Long-term survival of glial segments during nerve regeneration in the leech

Sex-Specific Microglial Responses to Glucocerebrosidase Inhibition: Relevance to GBA1-Linked Parkinson's Disease

Microglia are heterogenous cells characterized by distinct populations each contributing to specific biological processes in the nervous system, including neuroprotection. To elucidate the impact of sex-specific microglia heterogenicity to the susceptibility of neuronal stress, we video-recorded with time-lapse microscopy the changes in shape and motility occurring in primary cells derived from mice of both sexes in response to pro-inflammatory or neurotoxic stimulations. With this morpho-functional analysis, we documented distinct microglia subpopulations eliciting sex-specific responses to stimulation: male microglia tended to have a more pro-inflammatory phenotype, while female microglia showed increased sensitivity to conduritol-B-epoxide (CBE), a small molecule inhibitor of glucocerebrosidase, the enzyme encoded by the GBA1 gene, mutations of which are the major risk factor for Parkinson's Disease (PD). Interestingly, glucocerebrosidase inhibition particularly impaired the ability of female microglia to enhance the Nrf2-dependent detoxification pathway in neurons, attenuating the sex differences observed in this neuroprotective function. This finding is consistent with the clinical impact of GBA1 mutations, in which the 1.5-2-fold reduced risk of developing idiopathic PD observed in female individuals is lost in the GBA1 carrier population, thus suggesting a sex-specific role for microglia in the etiopathogenesis of PD-GBA1.

Natural mechanisms and artificial PEG-induced mechanism that repair traumatic damage to the plasmalemma in eukaryotes

Eukaryotic tissues are composed of individual cells surrounded by a plasmalemma that consists of a phospholipid bilayer with hydrophobic heads that bind cell water. Bound-water creates a thermodynamic barrier that impedes the fusion of a plasmalemma with other membrane-bound intracellular structures or with the plasmalemma of adjacent cells. Plasmalemmal damage consisting of small or large holes or complete transections of a cell or axon results in calcium influx at the lesion site. Calcium activates fusogenic pathways that have been phylogenetically conserved and that lower thermodynamic barriers for fusion of membrane-bound structures. Calcium influx also activates phylogenetically conserved sealing mechanisms that mobilize the gradual accumulation and fusion of vesicles/membrane-bound structures that seal the damaged membrane. These naturally occurring sealing mechanisms for different cells vary based on the type of lesion, the type of cell, the proximity of intracellular membranous structures to the lesion and the relation to adjacent cells. The reliability of different measures to assess plasmalemmal sealing need be carefully considered for each cell type. Polyethylene glycol (PEG) bypasses calcium and naturally occurring fusogenic pathways to artificially fuse adjacent cells (PEG-fusion) or artificially seal transected axons (PEG-sealing). PEG-fusion techniques can also be used to rapidly rejoin the closely apposed, open ends of severed axons. PEG-fused axons do not (Wallerian) degenerate and PEG-fused nerve allografts are not immune-rejected, and enable behavioral recoveries not observed for any other clinical treatment. A better understanding of natural and artificial mechanisms that induce membrane fusion should provide better clinical treatment for many disorders involving plasmalemmal damage.

Degeneration, Trophic Interactions, and Repair of Severed Axons: A Reconsideration of Some Common Assumptions

We suggest that several interrelated properties of severed axons (degeneration, trophic dependencies, initial repair, and eventual repair) differ in important ways from commonly held assumptions about those properties. Specifically, (1) axotomy does not necessarily produce rapid degeneration of distal axonal segments because (2) the trophic maintenance of nerve axons does not necessarily depend entirely on proteins transported from the perikaryon—but instead axonal proteins can be trophically maintained by slowing their degradation and/or by acquiring new proteins via axonal synthesis or transfer from adjacent cells (e.g., glia). (3) The initial repair of severed distal or proximal segments occurs by barriers (seals) formed amid accumulations of vesicles and/or myelin delaminations induced by calcium influx at cut axonal ends—rather than by collapse and fusion of cut axolemmal leaflets. (4) The eventual repair of severed mammalian CNS axons does not necessarily have to occur by neuritic outgrowths, which slowly extend from cut proximal ends to possibly reestablish lost functions weeks to years after axotomy—but instead complete repair can be induced within minutes by polyethylene glycol to rejoin (fuse) the cut ends of surviving proximal and distal stumps. Strategies to repair CNS lesions based on fusion techniques combined with rehabilitative training and induced axonal outgrowth may soon provide therapies that can at least partially restore lost CNS functions.

