Alzheimer's Disease: A Suitable Case for Treatment with Precision Medicine?

Alzheimer's disease (AD) is the most common cause of neurodegenerative impairment in elderly people. Clinical characteristics include short-term memory loss, confusion, hallucination, agitation, and behavioural disturbance. Owing to evolving research in biomarkers AD can be discovered at early onset, but the disease is currently considered a continuum, which suggests that pharmacotherapy is most efficacious in the preclinical phase, possibly 15 - 20 years before discernible onset. Present developments in AD therapy aim to respond to this understanding and go beyond the drug families that relieve clinical symptoms. Another important factor in this development is the emergence of precision medicine that aims to tailor treatment to specific patients or patient subgroups. This relatively new platform would categorize AD patients on the basis of parameters like clinical aspects, brain imaging, genetic profiling, clinical genetics and epidemiological factors. This review enlarges on recent progress in the design and clinical use of antisense molecules, antibodies, antioxidants, small molecules and gene editing to stop AD progress and possibly reverse the disease on the basis of relevant biomarkers.

Friends and Foes in Alzheimer's Disease.

Alzheimer's disease (AD) is a disabling neurodegenerative disease. The prognosis is poor, and currently there are no proven effective therapies. Most likely, the etiology is related to cerebral inflammatory processes that cause neuronal damage, resulting in dysfunction and apoptosis of nerve cells. Pathogens that evoke a neuroinflammatory response, collectively activate astrocytes and microglia, which contributes to the secretion of pro-inflammatory cytokines. This leads to the deposit of clustered fragments of beta-amyloid and misfolded tau proteins which do not elicit an adequate immune reaction. Apart from the function of astrocytes and microglia, molecular entities such as TREM2, SYK, C22, and C33 play a role in the physiopathology of AD. Furthermore, bacteria and viruses may trigger an overactive inflammatory response in the brain. Pathogens like Helicobacter pylori, Chlamydia pneumonia, and Porphyromonas gingivalis (known for low-grade infection in the oral cavity) can release gingipains, which are enzymes that can damage and destroy neurons. Chronic infection with Borrelia burgdorferi (the causative agent of Lyme disease) can co-localize with tau tangles and amyloid deposits. As for viral infections, herpes simplex virus 1, cytomegalovirus, and Epstein-Barr virus can play a role in the pathogenesis of AD. Present investigations have resulted in the development of antibodies that can clear the brain of beta-amyloid plaques. Trials with humanized aducanumab, lecanemab, and donanemab revealed limited success in AD patients. However, AD should be considered as a continuum in which the initial preclinical phase may take 10 or even 20 years. It is generally thought that this phase offers a window for efficacious treatment. Therefore, research is also focused on the identification of biomarkers for early AD detection. In this respect, the plasma measurement of neurofilament light chain in patients treated with hydromethylthionine mesylate may well open a new way to prevent the formation of tau tangles and represents the first treatment for AD at its roots.

Parkinson's Disease: A Tale of Many Players.

In 2020, more than 9 million patients suffering from Parkinson's disease (PD) were reported worldwide, and studies predict that the burden of this disease will grow substantially in industrial countries. In the last decade, there has been a better understanding of this neurodegenerative disorder, clinically characterized by motor disturbances, impaired balance, coordination, memory difficulties, and behavioral changes. Various preclinical investigations and studies on human postmortem brains suggest that local oxidative stress and inflammation promote misfolding and aggregation of alpha-synuclein within Lewy bodies and cause nerve cell damage. Parallel to these investigations, the familial contribution to the disease became evident from studies on genome-wide association in which specific genetic defects were linked to neuritic alpha-synuclein pathology. As for treatment, currently available pharmacological and surgical interventions may improve the quality of life but do not stop the progress of neurodegeneration. However, numerous preclinical studies have provided insights into the pathogenesis of PD. Their results provide a solid base for clinical trials and further developments. In this review, we discuss the pathogenesis, the prospects, and challenges of synolytic therapy, CRISPR, gene editing, and gene- and cell-based therapy. We also throw light on the recent observation that targeted physiotherapy may help improve the gait and other motor impairments.

The need for a standard for informed consent for collection of human fetal material.

