Accumulation and identification of lipofuscin-like pigment in the neurons of Bulla gouldiana (Gastropoda: Opisthobranchia)

How do neurons age? A focused review on the aging of the microtubular cytoskeleton

Aging is the leading risk factor for Alzheimer's disease and other neurodegenerative diseases. We now understand that a breakdown in the neuronal cytoskeleton, mainly underpinned by protein modifications leading to the destabilization of microtubules, is central to the pathogenesis of Alzheimer's disease. This is accompanied by morphological defects across the somatodendritic compartment, axon, and synapse. However, knowledge of what occurs to the microtubule cytoskeleton and morphology of the neuron during physiological aging is comparatively poor. Several recent studies have suggested that there is an age-related increase in the phosphorylation of the key microtubule stabilizing protein tau, a modification, which is known to destabilize the cytoskeleton in Alzheimer's disease. This indicates that the cytoskeleton and potentially other neuronal structures reliant on the cytoskeleton become functionally compromised during normal physiological aging. The current literature shows age-related reductions in synaptic spine density and shifts in synaptic spine conformation which might explain age-related synaptic functional deficits. However, knowledge of what occurs to the microtubular and actin cytoskeleton, with increasing age is extremely limited. When considering the somatodendritic compartment, a regression in dendrites and loss of dendritic length and volume is reported whilst a reduction in soma volume/size is often seen. However, research into cytoskeletal change is limited to a handful of studies demonstrating reductions in and mislocalizations of microtubule-associated proteins with just one study directly exploring the integrity of the microtubules. In the axon, an increase in axonal diameter and age-related appearance of swellings is reported but like the dendrites, just one study investigates the microtubules directly with others reporting loss or mislocalization of microtubule-associated proteins. Though these are the general trends reported, there are clear disparities between model organisms and brain regions that are worthy of further investigation. Additionally, longitudinal studies of neuronal/cytoskeletal aging should also investigate whether these age-related changes contribute not just to vulnerability to disease but also to the decline in nervous system function and behavioral output that all organisms experience. This will highlight the utility, if any, of cytoskeletal fortification for the promotion of healthy neuronal aging and potential protection against age-related neurodegenerative disease. This review seeks to summarize what is currently known about the physiological aging of the neuron and microtubular cytoskeleton in the hope of uncovering mechanisms underpinning age-related risk to disease.

Progressive accumulation of autofluorescent granules in macrophages in rat striatum after systemic 3-nitropropionic acid: a correlative light- and electron-microscopic study

A variety of tissue biomolecules and intracellular structures are known to be autofluorescent. However, autofluorescent signals in brain tissues often confound analysis of the fluorescent markers used for immunohistochemistry. While investigating tissue and cellular pathologies induced by 3-nitropropionic acid, a mitochondrial toxin selective for striatal neurons, we encountered many autofluorescent signals confined to the lesion core. These structures were excited by blue (wavelength = 488 nm) and yellow-orange (555 nm), but not by red (639 nm) or violet (405 nm) lasers, indicating that this autofluorescence overlaps with the emission spectra of commonly used fluorophores. Almost all of the autofluorescence was localized in activated microglia/macrophages, while reactive astrocytes emitted no detectable autofluorescence. Amoeboid brain macrophages filled with autofluorescent granules revealed very weak expression of the microglial marker, ionized calcium-binding adaptor molecule 1 (Iba1), while activated microglia with evident processes and intense Iba1 immunoreactivity contained scant autofluorescent granules. In addition, immunolabeling with two lysosomal markers, ED1/CD68 and lysosomal-associated membrane protein 1, showed a pattern complementary with autofluorescent signals in activated microglia/macrophages, implying that the autofluorescent structures reside within cytoplasm free of intact lysosomes. A correlative light- and electron-microscopic approach finally revealed the ultrastructural identity of the fluorescent granules, most of which matched to clusters of lipofuscin-like inclusions with varying morphology. Thus, autofluorescence in the damaged brain may reflect the presence of lipofuscin-laden brain macrophages, which should be taken into account when verifying any fluorescent signals that are likely to be correlated with activated microglia/macrophages after brain insults.

Sudan Black B, The Specific Histochemical Stain for Lipofuscin: A Novel Method to Detect Senescent Cells

The Sudan-Black-B (SBB) histochemical stain is well known to specifically react against lipofuscin, an aggregate of oxidized proteins, lipids, and metals. Lipofuscin is related to many ageing processes. It is also known to accumulate in senescent cells. We recently proved that lipofuscin detection, when applying the SBB staining, is highly specific for the visualization of senescent cells. Here, we present in detail this SBB method that can detect senescent cells in any material, irrespective of its preparation. This provides unique advantages not only in understanding physiological processes and the pathophysiology of various diseases but also in estimating the response to therapeutic interventions.

