Current Alzheimer Research - Volume 9, Issue 2, 2012

During the past 12 years, generation of intracellular signals by regulated cleavage of membrane proteins has made its way from a newcomer on the scientific stage (introduced by a battery of key papers appearing between 1998 and 2000, as reviewed in [1]), to a central signaling mechanism, termed Regulated Intramembrane Proteolysis (RIP), now presented in all standard textbooks of cell biology. In retrospect, it is intriguing to see how during the roughly 10 years preceding this breakthrough quite diverse lines of research have laid down the foundation for both the concept itself as well as for the identification of the three APP secretases, which are now accepted as an important machinery for the RIPping of a growing list of functionally important proteins. This story is the more fascinating, as en route with the new signaling paradigm two likewise novel enzyme architectures, disintegrin proteases and i-CliPs, had to enter the scientific stage as well. To start from the perspective of Alzheimers Disease, isolation of Aβ [2] and later APP provided the first example for the constitutive release of protein fragments from the membrane including the then almost unbelievable observation of proteolysis within the membrane plane [3, 4, reviewed in 5]. Further conceptual advances were made possible by the understanding of the SREBP signaling pathway, not only because it was the first one for which the two - step proteolysis scheme was elucidated as a signaling event, but also because it opened the road for the identification of enzymes similar to S2P and thus to RIP in eubacteria, documenting the phylogenetic age and conservation of such signaling techniques (reviewed in [1]). Then - almost ironically - two of the severe familial forms of AD offered a crucial genetic approach to identify and analyze PSEN1 and later PSEN2 and thus the active core of γ-secretase. Specifically, knockout data demonstrated the essential role of PSENs in APP (and later Notch) γ-secretase cleavage, while inhibitor data pointed towards their intramembrane aspartyl protease activity. In parallel, mutagenesis approaches targeting two conserved aspartates at the same transmembrane level in neighbouring TMDs of PSENs confirmed that this unusual proteolytic center is indeed crucial for γ secretase activity. Together, these approaches opened the way to our current understanding of this entirely novel type of an intramembrane - acting enzyme [6, 7], subsequently augmented by the demonstration that PSENs can only act as members of multiprotein complexes, whereby the composition of which would allow for the formation of different isocomplexes with - to some degree - different functional properties [8]....

Proteases regulate numerous physiological functions in all living organisms. Because of their contribution to βAPP processing, α-, β- and γ-secretases have focused particular attention of researchers in the field of Alzheimer's disease (AD) during the past 20 years. Whereas the β-secretase BACE1 and the heterotetrameric presenilin-dependent γ- secretase complex were identified between 1995 and 2002, α-secretase activity was attributed to previously described ADAM10 and ADAM17, two members of the type I integral membrane protein family called ADAMs (A Disintegrin And Metalloprotease). ADAM10 and/or ADAM17 target numerous substrates through various modes of action. This review focuses on the complex physiology of these α-secretases and will document their contribution to cancers, diabetes, rheumatoid arthritis, and prion diseases besides their well characterized role in Alzheimer's disease.

ADAM10 (A disintegrin and metalloproteinase 10) has been demonstrated as an enzyme with protective properties in Alzheimer's disease: in mouse models it not only lowered generation of toxic A-beta peptides and formation of senile plaques but also alleviated learning deficits and enhanced synaptic density. This is due to cleavage of the amyloid precursor protein (APP) within its A-beta stretch and to the release of the extracellular domain of APP with neuroprotective function. Aside from cleaving APP, ADAM10 has been linked to over 40 putative substrates at least in cell culture. These substrates are connected with important cellular functions such as cell migration, stress response and transport. For this contribution we focussed on ADAM10 acting on the APP-like proteins since for some of their representatives - in particular APLP2 and APP-L - a shedding by ADAM10 has been demonstrated in vivo. In addition, the importance of these proteins, especially of APLP2, has been repeatedly shown by intense analysis of double and triple transgenic mice which lack APP in combination with one or both APLPs: several phenotypes such as defects in migration of neuroblasts, in formation of synapses and synaptic transmission have been reported. However, the specific contribution of either the uncleaved full-length proteins or their extracellular domains secreted upon ADAM10 activity has not been elucidated. In this review, we report on recent findings concerning shedding of APP-like proteins and resulting functional consequences.

