The blood-brain barrier boundaries protect the brain by regulating the flow of molecules, peptides and cells, but many therapeutic agents are also blocked. Currently, researchers are exploiting the power of resident drug delivery companies, namely Microglia.

  • Researchers genetically modified human ipsc-microglia to deliver neprilysin when cells were close to amyloid plaques.
  • Both local injections of neprilysin secretory microglia and whole brain engraftment reduced the pathology of Aβ.
  • Only extensive engraftment helped to reduce pathology and improve neural density in the inferior coccyx membrane.

Matthew Bruton Jones of the University of California, Irvine University and his team have designed a protein therapy that ferries human IPSC-derived microglia (IMG) into the brain. They criticized immune cells for expressing the Aβ-degrading enzyme neprisin, but only expressed in regions where the cells encounter amyloid plaques. When researchers injected courier companies into a mouse model of Alzheimer's disease, cells not only reduced the amount of amyloid, but also improved the downstream aspect of Alzheimer's disease. “Of course there are other approaches to reduce it, but [Aβ] At the level, this study demonstrated the powerful potential of IPSC-Microglia to provide novel immune cell therapies for a wide range of neurological diseases,” senior author Blurton-Jones wrote in Alzforum.

Researchers praised the paper. “This work represents a shift in the potential paradigm of how we approach the challenge of providing therapeutics across the BBB,” writes Bart de Strooper of the University of London (comment below). “Replacing endogenous microglia with genetically modified microglia could be a game-changer in the treatment of brain disorders,” he writes.

“In my view, this proof-of-concept study brings to some important and remarkable discoveries,” writes PhD Eva Simonshikova. Students at Marie Event Rembrey Lab at the University of Victoria, British Columbia, Canada. “Utilizing microglia, the main immune cell of the brain that is normally present in pathology, the therapeutic medium brings novelty to the field of microglia targeting,” writes Simoncicova.

A few years ago, both the Blurton-Jones and De Strooper teams came up with a way to wipe out the microglia of the mouse and replace it with a human team of the mouse brain (April 2019 News). The former lab introduces mutations that allow microglia to resist inhibitors that normally kill them, and uses these engineered cells to resident microglia (January 2023 News). These implanted human microglia can prevent and even reverse aspects of neurodegenerative (and even reverse)June 2024 News). This similarly encouraged Brulton Jones of UC Irvine and first author Jean Paul Chadalevian to engineer these microglia to supply therapeutic proteins from within the central nervous system.

They have selected neprilysin, an enzyme that cleaves Aβ peptides among other substrates, and have been extensively studied as its mechanism and potential therapeutic agent (February 2001 News). In the past, researchers have tried several approaches to supply neprilysin, including viruses, neural stem cells, and non-viral structures (Spencer et al. , 2008; Blurton-Jones et al. , 2014; Rofo and Metzendorf et al. , 2022). Chadarevian et al. They wanted a safer, more controlled approach.

When microglia encounter amyloid plaques, they express a specific set of genes. Specifically, in microglia near plaques, promoter CD9 turns on this known plaque response signature. However, when the cells move just a few microns away, CD9 is completely turned off. “It's like this binary genetic switch,” said Chadarevian.

Select the switch. In brain regions of mice injected with human IPSC-derived microglia, CD9 (Y-axis) is expressed in proportion to the amount of amyloid plaque (X-axis) in a specific brain region. [Courtesy of Chadarevian et al., Cell Stem Cell, 2025.]

Scientists used CRISPR to insert a coding sequence for neprilysin under the control of the CD9 promoter of IMG. The hope was that when plaque activates CD9, microglia produce amyloid-degrading proteins. They injected both MITRG mice, older strains that can support human immune cells, and 5X-MITRG transgenic, a cross between MITRG and 5xFAD mice, and used engineered IMGs that generate either membrane-bound or secreted neprilysin. Both methods reduced amyloid and conserved synapses, albeit to a slightly different degree. Membrane-bound neprilysin reduced more amyloids, but secreted neprilysin reduced better protected synapses as measured by the presynaptic protein synaptophysin and postsynaptic protein ELISA. “Ultimately, that's what's important in Alzheimer's,” Bruton Jones said.

