ptl-1, a Caenorhabditis elegans gene whose products are homologous to the tau microtubule-associated proteins

Modelling the functional genomics of Parkinson's disease in Caenorhabditis elegans: LRRK2 and beyond

For decades, Parkinson's disease (PD) cases have been genetically categorised into familial, when caused by mutations in single genes with a clear inheritance pattern in affected families, or idiopathic, in the absence of an evident monogenic determinant. Recently, genome-wide association studies (GWAS) have revealed how common genetic variability can explain up to 36% of PD heritability and that PD manifestation is often determined by multiple variants at different genetic loci. Thus, one of the current challenges in PD research stands in modelling the complex genetic architecture of this condition and translating this into functional studies. Caenorhabditis elegans provide a profound advantage as a reductionist, economical model for PD research, with a short lifecycle, straightforward genome engineering and high conservation of PD relevant neural, cellular and molecular pathways. Functional models of PD genes utilising C. elegans show many phenotypes recapitulating pathologies observed in PD. When contrasted with mammalian in vivo and in vitro models, these are frequently validated, suggesting relevance of C. elegans in the development of novel PD functional models. This review will discuss how the nematode C. elegans PD models have contributed to the uncovering of molecular and cellular mechanisms of disease, with a focus on the genes most commonly found as causative in familial PD and risk factors in idiopathic PD. Specifically, we will examine the current knowledge on a central player in both familial and idiopathic PD, Leucine-rich repeat kinase 2 (LRRK2) and how it connects to multiple PD associated GWAS candidates and Mendelian disease-causing genes.

Non-Rodent Genetic Animal Models for Studying Tauopathy: Review of Drosophila, Zebrafish, and C. elegans Models

Tauopathy refers to a group of progressive neurodegenerative diseases, including frontotemporal lobar degeneration and Alzheimer's disease, which correlate with the malfunction of microtubule-associated protein Tau (MAPT) due to abnormal hyperphosphorylation, leading to the formation of intracellular aggregates in the brain. Despite extensive efforts to understand tauopathy and develop an efficient therapy, our knowledge is still far from complete. To find a solution for this group of devastating diseases, several animal models that mimic diverse disease phenotypes of tauopathy have been developed. Rodents are the dominating tauopathy models because of their similarity to humans and established disease lines, as well as experimental approaches. However, powerful genetic animal models using Drosophila, zebrafish, and C. elegans have also been developed for modeling tauopathy and have contributed to understanding the pathophysiology of tauopathy. The success of these models stems from the short lifespans, versatile genetic tools, real-time in-vivo imaging, low maintenance costs, and the capability for high-throughput screening. In this review, we summarize the main findings on mechanisms of tauopathy and discuss the current tauopathy models of these non-rodent genetic animals, highlighting their key advantages and limitations in tauopathy research.

Axon Injury-Induced Autophagy Activation Is Impaired in a C. elegans Model of Tauopathy

Autophagy is a conserved pathway that plays a key role in cell homeostasis in normal settings, as well as abnormal and stress conditions. Autophagy dysfunction is found in various neurodegenerative diseases, although it remains unclear whether autophagy impairment is a contributor or consequence of neurodegeneration. Axonal injury is an acute neuronal stress that triggers autophagic responses in an age-dependent manner. In this study, we investigate the injury-triggered autophagy response in a C. elegans model of tauopathy. We found that transgenic expression of pro-aggregant Tau, but not the anti-aggregant Tau, abolished axon injury-induced autophagy activation, resulting in a reduced axon regeneration capacity. Furthermore, axonal trafficking of autophagic vesicles were significantly reduced in the animals expressing pro-aggregant F3ΔK280 Tau, indicating that Tau aggregation impairs autophagy regulation. Importantly, the reduced number of total or trafficking autophagic vesicles in the tauopathy model was not restored by the autophagy activator rapamycin. Loss of PTL-1, the sole Tau homologue in C. elegans, also led to impaired injury-induced autophagy activation, but with an increased basal level of autophagic vesicles. Therefore, we have demonstrated that Tau aggregation as well as Tau depletion both lead to disruption of injury-induced autophagy responses, suggesting that aberrant protein aggregation or microtubule dysfunction can modulate autophagy regulation in neurons after injury.

Caenorhabditis elegans Models to Investigate the Mechanisms Underlying Tau Toxicity in Tauopathies

The understanding of the genetic, biochemical, and structural determinants underlying tau aggregation is pivotal in the elucidation of the pathogenic process driving tauopathies and the design of effective therapies. Relevant information on the molecular basis of human neurodegeneration in vivo can be obtained using the nematode Caenorhabditis elegans (C. elegans). To this end, two main approaches can be applied: the overexpression of genes/proteins leading to neuronal dysfunction and death, and studies in which proteins prone to misfolding are exogenously administered to induce a neurotoxic phenotype. Thanks to the easy generation of transgenic strains expressing human disease genes, C. elegans allows the identification of genes and/or proteins specifically associated with pathology and the specific disruptions of cellular processes involved in disease. Several transgenic strains expressing human wild-type or mutated tau have been developed and offer significant information concerning whether transgene expression regulates protein production and aggregation in soluble or insoluble form, onset of the disease, and the degenerative process. C. elegans is able to specifically react to the toxic assemblies of tau, thus developing a neurodegenerative phenotype that, even when exogenously administered, opens up the use of this assay to investigate in vivo the relationship between the tau sequence, its folding, and its proteotoxicity. These approaches can be employed to screen drugs and small molecules that can interact with the biogenesis and dynamics of formation of tau aggregates and to analyze their interactions with other cellular proteins.

Experimental Models of Tauopathy - From Mechanisms to Therapies

Animal models have been instrumental in reproducing key aspects of human tauopathy. In pursuing these efforts, the mouse continues to have a prominent role. In this chapter, we focus on models that overexpress wild-type or mutant forms of tau, the latter being based on mutations found in familial cases of frontotemporal dementia. We review some of these models in more detail and discuss what they have revealed about the underlying pathomechanisms, as well as highlighting new developments that exploit gene editing tools such as TALEN and CRISPR. Interestingly, when investigating the role of tau in impairing cellular functions, common themes emerge. Because tau is a scaffolding protein that aggregates in the somatodendritic domain under pathological conditions, it traps proteins such as parkin and JIP1, preventing them from executing their normal function in mitophagy and axonal transport, respectively. Another aspect is the emerging role of tau in the translational machinery and the finding that the somatodendritic accumulation of tau in Alzheimer's disease may in part be due to the induction of the de novo synthesis of tau by amyloid-β via the Fyn/ERK/S6 pathway. We further discuss treatment strategies such as tau-based vaccinations and therapeutic ultrasound and conclude by discussing whether there is a future for animal models of tauopathies.

