Drosophila adhesion molecules and neural development

Novel mouse IgG-like immunoreactivity expressed by neurons in the moth Manduca sexta: developmental regulation and colocalization with crustacean cardioactive peptide

Immunoglobulin-related molecules have been shown to play important roles in cell-cell recognition events during the development of both vertebrate and invertebrate nervous systems. In the moth, Manduca sexta, we report the presence of novel, mouse, immunoglobulin G (mIgG)-like immunoreactivity in a discrete population of identified neurosecretory neurons (the NS-Ls also known as the cell 27s) and interneurons (the IN-704s). A number of polyclonal anti-mIgG antibodies were used to immunostain these cells in wholemount. The mIgG-like-immunoreactive (IR) neurons were present during embryogenesis through the developing adult stages, but disappeared in the postemerged adult. Biochemical analysis of M. sexta ventral nerve cords revealed that the mIgG-like antigen is a membrane-associated 27-kDa protein which is likely responsible for the mIgG-like immunostaining observed. Unambiguous identification of the mIgG-like-IR neurons was based on neuronal morphology and our ability to demonstrate conclusively that these neurons expressed immunoreactivity to an antiserum against crustacean cardioactive peptide (CCAP). The NS-Ls and IN-704s were both shown to colocalize the CCAP and mIgG-like immunoreactivities. The mIgG-like and CCAP-IR neurons were identical to a subset of CCAP-IR neurons recently described by Davis et al. [(1993) J. Comp. Neurol., 338:612-627] in pupae. We found these CCAP-IR neurons, however, also to be present in larvae. The mIgG-like- and CCAP-IR neurons included the NS-L pair of the subesophageal maxillary neuromere, which projected anteriorly to the corpora cardiaca, and the NS-L of the labial neuromere whose axons projected out the dorsal nerve of the next posterior ganglion. The mIgG-like and CCAP-IR NS-Ls were also observed throughout the three thoracic ganglia, and all shared strikingly similar structural features. These cells exited out the dorsal nerve of the next posterior ganglion and eventually projected to the neurohemal release sites of the perivisceral organs. These neurons appear to be the homologues of the abdominal CCAP-IR NS-Ls, neurons that in the adult switch their neurotransmitter and release the neuropeptide bursicon. Our description of the distribution and developmental expression of this novel mIgG-like immunoreactivity may provide new insights into the regulation of neurotransmitter plasticity and/or recognition-signaling events involved in the embryonic and postembryonic assembly of the nervous system.

In vitro analyses of neurite outgrowth indicate a potential role for tenascin-like molecules in the development of insect olfactory glomeruli

Tenascin-like material is associated with glial cells that form borders around developing glomerular units in the olfactory (antennal) lobe of the moth Manduca sexta and is present at critical stages of glomerulus formation (Krull et al., 1994, J. Neurobiol. 25:515-534). Tenascin-like immunoreactivity declines in the mature lobe, coincident with a wave of synapse formation within the glomeruli and glomerulus stabilization. Tenascin-like molecules associated with neuropilar glia are in the correct position to influence the branching patterns of growing neurites by constraining them to glomeruli. In this study, we examine the growth of cultured moth antennal-lobe neurons in response to mouse CNS tenascin. Uniform tenascin provides a poor substrate for cell-body attachment and neurite outgrowth. Neuronal cell bodies provided with a striped substratum consisting of tenascin and concanavalin-A (con-A)/laminin attach preferentially to con-A/laminin lanes. Most neurons restrict their branching to con-A/laminin lanes both at early and later times in culture but others send processes across multiple tenascin and con-/laminin lanes in an apparently indiscriminate manner. Tenascin can inhibit the neuritic outgrowth of most antennal-lobe neurons, and this raises the possibility that the tenascin-like molecules associated with neuropilar glia in vivo act to constrain growing neurites to glomeruli. Thus, glial cells, acting in concert with olfactory axons, might act to promote glomerular patterns of branching by antennal-lobe neurons.

