Myosin V is an unconventional myosin proposed to be processive on actin filaments, analogous to kinesin on a microtubule [Mehta, A. D., et al. (1999) Nature (London) 400, 590 -593]. To ascertain the unique properties of myosin V that permit processivity, we undertook a detailed kinetic analysis of the myosin V motor. We expressed a truncated, single-headed myosin V construct that bound a single light chain to study its innate kinetics, free from constraints imposed by other regions of the molecule. The data demonstrate that unlike any previously characterized myosin a single-headed myosin V spends most of its kinetic cycle (>70%) strongly bound to actin in the presence of ATP. This kinetic tuning is accomplished by increasing several of the rates preceding strong binding to actin and concomitantly prolonging the duration of the strongly bound state by slowing the rate of ADP release. The net result is a myosin unlike any previously characterized, in that ADP release is the rate-limiting step for the actin-activated ATPase cycle. Thus, because of a number of kinetic adaptations, myosin V is tuned for processive movement on actin and will be capable of transporting cargo at lower motor densities than any other characterized myosin. Myosin V was identified in chicken brain cytoplasmic extracts as a calmodulin binding protein with actin-activated MgATPase activity and the ability to translocate actin filaments (1-3). By electron microscopy of rotary shadowed images, native chicken brain myosin V has two heads and two heavy chains that associate through a long coiled-coil domain in its tail region. Unlike myosin II, myosin V does not form filaments and is believed to act as a single molecule (4).The unusual structure, cellular functions (5, 6), and steady-state biochemical properties (7), as well as single molecule mechanics (4) of myosin V support it being a processive motor. That is, for each diffusional encounter, a single (two-headed) myosin V molecule may be capable of going through multiple ATPase cycles and traveling long distances, equivalent to many individual steps of the motor, along its actin track before dissociating.The myosin molecule, whether in the sarcomere of a muscle (myosin II) or moving vesicles on actin tracks (myosin V), goes through a characteristic cyclic interaction with actin (Scheme 1; predominant pathway is shown in bold). The key steps include the rapid binding of ATP to actin-bound myosin, the hydrolysis of ATP, the sequential release of phosphate (P i ) and ADP, and the rebinding of ATP. During the cycle, the myosin populates either the weak-binding states or strong-binding states (Scheme 1). Weakbinding myosin states dynamically detach and rebind to actin with a low affinity (K d Ͼ 1 M), whereas the strong-binding myosin states remain bound to actin with a high affinity (K d Ͻ Ͻ 1 M). Mechanical force generation, work, and directed movement on actin are possible only during periods when the myosin is strongly bound to actin. The fraction of the ATPase cycle that the myosin spends in ...

Temozolomide demonstrated good single-agent activity, an acceptable safety profile, and documented HQL benefits in patients with recurrent AA.

Kinesin-1 is an ATP-driven, processive motor that transports cargo along microtubules in a tightly regulated stepping cycle. Efficient gating mechanisms ensure that the sequence of kinetic events proceeds in proper order, generating a large number of successive reaction cycles. To study gating, we created two mutant constructs with extended neck-linkers and measured their properties using single-molecule optical trapping and ensemble fluorescence techniques. Due to a reduction in the inter-head tension, the constructs access an otherwise rarely populated conformational state where both motor heads remain bound to the microtubule. ATP-dependent, processive backstepping and futile hydrolysis were observed under moderate hindering loads. Based on measurements, we formulated a comprehensive model for kinesin motion that incorporates reaction pathways for both forward and backward stepping. In addition to inter-head tension, we find that neck-linker orientation is also responsible for ensuring gating in kinesin.

Cell migration, which is central to many biological processes including wound healing and cancer progression, is sensitive to environmental stiffness, and many cell types exhibit a stiffness optimum, at which migration is maximal. Here we present a cell migration simulator that predicts a stiffness optimum that can be shifted by altering the number of active molecular motors and clutches. This prediction is verified experimentally by comparing cell traction and F-actin retrograde flow for two cell types with differing amounts of active motors and clutches: embryonic chick forebrain neurons (ECFNs; optimum ∼1 kPa) and U251 glioma cells (optimum ∼100 kPa). In addition, the model predicts, and experiments confirm, that the stiffness optimum of U251 glioma cell migration, morphology and F-actin retrograde flow rate can be shifted to lower stiffness by simultaneous drug inhibition of myosin II motors and integrin-mediated adhesions.

The ability of gliomas to invade the brain limits the efficacy of standard therapies. In this study, we have examined glioma migration in living brain tissue by using two novel in vivo model systems. Within the brain, glioma cells migrate like nontransformed, neural progenitor cells-extending a prominent leading cytoplasmic process followed by a burst of forward movement by the cell body that requires myosin II. In contrast, on a two-dimensional surface, glioma cells migrate more like fibroblasts, and they do not require myosin II to move. To explain this phenomenon, we studied glioma migration through a series of synthetic membranes with defined pore sizes. Our results demonstrate that the A and B isoforms of myosin II are specifically required when a glioma cell has to squeeze through pores smaller than its nuclear diameter. They support a model in which the neural progenitor-like mode of glioma invasion and the requirement for myosin II represent an adaptation needed to move within the brain, which has a submicrometer effective pore size. Furthermore, the absolute requirement for myosin II in brain invasion underscores the importance of this molecular motor as a potential target for new anti-invasive therapies to treat malignant brain tumors. INTRODUCTIONMalignant gliomas are a group of primary brain tumors that have remained resistant to therapy and that have a dismal prognosis (Buckner et al., 2007;Stupp et al., 2007). Part of the reason for this state of affairs is that although gliomas rarely metastasize outside the CNS, they are capable of spreading long distances within the brain (Sherer, 1940;Burger and Kleihues, 1989;Hoelzinger et al., 2007). This invasive behavior limits the effectiveness of local therapies and contributes to the high mortality rate seen in these tumors (Lim et al., 2007). Preventing glioma invasion therefore has the potential to convert this highly malignant tumor into a focal disease, which could then be effectively treated with focal therapies, such as radiation and surgery. Realizing this outcome, however, will require a detailed understanding how gliomas invade normal brain.Gliomas typically invade the brain by migrating long distances through white matter tracts and by infiltrating cortex and subcortical gray matter structures. Brain parenchyma (neuropil) is composed of tightly packed neuronal and glial processes, and it is characterized by extracellular spaces that are in the submicrometer range (Thorne and Nicholson, 2006). This environment therefore represents a particular mechanical challenge to motile cells, such as gliomas, that need to insinuate themselves through a highly constraining brain matrix to migrate. Unfortunately, there is very little information about how glioma cells accomplish this process in vivo. Some insight is provided by studies of embryonic and early postnatal brain, where neural progenitor cells migrate along radial glial cells, and to some extent through the white matter and cortex before stopping and differentiating into mature neurons and glia. Ti...