·        Upon injury, senescent cells accumulate next to the lesion in the spinal cord.
·        Targeting senescent cells favors a pro-repair microenvironment in the spinal cord after injury.
·        Administration of senolytic drugs promotes recovery of spinal cord mediated functions, such as locomotion and sensation.

Underlying many aging-related chronic disorders are cells that stop growing and replicating — a process called senescence. But senescent cells are now being recognized for their role in tissue remodeling events, especially as organs and tissues try to react and repair themselves in response to damage.

In a study published in Cell Reports, Paramos-de-Carvalho and colleagues from Nova Medical School (NOVA Medical School – Faculdade de Ciências Médicas) show, that following spinal cord injury, senescent cells accumulate over the ensuing weeks and months in mice. This build-up of non-proliferating cells appears to lead to the functional deterioration of the spinal cord. But when the Portuguese researchers depleted these post-injury senescent cells with different senolytic drugs, such as ABT-263 or the combination of dasatinib and quercetin, the locomotor, sensory, and bladder functions of these mice improved.

“Our data support the potential use of therapeutics targeting senescent cells to promote neuroprotection in the context of spinal cord injuries,” propose Paramos-de-Carvalho and colleagues. “Targeting senescent cells is a promising therapeutic strategy not only for spinal cord injuries but potentially for other organs that lack regenerative competence.”

Spinal cord injury causes cell senescence

Though we’ve learned quite a bit of what happens after spinal cord injury, little progress has been made on therapeutic options, suggesting that something might be missing. That’s why Paramos-de-Carvalho and colleagues took a look to see if the induction of senescent cells is a cellular response triggered by an injury in the spinal cord. The Portuguese research team found that the number of senescent cells increased in the spinal cord in the ensuing few weeks after spinal cord injury. These induced senescent cells were primarily seen in the gray matter — where nerve cell bodies lie.

(Paramos-de-Carvalho et al., 2021 | Cell Reports) The senolytic ABT-263 reduces senescent cell numbers following spinal cord injury in mice. (I) Senescent cells, identified by being positive for a marker of senescence called SA-β-gal, were quantified in the total sectional gray matter and only at the ventral horn — where the cell bodies of motor neurons reside. At 15 dpi, the ABT-263 treatment significantly decreased the number of senescent cells (SA-β-gal+) in the total gray matter and the ventral horn by 68.4% and 58.0%, respectively. At 60 dpi, a significant reduction (41.9%) of SA-β-gal+ cells in ABT-263-treated animals was still observed in the ventral horn.

Senolytics improve spinal cord function after injury

So, if senescent cells are induced after spinal cord injury, what happens if you administer drugs called senolytics that target these cells? After treating mice that had spinal cord injuries with the senolytic ABT-263, Paramos-de-Carvalho and colleagues saw significantly improved locomotor performance, an effect maintained until the end of the study. These mice treated with ABT-263 also had improvements in their ability to sense cold stimuli through touch.

(Paramos-de-Carvalho et al., 2021 | Cell Reports) Targeting senescent cells with ABT-263 improves motor, sensory, and bladder function recovery following a spinal cord injury in mice. Mice that underwent spinal cord injuries performed better than non-treated counterparts in tests for movement. When tested for general locomotor behavior by wandering through an open field (C), mice treated with ABT-263 had superior scores compared to the untreated mice. When they had to climb a ladder, the mice treated with ABT-263 made fewer mistakes (D) and made more successful steps (E).

Notably, the effects of ABT-263 on locomotor and sensory recovery were corroborated by a second independent assay using the Dasatinib plus Quercetin senolytic cocktail. In addition, the Portuguese researchers could show that similarly to ABT-263, the Dasatinib plus Quercetin cocktail resulted in efficient depletion of senescent cells at the spinal cord lesion periphery. Taken together, these results highlight the detrimental impact of the persistent accumulation of senescent cells on motor and sensory functions after a spinal cord injury.

(Paramos-de-Carvalho et al., 2021 | Cell Reports) Targeting senescent cells with Dasatinib plus Quercetin promotes motor and sensory recovery following a spinal cord injury in mice. (G) The locomotor performance in the Horizontal Ladder (HL) was assessed at -1 (control), 15, and 30 days post-injury (dpi). Paramos-de-Carvalho and colleagues quantified the total number of mistakes and the percentage of singular positive events (plantar step, toe step, and skip) measured. (H and I) When determining the temperature at which injured mice reacted to a cold or hot stimulus, mice treated with Dasatinib plus Quercetin (D+Q) showed significant improvements when reacting to cold stimuli and minor improvements when reacting to hot stimuli. (J) At 15 dpi, senescent (SA-β-gal+) cells were quantified in the total sectional gray matter. D+Q treatment significantly decreased the number of SA-β-gal+ cells in the total grey matter by 49.3%.

What drives the functional deterioration of the spinal cord post-injury?

Paramos-de-Carvalho and colleagues show that at the root of this functional recovery is an improved and attenuated inflammation, scarring, and demyelination — the loss of the conductive material wrapping nerve cells critical for the transmission of electrical signals in the nervous system. What’s more, the Portuguese researchers were able to pin down​ key senescence-inducing compounds called SASP (senescence-associated secretory phenotype) factors that contribute to the inhibitory microenvironment for repair in mammalian spinal cord injury settings.

(Paramos-de-Carvalho et al., 2021 | Cell Reports) Targeting senescent cells improves functional recovery after spinal cord injury. Senescent cells are induced at the lesion periphery upon spinal cord injury. Administration of senolytic drugs promotes locomotor and sensory recovery. ABT-263 treatment suppresses pro-inflammatory and pro-fibrotic SASP responses. Targeting senescent cells favors a pro-repair microenvironment after injury.

How should we treat spinal cord injury?

Senolytic drugs like ABT-263 selectively eliminate senescent cells by transiently disabling the pro-survival networks and inducing their death, but it does not prevent new senescent cells. The generation of new senescent cells becomes relevant when thinking in a translational approach to optimize the time window for treatment to achieve high efficacy with low toxicity. However, senolytics themselves may not be enough.

NMN supplementation is an effective therapy protecting against multiple related conditions, such as nerve damage to the eye. Also, deletion of the NMN-producing enzyme NAMPT in neurons projecting from the brain to the spinal cord in adult mice leads to motor dysfunction, neurodegeneration, and death; however, NMN treatment reduced motor deficits and increased the lifespan of these mice. A similar paradigm was shown in neurons that project from the spinal cord to muscles. Together, these findings point to using senolytics combined with NMN to prevent degeneration of the nervous system.