This is an intriguing study in mice in which researchers explore exactly which structures must be regrown and recovered in the brain in order to restore access to memories lost to the progression of Alzheimer's disease - or, indeed, any other form of neurodegeneration. It is a great example of the growing level of control over fine neural structures in living individuals that is made possible by the latest biotechnologies and an increased understanding of the biochemical basis for memory.
Loss of long-term memory for specific learned experiences is a hallmark of early Alzheimer's disease (AD) that is also exhibited by mice genetically engineered to develop AD-like symptoms. Building on their previous work that identified and activated memory cells, researchers have now shown that spines - small knobs on brain-cell dendrites through which synaptic connections are formed - are essential for memory retrieval in these AD mice. Moreover, fiber-optic light stimulation can re-grow lost spines and help mice remember a previous experience.
Mouse memory is often inferred from learned behavior, in this case associating an unpleasant footshock with a particular cage. Remembering and expecting shocks causes mice to freeze in this enclosure but not in a neutral one. Compared with normal mice, AD mice exhibited amnesia and reduced freezing behavior, indicating progressive memory loss. The engrams, or memory traces, of this particular experience are known to be located in the dentate gyrus of the hippocampus, a key brain area for memory processing. During fear conditioning, researchers used a virus to deliver a gene into the dentate gyrus, which labeled active engram cells. This allowed researchers to visually identify the neurons that made up the engram for that specific fear memory. A second virus contained a gene making only these engram neurons sensitive to light. When the engram cells were reactivated with light in the AD mice, memory of the footshock experience became retrievable and freezing behavior was restored.
Memories restored with this method faded away within a day, and the researchers next sought to understand why this happens. They noted a reduction in the number of spines as the mice aged and their Alzheimer's disease progressed. Their waning memory for the fear training was also linked to a loss of these spines. Previous work had shown that spines grow when neurons undergo long-term potentiation, a persistent strengthening of synaptic connectivity that happens naturally in the brain but can also be artificially induced through stimulation. Through repeated stimulation with high-frequency bursts of light to the hippocampal memory circuit in AD mice, the team was able to boost the number of spines to levels indistinguishable from those in control mice. The freezing behavior in the trained task also returned and remained for up to six days. The implication is that restoring lost spines in the hippocampal circuit facilitated retrieval of the specific fear experience and its associated freezing behavior. Light stimulation did not boost the number of spines in normal mice or strengthen the fear memory, nor did indiscriminately shining light in the dentate gyrus result in any long-term memory improvement. Only the precise stimulation of engram cells was able to increase the number of spines and bring about the memory improvement in AD mice.