Telomeres as biomarkers for ageing and age-related diseases

Telomeres in telomerase-negative cells shorten during DNA replication in vitro due to numerous causes including the inability of DNA polymerases to fully copy the lagging strand, DNA end processing and random damage, often caused by oxidative stress. Short telomeres activate replicative senescence, an irreversible cell cycle arrest. Thus, telomere length is an indicator of replicative history, of the probability of cell senescence, and of the cumulative history of oxidative stress. Telomeres in most human cells shorten during ageing in vivo as well, suggesting that telomere length could be a biomarker of ageing and age-related morbidity. There are two distinct possibilities: First, in a tissue-specific fashion, short telomeres might indicate senescence of (stem) cells, and this might contribute to age-related functional attenuation in this tissue. Second, short telomeres in one tissue might cause systemic effects or might simply indicate a history of high stress and damage in the individual and could thus act as risk markers for age-related disease residing in a completely different tissue. In recent years, data have been published to support both approaches, and we will review these. While they together paint a fairly promising picture, it needs to be pointed out that until now most of the evidence is correlative, that much of it comes from underpowered studies, and that causal evidence for essential pathways, for instance for the impact of cell senescence on tissue ageing in vivo, is still very weak.

Molecular analysis of nicotinic receptor expression in autism

Autism is a developmental disorder of unknown aetiopathology and lacking any specific pharmacological therapeutic intervention. Neurotransmitters such as serotonin, gamma-aminobutyric acid (GABA) and acetylcholine have been implicated. Abnormalities in nicotinic acetylcholine receptors have been identified including cortical loss of binding to the alpha4/beta2 subtype and increase in cerebellar alpha7 binding. Receptor expression (mRNA) has not so far been systematically examined. This study aims to further explore the role of nicotinic receptors in autism by analysing nicotinic receptor subunit mRNA in conjunction with protein levels and receptor binding in different brain areas. Quantitative RT-PCR for alpha4, alpha7 and beta2 subunit mRNA expression levels; alpha3, alpha4, alpha7 and beta2 subunit protein expression immunochemistry and specific radioligand receptor binding were performed in adult autism and control brain samples from cerebral cortex and cerebellum. Alpha4 and beta2 protein expression and receptor binding density as well as alpha4 mRNA levels were lower in parietal cortex in autism, while alpha7 did not change for any of these parameters. In cerebellum, alpha4 mRNA expression was increased, whereas subunit protein and receptor levels were decreased. Alpha7 receptor binding in cerebellum was increased alongside non-significant elevations in mRNA and protein expression levels. No significant changes were found for beta2 in cerebellum. The data obtained, using complementary measures of receptor expression, indicate that reduced gene expression of the alpha4beta2 nicotinic receptor in the cerebral cortex is a major feature of the neurochemical pathology of autism, whilst post-transcriptional abnormalities of both this and the alpha7 subtype are apparent in the cerebellum. The findings point to dendritic and/or synaptic nicotinic receptor abnormalities that may relate to disruptions in cerebral circuitry development.

Immunohistochemical localisation of nicotinic acetylcholine receptor subunits in human cerebellum

Neuronal nicotinic acetylcholine receptors are members of the ligand-gated ion channel superfamily composed of alpha and beta subunits with specific structural, functional and pharmacological properties. In this study we have used immunohistochemistry to investigate the presence of nicotinic acetylcholine receptor subunits in human cerebellum. Tissue was obtained at autopsy from eight adult individuals (aged 36-56 years). Histological sections were prepared from formalin-fixed paraffin-embedded material. alpha 3, alpha 4, alpha 6, alpha 7, beta 2, and beta 4 subunits were present in this brain area associated with both neuronal and non-neuronal cell types. Most Purkinje cells were immunoreactive for all the above subunits, but most strongly for alpha 4 and alpha 7. A proportion of granule cell somata were immunoreactive for all subunits except alpha 3. Punctate immunoreactivity in Purkinje cell and granule cell layers was evident with antibodies against alpha 3, alpha 4, alpha 6, and alpha 7 in parallel with synaptophysin immunoreactivity, suggesting the presence of these subunits on nerve terminals in the human cerebellum. All subunits were present in the dentate nucleus associated with neurones and cell processes. Strong immunoreactivity of neuropil in both the molecular and granule cell layers and within the dentate nucleus was noted with alpha 4, alpha 7 and beta 4 subunits. Astrocytes and astrocytic cell processes appeared to be immunoreactive for alpha 7 and cell processes observed in white matter, also possibly astrocytic, were immunoreactive for beta2. Immunoreactivity to all subunits was noted in association with blood vessels. We suggest that nicotinic acetylcholine receptor subunits may be involved in the modulation of cerebellar activity. Further investigations are warranted to evaluate the participation of nicotinic acetylcholine receptors in cerebellar pathology associated with both developmental and age-related disorders.

