Expression patterns of semaphorin7A and plexinC1 during rat neural development suggest roles in axon guidance and neuronal migration

Background

Although originally identified as embryonic axon guidance cues, semaphorins are now known to regulate multiple, distinct, processes crucial for neuronal network formation including axon growth and branching, dendritic morphology, and neuronal migration. Semaphorin7A (Sema7A), the only glycosylphosphatidylinositol-anchored semaphorin, promotes axon growth in vitro and is required for the proper growth of the mouse lateral olfactory tract in vivo. Sema7A has been postulated to signal through two unrelated receptors, an RGD-dependent α1β1-integrin and a member of the plexin family, plexinC1. β1-integrins underlie Sema7A-mediated axon growth and Sema7A function in the immune system. Sema7A-plexinC1 interactions have also been implicated in immune system function, but the neuronal role of this ligand-receptor pair remains to be explored. To gain further insight into the function(s) of Sema7A and plexinC1 during neural development, we present here a detailed analysis of Sema7A and plexinC1 expression in the developing rat nervous system.

Results

In situ hybridization revealed select expression of Sema7A and plexinC1 in multiple neuronal systems including: the olfactory system, the hypothalamo-hypophysial system, the hippocampus, the meso-diencephalic dopamine system, and the spinal cord. Within these systems, Sema7A and plexinC1 are often expressed in specific neuronal subsets. In general, Sema7A transcript levels increase significantly towards adulthood, whereas plexinC1 expression decreases as development proceeds.

PlexinC1, but not Sema7A, is strongly expressed by distinct populations of migrating neurons. In addition to neuronal expression, Sema7A and plexinC1 transcripts were detected in oligodendrocytes and ependymal cells, respectively.

Conclusion

Sema7A and plexinC1 expression patterns are consistent with these proteins serving both cooperative and separate functions during neural development. The prominent expression of plexinC1 in several distinct populations of migrating neurons suggests a novel role for this plexin family member in neuronal migration.

Semaphorins in axon regeneration: developmental guidance molecules gone wrong?

Semaphorins are developmental axon guidance cues that continue to be expressed during adulthood and are regulated by neural injury. During the formation of the nervous system, repulsive semaphorins guide axons to their targets by restricting and channelling their growth. They affect the growth cone cytoskeleton through interactions with receptor complexes that are linked to a complicated intracellular signal transduction network. Following injury, regenerating axons stop growing when they reach the border of the glial-fibrotic scar, in part because they encounter a potent molecular barrier that inhibits growth cone extension. A number of secreted semaphorins are expressed in the glial-fibrotic scar and at least one transmembrane semaphorin is upregulated in oligodendrocytes surrounding the lesion site. Semaphorin receptors, and many of the signal transduction components required for semaphorin signalling, are present in injured central nervous system neurons. Here, we review evidence that supports a critical role for semaphorin signalling in axon regeneration, and highlight a number of challenges that lie ahead with respect to advancing our understanding of semaphorin function in the normal and injured adult nervous system.

Semaphorin 7A initiates T-cell-mediated inflammatory responses through alpha1beta1 integrin

Semaphorins are axon guidance factors that assist growing axons in finding appropriate targets and forming synapses. Emerging evidence suggests that semaphorins are involved not only in embryonic development but also in immune responses. Semaphorin 7A (Sema7A; also known as CD108), which is a glycosylphosphatidylinositol-anchored semaphorin, promotes axon outgrowth through beta1-integrin receptors and contributes to the formation of the lateral olfactory tract. Although Sema7A has been shown to stimulate human monocytes, its function as a negative regulator of T-cell responses has also been reported. Thus, the precise function of Sema7A in the immune system remains unclear. Here we show that Sema7A, which is expressed on activated T cells, stimulates cytokine production in monocytes and macrophages through alpha1beta1 integrin (also known as very late antigen-1) as a component of the immunological synapse, and is critical for the effector phase of the inflammatory immune response. Sema7A-deficient (Sema7a-/-) mice are defective in cell-mediated immune responses such as contact hypersensitivity and experimental autoimmune encephalomyelitis. Although antigen-specific and cytokine-producing effector T cells can develop and migrate into antigen-challenged sites in Sema7a-/- mice, Sema7a-/- T cells fail to induce contact hypersensitivity even when directly injected into the antigen-challenged sites. Thus, the interaction between Sema7A and alpha1beta1 integrin is crucial at the site of inflammation. These findings not only identify a function of Sema7A as an effector molecule in T-cell-mediated inflammation, but also reveal a mechanism of integrin-mediated immune regulation.