The ISSCR has developed the Informed Consent Standards for Human Fetal Tissue Donation and Research to promote uniformity and transparency in tissue donation and collection. This standard is designed to assist those working with and overseeing the regulation of such tissue and reassure the wider community and public.

From microsurgery to nanosurgery: how viral vectors may help repair the peripheral nerve.

Reconstructive surgery of the peripheral nerve has undergone major technical improvements over the last decades, leading to a significant improvement in the clinical outcome of surgery. Nonetheless, functional recovery remains suboptimal in the majority of patients after nerve repair surgery. In this review, we first discuss the molecular mechanisms involved in peripheral nerve injury and regeneration, with a special emphasis on the role of neurotrophic factors. We then identify five major challenges that currently exist in the clinical practice of nerve repair and their molecular basis. The first challenge is the slow rate of axonal outgrowth after peripheral nerve repair. The second problem is that of scar formation at the site of nerve injury, which is detrimental to functional recovery. As a third issue, we discuss the difficulty in assessing the degree of injury in closed traction lesions without total loss of continuity of the involved nerve elements. The fourth challenge is the problem of misrouting of regenerating axons. As a fifth and final issue we discuss the potential drawbacks of using sensory nerve grafts to support the regeneration of motoneurons. For all these challenges, solutions are likely to emerge from (a) a better understanding of their molecular basis and (b) the ability to influence these processes at a molecular level, possibly with the aid of viral vectors. We discuss how lentiviral vectors have been applied in the peripheral nerve to express neurotrophic factors and summarize both the advantages and drawbacks of this approach. Finally, we discuss how lentiviral vectors can be used to provide new, molecular neurobiology-based, approaches to address the clinical challenges described above.

Differential effects of lentiviral vector-mediated overexpression of nerve growth factor and glial cell line-derived neurotrophic factor on regenerating sensory and motor axons in the transected peripheral nerve.

Even after reconstructive surgery, major functional impairments remain in the majority of patients with peripheral nerve injuries. The application of novel emerging therapeutic strategies, such as lentiviral (LV) vectors, may help to stimulate peripheral nerve regeneration at a molecular level. In the experiments described here, we examined the effect of LV vector-mediated overexpression of nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) on regeneration of the rat peripheral nerve in a transection/repair model in vivo. We showed that LV vectors can be used to locally elevate levels of NGF and GDNF in the injured rat peripheral nerve and this has profound and differential effects on regenerating sensory and motor neurons. For sensory neurons, increased levels of NGF and GDNF do not affect the number of regenerated neurons 1 cm distal to a lesion at 4 weeks post-lesion but do cause changes in the expression of markers for different populations of nociceptive neurons. These changes are accompanied by significant alterations in the recovery of nociceptive function. For motoneurons, overexpression of GDNF causes trapping of regenerating axons, impairing both long-distance axonal outgrowth and reinnervation of target muscles, whereas NGF has no effect on these parameters. These observations show the feasibility of combining surgical repair of the transected nerve with the application of viral vectors. Furthermore, they show a difference between the regenerative responses of motor and sensory neurons to locally increased levels of NGF and GDNF.

Neuroregenerative effects of lentiviral vector-mediated GDNF expression in reimplanted ventral roots.

Traumatic avulsion of spinal nerve roots causes complete paralysis of the affected limb. Reimplantation of avulsed roots results in only limited functional recovery in humans, specifically of distal targets. Therefore, root avulsion causes serious and permanent disability. Here, we show in a rat model that lentiviral vector-mediated overexpression of glial cell line-derived neurotrophic factor (GDNF) in reimplanted nerve roots completely prevents motoneuron atrophy after ventral root avulsion and stimulates regeneration of axons into reimplanted roots. However, over the course of 16 weeks neuroma-like structures are formed in the reimplanted roots, and regenerating axons are trapped at sites with high levels of GDNF expression. A high local concentration of GDNF therefore impairs long distance regeneration. These observations show the feasibility of combining neurosurgical repair of avulsed roots with gene-therapeutic approaches. Our data also point to the importance of developing viral vectors that allow regulated expression of neurotrophic factors.