Human brain nicotinic receptors, their distribution and participation in neuropsychiatric disorders

Mapping of nicotinic acetylcholine receptor (nAChR) subtypes and subunits in human brain is far from complete, however it is clear that multiple subunits are present (including alpha3, alpha4, alpha5, alpha6 and alpha7, beta2, alpha3 and beta4) and that these receptors are not solely distributed on neurones, but also on cerebral vasculature and astrocytes. It is important to elucidate subunit composition of receptors associated with different cell types and pathways within the human CNS in terms of potential nicotinic therapy for a range of both developmental and age-related disorders in which nAChR attenuation occurs. Reductions in nAChRs are reported in Alzheimer's and Parkinson's diseases, dementia with Lewy bodies, schizophrenia and autism, but may not be associated with reduced cortical cholinergic innervation observed in vascular dementia or occur at an early stage in Down's syndrome. Changes in nAChR expression in neuropsychiatric disorders appear to be brain region and subtype specific and have been shown in some instances to be associated with pathology and symptomatology. It is likely that deficits in alpha4-containing receptors predominate in cortical areas in Alzheimer's disease and autism, whereas reduction of alpha7 receptors may be more important in schizophrenia. Changes in astrocytic and vascular nAChR expression in neurodegenerative diseases should also be considered. Studies using both animal models and human autopsy tissue suggest that nAChRs can play a role in neuroprotection against age-related pathology. It is possible that the development of nAChR subtype specific drugs may lead to advances in therapy for both age-related and psychiatric disorders.

Cholinergic activity in autism: abnormalities in the cerebral cortex and basal forebrain

OBJECTIVE

Measures of cholinergic transmitter activity were investigated in patients with autism because of reported neuropathological abnormalities in cholinergic nuclei in the basal forebrain.

METHOD

Levels of cholinergic enzyme and receptor activity were measured in the frontal and parietal cerebral cortex of deceased autistic adults, similarly aged normal adults without mental retardation, and nonautistic mentally retarded adults. The immunoreactivity levels of brain-derived neurotrophic factor and nerve growth factor were measured in the basal forebrain.

RESULTS

There were no differences between the autistic and comparison groups in choline acetyltransferase or acetylcholinesterase activity in the cerebral cortex and basal forebrain or in muscarinic M(2) receptor or alpha-bungarotoxin binding within the cortex. Cortical M(1) receptor binding was up to 30% lower than normal in the autistic subjects, and the difference reached significance in the parietal cortex. In both the parietal and frontal cortices, differences in nicotinic receptors assessed by [(3)H]epibatidine binding were significant and extensive (65%-73% lower in the autistic group than in the normal subjects); there were no differences in nicotine binding in the basal forebrain. Immunochemical analysis indicated lower levels of both the alpha(4) and beta(2) nicotinic receptor subunits in the parietal cortex. The M(1) receptor abnormality was not evident in the nonautistic group with mental retardation, although the lower [(3)H]epibatidine binding was apparent. In the basal forebrain, the level of brain-derived neurotrophic factor in the autistic group was three times as high as the level of the normal group.

CONCLUSIONS

These neurochemical abnormalities implicate the cholinergic system in developmental disorders such as autism and suggest the potential for intervention based on cholinergic receptor modulation.