The leech nervous system: a valuable model to study the microglia involvement in regenerative processes

Microglia are intrinsic components of the central nervous system (CNS). During pathologies in mammals, inflammatory processes implicate the resident microglia and the infiltration of blood cells including macrophages. Functions of microglia appear to be complex as they exhibit both neuroprotective and neurotoxic effects during neuropathological conditions in vivo and in vitro. The medicinal leech Hirudo medicinalis is a well-known model in neurobiology due to its ability to naturally repair its CNS following injury. Considering the low infiltration of blood cells in this process, the leech CNS is studied to specify the activation mechanisms of only resident microglial cells. The microglia recruitment is known to be essential for the usual sprouting of injured axons and does not require any other glial cells. The present review will describe the questions which are addressed to understand the nerve repair. They will discuss the implication of leech factors in the microglial accumulation, the identification of nerve cells producing these molecules, and the study of different microglial subsets. Those questions aim to better understand the mechanisms of microglial cell recruitment and their crosstalk with damaged neurons. The study of this dialog is necessary to elucidate the balance of the inflammation leading to the leech CNS repair.

Arachidonic acid closes innexin/pannexin channels and thereby inhibits microglia cell movement to a nerve injury

Pannexons are membrane channels formed by pannexins and are permeable to ATP. They have been implicated in various physiological and pathophysiological processes. Innexins, the invertebrate homologues of the pannexins, form innexons. Nerve injury induces calcium waves in glial cells, releasing ATP through glial pannexon/innexon channels. The ATP then activates microglia. More slowly, injury releases arachidonic acid (ArA). The present experiments show that ArA itself reduced the macroscopic membrane currents of innexin- and of pannexin-injected oocytes; ArA also blocked K(+) -induced release of ATP. In leeches, whose large glial cells have been favorable for studying control of microglia migration, ArA blocked glial dye-release and, evidently, ATP-release. A physiological consequence in the leech was block of microglial migration to nerve injuries. Exogenous ATP (100 µM) reversed the effect, for ATP causes activation and movement of microglia after nerve injury, but nitric oxide directs microglia to the lesion. It was not excluded that metabolites of ArA may also inhibit the channels. But for all these effects, ArA and its non-metabolizable analog eicosatetraynoic acid (ETYA) were indistinguishable. Therefore, ArA itself is an endogenous regulator of pannexons and innexons. ArA thus blocks release of ATP from glia after nerve injury and thereby, at least in leeches, stops microglia at lesions.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

A homologous form of human interleukin 16 is implicated in microglia recruitment following nervous system injury in leech Hirudo medicinalis

In contrast to mammals, the medicinal leech Hirudo medicinalis can completely repair its central nervous system (CNS) after injury. This invertebrate model offers unique opportunities to study the molecular and cellular basis of the CNS repair processes. When the leech CNS is injured, microglial cells migrate and accumulate at the site of lesion, a phenomenon known to be essential for the usual sprouting of injured axons. In the present study, we demonstrate that a new molecule, designated HmIL-16, having functional homologies with human interleukin-16 (IL-16), has chemotactic activity on leech microglial cells as observed using a gradient of human IL-16. Preincubation of microglial cells either with an anti-human IL-16 antibody or with anti-HmIL-16 antibody significantly reduced microglia migration induced by leech-conditioned medium. Functional homology was demonstrated further by the ability of HmIL-16 to promote human CD4+ T cell migration which was inhibited by antibody against human IL-16, an IL-16 antagonist peptide or soluble CD4. Immunohistochemistry of leech CNS indicates that HmIL-16 protein present in the neurons is rapidly transported and stored along the axonal processes to promote the recruitment of microglial cells to the injured axons. To our knowledge, this is the first identification of a functional interleukin-16 homologue in invertebrate CNS. The ability of HmIL-16 to recruit microglial cells to sites of CNS injury suggests a role for HmIL-16 in the crosstalk between neurons and microglia in the leech CNS repair.