Genetic modification of human sural nerve segments by a lentiviral vector encoding nerve growth factor.

OBJECTIVE: Autologous nerve grafts are used to treat severe peripheral nerve injury, but recovery of nerve function after grafting is rarely complete. Exogenous application of neurotrophic factors may enhance regeneration, but thus far the application of neurotrophic factors has been hampered by fast degradation after local application and unwanted side effects after systemic application. These problems may be overcome with the use of lentiviral (LV) vectors that direct sustained local transgene expression in cells. METHODS: Human sural nerve segments were either submerged in or injected with LV vectors encoding green fluorescent protein and cultured in vitro. Production of nerve growth factor (NGF) by nerve segments after injection of LV-NGF was quantified. The effect of NGF produced by LV-transduced fibroblasts derived from human sural nerve segments was assessed on neurite outgrowth in vitro. RESULTS: The injection of vector into nerve segments is a more effective way to deliver the vector than submersion of the nerve in vector-containing medium, leading to large numbers of transduced fibroblasts over a significant extent inside the nerve. The injection of LV-NGF leads to a gradual increase of NGF production, reaching a plateau after 4 days. LV-NGF-transduced human fibroblasts promote neurite outgrowth in vitro. CONCLUSION: We have developed a method to transduce cells in human sural nerve segments with LV vector. This approach holds promise as a powerful novel adjuvant therapy for peripheral nerve surgery and can be performed without changing the routine practice of nerve grafting.

Human neuroma contains increased levels of semaphorin 3A, which surrounds nerve fibers and reduces neurite extension in vitro.

Neuroma formation after peripheral nerve injury is detrimental to functional recovery and is therefore a significant clinical problem. The molecular basis for this phenomenon is not fully understood. Here, we show that the expression of the chemorepulsive protein semaphorin 3A (sema3A), but not semaphorin 3F, is increased in human neuroma tissue that has formed in severe obstetric brachial plexus lesions. Sema3A is produced by fibroblasts in the epineurial space and appears to be secreted into the extracellular matrix. It surrounds fascicles, minifascicles, or single axons, suggesting a role in fasciculation and inhibition of neurite outgrowth. Lentiviral vector-mediated knock-down of Neuropilin 1, the receptor for sema3A, leads to increased neurite outgrowth of F11 cells cultured on neuroma tissue, but not of F11 cells cultured on normal nerve tissue. These findings demonstrate the putative inhibitory role of sema3A in human neuroma tissue. Our observations are the first demonstration of the expression of sema3A in human neural scar tissue and support a role for this protein in the inhibition of axonal regeneration in injured human peripheral nerves. These findings contribute to the understanding of the outgrowth inhibitory properties of neuroma tissue.

Lentiviral vector-mediated reporter gene expression in avulsed spinal ventral root is short-term, but is prolonged using an immune "stealth" transgene.

PURPOSE: Spinal root avulsions result in paralysis of the upper and/or lower extremities. Implanting a peripheral nerve bridge or reinsertion of the avulsed roots in the spinal cord are surgical strategies that lead to some degree of functional recovery. In the current study lentiviral (LV) vector-mediated gene transfer of a green fluorescent protein (GFP) reporter gene was used to study the feasibility of gene therapy in the reimplanted root to further promote regeneration of motor axons. METHODS: A total of 68 female Wistar rats underwent unilateral root avulsion of the L4, L5 and L6 ventral lumbar roots. From 23 rats intercostal nerves were dissected before ventral root avulsion surgery, injected with a lentiviral vector encoding GFP (LV-GFP) and inserted between the spinal cord and avulsed rootlet. In the remaining 45 rats, the avulsed ventral root was injected with either LV-GFP or a lentiviral vector encoding a fusion between a GlyAla repeat and GFP (LV-GArGFP), and reinserted into the spinal cord. Expression of GFP was evaluated at 1,2, 4 and 10 weeks, and one group at 4 months. RESULTS: LV-GFP transduction of either nerve implants or reimplanted ventral roots revealed high GFP expression during the first 2 post-lesion weeks, but virtually no expression at 4 weeks. Since this reduction coincided with the appearance of mononuclear cells at the repair site, an immune response against GFP may have occurred. In a subsequent experiment reimplanted ventral roots were transduced with a vector encoding GFP fused with the GlyAla repeat of Epstein-Barr virus Nuclear Antigen 1 known to prevent generation of antigenic peptides from transgene products. Expression of this "stealth" gene persisted for at least 4 months in the reimplanted root. CONCLUSION: Thus persistent transgene expression can be achieved with non-immunogenic transgene products in reimplanted ventral roots. This demonstrates the feasibility of combining neurosurgical repair with LV vector-mediated gene therapy. The current approach will be used in future experiments with LV vectors encoding neurotrophic factors to enhance the regeneration of spinal motor neurons after traumatic avulsion of spinal nerve roots.

