Semaphorin-2a and Semaphorin-2b


DEVELOPMENTAL BIOLOGY

Embryonic

SEMA II mRNA is expressed at high levels in a small subset of CNS neurons, at high levels in a single T3 muscle, and in the gonad. Several anterior sensory structures are stained (Kolodkin, 1993).

A combinatorial semaphorin code instructs the initial steps of sensory circuit assembly in the Drosophila CNS

Longitudinal axon fascicles within the Drosophila embryonic CNS provide connections between body segments and are required for coordinated neural signaling along the anterior-posterior axis. This study shows that establishment of select CNS longitudinal tracts and formation of precise mechanosensory afferent innervation to the same CNS region are coordinately regulated by the secreted semaphorins Sema-2a and Sema-2b. Both Sema-2a and Sema-2b utilize the same neuronal receptor, plexin B (PlexB), but serve distinct guidance functions. Localized Sema-2b attraction promotes the initial assembly of a subset of CNS longitudinal projections and subsequent targeting of chordotonal sensory afferent axons to these same longitudinal connectives, whereas broader Sema-2a repulsion serves to prevent aberrant innervation. In the absence of Sema-2b or PlexB, chordotonal afferent connectivity within the CNS is severely disrupted, resulting in specific larval behavioral deficits. These results reveal that distinct semaphorin-mediated guidance functions converge at PlexB and are critical for functional neural circuit assembly (Wu, 2011).

The establishment of CNS longitudinal tracts in Drosophila occurs sequentially, from medial to lateral, through a series of distinct guidance events. These include extension of processes that pioneer these trajectories, and subsequent fasciculation and defasciculation events that allow additional processes to join these pathways, cross segment boundaries, and establish connectives that span the rostrocaudal axis of the embryonic CNS. During this process, a repulsive Slit gradient acts over a long range to establish three distinct lateral regions for longitudinally projecting axons, the choice of which is determined by differential expression of Robo receptors. Once they settle within an appropriate lateral region, individual axons that are part of the same bundle must then adhere to one another and remain fasciculated. This study found that Sema-2b signals through PlexB accomplish this task for longitudinal connectives in the intermediate region. Interestingly, this Sema-2b-PlexB-mediated organization is inherently connected to Silt-Robo-mediated patterning. The lateral position of intermediate longitudinal processes, including the pathway marked by Semi 2b-tauMyc reporter, is initially determined by Robo3-mediated signaling. Therefore where Sema-2b is expressed reflects lateral positional information derived from the Robo code. Then, this lateral information is further conveyed by the continuous Sema-2b expression over the entire anterior/posterior axis, mediating local organization of both CNS interneurons and sensory afferent projections through the PlexB receptor. When PlexB signaling is disrupted, 2b-tauMyc axons still project across the CNS midline and turn rostrally at the appropriate medial-to-lateral position; however, they subsequently wander both medially and laterally, often crossing the medio-lateral regional boundaries set by the Robo code. Therefore, PlexB-mediated Sema-2b signaling solidifies specific projection positioning originally established by the Robo code. Together, these two distinct Robo and plexin guidance cue signaling modules function in a sequential and complementary fashion to specify both long range medial-tolateral positioning (Robo) and short-range local fasciculation (PlexB). PlexA, the other Drosophila plexin receptor, and its ligand Sema-1a are specifically required for the proper formation of the 1D4-l pathway. However, Sema-1a does not show restricted expression within the medio-lateral axis of the nerve cord analogous to that observed for Sema-2b, suggesting a different mechanism may underlie Sema-1a-PlexA regulation of fasciculation in the most lateral CNS longitudinal region (Wu, 2011).