MICAL flavoprotein monooxygenases: structure, function and role in semaphorin signaling

MICALs (for Molecule Interacting with CasL) form a recently discovered family of evolutionary conserved signal transduction proteins. They contain multiple well-conserved domains known for interactions with the cytoskeleton, cytoskeletal adaptor proteins, and other signaling proteins. In addition to their ability to bind other proteins, MICALs contain a large NADPH-dependent flavoprotein monooxygenase enzymatic domain. Although MICALs have already been implicated in a variety of cellular processes, their function during axonal pathfinding in the Drosophila neuromuscular system has been best characterized. During the establishment of neuromuscular connectivity in the fruit fly, MICAL binds the axon guidance receptor Plexin A and transduces semaphorin-1a-mediated repulsive axon guidance. Intriguingly, mutagenesis and pharmacological inhibitor studies suggest a role for MICAL flavoenzyme redox functions in semaphorin/plexin-mediated axonal pathfinding events. This review summarizes our current understanding of MICALs, with an emphasis on their role in semaphorin signaling.

High-resolution structure of the catalytic region of MICAL (molecule interacting with CasL), a multidomain flavoenzyme-signaling molecule

Semaphorins are extracellular cell guidance cues that govern cytoskeletal dynamics during neuronal and vascular development. MICAL (molecule interacting with CasL) is a multidomain cytosolic protein with a putative flavoprotein monooxygenase (MO) region required for semaphorin-plexin repulsive axon guidance. Here, we report the 1.45-A resolution crystal structure of the FAD-containing MO domain of mouse MICAL-1 (residues 1-489). The topology most closely resembles that of the NADPH-dependent flavoenzyme p-hydroxybenzoate hydroxylase (PHBH). Comparison of structures before and after reaction with NADPH reveals that, as in PHBH, the flavin ring can switch between two discrete positions. In contrast with other MOs, this conformational switch is coupled with the opening of a channel to the active site, suggestive of a protein substrate. In support of this hypothesis, distinctive structural features highlight putative protein-binding sites in suitable proximity to the active site entrance. The unusual juxtaposition of this N-terminal MO (hydroxylase) activity with the characteristics of a multiprotein-binding scaffold exhibited by the C-terminal portion of the MICALs represents a unique combination of functionality to mediate signaling.

MICAL flavoprotein monooxygenases: expression during neural development and following spinal cord injuries in the rat

MICALs comprise of a family of phylogenetically conserved, multidomain cytosolic flavoprotein monooxygenases. Drosophila (D-)MICAL binds the neuronal Sema1a receptor PlexA, and D-MICAL-PlexA interactions are required in vivo for Sema1a-induced axon repulsion. The biological functions of vertebrate MICAL proteins, however, remain unknown. Here, we describe three rodent MICAL genes and analyze their expression in the intact rat nervous system and in two models of spinal cord injury. MICAL-1, -2, and -3 expression patterns in the embryonic, postnatal, and adult nervous system support the idea that MICALs play roles in neural development and plasticity. In addition, MICAL expression is elevated in oligodendrocytes and in meningeal fibroblasts at sites of spinal cord injury but is unchanged in lesioned corticospinal tract neurons. Furthermore, we find that the selective monooxygenase inhibitor EGCG attenuates the repulsive effects of Sema3A and Sema3F in vitro, but not those of several other repulsive cues and substrates. These results implicate MICALs in neuronal regeneration and support the possibility of employing EGCG to attenuate Sema3-mediated axon repulsion in the injured spinal cord.

