Discovery could help scientists stop the "death cascade" of neurons after a stroke

Ask your doctor and hospital what has been done with this information since it came out in Jan 2009. Part of the solution to this problem is quite simple; reduce the size of the tPA bolus substantially because you are using magnetic nanoparticles to deliver it straight to the clot. 

Discovery could help scientists stop the "death cascade" of neurons after a stroke

(PhysOrg.com) -- Distressed swimmers often panic, sapping the strength they need to keep their heads above water until help arrives. When desperate for oxygen, neurons behave in a similar way. They freak out, stupidly discharging energy until they drown in a sea of their own extruded salts. Every year, millions of victims of stroke or brain trauma suffer permanent brain damage because of this mad rush to oblivion that begins once a part of the brain is deprived of blood.

It is well known that a ubiquitous cell receptor drives these oxygen-starved neurons’ lemming-like behavior. But this particular receptor, for the neurotransmitter glutamate, is also responsible for the rapid transmission of information between neurons required for all cognition, among other things. Shutting it off has serious consequences, like coma. Now, a team of scientists at The Rockefeller University has identified a single subunit of this receptor that drives neuronal death. This new discovery suggests that drugs targeting a specific subunit of the complex glutamate receptor might be able to slow brain damage without disrupting other crucial brain functions.

Saving neurons. When normal neurons (top) are subjected to stroke-like damage, they quickly deteriorate and die (center). New research shows that a small portion of the cell’s glutamate receptors, the KA1 subunit, is responsible for this …more

“We have found that you can make mice resistant to this kind of cell death by blocking one piece of the receptor without the terrible side effects you get by blocking the whole thing,” says Sidney Strickland, head of the Laboratory of Neurobiology and Genetics, who directed the research. “Now we can start exploring potential drugs to do that in humans.” The neuronal panic that occurs when a clot or other insult blocks the flow of blood to part of the brain is called excitotoxic neurodegeneration. It results in the brain cells spitting out glutamate, which then accumulates in the synapses between neurons and stimulates the release of more glutamate. It’s a vicious cycle that kills the cells quickly and continues until blood flow is restored. Doctors often treat stroke victims by administering a heavy dose of a clot-buster called tissue plasminogen activator (tPA), a protein that can stimulate the dissolution of clots. Ironically, however, the same drug that does this crucial clot-busting also accelerates the panicky process that kills neurons, research by Strickland and others has shown. Investigating exactly how tPA does that is what led Strickland’s team to the recent discovery.
Neurons are typically couched in laminin, an extracellular matrix protein known to be involved with tPA in the neuronal “death cascade.” The Strickland lab’s experiments, published in The Journal of Cell Biology, show that tPA produces an enzyme that degrades laminin into toxic products that kill the neurons in their midst, specifically by stimulating the production of one of five subunits for a particular kind of glutamate receptor. The overproduction of this specific subunit, KA1, makes the cells hypersensitive to glutamate, which fans the glutamate frenzy leading to their death.

To better understand the process, Zu-Lin Chen and other colleagues at Rockefeller in the Strickland lab and at the University of Leicester in England designed lines of genetically modified mice that lacked either tPA or laminin specifically in the hippocampus, a region of the brain often damaged by stroke. To their surprise, they found that mice without laminin were protected from the typical neural degeneration that follows a simulated stroke in regular mice. They also found that mice with laminin but not tPA were relatively protected.
But when they injected degraded laminin into mouse brains without laminin or tPA they found a similar overproduction of the subunit of the glutamate receptor that they measured when inducing stroke in normal mice. The problem: Laminin, once degraded by tPA, prompts the proliferation of the receptor subunit that makes the cells suicidally sensitive to glutamate. By preventively injecting a molecule that disables that particular subunit, they were able to dramatically reduce the cell death following a stroke. A big plus: The treated mice did not suffer the severe side effects that come with blocking the entire glutamate receptor.
Whether this will turn into a therapy that can be applied after a stroke is uncertain.
“Can you do it after the fact? That will be a question,” Strickland says. “Cell death happens pretty quickly. But it’s an interesting avenue to pursue.”
Reference: The Journal of Cell Biology 183(7): 1299-1313 (December 29, 2008), jcb.rupress.org/cgi/content/abstract/183/7/1299
Provided by Rockefeller University

