Morphoregulation

Overall Role of Contactins Expression in Neurodevelopmental Events and Contribution to Neurological Disorders

BACKGROUND

Neurodegenerative disorders may depend upon a misregulation of the pathways which sustain neurodevelopmental control. In this context, this review article focuses on Friedreich ataxia (FA), a neurodegenerative disorder resulting from mutations within the gene encoding the Frataxin protein, which is involved in the control of mitochondrial function and oxidative metabolism.

OBJECTIVE

The specific aim of the present study concerns the FA molecular and cellular substrates, for which available transgenic mice models are proposed, including mutants undergoing misexpression of adhesive/morphoregulatory proteins, in particular belonging to the Contactin subset of the immunoglobulin supergene family.

METHODS

In both mutant and control mice, neurogenesis was explored by morphological/morphometric analysis through the expression of cell type-specific markers, including -tubulin, the Contactin-1 axonal adhesive glycoprotein, as well as the Glial Fibrillary Acidic Protein (GFAP).

RESULTS

Specific consequences were found to arise from the chosen misexpression approach, consisting of a neuronal developmental delay associated with glial upregulation. Protective effects against the arising phenotype resulted from antioxidants (essentially epigallocatechin gallate (EGCG)) administration, which was demonstrated through the profiles of neuronal (-tubulin and Contactin 1) as well as glial (GFAP) markers, in turn indicating the concomitant activation of neurodegeneration and neuro repair processes. The latter also implied activation of the Notch-1 signaling.

CONCLUSION

Overall, this study supports the significance of changes in morphoregulatory proteins expression in the FA pathogenesis and of antioxidant administration in counteracting it, which, in turn, allows to devise potential therapeutic approaches.

Decellularization Detergents As Methodological Variables in Mass Spectrometry of Stromal Matrices

Collagens, elastin, fibrillin, decorin, and laminin are key constituents of the extracellular matrix and basement membrane of mammalian organs. Thus, changes in their quantities may influence the mechanochemical regulation of resident cells. Since maintenance of a native stromal composition is a requirement for three-dimensional (3D) matrix-based recellularization techniques in tissue engineering, we studied the influence of the decellularization detergents on these proteins in porcine kidney, liver, pancreas, and skin. Using a quick thawing/quick microwave-assisted decellularization protocol and two different detergents, sodium dodecyl sulfate (SDS) vs Triton X-100 (TX100), at identical concentration, variations in matrix conservation of stromal proteins were detected by liquid chromatography-mass spectrometry coupled to light and scanning electron microscopies, in dependence on each detergent. In all organs tested except pancreas, collagens were retained to a statistically significant level using the TX100-based protocol. In contrast fibrillin, elastin (except in kidney), and decorin (only in liver) were better preserved with the SDS-dependent protocol. Irrespective of the detergent used, laminin always remained at an irrelevant level. Our results prompt attention to the type of detergent in organ decellularization, suggesting that its choice may influence morphoregulatory inputs peculiar to the type of 3D bioartificial mammalian organ to be reconstructed. Impact statement Simple change of the protocol's main detergent leads to a very substantial difference in the panel of the stromal proteins detected by qualitative and semiquantitative mass spectrometry in acellular porcine matrices. This remarkable methodological variable promises to yield proteomic reference panels in a number of different species-specific acellular matrices allowing for selective retainment of peculiar mechanochemical inputs, to differently address the development of the seeded cells in relation to the type of organ to be bioartificially reconstructed.

FGF8-mediated signaling regulates tooth developmental pace during odontogenesis

The developing human and mouse teeth constitute an ideal model system to study the regulatory mechanism underlying organ growth control since their teeth share highly conserved and well-characterized developmental processes and their developmental tempo varies notably. In the current study, we manipulated heterogenous recombination between human and mouse dental tissues and demonstrate that the dental mesenchyme dominates the tooth developmental tempo and FGF8 could be a critical player during this developmental process. Forced activation of FGF8 signaling in the dental mesenchyme of mice promoted cell proliferation, prevented cell apoptosis via p38 and perhaps PI3K-Akt intracellular signaling, and impelled the transition of the cell cycle from G1- to S-phase in the tooth germ, resulting in the slowdown of the tooth developmental pace. Our results provide compelling evidence that extrinsic signals can profoundly affect tooth developmental tempo and the dental mesenchymal FGF8 could be a pivotal factor in controlling the developmental pace in a non-cell-autonomous manner during mammalian odontogenesis.

Morpho-regulation in diverse chicken feather formation: Integrating branching modules and sex hormone-dependent morpho-regulatory modules

Many animals can change the size, shape, texture and color of their regenerated coats in response to different ages, sexes, or seasonal environmental changes. Here, we propose that the feather core branching morphogenesis module can be regulated by sex hormones or other environmental factors to change feather forms, textures or colors, thus generating a large spectrum of complexity for adaptation. We use sexual dimorphisms of the chicken to explore the role of hormones. A long-standing question is whether the sex-dependent feather morphologies are autonomously controlled by the male or female cell types, or extrinsically controlled and reversible. We have recently identified core feather branching molecular modules which control the anterior-posterior (bone morphogenetic orotein [BMP], Wnt gradient), medio-lateral (Retinoic signaling, Gremlin), and proximo-distal (Sprouty, BMP) patterning of feathers. We hypothesize that morpho-regulation, through quantitative modulation of existing parameters, can act on core branching modules to topologically tune the dimension of each parameter during morphogenesis and regeneration. Here, we explore the involvement of hormones in generating sexual dimorphisms using exogenously delivered hormones. Our strategy is to mimic male androgen levels by applying exogenous dihydrotestosterone and aromatase inhibitors to adult females and to mimic female estradiol levels by injecting exogenous estradiol to adult males. We also examine differentially expressed genes in the feathers of wildtype male and female chickens to identify potential downstream modifiers of feather morphogenesis. The data show male and female feather morphology and their color patterns can be modified extrinsically through molting and resetting the stem cell niche during regeneration.

The role of Gpi-anchored axonal glycoproteins in neural development and neurological disorders

This review article focuses on the Contactin (CNTN) subset of the Immunoglobulin supergene family (IgC2/FNIII molecules), whose components share structural properties (the association of Immunoglobulin type C2 with Fibronectin type III domains), as well as a general role in cell contact formation and axonal growth control. IgC2/FNIII molecules include 6 highly related components (CNTN 1-6), associated with the cell membrane via a Glycosyl Phosphatidyl Inositol (GPI)-containing lipid tail. Contactin 1 and Contactin 2 share ~50 (49.38)% identity at the aminoacid level. They are components of the cell surface, from which they may be released in soluble forms. They bind heterophilically to multiple partners in cis and in trans, including members of the related L1CAM family and of the Neurexin family Contactin-associated proteins (CNTNAPs or Casprs). Such interactions are important for organising the neuronal membrane, as well as for modulating the growth and pathfinding of axon tracts. In addition, they also mediate the functional maturation of axons by promoting their interactions with myelinating cells at the nodal, paranodal and juxtaparanodal regions. Such interactions also mediate differential ionic channels (both Na⁺ and K⁺) distribution, which is of critical relevance in the generation of the peak-shaped action potential. Indeed, thanks to their interactions with Ankyrin G, Na⁺ channels map within the nodal regions, where they drive axonal depolarization. However, no ionic channels are found in the flanking Contactin1-containing paranodal regions, where CNTN1 interactions with Caspr1 and with the Ig superfamily component Neurofascin 155 in cis and in trans, respectively, build a molecular barrier between the node and the juxtaparanode. In this region K⁺ channels are clustered, depending upon molecular interactions with Contactin 2 and with Caspr2. In addition to these functions, the Contactins appear to have also a role in degenerative and inflammatory disorders: indeed Contactin 2 is involved in neurodegenerative disorders with a special reference to the Alzheimer disease, given its ability to work as a ligand of the Alzheimer Precursor Protein (APP), which results in increased Alzheimer Intracellular Domain (AICD) release in a γ-secretase-dependent manner. On the other hand Contactin 1 drives Notch signalling activation via the Hes pathway, which could be consistent with its ability to modulate neuroinflammation events, and with the possibility that Contactin 1-dependent interactions may participate to the pathogenesis of the Multiple Sclerosis and of other inflammatory disorders.