Neuroglial ATP release through innexin channels controls microglial cell movement to a nerve injury

Microglia, the immune cells of the central nervous system, are attracted to sites of injury. The injury releases adenosine triphosphate (ATP) into the extracellular space, activating the microglia, but the full mechanism of release is not known. In glial cells, a family of physiologically regulated unpaired gap junction channels called innexons (invertebrates) or pannexons (vertebrates) located in the cell membrane is permeable to ATP. Innexons, but not pannexons, also pair to make gap junctions. Glial calcium waves, triggered by injury or mechanical stimulation, open pannexon/innexon channels and cause the release of ATP. It has been hypothesized that a glial calcium wave that triggers the release of ATP causes rapid microglial migration to distant lesions. In the present study in the leech, in which a single giant glial cell ensheathes each connective, hydrolysis of ATP with 10 U/ml apyrase or block of innexons with 10 µM carbenoxolone (CBX), which decreased injury-induced ATP release, reduced both movement of microglia and their accumulation at lesions. Directed movement and accumulation were restored in CBX by adding ATP, consistent with separate actions of ATP and nitric oxide, which is required for directed movement but does not activate glia. Injection of glia with innexin2 (Hminx2) RNAi inhibited release of carboxyfluorescein dye and microglial migration, whereas injection of innexin1 (Hminx1) RNAi did not when measured 2 days after injection, indicating that glial cells’ ATP release through innexons was required for microglial migration after nerve injury. Focal stimulation either mechanically or with ATP generated a calcium wave in the glial cell; injury caused a large, persistent intracellular calcium response. Neither the calcium wave nor the persistent response required ATP or its release. Thus, in the leech, innexin membrane channels releasing ATP from glia are required for migration and accumulation of microglia after nerve injury.

ATP and NO dually control migration of microglia to nerve lesions

Microglia migrate rapidly to lesions in the central nervous system (CNS), presumably in response to chemoattractants including ATP released directly or indirectly by the injury. Previous work on the leech has shown that nitric oxide (NO), generated at the lesion, is both a stop signal for microglia at the lesion and crucial for their directed migration from hundreds of micrometers away within the nerve cord, perhaps mediated by a soluble guanylate cyclase (sGC). In this study, application of 100 microM ATP caused maximal movement of microglia in leech nerve cords. The nucleotides ADP, UTP, and the nonhydrolyzable ATP analog AMP-PNP (adenyl-5'-yl imidodiphosphate) also caused movement, whereas AMP, cAMP, and adenosine were without effect. Both movement in ATP and migration after injury were slowed by 50 microM reactive blue 2 (RB2), an antagonist of purinergic receptors, without influencing the direction of movement. This contrasted with the effect of the NO scavenger cPTIO (2-(4-carboxyphenyl)-4,4,5,5-teramethylimidazoline-oxyl-3-oxide), which misdirected movement when applied at 1 mM. The cPTIO reduced cGMP immunoreactivity without changing the immunoreactivity of eNOS (endothelial nitric oxide synthase), which accompanies increased NOS activity after nerve cord injury, consistent with involvement of sGC. Moreover, the sGC-specific inhibitor LY83583 applied at 50 microM had a similar effect, in agreement with previous results with methylene blue. Taken together, the experiments support the hypothesis that ATP released directly or indirectly by injury activates microglia to move, whereas NO that activates sGC directs migration of microglia to CNS lesions.

Nitric oxide influences injury-induced microglial migration and accumulation in the leech CNS

Damage to the leech or mammalian CNS increases nitric oxide (NO) production and causes accumulation of phagocytic microglial cells at the injury site. The aim of this study was to determine whether NO plays a role in microglial migration and accumulation at lesions in which NO is generated by a rapidly appearing endothelial nitric oxide synthase (eNOS) in leeches. Immunohistochemistry and cytochemistry demonstrated active eNOS before and throughout the period of microglial accumulation at the lesion. Decreasing NO synthesis by application of the NOS inhibitor N(w)-nitro-L-arginine methyl ester (1 mM) significantly reduced microglial accumulation, whereas its inactive enantiomer N(w)-nitro-D-arginine methyl ester (1 mM) resulted in microglial accumulation similar to that in crushed controls. Increasing NO with the donor spermine NONOate (SPNO) (1 mM) also inhibited accumulation, but not in the presence of the NO scavenger 2-(4-carboxyphenyl)-4,4,5, 5-teramethylimidazoline-oxyl-3-oxide (50 microM). The effect of SPNO was reversed by washout. SPNO application reduced average microglial migratory speeds and even reversibly arrested cell movement, as measured in living nerve cords. These results suggest that NO produced at a lesion may be a stop signal for microglia to accumulate there and that it can act on microglia early in their migration. Thus, NO may assume a larger role in nerve repair and recovery from injury by modulating accumulation of microglia, which appear to be important for axonal regeneration.