Following medio-lateral specification by Slit-Robo signaling and general organization of longitudinal regions by Sema-plexin signaling, additional cues are likely to mediate local interactions among neural processes already restricted to defined regions in the neuropile. Several cell surface proteins may serve such functions; for example, the cell adhesion molecule (CAM) connectin, like Sema-2b, shows exquisitely restricted expression along a subset of longitudinal projections. More widely expressed CAMs also play important roles in maintaining the fasciculated state of longitudinally projecting processes that are part of the same connective; indeed, in the absence of the Drosophila Ig super family member FasII, axons that contribute to the MP1 pathway show reduced association when examined at high resolution. Therefore, an ensemble of short-range cues expressed in distinct subsets of longitudinally projecting neurons allows for individual pathways to be established following more global restriction to appropriate locations, and as is demonstrated in this study, this process is critical for the neural circuit function. It seems likely that similar mechanisms underlie the segregation of complex trajectories, the establishment of laminar organization, and the formation of discrete neural maps in other regions of invertebrate and vertebrate nervous systems (Wu, 2011).

These analyses allow for a comparison between the effects of the secreted semaphorins Sema-2a and Sema-2b on both CNS interneuron trajectories and sensory afferent targeting within the CNS. It was observed in both LOF and GOF genetic paradigms that Sema-2a acts as a repellent, consistent with previous observations. Sema-2b, in contrast, serves an opposite guidance function and promotes neurite fasciculation. The highly restricted expression of Sema-2b within the intermediate domain of the nerve cord serves to assemble select longitudinal tracts and ch sensory afferents in this region, strongly suggesting that Sema-2b functions as a local attractive cue to define a specific CNS subregion and influence the organization of specific circuits (Wu, 2011).

Although both Sema-2b and Sema-2a signal through the same receptor, PlexB, they appear to do so independently. In the absence of Sema-2a, Sema-2b is still required for fasciculation and organization of the 2b-tMyc and 1D4-i tracks, and also for correct ch afferent innervation in the intermediate region of the nerve cord. In the absence of Sema-2b, Sema-2a expression alone results in potent repellent effects within the CNS for both the Sema-2b-tauMyc pathway and ch sensory afferent targeting. The distinct attractive and repulsive functions of Sema-2b and Sema-2a, respectively, are further revealed by the different phenotypes observed in GOF experiments. In the CNS of Sema-2b-/- mutant embryos, expression of Sema-2a under the control of the Sema-2b promoter results in both 2b-tauMyc and 1D4+ tract defasciculation much more severe than what is observed in the Sema-2b mutant alone; similar expression of Sema-2b fully rescues the discontinuous and disorganized Sema-2b-/- longitudinal connective phenotypes. Moreover, membrane-tethered Sema-2b is similarly capable of rescuing the Sema-2b-/- mutant phenotype, further supporting the idea that Sema-2b is a short-range attractant. In the periphery, misexpression of transmembrane versions of both Sema-2b and Sema-2a in a single body wall muscle demonstrates that Sema-2bTM overexpression results in motor neuron attraction, whereas Sema-2aTM in this same misexpression paradigm functions as a motor axon repellent (Wu, 2011).

This study also shows that PlexB is the receptor that mediates both Sema-2a and Sema-2b functions in the intermediate region of the developing nerve cord. Only Sema-2a-/- Sema-2b-/- double null mutants, and not either single mutant, fully recapitulates the PlexB-/-mutant phenotype, and ligand binding experiments demonstrate that PlexB is the endogenous receptor for both Sema-2a and Sema-2b in the embryonic nerve cord. However, both ligands exert opposing guidance functions despite sharing over 68% amino acid identity and also very similar protein structures. In vertebrates, distinct plexin coreceptors often bias the sign of semaphorin-mediated guidance events. The Drosophila ortholog of Off-Track, a transmembrane protein implicated in modulation of vertebrate and invertebrate plexin signaling apparently does not function in the Drosophila PlexB-mediated guidance events investigated here (data not shown). It will be important to define the relevant differences between the Sema-2a and Sema-2b proteins that are critical for affecting divergent PlexB signaling, and whether or not unique ligand-receptor protein-protein interactions result in differential PlexB activation of signaling cascades with diametrically opposed effects on cytoskeletal components (Wu, 2011).

