HIRAX:A Probe of Dark Energy and Radio Transients

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a new 400-800MHz radio interferometer under development for deployment in South Africa. HIRAX will comprise 1024 six meter parabolic dishes on a compact grid and will map most of the southern sky over the course of four years. HIRAX has two primary science goals: to constrain Dark Energy and measure structure at high redshift, and to study radio transients and pulsars. HIRAX will observe unresolved sources of neutral hydrogen via their redshifted 21-cm emission line (`hydrogen intensity mapping'). The resulting maps of large-scale structure at redshifts 0.8-2.5 will be used to measure Baryon Acoustic Oscillations (BAO). HIRAX will improve upon current BAO measurements from galaxy surveys by observing a larger cosmological volume (larger in both survey area and redshift range) and by measuring BAO at higher redshift when the expansion of the universe transitioned to Dark Energy domination. HIRAX will complement CHIME, a hydrogen intensity mapping experiment in the Northern Hemisphere, by completing the sky coverage in the same redshift range. HIRAX's location in the Southern Hemisphere also allows a variety of cross-correlation measurements with large-scale structure surveys at many wavelengths. Daily maps of a few thousand square degrees of the Southern Hemisphere, encompassing much of the Milky Way galaxy, will also open new opportunities for discovering and monitoring radio transients. The HIRAX correlator will have the ability to rapidly and eXperimentciently detect transient events. This new data will shed light on the poorly understood nature of fast radio bursts (FRBs), enable pulsar monitoring to enhance long-wavelength gravitational wave searches, and provide a rich data set for new radio transient phenomena searches. This paper discusses the HIRAX instrument, science goals, and current status.Comment: 11 pages, 5 figure

Recovery of mouse neuromuscular junctions from single and repeated injections of botulinum neurotoxin A

Botulinum neurotoxin type A (BoNT/A) paralyses muscles by blocking acetylcholine (ACh) release from motor nerve terminals. Although highly toxic, it is used clinically to weaken muscles whose contraction is undesirable, as in dystonias. The effects of an injection of BoNT/A wear off after 3–4 months so repeated injections are often used. Recovery of neuromuscular transmission is accompanied by the formation of motor axon sprouts, some of which form new synaptic contacts. However, the functional importance of these new contacts is unknown. Using intracellular and focal extracellular recording we show that in the mouse epitrochleoanconeus (ETA), quantal release from the region of the original neuromuscular junction (NMJ) can be detected as soon as from new synaptic contacts, and generally accounts for > 80% of total release. During recovery the synaptic delay and the rise and decay times of endplate potentials (EPPs) become prolonged approximately 3-fold, but return to normal after 2–3 months. When studied after 3–4 months, the response to repetitive stimulation at frequencies up to 100 Hz is normal. When two or three injections of BoNT/A are given at intervals of 3–4 months, quantal release returns to normal values more slowly than after a single injection (11 and 15 weeks to reach 50% of control values versus 6 weeks after a single injection). In addition, branching of the intramuscular muscular motor axons, the distribution of the NMJs and the structure of many individual NMJs remain abnormal. These findings highlight the plasticity of the mammalian NMJ but also suggest important limits to it

Novel Mouse Model Reveals Distinct Activity-Dependent and –Independent Contributions to Synapse Development

The balanced action of both pre- and postsynaptic organizers regulates the formation of neuromuscular junctions (NMJ). The precise mechanisms that control the regional specialization of acetylcholine receptor (AChR) aggregation, guide ingrowing axons and contribute to correct synaptic patterning are unknown. Synaptic activity is of central importance and to understand synaptogenesis, it is necessary to distinguish between activity-dependent and activity-independent processes. By engineering a mutated fetal AChR subunit, we used homologous recombination to develop a mouse line that expresses AChR with massively reduced open probability during embryonic development. Through histological and immunochemical methods as well as electrophysiological techniques, we observed that endplate anatomy and distribution are severely aberrant and innervation patterns are completely disrupted. Nonetheless, in the absence of activity AChRs form postsynaptic specializations attracting motor axons and permitting generation of multiple nerve/muscle contacts on individual fibers. This process is not restricted to a specialized central zone of the diaphragm and proceeds throughout embryonic development. Phenotypes can be attributed to separate activity-dependent and -independent pathways. The correct patterning of synaptic connections, prevention of multiple contacts and control of nerve growth require AChR-mediated activity. In contrast, myotube survival and acetylcholine-mediated dispersal of AChRs are maintained even in the absence of AChR-mediated activity. Because mouse models in which acetylcholine is entirely absent do not display similar effects, we conclude that acetylcholine binding to the AChR initiates activity-dependent and activity-independent pathways whereby the AChR modulates formation of the NMJ