Transformation of leech microglial cell morphology and properties following co-culture with injured central nervous system tissue

When the leech central nervous system (CNS) is injured, microglial cells migrate to the site of the lesion. It is possible that the injured CNS releases diffusible substances that alter the properties of microglial cells; to investigate this, microglial cells were cultured in the presence of injured or uninjured CNS tissue. Grown on Concanavalin A (Con-A), 75 % of microglial cells are rounded in shape and are avoided by growing neurites. However, when chains of leech ganglia with damaged connectives were cultured on Con-A next to microglial cells, many of the microglial cells changed their morphology. The number of rounded cells present decreased to 48 %, 4 % became spindle-shaped and 48 % had an intermediate form. In addition, the presence of crushed ganglionic chains allowed more growth of neurites across microglial cells than occurred under control conditions, although round-shaped microglia were still avoided by growing neurites. Similar changes in microglial cells were produced in cells plated on Con-A in the presence of conditioned medium from crushed ganglionic chains. Hence, a diffusible substance from injured CNS tissue caused the morphology of the microglial cells plated on Con-A to become more like that of microglia plated on laminin, on which only 22 % of the cells are rounded while the remainder are spindle-shaped and are readily crossed by neurites. Changes in morphology were not observed when microglial cells were cultured with frozen and crushed ganglionic chains or with uncrushed chains. These experiments demonstrate that substances released from damaged leech CNS cause microglial cells plated on Con-A to change their morphology and the way in which they interact with growing neurites.

Macrophages and glia participate in the removal of apoptotic neurons from the Drosophila embryonic nervous system

Cell death in the Drosophila embryonic central nervous system (CNS) proceeds by apoptosis, which is revealed ultrastructurally by nuclear condensation, shrinkage of cytoplasmic volume, and preservation of intracellular organelles. Apoptotic cells do not accumulate in the CNS but are continuously removed and engulfed by phagocytic haemocytes. To determine whether embryonic glia can function as phagocytes, we studied serial electronic microscopic sections of the Drosophila CNS. Apoptotic cells in the nervous system are engulfed by a variety of glia including midline glia, interface (or longitudinal tract) glia, and nerve root glia. However, the majority of apoptotic cells in the CNS are engulfed by subperineurial glia in a fashion similar to the microglia of the vertebrate CNS. A close proximity between macrophages and subperineurial glia suggests that glia may transfer apoptotic profiles to the macrophages. Embryos affected by the maternal-effect mutation Bicaudal-D have no macrophages. In the absence of macrophages, most apoptotic cells are retained at the outer surfaces of the CNS, and subperineurial glia contain an abundance of apoptotic cells. Some apoptotic cells are expelled from the CNS, which suggests that the removal of apoptotic cells can occur in the absence of macrophages. The number of subperineurial glia is unaffected by changes in the rate of neuronal apoptosis.

Repair of the central nervous system: lessons from lesions in leeches

In contrast to the limited repair observed in the mammalian central nervous system (CNS), injured neurons in the leech reliably regenerate synapses and restore function with remarkable accuracy at the level of individual neurons. New and recent results reveal important roles for microglial cells and extracellular matrix components, including laminin, in repair. Tissue culture experiments have permitted isolation of neurons and manipulation of their environment, providing insights into the influence of substrate, electrical activity, and other cells, including microglia, on axon growth and synapse formation. The results account for distinctive features of successful repair in the adult leech, where axonal sprouting and target selection can be influenced by unequal competition between neurons. Differences between the formation of connections during embryonic development and repair in the adult include dissimilarities in the roles of glia and microglia in adults and embryos, suggesting that axon growth during regeneration in the CNS is not simply a recapitulation of processes observed during embryonic development. It may be possible in the future to improve mammalian CNS regeneration by recruiting cells whose counterparts in the leech have been identified as instrumental in repair.

