a/Rykmediated guidance in the establishment of two major axon tracts in the mammalian central nervous system,the corticospinal tract and the corpus callosum.Recent studies have also identified Ryk as a major suppressor of axonal regeneration after spinal cord injury.Thus,we also discuss this opposing aspect of Ryk function in axonal regeneration where its activity is a major impediment to axon regrowth(the Yang function).     

Axon guidance at the midline: from mutants to mechanisms

How axons in the developing nervous system successfully navigate to their correct targets is a fundamental problem in neurobiology. Understanding the mechanisms that mediate axon guidance will give important insight into how the nervous system is correctly wired during development and may have implications for therapeutic approaches to developmental brain disorders and nerve regeneration. Achieving this understanding will require unraveling the molecular logic that ensures the proper expression and localization of axon guidance cues and receptors, and elucidating the signaling events that regulate the growth cone cytoskeleton in response to guidance receptor activation. Studies of axon guidance at the midline of many experimental systems, from the ventral midline of Drosophila to the vertebrate spinal cord, have led to important mechanistic insights into the complex problem of wiring the nervous system. Here we review recent advances in understanding the regulation of midline axon guidance, with a particular emphasis on the contributions made from molecular genetic studies of invertebrate model systems.     

Axon Guidance at the Midline: From Mutants to Mechanisms

How axons in the developing nervous system successfully navigate to their correct targets is a fundamental problem in neurobiology. Understanding the mechanisms that mediate axon guidance will give important insight into how the nervous system is correctly wired during development and may have implications for therapeutic approaches to developmental brain disorders and nerve regeneration. Achieving this understanding will require unraveling the molecular logic that ensures the proper expression and localization of axon guidance cues and receptors, and elucidating the signaling events that regulate the growth cone cytoskeleton in response to guidance receptor activation. Studies of axon guidance at the midline of many experimental systems, from the ventral midline of Drosophila to the vertebrate spinal cord, have led to important mechanistic insights into the complex problem of wiring the nervous system. Here we review recent advances in understanding the regulation of midline axon guidance, with a particular emphasis on the contributions made from molecular genetic studies of invertebrate model systems.     

The DLK signalling pathway—a double‐edged sword in neural development and regeneration

Dual leucine zipper kinase (DLK), a mitogen‐activated protein kinase kinase kinase, controls axon growth, apoptosis and neuron degeneration during neural development, as well as neurodegeneration after various insults to the adult nervous system. Interestingly, recent studies have also highlighted a role of DLK in promoting axon regeneration in diverse model systems. Invertebrates and vertebrates, cold‐ and warm‐blooded animals, as well as central and peripheral mammalian nervous systems all differ in their ability to regenerate injured axons. Here, we discuss how DLK‐dependent signalling regulates apparently contradictory functions during neural development and regeneration in different species. In addition, we outline strategies to fine‐tune DLK function, either alone or together with other approaches, to promote axon regeneration in the adult mammalian central nervous system.     

Towards an understanding of semaphorin signalling in the spinal cord

Semaphorins are a large family of proteins that are classically associated with axon guidance. These proteins and their interacting partners, the neuropilins and plexins are now known to be key mediators in a variety of processes throughout the nervous system ranging from synaptic refinement to the correct positioning of neuronal and glial cell bodies. Recently, much attention has been given to the roles semaphorins play in other body tissues including the immune and vascular systems. This review wishes to draw attention back to the nervous system, specifically focusing on the role of semaphorins in the development of the spinal cord and their proposed roles in the adult. In addition, their functions in spinal cord injury at the glial scar are also discussed.     

Role of nonsynaptic communication in regulating the immune response

The discovery of nonsynaptic communication in the 1960s and 1970s was an important milestone in investigating the function of the nervous system, and it revolutionized our view about information transmission between neurons. In addition, nonsynaptic communication has a practical importance not only within the nervous system, but in the communication between the peripheral nervous system and other organ systems. Nonsynaptic communication takes place in different immune organs, which are innervated by sympathetic nerve terminals. In addition, the function of microglia, one of the immunocompetent cell types of the brain, can also be affected by neurotransmitters released from axon varicosities. The various functions of immune cells are modulated by released neurotransmitters without any direct synaptic contact between nerve endings and targeted immune cells requiring only functional neurotransmitter receptors on immune cells. Here, we briefly overview the role of the various receptor subtypes mediating nonsynaptic modulation of the function of immunocompetent cells both in the periphery and in the central nervous system.     

Dissemination pathways for poliovirus cells to animals models

It is considered there are two main pathways for poliovirus dissemination towards the central nervous system in humans. One is the pathway through the blood brain barrier. The orally ingested virus invades into the blood circulation, and then the virus permeates into the central nervous system through the blood brain barrier. The other is the neural pathway. In this pathway, the intramuscularly-inoculated virus is transported through the axons from the synapse to the cell body in the central nervous system. We have developed the oral infection system using the mouse models. Moreover, we proposed the possibility that PV is transcytosed through the brain capillary epithelia in a specific manner. As for the neural pathway, we have proved that PV is endocytosed into CD155 containing vesicles and the vesicles are retrogradely transported in the axon of rat primary motor neuron. We have also shown that the cytoplasmic dynein takes part in the transport.     

