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
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.
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“).
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.
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).
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?
In their 2016 article, Sacktor et al present several types of data:
- ZIP blocks L-LTP in both normal mice and mice lacking PKMζ.
- ZIP inhibits both the purified PKMζ enzyme and purified PKCι/λ.
- PKMζ -antisense oligonucleotides block L-LTP in normal mice but not in mice lacking PKMζ.
- An antagonist for PKCι/λ (ICAP) blocks the maintenance of L-LTP in mice lacking PKMζ.
- ICAP blocks spatial long-term memory maintenance in mice lacking PKMζ but not in normal mice.
- PKCι/λ is persistently up-regulated in brain tissue lacking PKMζ when L-LTP is induced, unlike the situation for normal mice.
- 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“.