Ectopic Neurite Growth Research Articles

Distinct effects of tubulin isotype mutations on neurite growth in Caenorhabditis elegans.

Tubulins, the building block of microtubules (MTs), play a critical role in both supporting and regulating neurite growth. Eukaryotic genomes contain multiple tubulin isotypes, and their missense mutations cause a range of neurodevelopmental defects. Using the Caenorhabditis elegans touch receptor neurons, we analyzed the effects of 67 tubulin missense mutations on neurite growth. Three types of mutations emerged: 1) loss-of-function mutations, which cause mild defects in neurite growth; 2) antimorphic mutations, which map to the GTP binding site and intradimer and interdimer interfaces, significantly reduce MT stability, and cause severe neurite growth defects; and 3) neomorphic mutations, which map to the exterior surface, increase MT stability, and cause ectopic neurite growth. Structure-function analysis reveals a causal relationship between tubulin structure and MT stability. This stability affects neuronal morphogenesis. As part of this analysis, we engineered several disease-associated human tubulin mutations into C. elegans genes and examined their impact on neuronal development at the cellular level. We also discovered an α-tubulin (TBA-7) that appears to destabilize MTs. Loss of TBA-7 led to the formation of hyperstable MTs and the generation of ectopic neurites; the lack of potential sites for polyamination and polyglutamination on TBA-7 may be responsible for this destabilization.

Chronic neurotrophin delivery promotes ectopic neurite growth from the spiral ganglion of deafened cochleae without compromising the spatial selectivity of cochlear implants

Cochlear implants restore hearing cues in the severe-profoundly deaf by electrically stimulating spiral ganglion neurons (SGNs). However, SGNs degenerate following loss of cochlear hair cells, due at least in part to a reduction in the endogenous neurotrophin (NT) supply, normally provided by hair cells and supporting cells of the organ of Corti. Delivering exogenous NTs to the cochlea can rescue SGNs from degeneration and can also promote the ectopic growth of SGN neurites. This resprouting may disrupt the cochleotopic organization upon which cochlear implants rely to impart pitch cues. Using retrograde labeling and confocal imaging of SGNs, we determined the extent of neurite growth following 28 days of exogenous NT treatment in deafened guinea pigs with and without chronic electrical stimulation (ES). On completion of this treatment, we measured the spread of neural activation to intracochlear ES by recording neural responses across the cochleotopically organized inferior colliculus using multichannel recording techniques. Although NT treatment significantly increased both the length and the lateral extent of growth of neurites along the cochlea compared with deafened controls, these anatomical changes did not affect the spread of neural activation when examined immediately after 28 days of NT treatment. NT treatment did, however, result in lower excitation thresholds compared with deafened controls. These data support the application of NTs for improved clinical outcomes for cochlear implant patients.

Distribution of ectopic neurite growth and other geometrical distortions of CNS neurons in feline GM2 gangliosidosis

Golgi and combined Golgi-electron microscopic (EM) studies were carried out on cats in the terminal stages of GM2 ganglioside storage disease and the resulting data were compared with those from similar studies other neuronal storage diseases in cats, including GM1 gangliosidosis. The results support the view that only limited types of neurons affected by the lysosomal hydrolase deficiency and subsequent intracellular storage have the capacity to sprout new dendritic-like growth processes from their axon hillocks, and that these neurons are essentially the same in all of these diseases studied to date. Golgi studies of CNS tissues from GM2 gangliosidosis cats revealed ectopic neurite growth on pyramidal neurons of cerebral cortex and multipolar cells of amygdala and claustrum, whereas other types of neurons responded to the metabolic defect with aspiny meganeurite formation or somatic enlargement, or appeared normal in terms of soma-dendritic morphology. Combined Golgi-EM studies of cortical pyramidal neurons revealed that ectopic, axon hillock neurites commonly possessed asymmetrical synapses which were similar to those observed in other storage disorders.

Initiation and growth of ectopic neurites and meganeurites during postnatal cortical development in ganglioside storage disease

The incidence of cortical pyramidal neurons displaying meganeurites or enlarged axon hillocks with ectopic spines and neurites was evaluated developmentally using feline models of GM1 and GM2 gangliosidosis. Results of these studies demonstrated that the onset of ectopic neurite growth occurred after the elaboration of dendrites on cortical pyramidal neurons, and that the time of onset of this renewed dendritogenesis was similar in the two diseases. Initiation and growth of ectopic neurites also correlated in a general way with onset and progression of clinical deterioration in both diseases. In GM1 gangliosidosis there was a greater tendency toward formation of meganeurites, whereas in cats with GM2 gangliosidosis the growth of ectopic axon hillock neurites without meganeurites predominated. At end-stage disease in GM2 gangliosidosis, nearly 90% of pyramidal cells displayed some degree of axon hillock neurite growth as opposed to less than half this number for GM1 gangliosidosis cats at the same age. These data are consistent with the hypothesis that there are two separate driving forces behind these somadendritic abnormalities of pyramidal neurons in the gangliosidoses. Excessive intraneuronal accumulation of storage vacuoles accounts for the formation of meganeurites, whereas some type of intrinsic metabolic defect results in axon hillock neurite growth which in turn offers new surface area for synaptic input. Currently available data indicate that GM2 or GM3 ganglioside, or a closely related metabolic product other than GM1 ganglioside, may be primarily associated with the growth of ectopic dendritic processes on morphologically mature neurons in storage diseases.