Soluble CD100 functions on human monocytes and immature dendritic cells require plexin C1 and plexin B1, respectively

CD100 represents the first semaphorin described in the immune system. It is expressed as a 300-kDa homodimer at the surface of most hematopoietic cells, but is also found in a soluble form following a proteolytic cleavage upon cell activation. We herein established that soluble CD100 (sCD100) impaired the migration of human monocytes and immature dendritic cells (DCs), but not of mature DCs. Performing competition assays, we identified plexin C1 (VESPR/CD232) as being involved in sCD100-mediated effects on human monocytes. Interestingly, we observed a complete down-regulation of plexin C1 expression during the in vitro differentiation process of monocytes to immature DCs, while concomitantly the surface expression of plexin B1 was induced. The latter receptor then binds sCD100 on immature DCs, mediating its inhibitory effect on cell migration. Finally, we showed that sCD100 modulated the cytokine production from monocytes and immature DCs. Together these results suggest that sCD100 plays a critical role in the regulation of antigen-presenting cell migration and functions via a tightly regulated process of receptor expression.

R-Ras fills another GAP in semaphorin signalling

Plexins are cell-surface receptors for the semaphorin family of neuronal guidance cues. Following semaphorin binding, the plexin cytoplasmic region initiates poorly understood signal-transduction events that lead to modifications of the cytoskeleton. Recent findings shed new light on the signalling network downstream of semaphorins and plexins by demonstrating that one of the plexins, plexin-B1, possesses an intrinsic GTPase-activating protein (GAP) activity towards R-Ras. Inactivation of R-Ras by the plexin-B1 GAP domains is required for plexin-B1-mediated effects on the cytoskeleton. These results indicate that plexins not only bind to but also regulate directly the activity of some of their downstream effectors.

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.

Semaphorin junction: making tracks toward neural connectivity

Semaphorins constitute one of the largest families of repulsive and attractive growth cone guidance proteins. They affect the growth cone's actin cytoskeleton through interactions with receptor complexes composed of ligand-binding, signal-transducing, and modulatory subunits. Our understanding of the intracellular signal transduction machinery linking semaphorins to actin dynamics is limited; however, recent advances provide a more comprehensive view of the molecular basis of neuronal semaphorin signaling.

MICALs, a family of conserved flavoprotein oxidoreductases, function in plexin-mediated axonal repulsion

Members of the semaphorin family of secreted and transmembrane proteins utilize plexins as neuronal receptors to signal repulsive axon guidance. It remains unknown how plexin proteins are directly linked to the regulation of cytoskeletal dynamics. Here, we show that Drosophila MICAL, a large, multidomain, cytosolic protein expressed in axons, interacts with the neuronal plexin A (PlexA) receptor and is required for Semaphorin 1a (Sema-1a)-PlexA-mediated repulsive axon guidance. In addition to containing several domains known to interact with cytoskeletal components, MICAL has a flavoprotein monooxygenase domain, the integrity of which is required for Sema-1a-PlexA repulsive axon guidance. Vertebrate orthologs of Drosophila MICAL are neuronally expressed and also interact with vertebrate plexins, and monooxygenase inhibitors abrogate semaphorin-mediated axonal repulsion. These results suggest a novel role for oxidoreductases in repulsive neuronal guidance.

Injury-induced class 3 semaphorin expression in the rat spinal cord

In this study we evaluate the expression of all members of the class 3 semaphorins and their receptor components following complete transection and contusion lesions of the adult rat spinal cord. Following both types of lesions the expression of all class 3 semaphorins is induced in fibroblast in the neural scar. The distribution of semaphorin-positive fibroblasts differs markedly in scars formed after transection or contusion lesion. In contusion lesions semaphorin expression is restricted to fibroblasts of the meningeal sheet surrounding the lesion, while after transection semaphorin-positive fibroblast penetrate deep into the center of the lesion. Two major descending spinal cord motor pathways, the cortico- and rubrospinal tract, continue to express receptor components for class 3 semaphorins following injury, rendering them potentially sensitive to scar-derived semaphorins. In line with this we observed that most descending spinal cord fibers were not able to penetrate the semaphorin positive portion of the neural scar formed at the lesion site. These results suggest that the full range of secreted semaphorins contributes to the inhibitory nature of the neural scar and thereby may inhibit successful regeneration in the injured spinal cord. Future studies will focus on the neutralization of class 3 semaphorins, in order to reveal whether this creates a more permissive environment for regeneration of injured spinal cord axons.