Molecular Pathogenesis of the Tauopathies

The tauopathies constitute a group of diseases that have Tau inclusions in neurons or glia as their common denominator. In this review, we describe the biochemical and histological differences in Tau pathology that are characteristic of the spectrum of frontotemporal lobar degeneration as primary tauopathies and of Alzheimer's disease as a secondary tauopathy, as well as the commonalities and differences between the familial and sporadic forms. Furthermore, we discuss selected advances in transgenic animal models in delineating the different pathomechanisms of Tau.

V363I and V363A mutated tau affect aggregation and neuronal dysfunction differently in C. elegans

Mutations in the microtubule-associated protein tau (MAPT) gene have been linked to a heterogeneous group of progressive neurodegenerative disorders commonly called tauopathies. From patients with frontotemporal lobar degeneration with distinct atypical clinical phenotypes, we recently identified two new mutations on the same codon, in position 363 of the MAPT gene, which resulted in the production of Val-to-Ala (tau_V363A) or Val-to-Ile (tau_V363I) mutated tau. These substitutions specifically affected microtubule polymerization and propensity of tau to aggregate in vitro suggesting that single amino acid modification may dictate the fate of the neuropathology. To clarify whether tau_V363A and tau_V363I affect protein misfolding differently in vivo driving certain phenotypes, we generated new transgenic C. elegans strains. Human 2N4R tau carrying the mutation was expressed in all the neurons of worms. The behavioral defects, misfolding and proteotoxicity caused by the tau_V363A and tau_V363I mutated proteins were compared to that induced by the expression of wild-type tau (tau_wt). Pan-neuronal expression of human 2N4R tau_WT in worms resulted in a neuromuscular defect with characteristics of a neurodegenerative phenotype. This defect was worsened by the expression of mutated proteins which drive distinct neuronal dysfunctions and synaptic impairments involving, in transgenic worms expressing tau_V363A (V363A) also a pharyngeal defect never linked before to other mutations. The two mutations differently affected the tau phosphorylation and misfolding propensities: tau_V363I was highly phosphorylated on epitopes corresponding to Thr231 and Ser202/Thr205, and accumulated as insoluble tau assemblies whereas tau_V363A showed a greater propensity to form soluble oligomeric assemblies. These findings uphold the role of a single amino acid substitution in specifically affecting the ability of tau to form soluble and insoluble assemblies, opening up new perspectives in the pathogenic mechanism underlying tauopathies.

Caenorhabditis elegans models of tauopathy

One of the hallmarks of the tauopathies, which include the neurodegenerative disorders, such as Alzheimer disease (AD), corticobasal degeneration, frontotemporal dementia, and progressive supranuclear palsy (PSP), is the abnormal accumulation of post-translationally modified, insoluble tau. The result is a loss of neurons, decreased mental function, and complete dependence of patients on others. Aggregation of tau, which under physiologic conditions is a highly soluble protein, is thought to be central to the pathogenesis of these diseases. Indeed one of the strongest lines of evidence is the MAPT gene polymorphisms that lead to the familial forms of tauopathy. Extensive research in animal models over the years has contributed some of the most important findings regarding the pathogenesis of these diseases. Despite this, the precise molecular mechanisms that lead to abnormal tau folding, accumulation, and spreading remain unknown. Owing to the fact that most of the biochemical pathways are conserved, Caenorhabditis elegans provides an alternative approach to identify cellular mechanisms and druggable genes that operate in such disorders. Many human genes implicated in neurodegenerative diseases have counterparts in C. elegans, making it an excellent model in which to study their pathogenesis. In this article, we review some of the important findings gained from C. elegans tauopathy models.-Pir, G. J., Choudhary, B., Mandelkow, E. Caenorhabditiselegans models of tauopathy.

Genetics of Frontotemporal Dementia

Frontotemporal dementia (FTD) is the second most common cause of dementia following Alzheimer's disease (AD). Between 20 and 50% of cases are familial. Mutations in MAPT, GRN and C9orf72 are found in 60% of familial FTD cases. C9orf72 mutations are the most common and account for 25%. Rarer mutations (<5%) occur in other genes such as VPC, CHMP2B, TARDP, FUS, ITM2B, TBK1 and TBP. The diagnosis is often challenging due to symptom overlap with AD and other conditions. We review the genetics, clinical presentations, neuroimaging, neuropathology, animal studies and therapeutic trials in FTD. We describe clinical scenarios including the original family with the tau stem loop mutation (+14) and also the recently discovered 'missing tau' mutation +15 that 'closed the loop' in 2015.

Tau physiology and pathomechanisms in frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) has been associated with toxic intracellular aggregates of hyperphosphorylated tau (FTLD-tau). Moreover, genetic studies identified mutations in the MAPT gene encoding tau in familial cases of the disease. In this review, we cover a range of aspects of tau function, both in the healthy and diseased brain, discussing several in vitro and in vivo models. Tau structure and function in the healthy brain is presented, accentuating its distinct compartmentalization in neurons and its role in microtubule stabilization and axonal transport. Furthermore, tau-driven pathology is discussed, introducing current concepts and the underlying experimental evidence. Different aspects of pathological tau phosphorylation, the protein's genomic and domain organization as well as its spreading in disease, together with MAPT-associated mutations and their respective models are presented. Dysfunction related to other post-transcriptional modifications and their effect on normal neuronal functions such as cell cycle, epigenetics and synapse dynamics are also discussed, providing a mechanistic explanation for the observations made in FTLD-tau cases, with the possibility for therapeutic intervention. In this review, we cover aspects of tau function, both in the healthy and diseased brain, referring to different in vitro and in vivo models. In healthy neurons, tau is compartmentalized, with higher concentrations found in the distal part of the axon. Cargo molecules are sensitive to this gradient. A disturbed tau distribution, as found in frontotemporal lobar degeneration (FTLD-tau), has severe consequences for cellular physiology: tau accumulates in the neuronal soma and dendrites, leading among others to microtubule depolymerization and impaired axonal transport. Tau forms insoluble aggregates that sequester additional molecules stalling cellular physiology. Neuronal communication is gradually lost as toxic tau accumulates in dendritic spines with subsequent degeneration of synapses and synaptic loss. Thus, by providing a mechanistic explanation for the observations made in FTLD-tau cases, arises a possibility for therapeutic interventions. This article is part of the Frontotemporal Dementia special issue.

