Classification and pathogenesis of neurodegenerative diseases

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Classification and pathogenesis of neurodegenerative diseases

The neurodegenerative diseases are characterized by progressive dysfunction and death of neurons. The degeneration often affects specific systems, implying some form of selective neuronal vulnerability. Diseases with a known vascular, toxic, metabolic, infective, or autoimmune cause are by convention excluded from this group.

CLASSIFICATION

There are two main groups of neurodegenerative diseases: the movement disorders and the dementia syndromes (Table 26.1).

Table 26.1

Different types of neurodegenerative disease

There is considerable overlap in the clinical and pathological features of these diseases. Apparently distinct clinical phenotypes are associated with several different types of pathologic abnormality. For example:

The clinical features of Parkinson’s disease are most commonly due to Lewy body pathology, but can also be caused by multiple system atrophy or by several disorders characterized by neurofibrillary tangle formation.

The syndrome of progressive supranuclear palsy can be associated with neurofibrillary tangles in neurons, Lewy body pathology, or cerebrovascular disease.

The syndrome of frontotemporal dementia can be associated with several distinct neuropathologic patterns of neurodegenerative disease.

On the other hand, the same neurodegenerative disease process can have several clinical phenotypes. The region affected by disease determines the clinical manifestations. For example:

The entity of multiple system atrophy as defined by the presence of large numbers of glial cytoplasmic inclusions can present with parkinsonism, laryngeal palsy, autonomic failure, cerebellar ataxia, or a combination of features.

Lewy body pathology can cause Parkinson’s disease, dementia, autonomic failure, focal dystonia, or isolated dysphagia.

PATHOGENESIS

Most neurodegenerative diseases are of unknown cause, but recent work has provided some insights into the mechanisms of neurodegeneration. Many environmental factors, such as toxins and viruses, have in the past been considered as possible causes of neurodegenerative diseases (Table 26.2), but the search for such factors has been largely disappointing and none has been established as causative.

Table 26.2

Environmental factors in neurodegenerative diseases

PROTEIN ACCUMULATION AND DEGRADATION

These are key features of several neurodegenerative diseases. The accumulations of abnormal proteins occur either intracellularly as inclusion bodies or extracellularly, usually as amyloid. Cellular inclusion bodies have long been recognized as characteristic of certain degenerative processes, and in many cases are required features for the histologic diagnosis of specific diseases. Important examples of such inclusions are:

the neurofibrillary tangles of Alzheimer’s disease

Lewy bodies of Parkinson’s disease.

Most inclusions were originally defined by special tinctorial staining techniques. Cellular and molecular biologic studies have characterized the constituent proteins of many inclusions and defined some of the steps involved in their biogenesis. Some of the intracellular inclusions of degenerative diseases are derived from cytoskeletal or cytoskeleton-related proteins, in many instances modified by abnormal patterns of phosphorylation and other post-translational processes. It is now becoming common for neurodegenerative diseases to be named according to the protein involved, for example synucleinopathy, tauopathy.

In certain diseases, mutations in the genes coding for specific proteins leads directly to their accumulation in inclusion bodies. Tau protein accumulates in neuronal and glial cells in a variety of neurodegenerative diseases, some of which are now recognized to be associated with mutations in the tau gene (MAPT), on chromosome 17. These insights have led to the collective designation of these conditions as tauopathies, some of which are familial and some sporadic. There are several isoforms of tau, formed by differential splicing. Of particular relevance in neurodegenerative disease is the difference between the forms of tau with three and those with four microtubule-binding domains. Alzheimer’s disease is the commonest condition characterized by tau protein accumulation but is not linked to MAPT mutations, and is hence an example of a sporadic tauopathy. Other sporadic conditions characterized by tau protein accumulation in neurons or glia include progressive supranuclear palsy, corticobasal degeneration, and grain dementia. A subgroup of the frontotemporal lobar degenerations is linked to mutations in the tau gene, and certain haplotypes of the tau gene are associated with progressive supranuclear palsy.

Transactivation response DNA binding protein 43 (TDP43) is normally located in the nucleus of cells and acts as a transcriptional regulator. This protein has been found to be the main constituent of inclusion bodies in amyotrophic lateral sclerosis/motor neuron disease, and in some of the cases of frontotemporal lobar degeneration.

