Current Enzyme Inhibition - Volume 9, Issue 1, 2013

The number of individuals afflicted by Alzheimer's disease (AD) worldwide is rapidly rising and we, as a society, are approaching a critical junction in the development of new therapeutics aimed at slowing or preventing the onset of this devastating neurodegenerative disease. The prevailing theory in the field pinpoints the amyloid β peptide (Aβ) as the causal agent of the disease and thus strategies that target the production, deposition, and clearance of Aβ in the brain continue to be at the forefront of therapeutic interventions. Aβ is produced through a series of proteolytic cleavage events, with β;-secretase (BACE1) being the rate-limiting enzymatic activity needed to begin the liberation of the Aβ peptide from the larger amyloid precursor protein (APP). As such, BACE1 itself has become a prime therapeutic target for AD. Still, over ten years after its discovery, only a handful of compounds targeting BACE1 have reached clinical trial, indicating that complex challenges continue to persist. This short review will focus on the cellular and molecular characteristics of BACE1, its role in APP processing and in AD, and its potential as a therapeutic target for AD.

γ-secretase is a pleiotropic intramembrane-cleaving protease. It is a multiprotein complex that comprises presnilin (PS), nicastrin (NCT), presnilin enhancer-2 (Pen-2), and anterior pharynx-defective-1 (Aph-1)—all of which are essential for its proteolytic activity. Promiscuously diverse substrates are susceptible to cleavage by γ-secretase, of which amyloid precursor protein (APP) and notch are the well-characterized substrates. Specifically, APP processing by γ - secretase has garnered much attention, because the generation of amyloidogenic amyloid β peptide (Aβ) is the hallmark of the pathogenesis of Alzheimer's disease (AD). Thus, the regulatory mechanisms and substrate specificities of γ-secretase proteolytic activity have been studied extensively as therapeutic targets of AD. Further, the processing of other substrates has broad biological implications, releasing an intracellular domain that functions as a signaling molecule. Hence, γ - secretase regulates many physiological functions and pathologies. We summarize the current literatures on γ-secretase, including its assembly and substrates. Its diverse substrates and the downstream events that are initiated by the release of the intracellular domains of substrates after γ-secretase cleavage are discussed, as is their significance under normal and aberrant physiological conditions. Further, the regulation of γ - secretase activity is broached to yield insights into using this promiscuous enzyme to develop therapeutic agents for AD.

Increasing evidence supports the idea that frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) share underlying causes and lie on opposite ends of a disease spectrum that leads to these disorders. The discovery of two different RNA binding proteins, TDP-43 and FUS/TLS, as common neuronal inclusions in FTD and ALS patients has strengthened this connection. Subsequent identification of mutations in TDP-43 and FUS that cause ALS, and FTD in rare cases, provides additional linkage between these diseases. Furthermore, autosomal dominant forms of FTD, ALS, or a combined phenotype can occur in the same family. Genetic studies have linked some of these cases to chromosome 9p21 and are known as c9FTD/ALS. Recently, an expanded GGGGCC hexanucleotide repeat in the C9ORF72 gene has been identified in c9FTD/ALS cases as well as sporadic forms of both diseases. Preliminary data suggest that the increased nucleotide repeats in C9ORF72 lead to RNA inclusions or foci in the nucleus of affected cells. Analogous to other repeat disorders such as myotonic dystrophy, expanded repeats in C9ORF72 may cause disease pathogenesis by sequestering RNA binding proteins and/or perturbing the splicing and regulation of key proteins regulating neuronal health and survival. Intriguingly, TDP-43 and FUS/TLS play fundamental roles in RNA regulation and splicing, and their mutation leads to dysfunctional regulation of their targets in model systems. Finally, defects in RNA splicing have been reported in sporadic ALS and a number of other genes involved in RNA metabolism (ANG, SETX, TAF15, ELP3, ATXN2) are associated with ALS. Based on these recent findings, we propose that defects in RNA metabolism are a common pathway linking FTD and ALS, and are responsible for disease pathogenesis in a significant portion of cases. Further research to test this hypothesis and determine if these proteins function within common biological pathways and share similar pathogenic mechanisms will ultimately open up new routes of therapy for these devastating disorders.

