Neurodegenerative diseases (NDDs) are nervous system disorders that impact around 30 million people globally. Loss of brain tissue is a hallmark symptom of NDDs. Amyotrophic lateral sclerosis (ALS), frontotemporal dementia, Parkinson's disease, Alzheimer's disease, and Huntington's disease are among the NDDs caused by protein misfolding and inappropriate processing of proteins. In addition, neurodegeneration has also been linked to oxidative stress, mitochondrial malfunction, and/or environmental variables strongly correlated with aging. Significant evidence has been obtained after years of intensive research that shows these factors have a crucial role in the etiology of prevalent neurodegenerative disorders. Many clues have been identified regarding neurodegenerative illnesses, but the complexities of these conditions still make them difficult to understand. This chapter presents a more straightforward explanation to help individuals better understand NDDs, their etiology, clinical symptoms, and pathogenesis

Neurodegenerative diseases are a heterogeneous group of disorders and are the leading cause of morbidity and disability. These are described by the progressive degeneration of the neurons and impaired function of the central nervous system. Prevailing neurodegenerative diseases in the world include Alzheimer's disease and Parkinson's disease and reports predict that on average, the prevalence of both diseases will double in a span of the next twenty years. Pieces of evidence showed that the immune system is profoundly involved in brain development, maintenance, and repair as well as in damage, therefore, may provide a wide scope to focus on the neuroinflammation-based therapeutic approaches. In this chapter, the various neuroinflammatory responses will be discussed during the onset and progression of both Alzheimer’s and Parkinson’s disease pathologies. We will be focusing on both central and peripheral inflammatory responses and their consideration for disease diagnosis and therapeutics.

 Neurodegenerative diseases are categorized mostly by protein deposits or known hereditary mechanisms, despite recent studies showing overlap and intraindividual variations in these symptoms. A synergistic interaction between pathological proteins advises extensive pathogenic pathways. Animal models and other studies have uncovered the fundamental mechanisms underlying neurodegeneration and cell death, opening up new avenues for future prevention and therapy plans. A multidomain therapy approach that emphasizes the underlying reasons why diseases alike Parkinson's, Alzheimer's, etc. occur. Neurodegenerative diseases like Parkinson's disease (PD) and Alzheimer's disease (AD) are becoming far more common in the Western world. Neuronal inflammation, gut microbiota, extracellular misfolded protein accumulation, hallmarks of various neurodegenerative nephropathies, and failure of the systemic and cerebral immune systems are some of the elements that affect the immunopathogenesis of neurodegenerative diseases. Deficits in the ubiquitin proteasome autophagy system, abnormal protein dynamics brought on by oxidative stress and free radical formation, mitochondrial dysfunction, impaired bioenergetics, neurotrophins dysfunction, “neuroinflammatory” processes, and (secondary) distractions of neuronal Golgi apparatus and axonal passage are some of the fundamental mechanisms that contribute to immunopathogenesis. Long-term cooperation between these interconnected systems results in programmed cell death. In this review, we discussed every idea and hypothesis that have been put up on the pathophysiology of neurodegenerative disorders.

Huntington’s disease (HD) is a progressive neurodegenerative complication of the brain that causes uncontrolled choreatic movements, memory loss, abnormal motor function, emotional changes, and a decline in cognition as well as an inability to perform daily routine tasks. The development of advanced techniques, including genetics, molecular biology, and genetic engineering, is beginning to discover an anomalous role of immune modulatory molecules in HD onset and pathophysiological complications. However, the role of immunoregulatory molecules, which are the key chemical messengers that mediate intracellular communication to regulate cellular and nuclear functions in HD pathogenesis, is still being unexplored. Here we present recent immunological association studies on HD and emerging mechanisms for the immunotherapies implicated in HD pathogenesis. The implications of immunotherapies are very critical under both healthy and HD disease conditions. Recently, research work has established new functional aspects of their pathways. Moreover, we propose future directions for immune-related research in HD pathogenesis and potential therapeutic approaches for immune-related therapies.

Human gut microbiota (GM) research has emerged as one of the most promising fields in recent years. Moreover, a major area of interest is the connection between GM and several human disorders. Numerous recent studies have demonstrated the vital roles that the gut microbiome plays in human physiology and pathology. Additionally, microbiome-based medicines have been used to cure illnesses. In biomedical research, aging and neurodegenerative conditions such as Alzheimer's and Parkinson's disease have also attracted a lot of attention. To explore the potential pathogenic or therapeutic impacts of GM in diseases, several researchers have examined the connections between these factors. Numerous biologically active chemicals produced by microbiota have an impact on neurochemistry via neuroendocrine, immunological, and metabolic pathways. Gastrointestinal functional disturbances can manifest well in advance of the onset of neurodegenerative disorders. Furthermore, recent advancements in both preclinical and clinical research have indicated that the composition of the GM assumes a significant role in governing the dynamic interplay between the gut-brain axis, potentially bearing relevance to the etiology of neurodegenerative maladies. This chapter focuses on the relationship between the microbiota and neurodegeneration, as well as the pertinent mechanisms, present applications, and potential future prospects for microbiome-based therapy. 