Emerging roles for semaphorins in neural regeneration

Progressive axon outgrowth during neural development contrasts with the failure of regenerative neurite growth in the mature mammalian central nervous system (CNS). During neuroembryogenesis, spatiotemporal patterns of repellent and attractant activities in the vicinity of the growth cone favor neurite outgrowth. In the mature CNS, however, a relative balance between forces supporting and restricting axon growth has been established, only allowing subtle morphological changes in existing neuritic arbors and synapses. Following CNS injury, this balance shifts towards enhanced expression of growth-inhibiting molecules and diminished availability of their growth-promoting counterparts. Evidence is now emerging that the proteins governing developmental axon guidance critically contribute to the failure of injured central neurons to regenerate. As a first step toward elucidation of the role of chemorepulsive axon guidance signals in axonal regeneration, the effects of lesions of the central and peripheral nervous system on the expression of Semaphorin3A, the prototype and founding member of the semaphorin family of axon guidance signals, and of the Semaphorin3A receptor proteins neuropilin-1 and plexin-A1 have recently been examined. Here we review the first evidence indicating that (i) lesion-induced changes in the expression of chemorepulsive semaphorins relate to the success or failure of injured neurons to regenerate and (ii) semaphorins may represent important molecular signals controlling multiple aspects of the cellular response that follows CNS injury. In the future, genetic manipulation of the injury-induced changes in the availability of semaphorins and/or of their receptors will provide further insight into the mechanisms by which semaphorins influence neural regeneration.

Peripheral nerve injury fails to induce growth of lesioned ascending dorsal column axons into spinal cord scar tissue expressing the axon repellent Semaphorin3A

We have investigated the hypothesis that the chemorepellent Semaphorin3A may be involved in the failure of axonal regeneration after injury to the ascending dorsal columns of adult rats. Following transection of the thoracic dorsal columns, fibroblasts in the dorsolateral parts of the lesion site showed robust expression of Semaphorin3A mRNA. In addition, dorsal root ganglion (DRG) neurons with projections through the dorsal columns to the injury site persistently expressed both Semaphorin3A receptor components, neuropilin-1 and plexin-A1. These ascending DRG collaterals failed to invade scar regions occupied by Semaphorin3A-positive fibroblasts, even in animals which had received conditioning lesions of the sciatic nerve to enhance regeneration. Other axon populations in the dorsal spinal cord were similarly unable to penetrate Semaphorin3A-positive scar tissue. These data suggest that Semaphorin3A may create an exclusion zone for regenerating dorsal column fibres and that enhancing the intrinsic regenerative response of DRG neurons has only limited effects on axonal regrowth. Tenascin-C and chondroitin sulphate proteoglycans were also detected at the injury site, which was largely devoid of central nervous system (CNS) myelin, showing that several classes of inhibitory factors, including semaphorins, with only partially overlapping spatial and temporal patterns of expression are in a position to participate in preventing regenerative axonal growth in the injured dorsal columns. Interestingly, conditioning nerve injuries enabled numerous ascending DRG axons to regrow across areas of strong tenascin-C and chondroitin sulphate proteoglycan expression, while areas containing Semaphorin3A and CNS myelin were selectively avoided by (pre)primed axonal sprouts.

Ectopic adenoviral vector-directed expression of Sema3A in organotypic spinal cord explants inhibits growth of primary sensory afferents

Sema3A (Sema III, SemD, collapsin-1) can induce neuronal growth cone collapse and axon repulsion of distinct neuronal populations. To study Sema3A function in patterning afferent projections into the developing spinal cord, we employed the recombinant adenoviral vector technique in embryonic rat spinal cord slices. Virus solution was injected in the dorsal aspect of organotypic spinal cord cultures with segmentally attached dorsal root ganglia (sc-DRG). In cultures grown in the presence of nerve growth factor (NGF), injected either with the control virus AdCMVLacZ or with vehicle only, afferent innervation patterns were similar to those of control. However, unilateral injection of AdCMVSema3A/AdCMVLacZ in sc-DRG slices revealed a strong inhibitory effect on NGF-dependent sensory afferent growth. Ectopic Sema3A in the dorsal spinal cord, the target area of NGF-responsive DRG fibers in vivo, created an exclusion zone for these fibers and as a result they failed to reach and innervate their appropriate target zones. Taken together, gain of Sema3A function in the dorsal aspect of sc-DRG cultures revealed a dominant inhibitory effect on NGF-dependent, nociceptive sensory DRG afferents, an observation in line with the model proposed by E. K. Messersmith et al. (1995, Neuron 14, 949-959), suggesting that Sema3A secreted by spinal cord cells can act to repel central sensory fibers during the formation of lamina-specific connections in the spinal cord.

