Grazyna Niewiadomska

Tau Protein Modifications and Interactions: Their Role in Function and Dysfunction

Tau protein is abundant in the central nervous system and involved in microtubule assembly and stabilization. It is predominantly associated with axonal microtubules and present at lower level in dendrites where it is engaged in signaling functions. Post-translational modifications of tau and its interaction with several proteins play an important regulatory role in the physiology of tau. As a consequence of abnormal modifications and expression, tau is redistributed from neuronal processes to the soma and forms toxic oligomers or aggregated deposits. The accumulation of tau protein is increasingly recognized as the neuropathological hallmark of a number of dementia disorders known as tauopathies. Dysfunction of tau protein may contribute to collapse of cytoskeleton, thereby causing improper anterograde and retrograde movement of motor proteins and their cargos on microtubules. These disturbances in intraneuronal signaling may compromise synaptic transmission as well as trophic support mechanisms in neurons.

The cholinergic system, nerve growth factor and the cytoskeleton.

Cholinergic neurons of the basal forebrain provide the major cholinergic innervation to the cortex and hippocampus, and play a key role in memory and attentional processes. Dysfunction of basal forebrain cholinergic neurons (BFCN) is a cardinal feature of Alzheimers disease (AD) and correlates with cognitive decline. Survival of BFCN neurons depends upon binding of nerve growth factor (NGF), which is synthesized and secreted by cells in the cortex and hippocampus, with high-affinity (TrkA) and low-affinity (p75(NTR)) neurotrophin receptors produced within BFCN neurons. NGF released from target cells activates TrkA on axon terminals and triggers activation of PI3K/Akt, MEK/ERK, and PLCγ (phospholipase C) signaling pathways. The signal then travels retrogradely along axon to cell body to promote neuronal survival. However, the nature of the retrograde signal remains mysterious. p75(NTR) receptors could mediate a fundamentally different signaling pathway leading to apoptic cell death. Dysfunction of NGF and its receptors has been suggested to underlie the selective degeneration of the BFCN in end stage Alzheimer disease. In this regard, NGF, the founding member of the neurotrophin family, has generated great interest as a potential target for the treatment of AD. This review focuses on NGF-cholinergic dependency, NGF/receptor binding, signal transduction, retrograde transport, regulation of specific cellular endpoints, and the potential involvement of cytoskeleton dysfunction in defected NGF signaling.

The septo-hippocampal system, learning and recovery of function.

We understand this review as an attempt to summarize recent advances in the understanding of cholinergic function in cognition. Such a role has been highlighted in the 1970s by the discovery that dementia patients have greatly reduced cholinergic activity in cortex and hippocampus. A brief anatomical description of the major cholinergic pathways focuses on the basal forebrain and its projections to cortex and hippocampus. From this distinction, compelling evidence suggests that the basal forebrain --> cortex projection regulates the excitability of principal cortical neurons and is thereby critically involved in attention, stimulus detection and memory function, although the biological conditions for these functions are still debated. Similar uncertainties remain for the septo-hippocampal cholinergic system. Although initial lesions of the septum caused memory deficits reminiscent of hippocampal ablations, recent and more refined neurotoxic lesion studies which spared non-cholinergic cells of the basal forebrain failed to confirm these memory impairments in experimental animals despite a near total loss of cholinergic labeling. Yet, a decline in cholinergic markers in aging and dementia still stands as the most central piece of evidence for a link between the cholinergic system and cognition and appear to provide valuable targets for therapeutic approaches.

Transient brain ischemia due to cardiac arrest causes irreversible long-lasting cognitive injury.

