Causative Genes in Amyotrophic Lateral Sclerosis and Protein Degradation Pathways: a Link to Neurodegeneration

Amyotrophic lateral sclerosis (ALS) is a disease caused by the degeneration of motor neurons (MNs) leading to progressive muscle weakness and atrophy. Several molecular pathways have been implicated, such as glutamate-mediated excitotoxicity, defects in cytoskeletal dynamics and axonal transport, disruption of RNA metabolism, and impairments in proteostasis. ALS is associated with protein accumulation in the cytoplasm of cells undergoing neurodegeneration, which is a hallmark of the disease. In this review, we focus on mechanisms of proteostasis, particularly protein degradation, and discuss how they are related to the genetics of ALS. Indeed, the genetic bases of the disease with the implication of more than 30 genes associated with familial ALS to date, together with the important increase in understanding of endoplasmic reticulum (ER) stress, proteasomal degradation, and autophagy, allow researchers to better understand the mechanisms underlying the selective death of motor neurons in ALS. It is clear that defects in proteostasis are involved in this type of cellular degeneration, but whether or not these mechanisms are primary causes or merely consequential remains to be clearly demonstrated. Novel cellular and animal models allowing chronic expression of mutant proteins, for example, are required. Further studies linking genetic discoveries in ALS to mechanisms of protein clearance will certainly be crucial in order to accelerate translational and clinical research towards new therapeutic targets and strategies.     

Influence of Zinc on Calcium-Dependent Signal Transduction Pathways During Aluminium-Induced Neurodegeneration

Metals perform important functions in the normal physiological system, and alterations in their levels may lead to a number of diseases. Aluminium (Al) has been implicated as a major risk factor, which is linked to several neurodegenerative diseases including Alzheimer’s disease and Parkinson’s disease. On the other hand, zinc (Zn) is considered as a neuromodulator and an essential dietary element that regulates a number of biological activities in our body. The aim of the present study was to investigate the effects of Zn supplementation, if any, in ameliorating the changes induced by Al on calcium signalling pathway. Male Sprague Dawley rats weighing 140–160 g were divided into four different groups viz.: normal control, aluminium treated (100 mg/kg b.wt./day via oral gavage), zinc treated (227 mg/l in drinking water) and combined aluminium and zinc treated. All the treatments were carried out for a total duration of 8 weeks. Al treatment decreased the Ca²⁺ ATPase activity whereas increased the levels of 3′, 5′-cyclic adenosine monophosphate, intracellular calcium and total calcium content in both the cerebrum and cerebellum, which, however, were modulated upon Zn supplementation. Al treatment exhibited a significant elevation in the protein expressions of phospholipase C, inositol triphosphate and protein kinase A but decreased the expression of protein kinase C, which, however, was reversed upon Zn co-treatment. Al treatment also revealed alterations in neurohistoarchitecture in the form of calcium deposits, which were improved upon zinc co-administration. The present study, therefore, suggests that zinc regulates the intracellular calcium signalling pathway during aluminium-induced neurodegeneration.     

Neurodegeneration: nicked to death

Ataxia oculomotor apraxia-1 is a neurological disorder that arises from mutations in the gene encoding the protein aprataxin. A recent study demonstrates that aprataxin is critical for the processing of obstructive DNA termini, suggesting a broader role for DNA single-strand break repair in neurodegenerative disease.     

Photosynthetic Membranes of Synechocystis or Plants Convert Sunlight to Photocurrent through Different Pathways due to Different Architectures