Nicotinic receptor subtypes in human brain related to aging and dementia

Neuronal nicotinic receptors are attracting increasing interest, beyond their role in relation to tobacco use, in the areas of human brain aging and disorders associated with dementia. Of the different receptor subtypes in the mammalian brain, many decline with normal aging in several different areas, including particularly cerebral cortex and hippocampus. There are further select subtype changes in the two most common forms of dementia in the elderly: Alzheimer's disease and dementia with Lewy bodies. The alpha4 subunit is most extensively reduced in the cortex in Alzheimer's disease, reflected in the loss of the high affinity binding site. There are also reductions in the low affinity binding site (alpha-bungarotoxin binding) in the thalamus in both disorders, which are likely to reflect the loss of the homomeric (most commonly alpha7) receptor subtype. Correlations exist between some of these receptor abnormalities and clinical and pathological features of the diseases. Targeting such receptors is a current therapeutic objective.

Alpha and beta nicotinic acetylcholine receptors subunits and synaptophysin in putamen from Parkinson's disease

It is well established that nicotinic receptors in the mammalian striatum are involved in modulation of the release of several neurotransmitters, including dopamine. In addition, nicotinic receptors with high affinity for agonists have generally been found to be reduced in the striatum in Parkinson's disease. In the present study antibodies have been used to examine which subunits contribute to the striatal nicotinic receptor loss in Parkinson's disease, and whether the reduction in [(3)H]nicotine binding correlates with synaptic loss. Autopsy tissue from the putamen of 12 Parkinson's disease cases and 12 age-matched control subjects was analysed by immunoblotting using antibodies against recombinant peptides specific for alpha3, alpha4, alpha7, beta2 and beta4 nicotinic acetylcholine receptor (nAChR) subunits and the synaptic marker synaptophysin, in conjunction with assessment of [(3)H]nicotine binding by autoradiography. The data indicate that there is no loss of alpha3, alpha4, alpha7 and beta2 immunoreactivity in the putamen in Parkinson's disease, despite a highly significant reduction in [(3)H]nicotine binding. An intense signal of beta4 immunoreactivity was found in human dorsal root ganglia, but not in temporal cortex or putamen samples. Synaptophysin immunoreactivities were also similar in Parkinson's disease and control cases. These results suggest that the loss of nicotine binding in the putamen in Parkinson's disease may involve an nAChR subunit (e.g., alpha5 and/or alpha6) other than those investigated. Alternatively, the results could reflect impaired subunit assembly at the plasma membrane.

Alpha4 but not alpha3 and alpha7 nicotinic acetylcholine receptor subunits are lost from the temporal cortex in Alzheimer's disease

Neuronal nicotinic acetylcholine receptors labelled with tritiated agonists are reduced in the cerebral cortex in Alzheimer's disease (AD), but to date it has not been demonstrated which nicotinic receptor subunits contribute to this deficit. In the present study, autopsy tissue from the temporal cortex of 14 AD cases and 15 age-matched control subjects was compared using immunoblotting with antibodies against recombinant peptides specific for alpha3, alpha4, and alpha7 subunits, in conjunction with [3H]epibatidine binding. Antibodies to alpha3, alpha4, and alpha7 produced one major band on western blots at 59, 51, and 57 kDa, respectively. [3H]Epibatidine binding and alpha4-like immunoreactivity (using antibodies against the extracellular domain and cytoplasmic loop of the alpha4 subunit) were reduced in AD cases compared with control subjects (p < 0.02) and with a subgroup of control subjects (n = 9) who did not smoke prior to death (p < 0.05) for the former two parameters. [3H]Epibatidine binding and cytoplasmic alpha4-like immunoreactivity were significantly elevated in a subgroup of control subjects (n = 4) known to have smoked prior to death (p < 0.05). There were no significant changes in alpha3- or alpha7-like immunoreactivity associated with AD or tobacco use. The selective involvement of alpha4 has implications for understanding the role of nicotinic receptors in AD and potential therapeutic targets.