Protein Kinase M and LTP

My previous blog post was also about LTP: long-term potentiation of synapses. Here, the topic is what has been called late-LTP (L-LTP), a strengthening of synapses that can last for months in the hippocampus of laboratory animals. Molecular and cellular mechanisms of L-LTP are prized among neurobiology researchers because LTP seems to be one form of synaptic change that helps animals store long-lasting memories. Does Protein kinase M play an important role in making L-LTP and long-lasting memories?

When Francis Crick was in his later years, he dabbled in theoretical neurobiology. One mechanism for memory that Crick discussed was the possibility that permanent changes in synaptic connections might involve enzymes (such as protein kinases) that would remain active and continually lock a particular neuronal synapse in a state that is required for storing a memory (see “Memory and molecular turnover“). Protein kinase M is proposed to provide such a molecular memory mechanism.

My favorite neurobiology research article from 2016 is “Compensation for PKMζ in long-term potentiation and spatial long-term memory in mutant mice“. The “compensation” that is explored in this article concerns a second protein kinase called PKCiota/lambda (PKCι/λ). [The same orthologous gene is called lambda in mice and iota in humans.] The authors of this 2016 article present evidence in support of the idea that in the absence of protein kinase M (PKM), PKCι/λ can act as a substitute kinase and provide mammals with some capacity to store memories. However, they believe that PKM is normally the protein kinase that is most important for L-LTP and long-lasting memories. In this model, PKCι/λ can provide a type of emergency backup memory system that is not as effective as the PKM system. The “mutant mice” in this study are transgenic mice that have been engineered to lack PKM.

Protein Kinase M

pkm

PKM is the c-terminal end of PKC (source)

What is protein kinase M? Protein kinase M got its name back in the 1970s when Yasutomi Nishizuka and his co-workers were studying a new protein kinase that came to be called protein kinase C (PKC). They noticed that brain tissue contained a protein kinase that only required magnesium for its activity, and they called it PKM. In fresh brain tissue, there was very little PKM, but PKM could be produced from PKC by proteolysis.

pkm-synthesis

Synthesis of PKM (source)

Later, it was found that there are several different human genes that code for members of the PKC family of enzymes. Protein kinase C was so named because some forms (the “classical” members of the family) of the enzyme require calcium for activation. In most cases, the N-terminal regulatory domain inhibits the substrate-binding  catalytic domain unless activators such as diacylglycerol (DAG) bind to the regulatory domain and relieve the inhibition.

One of the genes in the PKC gene family codes for a version of the enzyme that is called PKCzeta (PKCζ). In 1993, Todd Sacktor reported that a form of this enzyme (that was called PKMζ) could be detected in brain tissue, particularly following the induction of LTP (see “Persistent activation of the ζ isoform of protein kinase C in the maintenance of long-term potentiation“).

can2

PKCzeta is an atypical protein kinase C (image source). PS: the pseudosubstrate domain

In 2003, it was shown that there is a PKCzeta primary RNA transcript that can undergo alternate splicing in the brain (see “Protein Kinase Mζ Synthesis from a Brain mRNA Encoding an Independent Protein Kinase Cζ Catalytic Domain“). They obtained evidence for a special mRNA that can directly code for just the c-terminal catalytic domain of the PKCzeta enzyme, thus allowing for the synthesis of PKMζ as a translation product, and generating a persistently-activated form of PKC that is free of any need for stimulation of the enzyme by second messengers. Sacktor et al concluded: “These results indicated that brain PKMζ was not formed by a proteolytic mechanism but perhaps as a distinct ζ gene product.” In their model, synthesis of a persistently-activated PKMζ  is an important molecular mechanism for L-LTP and long-lasting memories.

pseudosubstrate

Model for activation of a Classical PKC (image source)

ZIP
Support for Sacktor’s model of memory storage came from inhibitor studies. The zeta inhibitory peptide (ZIP) is a potent competitive inhibitor of PKMζ in neurons (see “Matching biochemical and functional efficacies confirm ZIP as a potent competitive inhibitor of PKMζ in neurons“). ZIP can block L-LTP and also block memories (see the ZIP literature reviewed in this recent article).