Profound differences in spontaneous long-term functional recovery after defined spinal tract lesions in the rat.

The purpose of this study was to compare spontaneous functional recovery after different spinal motor tract lesions in the rat spinal cord using three methods of analysis, the BBB, the rope test, and the CatWalk. We transected the dorsal corticospinal tract (CSTx) or the rubrospinal tract (RSTx) or the complete dorsal half of the spinal cord (Hx) at thoracic level T8. Functional recovery was monitored for 31 weeks. We found no recovery of consistent inter limb coordination in any experimental group over time using the BBB locomotor rating scale. Quantitative CatWalk analysis revealed significant differences between experimental groups for inter limb coordination (RI). RSTx and Hx animals showed a significant decrease in the RI, and only in the RSTx group did the RI improve from 6 weeks post-lesion onward. Significant differences between experimental groups in step sequence patterns and base of support were also observed. In the rope test all experimental groups had significantly higher error percentages compared to control animals. Tracing of the CST revealed enhanced collateral formation rostral to the lesion in the CSTx group, not in other groups. The results presented here show that locomotor function in all, but CSTx groups gradually improved over time. This is important for studies that employ pharmacological, cell-, and/or gene therapy- based interventions to improve axonal regeneration and functional recovery after spinal cord injury.

Rescue and sprouting of motoneurons following ventral root avulsion and reimplantation combined with intraspinal adeno-associated viral vector-mediated expression of glial cell line-derived neurotrophic factor or brain-derived neurotrophic factor.

Following avulsion of a spinal ventral root, motoneurons that project through the avulsed root are axotomized. Avulsion between, for example, L2 and L6 leads to denervation of hind limb muscles. Reimplantation of an avulsed root directed to the motoneuron pool resulted in re-ingrowth of some motor axons. However, most motoneurons display retrograde atrophy and subsequently die. Two neurotrophic factors, glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF), promote the survival of motoneurons after injury. The long-term delivery of these neurotrophic factors to the motoneurons in the ventral horn of the spinal cord is problematic. One strategy to improve the outcome of the neurosurgical reinsertion of the ventral root following avulsion would involve gene transfer with adeno-associated viral (AAV) vectors encoding these neurotrophic factors near the denervated motoneuron pool. Here, we show that AAV-mediated overexpression of GDNF and BDNF in the spinal cord persisted for at least 16 weeks. At both 1 and 4 months post-lesion AAV-BDNF- and -GDNF-treated animals showed an increased survival of motoneurons, the effect being more prominent at 1 month. AAV vector-mediated overexpression of neurotrophins also promoted the formation of a network of motoneuron fibers in the ventral horn at the avulsed side, but motoneurons failed to extent axons into the reinserted L4 root towards the sciatic nerve nor to improve functional recovery of the hind limbs. This suggests that high levels of neurotrophic factors in the ventral horn promote sprouting, but prevent directional growth of axons of a higher number of surviving motoneurons into the implanted root.

Adeno-associated viral vector-mediated gene transfer of brain-derived neurotrophic factor reverses atrophy of rubrospinal neurons following both acute and chronic spinal cord injury.

