Protein Misfolding and Accumulation as Root Cause in Neurodegeneration

Review Article

Austin Alzheimers J Parkinsons Dis. 2014;1(3): 7.

Protein Misfolding and Accumulation as Root Cause in Neurodegeneration

Lonati E1, Sala G2and Bulbarelli A1*

1Department of Health Sciences, Milan Center for Neuroscience University of Milano-Bicocca, Italy

2Department of Surgery and Translational Medicine,Milan Center for Neuroscience University of Milano-Bicocca, Italy

*Corresponding author: Bulbarelli A, Department of Health Sciences Milan Center for Neuroscience University of Milano-Bicocca via Cadore 48, 20900 Monza, Italy.

Received: November 01, 2014; Accepted: November 25, 2014; Published: November 26, 2014


The current concept that accumulation and aggregation of misfolded proteins could represent a basic requirement for the neurodegenerative processes, has raised the attention to the efficiency of cell clearance machinery in influencing neuronal protein homeostasis. Indeed, although multifactorial etiology of Alzheimer’s disease (AD) and Parkinson disease (PD) progression, molecular events activated by environmental conditions and epigenetic mechanisms are strongly associated to oxidative stress and inflammatory damage that in turn seems to find a common origin in the anomalous accumulation of misfolded proteins.

Amyloid-β and tau for AD and alpha-synuclein for PD have been proposed as the central and most specific factors implied in the pathogenesis of these syndromes, which, as a consequence, have been classified as a proteinopathies.

Aggregated structures result inappropriate for proteasomal degradation and justify recent studies highlighting the importance of autophagy, a lysosomal degradation pathway, in misfolded proteins degradation in neurons.

Therefore, the poor efficiency of autophagy might be a primum movens in the physiopathology of neurodegenerative diseases. When other pathogenic mechanisms arise, they determine a compensatory induction of autophagy pathways; if this activation is not appropriate to reestablish neuronal homeostasis and prevent accumulation of neurotoxic products, the neurodegenerative process develops.

According to a recent theory regarding cell aging, a major responsible for the death of neurons, would be the neurotoxic effect of aberrant proteins and mitochondria, accumulating within senescent cells as a consequence of an agerelated progressive dysfunction of different catabolic pathways.

Keywords: Alzheimer’s disease; Parkinson’s disease; Amyloid-beta; tau; Alpha-synuclein; Autophagy


UPS: Ubiquitin/proteasome System; AD: Alzheimer’s disease; PrP: Prion Protein; Aβ: Amyloid beta; PD: Parkinson disease; ALP: Autophagy-Lysosome Pathway; CMA: Chaperone-Mediated Autophagy; Atg: Autophagy Related Genes; hsc70: Heat Shock Cognate Protein; lamp2A: Lysosomal-Associated Membrane Protein 2A; NFTs: Neurofibrillary Tangles; APP: Amyloid Precursor Protein; BACE1: β-secretase; ER: Endoplasmic Reticulum; TGN: Trans-Golgi Network; ROS: Reactive Oxygen Species; (PrP(C)): Cellular Prion Protein, MAP: Microtubules Associated Protein; MTs: Microtubules; PHFs: Paired Helical Filaments; Avs: Autophagosomes; MVBs: Multivesicular Bodies


Neurodegeneration referred to neuronal loss is strongly associated with abnormal aggregation of proteins in extra- or intracellular space [1], an event that characterizes the proteinopathies. Pathological protein misfolding and aggregation or aggregation-associated structures (inclusions) have come to be regarded as a deleterious process linked to dysfunction of different neuronal populations and synaptic loss [2,3] that eventually result in cognitive decline/dementia [4] or motor symptom onset [5].

Indeed, among differentiated cell types, neurons are unique in that, because of their extreme polarization, size and post-mitotic nature may be particularly sensitive to the accumulation of aggregated or damaged cytosolic compounds. Hence, protein turnover mechanism alteration, including Ubiquitin-Proteasome System (UPS) and Autophagy-Lysosome Pathway (ALP), becomes a strong detrimental event accompanied by abnormal phosphorylation, ubiquitination, covalent crosslinking and activation of autolytic proteases [6].