α-secretase is the name for a metalloprotease activity, which is assumed to play a key role in the prevention of the molecular mechanisms underlying Alzheimer's disease (AD). Proteases similar to α-secretase are essential for a wide range of biological processes, such as cell adhesion and embryonic development. The molecular culprit in AD is the amyloid β peptide (Aβ), which derives from the amyloid precursor protein (APP) through sequential cleavage by the two proteases β- and γ-secretase. In contrast, α-secretase, which is the metalloprotease ADAM10, cleaves APP within the Aβ domain, thus preventing Aβ generation. Additionally, it produces a secreted APP ectodomain with neurotrophic and neuroprotective properties. An increase in α-secretase cleavage is considered a therapeutic approach for AD, but the molecular mechanisms regulating α-secretase cleavage are only partly known. Protein kinase C and mitogen-activated protein kinase constitute central signaling hubs for the regulation of α-secretase cleavage. Additionally, recent studies increasingly demonstrate that the correct spatial and temporal localization of the two membrane proteins APP and α-secretase is essential for efficient α-secretase cleavage of APP. This review highlights the role of signaling pathways and protein trafficking in the control of APP α-secretase cleavage.

Neuregulin-1 (NRG1), known also as heregulin, acetylcholine receptor inducing activity (ARIA), glial growth factor (GGF), or sensory and motor neuron derived factor (SMDF), plays essential roles in several developmental processes, and is required also later in life. Many variants of NRG1 are produced via alternative splicing and usage of distinct promoters. All contain an epidermal growth factor (EGF)-like domain, which alone is sufficient to bind and activate the cognate receptors, members of the ErbB family. NRG1 mediated signaling is crucial for cardiogenesis and the development of the mammary gland and ErbB2 (HER2), an orphan co-receptor for NRG1 is the target of the drug Herceptin® (trastuzumab) used for treatment of metastatic breast cancer. In the nervous system, NRG1 controls the early development of subpopulations of neural crest cells. In particular, NRG1 acts as an essential paracrine signaling molecule expressed on the axonal surface, where it signals to Schwann cells throughout development and regulates the thickness of the myelin sheath. NRG1 is required also by other cell types in the nervous system, for instance as an axonal signal released by proprioceptive afferents to induce development of the muscle spindle, and it controls aspects of cortical interneuron development as well as the formation of thalamocortical projections. Work from several laboratories implicates dysregulation of NRG1/ErbB4 signaling in the etiology of schizophrenia. Biochemical studies have shown that the precursor proteins of NRG1 can be released from the membrane through limited proteolysis. In addition, most NRG1 isoforms contain a transmembrane domain, which is processed by γ-secretase after shedding. Thereby the intracellular domain is released into the cytoplasm. Despite this, the importance of NRG1 cleavage for its functions in vivo remained unclear until recently. β- Secretase (β-site amyloid precursor protein cleaving enzyme 1, BACE1) was first identified through its function as the rate limiting enzyme of amyloid-β-peptide (Aβ) production. Aβ is the major component of amyloid plaques in Alzheimer's disease (AD). More recently it was shown that Neuregulin-1 is a major physiological substrate of BACE1 during early postnatal development. Mutant mice lacking BACE1 display severe hypomyelination of peripheral nerves similar to that seen in mice lacking NRG1/ErbB signaling in Schwann cells, and a BACE1-dependent activation of NRG1 in the process of peripheral myelination was proposed. Here we summarize the current knowledge about the role of NRG1 proteolysis for ErbB receptor mediated signaling during development and in Alzheimer's disease.