Steering Neuronal Growth Cones by Shifting the Imbalance between Exocytosis and Endocytosis

You do expect your doctor to know how to manipulate this axon guidance to redirect neurons around dead areas to connect up sections of your brain again? Don't you?
http://www.jneurosci.org/content/34/21/7165.short

1. Takuro Tojima¹,²,*,
2. Rurika Itofusa¹,*, and
3. Hiroyuki Kamiguchi¹

+Show Affiliations

1. Author contributions: T.T., R.I., and H.K. designed research; T.T. and R.I. performed research; T.T. and R.I. analyzed data; T.T., R.I., and H.K. wrote the paper.
2. ↵*T.T. and R.I. contributed equally to this work.

1. The Journal of Neuroscience, 21 May 2014, 34(21): 7165-7178; doi: 10.1523/JNEUROSCI.5261-13.2014

• Abstract
• Full Text
• Full Text (PDF)

Abstract

Extracellular molecular cues guide migrating growth cones along specific routes during development of axon tracts. Such processes rely on asymmetric elevation of cytosolic Ca²⁺ concentrations across the growth cone that mediates its attractive or repulsive turning toward or away from the side with Ca²⁺ elevation, respectively. Downstream of these Ca²⁺ signals, localized activation of membrane trafficking steers the growth cone bidirectionally, with endocytosis driving repulsion and exocytosis causing attraction. However, it remains unclear how Ca²⁺ can differentially regulate these opposite membrane-trafficking events. Here, we show that growth cone turning depends on localized imbalance between exocytosis and endocytosis and identify Ca²⁺-dependent signaling pathways mediating such imbalance. In embryonic chicken dorsal root ganglion neurons, repulsive Ca²⁺ signals promote clathrin-mediated endocytosis through a 90 kDa splice variant of phosphatidylinositol-4-phosphate 5-kinase type-1γ (PIPKIγ90). In contrast, attractive Ca²⁺ signals facilitate exocytosis but suppress endocytosis via Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and cyclin-dependent kinase 5 (Cdk5) that can inactivate PIPKIγ90. Blocking CaMKII or Cdk5 leads to balanced activation of both exocytosis and endocytosis that causes straight growth cone migration even in the presence of guidance signals, whereas experimentally perturbing the balance restores the growth cone's turning response. Remarkably, the direction of this resumed turning depends on relative activities of exocytosis and endocytosis, but not on the type of guidance signals. Our results suggest that navigating growth cones can be redirected by shifting the imbalance between exocytosis and endocytosis, highlighting the importance of membrane-trafficking imbalance for axon guidance and, possibly, for polarized cell migration in general.

Glutamate Controls tPA Recycling by Astrocytes, Which in Turn Influences Glutamatergic Signals

I wish they would write in understandable sentences. Are they trying to say tPA is naturally present in the brain and is useful or not?
http://www.jneurosci.org/content/32/15/5186.abstract

Abstract

Tissue-type plasminogen activator (tPA) regulates physiological processes in the brain, such as learning and memory, and plays a critical role in neuronal survival and neuroinflammation in pathological conditions. Here we demonstrate, by combining mouse in vitro and in vivo data, that tPA is an important element of the cross talk between neurons and astrocytes. The data show that tPA released by neurons is constitutively endocytosed by astrocytes via the low-density lipoprotein-related protein receptor, and is then exocytosed in a regulated manner. The exocytotic recycling of tPA by astrocytes is inhibited in the presence of extracellular glutamate. Kainate receptors of astrocytes act as sensors of extracellular glutamate and, via a signaling pathway involving protein kinase C, modulate the exocytosis of tPA. Further, by thus capturing extracellular tPA, astrocytes serve to reduce NMDA-mediated responses potentiated by tPA. Overall, this work provides the first demonstration that the neuromodulator, tPA, may also be considered as a gliotransmitter.