Knockout
Confidence in the importance of PKMζ for memory was shaken when it was observed that mice engineered to lack PKCzeta still have some capacity to learn and form memories (see this and this). These results for PKCzeta knockout mice were not very alarming for me because other important protein kinases in the brain display overlap/redundancy in their functions (example).

In 2015, Sacktor et al published a theoretical model in which PKCι/λ could possibly take the place of  PKMζ  in mice that  lack a functional PKCzeta gene (see “Atypical PKCs in memory maintenance: the roles of feedback and redundancy“). Are there data to support this model?

2016 Data

zip

ZIP inhibits PKCι/λ (source)

In their 2016 article, Sacktor et al present several types of data:

  1. ZIP blocks L-LTP in both normal mice and mice lacking PKMζ.
  2. ZIP inhibits both the purified PKMζ enzyme and purified PKCι/λ.
  3. PKMζ -antisense oligonucleotides block L-LTP in normal mice but not in mice lacking PKMζ.
  4. An antagonist for PKCι/λ (ICAP) blocks the maintenance of L-LTP in mice lacking PKMζ.
  5. ICAP blocks spatial long-term memory maintenance in mice lacking PKMζ  but not in normal mice.
  6. PKCι/λ is persistently up-regulated in brain tissue lacking PKMζ when L-LTP is induced, unlike the situation for normal mice.
  7. They also show that memory storage in mice lacking PKMζ  is not entirely normal. Mice that depend on PKCι/λ for memory storage do not learn as efficiently as normal mice.

Sacktor et al suggest that for some types of memory, such as remembering the location of danger in the environment, PKCι/λ normally functions in short-term memory formation while PKMζ  is normally most important for the persistence of long-lasting memories. In mice that lack PKMζ, PKCι/λ can apparently function for both purposes, but its on-going involvement in short-term memory processes and responding to second messengers might partially disrupt the long-term memory storage function. For an interesting evolutionary perspective on the origins of PKM see: “Memory maintenance by PKMζ–an evolutionary perspective“.

Related reading:
1. Distinct Roles of PKCι/λ and PKMζ in the Initiation and Maintenance of Hippocampal Long-Term Potentiation and Memory.
2. commentary by Richard Morris.
3. Persistent increased PKMζ in long-term and remote spatial memory.
4. Role of Atypical Protein Kinases in Maintenance of Long-Term Memory and Synaptic Plasticity.

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Presynaptic Endocannabinoid-initiated LTP

cb1

presynaptic cannabinoid receptors (source)

“LTP” is long-term potentiation, a type of synaptic plasticity found in the hippocampus that is involved in episodic memory storage. A recent research article reports on the mechanism by which endocannabinoid signaling affects the induction of hippocampal long-term potentiation (LTP).

The new study concerns the role of presynaptic cannabinoid receptors (CB1R, see the image to the right) that respond to the endogenous cannabinoid 2-arachidonoyl-sn-glycerol (2AG). Two arachidonic acid derivatives, N-arachidonoylethanolamine (anandamide, AEA) and 2AG are the best studied endogenous cannabinoids. Both Δ9-tetrahydrocannabinol (THC), the main psychoactive molecule from cannabis and these two endogenous cannabinoids activate the CB1R.

thc

The endogenous cannabinoid 2AG can be made in dendrites by two enzymes, DGL or phospholipase C (PLC). The new research from the laboratory of Gary Lynch concerns production of LTP in the lateral perforant path (LPP), one of the cortical inputs to the hippocampus. Endogenous cannabinoids such as  2AG play normal roles in brain function and the actions of THC at CB1R are probably involved in the influence of cannabinoid drugs on orderly thought.

One way of activating PLC and 2AG synthesis in dendrites is via the action of glutamate on GluR5, a G protein-coupled receptor (see the diagram at the top right of this blog post). Glutamate is a common neurotransmitter at synapses in the hippocampus. In some parts of the brain, glutamate actions to activate NMDA receptors has been shown to lead to LTP, often via postsynaptic mechanisms involving altered levels of postsynaptic membrane receptors. Lynch et al provide evidence concerning how glutamate actions on GluR5, production of 2AG and activation of presynaptic CB1R can cause another form of LTP.