Substrate-dependent interactions of leech microglial cells and neurons in culture

The principal aim of the present experiments has been to analyze the properties of microglial cells and their role in nerve regeneration. In the leech, damage to the CNS has been shown to be followed by accumulation of laminin and microglial cells at the site of injury (Masuda-Nakagawa et al., 1990. Proc. R. Soc. Lond. B. 241:201-206; and 1993. Proc. Natl. Acad. Sci. USA 90:4966-4970). Procedures were devised for isolating these small, wandering cells from the CNS of the leech. In culture, they were reliably identified by their sizes, shapes, and phagocytotic activity. Their morphology, motility, and interactions with neurons were influenced by the substrate molecules on which they were plated. On the plant lectin concanavalin A (Con A) microglia had a rounded shape and remained stationary. By contrast on extracts of leech extracellular matrix (ECM) enriched with laminin the cells were mobile and spindle-shaped with long processes. On Con A, neuronal growth cones avoided microglial cells, whereas on ECM extract the presence of a microglial cell did not influence neurite growth. Microglial cells showed immunoreactivity on both substrates when stained with a monoclonal antibody against leech laminin. Together these results suggest that microglial cells are influenced in their properties by molecules in the environment and that they could contribute to neuronal outgrowth at the site of an injury.

Identification of a 70 kD protein with sequence homology to squid neurofilament protein in glial cells of the leech CNS

A monoclonal antibody G39, generated against a protein extract of leech central nervous system, labels specific cell types in adult, embryonic, and regenerating preparations. The antibody stained glial cells, microglial cells, and connective tissue cells, but not neurons or muscle on cryosections. The staining pattern resembled that of an intracellular network. Affinity purification of the antigen revealed a 70 kD protein. Peptide sequencing showed significant homology of a stretch of 15 amino acids to squid neural filament protein. The same mAb G39 delineated glial cells as they formed during development of the CNS and showed that the giant neuropil glial cells appear before those in the packets. The antigen recognized by mAb G39 represents a nonneuronal intermediate filament of the leech Hirudo medicinalis found in various cell-types such as glia, microglia, and some cells of the connective tissue.

The effects of the loss of target cells upon photoreceptor inputs in the fly's optic lobe

The sensitivity of sensory neurons to target cell denervation varies in the CNS. We have examined the effects of surgically interrupting the output axons of the first optic neuropil, or lamina, in the optic lobe of the fly (Musca domestica), upon the receptor terminal inputs to the lamina. Two of the output interneurons are the monopolar cells L1 and L2, which are found as a pair in each of the unit modules or cartridges of the lamina neuropil. The lamina axons of L1 and L2 degenerate rapidly (within 0.5 h) in a retrograde direction from their lesion site, but there is no sign of retrograde transneuronal degeneration to the receptor terminals, across the input synapse. At each of these synaptic sites, L1 and L2 are invariable contributors to two of the four elements of a postsynaptic tetrad. Not only do the receptor terminals persist, but the presynaptic ribbons at the tetrad sites do also, opposite the degenerated spines of L1 and L2, indicating their lack of target dependence at least over the longest period of post-lesion recovery (48 h) examined. The areal density of presynaptic sites was conserved in the face of the degenerative loss of L1 and L2, as were the numbers of capitate projections (glial invaginations into receptor terminals). The stability of both synaptic density and capitate projection number indicates that they are predominantly influenced by the receptor terminals, which are still intact. A reduction in the number of mitochondrial profiles was one of the few observed changes in the receptor terminals. The results reflect the autonomy which the terminals have, during development, from their interneurons; they especially reflect the role of the terminals in the adult, in maintaining the presynaptic site of their afferent synapses, the tetrads.

Long-term survival of anucleate axons and its implications for nerve regeneration

Severed distal segments of nerve axons (anucleate axons) have now been reported to survive for weeks to years in representative organisms from most phyla, including the vertebrates. Among invertebrates (especially crustaceans), such long-term survival might involve transfer of proteins from adjacent intact cells to anucleate axons. In lower vertebrates and mammals, long-term survival of anucleate axons is more likely attributed to a slow turnover of axonal proteins and/or a lack of phagocytosis by macrophages or other cell types. Invertebrate anucleate axons that exhibit long-term survival are often reactivated by neurites that have grown from proximal nucleate segments. In mammals, induction of long-term survival in anucleate axons might allow more time to use artificial mechanisms to repair nerve axons by fusing the two severed halves with polyethylene glycol, a technique recently developed to fuse severed halves of myelinated axons in earthworms.

