dAmyloid ␤ (A␤) peptides originating from amyloid precursor protein (APP) in the endosomal-lysosomal compartments play a critical role in the development of Alzheimer's disease (AD), the most common type of senile dementia affecting the elderly. Since insulin-like growth factor II (IGF-II) receptors facilitate the delivery of nascent lysosomal enzymes from the trans-Golgi network to endosomes, we evaluated their role in APP metabolism and cell viability using mouse fibroblast MS cells deficient in the murine IGF-II receptor and corresponding MS9II cells overexpressing the human IGF-II receptors. Our results show that IGF-II receptor overexpression increases the protein levels of APP. This is accompanied by an increase of ␤-site APP-cleaving enzyme 1 levels and an increase of ␤-and ␥-secretase enzyme activities, leading to enhanced A␤ production. At the cellular level, IGF-II receptor overexpression causes localization of APP in perinuclear tubular structures, an increase of lipid raft components, and increased lipid raft partitioning of APP. Finally, MS9II cells are more susceptible to staurosporine-induced cytotoxicity, which can be attenuated by ␤-secretase inhibitor. Together, these results highlight the potential contribution of IGF-II receptor to AD pathology not only by regulating expression/processing of APP but also by its role in cellular vulnerability. Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by severe memory loss followed by the deterioration of higher cognitive functions. Although most cases of AD occur sporadically after the age of 60 years, a small proportion of cases correspond to the early-onset (Ͻ60 years) autosomal dominant form of the disease. To date, mutations in three genes, the amyloid precursor protein (APP) gene on chromosome 21, the presenilin 1 (PSEN1) gene on chromosome 14, and the presenilin 2 (PSEN2) gene on chromosome 1, have been identified as the cause of a large proportion of early-onset familial AD (1-3). Additionally, inheritance of the ε4 allele of the apolipoprotein E (APOE) gene on chromosome 19 increases the risk of late-onset and sporadic AD. The neuropathological features associated with AD include the presence of extracellular ␤-amyloid (A␤) peptidecontaining neuritic plaques, intracellular tau-positive neurofibrillary tangles, and the loss of synapses and neurons in defined brain regions. Several lines of in vivo evidence suggest that A␤ peptides initiate or contribute to the neuronal loss and development of AD pathology (2, 4). A␤ peptides are generated from APP, a type I transmembrane protein, which can be processed either by nonamyloidogenic ␣-secretase or amyloidogenic ␤-secretase pathways (5, 6). The ␣-secretase cleaves APP within the A␤ domain, yielding soluble APP␣ and a 10-kDa C-terminal fragment (CTF-␣), which then can be processed by ␥-secretase to generate A␤ 17-40 /A␤ 17-42 fragments. The ␤-secretase, on the other hand, cleaves APP to generate soluble APP␤ and an A␤-containing Cterminal fragment (CTF-␤), which is fu...

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) binds M6P-capped ligands and IGF-II at different binding sites within the ectodomain and mediates ligand internalization and trafficking to the lysosome. Multivalent M6P-based ligands can cross-bridge the M6P/IGF2R, which increases the rate of receptor internalization, permitting IGF-II binding as a passenger ligand and subsequent trafficking to the lysosome, where the IGF-II is degraded. This unique feature of the receptor may be exploited to design novel therapeutic agents against IGF-II-dependent cancers that will lead to decreased bioavailable IGF-II within the tumor microenvironment. We have designed a panel of M6P-based ligands that bind to the M6P/IGF2R with high affinity in a bivalent manner and cause decreased cell viability. We present evidence that our ligands bind through the M6P-binding sites of the receptor and facilitate internalization and degradation of IGF-II from conditioned medium to mediate this cellular response. To our knowledge, this is the first panel of synthetic bivalent ligands for the M6P/IGF2R that can take advantage of the ligand-receptor interactions of the M6P/IGF2R to provide proof-of-principle evidence for the feasibility of novel chemotherapeutic agents that decrease IGF-II-dependent growth of cancer cells.

Individuals with poor postnatal growth are at risk for cardiovascular and metabolic problems as adults. Here we show that disruption of the molecular scaffold Kinase Suppressor of Ras 2 (KSR2) causes selective inhibition of hepatic GH signaling in neonatal mice with impaired expression of IGF-1 and IGFBP3. ksr2−/− mice are normal size at birth but show a marked increase in FGF21 accompanied by reduced body mass, shortened body length, and reduced bone mineral density (BMD) and content (BMC) first evident during postnatal development. However, disrupting FGF21 in ksr2−/− mice does not normalize mass, length, or bone density and content in fgf21−/−ksr2−/− mice. Body length, BMC and BMD, but not body mass, are rescued by infection of two-day-old ksr2−/− mice with a recombinant adenovirus encoding human IGF-1. Relative to wild-type mice, GH injections reveal a significant reduction in JAK2 and STAT5 phosphorylation in liver, but not in skeletal muscle, of ksr2−/− mice. However, primary hepatocytes isolated from ksr2−/− mice show no reduction in GH-stimulated STAT5 phosphorylation. These data indicate that KSR2 functions in a cell non-autonomous fashion to regulate GH-stimulated IGF-1 expression in the liver of neonatal mice, which plays a key role in the development of body length.

SUMMARY Oligomerization of the mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) is important for optimal ligand binding and internalization. M6P/IGF2R is a tumor suppressor gene that exhibits loss of heterozygosity and is mutated in several cancers. We tested the potential dominant-negative effects of two cancer-associated mutations that truncate the M6P/IGF2R in ectodomain repeats 9 and 14. Our hypothesis was that co-expression of the truncated receptors with wild-type/endogenous, full-length M6P/IGF2R would interfere with M6P/IGF2R function by heterodimer interference. Immunoprecipitation confirmed formation of heterodimeric complexes between full-length M6P/IGF2Rs and the truncated receptors, termed Rep9F and Rep14F. Remarkably, increasing expression of either Rep9F or Rep14F provoked decreased levels of full-length M6P/IGF2Rs in both cell lysates and plasma membranes, indicating a dominant-negative effect on receptor availability. Loss of full-length M6P/IGF2R was not due to increased proteasomal or lysosomal degradation, but instead arose from increased proteolytic cleavage of cell-surface M6P/IGF2Rs resulting in ectodomain release, by a mechanism that was inhibited by metal ion chelators. These data suggest that M6P/IGF2R truncation mutants may contribute to the cancer phenotype by decreasing availability of full-length M6P/IG2Rs to perform tumor-suppressive functions such as binding/internalization of receptor ligands like IGF-II.

A generalized strategy is presented for the rapid assembly of a set of bivalent ligands with a variety of linking functionalities from a common monomer. Herein, an array of phosphatase-inert mannose-6-phosphonate-presenting ligands for the cation-independent-mannose 6-phosphate receptor (CI-MPR) is constructed. Receptor binding affinity varies with linking functionality—the simple amide and 1,5-triazole(tetrazole) being preferred over the 1,4-triazole. This approach is expected to find application across chemical biology, particularly in glycoscience, wherein multivalency often governs molecular recognition.