Aging of neurons in the mollusc Lymnaea stagnalis small parietal ganglion: a morpho-functional comparison in the same neuron

The aim of this study was to compare the functional and structural changes in similarly identified neurons of the small parietal ganglion in 56 molluscs (Lymnaea stagnalis) of two age groups: adult (10-12 months) and old (20-22 months). No age changes were found in the values of membrane potential, resistance of the neuronal membrane, amplitude, duration, or rate of increase of the anterior action potential front. With aging, the thresholds of direct stimulation were significantly increased, the rate of action potential repolarization decreased, and the amplitude of trace hyperpolarization decreased. The most marked age-dependent changes were observed in the frequency of neuronal spontaneous activity. A clear relationship was established between the frequency of action potentials of the neuron and its structure in adult and old individuals alike. In the molluscs of both age groups, the neurons with a high frequency of action potential displayed ultrastructural features of high activity in the organelles involved in protein biosynthesis. The cytoplasm of these neurons was filled with numerous ribosomes and had a well-developed rough endoplasmic reticulum. The structure of cells with low spontaneous activity in old molluscs differed considerably from that of the corresponding neurons of the adult individuals. The former had significantly marked morphological signs of reduction of the protein-synthesizing processes, as well as of destructive and dystrophic changes. A decrease in the lability of neurons may be an important mechanism of aging.

Reaction of malonaldehyde with mitochondrial membranes

Malonaldehyde formed by lipid oxidation is regarded as a main crosslinker in the formation of natural age pigment. To elucidate the mechanism of pigment formation the reaction of malonaldehyde with biomembranes using fluorescence spectroscopy has been studied. Rat liver mitochondrial ghosts or bovine serum albumin were reacted with malonaldehyde. In both cases two main fluorescence changes were observed: protein fluorescence decreased to 50% of its initial value in about two hours; aminoiminopropene fluorescence reached a maximum at a much slower rate. The kinetics support a two-step reaction hypothesis. First, malonaldehyde reacts with protein quenching its fluorescence. Next fluorescent interprotein aminoiminopropene (AIP) crosslinks are formed. The fluorescence lifetime value of the induced AIP fluorophore was shown to be similar to the lifetime of naturally occurring age pigment previously reported for mitochondrial ghosts prepared from aged animals (5.4 ns +/- 0.3 and 5.9 ns +/- 0.6, respectively).

Age-dependent anatomical changes in an identified neuron in the CNS of Aplysia californica

Neurons of Aplysia californica are naturally pigmented and the pigment accumulates with age. In the present study the pigment was examined in the same neuron from Aplysia of three postmetamorphic ages: young, sexually mature, and old. The large central neuron, R2, was examined by light and electron microscopy to determine if the pigment possessed properties similar to lipofuscin pigment seen in aging mammalian neurons. We used the same microscopic techniques that demonstrate the presence of lipofuscin in mammalian neurons. Light microscopic studies demonstrated a regional correlation between autofluorescence, staining with Sudan Black, and the naturally occurring pigment in old R2s. Electron microscopic studies revealed the presence of large vacuolated and lamellated membrane-bound bodies in the peripheral cytoplasm of old R2s, similar to those found in mammalian neurons. The bodies were located in the same region in which autofluorescence and Sudan Black staining were observed. Although the naturally occurring pigment accumulates with age, it acquires characteristics of lipofuscin pigment in the neurons of older sexually mature animals. The presence of these pigment characteristics can be used as an index of aging in Aplysia neurons as they are in mammalian neurons.

Aging model for unexposed human dermis

Young human dermis is charterized by the presence of many fibroblasts with distended endoplasmic reticulum, extensive Golgi bodies, and orderly and "clean"-appearing groundwork. Older dermis contains inactive fibrocytes with lipofuscin-like granules, macrophages with dense granules, and extracellular spaces with evidence of degeneration. We have found the sex skin of cycling pigtail macaques (Macaca nemestrina) a predictable model for studies on aging changes in human dermis. Swollen sex skin has many biochemical and ultrastructural properties that are similar to those of young dermis, and deflated sex skin resembles older human skin. Sex skin, however, is unique in that it becomes "rejuvenated" with each succeeding ovarian cycle. This animal model may prove useful to researchers attempting to increase their understanding of aging in connective tissue.