Gas7-Deficient Mouse Reveals Roles in Motor Function and Muscle Fiber Composition during Aging

Background: Growth arrest-specific gene 7 (Gas7) has previously been shown to be involved in neurite outgrowth in vitro; however, its actual role has yet to be determined. To investigate the physiological function of Gas7 in vivo, here we generated a Gas7-deficient mouse strain with a labile Gas7 mutant protein whose functions are similar to wild-type Gas7. Methodology/Principal Findings: Our data show that aged Gas7-deficient mice have motor activity defects due to decreases in the number of spinal motor neurons and in muscle strength, of which the latter may be caused by changes in muscle fiber composition as shown in the soleus. In cross sections of the soleus of Gas7-deficient mice, gross morphological features and levels of myosin heavy chain I (MHC I) and MHC II markers revealed significantly fewer fast fibers. In addition, we found that nerve terminal sprouting, which may be associated with slow and fast muscle fiber composition, was considerably reduced at neuromuscular junctions (NMJ) during aging. Conclusions/Significance: These findings indicate that Gas7 is involved in motor neuron function associated with muscle strength maintenance

Altered mRNA expression of genes related to nerve cell activity in the fracture callus of older rats: A randomized, controlled, microarray study

BACKGROUND: The time required for radiographic union following femoral fracture increases with age in both humans and rats for unknown reasons. Since abnormalities in fracture innervation will slow skeletal healing, we explored whether abnormal mRNA expression of genes related to nerve cell activity in the older rats was associated with the slowing of skeletal repair. METHODS: Simple, transverse, mid-shaft, femoral fractures with intramedullary rod fixation were induced in anaesthetized female Sprague-Dawley rats at 6, 26, and 52 weeks of age. At 0, 0.4, 1, 2, 4, and 6 weeks after fracture, a bony segment, one-third the length of the femur, centered on the fracture site, including the external callus, cortical bone, and marrow elements, was harvested. cRNA was prepared and hybridized to 54 Affymetrix U34A microarrays (3/age/time point). RESULTS: The mRNA levels of 62 genes related to neural function were affected by fracture. Of the total, 38 genes were altered by fracture to a similar extent at the three ages. In contrast, eight neural genes showed prolonged down-regulation in the older rats compared to the more rapid return to pre-fracture levels in younger rats. Seven genes were up-regulated by fracture more in the younger rats than in the older rats, while nine genes were up-regulated more in the older rats than in the younger. CONCLUSIONS: mRNA of 24 nerve-related genes responded differently to fracture in older rats compared to young rats. This differential expression may reflect altered cell function at the fracture site that may be causally related to the slowing of fracture healing with age or may be an effect of the delayed healing

Aspects of the chemical embryology of the electromotor system of Torpedo marmorata with special reference to synaptogenesis

The functional development of the cholinergic electromotor system of Torpedo marmorata has been followed by measuring four proteins, choline acetyltransferase, acetylcholinesterase, myosin and nicotinic acetylcholine receptor, and the discharge of the electric organ in response to presynaptic stimulation in embryos from 28mm in length to new-born (~ 130 mm). The first two, as indicators of cholinergic function, were followed, both in the electric lobes, sites of the perikarya of the electromotor neurones, and in the electric organ; the last three, indicative respectively of the myofibrillar structures of the myotubes which are the precursors of the electrocytes, the cholinoceptive function of the differentiating postsynaptic membrane, and the existence of functional synapses, were followed in the electric organ. A fall in the tissue concentration of myosin heavy chain protein in the electric organ between 37 ± 2 and 57 ± 2 mm of embryo length correlated well with the transition from vertically orientated myotubes to horizontally flattened electrocytes, with loss in myofibrillar structure. The finding that receptor protein had reached almost 100% of the adult value at an embryo length at which discharges of the electric organ were barely detectable showed that synaptogenesis, seen morphologically as a continuous process commencing with the ingrowth of axons at 55 mm, occurs functionally in two stages in which the formation of a receptor- (and esterase-) rich postsynaptic membrane precedes that of transmitting synapses. Choline acetyltransferase activity was first detected in the lobe at about 40mm embryo length, 4–6 weeks after the electromotor perikarya had stopped dividing. It rose steadily to 125% of the adult value at birth. By contrast the rise in the electric organ was much slower; it reached only 50% of the adult value at birth and showed evidence of two phases, an initial small rise coincident with and perhaps due to the invasion of the electric organ by cholinergic electromotor axons and a more pronounced one from 90 mm accompanied by the formation of transmitting synapses. Significant (1V) discharges of electric organs in response to single stimuli were first detected at 90mm and rose exponentially to reach adult values at birth. However, the organ remained more fatiguable than that of the adult perhaps due to limited transmitter synthesis by the sub-adult concentrations of choline acetyltransferase at the nerve terminals