A nematode microtubule-associated protein, PTL-1, closely resembles its mammalian counterparts in overall molecular architecture

The mammalian microtubule-associated proteins (MAPs), MAP2, MAP4, and τ, are structurally similar and considered to be evolutionarily related. The primary structure of a nematode MAP, PTL-1, also reportedly resembles those of the MAPs, but only in a small portion of the molecule. In this study, we elucidated the overall domain organization of PTL-1, using a molecular dissection technique. Firstly, we isolated nematode microtubules and proved that the recombinant PTL-1 binds to nematode and porcine microtubules with similar affinities. Then, the recombinant PTL-1 was genetically dissected to generate four shorter polypeptides, and their microtubule-binding and assembly promoting activities were assessed, using porcine microtubules and tubulin. PTL-1 was found to consist of two parts, microtubule-binding and projection domains, with the former further divided into three functionally distinct subdomains. The molecular architecture of PTL-1 was proved to be quite analogous to its mammalian counterparts, MAP2, MAP4, and τ, strongly supporting their evolutionary relationships.

Use of Caenorhabditis elegans as a model to study Alzheimer's disease and other neurodegenerative diseases

Advances in research and technology has increased our quality of life, allowed us to combat diseases, and achieve increased longevity. Unfortunately, increased longevity is accompanied by a rise in the incidences of age-related diseases such as Alzheimer’s disease (AD). AD is the sixth leading cause of death, and one of the leading causes of dementia amongst the aged population in the USA. It is a progressive neurodegenerative disorder, characterized by the prevalence of extracellular Aβ plaques and intracellular neurofibrillary tangles, derived from the proteolysis of the amyloid precursor protein (APP) and the hyperphosphorylation of microtubule-associated protein tau, respectively. Despite years of extensive research, the molecular mechanisms that underlie the pathology of AD remain unclear. Model organisms, such as the nematode, Caenorhabditis elegans, present a complementary approach to addressing these questions. C. elegans has many advantages as a model system to study AD and other neurodegenerative diseases. Like their mammalian counterparts, they have complex biochemical pathways, most of which are conserved. Genes in which mutations are correlated with AD have counterparts in C. elegans, including an APP-related gene, apl-1, a tau homolog, ptl-1, and presenilin homologs, such as sel-12 and hop-1. Since the neuronal connectivity in C. elegans has already been established, C. elegans is also advantageous in modeling learning and memory impairments seen during AD. This article addresses the insights C. elegans provide in studying AD and other neurodegenerative diseases. Additionally, we explore the advantages and drawbacks associated with using this model.

Regulation of age-related structural integrity in neurons by protein with tau-like repeats (PTL-1) is cell autonomous

PTL-1 is the sole homolog of the MAP2/MAP4/tau family in Caenorhabditis elegans. Accumulation of tau is a pathological hallmark of neurodegenerative diseases such as Alzheimer's disease. Therefore, reducing tau levels has been suggested as a therapeutic strategy. We previously showed that PTL-1 maintains age-related structural integrity in neurons, implying that excessive reduction in the levels of a tau-like protein is detrimental. Here, we demonstrate that the regulation of neuronal ageing by PTL-1 occurs via a cell-autonomous mechanism. We re-expressed PTL-1 in a null mutant background using a pan-neuronal promoter to show that PTL-1 functions in neurons to maintain structural integrity. We next expressed PTL-1 only in touch neurons and showed rescue of the neuronal ageing phenotype of ptl-1 mutant animals in these neurons but not in another neuronal subset, the ventral nerve cord GABAergic neurons. Knockdown of PTL-1 in touch neurons also resulted in premature neuronal ageing in these neurons but not in GABAergic neurons. Additionally, expression of PTL-1 in touch neurons alone was unable to rescue the shortened lifespan observed in ptl-1 mutants, but pan-neuronal re-expression restored wild-type longevity, indicating that, at least for a specific group of mechanosensory neurons, premature neuronal ageing and organismal ageing can be decoupled.

Neuronal aging: learning from C. elegans

The heterogeneity and multigenetic nature of nervous system aging make modeling of it a formidable task in mammalian species. The powerful genetics, simple anatomy and short life span of the nematode Caenorhabditis elegans offer unique advantages in unraveling the molecular genetic network that regulates the integrity of neuronal structures and functions during aging. In this review, we first summarize recent breakthroughs in the morphological and functional characterization of C. elegans neuronal aging. Age-associated morphological changes include age-dependent neurite branching, axon beading or swelling, axon defasciculation, progressive distortion of the neuronal soma, and early decline in presynaptic release function. We then discuss genetic pathways that modulate the speed of neuronal aging concordant with alteration in life span, such as insulin signaling, as well as cell-autonomous factors that promote neuronal integrity during senescence, including membrane activity and JNK/MAPK signaling. As a robust genetic model for aging, insights from C. elegans neuronal aging studies will contribute to our mechanistic understanding of human brain aging.

March separate, strike together--role of phosphorylated TAU in mitochondrial dysfunction in Alzheimer's disease

The energy demand and calcium buffering requirements of the brain are met by the high number of mitochondria in neurons and in these, especially at the synapses. Mitochondria are the major producer of reactive oxygen species (ROS); at the same time, they are damaged by ROS that are induced by abnormal protein aggregates that characterize human neurodegenerative diseases such as Alzheimer's disease (AD). Because synaptic mitochondria are long-lived, any damage exerted by these aggregates impacts severely on neuronal function. Here we review how increased TAU, a defining feature of AD and related tauopathies, impairs mitochondrial function by following the principle: 'March separate, strike together!' In the presence of amyloid-β, TAU's toxicity is augmented suggesting synergistic pathomechanisms. In order to restore mitochondrial functions in neurodegeneration as a means of therapeutic intervention it will be important to integrate the various aspects of dysfunction and get a handle on targeting distinct cell types and subcellular compartments.