α-synuclein is a normal synaptic protein which has been recognized to be a major constituent of Lewy bodies in Parkinson’s disease and glial cytoplasmic inclusions in multiple system atrophy. In most cases, there is no mutation in the α-synuclein gene but this gene is mutated in rare familial forms of Parkinson’s disease. These conditions have been collectively termed synucleinopathies. There are also other, much rarer diseases associated with α-synuclein inclusion bodies (such as some forms of neuroaxonal dystrophy, and diffuse neurofibrillary tangles with calcification – a disease in which both tau and α-synuclein inclusions are present).

In diseases which are caused by triplet repeat expansions of CAG repeats, such as Huntington disease and some spinocerebellar degenerations, inclusions composed of polyglutamine, coded by a CAG repeat, develop within neuronal nuclei. These disorders have been referred to as the polyglutamine disorders.

The prion diseases (see Chapter 32) represent a biological phenomenon that is probably unique in which a protein-only transmissible agent is responsible for disease.

In many inclusion bodies, a common factor is accumulation of the protein ubiquitin, together with other proteins that are part of the ubiquitin-proteasome system (UPS), for example P62 (sequestosome 1). This system normally functions to eliminate short-lived or misfolded proteins. The present of UPS proteins in many different types of inclusion probably represents a cellular response aimed at the elimination of abnormal proteins. In some familial neurodegenerative diseases, mutations in genes coding for proteins involved in the UPS have been found, suggesting that inefficiencies in protein degradation can lead to neurodegeneration. Abnormalities in autophagic pathways and endosomal recycling may also contribute to the pathogenesis of this group of diseases.

A practical consequence of these insights is the increasing use of specific immunohistochemical techniques rather than empirical stains such as various types of silver impregnation to detect inclusions. Immunohistochemical detection of phosphorylated tau protein, α-synuclein, TDP43, P62 and ubiquitin are now routinely used to establish a histological diagnosis in many neurodegenerative diseases.

The accumulation of abnormal extracellular proteins in the form of amyloid is a characteristic feature of Alzheimer’s disease as well as prion diseases. The pathogenesis of the accumulation of amyloid is beginning to be uncovered. Amyloids are extracellular fibrillar proteins defined by their physicochemical characteristics, including:

an affinity for Congo red (and related stains)

a yellow–green birefringence when the stained material is viewed under polarized light.

These properties are due to the arrangement of the constituent peptides in an antiparallel β-pleated sheet, into which the Congo red dye intercalates in a precise and aligned orientation.

The formation of amyloid is facilitated by:

the overproduction of precursor peptide

the presence of accessory proteins, which promote nucleation of the β-pleated sheet

possible imbalances in the systems that regulate extracellular proteolysis (Fig. 26.1).

26.1 Deposition of amyloid.

OXIDATIVE STRESS AND MITOCHONDRIAL INTEGRITY

In several diseases oxidative stress has been implicated in neurodegeneration. The formation of free radicals is a normal event in metabolism and there are several physiological antioxidant defence systems that scavenge free radicals, minimizing possible toxic side effects. There is evidence that increased production of free radicals or inefficiencies in antioxidant defence systems may contribute to neurodegeneration. Exposure to environmental agents that are known to generate free radicals may be a contributing factor.

Increasingly, defects in mitochondrial biogenesis or degradation are being detected in neurodegenerative diseases; mutations in genes that regulate these aspects of mitochondrial function are responsible for some forms of familial Parkinson’s disease. Factors that are responsible for modulating mitochondrial integrity through fusion, fission and elimination in the process termed ‘mitophagy’ may all be involved in neurodegeneration, especially that linked to Parkinson’s disease.

EXCITOTOXICITY

Neuronal death may be induced by excessive glutamatergic stimulation of neurons, a process termed excitotoxicity, which has been implicated in the pathogenesis of several degenerative diseases. There are several forms of glutamate receptor, distinguished by their responses to different pharmacologic agonists. In neurodegenerative disease, the vulnerability of certain neuronal systems to damage has been linked to the expression of certain types of glutamate receptor that render neurons vulnerable to excitotoxic damage (Fig. 26.2).

26.2 Excitotoxicity.

INDUCTION OF PROGRAMMED CELL DEATH

The death of neurons in at least some neurodegenerative diseases has been linked to the form of programmed cell death termed apoptosis (Fig. 26.3). The stimuli that trigger this in the different diseases are at present uncertain. Initiating factors under consideration include excitotoxicity, deprivation of neurotrophic growth factors, toxicity of local cytokines, and toxic effects of accumulated proteins. The end result of mitochondrial damage can also result in signaling for apoptosis. However it is important to note that other forms of cell death which are also energy dependent and ‘programmed’ may take place in some situations and are not the same process as conventional apoptotic cell death.