Alcohols and peptides can block ethanol-induced teratogenesis and inhibition of cell adhesion.

Development of the nervous system depends on patterned neuronal migration and axonal growth, which are regulated by cell–cell and cell–substrate interactions (1). These interactions are mediated through cell adhesion molecules (CAMs), which include cadherins, selectins, integrins, mucins, and members of the IgG superfamily. Within the latter group are proteins, such as neural cell adhesion molecule, L1, and deleted in colon cancer (DCC), that are prominently expressed in the nervous system (2). In this issue of PNAS, investigators from the laboratory of Dr. Michael Charness (3) report findings that link these molecules to an important, preventable mental retardation syndrome.

The role of CAMs in neural development has been established partly through the study of experiments of nature, spontaneous mutations in CAM genes that produce neurodevelopmental disorders in humans. The best example of such a gene is L1, which is located on human chromosome Xq28 and codes for a 143-kDa transmembrane glycoprotein containing six IgG-like and five fibronectin type III-like extracellular domains, a transmembrane segment, and a short cytoplasmic C terminus (Fig. 1). Signaling through L1 activates transduction pathways that involve inositol phosphates, mobilization of intracellular calcium, voltage-gated calcium influx, and cAMP (4–6). Activation of these pathways may underlie the effects of L1 in cell adhesion, neurite extension, neuronal migration, and axonal fasciculation (7).

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Fig. 1.

Inhibition of L1-mediated cell adhesion by ethanol and its antagonism by alcohols and peptides. The C terminus (C), intracytoplasmic region (IC), transmembrane segment (TM), fibronectin-like domains (FN), IgG-like domains (IgG), and N terminus (N) of L1 are shown. Ethanol interferes with the interaction between L1 and its homophilic or heterophilic binding partners (L1CAM binding site) by acting at an as-yet-undetermined site on L1. Long, straight-chain alcohols such as octanol, the NAPVSIPQ fragment of activity-dependent neuroprotective protein (NAP), and the SALLRSIPA fragment of activity-dependent neurotrophic factor (SAL) block the effect of ethanol and thereby preserve L1 function.

Mutations in L1 produce a range of X-linked neurological syndromes (8). Hydrocephalus caused by congenital stenosis of the aqueduct of Sylvius (HSAS) is associated with enlarged cerebral ventricles, mental retardation, spastic paraparesis, and adducted thumbs. In some cases, these features coexist with Hirschsprung's disease (aganglionic megacolon), with or without cleft palate. Other L1 mutations produce MASA syndrome, which comprises mental retardation, aphasia, shuffling gait, and adducted thumbs, or CRASH syndrome, consisting of corpus callosum hypoplasia, retardation, adducted thumbs, spastic paraplegia, and hydrocephalus.

The ability to relate gene defects to clinical disorders can help to determine gene function and illuminate the pathophysiology of acquired disorders that target the gene product. For example, the offspring of women who drink to excess during pregnancy may be afflicted with the fetal alcohol syndrome (FAS), which is characterized classically by mental retardation, microcephaly, irritability, growth deficiency, and facial dysmorphism (short palpebral fissures, hypoplastic philtrum, thin upper lip, retrognathia) (9). Hydrocephalus and agenesis of the corpus callosum also occur in some cases of FAS, and together with mental retardation these features are reminiscent of those associated with L1 mutations. FAS is estimated to occur in 0.3–2.2 births per 1,000 in the United States, so it is not a rare disease. The resemblance of FAS to the spectrum of disorders caused by L1 mutations has led investigators to explore the possible role of L1 in FAS.

This is a line of investigation that the Charness laboratory has been pursuing for several years. First, they found that ethanol inhibits cell–cell adhesion induced by osteogenic protein-1 (OP-1) in NG108–15 neuroblastoma × glioma hybrid cultures (10). Because OP-1 induces the expression of CAMs, including neural cell adhesion molecule (NCAM)-140 and L1, they next asked whether ethanol interfered with the function of either of these proteins. Mouse fibroblasts were transfected with human L1 or NCAM-140, and the effects of ethanol on cell–cell adhesion were observed. Ethanol inhibited adhesion of L1-transfected, but not NCAM-140-transfected, fibroblasts, suggesting that L1 was ethanol's target (11). A subsequent study showed that the neuronal isoform of L1 was the form most consistently inhibited by ethanol (12).