2ag

synthesis and degradation pathways for 2AG (source)

lynch-cb1

proposed model (source)

The image to the right illustrates the proposed presynaptic mechanism of CB1R-mediated LTP at the glutamanergic synapses formed by the LPP with granule cells of the dentate gyrus (DG, an important input region of the hippocampal formation).

In this model, the 2AG synthesizing enzyme diacylglycerol lipase (DGL) requires calcium for activity and is localized to dendritic spines, where it forms a multimolecular complex with mGluR5. In the model, glutamate (green triangles) activates mGluR5 and NMDA receptors leading to increases in postsynaptic calcium levels, activation of DGL and the production of 2AG (pink circles).

la

latrunculin A

The 2AG then binds to presynaptic CB1R triggering a long-lasting increase in glutamate release. Block of actin filament reorganization with latrunculin A inhibited this LTP, suggesting that reorganization of the actin cytoskeleton in LPP axon terminals is involved in facilitating glutamate release during LTP. They propose that activated presynaptic CB1 receptors can promote cytoskeletal reorganization via a signaling cascade that involves a  small GTPases (RhoA, Rac or Rap) that can regulate actin polymerization.

actin2

actin filament regulation via CB1R interactions with the WAVE1 complex (source)

Related reading: A cannabinoid link between mitochondria and memory

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Circadian Pacemaker

Figure 1

constant dark (source)

In my previous blog post, I mentioned that the GHSR1 ghrelin receptor has a high level of activity independent of ghrelin binding.  Recently, an “orphan” G Protein-coupled Receptor (GPCR) (Gpr176) has been implicated in the function of the circadian pacemaker of the suprachiasmatic nucleus (SCN). Gpr176 might not have a ligand and it might function simply by having daily swings in the level of its expression.

staining for Gpr176 in the SCN

Gpr176 in the SCN (source)

In gene knock-out mice without this particular GPCR (Gpr176-/-) the circadian period is slower than in normal mice. This function of Gpr176 was discovered by searching through the many (about 140) orphan GPCRs for those that have high levels of expression in the suprachiasmatic nucleus. Gpr176 expression was shown to be regulated with a circadian pattern.

circadian

circadian expression of Gpr176 mRNA [cry1] (source)

It was previously known that vasoactive intestinal peptide (VIP) regulates the circadian rhythm.

The Vipr2 GPCR for VIP stimulates adenylate cyclase and  increased levels of cAMP in neurons of the SCN circadian pacemaker. Gpr176 inhibits adenylate cyclase through the inhibitory G-protein, Gz.

cAMP levels

High cAMP in Gpr176 knockout mice (source)

In gene knockout mice that do not express Gpr176, the measured levels of cAMP in the SCN are higher than for control mice.

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Ghrelin

Ghrelin in humans

Known actions of ghrelin on various target tissues (source).

Ghrelin is a vertebrate peptide hormone that has been most heavily studied for its role as a signalling molecule that is made in the gastrointestinal tract and which can modulate the behavior of ghrelin receptors on neurons, both in the peripheral and the central nervous system. One of the most well-studied functions of ghrelin in the nervous system is for regulation of eating behavior. Soon after its discovery (2001), exogenous ghrelin was shown to increase food consumption in humans.

GOAT

acylation of ghrelin by GOAT (source)

One of the interesting features of ghrelin as a peptide hormone is that acylation of the ghrelin peptide is required for binding of the hormone to its receptor, GHSR1.

The acylation of ghrelin seems to be regulated. An hypothesis was proposed (2009) that ghrelin might function as part of a regulatory system for supporting and enhancing food consumption under conditions where high-energy sources are available in the environment.

How ghrelin acts through binding to the GHSR1 G Protein-coupled receptor is being studied in both peripheral sensory neurons and neurons of the CNS. Some of ghrelin’s action to increase feeding behavior might be due to blockage of appetite suppressing signals that reach the brain by way of sensory neurons that transmit signals from the gut.