It was found here that Sema-plexin signaling critical for specifying a subset of intermediate longitudinal pathways is also utilized to generate precise mapping of ch sensory input onto CNS neurons. In Drosophila, different classes of sensory axons target to distinct regions of the nerve cord neuropile, and the same Robo code essential for positioning CNS axons also regulates the medio-lateral positioning of sensory axons within the CNS. In addition to slit-mediated repulsive effects on sensory afferent targeting, Sema-1a and Sema-2a also restrict the ventrally and medially projecting afferents of the pain sensing Class IV neurons within the most ventral and most medial portions of the nerve cord neuropile. This is reminiscent of recent observations in the mammalian spinal cord showing that a localized source of secreted Sema3e directs proprioceptive sensory input through plexin D1 signaling, ensuring the specificity of sensory-motor circuitry in the spinal cord through repellent signaling. In addition, the transmembrane semaphorins Sema-6C and 6D provide repulsive signals in the dorsal spinal cord that direct appropriate proprioceptive sensory afferent central projections. However, little is known about the identity of cues that serve to promote selective association between sensory afferents and their appropriate central targets in vertebrates or invertebrates. This study finds that PlexB signaling guides ch sensory terminals to their target region in the CNS through Sema-2b-mediated attraction. Selective disruption of PlexB function in ch neurons severely abolishes normal ch afferent projection in the CNS. Using a high-throughput assay for quantifying larval behavioral responses to vibration, a role was confirmed for ch sensory neurons in larval mechanosensation. Using this assay it waa also possible to show that precise ch afferent targeting is required for central processing of vibration sensation and subsequent initiation of appropriate behavioral output. At present, the precise postsynaptic target of ch axons are not known, though the analysis suggests the Sema-2b+ neurons are good candidates. Combining vibration response assays with visualization of activated constituents of the ch vibration sensation circuit will allow for a comprehensive determination of input and output following proprioceptive sensation (Wu, 2011).

The formation of a functional circuit relies on the precise assembly of a series of pre- and postsynaptic components. Robo3-mediated signaling is required both for the targeting of ch axons and a subset of the longitudinally projecting interneurons to the same broad intermediate domain of the neuropile. This study shows that PlexB-mediated signaling is important for both the assembly of distinct longitudinal projections and also the targeting of ch sensory axon terminal arborizations within the same restricted subregion of the Robo3-defined intermediate domain of the Drosophila embryonic nerve cord. The secreted semaphorin Sema-2b is a PlexB ligand that plays a central role in both of these guidance events during Drosophila neural development. Sema-2b-PlexB signaling promotes selective fasciculation of the small population of longitudinally projecting axons that express Sema-2b and also immediately adjacent longitudinal projections in the intermediate medio-lateral region of the development CNS. Sema-2b also facilitates targeting of ch afferent terminals that subsequently arrive and establish synaptic contacts in this intermediate region of the developing nerve cord. Sema-2b-PlexB signaling ensures the correct assembly of the circuit that processes ch sensory information, and in its absence larval vibration responses are dramatically compromised. Interestingly, the other PlexB ligand within the CNS, Sema-2a, plays an opposing role to Sema-2b by preventing aberrant targeting through repulsion; together, these two secreted semaphorin ligands act in concert to assure precise neural projection in the developing CNS. Therefore, a combinatorial guidance code utilizes both repulsive and attractive semaphorin cues to mediate the accurate connection of distinct CNS structures and, ultimately, to ensure functional neural circuit assembly (Wu, 2011).