Imbalanced protein load within cells is a critical aspect for most diseases of aging. In particular, the accumulation of proteins into neurotoxic aggregates is a common thread for a host of neurodegenerative diseases. Recent work demonstrates that age-related changes to the cellular chaperone repertoire contribute to abnormal buildup of the microtubule- associated protein tau that accumulates in a group of diseases termed tauopathies, the most common being Alzheimer's disease (AD). The Hsp90 co-chaperone repertoire has diverse effects on tau stability; some co-chaperones stabilize tau while others facilitate its clearance. We propose that each of these proteins may be novel therapeutic targets. While targeting Hsp90 directly may be deleterious at the organismal level, perhaps targeting individual co-chaperone activities will be more tolerable.

The appearance of protein aggregates is a common pathological hallmark that is associated with Alzheimer's disease and other major neurodegenerative disorders. Misfolded and damaged proteins that accumulate in neurodegenerative disease can be cleared through the ubiquitin proteasome system (UPS), a highly regulated pathway whose proper function is of paramount importance in the selective degradation of protein. Studies of the UPS have shown that alterations in the activity of this complex system may be contributors in the etiology of specific neurodegenerative disorders. Studies of familial genetic mutations and through experimental manipulation have suggested that a failure to clear toxic proteins through the UPS may exert important effect on the progression of these disorders; in addition, the accumulation of hallmark proteins themselves may in turn impair clearance by this pathway. This review will discuss recent observations indicating that alterations in the function and efficiency of the UPS are contributors in pathogenesis of Alzheimer's disease, and discuss whether modulation of the UPS may be an appropriate therapeutic target in Alzheimer's disease and similar neurological disorders.

Autophagy is a tightly regulated lysosomal degradation/recycling pathway, critical for cellular homeostasis, such as neuronal survival and death. Impaired autophagic function has been reported in several neurodegenerative diseases, such as Parkinson's, Huntington's and Alzheimer's disease (AD). AD is the most common cause of dementia in the elderly and it is characterized by progressive memory loss and cognitive decline along with synaptic dysfunction. Accumulations of amyloid β (Aβ) and tau proteins are two major neuropathological hallmarks of AD. In addition, accumulation of autophagic vacuoles and other autophagic pathology are evident in dystrophic neurites of AD brains. A series of studies has suggested that autophagy is involved in metabolism of Aβ and tau. Moreover, presenilin (PS), a core subunit of γ- secretase complex, has been demonstrated to play an important role in autophagy at the level of lysosomal proteolysis. In spite of several therapeutic approaches through modulation of autophagic pathway, inconsistent results among studies have made it difficult to determine whether autophagy induction will be beneficial or detrimental for AD pathogenesis. Therefore, roles of autophagy in AD need to be further investigated to develop therapeutic strategies in the future.

Neurophysiological indicators of Alzheimer';s disease include the presence of aggregated amyloid-beta (Aβ) peptide plaques and low levels of the neurotransmitter, acetylcholine, resulting in cognitive impairment. Acetylcholinesterase has been implicated in not only the aggregation of extracellular Aβ peptides through the enzyme's peripheral anionic site, but also the hydrolysis of acetylcholine at its catalytic active site. Two components in a methanolic extract of black walnuts (Juglians nigra), gallic and ellagic acids, were found to prevent aggregation of Abgr; peptides in the presence of acetylcholinesterase and to disaggregate previously formed oligomers. In addition, both compounds inhibited acetylcholinesterase activity, thus retaining higher acetylcholine concentrations. Analysis of phenolic structures related to gallic acid indicated that 1,3,5-trisubstituted phenolic rings conferred these inhibitory effects on acetylcholinesterase, and thus may provide a structural model on which to design dual inhibitors for both acetylcholinesterase catalytic activity and Aβ aggregation and disaggregation.

Lipase inhibitors have generated a great interest because they could help in the prevention or the therapy of lipase-related diseases. Therefore, the aim of this work was to evaluate the inhibitory effect of secondary metabolites extracts such as phenolic compounds and saponins of three Algerian medicinal plants: Achillea santolina, Inonotus hispidus and Zizyphus lotus, indeed their antiradicalaire activity using DPPH• (1, 1-diphenyl-2-picryl-hydrazyl). The phenolic extracts have shown a strong antiradicalaire activity than the saponin extracts with EC50 values ranged from 6 to 11 μg/ml and from 51 to 82 μg/ml, respectively. The enzymatic inhibition produced by these plant extracts is described here for the first time. The results have shown that the phenolic extracts are more potent than the saponin extracts with Ki values ranged from 0.011 mg/ml to 0.027 mg/ml for phenolic extracts, and ranged from 0.071 mg/ml to 0.69 mg/ml for saponin extracts. The nature, mechanism and possible physiological relevance of lipase inhibition by extract components are discussed.