The anomalous turnover of aggregation-prone proteins such as alpha-synuclein, Prion Protein (PrP), amyloid-beta (Aβ) peptide, and tau is the basis of 95% neurodegenerative diseases [6]. Alzheime’’s disease (AD) is characterized by the accumulation of Aβ and tau into plaques and tangles, respectively, while Parkinson’s disease (PD) is broadly characterized by the accumulation of alpha-synuclein into Lewy bodies [7]. Oxidative stress, an important pathogenic mechanism in AD [8] and PD [9] contributes to protein misfolding through a double mechanism: oxidative species directly generate post-translational modifications in protein residues, enhancing the tendency of the protein to aggregate; furthermore, high levels of free radicals alter proteins regulating/mediating UPS and ALP and produces ATP depletion through damage of mitochondrial membranes. These effects result in impairment the entire catabolic system of pathogenic proteins such as Aβ, tau and alpha-synuclein. Oxidative stress also represents an important factor both favoring and mediating glutamate-induced excitotoxicity occurring in AD and PD [10]. The ROS-induced lipid peroxidation of presynaptic membranes impairs the function of transporters involved in the maintenance of calcium homeostasis, with consequent glutamate release into the synaptic cleft, and of glutamate transporters, leading to an increase of extracellular glutamate, which can bind postsynaptic receptors and mediate excitotoxicity. Although pathogenic proteins associated to AD and PD are different, their ‘intrinsically disordered’ structure may contribute to clearance mechanism disruption, leading to the formation of microaggregates, that, in turn, can be combined and deposited into larger aggregates of various size and structure [11].

The recently emerged concept that the accumulation and aggregation of misfolded proteins could represent a basic requirement for the neurodegenerative process has raised the challenge of establishing the pathogenic role of the biological systems influencing neuronal protein homeostasis [12].

Intracellular catabolic systems

The Ubiquitin-Proteasome System (UPS) and the Autophagy- Lysosome Pathway (ALP) are the two major systems responsible for intracellular protein degradation.

The UPS is a major cellular mechanism required for the targeted degradation of most short-lived proteins, thus ensuring protein quality control in both the cytoplasm and the nucleus. The UPS is a ~ 2.5 MDa holoenzyme complex that comprises the 28-subunit core particle (20S subunit), and the 19-subunit regulatory particle (19S subunit). UPS recognizes, unfolds, and degrades polyubiquitinated substrates into small peptides of 3-24 amino acids. For degradation, UPS substrates are covalently conjugated with ubiquitin, a highly conserved 76-residue protein, through a complex enzymatic cascade involving E1, E2, and E3 enzymes [13]. E1 ubiquitin-activating enzymes generate a high-energy thioester bond between the C-terminus of ubiquitin and cysteine residues in E1. E2 ubiquitinconjugating enzymes transfer the activated ubiquitin to the E3- substrate complex. The abundance and specificity of the currently identified E3 ligases suggest that these enzymes determine the substrate selectivity of the UPS.

The ALP is a finely regulated and conserved cellular “selfeating” process that involves sequestration and delivery of cytosolic components to the lysosomes for degradation and recycling. Its function allows the clearance of substrates characterized by alterations limiting their physiologic function or responsible for a cytotoxic effect. This degradative process exerts a cytoprotective role that is probably dependent on the clearance of toxic intracellular structures and the catabolism of substrates in order to obtain energy during starvation. Anyway, in particular situations autophagy seems to mediate a specific pathway of programmed cell death; this function requires a strong activation of autophagy and until now, in vivo, has been identified only during involutional physiologic processes in embryonic tissues [14].