Introduction: The body of scientific evidence linking the microbiome to many diseases has grown dramatically over the past several years; neurological diseases have also shown a similar tendency. As a result, the gut-brain axis theory as well as the notion that there could be a connection between the gut microbiome and several CNS-related disorders whose pathophysiology is still not known have both emerged.

Development: We look at the role played by gut microbiomes in the gut-brain axis as well as the neurological conditions neuromyelitis optica, Alzheimer's, amyotrophic lateral sclerosis, Parkinson's, and multiple sclerosis, where changes in the gut microbiota have been linked to human studies.

Conclusions: The amount of data connecting gut microbiota to different neurological illnesses has significantly increased. Today, there is no longer any doubt that the gut microbiota of the host influences brain function. This review assembles a sizable body of credible research that is essential in emphasizing the crucial role of microbiota colonization in neurodevelopment and how changes in microbiota dynamics might have an age-dependent effect on brain function.

Microglia, the brain resident macrophages perform a wide range of functions ranging from brain development, equilibrium, maintenance in the brain microenvironment, injury repair and the preservation of neuronal networks. Cellular elasticity is a prerequisite for the multi-dimensional tasks performed by microglia. Epigenetic modulations are critically involved in altering gene expression finely coordinating with the phenotypic plasticity in microglia. Any change in its tuning favors the inflammatory state of the brain, which leads to detrimental effects on the nervous system. The present review offers an insightful exploration into the origin of microglia, shedding light on its vital functions and the intricate mechanisms that govern the epigenetic modification of microglia in neurodegenerative disorders. Furthermore, it explores potential avenues for mitigating these diseases.

Epigenetics is a field that is concerned with the investigation of heritable modifications in gene expression that transpire without DNA sequence alterations, thereby establishing a connection between the genome and its surroundings. Epigenetics simply analyzes gene expression amendment beyond variation to the DNA sequence. The gradual accumulation of epigenetic changes over the course of an individual's life span may contribute to neurodegeneration. This chapter deals with epigenetic alteration, which affects the progress of neurodegeneration with age. Epigenetic regulation, encompassing DNA methylation and histone modification, has been implicated in the anomalous alterations in gene expression that occur during the progression of neurodegeneration. The concept of epigenetics is useful to synthesize novel medications to target these disorders. In recent times, a plethora of epigeneticsbased medications have been developed for the treatment of neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. Due to a major lack of early screening processes that allow therapeutic agents to be distributed to afflicted neurons paramount to cell death, many neurological conditions have severely restricted options for treatment. Significant progress has been seen in neurodegenerative disease biomarkers. These biomarkers have been unfortunate, due to substantial disparities amidst the tissues acclimated to source biomarkers and biomarkers of disease. Neurodegeneration may be exacerbated by epigenetic changes that develop gradually. Epigenetic biomarkers could aid in the diagnosis, and monitoring, of neurodegenerative diseases.

Neurodegenerative diseases are a vast collection of neurological disorders with various clinical and pathological manifestations that impact particular subsets of neurons in distinct functional anatomic systems; they begin for unexplained reasons and advance inexorably. Alzheimer’s disease, Parkinson's disease, Amyotrophic lateral sclerosis, Huntington’s disease, Friedreich ataxia, and Spinal muscular atrophy are the major neurodegenerative diseases. The prevalence and incidence of these diseases rise dramatically with age; thus, the number of cases is expected to increase for the foreseeable future as life spans in many countries continue to increase. Although there are several medicines currently approved for managing neurodegenerative disorders, a large majority of them only help with associated symptoms. The limitations of pharmacotherapy in these disorders have led to an urgent shift towards the development of novel compounds, interventions, and methods that target shared features across the spectrum of neurodegenerative diseases. Drug repurposing is a novel strategy where existing drugs that have already been approved as safe in patients for the management of certain diseases are redeployed to treat other, unindicated diseases. In this chapter, we have covered the current therapeutic options and drugs that can be repurposed or have the potential to be repurposed for the management of various neurodegenerative diseases.