Tissue patterning and cellular mechanics

In development, cells organize into biological tissues through cell growth, migration, and differentiation. Globally, this process is dictated by a genetically encoded program in which secreted morphogens and cell-cell interactions prompt the adoption of unique cell fates. Yet, at its lowest level, development is achieved through the modification of cell-cell adhesion and actomyosin-based contractility, which set the level of tension within cells and dictate how they pack together into tissues. The regulation of tension within individual cells and across large groups of cells is a major driving force of tissue organization and the basis of all cell shape change and cell movement in development.

Mitochondrial DNA, chloroplast DNA and the origins of development in eukaryotic organisms

Background

Several proposals have been made to explain the rise of multicellular life forms. An internal environment can be created and controlled, germ cells can be protected in novel structures, and increased organismal size allows a "division of labor" among cell types. These proposals describe advantages of multicellular versus unicellular organisms at levels of organization at or above the individual cell. I focus on a subsequent phase of evolution, when multicellular organisms initiated the process of development that later became the more complex embryonic development found in animals and plants. The advantage here is realized at the level of the mitochondrion and chloroplast.

Hypothesis

The extreme instability of DNA in mitochondria and chloroplasts has not been widely appreciated even though it was first reported four decades ago. Here, I show that the evolutionary success of multicellular animals and plants can be traced to the protection of organellar DNA. Three stages are envisioned. Sequestration allowed mitochondria and chloroplasts to be placed in "quiet" germ line cells so that their DNA is not exposed to the oxidative stress produced by these organelles in "active" somatic cells. This advantage then provided Opportunity, a period of time during which novel processes arose for signaling within and between cells and (in animals) for cell-cell recognition molecules to evolve. Development then led to the enormous diversity of animals and plants.

Implications

The potency of a somatic stem cell is its potential to generate cell types other than itself, and this is a systems property. One of the biochemical properties required for stemness to emerge from a population of cells might be the metabolic quiescence that protects organellar DNA from oxidative stress.

Reviewers

This article was reviewed by John Logsdon, Arcady Mushegian, and Patrick Forterre.

Chemokines and chemokine receptors in mucosal homeostasis at the intestinal epithelial barrier in inflammatory bowel disease

Chemokines, a large family of small chemoattractive cytokines, and their receptors play an integral role in the regulation of the immune response and homeostasis. The ability of chemokines to attract specific populations of immune cells sets them apart from other chemoattractants. Chemokines produced within the gastrointestinal mucosa are critical players in directing the balance between physiological and pathophysiological inflammation in health, inflammatory bowel disease (IBD), and the progression to colon cancer. In addition to the well-characterized role of chemokines in directed trafficking of immune cells to the gut mucosa, the expression of chemokine receptors on the cells of the epithelium makes them active participants in the chemokine signaling network. Recent findings demonstrate an important role for chemokines and chemokine receptors in epithelial barrier repair and maintenance as well as an intricate involvement in limiting metastasis of colonic carcinoma. Increased recognition of the association between barrier defects and inflammation and the subsequent progression to cancer in IBD thus implicates chemokines as key regulators of mucosal homeostasis and disease pathogenesis.

Morphoregulation of teeth: modulating the number, size, shape and differentiation by tuning Bmp activity

During development and evolution, the morphology of ectodermal organs can be modulated so that an organism can adapt to different environments. We have proposed that morphoregulation can be achieved by simply tilting the balance of molecular activity. We test the principles by analyzing the effects of partial downregulation of Bmp signaling in oral and dental epithelia of the keratin 14-Noggin transgenic mouse. We observed a wide spectrum of tooth phenotypes. The dental formula changed from 1.0.0.3/1.0.0.3 to 1.0.0.2(1)/1.0.0.0. All mandibular and M3 maxillary molars were selectively lost because of the developmental block at the early bud stage. First and second maxillary molars were reduced in size, exhibited altered crown patterns, and failed to form multiple roots. In these mice, incisors were not transformed into molars. Histogenesis and differentiation of ameloblasts and odontoblasts in molars and incisors were abnormal. Lack of enamel caused misocclusion of incisors, leading to deformation and enlargement in size. Therefore, subtle differences in the level, distribution, and timing of signaling molecules can have major morphoregulatory consequences. Modulation of Bmp signaling exemplifies morphoregulation hypothesis: simple alteration of key signaling pathways can be used to transform a prototypical conical-shaped tooth into one with complex morphology. The involvement of related pathways and the implication of morphoregulation in tooth evolution are discussed.

Methylmercury Alters Eph and Ephrin Expression During Neuronal Differentiation of P19 Embryonal Carcinoma Cells

Developmental exposure to methylmercury (MeHg) induces a spectrum of neurological impairment characterized by cognitive disturbance, sensory/motor deficit, and diffuse structural abnormalities of the brain. These alterations may arise from neural path-finding errors during brain development, resulting from disturbances in the function of morphoregulatory guidance molecules. The Eph family of tyrosine kinase receptors and their ligands, the ephrins, guide neuronal migration and neurite pathfinding mainly via repulsive intercellular interactions. The present study examined the effects of MeHg on mRNA and protein expression profiles of Ephs and ephrins in the P19 embryonal carcinoma (EC) cell line and its neuronal derivatives. Undifferentiated control P19 cells displayed low- to undetectable levels of mRNA for ephrins or Ephs, with the sole exception of EphA2 which was highly expressed. Upon differentiation into neurons, the ephrin expression increased progressively through day 10. Similarly, expression of the Ephs, including EphsA3, -A4, -A8, -B2, -B3, -B4, and -B6, increased significantly. In contrast, EphA2 expression decreased in day 2, 6 and 10 control neurons. Treatment with MeHg did not affect the expression of mRNA for ephrins or Ephs in undifferentiated P19 cells. However, treatment of differentiating neurons with MeHg for 24 h caused consistent increases in ligand mRNA expression, particularly ephrin-A5, -A6, -B1, and -B2. Similarly, MeHg induced variable increases in mRNA expression of receptors EphA2, -A3, -B3, and -B6. A trend toward a concentration-response relationship was observed for the alterations in Eph receptor mRNA expression although increases at the low and mid concentrations did not reach statistical significance. Immunoblots for ligand and receptor proteins mirrored the increases in the mRNA levels at the 0.5 and 1.5 microM MeHg concentrations but showed decreased protein levels compared to controls at the 3.0 microM concentration. Alterations in the Eph/ephrin family of repulsion molecules may represent an important mechanism in developmental MeHg neurotoxicity.