Developmental regulation of axon branching in the vertebrate nervous system

During nervous system development, axons generate branches to connect with multiple synaptic targets. As with axon growth and guidance, axon branching is tightly controlled in order to establish functional neural circuits, yet the mechanisms that regulate this important process are less well understood. Here, we review recent advances in the study of several common branching processes in the vertebrate nervous system. By focusing on each step in these processes we illustrate how different types of branching are regulated by extracellular cues and neural activity, and highlight some common principles that underlie the establishment of complex neural circuits in vertebrate development.     

Netrin-3 protein is localized to the axons of motor, sensory, and sympathetic neurons

The netrin family of axon guidance cues has been shown to play a pivotal role in the guidance of a variety of axon projections during embryonic development, both in the vertebrate and invertebrate. While the guidance potential of netrin-1 has been examined in depth in many regions of the developing mouse brain very little information is available on the expression and activity of netrin-3. Here we show that the netrin-3 protein is present on motor neurons and subpopulations of neurons within sensory and sympathetic ganglia. Moreover, significant levels of netrin-3 protein were found to be associated with the axons projecting from these neurons suggesting a role for netrin-3 in axon pathfinding and fasciculation within the peripheral nervous system.     

Neuronal polarization is impaired in mice lacking RhoE expression

Proper development of neuronal networks relies on the polarization of the neurons, thus the establishment of two compartments, axons and dendrites, whose formation depends on cytoskeletal rearrangements. Rnd proteins are regulators of actin organization and they are important players in several aspects of brain development as neurite formation, axon guidance and neuron migration. We have recently demonstrated that mice lacking RhoE/Rnd3 expression die shortly after birth and have neuromotor impairment and neuromuscular alterations, indicating an abnormal development of the nervous system. In this study, we have further investigated the specific role played by RhoE in several aspects of neuronal development by using hippocampal neuron cultures. Our findings show that neurons from a mice lacking RhoE expression exhibit a decrease in the number and the total length of the neurites. We also show that RhoE-deficient neurons display a reduction in axon outgrowth and a delay in the process of neuronal polarization. In addition, our results suggest an involvement of the RHOA/ROCK/LIMK/COFILIN signaling pathway in the neuronal alterations induced by the lack of RhoE. These findings support our previous report revealing the important role of RhoE in the normal development of the nervous system and may provide novel therapeutic targets in neurodegenerative disorders.     

Drosophila Plexin B is a Sema-2a receptor required for axon guidance

Plexin receptors play a crucial role in the transduction of axonal guidance events elicited by semaphorin proteins. In Drosophila, Plexin A (PlexA) is a receptor for the transmembrane semaphorin semaphorin-1a (Sema-1a) and is required for motor and central nervous system (CNS) axon guidance in the developing embryonic nervous system. However, it remains unknown how PlexB functions during neural development and which ligands serve to activate this receptor. Here, we show that plexB, like plexA, is robustly expressed in the developing CNS and is required for motor and CNS axon pathfinding. PlexB and PlexA serve both distinct and shared neuronal guidance functions. We observe a physical association between these two plexin receptors in vivo and find that they can utilize common downstream signaling mechanisms. PlexB does not directly bind to the cytosolic semaphorin signaling component MICAL (molecule that interacts with CasL), but requires MICAL for certain axonal guidance functions. Ligand binding and genetic analyses demonstrate that PlexB is a receptor for the secreted semaphorin Sema-2a, suggesting that secreted and transmembrane semaphorins in Drosophila use PlexB and PlexA, respectively, for axon pathfinding during neural development. These results establish roles for PlexB in central and peripheral axon pathfinding, define a functional ligand for PlexB, and implicate common signaling events in plexin-mediated axonal guidance.     

Longitudinal axon guidance

Our knowledge about molecular mechanisms underlying axon guidance along the antero-posterior axis in contrast to the dorso-ventral axis of the developing nervous system is very limited. During the past two years in vitro and in vivo studies have indicated that morphogens have a role in longitudinal axon guidance. Morphogens are secreted proteins that act in a concentration-dependent manner on susceptible groups of precursor cells and induce their differentiation to a specific cell fate. Thus, gradients of morphogens are responsible for the appropriate patterning of the nervous system during early phases of neural development. Therefore, it was surprising to find that gradients of two of these morphogens, Wnt4 and Shh, can be re-used for longitudinal axon guidance during later stages of nervous system development.     

