Isoform Specific Deophosphorylation of Dynamin1 by Calcineurin...

Question 1

How do neurotrophins mediate their effects?

Answer 1

1. Through local signaling - in distal axons and growth cones - which mediates acute responses including rapid axon growth, branching, and guidance
2. Through retrograde signaling to the cell body and nucleus - which elicits long term changes in gene expression necessary for neuronal survival and differentiation.

Question 2

NGF’s receptors

Answer 2

TrkA

Question 3

NT3 in the mature nervous system binds to which tyrosine kinase? What about in the developing sympathetic nervous system?

Answer 3

mature - trkC

developing - TrkA, like NGF

Question 4

where is NT-3 highly expressed? Where is NGF highly expressed?

Answer 4

NT3 - intermediate targets like the vasculature, and it promotes early stages of axon growth
NGF - final peripheral targets, and it supports final target innervation.

Question 5

Which on - NGF or NT3 - promotes endocytosis and retrograde transport of TrkA?

Answer 5

NGF

Question 6

Calcineeurin

Answer 6

A calcium responsive serine/threonine phosphatase consisting of a catalytic subunit (calcineurin A) and a regulatory subunit (calcineurin B)

Responds to NGF and NT-3

Acts on NFATs (transcription factors)

Question 7

What happens when Calcineurin is activated?

Answer 7

Ca2+ dependent activation of calcineurin results in dephosphrylation and nuclear import of NFAT transcription factors NFAT1-4

Question 8

What did the authors of this paper discover about dynamin1?

Answer 8

that it is a local target of calcineurin signaling in axons and is critical for NGF-mediated growth in a manner independent of transcription.

this is possibly mediated by a PxIxIT box present within dynamin1 splicing isoforms that interacts with calcineurin.

Calcineurin interacts with the box on dynamin, and through this interaction TrkA is internalized and axons grow.

Question 9

CalcineurinB - what are the isoforms? what is it?

Answer 9

CaNB1 - ubiquitously expressed
CaNB2 - only expressed in testes

It is the calcineurin regulatory subunit… Calcineurin A is the catalytic subunit (caNA)

Question 10

What did they find for CaNB1 mutants? (Gross observations).

Answer 10

Sympathetic axons were shorter and less branched compared to WT.
But axon outgrowth from sympathetic ganglia and projections along the vasculature appeared normal in these embryos, and there were no differences in morphology of the sympathetic chain between mutant and WT embryos.

Suggests calcineurin is required for sympathetic innervation of final target tissues but that axon growth along the vasculature occurs via calcineurin independent mechanisms.

Question 11

Cyclosporin A

Answer 11

a calcineurin inhibitor

It is an immunosuppressant too.
binds to the cytosolic protein cyclophilin (immunophilin) of lymphocytes, especially T cells. This complex of ciclosporin and cyclophilin inhibits calcineurin, which, under normal circumstances, is responsible for activating the transcription of interleukin 2. In T-cells, activation of the T-cell receptor normally increases intracellular calcium, which acts via calmodulin to activate calcineurin. Calcineurin then dephosphorylates the transcription factor nuclear factor of activated T-cells (NFATc), which moves to the nucleus of the T-cell and increases the activity of genes coding for IL-2 and related cytokines. Ciclosporin prevents the dephosphorylation of NF-AT by binding to cyclophilin.[17] It also inhibits lymphokine production and interleukin release and, therefore, leads to a reduced function of effector T-cells. It does not affect cytostatic activity.