Abnormal neurofilament transport caused by targeted disruption of neuronal kinesin heavy chain KIF5A

Arrival, reversal, and departure of neurofilaments at the tips of growing axons

We have investigated the movement of green fluorescent protein-tagged neurofilaments at the distal ends of growing axons by using time-lapse fluorescence imaging. The filaments moved in a rapid, infrequent, and asynchronous manner in either an anterograde or retrograde direction (60% anterograde, 40% retrograde). Most of the anterograde filaments entered the growth cone and most of the retrograde filaments originated in the growth cone. In a small number of cases we were able to observe neurofilaments reverse direction, and all of these reversals occurred in or close to the growth cone. We conclude that neurofilament polymers are delivered rapidly and infrequently to the tips of growing axons and that some of these polymers reverse direction in the growth cone and move back into the axon. We propose that 1) growth cones are a preferential site of neurofilament reversal in distal axons, 2) most retrograde neurofilaments in distal axons originate by reversal of anterograde filaments in the growth cone, 3) those anterograde filaments that do not reverse direction are recruited to form the neurofilament cytoskeleton of the newly forming axon, and 4) the net delivery of neurofilament polymers to growth cones may be controlled by regulating the reversal frequency.

Intermediate filaments are dynamic and motile elements of cellular architecture

Recent evidence showing that intermediate filaments (IFs) are dynamic, motile elements of the cytoskeletal repertoire of vertebrate cells has overturned the long-standing view that they simply form static 'space filling' cytoplasmic networks. In fact, many types of IF are now known to engage in a remarkable array of movements that are closely associated with their assembly, disassembly and subcellular organization. Some of these motile properties are intrinsic to IFs and others are attributable to molecular crosstalk with either microtubules or actin-containing microfilaments. This crosstalk is, to a large extent, mediated by molecular motors, including conventional kinesin and cytoplasmic dynein. These motors are responsible for the high-speed delivery of nonfilamentous IF precursors and short filaments to specific regions of the cytoplasm, where they assemble into long IFs. Interestingly, the patterns and speeds of IF movements vary in different cell types and even within different regions of the same cell. These differences in motility may be related to their interactions with different types of molecular motor and/or other factors, such as IF-associated proteins.

Neurofilament subunits undergo more rapid translocation within retinas than in optic axons

Axonal transport of neurofilaments (NFs) has long been considered to be regulated by phosphorylation, although recent studies have challenged this hypothesis. Our prior analyses of axonal transport in optic axons demonstrated two distinct NF transport rates that spatially and temporally correlated with changes in NF phosphorylation. In our prior studies, we focused on subunits already within axons. Re-examination of these data using additional approaches and examining additional earlier time points have allowed us to calculate rates at which subunits transport out of retinas and into optic axons. NF subunits were radiolabeled by intravitreal injection of 35S-methionine. NF axonal transport was monitored by following the location of the front of radiolabeled subunits immunoprecipitated from retinas and segments of optic axons, which demonstrated four distinct transport rates. Subunits within retinas exhibited the fastest rate, and underwent a 50% slowing upon exiting the retina and entering optic axons. While this slowing could be due to a regional caliber increase and/or regional increase in NF phosphorylation within the first segment, prior studies indicated that inhibition of phosphatase activities increased NF phosphorylation within retinas and slowed NF subunit exit from retinas to a degree similar to that normally observed within the first segment of axons, suggesting that regional phosphorylation played a major role in slowing of NF transport following their exit from the retina. These findings provide additional support for the notion that phosphorylation regulates NF axonal transport.

The dynamic and motile properties of intermediate filaments

For many years, cytoplasmic intermediate filaments (IFs) were considered to be stable cytoskeletal elements contributing primarily to the maintenance of the structural and mechanical integrity of cells. However, recent studies of living cells have revealed that IFs and their precursors possess a remarkably wide array of dynamic and motile properties. These properties are in large part due to interactions with molecular motors such as conventional kinesin, cytoplasmic dynein, and myosin. The association between IFs and motors appears to account for much of the well-documented molecular cross talk between IFs and the other major cytoskeletal elements, microtubules, and actin-containing microfilaments. Furthermore, the associations with molecular motors are also responsible for the high-speed, targeted delivery of nonfilamentous IF protein cargo to specific regions of the cytoplasm where they polymerize into IFs. This review considers the functional implications of the motile properties of IFs and discusses the potential relationships between malfunctions in these motile activities and human diseases.

A Novel Actin-bundling Kinesin-related Protein from Dictyostelium discoideum

Actin filaments and microtubules are two major cytoskeletal systems involved in wide cellular processes, and the organizations of their filamentous networks are regulated by a large number of associated proteins. Recently, evidence has accumulated for the functional cooperation between the two filament systems via associated proteins. However, little is known about the interactions of the kinesin superfamily proteins, a class of microtubule-based motor proteins, with actin filaments. Here, we describe the identification and characterization of a novel kinesin-related protein named DdKin5 from Dictyostelium. DdKin5 consists of an N-terminal conserved motor domain, a central stalk region, and a C-terminal tail domain. The motor domain showed binding to microtubules in an ATP-dependent manner that is characteristic of kinesin-related proteins. We found that the C-terminal tail domain directly interacts with actin filaments and bundles them in vitro. Immunofluorescence studies showed that DdKin5 is specifically enriched at the actin-rich surface protrusions in cells. Overexpression of the DdKin5 protein affected the organization of actin filaments in cells. We propose that a kinesin-related protein, DdKin5, is a novel actin-bundling protein and a potential cross-linker of actin filaments and microtubules associated with specific actin-based structures in Dictyostelium.

