The Potential of Optogenetics to Implant, Modify and Erase Memories.

Ever since Boyden et al. pioneered light-dependent neuromodulation in 2005, a plethora of studies have elucidated the various facets of optogenetics. The subject of this essay is to demonstrate the significance of this research by exemplifying the use of optogenetics in memory research while critically evaluating its implications in the light of technical and ethical challenges.

Optogenetics

Optogenetic control of neural activity stands out through unprecedented spatiotemporal accuracy (Figure 1) and its rapid development into a highly sophisticated neuromodulation technology (Figure 2) has enabled it to venture into associational and interventional research while tackling unmet clinical needs. This essay will focus on comprehensively elucidating the application of optogenetics to implanting, retrieving and even erasing memories

Figure 1: Simplified illustration of optogenetic control of cell activity. Optogenetic control of cellular activity the functional integration of light-responsive proteins (opsins), sophisticated optics and gene editing technologies. At its core, optogenetics enables targeted control of activity and more general effector functions in neural (and non-neural) cells following specific light-stimuli on millisecond timescales (1). This is mediated by delivery of light-sensitive ion-channels, pumps and enzymes into naturally non-light-sensitive cells (2). The most prominent optogenetic actuators of single-component optogenetic systems, as shown in this figure, which allow the bidirectional modulation (excitation & inhibition) of transmembrane currents in excitable cells, are channelrhodopsin (light-gated ion channels), halorhodopsins (light-gated ion-pump) and bacteriorhodopsin (light-gated proton pump), which can be delivered with spatial specificity to selected cellular subpopulations (e.g. specific neuron-types) by various genetic-targeting methods including in-utero electroporation, inserting of rhodopsin-genes behind cell-specific promoters and the usage of compatible viral vector such as adeno-associated viral vectors (see figure). Applying light (photons of a specific wavelength) induces conformational changes in the opsins, effectively opening them to ions they are selective for. Finally, behavioural observations, electrical recordings, calcium imaging or more sophisticated biosensing modalities allow a rapid read-out of the cellular activity (here, intracellular electrophysiological measurements of neuronal activity are depicted), which is crucial for appropriate modulation of the stimulus. Own figure.

Figure 2: The past, present and future of optogenetics. Rather than pledging to be an exhaustive chronology, this figure merely illustrates key advances and milestones from the early days of light-dependent neuromodulation to modern-day optogenetics. As postulated by Deitheroth in 2015 (3), optogenetics is a “three-body problem”, referring to opsin genomics, optics and engineering whose cross-integration, enabled by a large body of preceding literature (3,4,5,6,7,8), effectively paved the way for Boyden et al.’s paradigmatic research, which constituted a tipping point in the developmental trajectory of optogenetics. While in-vitro activation of ion-channels by light had been demonstrated (9) and the optical control of neurons had been intended before employing various strategies (10,11), Boyden et. al first successfully transduced opsins into neurons and activated them (1). Consequently, widespread scepticism tipped into enthusiasm and a multitude of studies followed up, not only replicating Boyden’s findings(12,13), but improving and sophisticating various parts of the optogenetic tool-box; Mentionable milestones were the first successful application to mammalian(14,15,16) and primate (17) behavioural control, multiple-feature targeting(18) the development and discovery of novel rhodopsin variants with modified properties like responding to different wavelengths, altered kinetics, and ion-conductances and permitting inhibitory control(19,20,21,22,23) and its application on non-neural cell types such as ES-cells(24, 25) and cardiomyocytes(26), corroborating its universality. Its potential in a therapeutic context has been demonstrated extensively (27), particularly regarding eye-related disorders(27, 28, 29) and first clinical trials for retinitis pigmentosa(30) and urinary bladder syndrome have been approved. More recent breakthroughs include the development of minimally invasive, wireless, and transdermal optoelectronic systems (31,32), single-cell resolution optogenetics(33) , highly specific viral targeting of neuron-subclasses(34) , multi-site targeting(35) and closed-loop optogenetics in which stimulation parameters are automatically modulated to match the desired stimulation based on near-real-time read-outs of neural activity resulting in enhanced temporal specificity(36,37). The widely shared vision of this technology’s future is of a minimal-invasive tool allowing the separate control of multiple cell types with an optimal spatial and temporal resolution allowing to mimic and, if needed, replace natural physiological mechanisms providing almost limitless opportunities to investigate and control biological systems. Own figure.