The prenatal development of the anterior commissure in hamsters: pioneer fibers lead the way

The prenatal development of the anterior commissure (AC) was studied in 130 hamster embryos with ages varying from E12 to E16 (E1 = day of conception and E16 = P1 = day of birth) by use of carbocyanine crystals (DiI, DiA and/or DiO) implanted into different rostrocaudal segments of the paleocortex. On E12 and E13, many AC axons were seen with tortuous trajectories pointing towards the midline (precrossing stage). On E13.5 and E14, most AC fibers abutted the midsagittal plane, led by a few pioneer axons that grew as far as 500 microns ahead into the opposite hemisphere (crossing stage). Pioneers were present in most brains at these ages irrespective of the rostrocaudal position of the carbocyanine crystal. Somata of pioneer axons could be identified by retrograde labelling. They were characteristically immature neurons, located either in the olfactory peduncle or in the superficial layers of the olfactory cortex. On E14.5 and E15, pioneers and followers were seen close to the targets and on E15.5 and E16 interstitial budding occurred, and arborization started within the olfactory peduncle and the paleocortex (postcrossing stage). If the existence of pioneer fibers represents something more than a stochastic phenomenon, their appearance in the developing AC may reflect the operation of signals at the midline and/or in the contralateral hemisphere that either accelerate the growth of pioneers, or decelerate the growth of followers.

In vitro growth of locust embryonic pars intercerebralis neurosecretory cells

Neurosecretory brain cells from embryonic locusts cultured in serum-free medium failed to show any visible signs of growth. In contrast, the same neurons co-cultured with CNS explants (brain-retrocerebral complexes and thoracic ganglia) show excellent axonal growth: sprouting occurs after one day of co-culture and increases within the first week. These results indicate the production of an active neurite outgrowth stimulating factor by co-cultured CNS explants. The similarity of the stimulating effects by the two explants on neurite outgrowth rule out brain neurohormones as probable candidates for the stimulating factor. In addition, neither insulin nor neuroparsin added to the culture medium to test their trophic effect improves the growth of the cells. Conditioned medium derived from cultures of brain-retrocerebral complexes produced no neurite outgrowth, suggesting that the active factor released in the medium by brain explants does not remain free in solution but binds to the substratum. Finally, neurons co-cultured with CNS explants attached to the bottom of the culture dish develop neurites only when in close proximity to the explants. This observation strongly suggests the binding of an active neurite outgrowth stimulating factor to the substratum in the vicinity of the explants. As a control for CNS explants, the action of non-nervous tissue was tested: a similar, but less extensive neurotrophic effect, was observed with esophagus segments co-cultured with neurosecretory brain cells. These results demonstrate that locust neurosecretory neurons isolated in cell culture require combined explants for elaborating processes and suggest that the neurite promoting effect is mediated by a substrate-associated molecule(s).(ABSTRACT TRUNCATED AT 250 WORDS)

An antigen present in the Drosophila central nervous system only during embryonic and metamorphic stages

We report here about an antigen that is expressed in the central nervous system (CNS) of Drosophila only during the embryonic and metamorphic stages. In Drosophila, axonogenesis and synaptogenesis occur twice during the development: first in the embryonic and second in the metamorphic stages. We generated monoclonal antibodies (MAbs) in order to obtain molecular probes for analyzing axonogenesis or synaptogenesis in the CNS on the assumption that good candidates for molecules responsible for such phenomena must be present in the neuropil during those stages exclusively. As a result, we found MAb 66B2 whose intense immunoreactivity in the neuropil of the CNS was observed exclusively in the embryo and pupa, and not in the larva and adult. Immunoblot analyses showed that MAb 66B2 binds specifically to a protein with an apparent molecular weight of 350 K and neutral pI in the prepupal CNS. A significant amount of the antigen was isolated in forms that were soluble without detergent. Results of immunohistochemistry with MAb 66B2 in a primary culture of embryos showed that some live cells in the ganglion-like cluster were stained, and that neuronal cell bodies and neurites emanating from there were negative. These results strongly suggest that the 66B2 antigen observed in the CNS is an extracellular matrix component secreted from nonneuronal cells. These developmental changes in the 66B2 immunoreactivity in the CNS presumably reflect dynamic changes of an extracellular matrix in the CNS that are accompanied by axonogenesis or synaptogenesis.

Microtubule behavior during guidance of pioneer neuron growth cones in situ

The growth of an axon toward its target results from the reorganization of the cytoskeleton in response to environmental guidance cues. Recently developed imaging technology makes it possible to address the effect of such cues on the neural cytoskeleton directly. Although high resolution studies can be carried out on neurons in vitro, these circumstances do not recreate the complexity of the natural environment. We report here on the arrangement and dynamics of microtubules in live neurons pathfinding in response to natural guidance cues in situ using the embryonic grasshopper limb fillet preparation. A rich microtubule network was present within the body of the growth cone and normally extended into the distal growth cone margin. Complex microtubule loops often formed transiently within the growth cone. Branches both with and without microtubules were regularly observed. Microtubules did not extend into filopodia. During growth cone steering events in response to identified guidance cues, microtubule behaviour could be monitored. In turns towards guidepost cells, microtubules selectively invaded branches derived from filopodia that had contacted the guidepost cell. At limb segment boundaries, microtubules displayed a variety of behaviors, including selective branch invasion, and also invasion of multiple branches followed by selective retention in branches oriented in the correct direction. Microtubule invasion of multiple branches also was seen in growth cones migrating on intrasegmental epithelium. Both selective invasion and selective retention generate asymmetrical microtubule arrangements within the growth cone, and may play a key role in growth cone steering events.

