How are the nonmotor features related (connected) to the motor features of PD?

The diagnosis of Parkinson’s disease (PD) often is based on the clinical effects of dopamine deficiency and the development of a motor disorder. However, the so-called “non-motor” features may develop before any motor signs. Discuss and explain what the non-motor features are, how they arise, what organs, cells and molecules may be involved and if the non-motor features can be used as biomarkers for PD. How are the nonmotor features related (connected) to the motor features of PD?