Brain’s Tiny Architect: Unpacking the Secrets of MPP6 in Mouse Synapses!

Hey there, fellow brain explorers! Ever get curious about the microscopic movers and shakers that make our brains tick? Well, I’ve been diving into some fascinating research about a particular protein, Membrane Palmitoylated Protein 6 (MPP6), and its adventures in the synapses of the mouse cerebrum. It sounds like a mouthful, I know, but trust me, this little guy seems to be playing a pretty important role in how our brain cells chat with each other!

So, What’s the Big Deal with MPP6?

MPP6 isn’t just any protein; it’s part of a family called, you guessed it, the MPP family (MPP1-MPP7). Think of them as versatile construction workers within our cells. They have these cool domains – like specialized tools on a Swiss Army knife – called L27, PDZ, SH3, HOOK, and GK domains. These bits allow them to grab onto and interact with all sorts of other proteins, especially those embedded in cell membranes, like adhesion molecules, and even the cell’s internal scaffolding, like actin. It’s all about teamwork at the cellular level!

Now, we already had some clues about MPP6 from the peripheral nervous system (PNS) – that’s the network of nerves outside your brain and spinal cord. There, it’s known to team up with proteins like 4.1G and CADM4, helping with things like myelin formation (the fatty sheath around nerve fibers). But what about its job in the central nervous system (CNS) – the brain and spinal cord? That’s been a bit more of a mystery.

What really piqued my interest, and that of many researchers, is that recent big-data studies (Genome-Wide Association Studies, or GWAS) have started pointing fingers at MPP6 as potentially being involved in human mental disorders like schizophrenia. Plus, it’s even been flagged as a novel gene involved in regulating our sleep! So, you can see why we’re keen to figure out exactly what MPP6 is doing in the brain.

Our Mission: Finding MPP6 in the Mouse Brain

To get to the bottom of this, we embarked on a study using some very special mice – some were normal (wild-type, or Mpp6+/+), and others were genetically engineered to lack the MPP6 protein (Mpp6−/−). This way, we could compare what happens when MPP6 is present versus when it’s missing.

First off, we needed to confirm MPP6 was even in the mouse cerebrum. Using a technique called Western blotting (which is like a protein line-up), we found clear signals for MPP6 in the brain lysates and, importantly, in the synaptic membrane fractions. Synapses, if you remember your biology, are those crucial junctions where nerve cells communicate. So, finding MPP6 there was a big clue!

We didn’t stop there. We wanted to see who MPP6 was hanging out with. Through immunoprecipitation (a fancy way of fishing out a protein and seeing what’s stuck to it), we discovered that MPP6 forms protein complexes with other members of its family, specifically MPP1 and MPP2, and another important synaptic player called calcium/calmodulin-dependent serine protein kinase (CASK). It’s like a little molecular clique at the synapse!

Interestingly, even though these complexes exist, when we looked at the brains of mice lacking MPP6, the amounts of MPP1, MPP2, Lin7 (another protein that interacts with MPP6 in the PNS), and CASK didn’t really change. This suggests that in the CNS, MPP6 might not be the main transporter or anchor for these buddies, unlike its role in the PNS for Lin7.

Where Exactly is MPP6? A Microscopic Hunt

Knowing MPP6 is at the synapse is good, but “at the synapse” is a bit like saying “in the city.” We wanted a more precise address. So, we turned to immunohistochemistry, a technique that uses antibodies to light up specific proteins in tissue slices.

And voilà! We saw MPP6 in a dot-like pattern throughout the gray matter of the mouse brain – in the cerebral cortex, hippocampus, amygdala, and more. It was pretty scarce in the white matter (the brain’s wiring), which strongly suggested its job was neuron-related. To get even closer, we did double-immunostaining, looking for MPP6 and a known marker for excitatory synapses, GluN1. Guess what? They often appeared together, telling us MPP6 is right there in those excitatory communication hubs, particularly in the postsynaptic regions – the receiving end of the synaptic conversation.