Rubrospinal neurons (RSNs) undergo marked atrophy after cervical axotomy. This progressive atrophy may impair the regenerative capacity of RSNs in response to repair strategies that are targeted to promote rubrospinal tract regeneration. Here, we investigated whether we could achieve long-term rescue of RSNs from lesion-induced atrophy by adeno-associated viral (AAV) vector-mediated gene transfer of brain-derived neurotrophic factor (BDNF). We show for the first time that AAV vectors can be used for the persistent transduction of highly atrophic neurons in the red nucleus (RN) for up to 18 months after injury. Furthermore, BDNF gene transfer into the RN following spinal axotomy resulted in counteraction of atrophy in both the acute and chronic stage after injury. These novel findings demonstrate that a gene therapeutic approach can be used to reverse atrophy of lesioned CNS neurons for an extended period of time.

Viral vector-mediated gene transfer of neurotrophins to promote regeneration of the injured spinal cord.

Injuries to the adult mammalian spinal cord often lead to severe damage to both ascending (sensory) pathways and descending (motor) nerve pathways without the perspective of complete functional recovery. Future spinal cord repair strategies should comprise a multi-factorial approach addressing several issues, including optimalization of survival and function of spared central nervous system neurons in partial lesions and the modulation of trophic and inhibitory influences to promote and guide axonal regrowth. Neurotrophins have emerged as promising molecules to augment neuroprotection and neuronal regeneration. Although intracerebroventricular, intrathecal and local protein delivery of neurotrophins to the injured spinal cord has resulted in enhanced survival and regeneration of injured neurons, there are a number of drawbacks to these methods. Viral vector-mediated transfer of neurotrophin genes to the injured spinal cord is emerging as a novel and effective strategy to express neurotrophins in the injured nervous system. Ex vivo transfer of neurotrophic factor genes is explored as a way to bridge lesions cavities for axonal regeneration. Several viral vector systems, based on herpes simplex virus, adenovirus, adeno-associated virus, lentivirus, and moloney leukaemia virus, have been employed. The genetic modification of fibroblasts, Schwann cells, olfactory ensheathing glia cells, and stem cells, prior to implantation to the injured spinal cord has resulted in improved cellular nerve guides. So far, neurotrophic factor gene transfer to the injured spinal cord has led to results comparable to those obtained with direct protein delivery, but has a number of advantages. The steady advances that have been made in combining new viral vector systems with a range of promising cellular platforms for ex vivo gene transfer (e.g., primary embryonic neurons, Schwann cells, olfactory ensheating glia cells and neural stem cells) holds promising perspectives for the development of new neurotrophic factor-based therapies to repair the injured nervous system.

Ex vivo adenoviral vector-mediated neurotrophin gene transfer to olfactory ensheathing glia: effects on rubrospinal tract regeneration, lesion size, and functional recovery after implantation in the injured rat spinal cord.

The present study uniquely combines olfactory ensheathing glia (OEG) implantation with ex vivo adenoviral (AdV) vector-based neurotrophin gene therapy in an attempt to enhance regeneration after cervical spinal cord injury. Primary OEG were transduced with AdV vectors encoding rat brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or bacterial marker protein beta-galactosidase (LacZ) and subsequently implanted into adult Fischer rats directly after unilateral transection of the dorsolateral funiculus. Implanted animals received a total of 2 x 105 OEG that were subjected to transduction with neurotrophin-encoding AdV vector, AdV-LacZ, or no vector, respectively. At 4 months after injury, lesion volumes were smaller in all OEG implanted rats and significantly reduced in size after implantation of neurotrophin-encoding AdV vector-transduced OEG. All OEG grafts were filled with neurofilament-positive axons, and AdV vector-mediated expression of BDNF by implanted cells significantly enhanced regenerative sprouting of the rubrospinal tract. Behavioral analysis revealed that OEG-implanted rats displayed better locomotion during horizontal rope walking than unimplanted lesioned controls. Recovery of hind limb function was also improved after implantation of OEG that were transduced with a BDNF- or NT-3-encoding AdV vector. Hind limb performance during horizontal rope locomotion did directly correlate with lesion size, suggesting that neuroprotective effects of OEG implants contributed to the level of functional recovery. Thus, our results demonstrate that genetic engineering of OEG not only resulted in a cell that was more effective in promoting axonal outgrowth but could also lead to enhanced recovery after injury, possibly by sparing of spinal tissue.

Adeno-associated viral vectors as agents for gene delivery: application in disorders and trauma of the central nervous system.

