Senescent Microglia Release DLK1 and Deteriorate the Aging Brain

Research in mice and human models derived from induced pluripotent stem cells points to the protein DLK1 as a possible link between microglial senescence, myelin loss, and age-associated neuronal dysfunction.

• Microglia with shortened telomeres showed signs of senescence and an altered secretion profile.
• Soluble DLK1 increased in the cerebrospinal fluid of aged mice, while its removal reduced that signal.
• The molecule affected oligodendrocyte maturation and calcium signaling in excitatory neurons.

Senescent cells accumulate over time and release substances capable of altering the tissues where they reside. A study focused on the aging brain now points to a specific signal, soluble DLK1, as one of the mechanisms by which senescent microglia can harm myelin and neuronal function. The results come from experiments with mice, brain tissue analysis, and human models derived from induced pluripotent stem cells.

Cellular senescence occurs when a cell permanently stops dividing after experiencing stress or reaching its replicative limit. During youth, the immune system typically removes these cells more efficiently, but aging defenses allow them to survive, accumulate, and maintain an inflammatory environment. The finding described does not yet demonstrate a therapy for cognitive decline, although it does propose a biological pathway that could guide new research.

The work was presented by Fight Aging! based on an open-access study on microglia with shortened telomeres and their relationship with brain aging. Its authors describe microglia as the resident innate immune cells of the central nervous system, with functions that go far beyond attacking pathogens or removing damaged cells. They also monitor tissue, participate in its maintenance, and contribute to preserving neural circuits, so a sustained alteration of their behavior can have broad effects.

Telomeres and Senescence in the Brain

Telomeres protect the ends of chromosomes but gradually shorten due to the problem of terminal replication. When shortening reaches a critical point, the cell activates a brake on the cell cycle and enters replicative senescence, a state in which it stops multiplying but continues to release signals capable of influencing neighboring cells.

The phenomenon is particularly relevant in glial cells, including microglia, astrocytes, and oligodendrocytes, because these retain proliferative capacity after development. Moreover, they can increase their replicative activity in response to central nervous system injuries and other stress factors, a reaction that initially helps repair tissue but can also impose sustained pressure on their telomeres.

The cited analysis observed that telomeric shortening is detected in the white matter of the brain, while the length of telomeres in the gray matter remains relatively stable. That difference matters because white matter relies on myelin-covered structures to transmit signals between different brain regions, and any alteration of that insulation can affect neuronal communication without immediately destroying the neurons.

Research on neurodegenerative diseases has increasingly focused on overly inflammatory microglia. These cells can respond to pathologies such as tauopathy and the accumulation of beta-amyloid by reactivating proliferative programs, meaning that the same process aimed at containing damage could increase replicative stress and promote a senescent state associated with pathological aging.

DLK1 as a Damage Signal

To examine the consequences of this state, researchers studied the brains of mice with shortened telomeres. In these animals, they found lipofuscinosis, hypomyelination, microglial atrophy, and cognitive deficits, as well as signals compatible with accelerated glial aging detected through single-nucleus RNA sequencing.

Molecular analysis identified elevated pathways related to microglial senescence and allowed the localization of the delta-like non-canonical Notch ligand 1, known as DLK1, as a signal associated with this cellular state. In a model of human microglia derived from induced pluripotent stem cells, the soluble form of the molecule, called sDLK1, appeared increased when the cells adopted characteristics of senescence.

The signal was not limited to cell culture. sDLK1 also increased in the cerebrospinal fluid of mice with shortened telomeres and naturally aged animals, while microglial depletion eliminated this increase. This relationship suggests that microglia constitute a relevant source of the molecule in the aging brain, although it does not allow us to conclude by itself that it is the only source or that the same behavior occurs in all individuals.

Subsequent experiments evaluated what happens when sDLK1 is elevated within the organism. The increase caused hypomyelination and blocked the progression of the oligodendrocyte cell line, cells responsible for producing myelin in the central nervous system; in other words, the signal interfered with the maturation necessary to sustain the insulation of neuronal fibers.

Effects on Oligodendrocytes and Neurons

Myelin functions as a cover that facilitates the rapid and coordinated transmission of impulses between neurons. When it decreases, brain networks can lose efficiency, and the problem may manifest as a disruption of communication between regions rather than as a visible lesion in a single structure.

In human systems based on induced pluripotent stem cells, sDLK1 impaired the maturation of oligodendrocytes. This result aligns with observations made in mice and reinforces the hypothesis that excess signaling can act on support cells essential for maintaining the functional architecture of the brain.

Human models also showed changes in calcium signaling of excitatory neurons. Calcium participates in essential processes of neuronal communication, so a disruption of its dynamics can modify the response of these cells, although the described results do not themselves equate to a complete explanation of the cognitive deficits observed during aging.

The combination of both effects offers a possible explanation for the broad influence of senescent microglia: sDLK1 could simultaneously impair the maturation of myelin-producing cells and the activity of neurons that depend on precise signaling. Nevertheless, the findings should be interpreted as experimental evidence of a specific pathway, not as proof that DLK1 explains all processes associated with Alzheimer’s, cognitive loss, or brain degeneration.

Implications for Brain Aging

The study places microglial senescence at the center of a chain that begins with replicative stress and telomere shortening. Subsequently, the cells adopt an altered secretion profile, release sDLK1, and affect other cell populations that maintain the integrity of white matter and neuronal activity.

This interpretation also helps to separate two functions of microglia that are often presented as opposites. Immune surveillance and damage response are necessary for the central nervous system, but persistent activation or stable senescence can turn a protective response into a source of harmful signals for the tissue.

The possibility that sDLK1 serves as an indicator of age-associated changes will depend on future research that measures its behavior in more contexts and accurately establishes its origin. The available material only indicates increases in cerebrospinal fluid from murine models and effects on human stem cell systems, so it still does not allow for proposing a clinical threshold, treatment, or diagnostic test.

The results open several lines of study, including determining whether blocking DLK1 can preserve myelin without compromising the immune functions of microglia. It will also be necessary to establish whether reducing cellular senescence modifies cognitive deficits and if the observed effects depend on age, type of brain damage, or degree of telomere shortening.

The importance of the finding lies in identifying a specific signal within a complex biological process. If future work confirms that sDLK1 reproducibly connects aged microglia with oligodendrocyte and neuron dysfunction, the molecule could become a central piece in understanding why the brain loses adaptability with age.

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