Research / Clinical Summary

Don Cleveland, PhD Professor, Medicine / Cellular & Molecular Medicine Cancer Genetics Program Contact by Email

This laboratory is focused on two general areas: 1) how spindles are assembled and chromosomes faithfully moved into each daughter cell just prior to division, and  2) the molecular genetics of axonal growth and mechanisms of human motor neurons disease, especially the disease familiarly known as Lou Gehrig’s disease, or ALS.

Microtubules, Motors and Mitosis: Dissecting the Principles of Mitotic Spindle Assembly and Chromosome Movement

One key interest is deciphering the principles involved in moving chromosomes during mitosis.  Our primary emphasis has been on identifying components of mammalian centromeres, especially on components of the kinetochores, the structures to which microtubules attach and that contain the motor molecules for translocation of chromosomes along spindle microtubules.  Our initial identification of CENP-E (centromere protein E) showed it to be a candidate motor for powering chromosome movement during mitosis.  Using Xenopus egg extracts that are capable of reproducing the cell cycle in vitro, including spindle assembly and faithful chromosome segregation, we have demonstrated that CENP-E is essential for initial chromosome alignment.  Moreover, injection of antibodies to inhibit CENP-E function in mouse eggs has demonstrated that disruption of CENP-E function leads to a failure to progress past first meiotic metaphase.  Collectively, this demonstrates that CENP-E provides an important function in powering chromosome movement both in mammalian mitosis and meiosis.

Presently, a convergence of genetics, cell biology and biochemistry has focused attention on kinetochores as the source of cell cycle signals that work as checkpoints to block cell cycle advance until all chromosomes have successfully attached to spindle microtubules.  With demonstration by immunoelectron microscopy that CENP-E is one of the components that directly link kinetochores to microtubules, this places CENP-E as a central player in this signaling cascade. Current efforts, using antisense methods to eliminate CENP-E accumulation in cultured cells, and using gene disruption methods to examine the in vivo consequence of loss of CENP-E function in mice, as well as similar approaches to determining the functional properties of companion kinetochore components (including CENP-F, zw10, MAD2, BUB1 and BUB3), are underway to dissect this cell cycle control pathway.

An additional focus is on how the mitotic spindle poles nucleate and organize spindle microtubules. Initially, our focus was on the protein NuMA, an abundant nuclear protein during interphase, but one which redistributes to the spindle poles during early mitosis.  Immunodepletion of NuMA from frog egg extracts that can faithfully reproduce the cell cycle in vitro revealed that NuMA is one central mitotic cargo for the motor protein dynein and that bi-functional complexes of NuMA and dynein/dynactin are the essential components for tightly tethering spindle microtubules at the poles and tethering the poles to the spindle.

Mechanisms of Neuronal Growth and Death: Molecular Genetics of Axonal Growth and Motor Neuron Disease

Unlike most eukaryotic cells, an intrinsic feature of neurons is their extreme asymmetry.  For example, in the human peripheral nervous system single motor neurons extend over a meter in length.  Asymmetry is achieved in two phases.  The first is when a neurite extends toward its target.  After stable synapse formation, a second phase initiates in which the axon grows up to ten fold in diameter.  This radial growth phase, concomitant with myelination and essential for establishment of proper conduction velocity, yields an enormous increase in axonal volume and a huge cell, 99.9% of which is in the axon.  We have focused part of our effort on using molecular genetics and transgenic mice to test how radial growth of axons is achieved.  Already we have shown that neurofilaments, the most abundant structural element in axons, are essential for radial growth.  By deleting each of the three neurofilament genes or by over expressing them in transgenic mice, we are now examining the molecular principles that underlie neurofilament-dependent axonal growth.

Beyond this, essentially all human motor neuron disorders are characterized by the maldistribution of neurofilaments, a finding clearly suggesting that they may play an essential role in disease pathogenesis.  This is particularly true in the most prominent motor neuron disease, amyotrophic lateral sclerosis, or ALS, which is characterized by selective death of motor neurons.  By producing transgenic mice expressing mutations in neurofilaments, we have proven that such mutations can cause ALS in mice and we are now searching for the presence of similar mutations in human patients.  The cause of only 1.5% of human ALS is known and this is point mutations in an enzyme superoxide dismutase.  By expression of these mutations in mice, we have proven that disease arises from a toxic property of the mutant proteins and we are now looking for what that property is and what is the cascade of events leading to selective motor neuron death.

Keywords: mitosis, aneuploidy, mitotic checkpoint, centromeres

Update Summary via ONcLINE (password required)
Click here to request a new or forgotten password