If a defect in L1-mediated cell adhesion contributes to FAS, then drugs that block the antiadhesive action of ethanol might have therapeutic value when given to pregnant women whose fetuses are at risk for FAS. In recent studies, the Charness laboratory has focused on identifying such drugs. The first example came from investigations of the effects of alcohols of different carbon-chain lengths on cell adhesion. As is true of many alcohol effects, the potency with which straight-chain alcohols reduced cell adhesion in L1-transfected fibroblasts and NG108–15 cells treated with bone morphogenetic protein-7 (an inducer of L1) increased with increasing chain length from C1 through C4 (13). Longer chain-length alcohols such as octanol had no effect on adhesion themselves, but irreversibly blocked the effect of ethanol. More recently, Charness and colleagues (14) identified peptide antagonists of the antiadhesive action of ethanol. Previous studies had shown that administration of small fragments of activity-dependent neuroprotective protein (ADNP) or activity-dependent neurotrophic factor (ADNF) could prevent ethanol-induced teratogenesis in mouse whole-embryo cultures (15). Reasoning that these protective effects might result from antagonizing the effect of ethanol on L1-mediated cell adhesion, Charness and coworkers (16) treated L1-transfected fibroblasts with the octapeptide, ADNP fragment NAPVSIPQ (NAP), or the nonapeptide, ADNF fragment SALLRSIPA (SAL), and found that, at femtomolar concentrations, both peptides blocked the inhibition of L1-mediated adhesion by ethanol. The observation that two structurally distinct classes of compounds (alcohols and peptides) block both ethanol-induced teratogenesis and inhibition of cell adhesion by ethanol suggests that the two effects may be related.

In this issue of PNAS, Charness and coworkers (3) provide new insight into the molecular mechanisms that underlie ethanol-induced embryotoxicity and may be involved in producing FAS in humans. They report that mutations in the SIP region of NAP have parallel effects on the ability of NAP to antagonize (i) ethanol-induced inhibition of L1-mediated cell adhesion and (ii) ethanol-induced developmental abnormalities in mouse whole-embryo cultures. In contrast, it is possible through such mutations to dissociate these effects of NAP from its protective effect against tetrodotoxin-mediated neuronal cell death. Because the structural requirements for inhibition of L1-mediated cell adhesion and for embryotoxicity are similar, these two effects of NAP (and, by extension, of ethanol) may be causally related.

Given the critical role of CAMs in guiding brain development, it is not surprising that impaired dysfunction of CAMs should result in neurodevelopmental disorders. In fact, mutations in CAM or CAM-related genes other than L1 are also associated with mental retardation syndromes. These include 3p-deletion syndrome, which affects L1CAM2; congenital disorder of glycosylation type IIc, caused by mutations in the gene for GDP-fucose transporter-1, which regulates glycosylation of the E- and P-selectin ligand CD15; cleft lip/ palate-ectodermal dysplasia syndrome, from single-base substitution, deletion, or duplication in the CAM nectin-1; Hirschsprung's disease with mental retardation, from substitutions, deletions, or insertions in the zinc finger homeobox gene1B, which regulates E-cadherin expression; and Down's syndrome, in which duplication of Down's syndrome CAM may play a role.

What distinguishes FAS from these disorders is, of course, that it is caused by embryotoxicity from an exogenous toxin, ethanol, and is therefore preventable. Efforts at achieving this by promoting contraception in alcohol-using women of child-bearing age and abstinence from alcohol during pregnancy appear to have had some, but only limited, success (17). However, the ability to selectively antagonize particular effects of ethanol, such as inhibition of L1 function and the apparently related embryotoxicity, raises the possibility that drugs can be developed to reduce the incidence of FAS, even in women who are unable to stop drinking during pregnancy. Efforts to achieve this would undoubtedly be aided by more information regarding the precise molecular mechanism through which ethanol perturbs the function of L1. Another implication of the work reported by Charness and colleagues (3) is that, just as the phenotypic resemblance of FAS to genetic disorders involving L1 provided clues to the involvement of L1 in FAS, similarities between the syndromes produced by other human teratogens (such as anticonvulsants) and by mutations in other CAMs may be similarly fruitful.