Why do axons differ in caliber? Perge, János A; Niven, Jeremy E; Mugnaini, Enrico ...
The Journal of neuroscience,
2012-Jan-11, 2012-01-11, 20120111, Letnik:
32, Številka:
2
Journal Article
Recenzirano
Odprti dostop
CNS axons differ in diameter (d) by nearly 100-fold (∼0.1-10 μm); therefore, they differ in cross-sectional area (d(2)) and volume by nearly 10,000-fold. If, as found for optic nerve, mitochondrial ...volume fraction is constant with axon diameter, energy capacity would rise with axon volume, also as d(2). We asked, given constraints on space and energy, what functional requirements set an axon's diameter? Surveying 16 fiber groups spanning nearly the full range of diameters in five species (guinea pig, rat, monkey, locust, octopus), we found the following: (1) thin axons are most numerous; (2) mean firing frequencies, estimated for nine of the identified axon classes, are low for thin fibers and high for thick ones, ranging from ∼1 to >100 Hz; (3) a tract's distribution of fiber diameters, whether narrow or broad, and whether symmetric or skewed, reflects heterogeneity of information rates conveyed by its individual fibers; and (4) mitochondrial volume/axon length rises ≥d(2). To explain the pressure toward thin diameters, we note an established law of diminishing returns: an axon, to double its information rate, must more than double its firing rate. Since diameter is apparently linear with firing rate, doubling information rate would more than quadruple an axon's volume and energy use. Thicker axons may be needed to encode features that cannot be efficiently decoded if their information is spread over several low-rate channels. Thus, information rate may be the main variable that sets axon caliber, with axons constrained to deliver information at the lowest acceptable rate.
Amyotrophic lateral sclerosis (ALS) is a paralytic and usually fatal disorder caused by motor-neuron degeneration in the brain and spinal cord. Most cases of ALS are sporadic but about 5-10% are ...familial. Mutations in superoxide dismutase 1 (SOD1), TAR DNA-binding protein (TARDBP, also known as TDP43) and fused in sarcoma (FUS, also known as translocated in liposarcoma (TLS)) account for approximately 30% of classic familial ALS. Mutations in several other genes have also been reported as rare causes of ALS or ALS-like syndromes. The causes of the remaining cases of familial ALS and of the vast majority of sporadic ALS are unknown. Despite extensive studies of previously identified ALS-causing genes, the pathogenic mechanism underlying motor-neuron degeneration in ALS remains largely obscure. Dementia, usually of the frontotemporal lobar type, may occur in some ALS cases. It is unclear whether ALS and dementia share common aetiology and pathogenesis in ALS/dementia. Here we show that mutations in UBQLN2, which encodes the ubiquitin-like protein ubiquilin 2, cause dominantly inherited, chromosome-X-linked ALS and ALS/dementia. We describe novel ubiquilin 2 pathology in the spinal cords of ALS cases and in the brains of ALS/dementia cases with or without UBQLN2 mutations. Ubiquilin 2 is a member of the ubiquilin family, which regulates the degradation of ubiquitinated proteins. Functional analysis showed that mutations in UBQLN2 lead to an impairment of protein degradation. Therefore, our findings link abnormalities in ubiquilin 2 to defects in the protein degradation pathway, abnormal protein aggregation and neurodegeneration, indicating a common pathogenic mechanism that can be exploited for therapeutic intervention.
Objective
Amyotrophic lateral sclerosis (ALS) is a fatal disorder of motor neuron degeneration. Most cases of ALS are sporadic (SALS), but about 5 to 10% of ALS cases are familial (FALS). Recent ...studies have shown that mutations in FUS are causal in approximately 4 to 5% of FALS and some apparent SALS cases. The pathogenic mechanism of the mutant FUS‐mediated ALS and potential roles of FUS in non‐FUS ALS remain to be investigated.
Methods
Immunostaining was performed on postmortem spinal cords from 78 ALS cases, including SALS (n = 52), ALS with dementia (ALS/dementia, n = 10), and FALS (n = 16). In addition, postmortem brains or spinal cords from 22 cases with or without frontotemporal lobar degeneration were also studied. In total, 100 cases were studied.
Results
FUS‐immunoreactive inclusions were observed in spinal anterior horn neurons in all SALS and FALS cases, except for those with SOD1 mutations. The FUS‐containing inclusions were also immunoreactive with antibodies to TDP43, p62, and ubiquitin. A fraction of tested FUS antibodies recognized FUS inclusions, and specific antigen retrieval protocol appeared to be important for detection of the skein‐like FUS inclusions.
