Localized regulation of axonal RanGTPase controls retrograde injury signaling in peripheral nerve.

Peripheral sensory neurons respond to axon injury by activating an importin-dependent retrograde signaling mechanism. How is this mechanism regulated? Here, we show that Ran GTPase and its associated effectors RanBP1 and RanGAP regulate the formation of importin signaling complexes in injured axons. A gradient of nuclear RanGTP versus cytoplasmic RanGDP is thought to be fundamental for the organization of eukaryotic cells. Surprisingly, we find RanGTP in sciatic nerve axoplasm, distant from neuronal cell bodies and nuclei, and in association with dynein and importin-alpha. Following injury, localized translation of RanBP1 stimulates RanGTP dissociation from importins and subsequent hydrolysis, thereby allowing binding of newly synthesized importin-beta to importin-alpha and dynein. Perturbation of RanGTP hydrolysis or RanBP1 blockade at axonal injury sites reduces the neuronal conditioning lesion response. Thus, neurons employ localized mechanisms of Ran regulation to control retrograde injury signaling in peripheral nerve.

Retrograde signaling in injured nerve--the axon reaction revisited.

Injury to axons elicits changes in macromolecule synthesis in the corresponding cell bodies that are critical for an effective regenerative response. For decades the most easily studied aspect of this phenomenon was the onset of chromatolysis, a suite of structural changes in the cell body characterized by swelling, shifting of the nucleus and dispersal of Nissl bodies. The question: 'what is the signal for chromatolysis?' received no less than 10 possible answers in a comprehensive review article published more than three decades ago. Here we come back to this 36 years old question, and review progress on understanding the mechanism of retrograde injury signaling in lesioned peripheral nerves. Recent work suggests that this is based on local axonal synthesis of critical carrier proteins, including importins and vimentin that link diverse signaling molecules to the dynein retrograde motor. A multiplicity of binding sites and of potential signaling molecules, including transcription factors and MAP kinases (Erk, Jnk), may allow diverse options for information-rich encoding of the injury status of the axon for transmission to the cell body.

Vimentin-dependent spatial translocation of an activated MAP kinase in injured nerve.

How are phosphorylated kinases transported over long intracellular distances, such as in the case of axon to cell body signaling after nerve injury? Here, we show that the MAP kinases Erk1 and Erk2 are phosphorylated in sciatic nerve axoplasm upon nerve injury, concomitantly with the production of soluble forms of the intermediate filament vimentin by local translation and calpain cleavage in axoplasm. Vimentin binds phosphorylated Erks (pErk), thus linking pErk to the dynein retrograde motor via direct binding of vimentin to importin beta. Injury-induced Elk1 activation and neuronal regeneration are inhibited or delayed in dorsal root ganglion neurons from vimentin null mice, and in rats treated with a MEK inhibitor or with a peptide that prevents pErk-vimentin binding. Thus, soluble vimentin enables spatial translocation of pErk by importins and dynein in lesioned nerve.

Integration of retrograde axonal and nuclear transport mechanisms in neurons: implications for therapeutics.

The elongated morphology of neuronal processes imposes a significant challenge for effective intracellular communication between the neurites and the cell body. This problem is especially acute upon injury, when the cell body must receive accurate and timely information on the site and extent of axonal damage to mount an appropriate response. Recent work has demonstrated that nuclear import factors from the importin (karyopherin) alpha and beta families provide a mechanism for retrograde injury signaling. Importins are found throughout axons and dendrites at significant distances from the cell body, and importin beta protein is increased after nerve lesion by local translation of axonal mRNA. This leads to formation of a high-affinity nuclear localization signal (NLS) binding complex that traffics retrogradely due to an interaction of importin alpha with the motor protein dynein. Disruption of the complex with excess NLS peptides delays regeneration of injured sensory neurons. The dual role of importins in retrograde transport in axons and nuclear import in cell bodies suggests new avenues for manipulating intrinsic regeneration mechanisms in the nervous system and may provide a novel route for drug delivery to the CNS.

From snails to sciatic nerve: Retrograde injury signaling from axon to soma in lesioned neurons.

The cell body of a lesioned neuron must receive accurate and timely information on the site and extent of axonal damage, in order to mount an appropriate response. Specific mechanisms must therefore exist to transmit such information along the length of the axon from the lesion site to the cell body. Three distinct types of signals have been postulated to underlie this process, starting with injury-induced discharge of axon potentials, and continuing with two distinct types of retrogradely transported macromolecular signals. The latter include, on the one hand, an interruption of the normal supply of retrogradely transported trophic factors from the target; and on the other hand activated proteins emanating from the injury site. These activated proteins are termed "positive injury signals", and are thought to be endogenous axoplasmic proteins that undergo post-translational modifications at the lesion site upon axotomy, which then target them to the retrograde transport system for trafficking to the cell body. Here, we summarize the work to date supporting the positive retrograde injury signal hypothesis, and provide some new and emerging proteomic data on the system. We propose that the retrograde positive injury signals form part of a complex that is assembled by a combination of different processes, including post-translational modifications such as phosphorylation, regulated and transient proteolysis, and local axonal protein synthesis.

Axoplasmic importins enable retrograde injury signaling in lesioned nerve.

Axoplasmic proteins containing nuclear localization signals (NLS) signal retrogradely by an unknown mechanism in injured nerve. Here we demonstrate that the importin/karyopherin alpha and beta families underlie this process. We show that importins are found in axons at significant distances from the cell body and that importin beta protein is increased after nerve lesion by local translation of axonal mRNA. This leads to formation of a high-affinity NLS binding complex that traffics retrogradely with the motor protein dynein. Trituration of synthetic NLS peptide at the injury site of axotomized dorsal root ganglion (DRG) neurons delays their regenerative outgrowth, and NLS introduction to sciatic nerve concomitantly with a crush injury suppresses the conditioning lesion induced transition from arborizing to elongating growth in L4/L5 DRG neurons. These data suggest a model whereby lesion-induced upregulation of axonal importin beta may enable retrograde transport of signals that modulate the regeneration of injured neurons.