Paul Wen

A presenilin-1/γ-secretase cleavage releases the E-cadherin intracellular domain and regulates disassembly of adherens junctions

E‐cadherin controls a wide array of cellular behaviors including cell–cell adhesion, differentiation and tissue development. Here we show that presenilin‐1 (PS1), a protein involved in Alzheimers disease, controls a γ‐secretase‐like cleavage of E‐cadherin. This cleavage is stimulated by apoptosis or calcium influx and occurs between human E‐cadherin residues Leu731 and Arg732 at the membrane–cytoplasm interface. The PS1/γ‐secretase system cleaves both the full‐length E‐cadherin and a transmembrane C‐terminal fragment, derived from a metalloproteinase cleavage after the E‐cadherin ectodomain residue Pro700. The PS1/γ‐secretase cleavage dissociates E‐cadherins, β‐catenin and α‐catenin from the cytoskeleton, thus promoting disassembly of the E‐cadherin–catenin adhesion complex. Furthermore, this cleavage releases the cytoplasmic E‐cadherin to the cytosol and increases the levels of soluble β‐ and α‐catenins. Thus, the PS1/γ‐secretase system stimulates disassembly of the E‐cadherin– catenin complex and increases the cytosolic pool of β‐catenin, a key regulator of the Wnt signaling pathway.

A CBP Binding Transcriptional Repressor Produced by the PS1/ϵ-Cleavage of N-Cadherin Is Inhibited by PS1 FAD Mutations

Presenilin1 (PS1), a protein implicated in Alzheimers disease (AD), forms complexes with N-cadherin, a transmembrane protein with important neuronal and synaptic functions. Here, we show that a PS1-dependent gamma-secretase protease activity promotes an epsilon-like cleavage of N-cadherin to produce its intracellular domain peptide, N-Cad/CTF2. NMDA receptor agonists stimulate N-Cad/CTF2 production suggesting that this receptor regulates the epsilon-cleavage of N-cadherin. N-Cad/CTF2 binds the transcription factor CBP and promotes its proteasomal degradation, inhibiting CRE-dependent transactivation. Thus, the PS1-dependent epsilon-cleavage product N-Cad/CTF2 functions as a potent repressor of CBP/CREB-mediated transcription. Importantly, PS1 mutations associated with familial AD (FAD) and a gamma-secretase dominant-negative mutation inhibit N-Cad/CTF2 production and upregulate CREB-mediated transcription indicating that FAD mutations cause a gain of transcriptional function by inhibiting production of transcriptional repressor N-Cad/CTF2. These data raise the possibility that FAD mutation-induced transcriptional abnormalities maybe causally related to the dementia associated with FAD.

PS1 activates PI3K thus inhibiting GSK‐3 activity and tau overphosphorylation: effects of FAD mutations

Phosphatidylinositol 3‐kinase (PI3K) promotes cell survival and communication by activating its downstream effector Akt kinase. Here we show that PS1, a protein involved in familial Alzheimers disease (FAD), promotes cell survival by activating the PI3K/Akt cell survival signaling. This function of PS1 is unaffected by γ‐secretase inhibitors. Pharmacological and genetic evidence indicates that PS1 acts upstream of Akt, at or before PI3K kinase. PS1 forms complexes with the p85 subunit of PI3K and promotes cadherin/PI3K association. Furthermore, conditions that inhibit this association prevent the PS1‐induced PI3K/Akt activation, indicating that PS1 stimulates PI3K/Akt signaling by promoting cadherin/PI3K association. By activating PI3K/Akt signaling, PS1 promotes phosphorylation/inactivation of glycogen synthase kinase‐3 (GSK‐3), suppresses GSK‐3‐dependent phosphorylation of tau at residues overphosphorylated in AD and prevents apoptosis of confluent cells. PS1 FAD mutations inhibit the PS1‐dependent PI3K/Akt activation, thus promoting GSK‐3 activity and tau overphosphorylation at AD‐related residues. Our data raise the possibility that PS1 may prevent development of AD pathology by activating the PI3K/Akt signaling pathway. In contrast, FAD mutations may promote AD pathology by inhibiting this pathway.