Voltage-dependent sodium channel complexes consist of a pore-forming and voltage-sensing α-subunit and one or two β-subunits. The latter are type I transmembrane proteins with a broad spectrum of functions in channel expression and surface targeting, in channel electrophysiology and, notably, in cell-adhesion of excitable and non-excitable cells. Like the amyloid-precursor protein (APP), β-subunits are substrates for sequential cleavage either by α- and γ-secretase, or by β- and γ-secretase. Here, we focus on the processing of β-subunits by the amyloidogenic β-secretase, BACE1, which is up-regulated in Alzheimer's disease and is considered a highly promising pharmacologic target. Based on data from BACE1-deficient or over-expressing mice and from heterologous expression systems, this review summarizes our growing understanding of how BACE1-mediated cleavage of β-subunits interferes with their multiple physiological functions.

Genetic studies demonstrate that the ε4 allele of the apolipoprotein (apo) E is a risk factor for late onset Alzheimer's disease (AD). Apo E is the major component of lipoprotein particles in the brain that mediate transport of cholesterol and other lipids between neurons and glial cells, indicating an implication of cerebral lipid metabolism in the pathogenesis of AD. In addition, apo E is also involved in the metabolism and aggregation of the amyloid β-peptide (Aβ) that derives from proteolytic processing of the amyloid precursor protein (APP) and is found in plaques of AD brains. The generation of Aβ involves sequential cleavages of APP by proteases called β- and γ-secretase. γ-Secretase is a high molecular weight protein complex containing presenilins as catalytically active subunits. Importantly, mutations in the genes of APP and the two homologous PS proteins are a major cause of familial early onset AD, indicating that the metabolism of APP and generation of Aβ play critical roles in the initiation of the disease. This review focuses on the functional relation of γ-secretase complexes and the metabolism of lipoproteins in the brain. It is hypothesized that γ-secretase activity is critically involved in cellular lipid homeostasis and that impaired lipid metabolism contributes to the pathogenesis of AD.

Altered proteolytic processing of the β-amyloid precursor protein (APP) is a central event in familial and sporadic Alzheimer's disease (AD). In a process termed regulated intramembrane proteolysis (RIP), APP first undergoes ectodomain shedding executed either by α- secretases at the plasma membrane or by β-secretase in the endosomal compartment. The remaining membrane-anchored stubs are cleaved within the membrane plane by the γ-secretase complex, releasing the APP intracellular domain (AICD) into the cytosol and leading to the generation of the Aβ peptide in the amyloidogenic pathway that is initiated by β-secretase. The Aβ peptides aggregate to form soluble oligomers and finally deposit into amyloid plaques that are a hallmark of AD. Recent evidence indicates a role for Aβ oligomers in regulating synaptic plasticity with excess amounts of oligomers disrupting synaptic function. The amyloid cascade hypothesis of AD is centered on the Aβ peptide, the APP fragment that has been most intensely studied, while other cleavage products have been largely neglected. The secreted ectodomain generated after α-cleavage in the non-amyloidogenic pathway has neurotrophic and neuroproliferative activities, thus opposing the neurotoxicity observed with high concentrations of Aβ. Further, in analogy to many other membrane proteins that are subject to RIP, AICD can translocate to the nucleus to regulate transcription. Many RIP substrates are localized to the synapse and thus could convey a direct signal from the synapse to the nucleus upon cleavage. Evidence indicates that only the amyloidogenic pathway generates AICD capable of nuclear signaling, due to the subcellular compartmentalization of APP processing. In aging and sporadic AD there is an increase in β-secretase levels and activity generating more Aβ peptides and concomitantly leading to an increase in AICD nuclear signaling. In this review, I summarize the current knowledge on AICD nuclear signaling and propose mechanisms to explain how this physiological function of APP might impact the pathology seen in AD.