For an even closer look, we brought out the big guns: electron microscopy. This allowed us to see the ultrastructure, the super-fine details. Gold particles, attached to our MPP6 antibodies, showed up mostly on and just beneath the postsynaptic density (PSD) – a protein-rich area crucial for receiving signals. So, MPP6 seems to be a key resident of this postsynaptic neighborhood.

What Happens When MPP6 is Missing? Tiny Changes, Big Questions

Okay, so MPP6 is in the postsynaptic region. Does its absence change anything physically? When we compared the synapses of our Mpp6−/− mice with the normal Mpp6+/+ mice, we found some subtle differences. The synaptic cleft (the tiny gap between communicating neurons) was a bit narrower, and the PSD itself was slightly thinner in the mice lacking MPP6. These might seem like tiny changes, but in the world of brain function, even small structural alterations can have ripple effects.

MPP6 and Behavior: A Puzzling Picture

Given the links to human mental disorders and sleep, we were super curious about the behavior of our MPP6-deficient mice.

We started with an elevated plus-maze test, which is often used to assess anxiety. Mice can choose between open, exposed arms (scary!) or closed, sheltered arms (safe!). While there wasn’t a statistically significant difference in the time spent in open versus closed arms between the two groups of mice, one of our young Mpp6−/− mice did something pretty unusual – it climbed over the top struts of the “safe” enclosed arm for a good 50 seconds! The wild-type mice never did this. It’s just one mouse, so we can’t draw huge conclusions, but it’s a little quirk that makes you go “hmm.”

Next, we looked at locomotor activity, basically how much they move around, especially since MPP6 has been linked to sleep regulation. We implanted tiny devices under their skin to track their movement 24/7. And here, we saw something interesting! The Mpp6−/− mice were actually more active around midnight (when it’s dark and mice are usually busy) but less active from morning to noon compared to the normal mice. This definitely hints that MPP6 might be involved in the delicate dance of sleep-wake regulation.

Putting the Pieces Together: What Does It All Mean?

So, what’s the takeaway from all this detective work? We’ve established that MPP6 is indeed a synaptic protein in the mouse cerebrum, primarily chilling in the postsynaptic region. It forms molecular teams with other MPP family members like MPP1 and MPP2, and also with CASK.

The fact that MPP6 hangs out with GluN1 suggests it’s involved in excitatory synapses. But we also saw some MPP6 staining that didn’t overlap with GluN1. Could it also be in inhibitory synapses, maybe with GABA receptors, like its cousin MPP2? That’s an open question for future research!

The links between MPP6 and psychiatric disorders like schizophrenia, bipolar disorder, and autism spectrum disorder are compelling. It’s even been identified as one of the genes that changed as modern humans migrated out of Africa – how cool is that? The slight reduction in PSD thickness we saw in Mpp6−/− mice is also intriguing. Other proteins linked to social deficits, like Shank3b, also show reduced PSD thickness when absent. It makes you wonder how these structural changes contribute to behavioral differences.

Our behavioral tests gave us some tantalizing hints. While the anxiety test wasn’t conclusive, that one adventurous mouse was memorable! The altered sleep-wake activity, however, is pretty significant, especially given other research linking an MPP6-like gene in fruit flies to sleep patterns. It connects to a broader idea called the “phosphorylation hypothesis of sleep,” which suggests that changes in protein phosphorylation (adding or removing phosphate groups, a common way cells regulate protein activity) in synapses are key to sleep. Proteins like CASK, which MPP6 interacts with, are kinases, so they’re involved in this very process!

It seems MPP6 is part of a larger, complex network of proteins called Membrane-Associated Guanylate Kinases (MAGUKs), which also includes the Dlg family (like PSD95). These MAGUKs are master organizers at the synapse, and their dysfunction is increasingly linked to mental disorders. Understanding how all these players, including our friend MPP6, work together in the intricate nano-organization of a single synapse is a huge puzzle. But cracking it could give us incredible insights into how our brains work, and what goes wrong in neurological and psychological conditions.

So, while we’ve learned a lot about MPP6, there’s still so much more to uncover. It’s a tiny protein, but it seems to have a big role in the grand theater of the brain!

Source: Springer