MEK1 restores migration of polyamine-depleted cells by retention and activation of Rac1 in the cytoplasm

We previously showed that polyamines are required for proliferation and migration both in vivo and in a cultured intestinal epithelial cell (IEC-6) model. Wounding of the IEC-6 monolayer induced transient ERK activation, which was further enhanced by EGF. EGF stimulated migration in control and polyamine-depleted cells, but the degree of stimulation was significantly less in polyamine-depleted cells. Inhibition of MEK1 inhibited basal as well as EGF-induced ERK activation and migration. Expression of constitutively active (CA)-MEK and dominant-negative (DN)-MEK had significant effects on F-actin structure. CA-MEK increased stress fiber and lamellipodia formation, while DN-MEK showed loss of stress fibers and abnormal actin cytoskeletal structure. Unlike EGF, CA-MEK significantly increased migration of both control and polyamine-depleted cells. The most important and significant finding in this study was that polyamine depletion caused localization of Rac1 and RhoA to the nuclear as well as perinuclear regions. Interestingly, CA-MEK completely reversed the subcellular distribution of Rac1 and RhoA proteins in polyamine-depleted cells. Polyamine depletion increased Rac1 in the nuclear fraction and decreased it in the cytoplasmic and membrane fractions of vector-transfected cells. CA-MEK prevented accumulation of Rac1 in the nucleus. Polyamine depletion significantly decreased Rac1 activity during 6-h migration in vector-transfected cells. Cells transfected with CA-MEK had almost identical levels of activated Rac1 in all three groups. These results suggest that polyamine depletion prevents activation of Rac1 and RhoA by sequestering them to the nucleus and that expression of constitutively active MEK reverses this effect, creating the cellular localization required for activation.

Expression of Hoxb-5 during human lung development and in congenital lung malformations

BACKGROUND

We have previously shown that the Hox gene Hoxb-5 is necessary for normal mouse lung branching morphogenesis. Abnormal Hoxb-5 regulation causes specific alterations in airway branching. We hypothesized that Hoxb-5 is similarly involved in human lung branching morphogenesis, and is abnormally expressed in bronchopulmonary sequestration (BPS) and congenital cystic adenomatoid malformation (CCAM), both of which are congenital lung malformations with abnormal airway development.

METHODS

The temporal, spatial, and cellular expression of the Hoxb-5 protein was evaluated in normal human lung and BPS and CCAM tissue using Western blot analysis and immunocytochemistry.

RESULTS

The expression of Hoxb-5 during human lung development showed strong similarities to that during mouse lung development. Western blots showed high Hoxb-5 protein levels in the pseudoglandular period (PSG), decreased but sustained levels in the canalicular period (CAN), and negligible levels during the alveolar period (ALV). Immunocytochemistry showed Hoxb-5 protein expression in mesenchymal cells around branching airways in the pseuodglandular period, subepithelial fibroblast localization (especially at airway branch points) in the CAN and minimal expression in the ALV. In BPS and CCAM tissue, Hoxb-5 protein levels were increased compared to age- and developmentally-matched lung tissue, and were more similar to the PSG and CAN with Hoxb-5-positive cells in mesenchyme surrounding abnormally branched airways.

CONCLUSIONS

Hoxb-5 expression during human lung branching morphogenesis, which is similar to that observed in mouse lung development, indicates that it plays a role in controlling airway patterning. This notion is supported by results from BPS and CCAM tissue, in which Hoxb-5 is maintained in a manner typical of an earlier developmental stage and is associated with development of abnormal lung tissue.

Morpho-regulation of ectodermal organs: integument pathology and phenotypic variations in K14-Noggin engineered mice through modulation of bone morphogenic protein pathway

Ectodermal organs are composed of keratinocytes organized in different ways during induction, morphogenesis, differentiation, and regenerative stages. We hypothesize that an imbalance of fundamental signaling pathways should affect multiple ectodermal organs in a spatio-temporal-dependent manner. We produced a K14-Noggin transgenic mouse to modulate bone morphogenic protein (BMP) activity and test the extent of this hypothesis. We observed thickened skin epidermis, increased hair density, altered hair types, faster anagen re-entry, and formation of compound vibrissa follicles. The eyelid opening was smaller and ectopic cilia formed at the expense of Meibomian glands. In the distal limb, there were agenesis and hyperpigmentation of claws, interdigital webbing, reduced footpads, and trans-differentiation of sweat glands into hairs. The size of external genitalia increased in both sexes, but they remained fertile. We conclude that modulation of BMP activity can affect the number of ectodermal organs by acting during induction stages, influence the size and shape by acting during morphogenesis stages, change phenotypes by acting during differentiation stages, and facilitate new growth by acting during regeneration stages. Therefore during organogenesis, BMP antagonists can produce a spectrum of phenotypes in a stage-dependent manner by adjusting the level of BMP activity. The distinction between phenotypic variations and pathological changes is discussed.

Structure-specific DNA-binding proteins as the foundation for three-dimensional chromatin organization

Any functions of tandem repetitive sequences need proteins that specifically bind to them. Telomere-binding TRF2/MTBP attaches telomeres to the nuclear envelope in interphase due to its rod-domain-like motif. Interphase nuclei organized as a number of sponge-like ruffly round chromosome territories that could be rotated from outside. SAF-A/hnRNP-U and p68-helicase are proteins suitable to do that. Their location in the interchromosome territory space, ATPase domains, and the ability to be bound by satellite DNAs (satDNA) make them part of the wires used to help chromosome territory rotates. In case of active transcription p68-helicase can be involved in the formation of local "gene expression matrices" and due to its satDNA-binding specificity cause the rearrangement of the local chromosome territory. The marks of chromatin rearrangement, which have to be heritable, could be provided by SAF-A/hnRNP-U. During telophase unfolding the proper chromatin arrangement is restored according to these marks. The structural specificity of both proteins to the satDNAs provides a regulative but relatively stable mode of binding. The structural specificity of protein binding could help to find the "magic" centromeric sequence. With future investigations of proteins with the structural specificity of binding during early embryogenesis, when heterochromatin formation goes on, the molecular mechanisms of the "gene gating" hypothesis (Blobel, 1985) will be confirmed.

The requirement for polyamines for intestinal epithelial cell migration is mediated through Rac1

The rapid migration of intestinal epithelial cells is important to the healing of mucosal ulcers and wounds. This cell migration requires the presence of polyamines and the activation of RhoA. RhoA activity, however, is not sufficient for migration because polyamine depletion inhibited the migration of IEC-6 cells expressing constitutively active RhoA. The current study examines the role of Rac1 and Cdc42 in cell migration and whether their activities are polyamine-dependent. Polyamine depletion with alpha-difluoromethylornithine inhibited the activities of RhoA, Rac1, and Cdc42. This inhibition was prevented by supplying exogenous putrescine in the presence of alpha-difluoromethylornithine. IEC-6 cells transfected with constitutively active Rac1 and Cdc42 migrated more rapidly than vector-transfected cells, whereas cells expressing dominant negative Rac1 and Cdc42 migrated more slowly. Polyamine depletion had no effect on the migration of cells expressing Rac1 and only partially inhibited the migration of those expressing Cdc42. Although polyamine depletion caused the disappearance of actin stress fibers in cells transfected with empty vector, it had no effect on cells expressing Rac1. Constitutively active Rac1 increased RhoA and Cdc42 activity in both normal and polyamine-depleted cells. These results demonstrate that Rac1, RhoA, and Cdc42 are required for optimal epithelial cell migration and that Rac1 activity is sufficient for cell migration in the absence of polyamines due to its ability to activate RhoA and Cdc42 as well as its own effects on the process of cell migration. These data imply that the involvement of polyamines in cell migration occurs either at Rac1 itself or upstream from Rac1.

