Probing modifications of the neuronal cytoskeleton

Cerebral organoids derived from patients with Alzheimer's disease with PSEN1/2 mutations have defective tissue patterning and altered development

During the past two decades, induced pluripotent stem cells (iPSCs) have been widely used to study human neural development and disease. Especially in the field of Alzheimer's disease (AD), remarkable effort has been put into investigating molecular mechanisms behind this disease. Then, with the advent of 3D neuronal cultures and cerebral organoids (COs), several studies have demonstrated that this model can adequately mimic familial and sporadic AD. Therefore, we created an AD-CO model using iPSCs derived from patients with familial AD forms and explored early events and the progression of AD pathogenesis. Our study demonstrated that COs derived from three AD-iPSC lines with PSEN1(A246E) or PSEN2(N141I) mutations developed the AD-specific markers in vitro, yet they also uncover tissue patterning defects and altered development. These findings are complemented by single-cell sequencing data confirming this observation and uncovering that neurons in AD-COs likely differentiate prematurely.

Cytoskeleton derangement in brain of patients with Down Syndrome, Alzheimer’s disease and Pick’s disease

Although cytoskeleton derangement has been reported in brain of patients with neurodegenerative disorders, basic information on integral constituents forming this network including stoichiometric composition is missing. It was therefore the aim of the study to qualitatively and quantitatively evaluate individual proteins of the three major classes representing the cytoskeleton of human brain. Cytoskeleton proteins beta-actin (betaA), alpha-actinin (Act), tubulin beta-III (betaIII), microtubule associated protein 1 (MAP1), neurofilaments NF-L, NF-M and NF-H and neuron specific enolase (NSE), a marker for neuronal density, were determined by immunoblotting. Brain samples (frontal cortex) of controls (CO), patients with Down Syndrome (DS), Alzheimer's disease (AD) and Pick's disease (PD) were used for the study. In DS brain betaIII, NF-H and NF-M, in AD brain NF-M and NF-H and in PD brain NF-L, NF-M and NF-H were significantly reduced. Stoichiometry of cytoskeleton proteins in control brain revealed the following relations: betaA:Act:betaIII:MAP1:NF-L:NF-M:NF-H = 1.0:0.8:3.8:2.4:3.2:2.2. This stoichiometrical ratios were aberrant in DS, AD and PD with the main outcome that ratios of members of the neurocytoskeleton (betaIII, NF's) in relation to betaA were remarkably decreased. This finding confirms data of decreased neuronal density using NSE in DS and AD. We propose stoichiometry of cytoskeleton elements in normal brain and confirm and extend knowledge on cytoskeleton defects in neurodegenerative diseases. The finding of significantly decreased individual elements may well lead to or represent disassembly of the neurocytoskeleton observed in neurodegenerative diseases.

Suppression of the basic transcription element-binding protein in brain neuronal cultures inhibits thyroid hormone-induced neurite branching

The molecular mechanisms underlying the effect of thyroid hormone (T(3)) on neurite outgrowth are unknown. We recently identified the small GC-box binding protein BTEB (basic transcription element-binding protein) as a T(3)-regulated gene in the developing rat brain. BTEB mRNAs are rapidly (by 1 h) up-regulated by T(3) in primary rat embryonic neuronal cultures. Antisense oligodeoxynucleotides (ODNs), added to the cultures, reduced by 60% the level of BTEB mRNA. Addition of BTEB antisense ODNs to the cultures, before the onset of neurite polarity, had no effect on neurite elaboration but significantly decreased, in a dose-dependent manner, the effect of T(3) on neurite branching. We then examined the effects of antisense ODNs on a thyroid hormone target neuronal population, i.e. the acetylcholinesterase-positive neurons after the onset of neurite polarity. Exposure to BTEB antisense ODNs completely abolished the effects of T(3) on neurite branching and on the elaboration of neuritic filopodia-like structures in acetylcholinesterase cells. By contrast, antisense ODNs did not alter the effect of T(3) on neurite length. Our results show that titration of BTEB levels by T(3) regulates the degree of neurite branching and that the T(3)-induced neurite elongation and the T(3)-induced neurite branching are regulated by distinct mechanisms.

Effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on differentiating mouse N2a neuroblastoma cells

The effect of the neurotoxin 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) was investigated in mouse N2a neuroblastoma cells, induced to differentiate by serum withdrawal and addition of dibutyryl cyclic AMP, over a 24-h period. Addition of MPTP (10 microM) during differentiation caused a change in cell morphology characterised by an inhibition of axon outgrowth, in the absence of cell death. Biochemical characterisation by western blotting revealed that MPTP had no significant effects on the levels of actin, alpha-tubulin, or total heavy-chain neurofilament (NF-H). However, NF-H phosphorylation appeared to increase following MPTP treatment when blots were probed with the phosphorylation state-specific antibodies RMd09 and Ta51. In addition, indirect immunofluorescence analysis revealed an accumulation of phosphorylated NF-H in the cell perikaryon, suggesting that altered NF-H distribution was associated with the observed effects of MPTP on cell morphology. These changes may represent a useful in vitro marker of MPTP neurotoxicity within a simple differentiating neuronal cell model system.

