Natural history of mild cognitive impairment in older persons

Dementia update 2003

Clinical and scientific investigations in the field of dementia expanded greatly in the past two decades. Medline citations that addressed Alzheimer disease (AD), the most common dementing disorder, rose from 78 in 1980 to 987 in 1990 and to 1772 in 2000. Underscoring this increased body of knowledge, three practice parameters on dementia were published in 2001 by the Quality Standards Subcommittee of the American Academy of Neurology. Although some older studies are included for context, this review focuses on selected recent advances in dementia that are relevant for the practicing physician. Topics covered include: 1) diagnosis of dementia; 2) risk factors and biomarkers; 3) mild cognitive impairment; 4) the scientific basis for treatment advances in AD; and 5) other dementing disorders.

Cell cycle regulation of neuronal apoptosis in development and disease

Apoptosis of neurons is indispensable to the normal development of the nervous system and contributes to neuronal loss in neurologic injury and disease. Life and death decisions are imposed upon neurons by extracellular and intracellular stimuli including the lack of trophic support, exposure to neurotoxins, oxidative stress, and DNA damage. These stimuli induce signaling pathways that are integrated at the mitochondrial apoptotic machinery culminating in cell survival or death. Growing evidence suggests that cell cycle proteins are expressed in dying neurons in the developing and adult brain. However, the role and mechanisms by which re-activation of cell cycle pathways in postmitotic neurons propagates an apoptotic signal to the cell death machinery are just beginning to be characterized. Here, we will review the molecular mechanisms of neuronal cell death and survival with a focus on recent findings on cell cycle regulation of neuronal apoptosis in primary cultures of neurons, mouse models of neuronal diseases, and human neurodegenerative diseases.

Human cholinergic basal forebrain: chemoanatomy and neurologic dysfunction

The human cholinergic basal forebrain (CBF) is comprised of magnocellular hyperchromic neurons within the septal/diagonal band complex and nucleus basalis (NB) of Meynert. CBF neurons provide the major cholinergic innervation to the hippocampus, amygdala and neocortex. They play a role in cognition and attentional behaviors, and are dysfunctional in Alzheimer's disease (AD). The human CBF displays a continuum of large cells that contain various cholinergic markers, nerve growth factor (NGF) and its cognate receptors, calbindin, glutamate receptors, and the estrogen receptors, ERalpha and ERbeta. Admixed with these cholinergic neuronal phenotypes are smaller interneurons containing the m2 muscarinic acetylcholine receptor (mAChRs), NADPH-diaphorase, GABA, calcium binding proteins and several inhibitory neuropeptides including galanin (GAL), which is over expressed in AD. Studies using human autopsy material indicate an age-related dissociation of calbindin and the glutamate receptor GluR2 within CBF neurons, suggesting that these molecules act synergistically to induce excitotoxic cell death during aging, and possibly during AD. Choline acetyltrasnferease (ChAT) activity and CBF neuron number is preserved in the cholinergic basocortical system and up regulated in the septohippocampal system during prodromal as compared with end stage AD. In contrast, the number of CBF neurons containing NGF receptors is reduced early in the disease process suggesting a phenotypic silence and not a frank loss of neurons. In end stage AD, there is a selective reduction in trkA mRNA but not p75(NTR) in single CBF cells suggesting a neurotrophic defect throughout the progression of AD. These observations indicate the complexity of the chemoanatomy of the human CBF and suggest that multiple factors play different roles in its dysfunction in aging and AD.

Update on mild cognitive impairment

Mild cognitive impairment (MCI) refers to persons whose memory or other cognitive abilities are not normal, but who do not have clinically diagnosed dementia. MCI has received considerable attention in the medical literature over the past few years. There were 63 original reports in the English language literature with the term "mild cognitive impairment" in the title in 2001 and 2002, in contrast to only 26 articles in the prior decade. Although criteria for MCI are not a matter of secure agreement, a consensus is emerging that MCI is common, is associated with significant mortality and morbidity, particularly the development of AD, and is due, in large part, to the same pathologic processes responsible for Alzheimer's disease (AD). Current research efforts are geared towards understanding factors that contribute to the development of dementia among persons with MCI and towards intervention studies aimed at preventing the development of dementia among persons with MCI.