In addition to effects on neurons of the peripheral nervous system, ghrelin influences the activity of CNS neurons such as populations of neurons in the hypothalamus that regulate feeding behavior (2015a). In a study of hypothalamic neurons (2015), GHSR1 was shown to inhibit pre-synaptic calcium channel activity and inhibit GABA release. It is hypothesized that this could contribute to ghrelin’s ability to regulate parts of the hypothalamus that are involved in the regulation of eating behavior.

Ghrelin receptors in other parts of the brain in addition to the hypothalamus seem to be involved in the regulation of learned eating behaviors and calorie-rich food seeking behaviors. Since 1) the GHSR1 receptor has high levels of activation even in the absence of ghrelin binding (2009) and 2) GHSR1 can form complexes with other receptors (2013), it is not clear that all the functions of ghrelin receptors in the brain rely on the transport of ghrelin across the blood-brain barrier. Neurons in various brain regions such as the mesolimbic reward system display altered electrical activity in response to administered ghrelin under conditions where GHSR1 receptors interact in complex ways with other neurotransmitter systems (2016).

hypothalamus

Model for action of ghrelin in the CNS (source)

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Measles

From the World Health Organization’s Measles Fact sheet (Updated November 2014):

WHO measles facts

From the World Health Organization

Measles information from the U.S. Centers for Disease Control:

Manual for the Surveillance of Vaccine-Preventable Diseases, Chapter 7: Measles

The CDC measles outbreak webpage.

California Department of Public Health measles webpage with information for the current outbreak.

resent California measles cases

From the California Department of Public Health

mmrI recently turned 56 years of age and I was surprised to see that the majority of measles cases in California for the current outbreak are among older people. I’ve read that one measles patient in the current outbreak is 57 and another is 70 years old.

Vaccine GapYou’re squarely in that gap if you were born between 1957 and 1971

I was vaccinated for measles back in the 1960s, before the current two-dose-vaccination protocol became recommended, so today I went to the SCNM clinic and got a MMR (measles, mumps, and rubella) vaccination shot.

Looking to the future: what could happen in the United States?

figure 3

2008–2011 measles outbreak, France

The figure above is from Measles Elimination Efforts and 2008–2011 Outbreak, France. In 2011 there were more than 20,000 measles cases in France and almost 5,000 patients were hospitalized, including 1,023 for severe pneumonia and 27 for encephalitis/myelitis; 10 patients died.

Measles and the brain

Studies indicate that about 1 of every 1000 people infected by the measles virus will show evidence of the spread of virus to the central nervous system (infectious virus can be isolated from the cerebrospinal fluid or brain). The measles virus has a lipid envelope with two glycoproteins that have been shown to be important for infection of human cells: the fusion protein (F) and the receptor-binding protein (H).

Since some people with compromised immune system function cannot be vaccinated and since some vaccinated individuals develop encephalitis, additional treatments for measles are being sought. For example, it has been demonstrated that for laboratory animals, peptides with sequences corresponding to parts (such as the C-terminal heptad repeat region; HRC) of the measles virus fusion protein can be conjugated to cholesterol and upon injection into animals can protect against fatal measles virus infection.

Related Reading:

Feb. 2 2015 article about measles by Dr. Matthew Baral.

Feb. 3 2015 AZCentral.com “Gilbert pediatrician speaks out after kids’ measles exposure

nme

Invitation to Outbreak: Arizona does not enforce its own rules for documenting school children with non-medical reasons for not vaccinating (story at azcentral.com).

Outbreak timeline

December 17-20 2014 – Disneyland visitors infected (n=39, 5 from Arizona, 4 Pinal Co. residents (P0) & 1 Mericopa Co. resident (case M0))

January 11, 2015 – 1 additional Mericopa County woman (M1) was infected by P0 at a clinic in Mesa

Jan 20 & 21 – additional people (including unvaccinated children <1 year old) were exposed at a clinic in Mesa by M1

Jan 22 – Announcement of case M0

Jan 27 – Announcement that a total of 2 more people (P1, P2) had been infected in Pinal Co.