Morphological and biochemical changes of neurofilaments in aged rat sciatic nerve axons

We have made a detailed comparison of neurofilaments (NFs) in the axons of the sciatic nerves between young and aged rats. In young rats, NF density was similar between proximal and distal sciatic nerve, but it became higher in the proximal region of sciatic nerve of aged rats. In accordance with this morphological change, NF protein content decreased dramatically in the middle region of the sciatic nerves of aged rats. The ratio of NF-M to NF-H in aged rats was lower than that in young rats at the proximal region of sciatic nerves and further decreased in the distal region of sciatic nerve. We analyzed transcription and axonal transport of NF proteins in motor neurons in spinal cord which are the major constituents of sciatic nerve axons. Of the transcripts of the NF subunits, NF-M mRNA was particularly reduced in aged rats. Examination of slow axonal transport revealed that the transport rate for NF-M was slightly faster than that for NF-H in young rats, but slightly slower in aged rats. A decrease in both the synthesis and transport rate of NF-M with aging may contribute to the relative reduction in NF-M in the aged rat sciatic nerve. Although the relationship between NF packing and reduced NF-M is not clear at present, these changes in NFs may be associated with age-dependent axonal degeneration diseases.

The neurofilament middle molecular mass subunit carboxyl-terminal tail domains is essential for the radial growth and cytoskeletal architecture of axons but not for regulating neurofilament transport rate

The phosphorylated carboxyl-terminal "tail" domains of the neurofilament (NF) subunits, NF heavy (NF-H) and NF medium (NF-M) subunits, have been proposed to regulate axon radial growth, neurofilament spacing, and neurofilament transport rate, but direct in vivo evidence is lacking. Because deletion of the tail domain of NF-H did not alter these axonal properties (Rao, M.V., M.L. Garcia, Y. Miyazaki, T. Gotow, A. Yuan, S. Mattina, C.M. Ward, N.S. Calcutt, Y. Uchiyama, R.A. Nixon, and D.W. Cleveland. 2002. J. Cell Biol. 158:681-693), we investigated possible functions of the NF-M tail domain by constructing NF-M tail-deleted (NF-MtailDelta) mutant mice using an embryonic stem cell-mediated "gene knockin" approach that preserves normal ratios of the three neurofilament subunits. Mutant NF-MtailDelta mice exhibited severely inhibited radial growth of both motor and sensory axons. Caliber reduction was accompanied by reduced spacing between neurofilaments and loss of long cross-bridges with no change in neurofilament protein content. These observations define distinctive functions of the NF-M tail in regulating axon caliber by modulating the organization of the neurofilament network within axons. Surprisingly, the average rate of axonal transport of neurofilaments was unaltered despite these substantial effects on axon morphology. These results demonstrate that NF-M tail-mediated interactions of neurofilaments, independent of NF transport rate, are critical determinants of the size and cytoskeletal architecture of axons, and are mediated, in part, by the highly phosphorylated tail domain of NF-M.

Do disorders of movement cause movement disorders and dementia?

Neurons require long-distance microtubule-based transport systems to ferry vital cellular cargoes and signals between cell bodies and axonal or dendritic terminals. Considerable progress has been made on developing a molecular understanding of these processes and how they are integrated into normal neuronal functions. Recent work also suggests that these transport systems may fail early in the pathogenesis of a number of neurodegenerative diseases.

Neurotrophin-3 prevents the proximal accumulation of neurofilament proteins in sensory neurons of streptozocin-induced diabetic rats

The relation between neurofilament expression and/or phosphorylation in the proximal versus distal components of the sensory peripheral neuraxis was studied and related to disorders in structure and function of the distal axon of streptozocin (STZ)-induced diabetic rats studied for 14 weeks. The ability of neurotrophin-3 (NT-3) to prevent abnormalities in neurofilament biology was also investigated. Compared with age-matched controls, neurofilament heavy (NF-H) (3.3-fold) and neurofilament medium (NF-M) (2.5-fold), but not neurofilament light (NF-L), subunits accumulated in the proximal axon of sensory neurons of the lumbar dorsal root ganglia (DRG) in untreated diabetic rats. Neurofilament accumulation was prevented by NT-3. Small- and large-diameter sensory neurons exhibited elevated levels of NF-H protein accumulation and phosphorylation in the DRG of untreated diabetic rats, levels that were ameliorated by NT-3. The sural nerve of untreated diabetic rats showed a 50% decrease in the levels of NF-H and NF-M, but not NF-L, subunits; NT-3 only partially normalized the defect in NF-M expression. These observations were associated with significant lowering of motor and sensory nerve conduction velocity but no alteration in the mean axonal diameter of myelinated axons in the sural nerve in untreated diabetic rats. It is proposed that the accumulation of NF-H and NF-M subunits in the proximal axon is an etiologic factor in the distal axon degeneration observed in diabetes.