RhoA inactivation inhibits cell migration but does not mediate the effects of polyamine depletion

BACKGROUND & AIMS

Inhibition of RhoA activity and depletion of polyamines inhibits cell migration and causes changes in the actin cytoskeleton. In this article we have examined the effect of polyamine depletion on RhoA and evaluated these effects on cell migration.

METHODS

Polyamines were depleted in intestinal epithelial cell (IEC)-6 cells by incubating them for 4 days with 5 mmol/L alpha-difluoromethylornithine (DFMO), which inhibits ornithine decarboxylase, the first rate-limiting enzyme in the synthesis of polyamines. IEC-6 cells were then transfected with vectors containing HA tags and constitutively active (HA-V14) or dominant-negative (HA-N19) RhoA with pcDNA3 (vector).

RESULTS

DFMO caused a significant decrease in Rho levels in the cytoplasm and membranes of IEC-6 cells. This decrease was caused by an approximate 50% inhibition of RhoA protein synthesis. Neither the half-life of RhoA nor the level of RhoA messenger RNA (mRNA) was affected. HA-V14-RhoA cells migrated much more rapidly than vector-transfected cells, and HA-N19-RhoA cells exhibited almost no motility. The migration of HA-V14-RhoA cells, however, was inhibited markedly by polyamine depletion. Polyamine depletion did not affect the activity of RhoA in HA-V14-RhoA cells, but inhibited it dramatically in the vector-transfected cells. In the presence of DFMO, the HA-V14-RhoA cells lost stress fibers and gained the appearance of HA-N19-RhoA cells or wild-type cells treated with DFMO.

CONCLUSIONS

First, polyamines are essential for the activity and synthesis and, therefore, normal levels of RhoA protein. Second, RhoA does not mediate the inhibitory effects of polyamine depletion on cell migration.

E- and N-cadherin distribution in developing and functional human teeth under normal and pathological conditions

Cadherins are calcium-dependent cell adhesion molecules involved in the regulation of various biological processes such as cell recognition, intercellular communication, cell fate, cell polarity, boundary formation, and morphogenesis. Although previous studies have shown E-cadherin expression during rodent or human odontogenesis, there is no equivalent study available on N-cadherin expression in dental tissues. Here we examined and compared the expression patterns of E- and N-cadherins in both embryonic and adult (healthy, injured, carious) human teeth. Both proteins were expressed in the developing teeth during the cap and bell stages. E-cadherin expression in dental epithelium followed an apical-coronal gradient that was opposite to that observed for N-cadherin. E-cadherin was distributed in proliferating cells of the inner and outer enamel epithelia but not in differentiated cells such as ameloblasts, whereas N-cadherin expression was up-regulated in differentiated epithelial cells. By contrast to E-cadherin, N-cadherin was also expressed in mesenchymal cells that differentiate into odontoblasts and produce the hard tissue matrix of dentin. Although N-cadherin was not detected in permanent intact teeth, it was re-expressed during dentin repair processes in odontoblasts surrounding carious or traumatic sites. Similarly, N-cadherin re-expression was seen in vitro, in cultured primary pulp cells that differentiate into odontoblast-like cells. Taken together these results suggest that E- and N-cadherins may play a role during human tooth development and, moreover, indicate that N-cadherin is important for odontoblast function in normal development and under pathological conditions.

PYY-mediated fatty acid induced intestinal differentiation

An essential process for fatty acid digestion, absorption and assimilation is the constant replacement of mature intestinal epithelial cells by differentiating stem cells. Free fatty acids (FFA) and PYY may act in concert to alter mucosal cell differentiation through the cytoskeletal-extracellular matrix interactions. PYY induced expression of tetraspanins and intestinal fatty acid binding protein (I-FABP) may be part of a mechanism whereby FFA modulate expression of differentiation dependent proteins in the mucosa. This modulation provides a means for FFA to act as signal molecules in the feedback regulation of their own assimilation.

Disturbance of neuronal plasticity is a critical pathogenetic event in Alzheimer's disease

Brain areas affected by AD pathology are primarily those structures that are invovled in the regulation of "higher brain functions". The functions these areas subserve such as learning, memory, perception, self-awareness, and consciousness require a life-long re-fittng of synaptic contacts that allows for the acquistion of new epigenetic information, a process based on a particularly high degree of structural plasticity. Here, we outline a hypothesis that it is the "labile state fo differentiation" of a subset of neurons in the adult brain that allows for ongoing neuroplastic processes after development is completed but at the same time renders these neurons particularly vulnerable. Mechanisms of molecular and cellular control of neuronal differentiation and proliferation might, thus, not only play a role during development but critically involved in the pathogenesis of neurodegeneration.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

Towards a cellular and molecular understanding of neurulation

Neurulation occurs during the early embryogenesis of chordates, and it results in the formation of the neural tube, a dorsal hollow nerve cord that constitutes the rudiment of the entire adult central nervous system. The goal of studies on neurulation is to understand its tissue, cellular and molecular basis, as well as how neurulation is perturbed during the formation of neural tube defects. The tissue basis of neurulation consists of a series of coordinated morphogenetic movements within the primitive streak (e.g., regression of Hensen's node) and nascent primary germ layers formed during gastrulation. Signaling occurs between Hensen's node and the nascent ectoderm, initiating neurulation by inducing the neural plate (i.e., actually, by suppressing development of the epidermal ectoderm). Tissue movements subsequently result in shaping and bending of the neural plate and closure of the neural groove. The cellular basis of the tissue movements of neurulation consists of changes in the behavior of the constituent cells; namely, changes in cell number, position, shape, size and adhesion. Neurulation, like any morphogenetic event, occurs within the milieu of generic biophysical determinants of form present in all living tissues. Such forces govern and to some degree control morphogenesis in a tissue-autonomous manner. The molecular basis of neurulation remains largely unknown, but we suggest that neurulation genes have evolved to work in concert with such determinants, so that appropriate changes occur in the behaviors of the correct populations of cells at the correct time, maximizing the efficiency of neurulation and leading to heritable species- and axial-differences in this process. In this article, we review the tissue and cellular basis of neurulation and provide strategies to determine its molecular basis. We expect that such strategies will lead to the identification in the near future of critical neurulation genes, genes that when mutated perturb neurulation in a highly specific and predictable fashion and cause neurulation defects, thereby contributing to the formation of neural tube defects.

Expression of Otx homeodomain proteins induces cell aggregation in developing zebrafish embryos

In the zebrafish embryo, cells fated to give rise to the rostral brain move in a concerted fashion and retain tissue coherence during morphogenesis. We demonstrate here that Otx proteins have a dramatic effect on cell-cell interactions when expressed ectopically in the zebrafish embryo. Injection of zebrafish Otx1 or Drosophila otd RNAs into a single cell at the 16-cell stage results in aggregation of descendants of the injected cell. The Otx/Otd homeodomain is necessary for aggregation and appears to be sufficient for the effect when substituted for the homeodomain of an unrelated homeodomain protein. When cells containing injected zOtx1 RNA are limited to the area that is normally fated to become the anterior brain and neural retina, the induced aggregates contribute to anterior brain and retina tissues. In many other embryonic regions, which do not express endogenous zOtx1, the aggregates appear to be incompatible with normal development and do not integrate into developing tissues. By using an activatable Otx1-glutocorticoid receptor fusion protein that results in the stimulation of cell association, we demonstrate that cell aggregates can form as a result of Otx1 activity even after gastrulation is completed. Time-lapse analysis of cell movements show that cell aggregation occurs with only a slight inhibition of the rate of convergence. These results suggest that promotion of cell adhesion or mediation of cell repulsion may be one of the normal functions of the Otx proteins in the establishment of the anterior brain.

