The Outside-In Protocol

Henriette van Praag The link between plaques and cognitive decline is not as firmly established as many think. Post-mortem studies show that there are people with plaques who remained cognitively intact (PMID: 35332316). So having plaques is not always synonymous with dementia in terms of a person’s functioning. While we did not see a reduction per se in the number or density of plaques in our mouse model, we have not yet looked at the plaque composition itself — whether they are very dense, or what their measure of toxicity is.

What I can say is that in our proteomics analysis, we do see modifications in the processing of amyloid at the molecular level. There are reductions in amyloid precursor protein and the enzymes that produce toxic fragments, like BACE1. Meanwhile, the non-amyloidogenic pathway enzymes ADAM10 and ADAM17 are elevated. We also see that Psen1, the gamma-secretase involved in producing the most toxic amyloid fragments, is markedly upregulated in untreated Alzheimer’s mice — a pattern that the treatment modifies. So it is not a total yes or no story — it is a modification of the amyloid environment, even if the plaques appear unchanged histologically.

Henriette van Praag Several things may be happening. First, mRNA metabolic process proteins are upregulated resulting in modification of levels of proteins relevant to adult neurogenesis and synaptic plasticity, including those relevant to neural excitation and inhibition. In Alzheimer’s disease, there is often an over-excitability that leads to what we call excitotoxicity. Cathepsin B treatment reduces levels of glutamatergic proteins and increases GABAergic pathways — suggesting a rebalancing of the neural network from within.

Another observation is that in Alzheimer’s mice, we see an increase in myofibrillar proteins in the brain — proteins that are also expressed in muscle and have been previously reported to increase with aging and neurodegeneration in human brains. Our treatment reduces this presence, bringing the brain back toward healthy control levels.

Henriette van Praag I do not have a definitive answer, but the pattern we observe is striking. What happens in the muscle is very consistent with what happens in the brain. In the brain, we see increased RNA metabolism; in the muscle, we see improved cytoplasmic translation and metabolic processes. The brain is not doing one thing while the muscle does something completely different. They appear to work in concert, as one interconnected system.

Henriette van Praag This is a qualitative difference, not simply a dosage issue. Cathepsin B exists in three isoforms: a pro form that is not biologically active, a single chain, and a double chain form. In healthy mice, the treatment reduced the biologically active single-chain form, which may modify the muscle in a way that is not beneficial for function. In Alzheimer’s mice, the treatment resulted in significantly more active, mature Cathepsin B relative to the pro form. The disease state apparently changes how the muscle processes this protein — and further research is needed to determine whether and how that affects everything downstream.

Henriette van Praag That is a possibility, a gene therapy, rather than a pill, that expresses Cathepsin B in skeletal muscle. It could be of interest to study muscle tissue from people with cognitive impairment to see how their cells respond to the vector in a laboratory setting. This could allow medical professionals to screen patients — to determine whether the intervention is relevant for them before applying it. Someone who already shows changes in muscle, and who cannot exercise, could potentially be identified and treated this way. However, we are still very far away from a safe and applicable intervention for people. Much more preclinical research is needed first.

Henriette van Praag That is exactly what I was thinking and what we discussed above. A biopsy or blood test could be a way of detecting early changes. Whether risk can be identified twenty years in advance remains unknown. But predicting risk from muscle health, before cognitive symptoms appear, is very much in line with where this research is pointing.

Henriette van Praag Sarcopenia — the clinical loss of muscle mass that comes with ageing — is statistically linked to cognitive decline. Maintaining muscle health is not just about strength; it appears to matter for the brain as well. The general recommendation for older people is strength training, which helps not only mass but also metabolic activity and possibly the signaling signature that the muscle sends to the rest of the body.

Henriette van Praag Levels are the key. In certain cancers, concentrations of Cathepsin B in blood are higher than what we see with exercise, and can be associated with poorer disease outcome.

Henriette van Praag I very much agree that we should not treat the brain in isolation. A comprehensive approach — physical activity, healthy diet, improving metabolic health generally — would go a long way toward delaying or perhaps even preventing the onset of the disease. That is why I am talking to you: so you can make that recommendation to everybody, because I am just a scientist.

Henriette van Praag Oh yes, probably hundreds. Muscle is a vast endocrine organ, and even treating with Cathepsin B likely changes the whole proteomic profile of what else the muscle secretes — the other myokines, not just Cathepsin B itself. We have a long way to go to identify all of them, and especially the ones that are signaling to the brain.