The presenilin-1 familial Alzheimer disease mutant P117L impairs neurogenesis in the hippocampus of adult mice

The functions of presenilin 1 (PS1) and how PS1 mutations cause familial Alzheimers disease (FAD) are incompletely understood. PS1 expression is essential for neurogenesis during embryonic development and may also influence neurogenesis in adult brain. We examined how increasing PS1 expression or expressing an FAD mutant would affect neurogenesis in the adult hippocampus. A neuron-specific enolase (NSE) promoter was used to drive neuronal overexpression of either wild-type human PS1 or the FAD mutant P117L in transgenic mice, and the animals were studied under standard-housing conditions or after environmental enrichment. As judged by bromodeoxyuridine (BrdU) labeling, neural progenitor proliferation rate was mostly unaffected by increasing expression of either wild-type or FAD mutant PS1. However, in both housing conditions, the FAD mutant impaired the survival of BrdU-labeled neural progenitor cells leading to fewer new beta-III-tubulin-immunoreactive neurons being generated in FAD mutant animals during the 4-week postlabeling period. The effect was FAD mutant specific in that neural progenitor survival and differentiation in mice overexpressing wild-type human PS1 were similar to nontransgenic controls. Two additional lines of PS1 wild-type and FAD mutant transgenic mice showed similar changes indicating that the effects were not integration site-dependent. These studies demonstrate that a PS1 FAD mutant impairs new neuron production in adult hippocampus by decreasing neural progenitor survival. They also identify a new mechanism whereby PS1 FAD mutants may impair normal neuronal function and may have implications for the physiological functioning of the hippocampus in FAD.

Presenilin-1 binds cytoplasmic epithelial cadherin, inhibits cadherin/p120 association, and regulates stability and function of the cadherin/catenin adhesion complex

Here we show that presenilin-1 (PS1), a protein involved in Alzheimers disease, binds directly to epithelial cadherin (E-cadherin). This binding is mediated by the large cytoplasmic loop of PS1 and requires the membrane-proximal cytoplasmic sequence 604–615 of mature E-cadherin. This sequence is also required for E-cadherin binding of protein p120, a known regulator of cadherin-mediated cell adhesion. Using wild-type and PS1 knockout cells, we found that increasing PS1 levels suppresses p120/E-cadherin binding, and increasing p120 levels suppresses PS1/E-cadherin binding. Thus PS1 and p120 bind to and mutually compete for cellular E-cadherin. Furthermore, PS1 stimulates E-cadherin binding to β- and γ-catenin, promotes cytoskeletal association of the cadherin/catenin complexes, and increases Ca2+-dependent cell–cell aggregation. Remarkably, PS1 familial Alzheimer disease mutant ΔE9 increased neither the levels of cadherin/catenin complexes nor cell aggregation, suggesting that this familial Alzheimer disease mutation interferes with cadherin-based cell–cell adhesion. These data identify PS1 as an E-cadherin-binding protein and a regulator of E-cadherin function in vivo.

Human midsized neurofilament subunit induces motor neuron disease in transgenic mice.

Aberrant accumulation of neurofilaments is a feature of human motor neuron diseases. Experimentally motor neuron disease can be induced in transgenic mice by overexpressing the mouse neurofilament light subunit (NF-L), the human heavy subunit (NF-H), or mouse peripherin. Here we describe that mice harboring a bacterial artificial chromosome (BAC) transgene containing the human midsized neurofilament subunit (NF-M) gene develop a progressive hind limb paralysis associated with neurofilamentous accumulations in ventral horn motor neurons and axonal loss in ventral motor roots. Biochemical studies revealed that all three mouse neurofilament subunits along with the human NF-M contributed to filament formation, although filaments contained less peripherin. In addition the endogenous mouse NF-M became less phosphorylated in the presence of the human protein and accumulated in the cell bodies of affected neurons even though phosphorylated human NF-M did not. Remaining motor axons contained an increased density of neurofilaments and morphometric studies showed that principally small myelinated axons were lost in the transgenic animals. Removing half of the mouse NF-M by breeding the transgene onto the mouse NF-M heterozygous null background offered no protection against the development of disease, arguing that the effect is not simply due to elevation of total NF-M. Collectively these studies argue that the human and mouse NF-M proteins exhibit distinct biochemical properties and within mouse neurons are not interchangeable and that indeed the human protein may be toxic to some mouse neurons. These studies have implications for the use of human neurofilament transgenic mice as models of amyotrophic lateral sclerosis.