Three types of autophagy, depending on their respective sequestration and delivery mechanism, are known: microautophagy, macroautophagy and Chaperone-Mediated Autophagy (CMA) [15]. Microautophagy is a constitutive, non-selective process consisting on endocytosis of small amounts of cytoplasm into lysosomes through invagination of lysosomal membrane. Macroautophagy and CMA are inducible processes. Macroautophagy, responsible for the removal of misfolded proteins and aberrant organelles which are unsuitable for degradation via UPS, proceeds through various steps, each requiring the presence of specific Autophagy Related Genes (Atg). Macroautophagy starts with sequestration of a region of cytoplasm containing proteins and organelles designed for degradation within a double-membrane vacuole called autophagosome. Once formed, the autophagic vacuole undergoes a process of maturation, which is essential for the subsequent fusion with lysosome and the degradation of substrates.

CMA is a selective device for degradation of aberrant proteins, containing the consensus peptide sequence KFERQ, which are directly transported into the lysosomal lumen by a translocation system constituted by specific carrier proteins. CMA process requires the presence of two main proteins: cytosolic and lysosomal heat shock cognate protein 70 (hsc70) and lysosomal-associated membrane protein 2A (lamp2A). Cytosolic hsc70 binds the KFERQ sequence of substrate proteins and carries them to the lysosomal membrane, where lamp2A, after interaction with cytosolic hsc70, multimerizes and forms a translocation complex with lysosomal hsc70, thus mediating the transport of the substrate protein into the lysosomal lumen. The binding of the substrate protein to lamp2A represents the limiting step of CMA.

The current knowledge indicates that UPS and ALP are not parallel and independent proteolytic pathways but rather compensatory mechanisms cooperating in protein quality control. As a matter of fact, a functional interaction among them has been demonstrated. In particular, macroautophagy is able to modulate its own activity depending on the efficiency of the other two pathways. Both proteasome and CMA inhibition determine a cytoprotective activation of macroautophagy.

Alzheimer’s disease

AD is the most common cause of dementia in the elderly population, affecting approximately 7% of people older than 65 years and about 40% of people older than 80 years [16].

AD progression, indeed, leads slowly to loss of neurons in brain regions such as hippocampus, amygdala and cerebral cortex, destroying memory and cognitive functions.

Although its multifactorial etiology, molecular events activated by environmental conditions or epigenetic mechanisms converge altogether to the increase of hippocampal dystrophic neurites and synapse failure: number of synapses are already reduced in patients with mild cognitive impairment, a preclinical stage of AD, along with a compensatory increase in size of the remaining ones [17]. The described morphology changes are strongly associated to oxidative stress and inflammatory damage that in turn seem to find a shared origin in the anomalous accumulation of misfolded proteins [18].

In AD post-mortem brains, aggregates of nonfunctional protein tau called Neurofibrillary Tangles (NFTs) and cerebral plaques laden of the toxic Aβ peptide have been principally detected. Nonetheless, in the last years, intermediate oligomers and aggregates of Aβ as well as of abnormal tau have been reported as the most cytotoxic forms [18-20].

The imbalance between aggregation and clearance of these forms may easily lead to the following oxidative and inflammatory events underlying AD neurodegeneration [21]. Indeed, several defects in autophagy machinery (i.e. autophagosome formation, lysosomal acidification) has been identified in AD mouse models, suggesting the importance of its activation in preventing amyloidosis and tau aggregation [22].

Amyloid beta and tau: Aβ is a short peptide of about 39-42 amino acids generated by proteolytic cleavage of the membrane-bound Amyloid Precursor Protein (APP), by sequential enzymatic actions of β-secretase (BACE1), producing βAPPs and C99, and the following cleavage by a large multiprotein complex known as γ–secretase (Figure 1A) [23]. APP, synthesized in Endoplasmic Reticulum (ER), completes its maturation in the Trans-Golgi Network (TGN) to be delivered to the plasma membrane [24]. A quote of APP is recycled and internalized in endosome vesicles, where the amyloidogenic processing appears to prevalently occurs [24]. Indeed, in endosomal/ lysosomal vesicles, BACE1 working at the optimal pH is strongly activated.