Effects of Mutation or Deletion

Mutants for Sema-2a show a greatly reduced eclosion, flightlessness and other behavioral defects, and death 2 days after adult eclosion. The surviving adults have normal wings, which they hold in the normal resting posture; however, shortly before dying, many hold their wings up. They can walk and jump, but they do not fly, even after a jump. The mutant adult flies are also abnormal in a visual orientation test, even though they are normal in a phototaxis test, showing that they are not blind. There are no gross abnormalities in the overall patterning of axon pathways in Sema-2a mutant embryos (Kolodkin, 1993)

The molecular mechanisms controlling the ability of motor axons to recognize their appropriate muscle targets were dissected using Drosophila genetics to add or subtract Netrin A, Netrin B, Semaphorin II, and Fasciclin II, either alone or in combination. Discrete target selection by neurons might be specified in a point-to-point fashion such that each motor axon and its appropriate target have unique and complementary molecular labels. Alternatively, specificity might emerge from a dynamic and comparative process in which growth cones respond to qualitative and quantitative molecular differences expressed by neighboring targets and make their decisions based on the relative balance of attractive and repulsive forces. Fas II and Sema II are expressed by all muscles where they promote (Fas II) or inhibit (Sema II) promiscuous synaptogenesis. The level of Sema II expression, while not enough to stop growth cones from exploring their environment, nevertheless provides a threshold that specific attractive signals must overcome in order to permit synapse formation. Decreasing Sema II leads to an increase in innervation. In the absence of Sema II, targeting errors occur, usually in the form of additional ectopic connections to neighboring muscles, although in some cases the absence of the normal connection or inappropriate choice point decisions are observed as well. Increasing Sema II leads to a decrease in innervation. It is concluded that growth cones in this system apparently do not rely solely on single molecular labels on individual targets. Rather, these growth cones assess the relative balance of attractive and repulsive forces and select their targets based on the combinatorial and simultaneous input of multiple cues. Apparently a relative balance model is more valid in this system than a lock-and-key model (Winberg, 1998).

The modest and dynamic level of Fas II helps adjust the threshold for innervation. Prior to synapse formation, Fas II is expressed at a low level across the entire surface of the muscle, making it permissive for growth cone exploration and synapse formation. As the first synapse forms on a muscle, the Fas II level dramatically plummets over the muscle surface while Fas II clusters under the developing synapse. The first successful synapse leads to a rapid reduction in this general attractant, thereby shifting the relative balance in favor of Sema II-mediated repulsion and thus raising the hurdle over which attractive signals must pass in order to promote further synapse formation. In this way, the innervated muscle becomes more refractory to further innervation. Fas II, as a modulator of the balance of attraction and repulsion, becomes a temporal measure of the muscle's synaptic history (Winberg, 1998).

While Sema II generally prevents exuberant synapse formation, it can also play an important role in patterning connections. For example, the two axons that pioneer the transverse nerve (TN) normally meet and fasciculate near muscle 7. In the absence of Sema II, these axons often innervate muscles 7 and 6, and sometimes fail to fasciculate with one another. In this case, Sema II provides a repulsive force (from muscles 7 and 6) at a specific choice point, and in its absence, the TN growth cones make a different decision. Similarly, as the lateral branch of the segmental nerve branch a (SNa) extends posteriorly, one axon branch innervates muscle 5 while another continues posteriorly to innervate muscle 8. In the absence of Sema II, both sometimes stop and innervate muscle 5. In this case, Sema II provides a key repulsive force (from muscle 5) at a specific choice point, and in its absence, the growth cone that usually innervates muscle 8 instead makes a different decision. Both examples show how Sema II can do more than simply sharpen the pattern of innervation; Sema II can also influence specific targeting decisions in a dosage-dependent fashion. The Sema II experiments show that the pattern of expression (i.e., the differential levels expressed by neighboring muscles) can be more important than the absolute level. Simply increasing Sema II on all muscles has little influence on the SNa. But increasing Sema II expression on muscle 5 and not its neighboring muscles does influence the SNa axons, presumably because it presents these axons with a sharp repulsive boundary. This differential expression prevents the lateral branch of the SNa from extending towards muscles 5 and 8 (Winberg, 1998).