Extracellular matrix molecules in development and regeneration of the leech CNS

As neurons grow to their targets their processes elongate, branch and form specialized endings into which are inserted appropriate ion channels. Our aim has been to analyse the role of the extracellular matrix molecules laminin and tenascin in inducing growth and in determining the form and physiological properties of growing neurites. A preparation in which development and regeneration can be followed at the cellular and molecular level in the animal and in tissue culture is the central nervous system (CNS) of the leech. In leech extracellular matrix (ECM) both laminin and tenascin are present; the molecules are structurally similar but not identical to their vertebrate counterparts. Tenascin extracted from leech ECM shows a typical hexabrachial structure whereas laminin shows a typical cruciform structure in rotary shadowed preparations. Leech laminin purified by means of a monoclonal antibody is a molecule of about 1000 kDa, with a polypeptide composition of 340, 200, 180 and 160 kDa. Substrates that contain tenascin or laminin produce rapid and reliable outgrowth of neurites by identified cells. A remarkable finding is that the outgrowth pattern produced by an individual neuron depends in part on its identity, in part on the substrate upon which it is placed. For example, a Retzius cell grows in a quite different configuration and far more rapidly on laminin substrate than does another type of neuron containing the same transmitter (serotonin); and the pattern of outgrowth of the Retzius cell is different on laminin and on the plant lectin Con A (concanavalin A). Thus Con A induces the growth of processes that are shorter, thicker, more curved and contain fewer calcium channels than those grown on laminin. To determine whether laminin can also influence neurite outgrowth in the animal, immunocytological techniques have been used to follow its distribution in the extracellular matrix of normal, developing and regenerating leech CNS. In adult leeches neuronal processes in the CNS are not in contact with laminin which is confined to the surrounding extracellular matrix. In embryos however, laminin staining appears between ganglionic primordia along the pathways that neurons will follow. Similarly, after injury to the adult CNS, laminin accumulates at the very sites at which sprouting and regeneration begin. How the laminin becomes redistributed to appear in the region of injury has not yet been established. Together these findings suggest a key role for laminin and for other extracellular matrix molecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Accumulation of laminin and microglial cells at sites of injury and regeneration in the central nervous system of the leech

Profuse sprouting of leech neurons occurs in culture when they are plated on a substrate consisting of laminin molecules extracted from extracellular matrix that surrounds the central nervous system (CNS). To assess the role of laminin as a potential growth-promoting molecule in the animal, its distribution was compared in intact and regenerating CNS by light and electronmicroscopy, after it had been labelled with an anti-leech-laminin monoclonal antibody (206) and conjugated second antibodies. In frozen sections and electron micrographs of normal leeches the label was restricted to the connective-tissue capsule surrounding the connectives that link ganglia. Immediately after the connectives had been crushed the normal structure was disrupted but laminin remained in place. Two days after the crush, axons began to sprout vigorously and microglial cells accumulated in the lesion. At the same time, labelled laminin molecules were no longer restricted to the basement membrane but appeared within the connectives in the regions of neurite outgrowth. The distribution of laminin at these new sites within the CNS was punctate at two days, but changed over the following two weeks: the laminin became aggregated as condensed streaks running longitudinally within the connectives beyond the lesion. The close association of regenerating axons with laminin suggests that it may promote axonal growth in the CNS of the animal as in culture.

Individual microglia move rapidly and directly to nerve lesions in the leech central nervous system

Small cells called microglia, which collect at nerve lesions, were tracked as they moved within the leech nerve cord to crushes made minutes or hours before. The aim of this study was to determine whether microglia respond as a group and move en masse or instead move individually, at different rates, and whether they move along axons directly to the lesion or take another route, such as along the edges of the nerve cord. Cell nuclei in living nerve cords were stained with Hoechst 33258 dye and observed under dim ultraviolet illumination using fluorescence optics, a low-light video camera, and computer-assisted signal enhancement. Muscular movements of the cord were selectively reduced by bathing in 23 mM MgCl2. Regions of nerve cord within 300 microns of the crush were observed for 2-6 hr. Only a fraction of microglia, typically less than 50%, moved at any time, traveling toward the lesion at speeds up to 7 microns/min. Cells were moving as soon as observation began, within 15 min of crushing, and traveled directly toward the lesion along axons or axon tracts. Movements and roles of leech microglia are compared with their vertebrate counterparts, which are also active and respond to nerve injury.

