Dopaminergic neurodegeneration in the substantia nigra is associated with olfactory dysfunction in mice models of Parkinson's disease

Understanding the nose-brain axis and its role in related diseases: A conceptual review

The nose-brain axis (NBA), a critical component of the body-brain axis, not only serves as a drug transport route for the treatment of brain diseases but also mediates changes such as neuroimmune disorders, which may be an important mechanism in the occurrence and development of some nasal or brain diseases. Despite its importance, there are substantial gaps that remain in our understanding of the characteristics of NBA-mediated diseases and of the cellular and molecular mechanisms underlying the bidirectional NBA crosstalk. These gaps have limited the translational application of NBA-related research findings to some extent. Therefore, this review aims to address the conceptual framework of NBA and highlight its values in representative diseases by combining existing literature with new research results from our group. We hope that this paper will provide a basis for further in-depth research in this field, and facilitate the clinical translation of NBA.

Potential convergence of olfactory dysfunction in Parkinson's disease and COVID-19: The role of neuroinflammation

Parkinson's disease (PD) is a prevalent neurodegenerative disorder that affects 7-10 million individuals worldwide. A common early symptom of PD is olfactory dysfunction (OD), and more than 90% of PD patients suffer from OD. Recent studies have highlighted a high incidence of OD in patients with SARS-CoV-2 infection. This review investigates the potential convergence of OD in PD and COVID-19, particularly focusing on the mechanisms by which neuroinflammation contributes to OD and neurological events. Starting from our fundamental understanding of the olfactory bulb, we summarize the clinical features of OD and pathological features of the olfactory bulb from clinical cases and autopsy reports in PD patients. We then examine SARS-CoV-2-induced olfactory bulb neuropathology and OD and emphasize the SARS-CoV-2-induced neuroinflammatory cascades potentially leading to PD manifestations. By activating microglia and astrocytes, as well as facilitating the aggregation of α-synuclein, SARS-CoV-2 could contribute to the onset or exacerbation of PD. We also discuss the possible contributions of NF-κB, the NLRP3 inflammasome, and the JAK/STAT, p38 MAPK, TLR4, IL-6/JAK2/STAT3 and cGAS-STING signaling pathways. Although olfactory dysfunction in patients with COVID-19 may be reversible, it is challenging to restore OD in patients with PD. With the emergence of new SARS-CoV-2 variants and the recurrence of infections, we call for continued attention to the intersection between PD and SARS-CoV-2 infection, especially from the perspective of OD.

Experimental Animal Models of Prodromal Parkinson's Disease

There is an estimated 35-45% loss of striatal dopamine at the time of diagnosis of Parkinson's disease (PD), and cases clinically diagnosed in the early stages may already be pathologically in advanced stages. Recent large-scale clinical trials of disease-modifying therapies (DMT) also suggest the necessity of targeting patients at earlier stages of the disease. From this perspective, the prodromal phase of PD is currently the focus of attention, emphasizing the need for a prodromal mouse model that accurately reflects the pathophysiology, along with early biomarkers. To establish prodromal animal model of PD with high face validity that reflects the disease state, the model must possess high construct validity that accurately incorporates clinical and pathological features in the prodromal phase. Furthermore, as a preclinical model of DMT, the model must possess high predictive validity to accurately evaluate the response to intervention. This review provides an overview of animal models which reflect the characteristics of prodromal PD, including alpha-synuclein (aS) accumulation and associated early non-motor symptoms, with a focus on the aS propagation model and genetic model. In addition, we discuss the challenges associated with these models. The genetic model often fails to induce motor symptoms, while aS propagation models skip the crucial step of initial aS aggregate formation, thereby not fully replicating the entire natural course of the disease. Identifying factors that induce the transition from prodromal to symptomatic phase is important as a preclinical model for DMT to prevent or delay the onset of the disease.