No additional cases were reported: summary of the outbreak

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Narrated PowerPoint Presentations

PowerPoint animation options

I previously blogged about using ProfCast to add audio narration to PowerPoint presentations. I had better results recording audio using the internal microphone of a Macintosh than the internal microphone of an HP laptop, but I was left wanting to try a better microphone. Also, when I lecture, I tend to spend a significant amount of time discussing rather busy slides and doing a lot of pointing at specific features in the slide, so I needed a good way to automate all that pointing.

I finally got a headset with a noise-canceling microphone, which, amazingly, can be used right next to a running fan. I have also taken the time to start learning how to use the PowerPoint animation features, which can replace my habit of pointing at details on the slides.

I tried recording directly into PowerPoint using the Insert > Audio > Record Audio… option. I ran into the problem (discussed here) of the default audio quality for PowerPoint 2010 being set too low for me to get a good recording.

Another issue is that I like to be able to edit my audio. In the past I’ve used GarageBand, but I wanted to record the audio on the same HP laptop that has PowerPoint 2010. I’m now using Audacity to record audio from my headset’s microphone. I export the sound files from Audacity and then use PowerPoint’s “Insert> Audio > Audio from File…” option.

PowerPoint vs Video. I prefer to provide students with PowerPoint presentations that contain narration and automated pointing because I often insert hypertext links in my slides. However, it is possible to save a PowerPoint presentations as video and some students might prefer a video if they are using a hand-held device that will not play PowerPoint presentations. Select “Save and Send > Create a Video” from the file menu:

Here is an example of a video made from PowerPoint: http://youtu.be/9Veh6s3lbH8

If you use PowerPoint’s default size for presentation slides then you will never really be happy with how videos of your PowerPoint presentations look on YouTube. YouTube uses 16:9 (width:height) video, but the PowerPoint default is a 4:3 ratio. I suggest using PowerPoint >Display menu > Page Setup >Slides sized for: Custom > Width 13.33 inches, Height 7.5 inches. The PowerPoint option to save your videos as “Internet & DVD 852 x 480” will work well with YouTube’s 853 x 480 video format.

I’d like to try making media files available to students  via iTunes U.

I discovered that if you have a slide with animated elements and you add a video to the slide, the video will not play automatically even if you select Video Tools > Playback > Start Automatically. In order for this to work, the video has to be moved to the top (shown below for “gif to movie.wmv”) of the list of animated elements in the Animation Pane where all of the animated elements of the page are listed:

Related Reading about PowerPoint

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Amyloid β-protein and Synaptophysin

Amyloid β-peptide is normally produced in the brain from the amyloid precursor protein (APP). The amyloid cascade hypothesis of Alzheimer disease says that abnormal accumulation in the brain of amyloid β-peptide () causes disruption of synaptic neurotransmission and, eventually, the death of neurons and Alzheimer disease (see the image below).

Amyloid Cascade Hypothesis

During the past 20 years there has been a large amount of research into the normal physiological role of APP and the idea that  can disrupt synaptic neurotransmission.

In a previous blog post I mentioned that the WNT signaling pathway has been implicated in memory storage. There is evidence that the enzyme glycogen synthase kinase 3β (GSK-3β) is involved in memory deficits seen in Alzheimer disease.

A recently published article provides evidence that 42 binds to the synaptic vesicle protein synaptophysin42 disrupts the complex that is normally formed between synaptophysin and VAMP2 altering the way that neurons handle their synaptic vesicles.

If Aβ42 is involved in synaptic transmission by modulating several pre- and postsynaptic mechanisms then such research can identify potential targets for pharmacological intervention against age-associated memory loss.

In a previous blog post I mentioned the synaptic vesicle protein synaptogyrin-3. Synaptophysins and synaptogyrins are members of the tetraspan
vesicle membrane protein family and constitute major secretory vessicle proteins that bind to many other vessicle-associated proteins.

Related reading: The Alzheimer’s amyloid-degrading peptidase, neprilysin: can we control it?

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