Immunocytochemical changes of cytoskeleton components and calmodulin in the frog cerebellum and optic tectum during hibernation

During hibernation, variation in the metabolism of nerve cells occurs. Since the cytoskeleton plays an important role in nerve cell function, we have analyzed the immunocytochemical expression of two cytoskeleton components, i.e. phosphorylated 200 kDa neurofilament protein, and microtubule-associated protein 2 in the cerebellum and optic tectum of hibernating frogs (Rana esculenta) in comparison with active animals. In addition, we have considered the immunocytochemical expression of calmodulin, which is known to be involved in neurofilament phosphorylation. In hibernating animals, there was a decrease in the immunoreactivity for phosphorylated 200 kDa neurofilament protein and microtubule-associated protein 2 of fibers in both the cerebellum and in the optic tectum. In contrast, in the large neurons of the cerebellum, i.e. Purkinje neurons, there was an increase in the immunoreactivity for microtubule-associated protein 2. The changes in the cytoskeleton components were accompanied by a decrease in calmodulin immunoreactivity in the cytoplasm of nerve cells of the cerebellum. All the changes observed are consistent with a low neuronal activity during hibernation, as also indicated by previous microdensitometric and microfluorometric data. This shows a higher degree of chromatin condensation in hibernating animals and suggests that hibernation represents a simple form of neuronal plasticity.

Modulation of neurite branching by protein phosphorylation in cultured rat hippocampal neurons

The control of branching of axons and dendrites is poorly understood. It has been hypothesized that branching may be produced by changes in the cytoskeleton [F.J. Diez-Guerra, J. Avila, MAP2 phosphorylation parallels dendrite arborization in hippocampal neurones in culture, NeuroReport 4 (1993) 412-419; P. Friedrich, A. Aszodi, MAP2: a sensitive cross-linker and adjustable spacer in dendritic architecture, FEBS Lett. 295 (1991) 5-9]. The assembly and stability of microtubules, which are prominent cytoskeletal elements in both axons and dendrites, are regulated by microtubule-associated proteins, including tau (predominantly found in axons) and MAP2 (predominantly found in dendrites). The phosphorylation state of tau and MAP2 modulates their interactions with microtubules. In their low-phosphorylation states, tau and MAP2 bind to microtubules and increase microtubule assembly and/or stability. Increased phosphorylation decreases these effects. Diez-Guerra and Avila [F.J. Diez-Guerra, J. Avila, MAP2 phosphorylation parallels dendrite arborization in hippocampal neurones in culture, NeuroReport 4 (1993) 412-419] found that protein phosphorylation correlates with neurite branching in cultured rat hippocampal neurons, and hypothesized that increased protein phosphorylation stimulates neurite branching. To test this hypothesis, we cultured rat hippocampal neurons in the presence of specific modulators of serine-threonine protein kinases and phosphatases. Inhibitors of several protein kinases, which would be expected to decrease protein phosphorylation, reduced branching. KT5720, an inhibitor of cyclic AMP-dependent protein kinase, and KN62, an inhibitor of Ca(2+)-calmodulin-dependent protein kinases, inhibited branching of both axons and dendrites. Calphostin C and chelerythrine, inhibitors of protein kinase C, inhibited branching of axons but not dendrites. Treatments that would be expected to increase protein phosphorylation, including inhibitors of protein phosphatases (okadaic acid, cyclosporin A and FK506) and stimulators of PKA (SP-cAMPS) or PKC (phorbol 12-myristate 13-acetate), increased dendrite branching. Only FK506 and phorbol 12-myristate 13-acetate stimulated axon branching. A subset of these agents was tested to confirm their effects on protein phosphorylation in this preparation. Okadaic acid, FK506 and SP-cAMPS all increased protein phosphorylation; KT5720 and KN62 decreased protein phosphorylation. On Western blots, the position of MAP2c extracted from cultures exposed to okadaic acid was slightly shifted toward higher molecular weight, suggesting greater phosphorylation, while the position of MAP2c from cultures exposed to KT5720 and KN62 was slightly shifted toward lower molecular weight, suggesting less phosphorylation. We conclude that protein phosphorylation modulates both dendrite branching and axon branching, but with differences in sensitivity to phosphorylation and/or dephosphorylation by specific kinases and phosphatases.