Development of identified glia that ensheathe axons in Hirudo medicinalis

Interaction between neurons and glia may contribute to the formation of characteristic nerve bundles formed by axon elongation along stereotypic pathways. This study reports the temporal and spatial distribution of identified ensheathing glia during embryonic development in the leech. The development of connective glia was followed 1) using an immunohistochemical probe (monoclonal antibody Lan3-13), which recognized connective glia, and 2) using electron microscopy. Embryonic glia were initially located in the medial region of the lateral connectives and contained intermediate filaments. Glia cells continued to develop throughout embryogenesis; the number and size of glial processes increased, and they ensheathed smaller bundles of axons. The glial cell recognized by Lan3-13 first appeared after axons had already begun to form the connectives. This suggests that these particular glial cells may not function in the initial guidance of axons along stereotypic pathways. However, another cell that contained small bundles of intermediate filaments and glycogen granules was present at early stages of connective formation. These cells may be undifferentiated or transient glia, which could contribute to the formation of characteristic nerve bundles.

Morphology of chemosensory organs required for feeding in the leech Hirudo medicinalis

Sensilla that line the upper edge of the lip in the leech Hirudo medicinalis and that contain chemoreceptors required for feeding were examined in the scanning and transmission electron microscopes. The sensilla include two size-classes of ciliated button-like mounds--one about 35 microns in diameter and another about 10 microns in diameter. The larger sensilla are at the center of unpigmented patches of skin which are visible in the light microscope, while the smaller sensilla have not been previously described as distinct structures. Electron microscopy, though not light microscopy, shows that the lip sensilla differ markedly from the segmental sensilla of the leech, which have been shown to mediate mechanoreception and photoreception. In particular, the chemosensory lip sensilla contain multiciliated cells with cilia of a uniform length, whereas the segmental sensilla contain uniciliated cells with long, whip-like cilia, as well as multiciliated cells with short, stiff cilia. Thus, the two types of sensilla differ morphologically as well as functionally. In addition to the ciliated sensilla along the upper lip, structures consisting of a short, club-like process surrounded by granular material were observed inside the mouth. These structures may also be chemosensory organs.

The macroglial cells of the leech are molecularly heterogeneous

Monoclonal antibodies derived from fusions employing either whole leech nerve cords or fractionated proteins (gel bands) bind to the macroglial cells of the nerve cord. Three different antibodies bind to either one, two, or three of the four macroglial cell types in the leech CNS, serving as markers and showing that these four cell types, which differ primarily by anatomical position, all differ molecularly as well. Conventional microscopy confirms the existence of a novel macroglial cell type first noted because it binds antibody. Western (immunoblot) blot analyses demonstrate a polypeptide antigen of 77 kD in the macroglial cells of the connectives and a 130-kD polypeptide antigen associated with the macroglial cells of the connectives, of the root nerves, and of the ganglionic neuropil. The extensive molecular heterogeneity of leech neurons demonstrated by monoclonal antibody techniques is shared by macroglia.

Degenerative and regenerative changes in the trochlear nerve of goldfish

The features of unlesioned and lesioned trochlear nerves of goldfish have been examined electron microscopically. Lesioned nerves were studied between 1 and 107 days after cutting or crushing the nerve. Unlesioned nerves contained, on average, 77 myelinated axons and 19 unmyelinated axons. The latter were found in 1-2 fascicles per nerve. A basal lamina surrounded each myelinated axon and fascicle of unmyelinated axons. The numbers of myelinated axons, fascicles of unmyelinated axons and basal laminae varied by less than 5% over the intraorbital extramuscular segment of the nerve. Following interruption of the nerve, by either cutting or crushing, all of the axons and their myelin sheaths began to degenerate by 4 days in the distal nerve-stump. Both abnormally electron-dense and electron-lucent axons were observed. Both Schwann cells and macrophages appeared to phagocytose the myelin sheaths. Following a lesion, the Schwann cells and their basal laminae persisted in the distal nerve-stump. In crushed nerves, the basal laminae surrounding myelinated axons formed 97%, on average, of the Schwann tubes in the distal stump. The perimeters of the basal laminae were of similar size to those in the proximal stump, at least for the first 8 days after crush. In crushed nerves, single myelinated axons in the proximal nerve-stump gave rise to multiple sprouts, some of which reached the site of crush by 2 days, the distal stump by 4 days and the superior oblique muscle by 8 days. The regeneration of the unmyelinated axons was not examined. In both crushed and transected nerves, nearly all of the sprouts in the proximal and distal stumps were found within the basal laminae of Schwann cells, even though the spouts were disorganized in the transected region where there were no basal laminae. The growth cones of the regenerating axons were always found apposed to the inner surface of the basal laminae, which may have provided an adhesive substrate that directed their growth. Terminal sprouts from the ends of myelinated axons in the proximal stump accounted for the majority of the regenerating axons in the distal stump, as only a few collateral sprouts were found in the proximal stump, and only a small amount of axonal branching was found within the distal stump itself. The largest axons in the distal stump were remyelinated first, and the number of remyelinated axons increased progressively between 8 and 31 days after crush, at which time there were about twice as many as in unlesioned nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