Advanced with the secreted version, Chadarevian and his colleagues injected 5x the brains of mice at 2 months of age with engineered microglia. In one group, researchers injected genetically engineered cells into specific brain regions to maintain the mouse's own microglia. Another investigator replaced the mouse's original microglia with a neprilysin-producing IMG resistant to csf1RI, giving the mice an inhibitor, thus ensuring that only implanted microglia survive and survive and re-penetrate the mouse's brain.

Surprisingly, the former local delivery addressed the engraftment of the whole brain in a specific way. Both methods reduced Aβ monomers and oligomers as biochemically measured. Furthermore, both produced improvements in the pathology of secondary Alzheimer's disease, including astroglysis, neuroinflammation and reductions in plasma NFL.

However, in the subcaudal membrane, amyloid deposition began early and because these mice were part of the plaque-packed hippocampus, only widespread engraftment could reduce plaque load, dysplastic neurites, and astrogliopathy, and retain neogenesis density. Subcutaneously, it is located near the injection site, but was not injected directly. Therefore, mice with local engraftment had few human cells in their area. In the alternative group, microglia were generated by migrating throughout the lower tail. “In areas with a lot of amyloid pathology, we may need more of these cells to achieve an effect,” Blurton-Jones said.

Damage control. As amyloid plaques mature, they damage the nerve axons and begin to swell. In this diagram, halos of dystrophic neurons (yellow) are formed around the plaque (blue) of the outer culozone. Axonal damage is most reduced in whole brain engraftment (panel 4) of neprilysin-secreting human microglia (red). [Courtesy of Chadarevian et al., Cell Stem Cell, 2025.]

“We were surprised to see that this approach not only led to improvements in amyloid plaque pathology, which was somewhat anticipated based on specific biological uses, but also led to the normalization of several neuroinflammatory, neurotic damage, and astroglytic markers.” “This means that the strategies used can not only clear amyloid plaques, but can ultimately alter the complex networks downstream of cellular interactions underlying Alzheimer's disease.”

Importantly, for safety, plaque-induced neprilysin production did not lead to measurable cleavage of other substrates of neprilysin, scientists report.

Can IMG be designed to respond to the specific pathology of other diseases where microglia activation is known to play a role? Scientists addressed the questions using breast cancer brain metastasis and multiple sclerosis. They injected IMG into mouse models of these diseases and found that microglia carried a unique transcriptional response to each pathology. For each disease, the team identifies pathology-specific promoters and believes that CD9 can function similarly to how amyloid plaques are responding to. “The main thing we're trying to highlight is that there's a platform that's more widely applicable here,” says Chadarevian.

“This is a whole new paradigm for preclinical therapy development. It's very modular and customizable,” writes Chris Bennett at the University of Pennsylvania.

The team then wants to publish their papers on a less invasive route than the intracranial injections they discovered to put microglia in the brain. They are also investigating whether engineered microglia can work in lysosomal storage diseases. Regarding the approach of first depleting endogenous microglia and then replacing them with modified IMGs, Michael Simmons of DZNE in Munich suggested that nasu-hakola might be suitable for studying (see comments below).

Andrea Tamayo is a freelance writer in Brooklyn, New York.

News Quotes

  1. Chimeric mice: Can they model the response of human microglia?
  2. Healthy, drug-resistant microglia reactivating mouse brains
  3. Microglia implantation reverses aging-related pathology in mice
  4. Neprilysin comes out of the shadow

Paper quote

  1. Spencer B, Ra Ra, Rockst Room, Crews L, Adame A, Pot R, Patrick C, FH Gage, Im Verma, Massiah E.
    Long-term neprisin gene transfer is associated with reduced levels of intracellular Abeta and improved behavior in APP transgenic mice.
    BMC Neurosci. 2008; 9:109.
    PubMed.
  2. Blurton-Jones M, Spencer B, Michael S, Castle NA, Aa Agazaryan, Davis JL, FJ Music, Loring JF, Masliah E, Laferla FM.
    Correction: Genetically modified neural stem cells to express neprilysin reduce pathology in Alzheimer's disease transgenic models.
    Stem cells are res. 2024 March 25th; 15(1):88.
    PubMed.
    Modified paper.
  3. Rofo F, Metzendorf NG, Saubi C, Suominen L, Godec A, Sehlin D, SyväenS, HultQvist G.
    Blood-brain barrier penetration neprilysin degrades monomeric amyloid beta in a mouse model of Alzheimer's disease.
    Alzheimers Res. 2022 December 5th; 14(1): 180.
    PubMed.

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