Interpretation
Although mutations in FUS account for only a small fraction of FALS and SALS, our data suggest that FUS protein may be a common component of the cellular inclusions in non‐SOD1 ALS and some other neurodegenerative conditions, implying a shared pathogenic pathway underlying SALS, non‐SOD1 FALS, ALS/dementia, and related disorders. Our data also indicate that SOD1‐linked ALS may have a pathogenic pathway distinct from SALS and other types of FALS. ANN NEUROL 2010;67:739–748
Parkinson disease is a common neurodegenerative disorder that leads to difficulty in effectively translating thought into action. Although it is known that dopaminergic neurons that innervate the ...striatum die in Parkinson disease, it is not clear how this loss leads to symptoms. Recent work has implicated striatopallidal medium spiny neurons (MSNs) in this process, but how and precisely why these neurons change is not clear. Using multiphoton imaging, we show that dopamine depletion leads to a rapid and profound loss of spines and glutamatergic synapses on striatopallidal MSNs but not on neighboring striatonigral MSNs. This loss of connectivity is triggered by a new mechanism-dysregulation of intraspine Cav1.3 L-type Ca(2+) channels. The disconnection of striatopallidal neurons from motor command structures is likely to be a key step in the emergence of pathological activity that is responsible for symptoms in Parkinson disease.
Mutations in the gene encoding ubiquilin2 ( UBQLN2 ) cause amyotrophic lateral sclerosis (ALS), frontotemporal type of dementia, or both. However, the molecular mechanisms are unknown. Here, we show ...that ALS/dementia-linked UBQLN2 ᴾ⁴⁹⁷ᴴ transgenic mice develop neuronal pathology with ubiquilin2/ubiquitin/p62-positive inclusions in the brain, especially in the hippocampus, recapitulating several key pathological features of dementia observed in human patients with UBQLN2 mutations. A major feature of the ubiquilin2-related pathology in these mice, and reminiscent of human disease, is a dendritic spinopathy with protein aggregation in the dendritic spines and an associated decrease in dendritic spine density and synaptic dysfunction. Finally, we show that the protein inclusions in the dendritic spines are composed of several components of the proteasome machinery, including Ub ᴳ⁷⁶ⱽ–GFP, a representative ubiquitinated protein substrate that is accumulated in the transgenic mice. Our data, therefore, directly link impaired protein degradation to inclusion formation that is associated with synaptic dysfunction and cognitive deficits. These data imply a convergent molecular pathway involving synaptic protein recycling that may also be involved in other neurodegenerative disorders, with implications for development of widely applicable rational therapeutics.
Significance Mutations in the UBQLN2 gene, which encodes the ubiquitin-like protein ubiquilin2 (UBQLN2) have been shown to cause ALS and ALS/dementia. Ubiquilin2 links familial and sporadic forms of the disease through pathology observed in the spinal cords of all ALS cases and in the brains of ALS/dementia cases with or without UBQLN2 mutations. In this communication, we develop and characterize a mouse model of mutant UBQLN2 -linked dementia. We demonstrate that mutant mice develop impairment in the protein degradation pathway, abnormal protein aggregation, synaptic dysfunction, and cognitive deficits. This model provides a useful tool to further study dementia and develop rational therapies.
Transient receptor potential "canonical" cation channels (TRPC) are involved in many cellular activities, including neuronal synaptic transmission. These channels couple lipid metabolism, calcium ...homeostasis, and electrophysiological properties as they are calcium permeable and activated through the phospholipase C pathway and by diacylglycerol. The TRPC3 subunit is abundantly expressed in Purkinje cells (PCs), where it mediates slow metabotropic glutamate receptor-mediated synaptic responses. Recently, it has been shown that heterozygous moonwalker mice, which are a model of cerebellar ataxia, carry a dominant gain-of-function mutation (T635A) in the TRPC3 gene. This mutation leads to PC loss and dysmorphism, which have been suggested to cause the ataxia. However, the ataxic phenotype is present from a very early stage (before weaning), whereas PC loss does not appear until several months of age. Here we show that another class of cerebellar neurons, the type II unipolar brush cells (UBCs), express functional TRPC3 channels; intriguingly, these cells are ablated in moonwalker mice by 1 month of age. Additionally, we show that in moonwalker mice, intrinsic excitability of PCs is altered as early as 3 weeks after birth. We suggest that this altered excitability and the TRPC3-mediated loss of type II UBCs may both contribute to the ataxic phenotype of these mice and that different calcium handling in PCs and type II UBCs may account for the dramatic differences in sensitivity to the moonwalker mutation between these cell types.