The chemical embryology of the electromotor system of torpedo marmorata

Although detailed morphological studies of the embryological development of the electric organ of Torpedo were made in the nineteenth century with the light microscope (1,12,18) relatively little had been done with modern techniques with this important model cholinergic system until recently, when electron microscopic surveys were carried out (7,8,17). These have confirmed the conclusion of the earlier workers that electrocytes develop from vertically orientated myotubes and have documented the differentation of the postsynaptic membrane and the onset of synaptogenesis

Studies of glutamate dehydrogenase: chemical modification and quantitative determination of tryptophan residues

The effect of the modification of beef liver glutamate dehydrogenase with 2‐hydroxy‐5‐nitrobenzyl bromide was studied with respect to the association‐dissociation equilibrium as well as the enzymatic and regulatory properties. Upon incorporation of 2.1 molecules of the reagent per polypeptide chain, the association of the enzyme is strongly reduced, whereas the enzymatic activity is only slightly decreased (80% maximum velocity). The Km values and the regulation by ADP and GTP, respectively, remain unaltered. Evidence is presented that only tryptophan residues are modified. Magnetic circular dichroism measurements of the modified enzyme suggest partial disubstitntion of tryptophan residues. From the reduced incorporation of 2‐hydroxy‐5‐nitrobenzyl groups at high enzyme concentration it is concluded that tryptophan is located at the association areas of the enzyme. Quantitative determination of the tryptophan content of glutamate dehydrogenase with 2‐hydroxy‐5‐nitrobenzyl bromide, magnetic circular dichroism and peptide analysis yields four tryptophan residues per polypeptide chain contrary to three residues previously suggested from structural analysis. The fourth tryptophan, not present in the reported sequence, is recovered in a chymotryptie peptide Glu‐Trp

Cytoskeletal proteins at the cholinergic synapse: distribution of desmin, actin, fodrin, neurofilaments, and tubulin in Torpedo electric organ

Treatment of the electric organ of Torpedo marmorata with Triton X-100 in the presence of 2 mM MgCl2 generated a cytoskeletal fraction in which a 54 kDa polypeptide is a major constituent. This 54 kDa polypeptide accounted for about 8% of the cellular protein when total electric organ tissue was analyzed by two-dimensional gel electrophoresis. Immunoblotting experiments showed that this protein reacts with monoclonal antibodies to desmin, the major intermediate filament protein of avian and mammalian muscle tissue. Negative stain analysis revealed that filaments of about 10 nm diameter are the major structural elements of the electric organ cytoskeleton. In the presence of Ca2+ there was a rapid degradation of the desmin-like protein and intermediate filaments due to a Ca2+-activated protease. Some of the resulting fragments retained antigenic activity against the desmin antibodies. Immunoblotting of membrane fractions enriched in acetylcholine receptor revealed desmin in addition to some actin. A further cytoskeletal component was identified from biochemical and immunological properties as a homologue of the mammalian neurofilament L-polypeptide. Thus Torpedo expresses proteins homologous to the mammalian desmin and neurofilament L-protein which can be detected using immunological approaches. Immunofluorescence microscopy was used to map the location of various cytoskeletal proteins of the cholinergic synapse on paraffin sections and on en face preparations of membranes. Desmin staining was restricted to electrocytes and in en face preparations was seen associated with both the ventral receptor-containing membrane and with the non-innervated dorsal membrane. Antibodies to neurofilament L-protein stained only the axons and not the electrocytes. Staining for fodrin, a non-erythrocyte spectrin, resulted in submembraneous decoration of both the axons and the electrocytes. Axonal staining for neurofilaments and microtubules did not extend into the ends of the nerve terminal arborizations