Thylakoid membranes contain the redox active complexes catalyzing the light-dependent reactions of photosynthesis in cyanobacteria, algae and plants. Crude thylakoid membranes or purified photosystems from different organisms have previously been utilized for generation of electrical power and/or fuels. Here we investigate the electron transferability from thylakoid preparations from plants or the cyanobacterium Synechocystis. We show that upon illumination, crude Synechocystis thylakoids can reduce cytochrome c. In addition, this crude preparation can transfer electrons to a graphite electrode, producing an unmediated photocurrent of 15 μA/cm². Photocurrent could be obtained in the presence of the PSII inhibitor DCMU, indicating that the source of electrons is Q_A, the primary Photosystem II acceptor. In contrast, thylakoids purified from plants could not reduce cyt c, nor produced a photocurrent in the photocell in the presence of DCMU. The production of significant photocurrent (100 μA/cm²) from plant thylakoids required the addition of the soluble electron mediator DCBQ. Furthermore, we demonstrate that use of crude thylakoids from the D1-K238E mutant in Synechocystis resulted in improved electron transferability, increasing the direct photocurrent to 35 μA/cm². Applying the analogous mutation to tobacco plants did not achieve an equivalent effect. While electron abstraction from crude thylakoids of cyanobacteria or plants is feasible, we conclude that the site of the abstraction of the electrons from the thylakoids, the architecture of the thylakoid preparations influence the site of the electron abstraction, as well as the transfer pathway to the electrode. This dictates the use of different strategies for production of sustainable electrical current from photosynthetic thylakoid membranes of cyanobacteria or higher plants.     

Mitoptosis: Different Pathways for Mitochondrial Execution

Mitoptosis was described as a sort of mitochondrial death program. It could be associated with both necrosis and apoptosis, although degenerating mitochondria are also found in autophagic vacuoles. It was demonstrated that several molecules might contribute to the remodeling and rearrangement of mitochondrial membranes, leading to mitochondria rupture and disruption. Here, we hypothesize that, at least in T cells, two main pathways of mitoptosis can occur: an inner membrane mitoptosis (IMM), in which only the internal matrix and cristae are lost while the external mitochondrial envelope remains unaltered, and an outer membrane mitoptosis (OMM) where only swollen internal cristae are detected as remnants. We suggest that the study of these processes could provide useful insights not only to the field of cell death but also to the study of the pathogenic mechanisms of mitochondria-associated human diseases.

Addendum to:

Death Receptor Ligation Triggers Membrane Scrambling Between Golgi and Mitochondria

S. Ouasti, P. Matarrese, R. Paddon, R. Khosravi-Far, M. Sorice, A. Tinari, W. Malorni, M. Degli Esposti

Cell Death Differ 2006; Epub ahead of print     

Oral Microbiome Link to Neurodegeneration in Glaucoma

Background

Glaucoma is a progressive optic nerve degenerative disease that often leads to blindness. Local inflammatory responses are implicated in the pathology of glaucoma. Although inflammatory episodes outside the CNS, such as those due to acute systemic infections, have been linked to central neurodegeneration, they do not appear to be relevant to glaucoma. Based on clinical observations, we hypothesized that chronic subclinical peripheral inflammation contributes to neurodegeneration in glaucoma.

Methods

Mouthwash specimens from patients with glaucoma and control subjects were analyzed for the amount of bacteria. To determine a possible pathogenic mechanism, low-dose subcutaneous lipopolysaccharide (LPS) was administered in two separate animal models of glaucoma. Glaucomatous neurodegeneration was assessed in the retina and optic nerve two months later. Changes in gene expression of toll-like receptor 4 (TLR4) signaling pathway and complement as well as changes in microglial numbers and morphology were analyzed in the retina and optic nerve. The effect of pharmacologic blockade of TLR4 with naloxone was determined.

Findings

Patients with glaucoma had higher bacterial oral counts compared to control subjects (p<0.017). Low-dose LPS administration in glaucoma animal models resulted in enhancement of axonal degeneration and neuronal loss. Microglial activation in the optic nerve and retina as well as upregulation of TLR4 signaling and complement system were observed. Pharmacologic blockade of TLR4 partially ameliorated the enhanced damage.

Conclusions

The above findings suggest that the oral microbiome contributes to glaucoma pathophysiology. A plausible mechanism by which increased bacterial loads can lead to neurodegeneration is provided by experiments in animal models of the disease and involves activation of microglia in the retina and optic nerve, mediated through TLR4 signaling and complement upregulation. The finding that commensal bacteria may play a role in the development and/or progression of glaucomatous pathology may also be relevant to other chronic neurodegenerative disorders.     