Segregation of afferent projections in the central nervous system of the leech Hirudo medicinalis

Sensory axons originating in peripheral tissues converge onto each segmental ganglion in the central nervous system (CNS) of the leech, where they segregate into well-defined regions of the synaptic neuropil. Here we report on several aspects of the molecular and anatomical organizations of these afferent projections that bear upon the hypothesis that surface markers are involved in organizing these axons as they grow into the CNS. First, we show that the distribution of some surface markers in the adult is restricted to axons of peripheral origin and is not present on the neighboring axons of central neurons. Second, we demonstrate that the number of afferents increases postembryonically as the leech increases in size, suggesting that at least some of the cues employed by afferent axons to grow to appropriate central targets must be present throughout the life of the animal. We then show, using anterograde axonal tracing and immunohistochemistry, that there is both convergence and divergence of afferent axons into highly specific regions of the neuropil. Lastly, we examine the distribution of surface markers present on different subsets of afferents and show that axons having one type of marker segregate from those having the second type. Our results, considered together with previous observations in this system, provide new clues about the organization of afferent projections in the nervous system of the leech. They also suggest how a relatively small number of molecular markers might mediate fiber-fiber interactions to organize afferent axons as they grow into the CNS.

Adhesion molecules and animal development

In recent years considerable progress has been made in the identification and characterization of molecules that mediate cell adhesion during animal development. This review attempts to pick out from the vast amount of information in this rapidly expanding field some of the key features of adhesion molecules, to present ideas about their role in development, and to indicate the directions in which the field is now moving.

Pioneer neuron pathfinding from normal and ectopic locations in vivo after removal of the basal lamina

The contribution of the basal lamina to Ti1 pioneer axon guidance in grasshopper limb buds was investigated by allowing growth cones to migrate in 30%-31% stage limbs from which the basal lamina had been removed by enzymatic treatment. When the Ti1 axons extended from their normal location, the pathways established in the absence of basal lamina were normal. This indicates that the basal lamina is not required for initial proximal axon outgrowth, recognition of limb segment boundaries, or selective interaction with neuronal somata. Removal of the basal lamina from slightly older (32% stage) embryos resulted in displacement of the Ti1 somata to ectopic locations in approximately 50% of the limbs. Pathfinding from ectopic locations was aberrant in 45% of the cases observed. This demonstrates that if orienting information is present in the basal lamina-free epithelium at this stage, it is not the predominant factor in determining growth cone orientation from ectopic locations.

Selective recognition between embryonic afferent neurons of grasshopper appendages in vitro

Selective affinity between afferent neurons has been proposed as a major mechanism underlying the assembly of the insect peripheral nervous system during development. Afferent insect neurons establish adhesive interactions in vivo that are resistant to proteolytic degradation by elastase and independent of the basal lamina. We have tested whether afferent neurons express selective affinity for one another under more simplified and controlled conditions in vitro. We report here that (1) afferent neurons from dissociated embryonic tissue selectively aggregate within 80 min when incubated with rotation, (2) afferent axons establish and maintain fasciculation in vitro, and (3) afferent neuronal processes in vitro preferentially contact the somata of other afferent neurons in a mixed field of cells.

Antibodies to the mammalian adhesion molecule J1/tenascin and its carbohydrate epitope L2/HNK-1 label the receptor lymph cavities of insect sensilla

Data from light- and electronimmunocytochemistry gave evidence that the antibodies to the mammalian adhesion molecule J1/tenascin and its carbohydrate structure L2/HNK-1 react with immunoreactive structures present in the inner and outer receptor lymph cavities of antennal sensilla of the honey bee. Immunoreactivity was additionally present in the cytoplasm of the enveloping cells surrounding the receptor lymph cavities. Cell contacts between enveloping cells and between dendrites and enveloping cells were never observed to be antigen positive.