Aging in the nervous system of Caenorhabditis elegans

It has recently been described that aging in C. elegans is accompanied by the progressive development of morphological changes in the nervous system. These include novel outgrowths from the cell body or axonal process, as well as blebbing and beading along the length of the axon. The formation of these structures is regulated by numerous molecular players including members of the well-conserved insulin/insulin growth factor-like (IGF)-1 signaling and mitogen-activated protein (MAP) kinase pathways. This review summarizes the recent literature on neuronal aging in C. elegans, including our own findings, which indicate a role for protein with tau-like repeats (PTL-1), the homolog of mammalian tau and MAP2/4, in maintaining neuronal integrity during aging.

What Renders TAU Toxic

TAU is a microtubule-associated protein that under pathological conditions such as Alzheimer's disease (AD) forms insoluble, filamentous aggregates. When 20 years after TAU's discovery the first TAU transgenic mouse models were established, one declared goal that was achieved was the modeling of authentic TAU aggregate formation in the form of neurofibrillary tangles. However, as we review here, it has become increasingly clear that TAU causes damage much before these filamentous aggregates develop. In fact, because TAU is a scaffolding protein, increased levels and an altered subcellular localization (due to an increased insolubility and impaired clearance) result in the interaction of TAU with cellular proteins with which it would otherwise either not interact or do so to a lesser degree, thereby impairing their physiological functions. We specifically discuss the non-axonal localization of TAU, the role phosphorylation has in TAU toxicity and how TAU impairs mitochondrial functions. A major emphasis is on what we have learned from the four available TAU knock-out models in mice, and the knock-out of the TAU/MAP2 homolog PTL-1 in worms. It has been proposed that in human pathological conditions such as AD, a rare toxic TAU species exists which needs to be specifically removed to abrogate TAU's toxicity and restore neuronal functions. However, what is toxic in one context may not be in another, and simply reducing, but not fully abolishing TAU levels may be sufficient to abrogate TAU toxicity.

PTL-1 regulates neuronal integrity and lifespan in C. elegans

Protein with tau-like repeats (PTL-1) is the sole Caenorhabditis elegans homolog of tau and MAP2, which are members of the mammalian family of microtubule-associated proteins (MAPs). In mammalian neurons, tau and MAP2 are segregated, with tau being mainly localised to the axon and MAP2 mainly to the dendrite. In particular, tau plays a crucial role in pathology, as elevated levels lead to the formation of tau aggregates in many neurodegenerative conditions including Alzheimer's disease. We used PTL-1 in C. elegans to model the biological functions of a tau-like protein without the complication of functional redundancy that is observed among the mammalian MAPs. Our findings indicate that PTL-1 is important for the maintenance of neuronal health as animals age, as well as in the regulation of whole organism lifespan. In addition, gene dosage of PTL-1 is crucial because variations from wild-type levels are detrimental. We also observed that human tau is unable to robustly compensate for loss of PTL-1, although phenotypes observed in tau transgenic worms are dependent on the presence of endogenous PTL-1. Our data suggest that some of the effects of tau pathology result from the loss of physiological tau function and not solely from a toxic gain-of-function due to accumulation of tau.

Alzheimer's disease models and functional genomics-How many needles are there in the haystack?

Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD) are complex human brain disorders that affect an increasing number of people worldwide. With the identification first of the proteins that aggregate in AD and FTLD brains and subsequently of pathogenic gene mutations that cause their formation in the familial cases, the foundation was laid for the generation of animal models. These recapitulate essential aspects of the human conditions; expression of mutant forms of the amyloid-β protein-encoding APP gene in mice reproduces amyloid-β (Aβ) plaque formation in AD, while that of mutant forms of the tau-encoding microtubule-associated protein tau (MAPT) gene reproduces tau-containing neurofibrillary tangle formation, a lesion that is also prevalent in FTLD-Tau. The mouse models have been complemented by those in lower species such as C. elegans or Drosophila, highlighting the crucial role for Aβ and tau in human neurodegenerative disease. In this review, we will introduce selected AD/FTLD models and discuss how they were instrumental, by identifying deregulated mRNAs, miRNAs and proteins, in dissecting pathogenic mechanisms in neurodegenerative disease. We will discuss some recent examples, which includes miRNA species that are specifically deregulated by Aβ, mitochondrial proteins that are targets of both Aβ and tau, and the nuclear splicing factor SFPQ that accumulates in the cytoplasm in a tau-dependent manner. These examples illustrate how a functional genomics approach followed by a careful validation in experimental models and human tissue leads to a deeper understanding of the pathogenesis of AD and FTLD and ultimately, may help in finding a cure.

Tau/PTL-1 associates with kinesin-3 KIF1A/UNC-104 and affects the motor's motility characteristics in C. elegans neurons

Tauopathies are neurodegenerative diseases based on pathological tau-aggregation including Alzheimer's disease, frontotemporal dementia (FTD) and Pick's disease. In general, cargo (e.g., β-amyloid precursor protein, tau, neurofilaments) accumulation is a commonly observed phenomenon in degenerated neurons. Therefore, it is crucial to investigate the interaction between cargo, microtubule-binding proteins and molecular motors. We report the effect of tau/PTL-1 (protein with tau-like repeats) on the transport characteristics of the major axonal transporter kinesin-3 KIF1A/UNC-104 in the nervous system of Caenorhabditis elegans. Using confocal spinning disk time-lapse imaging we analyzed the motility of UNC-104::mRFP in ptl-1 knockout worms and found that predominantly retrograde moving characteristics are affected (rather than the motor's anterograde displacements). A similar motility pattern was observed for synaptobrevin-1-containing vesicles, a major cargo of UNC-104. Moreover, UNC-104 and PTL-1 colocalize and occasionally co-migrate. We further confirmed physical interactions between PTL-1 and UNC-104 in living animals using the bimolecular fluorescence complementation assay (BiFC) as well as in co-immunoprecipitation experiments. Though this study focuses on PTL-1/UNC-104 interactions, we extended our research on monitoring conventional kinesin-1 (UNC-116) as well as dynein motility pattern and found that in ptl-1 mutants retrograde displacements were also affected for UNC-116, while for dynein, interestingly, its anterograde movements were affected.