Developmental lead exposure disturbs expression of synaptic neural cell adhesion molecules in herring gull brains

Neurobehavioral testing of herring gull chicks (Larus argentatus) in both laboratory and field studies indicates that lead exposure during critical periods of development causes neurological deficits that may compromise survival in the wild. Accumulating evidence suggests that lead impairs neurodevelopment, in part, by altering the expression of cell adhesion molecules (CAMs) responsible for the proper formation and maintenance of neural structure and synaptic function. We examined the adhesion molecules NCAM, L1, and N-cadherin in gull brains to determine whether these CAMs are altered by lead exposure and might serve as markers of developmental neurotoxicity. One-day-old chicks were collected from nesting colonies and were laboratory housed. On post-hatching day (PHD) 2, chicks were given 100 mg/kg lead acetate or saline (intraperitoneally). Birds were killed on PHD 34, 44, or 55 (blood-lead levels averaged 27.4, 20.8, and 19.5 microg/dl, respectively). Brains were removed and stored at -70 degrees C until analysis. Expression of CAMs was determined in synaptosomal preparations by Western blotting and the activity of NCAM-associated sialyltransferase (ST) was determined in purified whole brain golgi apparatus. Elevation in synaptosomal polysialylated NCAM expression and a significant increase in golgi ST activity was observed in lead-treated animals at PHD 34. Reductions in synaptosomal N-cadherin were observed at PHD 34 and 44, while L1 expression appeared unaffected by lead at any time-point. By 55 days post-hatching, no differences in N-cadherin expression, polysialylated NCAM expression or NCAM-associated ST activity were seen in lead-treated animals as compared with age-matched control animals. Lead-induced disruption of CAM expression during early neurodevelopment may contribute to behavioral deficits observed in herring gulls in both the laboratory and the field, and may serve as a marker for heavy metal exposure during postnatal development.

Intercellular adhesion molecule-1 and hair follicle regression

Although the intercellular adhesion molecule-1 (ICAM-1) is recognized for its pivotal role in inflammation and immune responses, its role in developmental systems, such as the cyclic growth (anagen) and regression (catagen) of the hair follicle, remains to be explored. Here we demonstrate that ICAM-1 expression in murine skin is even more widespread and more developmentally regulated than was previously believed. In addition to endothelial cells, selected epidermal and follicular keratinocyte subpopulations, as well as interfollicular fibroblasts, express ICAM-1. Murine hair follicles express ICAM-1 only late during morphogenesis. Thereafter, morphologically identical follicles markedly differ in their ICAM-1 expression patterns, which become strikingly hair cycle-dependent in both intra- and extrafollicular skin compartments. Minimal ICAM-1 and leukocyte function-associated (LFA-1) protein and mRNA expression is observed during early anagen and maximal expression during late anagen and catagen. Keratinocytes of the distal outer root sheath, fibroblasts of the perifollicular connective tissue sheath, and perifollicular blood vessels exhibit maximal ICAM-1 immunoreactivity during catagen, which corresponds to changes of LFA-1 expression on perifollicular macrophages. Finally, ICAM-1-deficient mice display significant catagen acceleration compared to wild-type controls. Therefore, ICAM-1 upregulation is not limited to pathological situations but is also important for skin and hair follicle remodeling. Collectively, this suggests a new and apparently nonimmunological function for ICAM-1-related signaling in cutaneous biology.

Self-organization of periodic patterns by dissociated feather mesenchymal cells and the regulation of size, number and spacing of primordia

Periodic patterning is a fundamental organizing process in biology. Using a feather reconstitution assay, we traced back to the initial stage of the patterning process. Cells started from an equivalent state and self-organized into a periodic pattern without previous cues or sequential propagation. When different numbers of dissociated mesenchymal cells were confronted with a piece of same-sized epithelium, the size of feather primordia remained constant, not the number or interbud spacing, suggesting size determination is intrinsic to dissociated cells. Increasing bone morphogenetic protein (BMP) receptor expression in mesenchymal cells decreased the size of primordia while antagonizing the BMP pathway with Noggin increased the size of primordia. A threshold number of mesenchymal cells with a basal level of adhesion molecules such as NCAM were sufficient to trigger the patterning process. The process is best visualized by the progressive restriction of beta-catenin transcripts in the epidermis. Therefore, feather size, number and spacing are modulated through the available morphogen ligands and receptors in the system.

E- and P-cadherin expression during murine hair follicle morphogenesis and cycling

The role of adhesion molecules in the control of hair follicle (HF) morphogenesis, regression and cycling is still rather enigmatic. Since the adhesion molecules E- and P-cadherin (Ecad and Pcad) are functionally important, e.g. during embryonic pattern formation, we have studied their expression patterns during neonatal HF morphogenesis and cycling in C57/BL6 mice by immunohistology and semi-quantitative RT-PCR. The expression of both cadherins was strikingly hair cycle-dependent and restricted to distinct anatomical HF compartments. During HF morphogenesis, hair bud keratinocytes displayed strong Ecad and Pcad immunoreactivity (IR). While neonatal epidermis showed Ecad IR in all epidermal layers, Pcad IR was restricted to the basal layer. During later stages of HF morphogenesis and during anagen IV-VI of the adolescent murine hair cycle, the outer root sheath showed strong E- and Pcad IR. Instead, the outermost portion of the hair matrix and the inner root sheath displayed isolated Ecad IR, while the innermost portion of the hair matrix exhibited isolated Pcad IR. During telogen, all epidermal and follicular keratinocytes showed strong Ecad IR. This is in contrast to Pcad, whose IR was stringently restricted to matrix and secondary hair germ keratinocytes which are in closest proximity to the dermal papilla. These findings suggest that isolated or combined E- and/or Pcad expression is involved in follicular pattern formation by segregating HF keratinocytes into functionally distinct subpopulations; most notably, isolated Pcad expression may segregate those hair matrix keratinocytes into one functional epithelial tissue unit, which is particularly susceptible to growth control by dermal papilla-derived morphogens. The next challenge is to define which secreted agents implicated in hair growth control modulate these follicular cadherin expression patterns, and to define how these basic parameters of HF topobiology are altered during common hair growth disorders.

NF-kappaB activity is induced by neural cell adhesion molecule binding to neurons and astrocytes

The neural cell adhesion molecule, N-CAM, is expressed on the surface of astrocytes and neurons, and N-CAM homophilic binding has been shown to alter gene expression in both of these cell types. To determine mechanisms by which N-CAM regulates gene expression, we have analyzed DNA binding of and transcriptional activation by NF-kappaB after N-CAM binding to the cell surface. Addition of purified N-CAM, the recombinant third immunoglobulin domain of N-CAM, or N-CAM antibodies either to neonatal rat forebrain astrocytes or to cerebellar granule neurons increased NF-kappaB/DNA binding activity in nuclear extracts as measured by electrophoretic mobility shift assays. Analysis using supershifting antibodies indicated that, in both cell types, p50 and p65 but not p52, c-Rel, or Rel B were contained in the NF-kappaB-binding complex. NF-kappaB-mediated transcription was increased after N-CAM binding to astrocytes and neurons as demonstrated by the activation of two different luciferase reporter constructs containing NF-kappaB-binding sites. N-CAM binding also resulted in degradation of IkappaB-alpha protein followed by an increase in the levels of IkappaB-alpha mRNA and protein. These results indicate that N-CAM homophilic binding at the cell membrane leads to increased NF-kappaB binding to DNA and transcriptional activation in both neurons and astrocytes. Activation of NF-kappaB, however, did not influence the previously reported ability of N-CAM to inhibit astrocyte proliferation. These observations together support the hypothesis that N-CAM binding activates multiple pathways leading to changes in gene expression in both astrocytes and neurons.