Accurate regeneration of an electrical synapse between two leech neurones after destruction of the ensheathing glial cell

An interneurone, the S cell in the central nervous system of the leech, regenerates its severed axon and forms an electrical synapse with its target, another S cell, entirely within the ensheathment of two glial cells. After the two glial cells were killed selectively by intracellular injection of protease, axonal regeneration and synapse formation occurred in a normal fashion during the month following nerve injury. Soon after reconnexion of S cells, the conduction of impulses across the non-rectifying electrical junction between the cells was more reliable from the target than into it from the thinner regenerating axon. The distal segments of severed S-cell axons survived for weeks or months after destruction of their glial cells, indicating that the ensheathing glia is not required for long-term survival of axon segments. The distal axon segment of the S cell remained connected to the target axon at the normal region of synapse midway between ganglia within the nerve cord. In about half the cases in which reconnexion between injured S cell and target S cell occurred between 10 and 25 days following nerve crush, the regenerating neurone had formed an electrical synapse with its severed distal axon and had thereby become reconnected, indirectly, with its target. In the other cases, reconnexion was by direct contact. By 4 weeks, the proportion of injured S cells that were coupled and making direct contact with their targets rose to more than 80% of the total population, indicating that regeneration continued until the two S cells contacted one another directly. This is similar to the course of S-cell regeneration in the presence of the ensheathing glia. Microscopy of the regenerating neurone and both its distal axon segment and its target showed that the site of synapse formation in the absence of the usual glial sheath was normal. Fluorescence microscopy following intracellular injection of Lucifer Yellow dye, which crosses between S cells at the electrical synapse, showed that the regenerated synapse formed specifically between S cells. Moreover, the target did not form alternative synapses when regeneration failed.

Distinctions between gap junctions and sites of intermediate filament attachment in the leech C.N.S

Freeze-fracture studies on the nerve cord of the leech Hirudo medicinalis reveal that the plasma membranes of various cells, including glial and muscle cells, contain at least two distinct types of aggregated intramembrane particles, identified as hemidesmosomes and gap junctions. Hemidesmosomes consist of angular particles irregularly arranged in circular or elongate patches in external leaflets (E-faces), and are associated with a bundle of intermediate filaments extending into the cytoplasm. Hemidesmosomes of specific axons abut on extracellular space at openings in the surrounding glial sheath. Gap junctions are patches of rounder particles in cytoplasmic leaflets (P-faces) and are more uniformly spaced; they have a corresponding array of pits in the complementary E-face. Gap junctions connect processes of adjacent smooth muscle cells, and apparently interconnect glial processes. Thus, different types of cells in the leech C.N.S. have similar intramembrane specializations. Moreover, the hemidesmosomes and gap junctions might, on superficial examination, be confused.

Microglial movement to sites of nerve lesion in the leech CNS

The small glial cells in the central nervous system of the leech, Hirudo medicinalis, have been studied using two histological stains. Weak silver carbonate, a classic stain for vertebrate microglia, can selectively stain these small glial cells and shows that they are morphologically similar to vertebrate microglia. Feulgen's DNA-specific stain is useful for counting the compact and distinctive microglial nuclei. In uninjured connectives, which link segmental ganglia, there are 134 +/- 28 microglia per 210 micron of connective length. Within 24 h after the nerve cord is crushed leech microglia aggregate at the site of injury. This increase in cells, seen both in vivo and in culture, is approximately 5-fold. Although cells do not continue to accumulate at the injury site after the first day, their numbers continue to vary with time in the regions immediately adjacent to the crush for at least one week. A second crush made 24 h after the first shows that leech microglia are capable of responding to repeated injury.