Ablation of Vacuole Protein Sorting 18 (Vps18) Gene Leads to Neurodegeneration and Impaired Neuronal Migration by Disrupting Multiple Vesicle Transport Pathways to Lysosomes

Intracellular vesicle transport pathways are critical for neuronal survival and central nervous system development. The Vps-C complex regulates multiple vesicle transport pathways to the lysosome in lower organisms. However, little is known regarding its physiological function in mammals. We deleted Vps18, a central member of Vps-C core complex, in neural cells by generating Vps18^F/F; Nestin-Cre mice (Vps18 conditional knock-out mice). These mice displayed severe neurodegeneration and neuronal migration defects. Mechanistic studies revealed that Vps18 deficiency caused neurodegeneration by blocking multiple vesicle transport pathways to the lysosome, including autophagy, endocytosis, and biosynthetic pathways. Our study also showed that ablation of Vps18 resulted in up-regulation of β1 integrin in mouse brain probably due to lysosome dysfunction but had no effects on the reelin pathway, expression of N-cadherin, or activation of JNK, which are implicated in the regulation of neuronal migration. Finally, we demonstrated that knocking down β1 integrin partially rescued the migration defects, suggesting that Vps18 deficiency-mediated up-regulation of β1 integrin may contribute to the defect of neuronal migration in the Vps18-deficient brain. Our results demonstrate important roles of Vps18 in neuron survival and migration, which are disrupted in multiple neural disorders.     

Pathways to chromothripsis

Chromothripsis is a recently recognized mode of genetic instability that generates chromosomes with strikingly large numbers of segmental re-arrangements. While the characterization of these derivative chromosomes has provided new insights into the processes by which cancer genomes can evolve, the underlying signaling events and molecular mechanisms remain unknown. In medulloblastomas, chromothripsis has been observed to occur in the context of mutational inactivation of p53 and activation of the canonical Hedgehog (Hh) pathway. Recent studies have illuminated mechanistic links between these 2 signaling pathways, including a novel PTCH1 homolog that is regulated by p53. Here, we integrate this new pathway into a hypothetical model for the catastrophic DNA breakage that appears to trigger profound chromosomal rearrangements.     

Estrogen and Neurodegeneration

Although estrogen is best known for its effects on the maturation and differentiation of the primary and secondary sex organs, increasing evidence suggests that its influence extends beyond this system, and its activity in the CNS may initiate, or influence our susceptibility to neurodegenerative decline. Estrogen has been proposed to act as a general neuroprotectant at several levels and it is probable that deprivation of estrogen as a result of menopause exposes the aging, or diseased brain to several insults. In addition, estrogen deprivation is likely to initiate, or enhance degenerative changes caused by oxidative stress, and to reduce the brain's ability to maintain synaptic connectivity and cholinergic integrity leading to the cognitive decline seen in aged, and disease-afflicted individuals.     

Pathways to Parkinsonism

A novel gene for Parkinson's disease (PD), DJ-1, has been identified that encodes a multifunctional product with several known protein-protein interactions and effects on gene expression. Here, I outline how it is possible to construct hypotheses that place DJ-1 in different relationships to the other known PD genes, alpha-synuclein and parkin. The identification of multiple genetic causes will provide further impetus to describe the pathway leading to PD.     

Autophagy and Neurodegeneration

The proteasome and lysosome are sophisticated apparatuses capable of shredding unnecessary proteins in eukaryotic cells. The proteasome and its partner ubiquitin (which functions as a destination signal for proteolysis) play crucial roles in selective breakdown of not only short-lived regulatory proteins but also abnormal proteins that need to be rapidly eliminated from the cells. It is generally accepted that deficits of the proteasome-ubiquitin system are associated with various neurodegenerative diseases, since ubiquitin-positive inclusions frequently appear in neurons of patients and mice models of neurodegenerative diseases. However, investigators working in the field of neuronal diseases have focused their attention in recent years on autophagy (Greek for “the eating of oneself”) following the recent discovery that ablation of autophagy leads to accumulation of ubiquitin-positive inclusions, which are the pathological hallmark of neurodegenerative diseases. Here we discuss the consequences of autophagy deficiency in neurons.