Distinct patterns of NCAM expression are associated with defined stages of murine hair follicle morphogenesis and regression

Hair follicle development, growth (anagen), and regression (catagen) largely result from bidirectional epithelial-mesenchymal interactions whose molecular basis is still unclear. Because adhesion molecules are critically involved in pattern formation and because the fundamental importance of neural cell adhesion molecule (NCAM) for feather development has been demonstrated, we studied the protein expression patterns of NCAM during hair follicle development and regression in the C57BL/6 mouse model. During murine hair follicle development, NCAM immunoreactivity (IR) was first detected on epithelial hair placodes and later on selected keratinocytes in the distal outer root sheath. Mesenchymal NCAM immunoreactivity (IR) was noted on fibroblasts of the future dermal papilla (DP) and the perifollicular connective tissue sheath. Fetal hair follicle elongation coincided with strong, ubiquitous dermal NCAM IR, which remained strong until the follicles entered into their first neonatal catagen. At this time, the strong interfollicular dermal NCAM IR decreased substantially. During consecutive hair cycles, mesenchymal NCAM IR was seen exclusively on DP and perifollicular connective tissue sheath fibroblasts and on the trailing cells of regressing catagen hair follicles. These highly restricted and developmentally controlled expression patterns suggest an important role for NCAM in hair follicle topobiology during morphogenesis and cyclic remodeling of this miniorgan.

Molecular mechanisms of enteroendocrine differentiation

Passing through a complex series of developmental steps, the visceral endoderm differentiates into four intestinal epithelial lineages comprising enterocytes, goblet cells, paneth cells, and enteroendocrine cells. The intestinal enteroendocrine system consists of at least 15 different cell types, which can be classified on the basis of morphological criteria, expression of secretory products, and abundance of specific marker molecules. During intestinal development and in the adult gut, neuroendocrine subpopulations display strictly controlled differences in their geographical distribution that go along with dramatic differences in cell type-specific gene expression. Identification to transcription factors and regulatory DNA elements responsible for cell-specific gene expression in different neuroendocrine cell types as well as various transgenic and "knock-out" mouse models have largely added to our understanding of mechanisms controlling appropriate special and temporal activation of enteroendocrine differentiation programs. This article reviews current in vitro and in vivo studies analyzing different molecular aspects of enteroendocrine differentiation. In addition, the influence of intestinal diseases including malignant transformation on enteroendocrine differentiation and the underlying mechanisms will be discussed.

Epithelial vs mesenchymal contribution to the extracellular matrix in the human intestine

The basement membrane (BM) underlying the epithelium of the intestine is generally believed to be of both epithelial and mesenchymal origin but the exact contribution of each tissue has not been directly examined in the human. In this study, we have used a newly described procedure to dissociate the human intestine into pure epithelial and corresponding mesenchymal fractions. Northern blot and RT-PCR analyses of the fractions for the presence of transcripts encoding extracellular matrix molecules revealed that the epithelium produces the formal BM molecules such as the alpha 1, alpha 2, and beta 1 chains of laminin-1 and laminin-2 and the alpha 5(IV) and alpha 6(IV) chains of collagen as well as fibronectin, a BM-associated molecule. Interestingly, the alpha 1(IV) chain of collagen, which associates with the alpha 2(IV) chain to form the main BM collagen network, as well as tenascin-C and decorin, two BM-associated molecules, was found to be exclusively of mesenchymal origin. Taken together, these data support the concept that in the human, as in experimental animals, the intestinal BM is composed of components produced from both the epithelium and the mesenchyme.

Gene regulation of cell adhesion: a key step in neural morphogenesis

A mounting body of evidence suggests that cell adhesion molecules (CAMs) play important roles in morphogenetic patterning of the nervous system. The combined factors that control the expression of CAMs during early neural development are, however, largely unknown. We have hypothesized that the coordinate expression of homeobox (Hox) and paired box (Pax) proteins in the neural axis leads to the differential expression of particular CAM genes. Following this hypothesis, we have characterized the promoters and identified cis-regulatory sequences that bind to and respond to Hox and Pax proteins in the genes for three neurally expressed CAMs - the neural cell adhesion molecule, N-CAM, the neuron-glia cell adhesion molecule, Ng-CAM, and L1. Experiments on transgenic mice carrying N-CAM promoter/lacZ reporter gene constructs indicated that mutation of either the HBS or the PBS disrupted patterning of N-CAM expression in the embryonic spinal cord. To examine the factors that restrict the expression of certain CAMs to the nervous system, we identified regulatory elements that block expression of the Ng-CAM and L1 genes in non-neural cells. We characterized a 310 base pair region of the first intron of the Ng-CAM gene containing five neural restrictive silencer elements (NRSEs) and a binding site for the Pax-3 protein. These elements silenced activity of the Ng-CAM promoter in NIH3T3 fibroblasts, but had no effect on its activity in N2A neuroblastoma cells line. Similar analyses of the L1 gene revealed a single NRSE within the second intron that was important for silencing in this cellular transfection system. To analyze the role of the NRSE in vivo, we prepared transgenic mice containing two L1 gene/lacZ constructs, one containing the NRSE and another in which the NRSE was deleted. The wild type L1lacZ transgene showed a neurally restricted pattern of expression, whereas the NRSE-mutated L1 construct showed extensive extraneural expression of the L1 gene. Thus, neural specificity of CAM expression is controlled by the NRSE. The general significance of these observations is that they connect the expression of important families of transcriptional regulators with gene products capable of direct cellular mechanochemistry.

Local inhibitory action of BMPs and their relationships with activators in feather formation: implications for periodic patterning

The formation of periodic patterns is fundamental in biology. Theoretical models describing these phenomena have been proposed for feather patterning; however, no molecular candidates have been identified. Here we show that the feather tract is initiated by a continuous stripe of Shh, Fgf-4, and Ptc expression in the epithelium, which then segregates into discrete feather primordia that are more strongly Shh and Fgf-4 positive. The primordia also become Bmp-2 and Bmp-4 positive. Bead-mediated delivery of BMPs inhibits local feather formation in contrast with the activators, SHH and FGF-4, which induce feather formation. Both FGF-4 and SHH induce local expression of Bmp-4, while BMP-4 suppresses local expression of both. FGF-4 also induces Shh. Based on these findings, we propose a model that involves (1) homogeneously distributed global activators that define the field, (2) a position-dependent activator of competence that propagates across the field, and (3) local activators and inhibitors triggered in sites of individual primordia that act in a reaction-diffusion mechanism. A computer simulation model for feather pattern formation is also presented.