The netrins were initially discovered as long-range chemoattractants that are secreted by midline cells and that attract commissural growth cones toward the midline. Netrins might have another function, and strong evidence is presented supporting this notion. In addition to their CNS midline expression and function in axon guidance, NetA and NetB are also expressed by distinct subsets of muscles where they function as short-range target recognition molecules. Genetic analysis suggests that both types of Netrin-mediated attractive responses (i.e., pathfinding and targeting) require Frazzled, the DCC/UNC-40-like Netrin receptor. In contrast, Fra is not required for NetB-mediated repulsion of the segmental nerve. Even though they are expressed by distinct subsets of muscles and function as target recognition molecules, the two netrins, NetA and NetB, do not act alone in specifying any one of these muscle targets. NetB is expressed by muscles 7 and 6, but NetB is not the sole attractant used by RP3 to innervate these muscles. In the absence of NetB, in 35% of segments RP3 makes the correct pathfinding decisions in the periphery but fails to innervate muscles 7 and 6 properly. However, in the other 65% of segments it does innervate muscles 7 and 6. Clearly, other unknown cues must play a major role in this targeting decision. One potential candidate for an additional targeting cue is the Ig CAM Fasciclin III. However, removal of FasIII does not alter the penetrance of the RP3 phenotype of Netrin or frazzled mutants. NetB functions within the context of the relative balance of general attractants and repellents such as Fas II and Sema II. For example, since the TN axons are attracted by NetB, and muscles 7 and 6 express NetB, why do the TN axons not synapse on muscles 7 and 6? Evidently, they are sufficiently repelled by Sema II to prevent inappropriate synapse formation. Either increasing the level of NetA or NetB or decreasing the level of Sema II leads to ectopic TN synapses. The choice of synaptic partner by TN axons is controlled by the balance of NetB in relation to Sema II and Fas II (Winberg, 1998).

Distinct classes of motor axons respond differentially to NetA and NetB While all motor axons in this system appear to be attracted by Fas II and repelled by Sema II, the different types of motor axons respond differently to NetA and NetB. NetB is expressed by a subset of muscles (7 and 6) where it strongly attracts appropriate (RP3) axons, more weakly attracts certain inappropriate (TN) axons, and repels other inappropriate (SN) axons. RP3 and TN axons can also be strongly attracted by NetA, while SN axons are apparently indifferent to NetA. The TN axons display a stronger responsiveness to NetA than to NetB, as judged by the frequency of ectopic innervation of ventral muscles overexpressing either Netrin. This difference may make biological sense, as TN axons normally extend toward a dorsal stripe of epithelial cells expressing NetA but grow past NetB-expressing ventral muscles without innervating them (Winberg, 1998).

Although all of the molecular signals used for this targeting system are not yet known, four key components have been identified: the pan-muscle expression of Fas II and Sema II and the muscle-specific expression of NetA and NetB. Analysis of these four genes shows that the signals they encode are potent, function as short-range signals in a dosage-dependent fashion, and work in combinations that either amplify or antagonize one another. Fas II and Sema II help control the fidelity and precision of the targeting system, while NetA and NetB provide muscle-specific targeting cues. These results suggest that target selection in this system is not based on absolute attractants or repellents that either ensure or prevent synapse formation, but rather it is based on the balance of attractive and repulsive forces on any given target cell in relationship to its neighboring cells. Targeting molecules such as Netrins, Semaphorins, and IgCAMs sometimes function as antagonists and sometimes as collaborators. This model of target selection is very similar to the current view of axon guidance in terms of a relative balance of attractive and repulsive forces (Winberg, 1998).


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Semaphorin-2a and Semaphorin-2b: Biological Overview | Evolutionary Homologs | Developmental Biology | Effects of Mutation

date revised: 20 August 2014 

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