Semaphorins and their receptors in olfactory axon guidance

The mammalian olfactory system is capable of discriminating among a large variety of odor molecules and is therefore essential for the identification of food, enemies and mating partners. The assembly and maintenance of olfactory connectivity have been shown to depend on the combinatorial actions of a variety of molecular signals, including extracellular matrix, cell adhesion and odorant receptor molecules. Recent studies have identified semaphorins and their receptors as putative molecular cues involved in olfactory pathfinding, plasticity and regeneration. The semaphorins comprise a large family of secreted and transmembrane axon guidance proteins, being either repulsive or attractive in nature. Neuropilins were shown to serve as receptors for secreted class 3 semaphorins, whereas members of the plexin family are receptors for class 1 and V (viral) semaphorins. The present review will discuss a role for semaphorins and their receptors in the establishment and maintenance of olfactory connectivity.

Expression of the gene encoding the chemorepellent semaphorin III is induced in the fibroblast component of neural scar tissue formed following injuries of adult but not neonatal CNS

This study evaluates the expression of the chemorepellent semaphorin III (D)/collapsin-1 (sema III) following lesions to the rat CNS. Scar tissue, formed after penetrating injuries to the lateral olfactory tract (LOT), cortex, perforant pathway, and spinal cord, contained numerous spindle-shaped cells expressing high levels of sema III mRNA. The properties of these cells were investigated in detail in the lesioned LOT. Most sema III mRNA-positive cells were located in the core of the scar and expressed proteins characteristic for fibroblast-like cells. Neuropilin-1, a sema III receptor, was expressed in injured neurons with projections to the lesion site, in a subpopulation of scar-associated cells and in blood vessels around the scar. In contrast to lesions made in the mature CNS, LOT transection in neonates did not induce sema III mRNA expression within cells in the lesion and was followed by vigorous axonal regeneration. The concomitant expression of sema III and its receptor neuropilin-1 in the scar suggests that sema III/neuropilin-1-mediated mechanisms are involved in CNS scar formation. The expression of the secreted chemorepellent sema III following CNS injury provides the first evidence that chemorepulsive semaphorins may contribute to the inhibitory effects exerted by scars on the outgrowth of injured CNS neurites. The vigorous regrowth of injured axons in the absence of sema III following early neonatal lesions is consistent with this notion. The inactivation of sema III in scar tissue by either antibody perturbation or by genetic or pharmacological intervention could be a powerful means to promote long-distance regeneration in the adult CNS.

Evidence for a role of the chemorepellent semaphorin III and its receptor neuropilin-1 in the regeneration of primary olfactory axons

To explore a role for chemorepulsive axon guidance mechanisms in the regeneration of primary olfactory axons, we examined the expression of the chemorepellent semaphorin III (sema III), its receptor neuropilin-1, and collapsin response mediator protein-2 (CRMP-2) during regeneration of the olfactory system. In the intact olfactory system, neuropilin-1 and CRMP-2 mRNA expression define a distinct population of olfactory receptor neurons, corresponding to immature (B-50/GAP-43-positive) and a subset of mature (olfactory marker protein-positive) neurons located in the lower half of the olfactory epithelium. Sema III mRNA is expressed in pial sheet cells and in second-order olfactory neurons that are the target cells of neuropilin-1-positive primary olfactory axons. These data suggest that in the intact olfactory bulb sema III creates a molecular barrier, which helps restrict ingrowing olfactory axons to the nerve and glomerular layers of the bulb. Both axotomy of the primary olfactory nerve and bulbectomy induce the formation of new olfactory receptor neurons expressing neuropilin-1 and CRMP-2 mRNA. After axotomy, sema III mRNA is transiently induced in cells at the site of the lesion. These cells align regenerating bundles of olfactory axons. In contrast to the transient appearance of sema III-positive cells at the lesion site after axotomy, sema III-positive cells increase progressively after bulbectomy, apparently preventing regenerating neuropilin-1-positive nerve bundles from growing deeper into the lesion area. The presence of sema III in scar tissue and the concomitant expression of its receptor neuropilin-1 on regenerating olfactory axons suggests that semaphorin-mediated chemorepulsive signal transduction may contribute to the regenerative failure of these axons after bulbectomy.

