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Amyloid precursor protein (APP) is overexpressed in the developing brain and portions of its extracellular domain, especially amino acid residues 96-110, play an important role in neurite outgrowth and neural cell differentiation. In the current study, we evaluated the developmental abnormalities caused by administration of exogenous APP(96-110) in sea urchin embryos and larvae, which, like the developing mammalian brain, utilize acetylcholine and other neurotransmitters as morphogens; effects were compared to those of beta-amyloid 1-42 (Abeta42), the neurotoxic APP fragment contained within neurodegenerative plaques in Alzheimer's Disease. Although both peptides elicited dysmorphogenesis, Abeta42 was far more potent; in addition, whereas Abeta42 produced abnormalities at developmental stages ranging from early cleavage divisions to the late pluteus, APP(96-110) effects were restricted to the intermediate, mid-blastula stage. For both agents, anomalies were prevented or reduced by addition of lipid-permeable analogs of acetylcholine, serotonin or cannabinoids; physostigmine, a carbamate-derived cholinesterase inhibitor, was also effective. In contrast, agents that act on NMDA receptors (memantine) or alpha-adrenergic receptors (nicergoline), and that are therapeutic in Alzheimer's Disease, were themselves embryotoxic, as was tacrine, a cholinesterase inhibitor from a different chemical class than physostigmine. Protection was also provided by agents acting downstream from receptor-mediated events: increasing cyclic AMP with caffeine or isobutylmethylxanthine, or administering the antioxidant, a-tocopherol, were all partially effective. Our findings reinforce a role for APP in development and point to specific interactions with neurotransmitter systems that act as morphogens in developing sea urchins as well as in the mammalian brain.

Forty serotonin-related neurochemicals were tested on embryos and larvae of Lytechinus variegatus and other sea urchin species. Some of these substances (agonists of 5-HT1 receptors, antagonists of 5-HT2, 5-HT3 or 5-HT4 receptors, and inhibitors of the serotonin transporter, SERT) perturbed post-blastulation development, eliciting changes in embryonic/larval phenotypes typical for each class of receptor ligand. These developmental malformations were prevented completely or partially by serotonin (5-HT) or 5-HT analogs (5-HTQ, AA-5-HT), providing evidence for the putative localization of cellular targets. Immunoreactive 5-HT, 5-HT receptors and SERT were found in pre-nervous embryos and larvae of both L. variegatus and Strongylocentrotus droebachiensis. During gastrulation, these components of the serotonergic system were localized to the archenteron (primary gut), mesenchyme-like cells, and often the apical ectoderm. These results provide evidence that pre-nervous 5-HT may regulate early events of sea urchin embryogenesis, mediated by 5-HT receptors or the 5-HT transporter.

Abstract: Accumulation of β-amyloid protein is an Alzheimer''s disease hallmark but also may be mechanistically involved in neurodegeneration. One of its cleavage peptides, Aβ42, has been used to evaluate the mechanisms underlying amyloid-induced cytotoxicity and targeting of acetylcholine systems. We studied Sphaerechinus granularis sea urchin embryos which utilize acetylcholine and other neurotransmitters as morphogens. At a threshold concentration of 0.1μM Aβ42, there was damage to the larval skeleton, accumulation of ectodermal cells in the blastocoele and underdevelopment of larval arms. Raising the Aβ42 concentration to 0.2–0.4μM produced anomalies depending on the stage at which Aβ42 was introduced: at the first cleavage divisions, abnormalities appeared within 1–2 cell cycles; at the mid-blastula stage, the peak period of sensitivity to Aβ42, gastrulation was blocked; at later stages, there was progressive damage to the larval skeleton, digestive tract and larval spicules, as well as regression of larval arms. Each of these anomalies could be offset by the addition of lipid-permeable analogs of acetylcholine (arachidonoyl dimethylaminoethanol), serotonin (arachidonoyl serotonin) and cannabinoids (arachidonoyl vanillylamine), with the greatest activity exhibited by the acetylcholine analog. These results indicate that sea urchin embryos provide a model suitable to characterize the mechanisms underlying the cytotoxicity of Aβ42, as well as providing a system that enables the rapid screening of potential therapeutic interventions. The protection provided by neurotransmitter analogs, especially that for acetylcholine, points to unsuspected advantages of existing therapies that enhance cholinergic function, as well as indicating novel approaches that may prove protective in Alzheimer''s disease. [Copyright &y& Elsevier]