1.1 Photodynamic Therapy and Imaging
The purpose of this review is to present the current state of the role of imaging in photodynamic therapy (PDT). In order for the reader to fully appreciate the context of the discussions embodied in this article we begin with an overview of the PDT process, starting with a brief historical perspective followed by detailed discussions of specific applications of imaging in PDT. Each section starts with an overview of the specific topic and, where appropriate, ends with summary and future directions. The review closes with the authors’ perspective of the areas of future emphasis and promise. The basic premise of this review is that a combination of imaging and PDT will provide improved research and therapeutic strategies.

PDT is a photochemistry-based approach that uses a light-activatable chemical, termed a photosensitizer (PS), and light of an appropriate wavelength, to impart cytotoxicity via the generation of reactive molecular species (Figure 1a). In clinical settings, the PS is typically administered intravenously or topically, followed by illumination using a light delivery system suitable for the anatomical site being treated (Figure 1b). The time delay, often referred to as drug-light interval, between PS administration and the start of illumination with currently used PSs varies from 5 minutes to 24 hours or more depending on the specific PS and the target disease. Strictly speaking, this should be referred to as the PS-light interval, as at the concentrations typically used the PS is not a drug, but the drug-light interval terminology seems to be used fairly frequently. Typically, the useful range of wavelengths for therapeutic activation of the PS is 600 to 800 nm, to avoid interference by endogenous chromophores within the body, and yet maintain the energetics necessary for the generation of cytotoxic species (as discussed below) such as singlet oxygen (1O2). However, it is important to note that photosensitizers can also serve as fluorescence imaging agents for which activation with light in the 400nm range is often used and has been extremely useful in diagnostic imaging applications as described extensively in Section 2 of this review. The obvious limitation of short wavelength excitation is the lack of tissue penetration so that the volumes that are probed under these conditions are relatively shallow.






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Figure 1


(A) A schematic representation of PDT where PS is a photoactivatable multifunctional agent, which, upon light activation can serve as both an imaging agent and a therapeutic agent. (B) A schematic representation of the sequence of administration, localization and light activation of the PS for PDT or fluorescence imaging. Typically the PS is delivered systemically and allowed to circulate for an appropriate time interval (the “drug-light interval”), during which the PS accumulates preferentially in the target lesion(s) prior to light activation. In the idealized depiction here the PS is accumulation is shown to be entirely in the target tissue, however, even if this is not the case, light delivery confers a second layer of selectivity so that the cytotoxic effect will be generated only in regions where both drug and light are present. Upon localization of the PS, light activation will result in fluorescence emission which can be implemented for imaging applications, as well as generation cytotoxic species for therapy. In the former case light activation is achieved with a low fluence rate to generate fluorescence emission with little or no cytotoxic effect, while in the latter case a high fluence rate is used to generate a sufficient concentration of cytotoxic species to achieve biological effects.

/pdf/imaging-and-photodynamic-therapy-mechanisms-monitoring-and-76rc7acuq5.pdf

Imaging and photodynamic therapy: mechanisms, monitoring, and optimization.

Mutations in the gene encoding the transcriptional repressor methyl-CpG binding protein 2 (MeCP2) cause the neurodevelopmental disorder Rett syndrome Loss of function as well as increased dosage of the MECP2 gene cause a host of neuropsychiatric disorders To explore the molecular mechanism(s) underlying these disorders, we examined gene expression patterns in the hypothalamus of mice that either lack or overexpress MeCP2 In both models, MeCP2 dysfunction induced changes in the expression levels of thousands of genes, but unexpectedly the majority of genes (∼85%) appeared to be activated by MeCP2 We selected six genes and confirmed that MeCP2 binds to their promoters Furthermore, we showed that MeCP2 associates with the transcriptional activator CREB1 at the promoter of an activated target but not a repressed target These studies suggest that MeCP2 regulates the expression of a wide range of genes in the hypothalamus and that it can function as both an activator and a repressor of transcription

https://science.sciencemag.org/content/sci/320/5880/1224.full.pdf

MeCP2, a key contributor to neurological disease, activates and represses transcription.

https://www.rand.org/content/dam/rand/www/external/labor/aging/rsi/rsi_papers/2013/gage2.pdf

A Model for Neural Development and Treatment of Rett Syndrome Using Human Induced Pluripotent Stem Cells

https://www.cell.com/neuron/pdf/S0896-6273(07)00756-8.pdf

The Story of Rett Syndrome: From Clinic to Neurobiology

Rett syndrome is an autism spectrum disorder caused by mosaic expression of mutant copies of the X-linked MECP2 gene in neurons. However, neurons do not die, which suggests that this is not a neurodegenerative disorder. An important question for future therapeutic approaches to this and related disorders concerns phenotypic reversibility. Can viable but defective neurons be repaired, or is the damage done during development without normal MeCP2 irrevocable? Using a mouse model, we demonstrate robust phenotypic reversal, as activation of MeCP2 expression leads to striking loss of advanced neurological symptoms in both immature and mature adult animals.