Selective expression of presenilin 1 in neural progenitor cells rescues the cerebral hemorrhages and cortical lamination defects in presenilin 1-null mutant mice

Mice with a null mutation of the presenilin 1 gene (Psen1–/–) die during late intrauterine life or shortly after birth and exhibit multiple CNS and non-CNS abnormalities, including cerebral hemorrhages and altered cortical development. The cellular and molecular basis for the developmental effects of Psen1 remain incompletely understood. Psen1 is expressed in neural progenitors in developing brain, as well as in postmitotic neurons. We crossed transgenic mice with either neuron-specific or neural progenitor-specific expression of Psen1 onto the Psen1–/– background. We show that neither neuron-specific nor neural progenitor-specific expression of Psen1 can rescue the embryonic lethality of the Psen1–/– embryo. Indeed neuron-specific expression rescued none of the abnormalities in Psen1–/– mice. However, Psen1 expression in neural progenitors rescued the cortical lamination defects, as well as the cerebral hemorrhages, and restored a normal vascular pattern in Psen1–/– embryos. Collectively, these studies demonstrate that Psen1 expression in neural progenitor cells is crucial for cortical development and reveal a novel role for neuroectodermal expression of Psen1 in development of the brain vasculature.

Entorhinal cortex lesioning promotes neurogenesis in the hippocampus of adult mice

Hippocampal neurogenesis in adult mammals is influenced by many factors. Lesioning of the entorhinal cortex is a standard model used to study injury and repair in the hippocampus. Here we use bromodeoxyuridine (BrdU) labeling combined with immunohistochemical identification using cell type specific markers to follow the fate of neural progenitors in the hippocampus following entorhinal cortex lesioning in mice. We show that unilateral entorhinal cortex lesioning does not alter the rate of neural progenitor proliferation in the ipsilateral dentate gyrus during the first 3 days after lesioning. However it enhances cell survival at 42 days post-lesioning leading to an increased number of beta-III tubulin and calbindin-immunoreactive neurons being produced. By contrast, when BrdU was administered 21 days post-lesioning, the number of surviving cells 21 days later was similar on the lesioned and non-lesioned sides. Thus, acutely entorhinal cortex lesioning promotes neurogenesis by enhancing survival of either neural progenitors or their progeny. However, this stimulus to neurogenesis is not sustained into the recovery period.

Transgenic rescue of Krabbe disease in the twitcher mouse

The twitcher mouse is a natural model of Krabbe disease caused by galactocerebrosidase (GALC) deficiency. Previous attempts at rescuing the twitcher mouse by bone marrow transplantion, viral transduction, or transgenesis were only partially successful. Here, we report the transgenic (tg) rescue of the twitcher mouse with a BAC clone harboring the entire GALC. The twi/twi/hGALC tg mice exhibited growth, motor function, and fertility similar to those of nonaffected animals. These animals had normal levels of GALC activity in brain and were free of the typical twitcher demyelinating pathology. Surprisingly, GALC expression in twi/twi hGALC tg kidneys was low and galactocerebroside storage was only partially cleared. Nonetheless, these mice have been maintained for over 1 year without any sign of disease. Since pathological damage associated with GALC deficiency is confined to the nervous system, our work represents the first successful rescue of the twitcher mouse and opens the possibility of developing novel therapeutic approaches.

Cortical development in the presenilin-1 null mutant mouse fails after splitting of the preplate and is not due to a failure of reelin-dependent signaling.

Cortical development is disrupted in presenilin‐1 null mutant (Psen1−/−) mice. Prior studies have commented on similarities between Psen1−/− and reeler mice. Reelin induces phosphorylation of Dab1 and activates the phosphatidylinositol 3‐kinase (PI3K)/Akt pathway. Psen1 is known to modulate PI3K/Akt signaling and both known reelin receptors (apoER2 and VLDLR) are substrates for Psen1 associated γ‐secretase activity. The purpose of this study was to determine whether reelin signaling is disrupted in Psen1−/− mice. We show that, while Dab1 is hypophosphorylated late in cortical development in Psen1−/− mice, it is normally phosphorylated at earlier ages and reelin signaling is intact in Psen1−/− primary neuronal cultures. γ‐secretase activity was also not required for reelin‐induced phosphorylation of Dab1. Unlike reeler mice the preplate splits in Psen1−/− brain. Thus cortical development in Psen1−/− mice fails only after splitting of the preplate and is not due to an intrinsic failure of reelin signaling. Developmental Dynamics 237:2405–2414, 2008.