Joe Howard, PhD

Motor and Cytoskeletal Systems: From molecules to cells

Tubulin exchange in and out of the microtubule wall and its role in severing, rigidity & dynamics
Microtubules are tubular polymers whose protein subunits, tubulin, associate in a head-to-tail geometry to form protofilaments, thirteen of which form the microtubule’s cylindrical wall. The pipe-like geometry gives the microtubule high rigidity for its protein mass. High bending rigidity is essential for the structural roles that microtubules play in cellular architecture: as tracks for motor proteins such as kinesins and dyneins, and as scaffolds that support force-generating organelles such as the mitotic spindle and the cilium (Howard 2001).

Microtubules grow and shrink by addition and subtraction of tubulin dimers at their ends, processes that are regulated by a host a microtubule associated proteins (Howard and Hyman 2007, Bowne-Anderson et al. 2015). Recently, however, it has become clear that, in addition to removal and addition of tubulin at microtubule ends, significant tubulin exchange also occurs within the wall of the microtubule. Removal can be mediated by microtubule severing enzymes such as spastin and katanin (Kuo & Howard 2021, Kuo et al. 2022), by motor proteins such as kinesins and dyneins, and by mechanical forces applied to the microtubule. Removal of tubulin from the microtubule lattice leads to holes, whose enlargement leads to microtubule softening and eventual breakage, and whose repair by incorporation of new GTP-tubulin from solution can promote microtubule growth. Together, the growth and repair of these defects can profoundly rearrange the microtubule cytoskeleton in cells.

We are developing new techniques for visualizing microtubule defects and to study the kinetic and structural mechanisms of microtubule severing.

Branching morphogenesis of neurons
The architecture of the brain and its constituent neurons is staggeringly complex. This complexity is enabled by the highly branched morphologies of dendrites and axons, which allow each neuron to connect to thousands of other neurons. We recently showed, using Drosophila sensory neurons as a model system, that the branching, growth, and retraction of dendrite tips can generate many of the morphological features of dendrites including the rate of growth of their arbors during development, and the average length, density, and orientation of their branches (Shree et al. 2022, Ouyang et al. in preparation). Branch diameters, another important morphological feature of neurons, change systematically across branch points, which facilitates the distribution of materials and nutrients through the network (Liao et al. 2019). Furthermore, neuronal dendrites have a scale-invariant network architecture that optimizes their function and metabolism (Liao et al. 2023).

Currently, we are using genetic perturbations and high-resolution imaging to elucidate the role of the microtubule cytoskeleton in generating dendrite morphology.

The motility of cilia and flagella
A major open question in cell motility is how the dynein motors, which power the bending of cilia and flagella, are coordinated to give the periodic beating patterns that drive cell motion (Howard et al. 2022). We are using the single-celled alga Chlamydomonas reinhardtii as a model system to test different models of motor coupling (Geyer et al. 2016, Sartori et al. 2016, Geyer 2022).

Currently, we are analyzing waveforms of different mutants by high-speed light microscopy and high-resolution electron cryo-microscopy.