Addendum to:

Loss of Autophagy in the Central Nervous System Causes Neurodegeneration in Mice

Masaaki Komatsu, Satoshi Waguri, Tomoki Chiba, Shigeo Murata, Jun-ichi Iwata, Isei Tanida, Takashi Ueno, Masato Koike, Yasuo Uchiyama, Eiki Kominami and Keiji Tanaka

Nature 2006; In press (Published on line 19 April 2006)     

Mitochondria and Neurodegeneration

Many lines of evidence suggest that mitochondria have a central role in ageing-related neurodegenerative diseases. However, despite the evidence of morphological, biochemical and molecular abnormalities in mitochondria in various tissues of patients with neurodegenerative disorders, the question “is mitochondrial dysfunction a necessary step in neurodegeneration?” is still unanswered. In this review, we highlight some of the major neurodegenerative disorders (Alzheimer’s disease, Parkinson’s disease, Amyotrophic lateral sclerosis and Huntington’s disease) and discuss the role of the mitochondria in the pathogenetic cascade leading to neurodegeneration.     

老年性痴呆发病过程中内源性甲醛慢性损伤机制

通过原子力显微镜、荧光标记、Congo染色等方法,观察到低浓度甲醛可以诱导人类神经Tau蛋白错误折叠并形成具有细胞毒性的似球状聚积物;气相色谱和液相色谱等分析结果表明,神经鞘磷脂N-Acyl-4-sphingoine-1-phosphocholine (myelin的过氧化能够产生甲醛分子;脂质过氧化的代谢产物丙二醛(malondialdehyde)在修饰蛋白质(BSA)的过程中,亦可产生甲醛分子.以上结果为内源性甲醛的产生揭示了新的途径.值得注意的是,在生理条件下,血液中内源性甲醛的水平维持在一个动态平衡((0.087±0.004)retool/L),与体外培养神经细胞时甲醛产生毒性的浓度(～0.1 mmol/L)十分接近,甚至已经达到产生一定细胞毒性的水平.随着机体的衰老,内源性甲醛的调节机能下降,在氧化应激等相关因素的诱导作用下,内源性甲醛浓度可能升高,对中枢神经系统一定部位的神经细胞造成慢性损伤,这可能是散发性老年痴呆发病的机制之一.     

Brain-Specific Aminopeptidase: From Enkephalinase to Protector Against Neurodegeneration

The major breakthrough discovery of enkephalins as endogenous opiates led our attempts to determine their inactivation mechanisms. Because the NH2-terminal tyrosine is absolutely necessary for the neuropeptides to exert analgesic effects, and aminopeptidase activities are extraordinarily high in the brain, a specific “amino-enkephalinase” should exist. Several aminopeptidases were identified in the central nervous system during the search. In fact, our laboratory found two novel neuron-specific aminopeptidases: NAP and NAP-2. NAP is the only functionally active brain-specific enzyme known. Its synaptic location coupled with its limited substrate specificity could constitute a “functional” specificity and contribute to enkephalin-specific functions. In addition, NAP was found to be essential for neuron growth, differentiation, and death. Thus, aminopeptidases are likely important for mental health and neurological diseases. Recently, puromycin-sensitive aminopeptidase (PSA) was identified as a modifier of tau-induced neurodegeneration. Because the enzymatic similarity between PSA and NAP, we believe that the depletion of NAP in Alzheimer’s disease (AD) brains plays a causal role in the development of AD pathology. Therefore, use of the puromycin-sensitive neuron-aminopeptidase NAP could provide neuroprotective mechanisms in AD and similar neurodegenerative diseases. Special issue in honor of Naren Banik.     

Pathways to mutualism breakdown

Mutualisms are ubiquitous in nature despite the widely held view that they are unstable interactions. Models predict that mutualists might often evolve into parasites, can abandon their partners to live autonomously and are also vulnerable to extinction. Yet a basic empirical question, whether mutualisms commonly break down, has been mostly overlooked. As we discuss here, recent progress in molecular systematics helps address this question. Newly constructed phylogenies reveal that parasites as well as autonomous (non-mutualist) taxa are nested within ancestrally mutualistic clades. Although models have focused on the propensity of mutualism to become parasitic, such shifts appear relatively rarely. By contrast, diverse systems exhibit reversions to autonomy, and this might be a common and unexplored endpoint to mutualism.