A bow-tie genetic architecture for morphogenesis suggested by a genome-wide RNAi screen in Caenorhabditis elegans

During animal development, cellular morphogenesis plays a fundamental role in determining the shape and function of tissues and organs. Identifying the components that regulate and drive morphogenesis is thus a major goal of developmental biology. The four-celled tip of the Caenorhabditis elegans male tail is a simple but powerful model for studying the mechanism of morphogenesis and its spatiotemporal regulation. Here, through a genome-wide post-embryonic RNAi-feeding screen, we identified 212 components that regulate or participate in male tail tip morphogenesis. We constructed a working hypothesis for a gene regulatory network of tail tip morphogenesis. We found regulatory roles for the posterior Hox genes nob-1 and php-3, the TGF-β pathway, nuclear hormone receptors (e.g. nhr-25), the heterochronic gene blmp-1, and the GATA transcription factors egl-18 and elt-6. The majority of the pathways converge at dmd-3 and mab-3. In addition, nhr-25 and dmd-3/mab-3 regulate each others' expression, thus placing these three genes at the center of a complex regulatory network. We also show that dmd-3 and mab-3 negatively regulate other signaling pathways and affect downstream cellular processes such as vesicular trafficking (e.g. arl-1, rme-8) and rearrangement of the cytoskeleton (e.g. cdc-42, nmy-1, and nmy-2). Based on these data, we suggest that male tail tip morphogenesis is governed by a gene regulatory network with a bow-tie architecture.

Morphogenesis is a process in which cells change their shape and position to give rise to mature structures. Elucidation of the molecular basis of morphogenesis and its regulation would be a major step towards understanding organ formation and functionality. We focus on a powerful model for morphogenesis, the four-celled tail tip of the C. elegans male, which undergoes morphogenesis during the last larval stage. To comprehensively determine the components that regulate and execute male tail tip morphogenesis, we performed a genome-wide RNAi screen. We identified 212 genes that encode proteins with roles in fundamental processes like endocytosis, vesicular trafficking, cell–cell communication, and cytoskeletal organization. We determined the interactions among several of these genes to reconstruct a first draft of the genetic network underlying tail tip morphogenesis. The structure of this network is consistent with the "bow-tie architecture" that has been proposed to be universal and confers evolvability and robustness to biological systems. Bow-tie networks have a conserved core which is linked to numerous input and output components. Many components of the network underlying tail tip morphogenesis in C. elegans are conserved all the way to humans. Thus, understanding tail tip morphogenesis will inform us about morphogenesis in other organisms.

Neurodegenerative disorders: insights from the nematode Caenorhabditis elegans

Neurodegenerative diseases impose a burden on society, yet for the most part, the mechanisms underlying neuronal dysfunction and death in these disorders remain unclear despite the identification of relevant disease genes. Given the molecular conservation in neuronal signaling pathways across vertebrate and invertebrate species, many researchers have turned to the nematode Caenorhabditis elegans to identify the mechanisms underlying neurodegenerative disease pathology. C. elegans can be engineered to express human proteins associated with neurodegeneration; additionally, the function of C. elegans orthologs of human neurodegenerative disease genes can be dissected. Herein, we examine major C. elegans neurodegeneration models that recapitulate many aspects of human neurodegenerative disease and we survey the screens that have identified modifier genes. This review highlights how the C. elegans community has used this versatile organism to model several aspects of human neurodegeneration and how these studies have contributed to our understanding of human disease.

Understanding the molecular basis of Alzheimer's disease using a Caenorhabditis elegans model system

Alzheimer's disease (AD) is the major cause of dementia in the United States. At the cellular level, the brains of AD patients are characterized by extracellular dense plaques and intracellular neurofibrillary tangles whose major components are the beta-amyloid peptide and tau, respectively. The beta-amyloid peptide is a cleavage product of the amyloid precursor protein (APP); mutations in APP have been correlated with a small number of cases of familial Alzheimer's disease. APP is the canonical member of the APP family, whose functions remain unclear. The nematode Caenorhabditis elegans, one of the premier genetic workhorses, is being used in a variety of ways to address the functions of APP and determine how the beta-amyloid peptide and tau can induce toxicity. First, the function of the C. elegans APP-related gene, apl-1, is being examined. Although different organisms may use APP and related proteins, such as APL-1, in different functional contexts, the pathways in which they function and the molecules with which they interact are usually conserved. Second, components of the gamma-secretase complex and their respective functions are being revealed through genetic analyses in C. elegans. Third, to address questions of toxicity, onset of degeneration, and protective mechanisms, different human beta-amyloid peptide and tau variants are being introduced into C. elegans and the resultant transgenic lines examined. Here, we summarize how a simple system such as C. elegans can be used as a model to understand APP function and suppression of beta-amyloid peptide and tau toxicity in higher organisms.

Animal models for Alzheimer's disease and frontotemporal dementia: a perspective

In dementia research, animal models have become indispensable tools. They not only model aspects of the human condition, but also simulate processes that occur in humans and hence provide insight into how disease is initiated and propagated. The present review discusses two prominent human neurodegenerative disorders, Alzheimer's disease and frontotemporal dementia. It discusses what we would like to model in animals and highlights some of the more recent achievements using species as diverse as mice, fish, flies and worms. Advances in imaging and therapy are explored. We also discuss some anticipated new models and developments. These will reveal how key players in the pathogenesis of Alzheimer's disease and frontotemporal dementia, such as the peptide Aβ (amyloid β) and the protein tau, cause neuronal dysfunction and eventually, neuronal demise. Understanding these processes fully will lead to early diagnosis and therapy.

SUT-2 potentiates tau-induced neurotoxicity in Caenorhabditis elegans

Expression of human tau in Caenorhabditis elegans neurons causes accumulation of aggregated tau leading to neurodegeneration and uncoordinated movement. We used this model of human tauopathy disorders to screen for genes required for tau neurotoxicity. Recessive loss-of-function mutations in the sut-2 locus suppress the Unc phenotype, tau aggregation and neurodegenerative changes caused by human tau. We cloned the sut-2 gene and found it encodes a novel sub-type of CCCH zinc finger protein conserved across animal phyla. SUT-2 shares significant identity with the mammalian SUT-2 (MSUT-2). To identify SUT-2 interacting proteins, we conducted a yeast two hybrid screen and found SUT-2 binds to ZYG-12, the sole C. elegans HOOK protein family member. Likewise, SUT-2 binds ZYG-12 in in vitro protein binding assays. Furthermore, loss of ZYG-12 leads to a marked upregulation of SUT-2 protein supporting the connection between SUT-2 and ZYG-12. The human genome encodes three homologs of ZYG-12: HOOK1, HOOK2 and HOOK3. Of these, the human ortholog of SUT-2 (MSUT-2) binds only to HOOK2 suggesting the interaction between SUT-2 and HOOK family proteins is conserved across animal phyla. The identification of sut-2 as a gene required for tau neurotoxicity in C. elegans may suggest new neuroprotective strategies capable of arresting tau pathogenesis in tauopathy disorders.