Neuronal cell death is preceded by cell cycle events at all stages of Alzheimer's disease

Cell cycle events play a major role in the loss of neurons in advanced Alzheimer's disease (AD). It is currently unknown, however, whether the same is true for the neuronal losses in early disease stages. To explore this issue we analyzed brain autopsy material from individuals clinically categorized with mild cognitive impairment (MCI), many if not most of whom will progress to AD. Immunocytochemistry for three cell cycle-related proteins, proliferating cell nuclear antigen, cyclin D, and cyclin B, was performed on sections from hippocampus, basal nucleus of Meynert, and entorhinal cortex. The results obtained from MCI cases were compared with material from individuals diagnosed with AD and those without cognitive impairment. In both hippocampus and basal nucleus, there was a significant percentage of cell cycle immunopositive neurons in the MCI cases. These percentages were similar to those found in the AD cases but significantly higher than non-cognitively impaired controls. In entorhinal cortex, the density of cell cycle-positive neurons was greater in MCI than in AD. However, we observed large variations in the percentages of immunopositive neurons from individual to individual. These findings lend support to the hypothesis that both the mechanism of cell loss (a cell cycle-induced death) and the rate of cell loss (a slow atrophy over several months) are identical at all stages of the AD disease process. The implication of the findings for human clinical trials is discussed.

Age-related mild cognitive deficit: a ready-to-use concept?

For better management of mild cognitive impairment in elderly patients, clinicians should be provided with instruments to detect early changes and predict their progression. To define this cognitive status between optimal and pathological aging, many concepts have been proposed, which actually describe various conditions and provide more or less precise criteria, leaving room for variable implementation. As a consequence, application of these criteria gave highly variable prevalence rates, Neuropathological studies indicate that the different criteria have variable power in detecting incipient Alzheimer's disease (AD) and suggest that the transition between mild cognitive impairment and ÀD is not merely quantitative. Follow-up studies have produced, according to the criteria used, a 2.5% to 16,6% annual rate for progression toward dementia, and have also shown that the criteria differ in their stability and predictive power. Baseline cognitive performances have some predictive value, but are difficult to apply in first-line medicine. Investigational techniques (structural and functional imaging, magnetic resonance spectroscopy, magnetization transfer imaging, cerebrospinal fluid neuro-chemistry, and apolipoprotein E genotype) are promising tools in the early diagnosis of AD, which remains the most frequent type of dementia in elderly people and probably the most frequent type developed by patients with mild cognitive deficit. The final goal is to offer early treatment to those patients who will evolve towards dementia, once they can be identified, in the case of AD, recent findings question the adequacy of cholinergic replacement therapies. In its current state, the criteria for mild cognitive deficit are hardly transferable to first-line medicine. However, disseminating the concept could help increase the sensitivity of general practitioners to the importance of cognitive complaints and signs in their elderly patients.

Mild cognitive impairment: prevalence, prognosis, aetiology, and treatment

Mild cognitive impairment (MCI) is a recently described syndrome that is currently thought of as a transition phase between healthy cognitive ageing and dementia. Although this notion seems to be reasonable, the general nature of the term MCI--including its many definitions--makes accurate accounting of the prevalence, prognosis, and potential benefit from treatment somewhat difficult. The differences in cognitive profile and clinical progression among individuals with MCI are generally recognised. However, recent evidence also suggests that the aetiological heterogeneity among individuals with MCI could be greater than previously reported. For example, cerebrovascular disease seems to be underestimated as a potential cause of MCI. In this review, I attempt to recognise workable definitions of MCI to discuss the prevalence, pathophysiology, prognosis, and possibilities for treatment of this disorder.