The use of viral vectors as agents for gene delivery provides a direct approach to manipulate gene expression in the mammalian central nervous system (CNS). The present article describes in detail the methodology for the injection of viral vectors, in particular adeno-associated virus (AAV) vectors, into the adult rat brain and spinal cord to obtain reproducible and successful transduction of neural tissue. Surgical and injection procedures are based on the extensive experience of our laboratory to deliver viral vectors to the adult rat CNS and have been optimized over the years. First, a brief overview is presented on the use and potential of viral vectors to treat neurological disorders or trauma of the CNS. Next, methods to deliver AAV vectors to the rat brain and spinal cord are described in great detail with the intent of providing a practical guide to potential users. Finally, some data on the experimental outcomes following AAV vector-mediated gene transfer to the adult rat CNS are presented as is a brief discussion on both the advantages and limitations of AAV vectors as tools for somatic gene transfer.

Pharmacological, cell, and gene therapy strategies to promote spinal cord regeneration.

In this review, recent studies using pharmacological treatment, cell transplantation, and gene therapy to promote regeneration of the injured spinal cord in animal models will be summarized. Pharmacological and cell transplantation treatments generally revealed some degree of effect on the regeneration of the injured ascending and descending tracts, but further improvements to achieve a more significant functional recovery are necessary. The use of gene therapy to promote repair of the injured nervous system is a relatively new concept. It is based on the development of methods for delivering therapeutic genes to neurons, glia cells, or nonneural cells. Direct in vivo gene transfer or gene transfer in combination with (neuro)transplantation (ex vivo gene transfer) appeared powerful strategies to promote neuronal survival and axonal regrowth following traumatic injury to the central nervous system. Recent advances in understanding the cellular and molecular mechanisms that govern neuronal survival and neurite outgrowth have enabled the design of experiments aimed at viral vector-mediated transfer of genes encoding neurotrophic factors, growth-associated proteins, cell adhesion molecules, and antiapoptotic genes. Central to the success of these approaches was the development of efficient, nontoxic vectors for gene delivery and the acquirement of the appropriate (genetically modified) cells for neurotransplantation. Direct gene transfer in the nervous system was first achieved with herpes viral and El-deleted adenoviral vectors. Both vector systems are problematic in that these vectors elicit immunogenic and cytotoxic responses. Adeno-associated viral vectors and lentiviral vectors constitute improved gene delivery systems and are beginning to be applied in neuroregeneration research of the spinal cord. Ex vivo approaches were initially based on the implantation of genetically modified fibroblasts. More recently, transduced Schwann cells, genetically modified pieces of peripheral nerve, and olfactory ensheathing glia have been used as implants into the injured spinal cord.

Clinical neurotransplantation: core assessment protocol rather than sham surgery as control.

Basic neurotransplantation research evoked clinical trials of restorative brain surgery. Parkinson's disease was the first and primary test bed for this putative new therapeutic method. Various centers performed the grafting surgery and the behavioral evaluations in different ways, and observed a varying degree of symptomatic relief. This led to a plea for double blind placebo-controlled clinical trials, which have since been performed and of which the first outcomes were recently published. In the present paper this approach of experimental neurotransplantation in brain diseases is discussed and rejected. Neural grafting in the central nervous system is irreversible and is therefore not suitable for experimental approaches originally designed for and best suited to drug studies. For Parkinson's disease in particular, the technique is far from optimized to perform large-scale studies at this stage. Moreover, previous negative results of adrenal medulla tissue implantation in the brain of patients make placebo effects rather unlikely. Moral arguments concerning the validity of the informed consent, therapeutic misconception, and the risk/benefit ratio can be added in the plea against this control surgery. Finally, a recommendation is made for study designs that apply a disease-dedicated core assessment protocol (CAP) that can evaluate the period from pre-operative to post-convalescent stages quantitatively, and therefore, unbiased. The strength of these CAPs is that they allow comparisons of different grafting techniques, of results between centers and of other types of interventions and invasive treatments such as deep brain stimulation. On ethical grounds, it is unacceptable not to use a study design that circumvents sham or imitation surgery. It is a challenge for the neuroscience community to develop CAPs for brain diseases that are eligible for neurotransplantation in the future.