The invertebrate microtubule-associated protein PTL-1 functions in mechanosensation and development in Caenorhabditis elegans

PTL-1, a microtubule-associated protein of the structural MAP2/tau family, is the sole member of this gene family in Caenorhabditis elegans. Sequence analysis of available invertebrate genomes revealed a number of single, putative tau-like genes with high similarity to ptl-1. The ptl-1 gene is expressed in a number of cells, most notably mechanosensory neurons. We examined the role of ptl-1 in C. elegans in adult neurons as well as during development. A ptl-1 knockout strain of worms exhibited an egg-hatching defect, as well as a reduced sensitivity to touch stimuli. In addition, the knockout allele ptl-1(ok621) acts as a dominant enhancer of several temperature-sensitive alleles of mec-7 and mec-12, which code the isoforms of beta-tubulin and alpha-tubulin that together form the unusual 15 protofilament microtubules involved in touch sensation. These results demonstrate for the first time a functional role for this microtubule-associated protein in nematodes and suggest that PTL-1 is involved in mechanosensation as well as some aspect of embryogenesis.

SUT-1 enables tau-induced neurotoxicity in C. elegans

We previously reported a transgenic Caenorhabditis elegans model for tauopathies in which expression of human tau in neurons caused insoluble phosphorylated tau accumulation, neurodegeneration and uncoordinated movement (Unc). To identify genes participating in tau neurotoxicity, we conducted a forward genetic screen for mutations that ameliorate tau-induced uncoordination. The recessive mutation sut-1(bk79) partially suppresses the Unc phenotype, tau aggregation and neurodegenerative changes caused by tau. We identified the sut-1 gene and found it encodes a novel protein. We conducted a yeast two hybrid screen to identify SUT-1 binding partners and found UNC-34, the C. elegans homolog of the cytoskeletal regulatory protein Enabled (ENA). In vitro protein binding assays and genetic studies validated the interaction between SUT-1 and UNC-34. The SUT-1/UNC-34 protein-protein interaction plays a role in both the normal function of UNC-34 and in the tau-induced phenotype. Thus, we have found a conserved molecular pathway participating in tau neurotoxicity in C. elegans.

Invertebrate models of Alzheimer's disease

The intensely studied model organisms Caenorhabditis elegans and Drosophila melanogaster have been employed to study a number of neurodegenerative diseases, including Alzheimer's disease (AD). Although worms and flies are phylogenetically distant from humans, results of both classic genetic analyses and transgenic manipulation of these invertebrates suggest they are valid models for at least some aspects of AD. This review describes the rationale for AD-relevant studies in worms and flies and discusses both what has been learned from these studies and what may be discovered in the future.

Transgenic animal models of tauopathies

Tauopathies are a group of neurodegenerative disorders that include Alzheimer's disease, frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) and other related diseases with prominent tau pathology. Research advances in the last several decades have characterized and defined tau neuropathologies of both neuron and glia in these diverse disorders and this has stimulated development of animal models of tauopathies. Indeed, animal models ranging from invertebrate species such as C. elegan to Drosophila melanogaster and mammalian transgenic mouse models of tauopathies have been generated and reported. This review summarizes the salient features of many of the known models of tauopathies.

The MAP2/Tau family of microtubule-associated proteins

Microtubule-associated proteins (MAPs) of the MAP2/Tau family include the vertebrate proteins MAP2, MAP4, and Tau and homologs in other animals. All three vertebrate members of the family have alternative splice forms; all isoforms share a conserved carboxy-terminal domain containing microtubule-binding repeats, and an amino-terminal projection domain of varying size. MAP2 and Tau are found in neurons, whereas MAP4 is present in many other tissues but is generally absent from neurons. Members of the family are best known for their microtubule-stabilizing activity and for proposed roles regulating microtubule networks in the axons and dendrites of neurons. Contrary to this simple, traditional view, accumulating evidence suggests a much broader range of functions, such as binding to filamentous (F) actin, recruitment of signaling proteins, and regulation of microtubule-mediated transport. Tau is also implicated in Alzheimer's disease and other dementias. The ability of MAP2 to interact with both microtubules and F-actin might be critical for neuromorphogenic processes, such as neurite initiation, during which networks of microtubules and F-actin are reorganized in a coordinated manner. Various upstream kinases and interacting proteins have been identified that regulate the microtubule-stabilizing activity of MAP2/Tau family proteins.

Expression and distribution of distinct variants of E-MAP-115 during proliferation and differentiation of human intestinal epithelial cells

Epithelial cell proliferation and differentiation occur concomitant with striking remodeling of the cytoskeleton. Microtubules (MTs) play important roles in these processes, during which the MTs themselves are reorganized and stabilized by microtubule-associated proteins (MAPs). Among the proteins classified as structural MAPs, E-MAP-115 (also named ensconsin) is preferentially expressed in cells of epithelial origin. The aims of this study were, first, to determine if E-MAP-115, like other MAPs, is expressed as different isoforms during differentiation and, second, to perform a detailed analysis of the expression and distribution of any E-MAP-115 variants detected in intestinal epithelial cells during their polarization/differentiation. It was our expectation that these data would help us to develop hypotheses concerning the role of this MAP in epithelial development. We report the expression of three E-MAP-115 transcripts encoding isoforms of 115, 105, and 95 kDa; two display an expression gradient inverse to the third one as Caco-2 cells progress from proliferation through the stages of differentiation. To monitor the proteins produced from each transcript, we used purified polyclonal antibodies against synthetic peptides contained within the 115, 105, and 95 kDa isoforms to assay proliferating and differentiating CaCo-2 cells. Our results indicate that the expression and MT-binding capacity of the 115, 105, and 95 kDa isoforms vary upon proliferation/differentiation of the cells. E-MAP-115 proteins colocalize with MTs in proliferative and differentiated Caco-2 cells; in vivo, they are expressed in both crypt and villus epithelial cells where they are mainly concentrated at the apical pole of the cells.