Herein, we used a clinically-relevant model of 10 min cardiac arrest (CA) in Wistar rats. Histological analyses of the ischemic brains of old rats showed significant atrophy of CA(1) sector of hippocampus (Nissl and NeuN stainings) corresponding with increase of glial fibrillary acidic protein expression. The long-term behavioral consequences of above manipulation producing global brain ischemia were assessed in young, middle-aged and old rats, i.e., 3-, 6- and 18-months post-treatment, respectively. In young animals no differences were found in the context-dependent memory in Fear Conditioning test. The most striking behavioral abnormalities were found in middle-aged rats (6 months post-ischemia). Ischemic rats showed hyperactivity and decreased level of anxiety in Open Field and problems with spatial learning and memory in a Novel Object Location test, T-maze and Morris Water Maze. In old animals, a decline of motor and cognitive functions was found not only in ischemic but also in sham/control ones. This study describes consequences of global brain ischemia in aging animals.

Amelioration of cholinergic neurons dysfunction in aged rats depends on the continuous supply of NGF

The present study was designed to examine whether NGF-induced improvement in morphology of senile basal forebrain cholinergic neurons persist after discontinuation of NGF treatment. Trophic effect of continuous intraventricular infusion of NGF was tested in the 4- and 28 months old male Wistar rats immediately after cessation of NGF and 3 or 6 weeks after termination of treatment. Immunohistochemical procedure for ChAT, TrkA, and p75(NTR) receptor has been applied to identify cholinergic cells in the basal forebrain structures. Using the quantitative image analyzer, morphometric and densitometric parameters of cholinergic cells were measured. In untreated 28-month-old rats a reduction in the number, size and intensity of staining of cholinergic neurons was observed in all basal forebrain structures. NGF significantly improved morphological parameters of ChAT- and TrkA-positive cells in aged rats. In 28-month-old rats tested within 3 and 6 weeks after discontinuation of infusion a renewed progressive deterioration of cholinergic phenotype of basal forebrain neurons was observed when compared with the NGF-treated immediately tested group. The parallel staining for p75(NTR) revealed normal morphology of the basal forebrain neurons, despite of the age of rats or the NGF treatment. Analysis of Nissl stained sections also showed that 28-month-old rats did not display significant losses of neurons in the basal forebrain when compared with the young animals. These findings demonstrate that senile impairment of cholinergic neurons is induced by a loss of cholinergic phenotype rather than an acute degeneration of cell bodies. NGF may be beneficial in enhancing cholinergic neurochemical parameters, but the protective effects seem to be dependent on the continuous supply of NGF.

Altered cellular distribution of phospho-tau proteins coincides with impaired retrograde axonal transport in neurons of aged rats.

Abstract: We hypothesize that the age‐related degeneration of cytoskeleton in basal forebrain cholinergic neurons renders the NGF‐TrkA signaling system non‐functional and thereby impairs trophic support. Comparing young (4 months) and aged (28 months) rat brain, we examined immunohistochemically the compartmentalization of phosphorylated Tau protein using antibodies phospho‐Tau404 and phospho‐Tau231 of the GSK3β kinase, known to phosphorylate Tau, the neurotrophin NGF, and its receptor P‐TrkA. Retrograde labeling of basal forebrain cholinergic cells after injection of fluorogold into multiple sites in cortex and hippocampus revealed a significantly lower number of fluorogold‐positive cells in aged brain. Despite a lower density of P‐TrkA immunoreactivity in cortex and hippocampus of aged rats, there was no difference in NGF expression. In young animals phospho‐Tau404, phospho‐Tau231, and GSK3 immunoreactivity was observed mainly in neuronal fibers with lower staining in somata both in cortex and hippocampus. By contrast, Tau and GSK3 labeling were confined to the cell bodies in aged rats. This is confirmation that aging leads to a redistribution of cytoskeletal proteins. Since a somatic localization of phospho‐Tau is indicative of cytoskeletal breakdown, we suggest that failure of axonal trafficking may be responsible for the lack of trophic support in aged cholinergic neurons of the basal forebrain.