The apolipoprotein E epsilon 2 allele and decline in episodic memory

OBJECTIVES

The apolipoprotein E (apoE) epsilon 4 allele is related to decline in multiple cognitive domains, especially episodic memory, but the effect of the epsilon 2 allele on change in different forms of cognitive function has been difficult to establish.

METHODS

Participants are from the Religious Orders Study. At baseline, they were at least 65 years old and free of clinical evidence of dementia. For up to eight years, they underwent annual clinical evaluations that included detailed cognitive function assessment from which previously established summary measures of episodic memory, semantic memory, working memory, perceptual speed, and visuospatial ability were derived. Growth curve models were used to assess change in each measure and its relation to apoE genotype, controlling for age, sex, education, and baseline level of cognition. Follow up data were available in 669 persons (98% of those eligible). We treated those with the epsilon 3/3 genotype as the reference group (n=425), which was contrasted with epsilon 2 ( epsilon 2/2, epsilon 2/3; n=86), and epsilon 4 ( epsilon 3/4, epsilon 4/4; n=158) subgroups.

RESULTS

Rate of episodic memory change in the three subgroups significantly differed, with an average annual increase of 0.016 units in the epsilon 2 subgroup and annual decreases of 0.022 units in those with epsilon 3/3 and of 0.073 units in the epsilon 4 subgroup. The epsilon 2 subgroup did not differ from those with epsilon 3/3 in rate of decline in other cognitive systems. The epsilon 4 subgroup declined more rapidly than those with epsilon 3/3 in semantic memory and perceptual speed but not in working memory or visuospatial ability.

CONCLUSION

Possession of one or more apoE epsilon 2 alleles is associated with reduced decline in episodic memory in older persons.

Subcortical ischaemic vascular dementia

Vascular dementia is the second most common type of dementia. The subcortical ischaemic form (SIVD) frequently causes cognitive impairment and dementia in elderly people. SIVD results from small-vessel disease, which produces either arteriolar occlusion and lacunes or widespread incomplete infarction of white matter due to critical stenosis of medullary arterioles and hypoperfusion (Binswanger's disease). Symptoms include motor and cognitive dysexecutive slowing, forgetfulness, dysarthria, mood changes, urinary symptoms, and short-stepped gait. These manifestations probably result from ischaemic interruption of parallel circuits from the prefrontal cortex to the basal ganglia and corresponding thalamocortical connections. Brain imaging (computed tomography and magnetic resonance imaging) is essential for correct diagnosis. The main risk factors are advanced age, hypertension, diabetes, smoking, hyperhomocysteinaemia, hyperfibrinogenaemia, and other conditions that can cause brain hypoperfusion such as obstructive sleep apnoea, congestive heart failure, cardiac arrhythmias, and orthostatic hypotension. Cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL)and some forms of cerebral amyloid angiopathy have a genetic basis. Treatment is symptomatic and prevention requires control of treatable risk factors.

Gene expression profiles of cholinergic nucleus basalis neurons in Alzheimer's disease

Cholinergic neurons of the nucleus basalis (NB) are selectively vulnerable in Alzheimer's disease (AD), yet the molecular mechanisms associated with their dysfunction remain unknown. We used single cell RNA amplification and custom array technology to examine the expression of functional classes of mRNAs found in anterior NB neurons from normal aged and AD subjects. mRNAs encoding neurotrophin receptors, synaptic proteins, protein phosphatases, and amyloid-related proteins were evaluated. We found that trkB and trkC mRNAs were selectively down-regulated in NB neurons, whereas p75NTR mRNA levels remained stable in end stage AD. TrkA mRNA was reduced by approximately 28%, but did not reach statistical significance. There was a down-regulation of synaptophysin, synaptotagmin, and protein phosphatases PP1alpha and PP1beta mRNAs in AD. In contrast, we found a selective up-regulation of cathepsin D mRNA in NB neurons in AD brain. Thus, anterior NB neurons undergo selective alterations in gene expression in AD. These results may provide clues to the molecular pathogenesis of NB neuronal degeneration during AD.