Alterations in neuron morphology in mucopolysaccharidosis type I

Morphological changes in neurons with inborn defects of the lysosomal hydrolase, alpha-L-iduronidase, and with concomitant storage of glycosaminoglycans, were evaluated by Golgi staining in two animal models and compared to a similar study of a child with the same disease. Cortical pyramidal neurons in feline mucopolysaccharidosis type I often displayed axon hillock enlargements (meganeurites) and/or ectopic, secondary neuritic processes sprouting from this same region of the cell. The latter structures were prominent and often appeared longer than similar neurites reported in other neuronal storage diseases. Although most meganeurites were aspiny, a few were observed which possessed spine-like processes or neurites. Other than these morphological changes in cortical pyramidal neurons, few other cell types displayed abnormalities demonstrable by Golgi impregnation. In the canine model of this disorder, abnormal Golgi-impregnated cortical neurons resembled more closely those seen in human mucopolysaccharidosis. That is, they possessed meganeurites which typically were aspiny in appearance. Ectopic neurite growth was not observed on any Golgi-impregnated neurons in the cases of canine or human mucopolysaccharidosis used in this study. The latter finding, given the advanced ages of these cases, is consistent with the view that ectopic neuritogenesis seen in neuronal storage diseases may be subject to a developmental window, albeit one open well beyond the period of early postnatal maturation.

Further studies on ectopic dendrite growth and other geometrical distortions of neurons in feline GMl gangliosidosis

Systematic Golgi studies have been performed on major subcortical, diencephalic, brain stem and spinal cord regions from cats with the inherited neuronal storage disease, GM1 gangliosidosis. Resulting data have been compared with other Golgi studies of neuronal storage disorders in man and animals, including an earlier, more limited examination of this same disease model. These previous studies have shown that in human and feline gangliosidoses cortical pyramidal neurons undergo remarkable changes in soma-dendritic geometry. The latter include the formation of conspicuous cellular enlargements between somata and axonal initial segments (meganeurites) and the sprouting of secondary neuritic processes from this same region of the cell. Further, ultrastructural studies have revealed normal appearing synapses on the surface of this ectopically placed dendritic-like membrane. Results of the present study indicate that the distribution of meganeurites, secondary neurites and other geometrical distortions of neurons in GM1 gangliosidosis varies with cell type and brain region. This cell type-specific response to the metabolic error and subsequent storage could be categorized in three ways. Firstly, certain types of cells (e.g. multipolar neurons of the amygdala and claustrum) exhibited changes similar to those reported for cortical pyramidal neurons. That is, cells of these regions either displayed spine or neurite-bearing meganeurites, or enlarged axon hillocks which were covered with similar processes. Other types of neurons did not demonstrate ectopic neurite growth or spine-covered meganeurites, but did display prominent aspiny meganeurites (e.g. neurons of the superior colliculus, periaqueductal gray, hypothalamus and basal forebrain nuclei). A third category of neurons did not possess meganeurites or neurite growth but instead demonstrated massive somatic expansion which exceeded that observed in meganeurite-bearing cell types (e.g. certain brain stem and spinal cord neurons). These data have been compared with the more limited Golgi studies of other types of neuronal storage disorders and the same types of neurons appeared to respond in similar fashion across this spectrum of diseases. The data presented and discussed in this paper demonstrate three significant morphological events which occur in neurons as a result of lysosomal hydrolase deficiency. These are (i) storage, which occurs in all neurons but manifests as meganeurite formation or somatic enlargement depending on the cell type, (ii) axon hillock or meganeurite-associated spine and neurite growth, and (iii) new synapse formation on spine-covered meganeurites and on neurites. The independent occurrence of meganeurites and neurites is not consistent with the view that these morphological changes are simply different manifestations of a single defect in the regulation of neuronal surface membrane production, as has been suggested previously. Rather, this study offers the view that axon hillock neurites and meganeurite-associated neurites and spines are the morphological hallmark of increased axon hillock-asociated synapse formation, whereas meganeurites themselves simply manifest the storage process. Possible factors controlling the expression of these features of neurons are discussed.

Locoweed-induced neuronal storage disease characterized by meganeurite formation

Golgi staining was performed on cerebral cortex and thalamus of adult animals chronically intoxicated with an α-mannosidase inhibitor found in locoweed ( Astragalus lentiginosus). The widespread occurrence of large, aspiny meganeurites was discovered on cortical pyramidal and thalamic principal neurons but aberrant spines and neurite growth were not observed. Ectopic neurite growth is known to be characteristics of α-mannosidosis of early onset in inherited and induced feline models. The absence of neuritogenesis in a storage disease known to be so characterized when induced in younger animals suggests that this unusual phenomenon is in some way linked to normal developmental processes associated with brain maturation.