KSA antigen Ep-CAM mediates cell-cell adhesion of pancreatic epithelial cells: morphoregulatory roles in pancreatic islet development

Cell adhesion molecules (CAMs) are important mediators of cell-cell interactions and regulate cell fate determination by influencing growth, differentiation, and organization within tissues. The human pancarcinoma antigen KSA is a glycoprotein of 40 kD originally identified as a marker of rapidly proliferating tumors of epithelial origin. Interestingly, most normal epithelia also express this antigen, although at lower levels, suggesting that a dynamic regulation of KSA may occur during cell growth and differentiation. Recently, evidence has been provided that this glycoprotein may function as an epithelial cell adhesion molecule (Ep-CAM). Here, we report that Ep-CAM exhibits the features of a morphoregulatory molecule involved in the development of human pancreatic islets. We demonstrate that Ep-CAM expression is targeted to the lateral domain of epithelial cells of the human fetal pancreas, and that it mediates calcium-independent cell-cell adhesion. Quantitative confocal immunofluorescence in fetal pancreata identified the highest levels of Ep-CAM expression in developing islet-like cell clusters budding from the ductal epithelium, a cell compartment thought to comprise endocrine progenitors. A surprisingly reversed pattern was observed in the human adult pancreas, displaying low levels of Ep-CAM in islet cells and high levels in ducts. We further demonstrate that culture conditions promoting epithelial cell growth induce upregulation of Ep-CAM, whereas endocrine differentiation of fetal pancreatic epithelial cells, transplanted in nude mice, is associated with a downregulation of Ep-CAM expression. In addition, a blockade of Ep-CAM function by KS1/4 mAb induced insulin and glucagon gene transcription and translation in fetal pancreatic cell clusters. These results indicate that developmentally regulated expression and function of Ep-CAM play a morphoregulatory role in pancreatic islet ontogeny.

Allosteric modulation of AMPA-type glutamate receptors increases activity of the promoter for the neural cell adhesion molecule, N-CAM

To study regulation in vivo of the promoter for the neural cell adhesion molecule, N-CAM, we have used homologous recombination to insert the bacterial lacZ gene between the transcription and translation initiation sites of the N-CAM gene. This insertion disrupts the gene and places the expression of beta-galactosidase under the control of the N-CAM promoter. Animals homozygous for the disrupted allele did not express N-CAM mRNA or protein, but the pattern of beta-galactosidase expression in heterozygous and homozygous embryos was similar to that of N-CAM mRNA in wild-type animals. The homozygotes exhibited many of the morphological abnormalities observed in previously reported N-CAM knockout mice, with the exception that hippocampal long-term potentiation in the Schaffer collaterals was identical in homozygous, heterozygous, and wild-type animals. Heterozygous mice were used to examine the regulation of the N-CAM promoter in response to enhanced synaptic transmission. Treatment of the mice with an ampakine, an allosteric modulator of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that enhances normal glutamate-mediated synaptic transmission, increased the expression of beta-galactosidase in vivo as well as in tissue slices in vitro. Similar treatments also increased the expression of N-CAM mRNA in the heterozygotes. The effects of ampakine in slices were strongly reduced in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an AMPA receptor antagonist. Taken together, these results indicate that facilitation of AMPA receptor-mediated transmission leads to activation of the N-CAM promoter and provide support for the hypothesis that N-CAM synthesis is regulated in part by synaptic activity.

Morphostasis: an evolving perspective

In an earlier article, I proposed a pathway by which morphostasis (tissue homeostasis) may have evolved. It began in single-celled organisms and culminated in the mammalian immune system. This evolutionary path is now traced from its source--the intracellular surveillance within an isolated cell of its own internal health. Morphostasis sequentially incorporates heat shock proteins, apoptosis, cell adhesion molecules, complement components, gap junctions, phagocytes, natural killer cells, cytotoxic T-cells, helper cells and antibodies. I propose that the sequence leading to the insertion of gap junctions is an ancestor of the complement attack sequence. Although contentious, this deduction is intriguing, since numerous, minimal clues support the proposition. The broad hypothesis emphasizes a theme that may prove to be a useful framework on which to hang a better understanding of immunology and embryology. It highlights points where a concentrated research effort may rapidly advance our understanding of both.

Rho proteins play a critical role in cell migration during the early phase of mucosal restitution

In the intestine, several growth factors stimulate migration of epithelial cells, contributing to the maintenance of tissue integrity. The Ras-like GTPase Rho regulates a signal transduction pathway linking growth factor receptors to the formation of actin stress fibers and focal adhesions, presumed to be important for motility. Using an in vitro wound-induced migration assay, we have examined the role of Rho GTPases in the migration of IEC-6 and Caco-2 cells, and provide evidence that the Rho GTPases play an essential role in the initial phase of mucosal wound healing. Treatment of the cells with Clostridium difficile toxins A and B, inhibitors of the Rho family GTPases inhibited migration in a dose-dependent fashion. Microinjection of the inhibitory exchange factor Rho-guanine nucleotide dissociation inhibitor (GDI), or Clostridium botulinum C3 ADP-ribosyl transferase (C3) toxin, a Rho-ADP-ribosylating exoenzyme, potently inhibited migration. Microinjection of RhoT19N, a dominant negative form of RhoA, or in vitro ADP-ribosylated RhoA impaired the ability of cells to migrate. Rho-GDI and C3 exoenzyme also inhibited EGF-induced migration of IEC-6 cells. These results demonstrate that Rho is required for endogenous and EGF-induced migration of small intestinal crypt cells, and that Rho proteins are essential elements of a mechanism by which growth factors induce cell migration to restitute mucosal integrity.

Gene regulation of cell adhesion molecules in neural morphogenesis

A mounting body of evidence suggests that cell-cell adhesion molecules (CAMs) play critical roles in morphogenetic patterning and in laying down the initial tissue scaffold of the nervous system. Perturbations of CAM binding can lead to altered tissue pattern and interruption of tissue interactions to altered patterns of CAM expression. The combined factors that regulate the expression of CAMs and that drive early neural development are, however, largely unknown. We have hypothesized that the coordinate expression of homeobox (Hox) and paired box (Pax) transcription factors in various axes of the body plan leads to differential expression of particular CAM genes. Following this hypothesis, we have characterized the promoters and other regulatory regions of a number of genes specifying CAMs and have identified cis-regulatory elements that bind and respond to Hox and Pax proteins. Our recent experiments in vitro indicate, for example, that transcription factors encoded by Hox and Pax genes bind to specific DNA sequences in the N-CAM promoter and activate expression of the N-CAM gene. Experiments on transgenic mice carrying either the wild-type N-CAM promoter or a variant with mutations in the homeodomain binding sites (HBS) linked to a lac-Z reporter gene indicate that interactions with these elements are important in establishing and maintaining N-CAM expression in the spinal cord. We have also examined the regulatory sequences controlling expression of the gene for the neuron-glia adhesion molecule (Ng-CAM). Unlike N-CAM, which is also expressed in many non-neural sites, Ng-CAM is restricted to cells of the nervous system. After identifying this promoter for the Ng-CAM gene, we characterized a silencer region in the first intron of the gene that extinguishes the expression of Ng-CAM in fibroblasts but not in neuronal cells. Thus, a default mechanism can account for the restriction of Ng-CAM expression to the nervous system. The silencer region contains five neural-restrictive silencer elements and a binding site for the Pax3 protein, which also appears to have silencing activity. All of these findings suggest that Hox and Pax transcription factors can have both activating and silencing functions in regulating CAM gene expression. The general significance of these accumulated observations is that they connect the place-dependent expression of gene products capable of direct morphoregulation to the function of pattern-forming genes.