http://science.sciencemag.org/content/sci/315/5815/1143.full.pdf

Reversal of Neurological Defects in a Mouse Model of Rett Syndrome

Mutations in the X-linked methyl-CpG-binding protein 2 (MECP2), encoding a transcriptional repressor, cause Rett syndrome and a variety of related neurodevelopmental disorders. The vast majority of mutations associated with human disease are loss-of-function mutations, but precisely what aspect of MeCP2 function is responsible for these phenotypes remains unknown. We overexpressed wild-type human protein in transgenic mice using a large genomic clone containing the entire human MECP2 locus. Detailed neurobehavioral and electrophysiological studies in transgenic line MeCP2(Tg1), which expresses MeCP2 at approximately 2-fold wild-type levels, demonstrated onset of phenotypes around 10 weeks of age. Surprisingly, these mice displayed enhanced motor and contextual learning and enhanced synaptic plasticity in the hippocampus. After 20 weeks of age, however, these mice developed seizures, became hypoactive and approximately 30% of them died by 1 year of age. These data demonstrate that MeCP2 levels must be tightly regulated in vivo, and that even mild overexpression of this protein is detrimental. Furthermore, these results support the possibility that duplications or gain-of-function mutations in MECP2 might underlie some cases of X-linked delayed-onset neurobehavioral disorders.

Mild overexpression of MeCP2 causes a progressive neurological disorder in mice

Recessive mutations in genes encoding voltage-gated Ca 2+ channel subunits alter high-voltage-activated (HVA) calcium currents, impair neurotransmitter release, and stimulate thalamic low-voltage-activated (LVA) currents that contribute to a cortical spike-wave epilepsy phenotype in mice. We now report thalamic LVA current elevations in a non-Ca 2+ channel mutant. EEG analysis of Coloboma ( Cm/ +), an autosomal dominant mutant mouse lacking one copy of the gene for a synaptosomal-associated protein (SNAP25) that interacts with HVA channels, reveals abnormal spike-wave discharges (SWDs) in the behaving animal. We compared the biophysical properties of both LVA and HVA currents in Cm/ + and wild-type thalamic neurons and observed a 54% increase in peak current density of LVA currents evoked at –50 mV from –110 mV in Cm/ + before the developmental onset of seizures relative to control. The midpoint voltage for steady-state inactivation of LVA currents in Cm/ + was shifted in a depolarized direction by 8 mV before epilepsy onset, and the mean time constant for decay of LVA Ca 2+ currents at –50 mV was also prolonged. No significant differences were found in recovery from inactivation of LVA currents or in HVA current densities and kinetics. Our data demonstrate that a non-Ca 2+ channel subunit gene mutation leads to potentiated thalamic LVA currents that precede the appearance of SWDs and that altered somatodendritic HVA currents are not required for abnormal thalamocortical oscillations. We suggest that presynaptic release defects shared by these mutants lead to postsynaptic LVA excitability increases in thalamic pacemaker neurons that favor rebound bursting and absence epilepsy.

https://www.jneurosci.org/content/jneuro/24/22/5239.full.pdf

Elevated Thalamic Low-Voltage-Activated Currents Precede the Onset of Absence Epilepsy in the SNAP25-Deficient Mouse Mutant Coloboma

Apoptosis can be mediated by mechanisms other than the traditional caspase-mediated cleavage cascade. There is growing recognition that alternative proteolytic enzymes such as the lysosomal cathepsin proteases can initiate or propagate proapoptotic signals, but it is currently unclear how cathepsins achieve these actions. Recent in vitro evidence suggests that cathepsins cleave the proapoptotic Bcl-2 family member Bid, thereby activating it and allowing it to induce the mitochondrial release of cytochrome c and subsequent apoptosis. We have tested this hypothesis in vivo by breeding mice that lack cathepsin inhibition (cystatin B-deficient mice) to Bid-deficient mice, to determine whether the apoptosis caused by cathepsins is dependent on Bid signaling. We found that cathepsins are still able to promote apoptosis even in the absence of Bid, indicating that these proteases mediate apoptosis via a different pathway, or that some other molecule can functionally substitute for Bid in this system.

/pdf/apoptosis-caused-by-cathepsins-does-not-require-bid-279pt8xr4z.pdf

Alexander P. Vilaythong

Papers

Mild overexpression of MeCP2 causes a progressive neurological disorder in mice

Elevated Thalamic Low-Voltage-Activated Currents Precede the Onset of Absence Epilepsy in the SNAP25-Deficient Mouse Mutant Coloboma

Apoptosis caused by cathepsins does not require Bid signaling in an in vivo model of progressive myoclonus epilepsy (EPM1)