The role of kinetochore dynein in checkpoint silencing is restricted to disassembly of the corona

During mitosis, kinetochore-microtubule attachments are monitored by a molecular surveillance system known as the spindle assembly checkpoint. The prevailing model posits that dynein evicts checkpoint proteins (e.g., Mad1, Mad2) from stably attached kinetochores by transporting them away from kinetochores, thus contributing to checkpoint silencing. However, the mechanism by which dynein performs this function, and its precise role in checkpoint silencing remain unresolved. Here, we find that dynein's role in checkpoint silencing is restricted to evicting checkpoint effectors from the fibrous corona, and not the outer kinetochore. Dynein evicts these molecules from the corona in a manner that does not require stable, end-on microtubule attachments. Thus, by disassembling the corona through indiscriminate microtubule encounters, dynein primes the checkpoint signaling apparatus so it can respond to stable end-on microtubule attachments and permit cells to progress through mitosis. Accordingly, we find that dynein function in checkpoint silencing becomes largely dispensable in cells in which checkpoint effectors are excluded from the corona.

Cofilin and Actin Dynamics: Multiple Modes of Regulation and Their Impacts in Neuronal Development and Degeneration

Proteins of the actin depolymerizing factor (ADF)/cofilin family are ubiquitous among eukaryotes and are essential regulators of actin dynamics and function. Mammalian neurons express cofilin-1 as the major isoform, but ADF and cofilin-2 are also expressed. All isoforms bind preferentially and cooperatively along ADP-subunits in F-actin, affecting the filament helical rotation, and when either alone or when enhanced by other proteins, promotes filament severing and subunit turnover. Although self-regulating cofilin-mediated actin dynamics can drive motility without post-translational regulation, cells utilize many mechanisms to locally control cofilin, including cooperation/competition with other proteins. Newly identified post-translational modifications function with or are independent from the well-established phosphorylation of serine 3 and provide unexplored avenues for isoform specific regulation. Cofilin modulates actin transport and function in the nucleus as well as actin organization associated with mitochondrial fission and mitophagy. Under neuronal stress conditions, cofilin-saturated F-actin fragments can undergo oxidative cross-linking and bundle together to form cofilin-actin rods. Rods form in abundance within neurons around brain ischemic lesions and can be rapidly induced in neurites of most hippocampal and cortical neurons through energy depletion or glutamate-induced excitotoxicity. In ~20% of rodent hippocampal neurons, rods form more slowly in a receptor-mediated process triggered by factors intimately connected to disease-related dementias, e.g., amyloid-β in Alzheimer’s disease. This rod-inducing pathway requires a cellular prion protein, NADPH oxidase, and G-protein coupled receptors, e.g., CXCR4 and CCR5. Here, we will review many aspects of cofilin regulation and its contribution to synaptic loss and pathology of neurodegenerative diseases.

ADF/Cofilin-actin rods in neurodegenerative diseases

Dephosphorylation (activation) of cofilin, an actin binding protein, is stimulated by initiators of neuronal dysfunction and degeneration including oxidative stress, excitotoxic glutamate, ischemia, and soluble forms of beta-amyloid peptide (Abeta). Hyperactive cofilin forms rod-shaped cofilin-saturated actin filament bundles (rods). Other proteins are recruited to rods but are not necessary for rod formation. Neuronal cytoplasmic rods accumulate within neurites where they disrupt synaptic function and are a likely cause of synaptic loss without neuronal loss, as occurs early in dementias. Different rod-inducing stimuli target distinct neuronal populations within the hippocampus. Rods form rapidly, often in tandem arrays, in response to stress. They accumulate phosphorylated tau that immunostains for epitopes present in "striated neuropil threads," characteristic of tau pathology in Alzheimer disease (AD) brain. Thus, rods might aid in further tau modifications or assembly into paired helical filaments, the major component of neurofibrillary tangles (NFTs). Rods can occlude neurites and block vesicle transport. Some rod-inducing treatments cause an increase in secreted Abeta. Thus rods may mediate the loss of synapses, production of excess Abeta, and formation of NFTs, all of the pathological hallmarks of AD. Cofilin-actin rods also form within the nucleus of heat-shocked neurons and are cleared from cells expressing wild type huntingtin protein but not in cells expressing mutant or silenced huntingtin, suggesting a role for nuclear rods in Huntington disease (HD). As an early event in the neurodegenerative cascade, rod formation is an ideal target for therapeutic intervention that might be useful in treatment of many different neurological diseases.

