The response of Dual-leucine zipper kinase (DLK) to nocodazole: Evidence for a homeostatic cytoskeletal repair mechanism

Genetic and pharmacological perturbation of the cytoskeleton enhances the regenerative potential of neurons. This response requires Dual-leucine Zipper Kinase (DLK), a neuronal stress sensor that is a central regulator of axon regeneration and degeneration. The damage and repair aspects of this response are reminiscent of other cellular homeostatic systems, suggesting that a cytoskeletal homeostatic response exists. In this study, we propose a framework for understanding DLK mediated neuronal cytoskeletal homeostasis. We demonstrate that low dose nocodazole treatment activates DLK signaling. Activation of DLK signaling results in a DLK-dependent transcriptional signature, which we identify through RNA-seq. This signature includes genes likely to attenuate DLK signaling while simultaneously inducing actin regulating genes. We identify alterations to the cytoskeleton including actin-based morphological changes to the axon. These results are consistent with the model that cytoskeletal disruption in the neuron induces a DLK-dependent homeostatic mechanism, which we term the Cytoskeletal Stress Response (CSR) pathway.

The response of Dual-Leucine Zipper Kinase (DLK) to nocodazole: evidence for a homeostatic cytoskeletal repair mechanism

Genetic and pharmacological perturbation of the cytoskeleton enhances the regenerative potential of neurons. This response requires Dual-leucine Zipper Kinase (DLK), a neuronal stress sensor that is a central regulator of axon regeneration and degeneration. The damage and repair aspects of this response are reminiscent of other cellular homeostatic systems, suggesting that a cytoskeletal homeostatic response exists. In this study, we propose a framework for understanding DLK mediated neuronal cytoskeletal homeostasis. We demonstrate that a) low dose nocodazole treatment activates DLK signaling and b) DLK signaling mitigates the microtubule damage caused by the cytoskeletal perturbation. We also perform RNA-seq to discover a DLK-dependent transcriptional signature. This signature includes genes likely to attenuate DLK signaling while simultaneously inducing actin regulating genes and promoting actin-based morphological changes to the axon. These results are consistent with the model that cytoskeletal disruption in the neuron induces a DLK-dependent homeostatic mechanism, which we term the Cytoskeletal Stress Response (CSR) pathway.

Distinct developmental and degenerative functions of SARM1 require NAD+ hydrolase activity

SARM1 is the founding member of the TIR-domain family of NAD⁺ hydrolases and the central executioner of pathological axon degeneration. SARM1-dependent degeneration requires NAD⁺ hydrolysis. Prior to the discovery that SARM1 is an enzyme, SARM1 was studied as a TIR-domain adaptor protein with non-degenerative signaling roles in innate immunity and invertebrate neurodevelopment, including at the Drosophila neuromuscular junction (NMJ). Here we explore whether the NADase activity of SARM1 also contributes to developmental signaling. We developed transgenic Drosophila lines that express SARM1 variants with normal, deficient, and enhanced NADase activity and tested their function in NMJ development. We find that NMJ overgrowth scales with the amount of NADase activity, suggesting an instructive role for NAD⁺ hydrolysis in this developmental signaling pathway. While degenerative and developmental SARM1 signaling share a requirement for NAD⁺ hydrolysis, we demonstrate that these signals use distinct upstream and downstream mechanisms. These results identify SARM1-dependent NAD⁺ hydrolysis as a heretofore unappreciated component of developmental signaling. SARM1 now joins sirtuins and Parps as enzymes that regulate signal transduction pathways via mechanisms that involve NAD⁺ cleavage, greatly expanding the potential scope of SARM1 TIR NADase functions.

SARM1 is the central executioner of axon loss, and inhibition of SARM1 is a therapeutic target for many devastating neurodegenerative disorders. SARM1 is the founding member of the TIR-domain family of NAD⁺ cleaving enzymes, destroying the essential metabolite NAD⁺ and inducing an energetic crisis in the axon. This was a surprising finding, as previously studied TIR-domain proteins were characterized as scaffolds that bind signaling proteins to coordinate signal transduction cascades. Indeed, before the discovery of the role of SARM1 in axon degeneration, SARM1 was studied as a regulator of intracellular signaling in immunity and neurodevelopment where it was assumed to act as a scaffold. Here we investigate whether the recently described SARM1 enzymatic activity also regulates such signal transduction pathways. Indeed, we show that a developmental signaling pathway scales with the amount of NADase activity, suggesting an instructive role for NAD⁺ cleavage. While degenerative and developmental SARM1 signaling share a requirement for NAD⁺ cleavage, they utilize distinct upstream and downstream mechanisms. With these findings, SARM1 now joins sirtuins and Parps as enzymes that regulate signal transduction pathways via mechanisms that involve NAD⁺ cleavage, greatly expanding the potential scope of SARM1 TIR NADase functions.

