Chapter 9 Changes in Neurotransmitter Signal Transduction Pathways in the Aging Brain

■ REVIEW : The Presenilins

Presenilin-1 and presenilin-2 are highly homologous genes located on chromosomes 14 and 1, respectively, that have recently been linked to some cases of early-onset autosomal dominant inherited forms of Alzhei mer's disease (AD). Presenilins are integral membrane proteins localized in the endoplasmic reticulum of neurons throughout the nervous system. Studies of presenilin-1 knockout mice, and of invertebrate homo logues of presenilins and their interacting proteins, suggest major roles for presenilins in normal develop ment. Presenilin-1 mutant knockin mice do not exhibit developmental abnormalities, which indicates that the pathogenic mechanism of presenilin mutations involves gain of an adverse property of the mutant protein. Expression of presenilin mutations in cultured neurons and transgenic mice results in increased sensitivity to apoptosis induced by trophic factor withdrawal and exposure to oxidative and metabolic insults, and also alters gene expression. The pathogenic mechanism of presenilin mutations may involve perturbed endo plasmic reticulum calcium homeostasis resulting in enhanced oxidative stress, altered proteolytic processing of the amyloid precursor protein (APP), and increased neuronal vulnerability to excitotoxicity. Studies of presenilins are rapidly increasing our understanding the molecular and cellular underpinnings of AD and are also elucidating novel roles of the endoplasmic reticulum in neuronal plasticity and cell death. NEURO SCIENTIST 5:112-124, 1999

Chemical network of the living human brain. Evidence of reorganization with aging

We recently described the chemical network properties of the human brain using in vivo proton magnetic resonance spectroscopy ((1)H MRS). In a separate study of aging we found increased concentration of chemicals in the prefrontal and sensorimotor cortices up to the third decade of life, and subsequent decrease of chemical concentrations in the same brain regions after the third decade between young and middle age. We anticipated that these age-dependent differences in chemical concentrations might be a reflection of the chemical network reorganization of the brain during aging. The pattern of chemical connectivity within and across brain regions for all regional chemicals, and specific patterns of chemical connectivity for each chemical type were examined for young and middle age groups using (1)H MRS and correlation analysis. For all studied ages, the dominant positive correlations occurred within brain regions and negative correlations were seen across brain regions. However, the pattern of negative chemical connectivity across brain regions was weaker in middle-aged group (F = 40.4, P < 10(-7) comparing r-values between the two age groups, ANOVA). Within brain regions, the age effects on chemical correlations were seen in the cingulate cortex (46% decrease in the middle-aged group, F = 7.2, P < 0.007) and sensorimotor cortex (SMC) (27% decrease, F = 8.9, P<0.003). Between brain regions, the age effects on chemical correlations were seen in the chemical interactions between the thalamus (433.3% increase in the middle-aged group, F = 11.7, P < 0.003), SMC (280% increase, F=20.1, P < 10(-5)), cingulate cortex (100.7% increase, F = 21.3, P < 10(-7)), and other brain regions. We found also age-differential patterns of chemical connectivity across the studied brain regions for most chemical types. The results provide evidence that normal aging is associated with reorganization of chemical network of the human brain.

Modification of ion channels and calcium homeostasis of basal forebrain neurons during aging

In this paper we review the last several years of work from our lab with attention to changes in the properties of basal forebrain neurons during aging. These neurons play a central role in behavioral functions, such as: attention, arousal, cognition and autonomic activity, and these functions can be adversely affected during aging. Therefore, it is fundamental to define the cellular mechanisms of aging in order to understand the basal forebrain and to correct deficits associated with aging. We have examined changes in the physiological properties of basal forebrain neurons during aging with whole-cell and single-channel patch-clamp, as well as, microfluorimetric measurements of intracellular calcium concentrations. These studies contribute to the understanding of integration within the basal forebrain and to the identification of age-related changes within central mammalian neurons. Although extensive functional/behavioral decline is often assumed to occur during aging, our data support an interpretation of compensatory increases in function for excitatory amino acid receptors, GABA(A) receptors, voltage-gated calcium currents and calcium homeostatic mechanisms. We believe that these changes occur to compensate for decrements accruing with age, such as decreased synaptic contacts, ion imbalances or neuronal loss. The basal forebrain must retain functionality into late aging if senescence is to be productive. Thus, it is critical to recognize the potential cellular and subcellular targets for therapeutic interventions intended to correct age-related behavioral deficits.

Peroxisomal disease cell lines with cellular plasmalogen deficiency have impaired muscarinic cholinergic signal transduction activity and amyloid precursor protein secretion

We tested whether alterations in membrane lipid composition associated with peroxisomal diseases affect muscarinic cholinergic signal transduction activity and amyloid precursor protein (APP) secretion in cultured human skin fibroblasts and Chinese hamster ovary (CHO) mutants. We found that in cell lines from patients with peroxisomal disorders where plasmalogen levels were low, the low-Km GTPase activity was not induced by carbachol, and APP secretion was reduced. This effect on signal transduction activity was not associated with decreased levels of the M1-muscarinic cholinergic receptor or its associated heterotrimeric G-protein. Specifically, this decrease was associated with a plasmalogen deficiency since a CHO cell line with only a deficit in plasmalogens was as severely affected as were generalized peroxisomal disorder cell lines. Thus, plasmalogens appear to be implicated in muscarinic cholinergic signal transduction and secretion of APP. These results provide new insights about the pathophysiology of peroxisomal diseases and may be relevant to Alzheimer's disease where reduced plasmalogen levels have been reported.