Alzheimer's disease (AD) is the most common cause of dementia in aging populations. Although amyloid plaques are the hallmark of AD, loss of synapses and synaptic dysfunction are closely associated with the duration and severity of cognitive impairment in AD patients. Amyloid precursor protein (APP) and its cleavage products including Aβ have been suggested as homeostatic regulators of synaptic activity. APP manipulation and Aβ application, in vitro and in vivo, affect synapse formation and synaptic transmission. Moreover, synaptic dysfunction and learning deficits precede Aβ plaque deposition, suggesting that synaptic alterations may underlie the initial development of the disease. Because of the pivotal role of APP and Aβ in AD pathogenesis, it is essential to understand how APP and Aβ modulate synaptic function. Here, we review the roles that APP and Aβ play at the synapses, with particular focus on recent findings for the importance of APP in synaptogenesis and synaptic function.

The Notch pathway is a critical mediator of short-range cell-cell communication that is reiteratively used to regulate a diverse array of cellular processes during embryonic development and the renewal and maintenance of adult tissues. Most Notch-dependent processes utilize a core signaling mechanism that is dependent on regulated intramembrane proteolysis: Upon ligand binding, Notch receptors undergo ectodomain shedding by ADAM metalloproteases, followed by γ-secretase-mediated intramembrane proteolysis. This releases the Notch intracellular domain, which translocates to the nucleus to activate transcription. In this review, we highlight the roles of Notch signaling particularly in self-renewing tissues in adults and several human diseases and raise some key considerations when targeting ADAMs and γ-secretase as disease-modifying strategies for Alzheimer's Disease.

Cerebral hypometabolism and abnormal levels of amyloid beta (Aβ), total (t-tau) and phosphorylated tau (ptau) proteins in cerebrospinal fluid (CSF) are established biomarkers of Alzheimer's disease (AD). We examined the agreement between these biomarkers in a single center study of patients with AD of severity extending over a wide range. Forty seven patients (MMSE 21.4±3.6, range 13-28 points) with incipient and probable AD underwent positron emission tomography with [18F]-fluorodeoxyglucose (FDG-PET) and lumbar puncture for CSF assays of Aβ1-42, p-tau181, and t-tau. All findings were classified as either positive or negative for AD. Statistical analyses were performed for the whole sample (n=47) and for the subgroups stratified as mild (MMSE >20 points, n=30) and moderate (MMSE <21 points, n=17) AD. In the whole patient sample, the agreement with the FDG-PET finding was 77% (chance-corrected kappa [κ]=0.34, p=0.016) for t-tau, 68% (κ=0.10, n.s.) for p-tau181, and 68% (κ=0.04, n.s.) for Aβ1-42. No significant agreement was found in the mild AD subgroup, while there was a strong agreement for t-tau (94%, κ=0.77, p=0.001) and p-tau181 (88%, κ=0.60, p=0.014) in the moderate AD group. A significant agreement between the FDG-PET and CSF tau findings in patients with AD supports the view that both are markers of neurodegeneration. CSF tau proteins and FDG-PET might substitute each other as supportive diagnostic tools in patients with suspected moderate-to-severe Alzheimer's dementia, while this is not the case in subjects at an earlier disease stage.

Phosphorylation and, therefore, binding capacity of microtubule-associated protein tau is regulated by specific kinases and phosphatases. Activation of tau kinases plays a crucial role in tau- hyper-phosphorylation in Alzheimer disease (AD) and related tauopathies. Among phosphatases, protein phosphatase 2A, PP2A, is a principal tau dephosphorylating enzyme in the brain. PP2A acts as trimer composed of a catalytic (PP2A C), a scaffolding (PP2A A) and a regulatory (PP2 AB; B55α) subunit. Several abnormalities of PP2A have been reported in AD, including decreased mRNA and protein levels of the PP2A C (not replicated by other studies); decreased protein levels of the PP2A A and B55α; reduced PP2A C methylation at Leu309 due to impaired function methyltransferase type IV; increased PP2A C phosphorylation at Tyr307; up-regulation of the PP2A inhibitors I1 and I2; and loss of enzymatic activity. These observations indicate that PP2A is a putative target of therapeutic intervention considering that enhancing PP2A activity would decrease tau hyper-phosphorylation in AD. In spite of these achievements further studies are needed to replicate the reported individual different alterations converging in PP2A in AD.