Regulated phosphorylation and dephosphorylation of tau protein: effects on microtubule interaction, intracellular trafficking and neurodegeneration

This review attempts to summarize what is known about tau phosphorylation in the context of both normal cellular function and dysfunction. However, conceptions of tau function continue to evolve, and it is likely that the regulation of tau distribution and metabolism is complex. The roles of microtubule-associated kinases and phosphatases have yet to be fully described, but may afford insight into how tau phosphorylation at the distal end of the axon regulates cytoskeletal-membrane interactions. Finally, lipid and glycosaminoglycan modification of tau structure affords yet more complexity for regulation and aggregation. Continued work will help to determine what is causal and what is coincidental in Alzheimer's disease, and may lead to identification of therapeutic targets for halting the progression of paired helical filament formation.

Development of fetal hippocampal grafts in intact and lesioned hippocampus

Functional recovery observed in Parkinson's disease patients following grafting of fetal substantia nigra has encouraged the development of similar grafting therapy for other neurological disorders. Fetal hippocampal grafting paradigms are of considerable significance because of their potential to treat neurological disorders affecting primarily hippocampus, including temporal lobe epilepsy, cerebral ischemia, stroke, and head injury. Since many recent studies of hippocampal transplants were carried out with an aim of laying the foundation for future clinical applications, an overview of the development of fetal hippocampal transplants, and their capability for inducing functional recovery under different host conditions is timely. In this review, we will summarize recent developments in hippocampal transplants, especially the anatomical and/or functional integration of grafts within the host brain under specific host conditions, including a comparison of intact hippocampus with various types of hippocampal lesions or injury. Improvements in grafting techniques, methods for analysis of graft integration and graft function will be summarized, in addition to critical factors which enhance the survival and integration of grafted cells and alternative sources of donor cells currently being tested or considered for hippocampal transplantation. Viewed collectively, hippocampal grafting studies show that fetal hippocampal tissue/cells survive grafting, establish both afferent and efferent connections with the host brain, and are also capable of ameliorating certain learning and memory deficits in some models. However, the efficacy of intracerebral fetal hippocampal grafts varies considerably in different animal models, depending on several factors: the mode of donor tissue preparation, the method of grafting, the state of host hippocampus at the time of grafting, and the placement of grafts within the hippocampus. Functional improvement in many models appeared to be caused partially by re-establishment of damaged circuitry and partially by a trophic action of grafts. However, exact mechanisms of graft-mediated behavioral recovery remain to be clarified due to the lack of correlative analysis in the same animal between the degree of graft integration and behavioral recovery. Issues of mechanisms of action, degree of restoration of host circuitry and amelioration of host pathological conditions will need to be sorted out clearly prior to clinical use of fetal hippocampal transplants for susceptible neurological conditions.

Adenosine dialdehyde blocks the disappearance of two nerve growth factor-induced insoluble proteins

Two nonionic-detergent-insoluble proteins are induced early in the nerve growth factor (NGF)-induced neuronal differentiation of PC12 cells. The pools of these two proteins then disappear from the insoluble fraction after a few days of continued exposure of the cells to NGF. The methylation-inhibiting drug adenosine dialdehyde blocks the disappearance of these insoluble proteins, implicating a methylation-dependent step in the pathway that regulates the fate of these proteins.

Non-phosphorylated neurofilament protein immunoreactivity in adult and developing rat hippocampus: specificity and application in grafting studies

Neurofilament proteins are critical to the development and maintenance of neuronal shape in the nervous system. These proteins are developmentally regulated and several transition forms are expressed, prior to full neuronal stabilization. We have studied the spatial distribution and time course of expression of non-phosphorylated neurofilament protein (NPNFP) immunoreactivity in several preparations of rat hippocampus, using a mixture (SMI 311) of several monoclonal antibodies directed against NPNFP epitopes. Differential staining was observed in young and adult hippocampus. Large pyramidal neurons in CA3 and CA4 subfields were strongly immunoreactive in adult hippocampus whereas the smaller CA1 pyramidal neurons, most interneurons and dentate granule cells were immunonegative. SMI 311 staining initially appeared at postnatal day (P) 5 with positive staining in apical dendrites and soma in a few pyramidal neurons in CA3, but almost reached the adult pattern by P10. Compared to adult hippocampus, the number of immunoreactive interneurons in all subfields appeared increased at P10 and P15. In cultures of embryonic hippocampus, all neurons, regardless of their morphology, were SMI 311 positive, suggesting loss of differential expression in tissue culture conditions. However, SMI 311 expression in fetal hippocampal neurons grafted to adult hippocampus was similar to hippocampal neurons which had developed in situ. These results suggest that SMI 311 antibody identifies a distinct group of primarily CA3 and CA4 pyramidal cells in adult hippocampus. The application of SMI 311 immunostaining appears suitable for identification of large CA3 and CA4 pyramidal neurons within hippocampal transplants grafted to adult CNS but not in tissue culture.