Modulation of the membrane-binding projection domain of tau protein: splicing regulation of exon 3

Tau is a microtubule-associated protein whose transcript undergoes complex regulated splicing in the mammalian nervous system. The N-terminal domain of the protein interacts with the axonal membrane, and is modulated by differential inclusion of exons 2 and 3. These two tau exons are alternatively spliced cassettes, in which exon 3 never appears independently of exon 2. Previous work with tau minigene constructs indicated that exon 3 is intrinsically suboptimal and its primary regulator is a weak branch point. In this study, we confirm the role of the weak branch point in the regulation of exon 3 but also show that the exon is additionally regulated by a combination of exonic enhancers and silencers. Furthermore, we demonstrate that known splicing regulators affect the ratio of exon 3 isoforms, Lastly, we tentatively pinpoint the site of action of several splicing factors which regulate tau exon 3.

Analyzing defects in the Caenorhabditis elegans nervous system using organismal and cell biological approaches

The goal of this laboratory exercise is to increase student understanding of the impact of nervous system function at both the organismal and cellular levels. This inquiry-based exercise is designed for an undergraduate course examining principles of cell biology. After observing the movement of Caenorhabditis elegans with defects in their nervous system, students examine the structure of the nervous system to categorize the type of defect. They distinguish between defects in synaptic vesicle transport and defects in synaptic vesicle fusion with membranes. The synaptic vesicles are tagged with green fluorescent protein (GFP), simplifying cellular analysis. The expected outcome of this experiment is that students will better understand the concepts of vesicle transport, neurotransmitter release, GFP, and the relation between the nervous system and behavior.

An isoform of eIF4E is a component of germ granules and is required for spermatogenesis inC. elegans

Control of gene expression at the translational level is crucial for many developmental processes. The mRNA cap-binding protein, eIF4E, is a key player in regulation of translation initiation; appropriate levels of eIF4E are essential for normal cell-cycle regulation and tissue differentiation. The observation that eIF4E levels are elevated during gametogenesis in several organisms suggests that eIF4E might have a specific role in gamete formation as well. We show that one of the five isoforms of C. elegans eIF4E, IFE-1, is enriched in the germline and is a component of germ granules (P granules). The association of IFE-1 with P granules requires the P-granule protein PGL-1. In vitro PGL-1 interacts directly with IFE-1, but not with the other four isoforms of eIF4E. Analysis of animals depleted of IFE-1 by RNAi shows that IFE-1 is required for spermatogenesis, specifically for efficient progression through the meiotic divisions and for the production of functional sperm, in both hermaphrodites and males. The requirement for IFE-1 is highly sensitive to temperature. IFE-1 is not required for oogenesis, as ife-1(RNAi) hermaphrodites produce viable progeny when normal sperm are supplied. Consistent with a primary role in spermatogenesis, ife-1 mRNA levels are highest in regions of the gonad undergoing spermatogenesis. Our results suggest that C. elegans spermatogenesis requires either this specific isoform of eIF4E or an elevated level of eIF4E.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.

Functional characterization of five eIF4E isoforms in Caenorhabditis elegans

Recognition of the 5'-cap structure of mRNA by eIF4E is a critical step in the recruitment of most mRNAs to the ribosome. In Caenorhabditis elegans, approximately 70% of mRNAs contain an unusual 2,2,7-trimethylguanosine cap structure as a result of trans-splicing onto the 5' end of the pre-mRNA. The characterization of three eIF4E isoforms in C. elegans (IFE-1, IFE-2, and IFE-3) was reported previously. The present study describes two more eIF4E isoforms expressed in C. elegans, IFE-4 and IFE-5. We analyzed the requirement of each isoform for viability by RNA interference. IFE-3, the most closely related to mammalian eIF4E-1, binds only 7-methylguanosine caps and is essential for viability. In contrast, three closely related isoforms (IFE-1, IFE-2, and IFE-5) bind 2,2, 7-trimethylguanosine caps and are partially redundant, but at least one functional isoform is required for viability. IFE-4, which binds only 7-methylguanosine caps, is most closely related to an unusual eIF4E isoform found in plants (nCBP) and mammals (4E-HP) and is not essential for viability in any combination of IFE knockout. ife-2, ife-3, ife-4, and ife-5 mRNAs are themselves trans-spliced to SL1 spliced leaders. ife-1 mRNA is trans-spliced to an SL2 leader, indicating that its gene resides in a downstream position of an operon.

Microtubule-associated protein 4 (MAP4) regulates assembly, protomer-polymer partitioning and synthesis of tubulin in cultured cells

We depleted MAP4, a ubiquitously expressed microtubule (MT)-associated protein previously shown to be capable of stabilizing MTs, from HeLa cells by stably expressing antisense RNA. These HeLa-AS cells, in which the MAP4 level was decreased to 33% of the wild-type level, displayed decreased content of total tubulin (65% of the wild-type level). The partitioning of cellular tubulin into protomer and polymer was altered in HeLa-AS cells: polymeric tubulin was decreased to 46% of the level in control cells, while protomeric tubulin was increased to 226% of the level in control cells. Tubulin protein synthesis was decreased, consistent with the tubulin autoregulation model, which proposes that tubulin protomer inhibits its own synthesis. Following release from drug-induced depolymerization, MTs in HeLa-AS cells reformed more slowly, and showed an increased focus on the centrosome, as compared to control cells. HeLa-AS cells also appeared to be less bipolar in shape and flatter than control cells. Our data suggest that MAP4 regulates assembly level of MTs and, perhaps through this mechanism, is involved in controlling spreading and shape of cells.

ZipA is a MAP-Tau homolog and is essential for structural integrity of the cytokinetic FtsZ ring during bacterial cell division

The first visible event in prokaryotic cell division is the assembly of the soluble, tubulin-like FtsZ GTPase into a membrane-associated cytokinetic ring that defines the division plane in bacterial and archaeal cells. In the temperature-sensitive ftsZ84 mutant of Escherichia coli, this ring assembly is impaired at the restrictive temperature causing lethal cell filamentation. Here I present genetic and morphological evidence that a 2-fold higher dosage of the division gene zipA suppresses thermosensitivity of the ftsZ84 mutant by stabilizing the labile FtsZ84 ring structure in vivo. I demonstrate that purified ZipA promotes and stabilizes protofilament assembly of both FtsZ and FtsZ84 in vitro and cosediments with the protofilaments. Furthermore, ZipA organizes FtsZ protofilaments into arrays of long bundles or sheets that probably represent the physiological organization of the FtsZ ring in bacterial cells. The N-terminal cytoplasmic domain of membrane-anchored ZipA contains sequence elements that resemble the microtubule-binding signature motifs in eukaryotic Tau, MAP2 and MAP4 proteins. It is postulated that the MAP-Tau-homologous motifs in ZipA mediate its binding to FtsZ, and that FtsZ-ZipA interaction represents an ancient prototype of the protein-protein interaction that enables MAPs to suppress microtubule catastrophe and/or to promote rescue.