Compartmental protein expression of Tau, GSK-3β and TrkA in cholinergic neurons of aged rats

Summary.During aging basal forebrain cholinergic neurons (BFCNs) degenerate, and we hypothesize this to be the result of a degeneration of the cytoskeleton. As a corollary, retrograde transport of the complex of nerve growth factor (NGF) and its activated receptor phospho-TrkA (P-TrkA) is impaired. Using immunocytochemistry, we here compare young and aged rat brains in their subcellular localization of NGF and P-TrkA in relation to the compartmentalization of phosphorylation-dependent tau protein isoforms. Despite lower P-TrkA immunoreactivity in cortex and hippocampus of aged rats, NGF immunoreactivity was not altered in these areas, but was significantly lower in aged basal forebrain. In young animals, expression of tau isoforms and glycogen synthase kinase-3β (GSK-3β) was restricted to neuritic structures in cortex, hippocampus, and basal forebrain. In contrast, tau and GSK-3β labeling was confined to cell bodies in aged rats. Since a somatic localization of phospho-tau is indicative of cytoskeletal breakdown, we suggest this to be the mechanism the breakdown of trophic support in aging BFCNs.

Age-dependent changes in neuronal distribution of CacyBP/SIP: comparison to tubulin and the tau protein

CacyBP/SIP was originally identified as an S100A6 (calcyclin) target and later on as a Siah-1 interacting protein. Recently, we have shown that CacyBP/SIP interacts with tubulin, which suggests its involvement in the reorganization of microtubules. In this work we examined the localization of CacyBP/SIP in cultured neurons and in brain neurons of young and aged rats, and compared this localization with that of tubulin and the tau protein. We have found that in neurons of young rats CacyBP/SIP, tubulin and tau are present in the cytoplasm and in the neuronal processes, whereas in aged animals CacyBP/SIP and tau are mainly seen in the cytoplasm of the neuronal somata. In aged rats, these changes are also accompanied by a different localization pattern of tubulin. Thus, our results show that localization of CacyBP/SIP in brain neurons is similar to that observed for tau and tubulin, which points to the involvement of CacyBP/SIP in cytoskeletal physiology.

Cytoskeletal transport in the aging brain: focus on the cholinergic system.

There is now compelling evidence for the aging-related breakdown of cytoskeletal support in neurons. Similarly affected are the principal components of the intracellular microtubule system, the transport units involved in active shuttle of organelles and molecules in an antero- and retrograde manner, and the proteins stabilizing the cytoskeleton and providing trophic support. Here, we review the basic organization of the cytoskeleton, and describe its elements and their interactions. We then critically assess the role of these cytoskeletal proteins in physiological aging and aging-related malfunction. Our focus is on the microtubule-associated protein tau, for which comprehensive investigations suggest a critical role in neurodegenerative diseases, for instance tauopathies. These diseases frequently lead to cognitive decline and are often paralleled by reductions in cholinergic neurotransmission. We propose this reduction to be due to destabilization of the cytoskeleton and protein transport mechanisms in these neurons. Therefore, maintenance of the neuronal cytoskeleton during aging may prevent or delay neurodegeneration as well as cognitive decline during physiological aging.

Calcyclin binding protein and Siah-1 interacting protein in Alzheimer's disease pathology: neuronal localization and possible function.

The calcyclin binding protein and Siah-1 interacting protein (CacyBP/SIP) protein was shown to play a role in the organization of microtubules. In this work we have examined the neuronal distribution and possible function of CacyBP/SIP in cytoskeletal pathophysiology. We have used brain tissue from Alzheimers disease (AD) patients and from transgenic mice modeling 2 different pathologies characteristic for AD: amyloid and tau. In the brain from AD patients, CacyBP/SIP was found to be almost exclusively present in neuronal somata, and in control patients it was seen in the somata and neuronal processes. In mice doubly transgenic for amyloid precursor protein and presenilin 1 there was no difference in CacyBP/SIP neuronal localization in comparison with the nontransgenic animals. By contrast in tau transgenic mice, localization of CacyBP/SIP was similar to that observed for AD patients. To find the relation between CacyBP/SIP and tau we examined dephosphorylation of tau by CacyBP/SIP. We found that indeed it exhibited phosphatase activity toward tau. Altogether, our results suggest that CacyBP/SIP might play a role in AD pathology.