Optogenetics and memory

Memory is the capacity to store and retrieve information, and as such, shapes identities of “remembering-beings”, guides behaviour and is fundamental to moral and ethics and their experience-inducible transgression. It exists in various forms (short-term, long-term) and can be distinctively subdivided as a function of its behavioural manifestation (declarative, non-declarative)(38,39). Each form and category of memory has a neural basis and, consequently, finds its physical correlate in idiosyncratic neural architecture(40,41), thus, qualifying as a target for optogenetic intervention. In general but particularly regarding this specific application, the significance of optogenetics is twofold: Firstly, optogenetics has demonstrated its potential in driving epistemological progress in memory research such as challenging and substantiating hypotheses regarding the implication and identification of hippocampal, striatal, cortical, and amygdalar structures in mechanisms of memory formation, modification and retrieval(42,43,44) as it greatly facilitates the causal investigation of neural circuitry, thus, eclipsing traditional and commonly invasive methods such as physical and genetic lesions or pharmacological and electrical stimulations of these brain regions, which lack spatial(45) and temporal(46) definition. Secondly, drawing from the corpus of literature it has generated and its steadily increasing sophistication, optogenetic tools can be utilized to exert control over processes of memory formation, modification, and retrieval which manifests in various forms:

Creating memories

The “implantation” of untrue memories into humans was first demonstrated systematically by Dr Loftus, using deception and misinformation to alter the subject’s recollection of events(47,48,49). Almost 40 years later, optogenetic manipulation of neural circuitry has been used to transcend this error-prone, communication-dependent memory-modification method and to fabricate memories artificially via contextual fear-conditioning paradigms(50), in which hippocampal engrams of a fear-memory, activated naturally in a fear-evoking context, were reactivated optogenetically in a separate fear-unrelated context and sustainably induced for the non-fear context, effectively relating to the artificial insertion of a fear-memory trace. Since, multiple studies have incepted false memories using similar methods(51,52).

Changing memories

Memory valence refers to the emotional “content” of a memory, which arises by conditioning a memory with an emotional stimulus. Its neuroarchitectural substrate in the limbic system (53) can be perturbed to reverse the polarity of the valence and turn a negatively “tainted” memory into a positive memory by optogenetically reactivating these particular memory engrams in the context of new, emotionally opposite stimuli(54).

Deleting memories

Further research suggests that erasing specific memories is achievable, too (55,56). The capacity of optogenetics to bidirectionally modulate neural activity was exploited to induce synaptic plasticity-mechanisms (LTP, LTD) in memory-engrams to inactivate and reactivate specific memories in similar fear-conditioning experiments (55) and is applicable to recent memories and consolidated ones (57). In contrast to amnestic agents, optogenetics allows the erasure of specific memories as opposed to generally blocking the consolidation of new memories as seen in drug-induced amnesia(58). The capability to modify memories implicates exciting applications for post-traumatic stress disorders, in which persistent recollection of memories and their emotional component escalates to a debilitating pathology(59), which in parts could be reversed or prevented by such memory-modification(60).

Clinical applications (memory)

As PTSD is not the only pathology involving memory, optogenetics may help to treat other memory-impairing neurodegenerative pathologies (61). This was exemplified in multiple studies by optogenetically reactivating silent memory-engrams in retrograde amnesia (62), retrieving “lost” memories in mice genetically engineered to develop Alzheimer through engram-specific optical stimulation (63) or indirectly improving prima facie unrelated conditions such as depression by modifying negative memories and reactivating positive ones (64).

Memory enhancement

As with most biotechnological advances, these disruptive technologies can be reappropriated for enhancement. Various pharmacological stimulants advertise memory enhancement, yet, optogenetics incorporates a giant leap in efficacy through its ability to selectively target and repetitively stimulate memory engrams and consequently enhance the consolidation of memories(65), which may allow us to relive memories that would otherwise fade over time(66).

Challenges and constraints

Despite its widespread applications in research and promising advancement toward clinical use, optogenetics falls short of a panacea, which becomes particularly apparent when considering technical challenges and ethical constraints.