The espin actin-bundling proteins, which are the target of the jerker deafness mutation, caused a dramatic, concentration-dependent lengthening of LLC-PK1-CL4 cell microvilli and their parallel actin ...bundles. Espin level was also positively correlated with stereocilium length in hair cells. Villin, but not fascin or fimbrin, also produced noticeable lengthening. The espin COOH-terminal peptide, which contains the actin-bundling module, was necessary and sufficient for lengthening. Lengthening was blocked by 100 nM cytochalasin D. Espin cross-links slowed actin depolymerization in vitro less than twofold. Elimination of an actin monomer-binding WASP homology 2 domain and a profilin-binding proline-rich domain from espin did not decrease lengthening, but made it possible to demonstrate that actin incorporation was restricted to the microvillar tip and that bundles continued to undergo actin treadmilling at ∼ 1.5 s-1 during and after lengthening. Thus, through relatively subtle effects on actin polymerization/depolymerization reactions in a treadmilling parallel actin bundle, espin crosslinks cause pronounced barbed-end elongation and, thereby, make a longer bundle without joining shorter modules.
Unipolar brush cells (UBCs) are excitatory cerebellar granular layer interneurons whose brush-like dendrites receive one-to-one mossy fiber inputs. Subclasses of UBCs differ primarily by expressing ...metabotropic glutamate receptor (mGluR) 1α or calretinin. We used GENSAT Tg(Grp-EGFP) BAC transgenic mice, which selectively express enhanced green fluorescent protein (EGFP) in mGluR1α-positive UBCs to compare the functional properties of the two subclasses. Compared to EGFP-negative UBCs, which include the calretinin-positive cells, EGFP-positive UBCs had smaller somata (area 48 vs 63 μm
2
), lower specific membrane resistance (6.4 vs. 13.7 KΩ cm
2
), were less prone to intrinsic firing, and showed more irregular firing (in cell-attached ~49 % were firing vs. ~88 %, and the CV was 0.53 vs. 0.32 for EGFP-negative cells). Some of these differences are attributable to higher density of background K
+
currents in EGFP-positive cells (at −120 mV, the barium-sensitive current was 94 vs. 37 pA in EGFP-negative cells); Ih, on the contrary, was more abundantly expressed in EGFP-negative cells (at −140 mV, it was −122 vs. −54 pA in EGFP-positive neurons); furthermore, while group II mGluR modulation of the background potassium current in EGFP-negative UBCs was maintained after intracellular dialysis, mGluR modulation in EGFP-positive UBCs was lost in whole-cell recordings. Finally, cell-attached firing was reversibly abolished by the GABA
B
activation in EGFP-positive, but not in EGFP-negative UBCs. Immunohistochemistry showed that EGFP-negative UBCs express GIRK2 at high density, while mGluR1α UBCs are GIRK2 negative, suggesting that GIRK2 mediates the mGluR-sensitive current in EGFP-negative UBCs. These data suggest that the two subclasses perform different functions in the cerebellar microcircuits.
Neuronal firing patterns are determined by the cell's intrinsic electrical and morphological properties and are regulated
by synaptic interactions. While the properties of cerebellar neurons have ...generally been studied in much detail, little is
known about the unipolar brush cells (UBCs), a type of glutamatergic interneuron that is enriched in the granular layer of
the mammalian vestibulocerebellum and participates in the representation of head orientation in space. Here we show that UBCs
can be distinguished from adjacent granule cells on the basis of differences in membrane capacitance, input resistance and
response to hyperpolarizing current injection. We also show that UBCs are intrinsically firing neurons. Using action potential
clamp experiments and whole-cell recordings we demonstrate that two currents contribute to this property: a persistent TTX-sensitive
sodium current and a ruthenium red-sensitive, TRP-like cationic current, both of which are active during interspike intervals
and have reversal potentials positive to threshold. Interestingly, although UBCs are also endowed with a large I h current, this current is not involved in their intrinsic firing, perhaps because it activates at voltages that are more hyperpolarized
than those associated with autonomous activity.
The espins are actin-bundling proteins of brush border microvilli and Sertoli cell-spermatid junctions. We have determined that espins are also present in hair cell stereocilia and have uncovered a ...connection between the espin gene and jerker, a recessive mutation that causes hair cell degeneration, deafness, and vestibular dysfunction. The espin gene maps to the same region of mouse chromosome 4 as jerker. The tissues of jerker mice do not accumulate espin proteins but contain normal levels of espin mRNAs. The espin gene of jerker mice has a frameshift mutation that affects the espin C-terminal actin-bundling module. These data suggest that jerker mice are, in effect, espin null and that the jerker phenotype results from a mutation in the espin gene.
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