Production and use of replication-deficient adenovirus for transgene expression in neurons

Adenoviruses infect a wide range of cell types, do not require integration into the host cell genome, and can be produced as replication-deficient viruses capable of expressing transgenes behind any desired promoter. Thus, they are ideal for use in expressing transgenes in the postmitotic neuron. This chapter describes simplifications in the protocols for making recombinant adenoviruses and their use in expressing transgenes in primary neurons of several different types.

Alx-4, a transcriptional activator whose expression is restricted to sites of epithelial-mesenchymal interactions

We have recently demonstrated that the retinoblastoma family of negative cell cycle regulators can form complexes with a class of developmental factors which contain paired-like (PL) homeodomains (Wiggan et al. [1998] Oncogene 16:227-236). Our screens led to the isolation of a novel PL-homeodomain protein which had been isolated independently by another group and called Alx-4 (Qu et al. [1997] Development 124:3999-4008). Mice homozygous for a targeted null mutation of Alx-4 have several abnormalities, including preaxial polydactyly, suggesting that Alx-4 plays a role in pattern formation in limb buds. In data that we present here, we show that Alx-4 is expressed in mesenchymal condensations of a diverse group of tissues whose development is dependent on epithelial-mesenchymal interactions, many of which are additionally dependent on expression of the HMG-box-containing protein, LEF-1. Alx-4-expressing tissues include osteoblast precursors of most bones, the dermal papilla of hair and whisker follicles, the dental papilla of teeth, and a subset of mesenchymal cells in pubescent mammary glands. We show further that Alx-4 strongly activates transcription from a promoter containing the homeodomain binding site, P2. Optimal activation requires specific sequences in the N-terminal portion of Alx-4 as well as a proline-rich region downstream of the PL-homeodomain, but not the paired-tail at the C terminus. Taken together, our results demonstrate that Alx-4 is a potent transcriptional activator that is expressed at sites of epithelial-mesenchymal interactions during murine embryonic development.

Interaction of the pRB-family proteins with factors containing paired-like homeodomains

The specific loss of pRB or p107 together with p130 disrupts the normal development of only a very limited spectrum of tissues. These developmental defects have been attributed primarily to deregulation of E2F activity and consequent uncontrolled proliferation. We hypothesized, however, that the tissue-specific nature of these defects may also reflect deregulation of pRB-family associated factors that are specifically involved in determining cell fate. We report here that the pRB-family members interact with transcription factors which contain paired-like homeodomains such as MHox, Chx10 and Pax-3. The interaction between the pRB-family and the paired-like homeodomain proteins was initially identified in a yeast two-hybrid screen where the N-terminal portion of p130 was used to isolate interacting factors from an embryonic mouse library. This interaction was confirmed by in vitro binding and co-immunoprecipitation assays. We show further that co-expression of Pax-3 dependent pRB, p107 or p130 with Pax-3 causes repression of activated transcription from the c-met promoter. These data demonstrate that the pRB-family proteins can modulate the activity of factors which specifically control cell fate and/or differentiation as well as controlling cell cycle regulators.

Activity of the retinoblastoma family proteins, pRB, p107, and p130, during cellular proliferation and differentiation

Genetic evidence from retinoblastoma patients and experiments describing the mechanism of cellular transformation by the DNA tumor viruses have defined a central role for the retinoblastoma protein (pRB) family of tumor suppressors in the normal regulation of the eukaryotic cell cycle. These proteins, pRB, p107, and p130, act in a cell cycle-dependent manner to regulate the activity of a number of important cellular transcription factors, such as the E2F-family, which in turn regulate expression of genes whose products are important for cell cycle progression. In addition, inhibition of E2F activity by the pRB family proteins is required for cell cycle exit after terminal differentiation or nutrient depletion. The loss of functional pRB, due to mutation of both RB1 alleles, results in deregulated E2F activity and a predisposition to specific malignancies. Similarly, inactivation of the pRB family by the transforming proteins of the DNA tumor viruses overcomes cellular quiescence and prevents terminal differentiation by blocking the interaction of pRB, p107, and p130 with the E2F proteins, leading to cell cycle progression and, ultimately, cellular transformation. Together these two lines of evidence implicate the pRB family of negative cell cycle regulators and the E2F family of transcription factors as central components in the cell cycle machinery.