Regulation of semaphorin III/collapsin-1 gene expression during peripheral nerve regeneration

The competence of neurons to regenerate depends on their ability to initiate a program of gene expression supporting growth and on the growth-permissive properties of glial cells in the distal stump of the injured nerve. Most studies on intrinsic molecular mechanisms governing peripheral nerve regeneration have focussed on the lesion-induced expression of proteins promoting growth cone motility, neurite extension, and adhesion. However, little is known about the expression of intrinsic chemorepulsive proteins and their receptors, after peripheral nerve injury and during nerve regeneration. Here we report the effect of peripheral nerve injury on the expression of the genes encoding sema III/coll-1 and its receptor neuropilin-1, which are known to be expressed in adult sensory and/or motor neurons. We have shown that peripheral nerve crush or transection results in a decline in sema III/coll-1 mRNA expression in injured spinal and facial motor neurons. This decline was paralleled by an induction in the expression of the growth-associated protein B-50/GAP-43. As sema III/coll-1 returned to normal levels following nerve crush, B-50/GAP-43 returned to precrush levels. Thus, the decline in sema III/coll-1 mRNA coincided with sensory and motor neuron regeneration. A sustained decline in sema III/coll-1 mRNA expression was found when regeneration was blocked by nerve transection and ligation. No changes were observed in neuropilin-1 mRNA levels after injury to sensory and motor neurons, suggesting that regenerating peripheral neurons continue to be sensitive to sema III/coll-1. Therefore we propose that a decreased expression of sema III/coll-1, one of the major ligands for neuropilin-1, during peripheral nerve regeneration is an important molecular event that is part of the adaptive response related to the success of regenerative neurite outgrowth occurring following peripheral nerve injury.

Adenoviral vector-mediated gene delivery to injured rat peripheral nerve

Although much progress has been made, current treatments of peripheral nerve damage mostly result in only partial recovery. Local production of neurite outgrowth-promoting molecules, such as neurotrophins and/or cell adhesion molecules, at the site of damage may be used as a new means to promote the regeneration process. We have now explored the ability of an adenoviral vector encoding the reporter gene LacZ (Ad-LacZ) to direct the expression of a foreign gene to Schwann cells of intact and crushed rat sciatic nerves. Infusion of 8 x 10(7) PFU Ad-LacZ in the intact sciatic nerve resulted in the transduction of many Schwann cells with high levels of transgene expression lasting at least up to 12 days following viral vector administration. The efficacy of adenoviral vector delivery to a crushed nerve was investigated using three strategies. Injection of the adenoviral vector at the time of, or immediately after, a crush resulted in the transduction of only a few Schwann cells. Administration of the adenoviral vector the day after the crush resulted in the transduction of a similar number of Schwann cells 5 days after administration, as observed in uncrushed nerves. Regenerating nerve fibers were closely associated with beta-galactosidase-positive Schwann cells, indicating that the capacity of transduced Schwann cells to guide regenerating fibers was not altered. These results imply that the expression of growth-promoting proteins through adenoviral vector-mediated gene transfer may be a realistic option to promote peripheral nerve regeneration.