Translatability

A lack of translatability to humans is a common constraint of animal-based experimentation in laboratory environments and procedures as seen in most simple, optogenetic interventions involving contextual conditioning may curtail realism (67). However, the simplicity of these experiments should not be mistaken for a de-facto barrier to translatability, as in humans too, associative memory systems have been implicated in contextual conditioning (68) and, more generally, first optogenetics-based clinical trials for humans have shown promising results (Figure 2).

Technical issues and safety issues

Technical issues concerning general applications of optogenetics have been investigated in depth (69) such as the recent discovery that long-term expression of channelrhodopsin may induce abnormal axon morphology (70), however, some challenges are particularly critical in the context of memory modification and usually translate into a question of efficacy: Heterogeneity of optogenetic control may arise at the level of transgene-expression and optical stimulation, which often vary across a neuronal population (69). This may lead optogenetic stimulation to elicit different responses within a memory-engram by driving plasticity in some neurones and not in others, which is problematic because the accurate control of cellular plasticity-mechanisms is imperative to the modification of memories (71). Furthermore, the neural circuits in highly interconnected memory engrams, that are postulated to encode for a specific memory (41), may play a role in adjacent neurocircuits, and thus optogenetic alterations to modify memories may have undesirable side-effects such as shown in experiments where memory-enhancement in mice elicited a sensitization to pain(72).

Other, more practical, challenges may arise as a result of optogenetic memory-modification: While memories can be erased, the context in which these memories emerged is unalterable and the consequences of the event erased from the person’s memory may compound and lead to a divergence and conflict between the person’s actual “Umgebung” and the premisses this person holds about the world around her. Furthermore, the adaptive nature of memory and its purpose for survival is well established(73), raising the question of whether modifying memories or their valence, even if the relation to survival-related information is not apparent, may disinhibit a subject from engaging in dangerous situations. The validity of this concern has been confirmed empirically when pharmacological suppression of memory-associated emotionality in arachnophobic persons increased their tendency to engage with venomous spiders outside the laboratory environment(74).

Ethical concerns

While technical challenges may be overcome in the foreseeable future, a less tangible obstacle originates from ethical implications, which are plentiful in the realm of optogenetic memory-modification, shedding light on why concerns have gathered momentum in the neuro-ethics community (75,76).

As mentioned above, remembering does not simply refer to storing and retrieving information, but is tightly intertwined with our character, defines our identity, and calibrates our moral compass, raising considerable problems when tinkering with its very substrate, our memory.

The timeless intention to define identity has launched a centuries-old debate and various theories compete to this day (77,78) of which the “Memory Theory” by empiricist John Locke is widely supported, stating that a person’s identity is defined by what this person remembers (79). The notion that identity is composed by an amalgamate of an individual’s autobiographic and self-defining memories, which was partly demonstrated experimentally as qualitative alterations in Alzheimer’s patients’ identity (80), would suggest that optogenetic manipulation of memories relates to altering the very fundament of a person’s identity (77) and may, thus, compromise their authenticity. Beyond identity per se, evaluative frameworks and moral values, which are also theorized to contour a person’s identity(81), may change as a consequence, especially if the valence of a particular memory is changed. For example, changing the valence of a policeman’s memory of a firefight may incline him to find satisfaction in the latter. The opportunity to abuse this potential, by autocratic regimes for their soldiers, for example, is evident, which is why prohibitive legislation may be sensible. However, imposing a legislative mandate on people’s rights to exercise “control” over their minds may infringe the central value of Kantian tradition and utilitarian liberalism that are fundamental to western ethics and legislation. However, a pragmatic compromise between individual autonomy and society welfare, which is characteristic of biomedical ethics and anthropotechnology, may allow treating those debilitated by dysfunctional memory while limiting enhancement-related applications(82).

Conclusion

Considering this specific application of optogenetics, it is sensible to argue that Boyden’s paradigm-shifting research revolutionized the way biological questions are tackled which, in near future, is likely to provide a much deeper understanding of the functioning of neural circuitry and non-neural systems. Optogenetic memory modification is a testimony of this technology’s universality and potential for clinical applications although novel therapies are more likely to arise as a consequence of the scientific insights optogenetics generates. However, enthusiasm about this technology should be tempered with caution and should not drive the technology to outpace ethical debate as the latter should not be a reactive and ephemeral consideration but a proactive response to the vast array of opportunities optogenetics opens up.

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