Evolution of the VEGF-Regulated Vascular Network from a Neural Guidance System

The vascular network is closely linked to the neural system, and an interdependence is displayed in healthy and in pathophysiological responses. How has close apposition of two such functionally different systems occurred? Here, we present a hypothesis for the evolution of the vascular network from an ancestral neural guidance system. Biological cornerstones of this hypothesis are the vascular endothelial growth factor (VEGF) protein family and cognate receptors. The primary sequences of such proteins are conserved from invertebrates, such as worms and flies that lack discernible vascular systems compared to mammals, but all these systems have sophisticated neuronal wiring involving such molecules. Ancestral VEGFs and receptors (VEGFRs) could have been used to develop and maintain the nervous system in primitive eukaryotes. During evolution, the demands of increased morphological complexity required systems for transporting molecules and cells, i.e., biological conductive tubes. We propose that the VEGF–VEGFR axis was subverted by evolution to mediate the formation of biological tubes necessary for transport of fluids, e.g., blood. Increasingly, there is evidence that aberrant VEGF-mediated responses are also linked to neuronal dysfunctions ranging from motor neuron disease, stroke, Parkinson’s disease, Alzheimer’s disease, ischemic brain disease, epilepsy, multiple sclerosis, and neuronal repair after injury, as well as common vascular diseases (e.g., retinal disease). Manipulation and correction of the VEGF response in different neural tissues could be an effective strategy to treat different neurological diseases.     

Calcium/calmodulin-dependent protein kinase II regulates mammalian axon growth by affecting F-actin length in growth cone

While axon regeneration is a key determinant of functional recovery of the nervous system after injury, it is often poor in the mature nervous system. Influx of extracellular calcium (Ca²⁺) is one of the first phenomena that occur following axonal injury, and calcium/calmodulin-dependent protein kinase II (CaMKII), a target substrate for calcium ions, regulates the status of cytoskeletal proteins such as F-actin. Herein, we found that peripheral axotomy activates CaMKII in dorsal root ganglion (DRG) sensory neurons, and inhibition of CaMKII impairs axon outgrowth in both the peripheral and central nervous systems (PNS and CNS, respectively). Most importantly, we also found that the activation of CaMKII promotes PNS and CNS axon growth, and regulatory effects of CaMKII on axon growth occur via affecting the length of the F-actin. Thus, we believe our findings provide clear evidence that CaMKII is a critical modulator of mammalian axon regeneration.     

Cutting edge: MHC class I-Ly49 interaction regulates neuronal function

MHC class I molecules (MHC-I) have been implicated in nervous system development in the mouse. In this study we present evidence for the interaction of MHC-I with the NK cell receptor Ly49 in primary cortical neuronal cultures. We show that MHC-I and Ly49 are expressed on neuronal soma and axon surfaces, with Ly49 also present on dendrites. Anti-MHC-I Abs reduce synapsin-I expression and enhance neurite outgrowth and neuronal death. Conversely, anti-Ly49 mAbs increase synapsin-I expression, reduce neurite outgrowth, and promote neuron viability. Because we show that Ly49 genes are selectively expressed in the adult brain, these findings suggest an unsuspected role for the MHC-I-Ly49 interaction in the development and function of the brain.     

Celsr3 and Fzd3 in axon guidance

The assembly of functional neuronal circuits depends on the correct wiring of axons and dendrites. To reach their targets, axons are guided by a variety of extracellular guidance cues, including Netrins, Ephrins, Semaphorins and Slits. Corresponding receptors in the growth cone, the dynamic structure at the tip of the growing axon, sense and integrate these positional signals, and activate downstream effectors to regulate cytoskeletal organization. In addition to the four canonical families of axon guidance cues mentioned above, some proteins that regulate planar cell polarity were recently found to be critical for axon guidance. The seven-transmembrane domain receptors Celsr3 and Fzd3, in particular, control the development of most longitudinal tracts in the central nervous system, and axon navigation in the peripheral, sympathetic and enteric nervous systems. Despite their unequivocally important role, however, underlying molecular mechanisms remain elusive. We do not know which extracellular ligands they recognize, whether they have co-receptors in the growth cone, and what their downstream effectors are. Here, we review some recent advances and discuss future trends in this emerging field.     

Distinct expression patterns of syndecans in the embryonic zebrafish brain

Axon pathfinding in the neuroepithelium of embryonic brain is dependent on a variety of short and long range guidance cues. Heparan sulfate proteoglycans such as syndecans act as modulators of these cues and their importance in neural development is highlighted by their phylogenetic conservation. In Drosophilia, a single syndecan is present on the surface of axon growth cones and is required for chemorepulsive signalling during midline crossing. Understanding the role of syndecans in the vertebrate nervous system is challenging given that there are four homologous genes, syndecans 1–4. We show here that syndecans 2–4 are expressed in the zebrafish embryonic brain during the major period of axon growth. These genes show differing expression patterns in the brain which provides putative insights into their functional specificity.