BackgroundAlzheimer's disease (AD) is a neurodegenerative disorder primarily characterized by the deposition of β-amyloid plaques in the brain. Plaques are composed of the amyloid-β peptide derived from cleavage of the amyloid precursor protein (APP). Mutations in APP lead to the development of Familial Alzheimer's Disease (FAD), however, the normal function of this protein has proven elusive. The organism Caenorhabditis elegans is an attractive model as the amyloid precursor-like protein (APL-1) is the single ortholog of APP, and loss of apl-1 leads to a severe molting defect and early larval lethality.Methodology/Principal FindingsWe report here that lethality and molting can be rescued by full length APL-1, C-terminal mutations as well as a C-terminal truncation, suggesting that the extracellular region of the protein is essential for viability. RNAi knock-down of apl-1 followed by drug testing on the acetylcholinesterase inhibitor aldicarb showed that loss of apl-1 leads to aldicarb hypersensitivity, indicating a defect in synaptic function. The aldicarb hypersensitivity can be rescued by full length APL-1 in a dose dependent fashion. At the cellular level, kinesins UNC-104/KIF-1A and UNC-116/kinesin-1 are positive regulators of APL-1 expression in the neurons. Knock-down of the small GTPase rab-5 also leads to a dramatic decrease in the amount of apl-1 expression in neurons, suggesting that trafficking from the plasma membrane to the early endosome is important for apl-1 function. Loss of function of a different small GTPase, UNC-108, on the contrary, leads to the retention of APL-1 in the cell body.Conclusions/SignificanceOur results reveal novel insights into the intracellular trafficking of APL-1 and we report a functional role for APL-1 in synaptic transmission.

The amyloid precursor protein (APP) has been under intensive study in recent years, mainly due to its critical role in the pathogenesis of Alzheimer's disease (AD). β-Amyloid (Aβ) peptides generated from APP proteolytic cleavage can aggregate, leading to plaque formation in human AD brains. Point mutations of APP affecting Aβ production are found to be causal for hereditary early onset familial AD. It is very likely that elucidating the physiological properties of APP will greatly facilitate the understanding of its role in AD pathogenesis. A number of APP loss-and gainof-function models have been established in model organisms including Caenorhabditis elegans, Drosophila, zebrafish and mouse. These in vivo models provide us valuable insights into APP physiological functions. In addition, several knock-in mouse models expressing mutant APP at a physiological level are available to allow us to study AD pathogenesis without APP overexpression. This article will review the current physiological and pathophysiological animal models of APP.

Type I procollagen is a heterotrimer composed of two pro␣1(I) chains and one pro␣2(I) chain, encoded by the COL1A1 and COL1A2 genes, respectively. Mutations in these genes usually lead to dominantly inherited forms of osteogenesis imperfecta (OI) by altering the triple helical domains, but a few affect sequences in the pro␣1(I) C-terminal propeptide (C-propeptide), and one, which has a phenotype only in homozygotes, alters the pro␣2(I) C-propeptide. Here we describe four dominant mutations in the COL1A2 gene that alter sequences of the pro␣2(I) C-propeptide in individuals with clinical features of a milder form of the disease, OI type IV. Three of the four appear to interfere with disulfide bonds that stabilize the C-propeptide conformation and its interaction with other chains in the trimer. Cultured cells synthesized pro␣2(I) chains that were slow to assemble with pro␣1(I) chains to form heterotrimers and that were retained intracellularly. Some alterations led to the uncharacteristic formation of pro␣1(I) homotrimers. These findings show that the C-propeptide of pro␣2(I), like that of the pro␣1(I) C-propeptide, is essential for efficient assembly of type I procollagen heterotrimers. The milder OI phenotypes likely reflect a diminished amount of normal type I procollagen, small populations of overmodified heterotrimers, and pro␣1(I) homotrimers that are compatible with normal skeletal growth. Osteogenesis imperfecta (OI)4 (1, 2), commonly known as brittle bone disease, is usually caused by mutations in the COL1A1 and COL1A2 genes that encode the pro␣1(I) and pro␣2(I) chains, respectively, of type I procollagen. This heterotrimeric collagen is composed of two pro␣1(I) chains and one pro␣2(I) chain. Molecular assembly of the trimer is a multistep process that occurs following synthesis of the full-length chains and release from the ribosome. The C-terminal propeptide (C-propeptide) of each chain folds into a structure that is stabilized by intra-chain disulfide bonds and exposes a chain selectivity domain (3) that directs the interaction of the correct three chains into trimers. Following stabilization of the trimer by inter-chain disulfide bonds, the collagen triple helical domains are nucleated by sequences at their C-terminal end and the helix then propagates in an N-terminal direction.The vast majority of OI-causing mutations (4, 5) 5 affect the triple helical domains and perturb either the nucleation or the propagation of the triple helix N-terminal to the altered sequence but do not affect initial assembly of the pro␣ chains (6). The delay in helix propagation permits prolonged access of modifying enzymes to the chains N-terminal of the alteration (7), whereas the sequence alterations themselves alter the helical structure (8, 9) and reduce molecular thermal stability (10, 11). The phenotypic outcome of these mutations is, in part, due to the synthesis of structurally altered triple helices, and reflects both the location and nature of the change. These mutations result in the full spectrum of OI severity,...

Dickkopf-like1 (Dkkl1) encodes a glycoprotein secreted by postmeiotic male germ cells. We report here that adult Dkkl1-deficient males have elevated sperm counts caused by a decrease in postpubertal spermatocyte apoptosis and display, upon aging, increased local production of testosterone. Molecular analyses identified the Fas death ligand (FasL) as a target for Dkkl1 pro-apoptotic activity in adult mice. Accordingly, adult FasL-deficient gld mice display an increased sperm count and decreased spermatocyte apoptosis phenotype similar to the one observed in Dkkl1-deficient mice. We also show that the elevated testosterone level observed in aging Dkkl1-deficient males is secondary to increased expression in Leydig cells of CYP11A and CYP17, two genes implicated in steroidogenesis. Furthermore, treatment of Leydig cells with Dkkl1 decreases DNA binding and transcriptional activity of steroidogenic factor 1, a pivotal regulator of gene expression in testis. Thus, this study establishes Dkkl1 as a negative regulator of adult testis homeostasis and identifies a novel, Dkkl1/FasL-dependent, regulation that specifically controls the number of postpubertal spermatocytes.