Multiple isoforms of eukaryotic protein synthesis initiation factor 4E in Caenorhabditis elegans can distinguish between mono- and trimethylated mRNA cap structures

The rate-limiting step for cap-dependent translation initiation in eukaryotes is recruitment of mRNA to the ribosome. An early event in this process is recognition of the m7GTP-containing cap structure at the 5'-end of the mRNA by initiation factor eIF4E. In the nematode Caenorhabditis elegans, mRNAs from 70% of the genes contain a different cap structure, m32,2,7GTP. This cap structure is poorly recognized by mammalian elF4E, suggesting that C. elegans may possess a specialized form of elF4E that can recognize m32,2,7GTP. Analysis of the C. elegans genomic sequence data base revealed the presence of three elF4E-like genes, here named ife-1, ife-2, and ife-3. cDNAs for these three eIF4E isoforms were cloned and sequenced. Isoform-specific antibodies were prepared from synthetic peptides based on nonhomologous regions of the three proteins. All three eIF4E isoforms were detected in extracts of C. elegans and were retained on m7GTP-Sepharose. One eIF4E isoform, IFE-1, was also retained on m32,2,7GTP-Sepharose. Furthermore, binding of IFE-1 and IFE-2 to m7GTP-Sepharose was inhibited by m32,2,7GTP. These results suggest that IFE-1 and IFE-2 bind both m7GTP- and m32,2, 7GTP-containing mRNA cap structures, although with different affinities. In conjunction with IFE-3, these eIF4E isoforms would permit cap-dependent recruitment of all C. elegans mRNAs to the ribosome.

Microtubule polymerization dynamics

The polymerization dynamics of microtubules are central to their biological functions. Polymerization dynamics allow microtubules to adopt spatial arrangements that can change rapidly in response to cellular needs and, in some cases, to perform mechanical work. Microtubules utilize the energy of GTP hydrolysis to fuel a unique polymerization mechanism termed dynamic instability. In this review, we first describe progress toward understanding the mechanism of dynamic instability of pure tubulin and then discuss the function and regulation of microtubule dynamic instability in living cells.

The C. elegans gene pag-3 is homologous to the zinc finger proto-oncogene gfi-1

Mutations in the Caenorhabditis elegans gene pag-3 result in misexpression of touch receptor-specific genes in the BDU interneurons and in motility defects. We cloned pag-3 and found that the gene encodes a C2H2-type zinc finger protein related to the mammalian GFI-1 protein. Sequencing of the three pag-3 alleles showed that two apparent null alleles encode a nonsense mutation before the zinc fingers and a missense mutation in the fourth zinc finger that changes a coordinating histidine to a tyrosine. The third allele contains a nonsense mutation in the N-terminal region but is not a null allele. Northern analysis showed that a single pag-3 transcript of about 1.6 kb is present in embryos and L1, L2 and L3 larvae. pag-3 message levels were about twofold higher in pag-3 mutants than in wild-type animals, which suggested that pag-3 may negatively regulate its own expression. pag-3lacZ fusion genes were expressed in the BDU interneurons, the touch neurons, 11 VA and 11 VB ventral cord motor neurons, two AVF interneurons and in unidentified neurons of the retrovesicular ganglion. The BDU neurons and the ALM touch neurons are lineal sister cells in the AB.a lineage and the VA and VB motor neurons are lineal sister cells in the AB.p lineage. The VA motor neurons are required for backward movement and the VB motor neurons are required for forward movement. Mosaic analysis showed that the wild-type pag-3 gene is required in the AB.p lineage for coordinated movement and in the AB.a lineage to suppress touch neuron gene expression in the BDU neurons. Because pag-3 is expressed in both the BDU neurons and in the touch neurons, another protein(s) not expressed in the touch neurons may interact with pag-3 to repress touch neuron gene expression in the BDU neurons. Alternatively, another protein in the touch receptor cells may inactivate PAG-3 and allow expression of the touch receptor program. These results show that pag-3 is a temporally regulated gene that is expressed early in development and functions in multiple types of neurons. They also strongly suggest that the PAG3 protein is a DNA-binding protein with properties similar to the mammalian proto-oncogene product GFI-1.

MARK, a novel family of protein kinases that phosphorylate microtubule-associated proteins and trigger microtubule disruption

MARK phosphorylates the microtubule-associated proteins tau, MAP2, and MAP4 on their microtubule-binding domain, causing their dissociation from microtubules and increased microtubule dynamics. We describe the molecular cloning, distribution, activation mechanism, and overexpression of two MARK proteins from rat that arise from distinct genes. They encode Ser/Thr kinases of 88 and 81 kDa, respectively, and show similarity to the yeast kin1+ and C. elegans par-1 genes that are involved in the establishment of cell polarity. Expression of both isoforms is ubiquitous, and homologous genes are present in humans. Catalytic activity depends on phosphorylation of two residues in subdomain VIII. Overexpression of MARK in cells leads to hyperphosphorylation of MAPs on KXGS motifs and to disruption of the microtubule array, resulting in morphological changes and cell death.

Normal and pathological Tau proteins as factors for microtubule assembly

Tau proteins are microtubule-associated proteins. They regulate the dynamics of the microtubule network, especially involved in the axonal transport and neuronal plasticity. Tau proteins belong to a family of developmentally regulated isoforms generated by alternative splicing and phosphorylation. This generates several Tau variants that interact with tubulin and other proteins. Therefore, Tau proteins are influenced by many physiological regulations. Tau proteins are also powerful markers of the neuronal physiological state. Their degree of phosphorylation is a good marker of cell integrity. It is heavily disturbed in numerous neurodegenerative disorders, leading to a collapse of the microtubule network and the presence of intraneuronal lesions resulting from Tau aggregation. However, different biochemical and immunological patterns of pathological Tau proteins found among neurodegenerative disorders are useful markers for the understanding of the role of Tau protein isoforms and the diagnosis of these pathological conditions.