The role of Twist1 in mutant huntingtin-induced transcriptional alterations and neurotoxicity

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an abnormal expansion of polyglutamine repeats in the huntingtin protein (Htt). Transcriptional dysregulation is an early event in the course of HD progression and is thought to contribute to disease pathogenesis, but how mutant Htt causes transcriptional alterations and subsequent cell death in neurons is not well understood. RNA-Seq analysis revealed that expression of a mutant Htt fragment in primary cortical neurons leads to robust gene expression changes before neuronal death. Basic helix-loop-helix transcription factor Twist1, which is essential for embryogenesis and is normally expressed at low levels in mature neurons, was substantially up-regulated in mutant Htt-expressing neurons in culture and in the brains of HD mouse models. Knockdown of Twist1 by RNAi in mutant Htt-expressing primary cortical neurons reversed the altered expression of a subset of genes involved in neuronal function and, importantly, abrogated neurotoxicity. Using brain-derived neurotrophic factor (Bdnf), which is known to be involved in HD pathogenesis, as a model gene, we found that Twist1 knockdown could reverse mutant Htt-induced DNA hypermethylation at the Bdnf regulatory region and reactivate Bdnf expression. Together, these results suggest that Twist1 is an important upstream mediator of mutant Htt-induced neuronal death and may in part operate through epigenetic mechanisms.

Endosonography versus helical computed tomography for locoregional staging of gastric cancer

BACKGROUND AND STUDY AIMS

Endosonography (EUS) has been shown to be more accurate than incremental computed tomography (CT) in the local (T) and regional (N) staging of gastric carcinoma; however, EUS has never been compared with helical CT (HCT). The fifth edition of the TNM classification changed the guidelines for N-staging of gastric carcinoma. The accuracy of imaging methods in this new system remains unknown.

PATIENTS AND METHODS

Staging accuracy of EUS and HCT were compared prospectively with pathological or intraoperative findings in 88 gastric carcinoma patients. Staging was done according to the fourth and fifth editions of the TNM classification. EUS was done with a radial echo endoscope, and HCT with a scanner with two rows of detectors (two-phase contrast-enhanced scanning of a water-filled stomach).

RESULTS

The T-staging accuracy of EUS (63 %, CI 52 - 73 %) was superior to the accuracy of HCT (44 %, CI 34 - 55 %; P = 0.021). N-staging accuracy of both methods was similar when the fourth edition of the TNM classification was used (EUS 47 %, CI 34 - 60 %; HCT 52 %, CI 38 - 65 %). However, HCT was more accurate than EUS when the fifth edition of the classification was applied (EUS 30 %, CI 18 - 43 %, HCT 47 %, CI 34 - 60 %; P = 0.044). The accuracy of detection of lymph node metastases was similar for both methods (EUS 67 %, CI 54 - 78 %; HCT 77 %, CI 64 - 86 %).

CONCLUSIONS

EUS is more accurate than HCT in the T-staging of gastric carcinoma. Both methods are comparable for N-staging, when this is done according to the older, fourth edition of the TNM classification. If the fifth edition is used, EUS is less accurate than HCT.

Transabdominal ultrasound for visualizing gastric submucosal tumors diagnosed by endosonography: can surveillance be simplified?

BACKGROUND AND STUDY AIMS

Management options for gastric submucosal tumors (SMTs) include tumor removal or surveillance. If the latter is chosen, it requires repeated measurements of the tumor diameter. Although this can be achieved using endoscopy or endosonography (EUS), a less invasive and cheaper method would be welcome. The aim of this prospective study was to assess the proportion of gastric SMTs that can be visualized using transabdominal ultrasound of the water-filled stomach.

PATIENTS AND METHODS

Fifty-one consecutive patients with endosonographically diagnosed gastric SMTs underwent transabdominal ultrasound examinations of the water-filled stomach performed immediately after EUS; both procedures were carried out by the same investigator. Transabdominal ultrasound was considered positive only if: firstly, the tumor was visualized unequivocally; secondly, its dimensions could be measured; and thirdly, photographic documentation could be recorded. In each case, a positive result had to be confirmed by an independent investigator, who reviewed the photographic documentation.

RESULTS

The median size of SMTs on EUS was 25 mm (range 4 - 55 mm). Twelve tumors were located in the antrum, 25 in the gastric body, and 14 in the gastric fundus or cardia. Transabdominal ultrasound demonstrated the tumor in 35 of 51 patients (69 %). For tumors < or = 30 mm, the visualization rate was 61 % (22 of 36). The location of the tumor and its EUS features did not significantly affect the sensitivity of transabdominal ultrasound.

CONCLUSIONS

In 69 % of patients with endosonographically diagnosed gastric SMTs, the tumor can also be visualized (and measured) using transabdominal ultrasound of the water-filled stomach. This noninvasive and inexpensive method may potentially be useful for surveillance in patients with SMTs, and further evaluation in this setting is warranted.

Helical computed tomographic cholangiography versus endosonography for suspected bile duct stones: a prospective blinded study in non-jaundiced patients

BACKGROUND

Helical computed tomography performed after intravenous administration of a cholangiographic contrast material (HCT-cholangiography) may be useful for detecting bile duct stones in non-jaundiced patients. However, this method has never been compared with other non-invasive biliary imaging tests.

AIMS

To compare prospectively HCT-cholangiography and endosonography (EUS) in a group of non-jaundiced patients with suspected bile duct stones.

METHODS

Fifty two subjects underwent both HCT-cholangiography and EUS. Endoscopic retrograde cholangiography (ERCP), with or without instrumental bile duct exploration, served as a reference method, and was successful in all but two patients.

RESULTS

Thirty four patients (68%) were found to have choledocholithiasis at ERCP. The sensitivity for HCT-cholangiography in stone detection was 85%, specificity 88%, and accuracy 86%. For EUS the sensitivity was 91%, specificity 100%, and accuracy 94%. The differences were not significant. No serious complications occurred with either method.

CONCLUSIONS

HCT-cholangiography and EUS are safe and comparably accurate methods for detecting bile duct stones in non-jaundiced patients.