Biomedical subjects

Shanker Karunanithi

Publications and source records attributed to Shanker Karunanithi.

5 recordsLinked to original sources

PubMed2007

Morphological and functional effects of altered cysteine string protein at the Drosophila larval neuromuscular junction.

The synaptic vesicle-associated cysteine string protein (CSP) is critical for neurotransmitter release at the neuromuscular junction (NMJ) of Drosophila, where the approximately 4% of mutant flies lacking CSP that survive to adulthood exhibit spastic jumping and shaking, temperature-sensitive paralysis, and premature death. Previously, it has been shown that CSP is also required for nerve terminal growth and the prevention of neurodegeneration in Drosophila and mice. At larval csp null mutant NMJs of Drosophila, intracellular recordings from the muscle showed that evoked release is significantly reduced at room temperature. However, it remained unclear whether the reduction in evoked release might be due to a loss of synaptic boutons, loss of synapses, and alterations in trafficking of vesicles to synapses. To resolve these issues, we have examined synaptic structure and function of csp null mutant NMJs at the level of single boutons. csp null mutations proportionally reduce the number of synaptic boutons of both motor neurons (1s and 1b) innervating larval muscles 6 and 7, while the number of synapses per bouton remains normal. However, focal recordings from individual synaptic boutons show that nerve-evoked neurotransmitter release is also impaired in both 1s and 1b boutons. Further, our ultrastructural analyses show that the reduction in evoked release at low stimulation frequencies is not due to a loss of synapses or to alterations in docked vesicles at synapses. Together, these data suggest that CSP promotes synaptic growth and evoked neurotransmitter release by mechanistically independent signaling pathways.

Animals↗

PubMed2006

Thermoprotection of synaptic transmission in a Drosophila heat shock factor mutant is accompanied by increased expression of Hsp83 and DnaJ-1.

In Drosophila larvae, acquired synaptic thermotolerance after heat shock has previously been shown to correlate with the induction of heat shock proteins (Hsps) including HSP70. We tested the hypothesis that synaptic thermotolerance would be significantly diminished in a temperature-sensitive strain (Drosophila heat shock factor mutant hsf4), which has been reported not to be able to produce inducible Hsps in response to heat shock. Contrary to our hypothesis, considerable thermoprotection was still observed at hsf4 larval synapses after heat shock. To investigate the cause of this thermoprotection, we conducted DNA microarray experiments to identify heat-induced transcript changes in these organisms. Transcripts of the hsp83, dnaJ-1 (hsp40), and glutathione-S-transferase gstE1 genes were significantly upregulated in hsf4 larvae after heat shock. In addition, increases in the levels of Hsp83 and DnaJ-1 proteins but not in the inducible form of Hsp70 were detected by Western blot analysis. The mode of heat shock administration differentially affected the relative transcript and translational changes for these chaperones. These results indicate that the compensatory upregulation of constitutively expressed Hsps, in the absence of the synthesis of the major inducible Hsp, Hsp70, could still provide substantial thermoprotection to both synapses and the whole organism.

Acclimatization↗

PubMed2002

Quantal size and variation determined by vesicle size in normal and mutant Drosophila glutamatergic synapses.

Quantal size and variation at chemical synapses could be determined presynaptically by the amount of neurotransmitter released from synaptic vesicles or postsynaptically by the number of receptors available for activation. We investigated these possibilities at Drosophila glutamatergic neuromuscular synapses formed by two separate motor neurons innervating the same muscle cell. At wild-type synapses of the two neurons we found a difference in quantal size corresponding to a difference in mean synaptic vesicle volume. The same finding applied to two mutants (dlg and lap) in which synaptic vesicle size was altered. Quantal variances at wild-type and mutant synapses were similar and could be accounted for by variation in vesicular volume. The linear relationship between quantal size and vesicular volume for several different genotypes indicates that glutamate is regulated homeostatically to the same intravesicular concentration in all cases. Thus functional differences in synaptic strength among glutamatergic neurons of Drosophila result in part from intrinsic differences in vesicle size.

Animals↗

PubMed2002

Enhancement of presynaptic performance in transgenic Drosophila overexpressing heat shock protein Hsp70.

Prior heat shock confers protection to Drosophila synapses during subsequent heat stress by stabilizing quantal size and reducing the decline of quantal emission at individual synaptic boutons. The major heat shock protein Hsp70, which is strongly induced by high temperatures in Drosophila, may be responsible for this synaptic protection. To test this hypothesis, we investigated synaptic protection and stabilization at larval neuromuscular junctions of transgenic Drosophila which produce more than the normal amount of Hsp70 in response to heat shock. Overexpression of Hsp70 coincides with enhanced protection of presynaptic performance, assayed by measuring mean quantal content and percentage success of transmission. Quantal size was not selectively altered, indicating no effects of overexpression on postsynaptic performance. Thus, presynaptic mechanisms can be protected by manipulating levels of Hsp70, which would provide stability to neural circuits otherwise susceptible to heat stress.

Animals↗

PubMed2002

Diversification of synaptic strength: presynaptic elements.

Synapses are not static; their performance is modified adaptively in response to activity. Presynaptic mechanisms that affect the probability of transmitter release or the amount of transmitter that is released are important in synaptic diversification. Here, we address the diversity of presynaptic performance and its underlying mechanisms: how much of the variation can be accounted for by variation in synaptic morphology and how much by molecular differences? Significant progress has been made in defining presynaptic structural contributions to synaptic strength; by contrast, we know little about how presynaptic proteins produce normally observed functional differentiation, despite abundant information on presynaptic proteins and on the effects of their individual manipulation. Closing the gap between molecular and physiological synaptic diversification still represents a considerable challenge.

Animals↗