Cloning of a novel human neural cell adhesion molecule gene (NCAM2) that maps to chromosome region 21q21 and is potentially involved in Down syndrome

To contribute to the development of the transcription map of human chromosome 21 (HC21), we have used exon trapping to identify portions of HC21 genes. One trapped exon showed strong homology with members of the neural cell adhesion molecule (NCAM) family of genes from different species. We subsequently cloned the complete coding sequence from a human fetal brain cDNA library and determined its nucleotide sequence and predicted amino acid sequence. The predicted polypeptide of this novel NCAM2 gene contains 837 amino acids and shows 62% similarity to the NCAM homologs. It contains five immunoglobulin-like domains, two fibronectin type III domains, a transmembrane domain and a cytoplasmic domain. The gene is expressed most strongly in human adult and fetal brain. Using somatic cell hybrids, we mapped NCAM2 to 21q21, between markers D21S18 and D21S282. Radiation hybrid mapping localized this novel gene between polymorphic markers D21S1914 and D21S265. NCAMs are members of the immunoglobulin superfamily and are essential in the formation and maintenance of tissue structure. To date there are no candidate human disorders on HC21 that could be associated with mutations in NCAM2. In addition, the role of NCAM2 in the pathophysiology of Down syndrome is unknown. However, it is a good candidate for involvement in certain Down syndrome phenotypes because a slight overexpression of NCAMs increases many-fold the homotypic adhesion properties of cells.

Hormonal regulation of N-cadherin mRNA levels in rat granulosa cells

We have examined the ability of hormones to modulate the steady-state levels of N-cadherin mRNA transcripts in aggregated and dispersed rat granulosa cell populations. Estradiol and follicle-stimulating hormone (FSH) had no effect on the levels of N-cadherin mRNA transcripts in aggregated granulosa cells. In contrast, these two hormones stimulated N-cadherin mRNA levels in dispersed granulosa cells. This is the first report that estradiol and FSH are capable of regulating N-cadherin mRNA levels. The results also suggest that the N-cadherin mRNA levels in dispersed and aggregated granulosa cells are regulated by different mechanisms.

A binding site for Pax proteins regulates expression of the gene for the neural cell adhesion molecule in the embryonic spinal cord

The neural cell adhesion molecule (N-CAM) mediates cell-cell interactions and is expressed in characteristic spatiotemporal patterns during development. In previous studies of factors that control N-CAM gene expression, we identified a binding site for the paired domain of Pax proteins (designated PBS) in the mouse N-CAM promoter. In this study, we demonstrate that a transcription factor known to be important for development of the central nervous system, Pax-6, binds to the N-CAM PBS and show that the PBS can influence N-CAM expression in vivo. Pax-6, produced in COS-1 cells, bound to the PBS through two half-sites, PBS-1 and PBS-2; mutations in both of these sites completely disrupted binding. Moreover, nuclear extracts from embryonic day (E) 11.5 mouse embryos bound to the PBS, and this binding was inhibited by antibodies to Pax-6. To determine the role of the PBS in vivo, we generated transgenic mice with N-CAM promoter/lacZ gene constructs containing either a wild-type or a mutated PBS. Mutations in PBS-1 and PBS-2 decreased the extent of beta-galactosidase expression in the mantle layer of the spinal cord limiting it to ventral regions at E11.5. At E14.5, these mutations eliminated most of the expression that was seen in the wild-type spinal cord. Taken together with our previous observations that the PBS binds multiple Pax proteins, the data indicate that such binding contributes to the regulation of N-CAM gene expression during neural development.

Role of arsenic as a reproductive toxin with particular attention to neural tube defects

Arsenic has been recognized as a human toxicant for over 2000 years. More recently it has been readily accepted as a human carcinogen. Animal research has demonstrated arsenic's ability to have profound detrimental effects on the developing embryo in avian and mammalian species. This article comprehensively reviews the human and animal literature on the subject of the reproductive toxicity of arsenic. A variety of endpoints are considered, including spontaneous abortion, cardiovascular defects, and arsenic's role in the causation of neural tube defects (NTDs). A summary of the literature that has examined the various postulated mechanisms by which arsenic may produce NTDs is also considered. In addition, a discussion of literature relative to the presence of arsenic in the general environment and in the workplace is presented. This article reaches the conclusion that while further research is clearly needed, particularly on the potential toxicity of organic arsenical compounds, the current literature suggests it may be prudent and appropriate to treat inorganic arsenic as a probable human reproductive toxin.

Embryonic expression patterns of the neural cell adhesion molecule gene are regulated by homeodomain binding sites

During development of the vertebrate nervous system, the neural cell adhesion molecule (N-CAM) is expressed in a defined spatiotemporal pattern. We have proposed that the expression of N-CAM is controlled, in part, by proteins encoded by homeobox genes. This hypothesis has been supported by previous in vitro experiments showing that products of homeobox genes can both bind to and transactivate the N-CAM promoter via two homeodomain binding sites, HBS-I and HBS-II. We have now tested the hypothesis that the N-CAM gene is a target of homeodomain proteins in vivo by using transgenic mice containing native and mutated N-CAM promoter constructs linked to a beta-galactosidase reporter gene. Segments of the 5' flanking region of the mouse N-CAM gene were sufficient to direct expression of the reporter gene in the central nervous system in a pattern consistent with that of the endogenous N-CAM gene. For example, at embryonic day (E) 11, beta-galactosidase staining was found in postmitotic neurons in dorsolateral and ventrolateral regions of the spinal cord; at E14.5, staining was seen in these neurons throughout the spinal cord. In contrast, mice carrying an N-CAM promoter-reporter construct with mutations in both homeodomain binding sites (HBS-I and HBS-II) showed altered expression patterns in the spinal cord. At E11, beta-galactosidase expression was seen in the ventrolateral spinal cord, but was absent in the dorsolateral areas, and at E 14.5, beta-galactosidase expression was no longer detected in any cells of the cord. Homeodomain binding sites found in the N-CAM promoter thus appear to be important in determining specific expression patterns of N-CAM along the dorsoventral axis in the developing spinal cord. These experiments suggest that the N-CAM gene is an in vivo target of homeobox gene products in vertebrates.

Catenin-dependent and -independent functions of vascular endothelial cadherin

Vascular endothelial cadherin (VE-cadherin, cadherin-5, or 7B4) is an endothelial specific cadherin that regulates cell to cell junction organization in this cell type. Cadherin linkage to intracellular catenins was found to be required for their adhesive properties and for localization at cell to cell junctions. We constructed a mutant form of VE-cadherin lacking the last 82 amino acids of the cytoplasmic domain. Surprisingly, despite any detectable association of this truncated VE-cadherin to catenin-cytoskeletal complex, the molecule was able to cluster at cell-cell contacts in a manner similar to wild type VE-cadherin. Truncated VE-cadherin was also able to promote calcium-dependent cell to cell aggregation and to partially inhibit cell detachment and migration from a confluent monolayer. In contrast, intercellular junction permeability to high molecular weight molecules was severely impaired by truncation of VE-cadherin cytoplasmic domain. These results suggest that the VE-cadherin extracellular domain is enough for early steps of cell adhesion and recognition. However, interaction of VE-cadherin with the cytoskeleton is necessary to provide strength and cohesion to the junction. The data also suggest that cadherin functional regulation might not be identical among the members of the family.

Fibronectin peptide DRVPHSRNSIT and fibronectin receptor peptide DLYYLMDL arrest gastrulation of Rana pipiens

Gastrulation is characterized by dramatic cell migration which is thought to require the interaction of cell adhesion molecules with extracellular molecules. We have tested two novel peptides, a fibronectin peptide and a fibronectin receptor peptide, for their effects on gastrulation of the leopard frog Rana pipiens. The fibronectin peptide DRVPHSRNSIT corresponds to residues 1373-1383 of the cell-binding domain of fibronectin; the receptor peptide DLYYLMDL corresponds to residues 124-131 of beta 1 subunit of a variety of integrins including alpha 5 beta 1. Either of these peptides significantly inhibited gastrulation after being microinjected into mid-blastulae. These results indicate that these sequences may correspond to the ligand/receptor interaction sites of fibronectin and its receptor(s).