The amyloid-␤ precursor protein (A␤PP) is a ubiquitously expressed transmembrane protein whose cleavage product, the amyloid-␤ (A␤) protein, is deposited in amyloid plaques in neurodegenerative conditions such as Alzheimer disease, Down syndrome, and head injury. We recently reported that this protein, normally associated with neurodegenerative conditions, is expressed by human embryonic stem cells (hESCs). We now report that the differential processing of A␤PP via secretase enzymes regulates the proliferation and differentiation of hESCs. hESCs endogenously produce amyloid-␤, which when added exogenously in soluble and fibrillar forms but not oligomeric forms markedly increased hESC proliferation. The inhibition of A␤PP cleavage by ␤-secretase inhibitors significantly suppressed hESC proliferation and promoted nestin expression, an early marker of neural precursor cell (NPC) formation. The induction of NPC differentiation via the non-amyloidogenic pathway was confirmed by the addition of secreted A␤PP␣, which suppressed hESC proliferation and promoted the formation of NPCs. Together these data suggest that differential processing of A␤PP is normally required for embryonic neurogenesis.The amyloid-␤ precursor protein (A␤PP) 5 is a ubiquitously expressed transmembrane protein whose cleavage product, the amyloid-␤ (A␤) protein, is deposited in amyloid plaques in the aged brain, following head injury, and in the neurodegenerative conditions of Alzheimer disease (AD) and Down syndrome (DS). A␤PP has structural similarity to growth factors (1) and modulates several important neurotrophic functions, including neuritogenesis, synaptogenesis, and synaptic plasticity (2). The function of A␤PP during early embryogenesis and neurogenesis has not been well described.A␤PP is processed by at least two pathways, the non-amyloidogenic and amyloidogenic pathways. Non-amyloidogenic processing of A␤PP yields secreted A␤PP␣ (sA␤PP␣), the secreted extracellular domain of A␤PP that acts as a growth factor for many cell types and promotes neuritogenesis (3). Amyloidogenic processing of A␤PP releases sA␤PP␤, the A␤PP intracellular domain, and A␤ proteins. The A␤ protein has both neurotoxic and neurotrophic properties (4) dependent on the differentiation state of the neuron; A␤ is neurotoxic to differentiating neurons via a mechanism involving differentiation-associated increases in the phosphorylation of the microtubule-associated protein tau (5) but neurotrophic to undifferentiated embryonic neurons. Evidence supporting a neurotrophic function for A␤ during development include its neurogenic activity toward rat neural stem cells (4 -6). Consistent with these data, two studies have demonstrated increased hippocampal neurogenesis in young transgenic mice overexpressing human APP Sw,Ind (7,8).Recently we reported that human embryonic stem cells (hESCs) express A␤PP and that both the stemness of the cells and the pregnancy-associated hormone human chorionic gonadotropin alter A␤PP expression (9). These results suggest a functional role fo...

Senescence is characterized neurologically by a decline in cognitive function, which we propose is the result of degenerative processes initiated by the dysregulation of the hypothalamic-pituitary-gonadal (HPG) axis with menopause and andropause. Compelling epidemiologic evidence to support this assertion includes the increased prevalence of Alzheimer disease (AD) in women, the correlation of serum HPG hormones with disease and the decreased incidence, and delay in the onset of AD following hormone replacement therapy. Dysregulation of the axis at this time leads to alterations in the concentrations of all serum HPG hormones (decreased neuronal sex steroid signaling, but increased neuronal gonadotropin releasing hormone, luteinizing hormone, and activin signaling). Hormones of the HPG axis, receptors for which are present in the adult brain, are important regulators of cell proliferation and differentiation during growth and development. Based on this, we propose that dysregulated HPG hormone signaling with menopause/andropause leads to the abortive reentry of differentiated neurons into the cell cycle via a process we term "dyosis." Interestingly, the major biochemical and neuropathologic changes reported for the AD brain also are intimately associated with neuron division: altered AbetaPP metabolism, Abeta deposition, tau phosphorylation, mitochondrial alterations, chromosomal replication, synapse loss, and death of differentiated neurons. Recent evidence supports the premise that AD-related biochemical changes are likely the combined result of increased mitotic signaling by gonadotropins and GnRH, decreased differentiative and neuroprotective signaling via sex steroids, and increased differentiative signaling via activins. This results in a hormonal milieu that is permissive of cell cycle reentry but does not allow completion of metaphase. Partial resetting of the axis following administration of normal endogenous sex steroids delays the onset and decreases the incidence of AD. Ideally, supplementation with HPG hormones should mimic closely the serum concentrations of all HPG hormones in reproductive men and cycling women to prevent dyotic signaling and attempted neuron division.

IntroductionThe physiological signals that direct the division and differentiation of the zygote to form a blastocyst, and subsequent embryonic stem cell division and differentiation during early embryogenesis, are unknown. Although a number of growth factors, including the pregnancy-associated hormone human chorionic gonadotropin (hCG) are secreted by trophoblasts that lie adjacent to the embryoblast in the blastocyst, it is not known whether these growth factors directly signal human embryonic stem cells (hESCs).MethodsHere we used hESCs as a model of inner cell mass differentiation to examine the hormonal requirements for the formation of embryoid bodies (EB's; akin to blastulation) and neuroectodermal rosettes (akin to neurulation).ResultsWe found that hCG promotes the division of hESCs and their differentiation into EB's and neuroectodermal rosettes. Inhibition of luteinizing hormone/chorionic gonadotropin receptor (LHCGR) signaling suppresses hESC proliferation, an effect that is reversed by treatment with hCG. hCG treatment rapidly upregulates steroidogenic acute regulatory protein (StAR)-mediated cholesterol transport and the synthesis of progesterone (P4). hESCs express P4 receptor A, and treatment of hESC colonies with P4 induces neurulation, as demonstrated by the expression of nestin and the formation of columnar neuroectodermal cells that organize into neural tubelike rosettes. Suppression of P4 signaling by withdrawing P4 or treating with the P4-receptor antagonist RU-486 inhibits the differentiation of hESC colonies into EB's and rosettes.ConclusionsOur findings indicate that hCG signaling via LHCGR on hESC promotes proliferation and differentiation during blastulation and neurulation. These findings suggest that trophoblastic hCG secretion and signaling to the adjacent embryoblast could be the commencement of trophic support by placental tissues in the growth and development of the human embryo.

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