Anatomical distribution of the chemorepellent semaphorin III/collapsin-1 in the adult rat and human brain: predominant expression in structures of the olfactory-hippocampal pathway and the motor system

Alterations in neuronal connectivity of the mature central nervous system (CNS) appear to depend on a delicate balance between growth-promoting and growth-inhibiting molecules. To begin to address a potential role of the secreted chemorepulsive protein semaphorin(D)III/collapsin-1 (semaIII/coll-1) in structural plasticity during adulthood, we used high-resolution nonradioactive in situ hybridization to identify neural structures that express semaIII/coll-1 mRNA in the mature rat and human brain. SemaIII/coll-1 was expressed in distinct but anatomically and functionally linked structures of the adult nervous system. The olfactory-hippocampal pathway displayed semaIII/coll-1 expression in a continuum of neuronal structures, including mitral and tufted cells of the olfactory bulb, olfactory tubercle, and piriform cortex; and distinct nuclei of the amygdaloid complex, the superficial layers of the entorhinal cortex, and the subiculum of the hippocampal formation. In addition, prominent labeling was found in neuronal components of the motor system, particularly in cerebellar Purkinje cells and in subpopulations of cranial and spinal motoneurons. Retrograde tracing combined with in situ hybridization also revealed that the staining of semaIII/coll-1 within the entorhinal cortex was present in the stellate neurons that project via the perforant path to the molecular layer of the dentate gyrus. Like in the rat, the human brain displayed discrete expression of semaIII/coll-1. Among the structures examined, the most prominent staining was observed in the cellular islands of the superficial layers of the human entorhinal cortex. The constitutive expression of the chemorepellent semaIII/coll-1 in discrete populations of neurons in the mature rat and human CNS raises the possibility that, in addition to its function as repulsive axon guidance cue during development, semaIII/coll-1 might be involved in restricting structural changes that occur in the wiring of the intact CNS.

Differential expression of estrogen receptor mRNA and protein in the female rat preoptic area

The expression of estrogen receptor (ER) mRNA and ER protein in the medial preoptic area of ovariectomized rat was investigated at both cellular and regional levels using non-isotopic in situ hybridization and immunohistochemistry. ER mRNA was localized in the cytoplasm, while both liganded and unliganded forms of the ER protein were confined to the nucleus. Furthermore, ER mRNA containing cells were evenly distributed throughout the medial preoptic area, showing a homogeneous staining pattern compared to that of ER protein. ER immunoreactive cells were highly distributed in the medial, moderately in the lateral aspect of the medial preoptic area, showing a heterogeneous staining pattern with strongly and weakly labeled cells. These results suggest that ER protein levels are controlled by cellular posttranscriptional mechanisms.

The alteration of glucocorticoid receptor-immunoreactivity in the rat forebrain following short-term and long-term adrenalectomy

To examine the effect of short-term and long-term adrenalectomy (ADX) on the glucocorticoid receptor (GR) expression, we performed an immunohistochemical study on the rat forebrain. One day after ADX, the GR-immunoreactivity significantly decreased or disappeared in most forebrain structures, while relatively strong GR-immunoreactivity was still found within the hypothalamus especially in the arcuate nucleus (ARC) and the parvocellular paraventricular nucleus (PVN). Two weeks following ADX, GR-immunoreactive cells disappeared in many structures of the forebrain including most parts of hypothalamus while moderate GR-immunoreactivity was still observable in the ARC and PVN. More than 3 months after ADX, the rats still survived when they received replacement of corticosterone during the first 2 weeks following the operation. Moderate GR-immunoreactivity in the ARC and PVN of the hypothalamus was exhibited whereas no immunoreactive cells remained in the cerebral cortex, thalamus and other forebrain structures when these animals showed obvious cell death in the granule cells of the dentate gyrus, identified with the silver impregnation method for degenerating cells. Massive cell loss in this hippocampal region is an indicator of a complete ADX, in addition to the blood corticosterone level. These results demonstrate topographic differences of GR expression in the rat forebrain after ADX with only continuous immunoreactivity in the ARC and PVN of the hypothalamus, suggesting that some neurons in the ARC and PVN could keep active GR probably in order to maintain their survival after removing the adrenal gland.

The perinatal ontogeny of estrogen receptor-immunoreactivity in the developing male and female rat hypothalamus

Developmental expression of the estrogen receptor (ER) in rat hypothalamus was examined using immunohistochemistry. In the medial preoptic nucleus and ventromedial nucleus ER-immunoreactivity was detected as early as E17, whereas ER protein expression in the periventricular preoptic nucleus and arcuate nucleus was delayed until E19. These results show that following a region specific onset of the ER protein expression sex differences in ER levels are already detectable during the perinatal period.